Marine Engineering Practice.pdf 6l393y

Mumbai 23" October, 2000

k Qlatte rjee

C Eng.,F.I.Mar.E.,F.I.E. L

Extra First Class Engineer (U.K.) oday, Indian marine engineers are considered amongst the best in the Maritime Industry. Marine Engineers of today are required to have notonly "hands on" skill and competency, but also sound under pinning knowledge of the subject. This has been achieved through systematic training and education imparted by the premier Maritime Institutions in India.

This book "Marine Engineering Practice" written by Mr. Vikram Gokhale and Mr. N. Nanda, faculty of La1 Bahadur Shastri College of Advanced Maritime Studies and Research, Mumbai covers the subject "Marine Engineering Practice" (at Operational and Management levels) for Marine ~ngineerOfficer Certificates of Competency, Examinees. An attempt has been made to standardize the topics, with particular attention paid to practical difficulties, which are commonly experienced and important details, which are not readily available to seagoing engineers.

This book having a very readable balance of the "need to know" with the "nice to know" is, in my opinion, the first publication which attempts to cover the competency functions of "Maintenance and Repaif in the STCW'95 Code A-lll/l & A-II1/2. Mr. Vikram Gokhale and Mr. N. Nanda are both ex-Chief Engineers of Merctant Ships with a lot of practical experience, which they have used in writing this book for the benefit of students. 1 wish them a brilliant success in this quality publication.

4&L& (Ajoy Chatte jee) PRINCIPAL OFFICER & REGISTRAR OF INDIAN SHIES MERCANTILE MARINE DEPARTMENT MUMBAI DISTRICT.

he overwhelmingly good response to the first edition of this book, has prompted us to thoroughly revise and bring out this new edition. We have not only increased the coverage of topics, but also corrected some errors which had inadvertently crept in, despite our best efforts to keep the book error-free. Several of the sketches, which had not been printed clearly in the first edition, have been completely re-drawn and further simplified, so as to help students to understand the subject more easily. A lot of positive feed-back from our students has indicated that our first edition has successfully covered most of the topics of the new subject 'Marine Engineering Practice', at the Operational and Management levels of STCW 95. This type o i positive feed-back is most welcome, and will help us to hrther update the book. It is only through constant improvement, that a book can keep pace with the great changes that are taking place in this field. A 'good' text-book will renraitr 'good', only if it is keeping abreast with new technology, so that current practices are adequately covered. We earnestly hope that this will benefit all our Students.

Vikram Gokhale N. Nanda. LBS College of Advanced Maritime Studies and Research, Mumbai.

The authors gratefully acknowledge the help, information obtained and contributions from various of the Marine engineering fraternity, and in particular, Mr. N. V. Samant of the La1 Bahadur Shastri College of Advanced Maritime Studies & Research, Mumbai and Mr. V. P. Datar, of MAN B&W Diesel Ms. We are particularly appreciative of the information obtained through various books, technical magazines and other articles, which helped to cover the subject at an advanced level. We would not attempt to enumerate all those who have contributed in some measure to this effort, since the list would be endless. This is more the result of an industry effort.

Mr. Vikram Gokhale and Mr. N. Nanda are both Marine engineers from the Marine Engineering College (DMET) , with extensive experience not only as Specialists in the marine field, but also as ship-board engineers, tackling a variety of problems. They have the necessary technical background and training that makes a Marine engineer so unique.

The purpose of this book is to provide guidance. The authors cannot accept the responsibility or any consequences of use of this information for any other purposes. The opinions expressed here are those of the authors only.

Part I Maurine Diesel Engines Components Bedplate Forces and stresses Holding down bolts Resin chocks Checking the tightness of bolts Engine frame Tie rods Tie rod tensioning Consequences of running with loose tie rods Breakage of Tie rods Bottom end bolts - two stroke engines Bottom end bolts -four stroke engines Main Bearing bolts Camshaft Gear drive Chain drive Adjustment Maintenance and wear limits Crankshafts Defects Rebuilding ,Repairs Deflections Inspection Main Bearings Inspection Dismantling Criteria for rejection

Crosshead Bearings Clearances Failures Inspection Dismantling Connecting Rod Inspection Dismantling Piston and piston rings Material Differences between two and four stroke Decarbonisation Piston ring failure New advances Ring and liner compatibility Ring wear measurement (SIPWA)

.

.

Repairs of Engine components Stuffing box Relief valve Air Starting valve Distributor Turbocharger Cutting-off Running for longer periods with turbocharger out of operation Running for short periods with turbocharger out of opemion New advances Governor Overspeed Trip Fuel Injection pump Checking he1 pump timing -MAN B&W Adjusting he1 cam Checking fuel pump timing - Sulzer Fuel Injector Problems Testing 1 Inspection

Cylinder liner Inspection I gauging Causes of liner wear Clover-leaving Removal Cylinder lubrication Quill 0 Feed rate Thrust bearing Gears

Indicators Difference between peak / compression pressures Precautions Faults Engine alignment Piano wire measurement method Optical sighting method Basic troubleshooting Starting problems Exhaust temperatures high Rpm falling Smoky exhaust Hot spots in Crankcase Reduced compression pressure Loss of Power

Overspeed Knocking Alarms Main engine Auxiliary engines Auxiliary and exhaust gas boilers Condition monitoring

Repair methods Non destructive tests Failure of welded ts Repairs of Cast iron valves Welding of Cast iron components Cracks in crankshalls Leaks in heat exchangers Repairs on pumps Repairs of ts and fittings Repairing cracks on cylinder blocks Propeller repairs I Inspection, Repairs of Liners & Pintle cone. clearances, loss, repairs. Examination of dents. Rudder -

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Part II Safe working practices Ellectrial maintenance, troubteshooting Safe working practices Health Protective Clothing and equipment Working in machinery spaces Lifting gear Electrical maintenance, troublleshooting Safety

Do's and Don'ts First aid in case of electrical shock Insulation resistance testing Circuit testing Multimeten Insulation testing Continuity testing Fault finding Electrical system Circuit Breakers Transformers

Shore supply Circuit protection devices Generator maintenance Switchboard maintenance Electrical motors Care of motors Maintenance Single phasing Navigation lights Safety arrangements Maintenance Maintenance of Batteries Battery charging ~aintenaneeof electrical equipment in Tankers Electrical testing in hazardous areas Maintenance of Ex-protected equipment Surveys of electrical equipment Circuit troubleshooting Voltage drop Continuity Short circuit Earth fault Reading circuit diagrams Motor Starters - A.C. Magnetic Starter Troubleshooting Maintenance

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135 136

Part IIP - AuxiYiayr maclhmery k'umups, Heat exchangers, Refrigeration ik Air Conditioning system, Marine piping systems, Materials, Boiler water hreatment, Reverse osmosis plant, Deck machinery, Sewage plant, Valves and thek inspctiolri. Pumps Head, Power requirements, Casings.

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137 151

Impellers, shrouding and wear rings Bearings Shaft design Gland packing, Mechanical seals Priming, Pump overhaul, Troubleshooting Checking alignment, Vibration, overload and drop in capacity Overhauling Net positive suction head (NPSH) Pumping systems for special applications Crude oil tankers Chemical tankers Liquefied gas carriers Axial flow Pumps

Gaskets Belt drives Heat exchangers Heat transfer, Plate type heat exchanger Provisions for expansion, Aidocks T h e d conductivity, Tube renewal

172- 178 /'Refrigeration and Air conditioning Charging, Routine maintenance. Expansion valve, Maintenance planning Air conditioning problems, Calculations. Marine Piping systems 179- 184 Piping Drawings, Corrosion protection, Steam lines, Materials, Boiler water treatment. Reverse Osmosis plant [ ~ e c k Machinery Maintenance of winches, cranes, hatch covers Surveys Sewage plant Valves and their Inspection

185 - 186 187 - 189

Marine EngineeringPractice

Engiine Com1poirneirnts BEDPLATE There are different types of bedplates in slow-speed engines: They are completely enclosed on the underside, in order to form an oil sump, while the bearings for the crankshaft are ed by cross girders, which form part of the bedplate.

The Trestle type sits upon two parallel stools or raised portions of the ship's structure located in a fore-and-aft direction. The Deep box type of bedplate is with a flat bottom, which enables it to be bolted to the ship's flat tank top. Bedplates are usually fabricated from mild steel plates and steel castings, welded together; smaller engines may have cast-iron bedplates. The main advantage of the fabricated bedplate is its lightness and lower construction cost. Bedplates made of Cast-iron are able to absorb vibration better than fabricated bedplates, due to the good internal damping characteristics of Castiron. Usually the larger engine bedplates are of the deep box pattern with a flat bottom Smaller engines are either of the flat-bottom type or of the type that sits on stools forming part of the ship's structure.

Marine Engineering Practice

All parts of an engine bedplate contribute to its strength. The parts are made from mild steel plates and steel castings, which are assembled and welded together so that the bedplate is strong longitudinally (fore-and-aft) and transversely (port and starboard) with good resistance to twisting along its length. The longitudinal strength is obtained by making each iide of the bedplate in the form of a Girder. These girders may be box formed with two flanges and two webs, or as a normal girder with one web and two flanges. The cross girders in which the main bearings are housed give the bedplate transverse strength. The cast steel cross girders are welded to the sides of the bedplate, to give good resistance to twisting. The weight of the static engine parts is transferred to the bedplate through the A-frames. The sides of the bedplates, in way of the A-fiarne landings, are stiffened. Bedplates having the Deep box girder type construction have diaphragms between the webs and the flanges. The kansverse walls form the saddles for the main bearings. The strainer at the bottom prevents impurities fiom being canied over with the oil back to the sump. The sump contains the bulk of the lubricating oil, and its sounding is to be constantly monitored, so as to get a warning of oil leakage or contamination, which could adversely affect the engine.

SADDLE

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TYPICAL BEDPLATE OF A PROPULSION ENGINE

Marine Engineering Practice

Forces and stresses on a Bedplate The Bedplate has to withstand and transmit the stresses arising due to the following forces : 1. The 'firing' loads transmitted through the Tie-rods. 2. Primary and Secondary couples of the reciprocating masses. 3. Centrifugal inertia forces occurring when the vessel rolls in a heavy sea. Holding down bolts (Bedplate) A typical Holding down bolt (fitted type) is shown. The holes in the Tank-top are screw cut and the studs are screwed down, until the conical face, at the lower end of the enlarged part of the stud, seats on the tank-top to form a water-tight t with the Grommet as shown.

HOLDINGDOWN BOLT

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Holding down bolts for modem main propulsion low speed engines are of the 'long sleeve' type, and are hydraulically tensioned. Owing to its greater length, it has a greater elasticity and is less prone to cracking, than earlier bolts. These holding down bolts only withstand tensile stresses, and must not be subjected to shear stress.

13

Marine Engineering Practice

Resin Chocks An alternative to the traditional method of chocking using cast iron chocks, is the use of epoxy resin. The biggest advantage of using resin for chocking is the saving in time and man-power, which earlier methods required. Especially the time required for machining of foundation surfaces, which was very long, has now been cut-down. The engine needs to be correctly aligned, with respect to the shafting, taking suitable allowance for compression of the chock (about 111000of chock thickness).

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The surfaces are suitably cleaned with solvents, to remove all impurities like oil, rust and so on, which could affect the bonding. Dams are prepared to contain the resin, when it is poured in. The Holding down bolts are now fitted after spraying with a suitable releasing agent, to prevent the resin from adhesion. A slight head is given to the resin when pouring, so that it completely fills up the spaces. The deflections of the Crankshaft should be checked, especially after the resin has set, to confirm that they are within the limits. Resin chocking was initially developed as a repair technique, which considerably cut-down the time (which would have been required for machining metal chocks). Resin curing will take place in about 18 hours, if the ambient temperature is between 18' C and 25' C. This can take upto 48 hours, if ambient temperatures are lower. During the chocking operation, a sample of resin material from each batch is sent for testing purposes. This method lowers the bolt tension by a factor of 4, compared to metal chocks.

Marine Engirreerirrg Practice

HOLDINGWWN

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HOWIPiC DOWN BOLT

Checking Holding-down bolts and methods used to check tightness The Classification societies requirement is that Holding-down bolts be checked by a Surveyor, within each survey cycle. This interval of time may be too long and the bolts should preferably be checked at 6-monthly intervals, unless there is a case history of the bolts going slack more frequently. In new vessels, the bolts should be checked within one month of the commencement of the maiden voyage, or e d e r if possible. The interval may then be gradually increased if all is found in order. After a vessel has been through bad weather, the bolts should be checked as soon as possible. A rough method of checking Holding-down bolts is the hammer test. Hold the tip of the thumb on one side of the nut face and strike the nut on the opposite side. If the nut is slack, the nut and stud spring against the thumb and then retract. The movement can be felt against the thumb. If a holding-down bolt is of the fitted type, this test cannot be used, and a hydraulic jack must be used. Due to the presence of bilge water on the tank top at various times, the holding-down bolt nuts may rust and seize on the studs. In this case, the seized condition makes it seem as if the nut is tight. The hammer testing method, however, can be used in finding slack nuts, even when they are seized on a stud. Action to be taken if a number of holding-down bolts are found to be slack When chocks and their mating surfaces on the bedplate and tank top have fretted, the chocks cannot properly the engine. If the Holdingdown bolts are tightened, the crankshaft alignment may be seriously affected, with lesser effects being felt on crosshead guide and cylinder alignment.

15

Marine Engineering Practice

The seriousness ofthe situation will be depend on the amount of fretting that has occurred. Before any tightening of the Holding-down bolts is canied out, the Alignment of the crankshaft should be checked, by taking deflections with a dial gauge. If the crankshaft alignment is satisfactory, the slack chocks can be removed and smoothed on the mating surfaces and then replaced. The bolts can then be tightened, to harden the chock. After all the bolts and chocks have been tightened, the crankshaft alignment must be rechecked. ENGINE FRAMES A Frames are used to the cylinder block in most two stroke main propulsion engines. They are called A Frames, because they resemble the letter 'A' in shape. They are fitted at each transverse girder, starting fiom the ends. These are usually fabricated from steel plates.

'A' FRAME

[RTA]

In superlong stroke engines, high structural rigidity is of major importance. ~xtensivestress and deformation calculations are canied out, using full three-dimensional finite-element Computer models for different column designs, to optimum fiame configuration. The Bedplate and the Columns should be designed for minimum machining requirements. The cylinder jacket can be built for a sing!e unit, or may include more cylinders, depending upon the manufacturer's casting capacities.

TIE RODS The Cylinder entablature, 'A' Frames and the Bedplate are bound firmly together, by long steel Tie-rods, ing through hollow columns.

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TIE RODS EXHAUST VALVE HOUSIh'G

CYLINDER HEAD -$$&GAS

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JACK BOLTS

COMPRESSIVELOAD

The Tie rods are of ordinary Mild steel, and screwed at each end to take the nuts. The lower nut is squared and fits into an opening of similar shape cast in the Bedplate, so as to prevent the nut from turning, when the bolt is screwed into it, and the upper nut is tightened. For a 9 m height engine, the Cast iron parts of the engine would be compressed from 0.5 - 0.75 mm by tightening the Tie bolts. The Tie rods are pre-stressed at assembly, so that the Engine structure is under compression at all times. Two Tie-rods are fitted to each transverst: member, and through tubes. In large superlong stroke low speed propulsion engines, the Tie rods may be in two parts, to facilitate ease of removal. To prevent any lateral movement which could cause vibration problems, 'pinch' bolts are fitted.

Marine Engineering Practice

Tie rod tensioning The Tie rods need to be checked for proper tension at intervals laid down in the maintenance schedule, and also after any scavenge fire. This is done hydraulically for modem engines. Large tie-bolts are tightened with a hydraulic jack which loads the tiebolt in tension. The tie-bolt nut is usually drilled to take a toggle bar or, slotted to take a hook spanner, and when the correct pull is on the tie-bolt, the tie-nut is pulled up hand-tight; the pressure in the jack is then released leaving the tie-bolt tight. The load placed on the tie-bolt by the hydraulic jack is controlled by the hand-pump pressure, which is indicated on the pump pressure gauge.

TENSIONING OF TIE RODS

This is done by the following method : 1. Connect two pre-tensioning jacks to two adjacent tie bolts. 2. Obtain the required hydraulic pressure and maintain this. 3. Check the clearance between the nut and the intermediate ring and adjust if required. 4. Release the pressure and remove the jacks. By pre-tensioning the Tie rods, a pre-determined tensile load is induced in the Tie rod, and a pre-determined compressive load is induced in the Entablature, 'A' Frame and Bedplate. During the firing stroke, the tensile load on the Tie rod increases. The compressive stresses on the Entablature, 'A' Frame and Bedplate are reduced, thereby keeping the components at very low fatigue levels. In other words, if the Tie rods were not pre-tensioned properly, the fluctuation of stresses in the components between the firing and the non-firing periods could be higher, leading to fatigue failure of components. To minirnise bending, Tie rods are placed as close to the Crankshaft axis as possible. This reduces the Bending stress on Girders and prevents unbalanced loads from being transmitted to the welds.

Marine Englneerlng Pracrlce

I How to find loose tie-rods, while the engine is running. If tie-rods are not taking their proper load due to slackened nuts, the cylinder jackets adjacent to the slack tie-bolts can be seen lifting, when the piston is nearing the end of compression, or on firing. If no movement is visible, the thumb can be pressed on the cylinder jacket, with the thu'mb nail in with the tie-bolt nut. Small movements, too small to be visible, can be felt with the thumb nail. If washers are fitted between tie-bolt nuts and cylinder jackets, a washer may sometimes be twisted at the end of the firing strokes in each cylinder adjacent to the slack tie-bolt. Dial indicator gauges can also be used to detect relative movement between the tie-bolts and cylinder jackets. Tie-bolts should be kept under observation at all times, particularly when running at reduced power in bad weather and following power increases during moderating weather. Fretting of landings of the cylinder beam on the A-kames and of top tiebolt nut landings are signs of loose tie rods. Fretting of a surface is often indicated by a rust-red powder being present at the outside of the faces that are ketting. If the location is dry and k e e of oil, the red powder looks as if it has been dusted on the surfaces, being thickest at the t.

Consequences of running an engine with slack Tie-rods If an engine is run with slack tie-rods, the cylinder beam flexes and lifts at the location of the slack tie-bolt. In time the landing faces of the tie-bolt upper and lower nuts, and the landing faces of the cylinder beam on the Aframe, fret and the machined faces are eventually destroyed. The fitted bracing bolts between the cylinder jackets will also slacken and the fit of the bolts will be lost. If fietting has occurred in an uneven pattern, where the cylinder beam lands, and the tie-bolts are tightened, the alignment of the cylinders to the line of the piston stroke is destroyed. After fretting has occurred, nut landing faces may be out of square, and if tie-bolts are tightened on faces which are out of square, a bending moment will be induced in the tiebolt. This, in turn, causes an uneven stress pattern in the tie-boit, which could lead to early fatigue failure. Breakage of tie-rods If, during an inspection, a Tie-rod is found to be broken, it must be replaced, as soon as practicable. If the breakage leaves the lower portion short remove it through the Crankcase and withdraw the upper part from the top. If the breakage leaves a long lower portion, the Tie rod should be cut and removed

Marine Enginering Practice

*

BOTTOM END BOLTS ( SLOW SPEED ENGINES) These bolts are under repeated applied stress. The various stresses undergone by the bottom end bolts are shown below.

+

Stress

-

+-

Fluctuating

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+ Alternating

Reversed Stress : The stress range is symmetrical about the zero sees5 line.

Repeated Stress : The Bottom end bolt is stressed and then completely unloaded. Fluctuating Stress : The Bottom end bolt is stressed either compressive or tensile, but the stress range does not through zero. Alternating Stress : The Bonom end bolt undergoes a stress range, which es through the zero stress line. Hence it changes &om Tensile to Compressive. But it is asymmetrical about the zero stress line. These ranges of stresses ing through the zero line can have the effect of lowering the life span of the component, for the same stress range. It is prone to fatigue failure : One portion of the fiacture will be discoloured, and relatively smooth, whereas the other portion will be clean or crystalline. The fatigue limit is increased by using alloy steels, and is reduced by changing the section of the bolt, fillet and so on. Life of Bolts If the Bottom end bolt has completed approximately 18000 to 20000 running hours (4 stoke), it must be renewed, whatever its condition. They should also be renewed if a piston seizure takes place or a failure of the overspeed trip. For large highly stressed Bottom end bolts, the traditional means of tightening by hammer has been replaced by hydraulic tightening, which ensures more uniform tightening. There is no shock loading and bending moments (twistj.

Marine Engineering Practice

I

BOTTOM END BOLTS ( 4 STROKE ENGINES) Bottom end bolts of 4 stroke engines are subjected to fluctuating cyclic stresses and therefore exposed to potential fatigue failure. They experience large fluctuations of stress during cycles. This is due to inertia forces experienced in reversing the direction of the piston over the TDC,on the exhaust stroke. The forces experienced in this situation are very high. Load As Stress Area It implies that for a given load, the stress can be reduced by increasing the area, and therefore increasing the size and weight of the bolts in four stroke engines.

MAIN BEARING BOLTS These are subjected to heavy loads, fluctuating in magnitude and direction. They transmit the downwards force from the engine frame. Comparison between Bottom end and Main bearing Bolts Bottom end bolts, as compared to Main bearing bolts, have : Higher fatigue limit Higher tcllsilc duclilily, as conducive Lo higher fatigue strength Fatigue limit is increased by case carburising, nitriding or carbon nitriding. They are manufactured to higher standards of finish. They have increased diameter at mid-shank, to reduce vibration.

Marine Engineering Practicr

BOTTOM END BOLTS ( FOUR STROKE ENGINES) Piston scuffing stretches the Crankpin or bottom end bolts as the crank drags the piston along.

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'* The flexing of the Connecting rod foot causes the bottom end bolts to bend. (See figure below).

The only way to safeguard against failure of the bottom end bolts is to renew them every 30,000 hours, or earlier (check the engine manual). Every time the bottom end bolts are replaced, an entry is to be made in the Log, so that the running hours can be recorded.

Marine Engineering fiucticr

It is important not to exceed the Elongation limits of the bottom end bolt at any time. This value can be found in the engine manual. Before tightening the bottom end bolts, use a 'stretch' gauge a s shown above. The problem occurs when any bolt comes loose during running. The bolts can be checked during the Crankcase inspection, either by checking the split pins or by 'hammer' test, to confirm that they have not worked loose. Never over-tighten the bolts, as thread friction varies from nut to nut, and can cause deformation of the bolt.

Tighten both sides till the stretch gauge indicates exactly the same reading on each of them. Record the stretch of the bottom end bolts in the history for the engine, so that comparative figures are available at the next check of the bolts.

Marine Engineering Practice

CAMSHAFT There are two different types of drives - Gear drive and Chain drive. GEAR DRIVE A train of Gear wheels is fitted, which transmits the drive from the crankshaft to the camshaft. Appropriate speed and direction is achieved, by suitably arranging the intermediate gears in different positions and sizes. Unlike a chain, the gear does not lose tension; however, gears are subject to damage and difficult/ expensive to replace.

Large superlong stroke engines have Moment Compensators fitted to the gears. The balance weights rotate at the full engine speed or twice the engine speed, in synchronisation with the Crankshaft to take care of primary and secondary vibration.

Marine Engineering Practice

1

I

1

CHAIN DRIVE Chain drives are used by many manufacturers, to give more flexibility in location of camshaft, as well as ease of repair I replacement of parts at a lower cost; however this gives a more complex arrangement, with a large number of moving parts subject to wear and slackening, which can affect the timing of fuel a s well as other factors in engine operation.

CAMSHAFT

5-9

CHAIN DRIVE

The chain tension needs to be checked at intervals given in the maintenance schedule, or earlier if any doubt exists.

GRASP AND

PULL AWAY

CHECKING CHAIN TENSION

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To carry out the checking, turn the engine to slacken the longest free length. At the middle of the longest free length, grasp the chain and try to pull it away from the guide bar, by a distance of approximately half a chain link. If it is possible to pull more, the chain is slack and needs to be re-tensioned. Loosen the screw of the Chain Tightener, and by turning the 'loose' plate of the Chain Tightener, adjust the tension of the chain, as shown below. CHAIN TENSION ADJUSTMENT ADJUSTMEN

Excessive chain tension overloads the chain and the bearings and will wear unnecessarily. Insufficient chain tension may give rise to vibration, in the case of resonance, which, in the long run,may damage the chain. Lubricating the chain drive Special attention is to be paid to the securing of the lubricating oil pipes and spray nozzles. The spray nozzles must be directed so that the oil jets correctly hit the side bars. During operation, :he functioning of the spray nozzles must be periodically checked and the nozzles cleaned if necessary. Maintenance of the chain drive The chain drive is to be checked at the prescribed intervals given in the manual. At overhauls, the chain must be examined for signs of damage, such as cracks in the wheelsor side bars, jamming links, pitted wheels, traces of blows on the outer parts on s f catching, or scouring on the insides of the side bars, due to poorly aligned sprocket wheels. The causes of these defects must be put right immediately. Damaged or broken chains must be replaced as soon as possible.

Marine Engineering Practice 1

During operation, the teeth of the sprocket wheels acquire a polished or slightly worn band above the root circle. A slight polish is often produced on the inner side bars, in spite of correctly aligned wheels. If there are signs of marked wear, however, its cause is to be investigated and the chain closely inspected. Burrs on the edges of the teeth point to vibration of the chain. Sprocket wheels whose teeth sustained major deformation should be replaced as soon as possible. The bearings and bearing pins, their fastenings and their true and even running are to be checked at overhauls. issible wear of the chni~i

The chain wear is directly proportional to the lengthening of the chain. The maximum issible chain elongation is approximately 1 % of the length of the chain. Also check relative position of camshaft w.r.t crankshaft, the limiting maximum deviation allowed being 2'.

CRANKSEIAFTS Crankshafts of marine engines are made up of a number of cranks, which are built-up to form a single shaft. Lubrication of the crank bearing is important, and is usually from the Cross-head, through the connecting rod. This eliminates the bores, which act as stress raisers. Crankshafts are rotated by forces transmitted through the comecting rods and bonom end bearings. Every crank is made up of two crank webs ed by a common crank-pin, to which the bonom end bearing is fined.

I

Each web is connected to a journal to form pan of a continuous shaft. Main bearings the shaft, at each journal. One end of the shaft is connected to the Flywheel, to transmit the engine power. The other end is called as the 'free-end'.

Balancing of the engine can be done t y varying the Crankpin bore holes, thus eliminating the need for bolted-on counter weights. Gear drive or chain drive wheels drive the camshaft, which needs to be synchronised with the crankshaft movement. The crankshaft is subjected to fluctuating bending, torsion and shear stresses. The materials used have high strength, fatigue resistance and form good bearing surfaces. Carbon steels or low alloy steels are used. Two-stroke main propulsion engine crankshafts are too large for a single forging and are of a built-up construction. Large crankshafts are of solid welded construction. A number of parts are forged or cast separately and then welded together to form a complete shaft. Each part consists of a crank-pin and two webs, with a half length ofjournal on each web. Parts are heat treated and rough machined; they are placed end-to-end, at the correct crank angles and narrowgap submerged-arc welding s them to form complete journals. Post heat treatment is followed by finish machining of journals and their central bore holes. Welds are stress-relieved and subjected to non-destructive tests. Some Crankshafts are still built up with each throw of the crank-pin. having two webs, which is either forged or cast and then machined. Holcs arc bored in each web to allow them to be shrunk on to adjacent joumals which have also been forged and machined separately. The webs are heated to the correct temperature and then cooled slowly, shrinking them on the journals to form a continuous length of crankshaft with correct angles between units to correspond with the firing order of the engine. Shrinkage fits are well able to transmit the total engine torque without slippage. No oil holes are drilled in built-up shafts. Welded crankshafts allow a considerable saving in weight. Webs are of reduced width and thickness, allowing longer bearing length. Overlap permits large diameter journals and pins, reducing bearing pressures. Shafis have a higher natural frequency of torsional vibration which improves design.

CRANKSHAlT DEFECTS 1. Misalignment due to worn-out main bearings and excessive bending of engine frame.

Murinr Enginrrrittg Prudice -

2. Vibration due to incorrect power balance, running in Critical band, running vessel in Light condition for long periods leading to impulsive forces from propeller, excessive wear-down of Thrust bearing. 3. Defects during manufacture, such as slag inclusions, incorrect heat treatment and machining defects of oil holes and fillets. 4. Fretting corrosion, which increases with load, amplitude of movement and frequency. (Refer Advanced Marine Engineering Knowledge - Volume 1) 5. Bearing corrosion, if fuel and lubricating oil combine, resulting in weak acid formation, which could damage the bem'ng suhces, resulting in scoring.

RE-BUILDTNG CRANKSHAFT When a crankshaft is damaged, it may often be un-economical to emove and send ashore for repairs. In that case, a possible replace, or ev alternative is in-situ epairs. Sometimes it is sufficient to re-grind the journals and fit over-sl arings. If repair in-situ is not possible, if the wear is excessive and there is no suitable over-sized bearing available, the crankshaft must be sent for reconditioning. In that case, the journals are built-up, by arc welding, s-gedL..'y~. arc welding or TIG I MIG welding and Chrome plating. The welding machines used are automatic. The crankshaft is first c l e a n 4 witk+kaline solution. All *"I *.*l * scores are removed by grinding. The welding & is then positioned and the oil holes in the crankshaft are plugged with ceramic plugs, to prevent weld B r f from entering the openings. M e r completion of welding, the crankshaft must be stress-relieved. This may be done by butane burners. A temperature indicating crayon is used to mark the crankshaft. As the heat is applied, the colour of the mark changes, which indicates when the desired temperature is reached. This repair is acceptable for 4 stroke engines, however replacement is still the preferred option. After repairs, the crankshaft must be checked for kinetic and dynamic balance.

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: 1

Marine Engineering Practice

Repairs on Crankshafts Crankshafts and crank pins are usually r e p ~ r e dfor uneven wear, which makes them 'out of round'. One solution is to re-condition by hard chrome plating, which is then ground down to the correct size. If however, this is not possible, then 'metal spraying' is also applied to build-up the surface. Metal for spraying is usually special carbon steel. The journal are first prepared, and then the spraying is carried out as soon as possible, to avoid oxidation of the cleaned surfaces. After spraying, the journals are ground down to the correct size, as this steel coating is too hard for machining. Wet grinding is preferred so as to prevent the expansion of the metal layer from the heat of the grinding.

CRANKSHAFT DEFLECTIONS Misalignment of the crankshaft occurs due to wear of main bearings or distonion of the engine bedplate. This can also occur due to running aground, from damage to the ship's hull. It can be detected by measuring deflections of crankshaft webs, for each mit of the engine.

Marine E~:g!i:eeringPractice

If misalignment exists the crank webs will open and close slightly as the engine is rotated; this is measured by means of a clock or dial gauge fitted between adjacent webs at a point in line with the outside of the journals furthest from the crank pin. A spring extension rod will hold this in position. The first measurement is taken with the engine just beyond bottom dead centre position with the gauge close to the side of the connecting rod. It is usual to set the gauge to zero. The engine is now rotated by the turning gear and stopped at each quarter turn, where gauge readings are taken as plus or minus values. The final reading is taken near bottom centre, with the connecting rod on the opposite side of the gauge to the first reading. The first and last readings are averaged to use as an approximation for bottom centre position. This procedure is repeated for each unit in turn. Feeler gauges should be used to ascertain that the crankshaft has not lifted. It may he advisable to rotate the turning gear slightly in reverse direction after stopping for readings, this will ensure free positioning of the cranks, particularly for those adjacent to the turning gear. All readings are recorded and these should he compared with previous values, preferably with the ship in a similar load condition and at similar temperatures. Total deflection vertically and horizontally is calculated for each crank. The vertical total will be proportional to misalignment between the bearings due t o wear-down. The horizontal total indicates side wear in the bearings. By plotting all vertical deflections for the whole engine, it is possible to obtain information as to which main bearings are 'high' and which are 'low'. This may be assisted by bridge gauge readings from the bearings, but these do not take possible distortion of the bedplate into . It must be checked that the journal is in with the bottom surface of the bearing. Limiting values for maximum deflection are set by engine builders. These depend upon the stiffness of crankshaft, the engine's strokebore ratio, and so on. They indicate the limits to which misalignment may be permitted before renewal of bearings and realignment are necessary. Excessive misalignment will cause bending of the crankchaft and webs with fluctuating and alternating stresses, causing fatigue and the possibility of shaft failure. It will set up vibration and cause damage to main bearings. Frequency of inspections 9 Crankshaft deflections should be recorded at the time of installation. 9 After the first 1000 hrs. ofrunning. 9 Annual ( during over-hauls). During this period, ship should be in the loaded condition and the engine warm.

Marine Enginrering Practice

MGLN BEARINGS Main bearings are presently of the 'thin shell' type. They are white metal lined, as white metal provides greater safety and tolerance against misalignment, as compared to the conventional tin-aluminium bearings. They have a superior nmning-in characteristic as compared to tin-aluminium, which have a running-in layer of lead indium. Main bearings provide to the Crankshaft, and take the weight of the working parts, besides the loads due to combustion. The Main bearing clea-~ncesmust be regularly checked, to ensure that lubrication is not reduced and alignment is not affected. The steel backing shells are held in place by the bore of the housing and are designed to provide 'nip', i.e. a good interference fit, to provide adequate grip on the bearing shell, preventing it from turning in the housing. The nip provided is not great enough to cause distortion, which could adversely affect the running clearances.

Main Bearing of RTA-ST (Weight Saving 40%)

Marine Engineering Badice

Inspection Main bearings need to be inspected at regular inteivals, as per the requirements of the CSM, and also in case the Crankshaft deflections show this to be necessary. Main bearing clearance should be zero at the bottom, since the engine is stationary, and the journal is resting on the bottom shell. If this is not so, it means that the crankshaft is out-of-alignment. The clearance of the bearing can be checked by means of special feeler gauges, provided by the manufacturer for this purpose. CEIECKING MAIN BEARING CLEARANCE

Marine Engineering Badi

Main bearing dismantling The dismantling of the Main bearing is carried out as below : Upper half rr Open all Crankcase doors and ventilate. rr Disconnect the lubricating oil inlet pipes. rr Loosen lockings on the nuts and slacken the nuts using the hydraulic Pump. rr Remove the nuts and lockings. Lift the cover by means of chain blocks, adequately ed on the A-Frame. rr Fit the eye-bolt on the top half shell, and lift it by the chain block, as shown :

BOTTOM HALF rr After dismantling and removing the Upper main bearing shell, connect the oil pump and hoses to the bore below the Main bearing shell. rr Insert the supplied tool pin in the oil hole on the journal, and tighten it.

Marine Engineering Radice

u rr rr

Pump oil to lubricate the underside or lower half Turn engine by means of the turning gear, so that the pin drives the lower half shell around and out, to the upper side of the journal. Lift the lower half by the chain block, connected to the eye-bolt.

Alternative method :

Jack u p the webs as shown in the sketch, by means of the jacks mounted on the cross-piece,, by approximately 0.2 mm, so that the lower half is free of the load. u Attach the nylon wire rope, ing it around the ends, as shown. rr Pull the lower shell around, by means of the chain block and a wire ing over the small fairlead suspended over the bearing. rr The lower half shell will be rolled-over the main bearing journal when it turns through 1 8 0 ~ . rr The lower shell can now be lifted out by the chain-block arrangement. rr

Bearing shells are inspected at the time of dismantling. After cleaning, they must be inspected, to check if they are fit for further service, or whether they need to be renewed Criteria for rejection The bearing shell must be rejected i f : 9 Bearing metal is cracked or broken. 9 Bearing surface is grooved over more than 10 % of its area. 9 Fretting has taken place in the housing. 9 There is less than 90 % area, between the back of the bearing and the housing. )Z The 'arc of ' between the journal and the bearing shell is less than 140' and the area is less than 90 % within the arc. 9 Excessive wear due to adhesion. This occurs due to scuffing and scoring. This action is intensified by high working temperature and a high intensity of loading. The loss of metal is termed as 'Wiping-off . 9 Excessive wear due to abrasion. This occurs due to dust and other impurities in the lubricating oil. 9 Excessive wear due to corrosion. When the lubricating oil turns acidic, it attacks the bearing material. CROSSHEAD BEARINGS The reciprocating motion of the piston, is converted to the rotary motion of the crankshaft, through the Crosshead bearing. This bearing, in two-stroke engines, is extremely difficult to lubricate, since it is uni-directionally loaded. This means that there is no time when the load is relieved, for lubricating oil to enter easily. Various manufacturers have devised different ways to counter this. Some engines use separate higher pressure lube oil pumps for the Crossheads. The use of a larger area for the heavily loaded lower shell has resulted in a substantial reduction of the specific loading, and improved the load carrying ability considerably. Unlike most bearings, where hydrodynamic lubrication is easier to achieve to due high relative motion of rotation, the Crosshead bearing merely oscillates about its pin. This makes it difficult to generate a hydrodynamic film of oil, which can effectively cany the load. When the exhaust valve opens, the cylinder pressure drops to its lowest value. It is around this point, that the oil can enter to 'lift' the Crosshead pin, and create a suitable fluid film, in preparation for the next firing stroke and the subsequent high loading.

Marine Engineering &actice

Failure of Crosshead bearings Failure of the Crosshead bearing can occur due to various reasons : > Misalignment of the running gear. > Poor quality of the material. > Poor surface finish, causing early deterioration. > Starvation of lubricating oil. .. > Overload of the engine. Signs of failure are : Squeezing of the white metal leading to lubrication problems. Cracking of the white metal shell. Fatigue failure. Melting of the white metal, due to overload (overheating). Wear of the bearing due to abrasion or corrosion.

Marine Engineering *adice

t

Inspection of crosshead bearings The low sliding speed and uni-directional loading causes severe conditions in the operation of the Crosshead bearing. They require to be carefilly inspected, during fitting as well as during service. Checks without dismantling : J Feel the bearing, while still warm, for abnormal rise in temperature. J While the oil is still in circulation, after just stopping the engine, check that uniform jets appear from all the outlet grooves in the lower shells. J With oil circulation stopped, and the crank in BDC position, check the top clearance with a feeler gauge. J Check for signs of squeezed out metal, wiped out or loosened overlay. Also check for fragments of white metal in the oil. If the following are noticed, the Crosshead bearing needs to be dismantled : u Bearing running hot. rr Oil jets reduced 1 absent or twisted. Excessive clearance. u Signs of damaged white metal. Dismantling of the Crossbead bearing Turn the engine till you have access to the nuts on the piston rod, the telescopic pipe and the Crosshead bearing cap. 3 Loosen and remove locking from piston rod studs and piston rod foot. Loosen and remove the lock nuts from the Crosshead bearing cap. = Mount the spacer rings and hydraulic jacks for loosening the nuts on the piston rod and the Crosshead bearing cap. 2 After removing the nuts, and taking out the hydraulic jacks. 3 Remove the two studs for the piston rod from the Crosshead, by means of the stud setter tool. (see figure). 3

Murine Engineering B u d i c e

3 3

3

3

3

Loosen the screws of the Telescopic pipe and mount the lifting tool for suspending the Telescopic pipe. (Never turn the engine without doing this.) Turn the engine to TDC to dismount and suspend the Telescopic pipe.

w--

Turn the engine to take the Crosshead downwards, to give access to mount the chains and eye-bolts for suspending the piston from the cylinder frame. Suspend the piston rod, by turning the Crosshead downwards, as shown.

Turn the Crosshead to BDC. Hook on Bearing cap and raise to inspect the upper bearins shell. as shown.

Marine Engineering Practice

3

By means of tackles, raise the Crosshead from the Connecting rod, to inspect the lower half of the Crosshead bearing and Crosshead pin.

3

lf all is in order, the Crosshead may be re-assembled.

Damage to the Crosshead bearing The Crosshead Pin or Journal may become oval or roughened if the loaded part of the Journal has worn heavily,'or there are scratches on more than one third of the area, or the white metal has been 'wiped out' over a wide area. The Crosshead Pin then needs to be re-conditioned ashore. The Bearing shells may need to be replaced, if there are crack formations or the white metal is smeared out (wiped-out) due to heat softening and plastic deformation. CONNECTING ROD

The Connecting rod transmits the gas forces from the Piston to the Crankshaft. The bearings of the Connecting rod are the Top end (Crosshead) and Bottom end (Crankpin). They are machined from a steel forging shaped at each end to accommodate the bearings. Usually the oil hole is bored through the centre of the rod, for flow of the lubricating cum cooling oil. This flow is downwards in the Crosshead engines, while it is upwards in the Trunk piston engines. For Trunk piston engines, the connecting rod usually has an obliquely split bottom end or small palm type, with shims for adjustment of the compression pressure.

Marine Eigineering Badice

1,ARC:E TWO Sl'ROU. hUIN PROPlllSION ENGINE

CDNNK1lNG PISTON ENCINE

Marine Engineering Raclice

Inspection procedure The Connecting rod and the Bottom end bearing are inspected for fretting and cracks. Any defects noticed should be made good. In case of trunk i ~ t o nengines, the Connecting rod alignment is to be checked after piston a ' ? g In order to do this, the Connecting rod is to be I,,? :*,r placed on the surface plate, as shown, with suitable mandrds closely fitting in the top and bottom end bearings. By traversing a dial gauge on both ends of both mandrels, any twist in the Connecting rod can be detected.

&.

.,

n V X T LN C O N N E m G ROD

Dismantling of the Crankpin bearing Remove the Crankcase doors, and turn the crank of the unit to TDC. 3 Lockings to be removed, and nuts slackened by hydraulic jacks. 3 Suspend the chain blocks, as shown, on each side. With wire slings, shackles and eye-bolts, lower the Bottom half into the Crankcase. 3

Marine Engineering Ptudice

Lift out the bottom end bearing outside the crankcase, by appropriate shackles and chain blocks. a Fit the Crosshead holding pin device to the Guide as shown. Turn engine till the Crosshead rests on the holding pin. 3

a the Connecting rod adequately, and continue turning the engine, till the Crankpin is clear of the upper half, in order to have sufficient space for the inspection.

Marine Engineering R a d i c e

PISTON The piston transmits the gas forces to the Crankshaft through the piston rod. It must have a long fatigue life to survive the fluctuating mechanical and severe thermal stresses during its working life. The material must be resistant to high temperature 'creep', corrosion due to acids and erosion. The material depends on the size, the rating and the fuel used. Large two stroke engine pistons are made up of various parts - Crown, Skirt and Cooling element. Pistons are cooled either by water (e.g. Sulzer RTA %series) or oil (MAN E&W MC engines and Sulzer RTA 2-series).

OIL COOLED PISTON

WATER COOLED PISTON

In both the cooling systems, periodic inspection must be carried out. Fresh water cooling has the advantage that fresh water has twice the thermal capacity of oil, and a higher outlet temperature may be maintained (oil gets carbonised at high temperatures). However, water leakage would lead to contamination of the lubricating oil with disastrous consequences.

Marine Engineering h a i c e

Material Cast Steel for piston crowns.(for 2 stroke Crosshead engines.) (for 4 stroke Trunk piston engines.) Aluminium for pistons Forged steel for piston rod. Cast iron for piston skirts. DiNerences behveen Two stroke and Four stroke pistons A Four stroke piston is usually of the trunk type, i.e. it is directly connected to the crankshaft, via connecting rod. It has a Gudgeon pin, which helps in converting the reciprocating motion of the piston to the rotiuy motion of the crankshaft. A Two stroke piston is usually of the cross-head type, i.e. it is connected via the piston rod to the Cross-head bearing, which reciprocates along with the piston. The connecting rod connects the crosshead tqthe crankshaft. The Four stroke piston is usually shorter i&ngth. It is cast in one piece, and carries compression as well as oil-scraper rings. The piston is usually splash-lubricated, and hence the need for oil scraper rings. All of the above components are normally of different materials. In the latest engines, oil cooling is enhanced by the so called 'Jet shaker' principle, which increases the heat transfer rates to improve cooling efficiency. I

COOLMO

, ! ! RTA-2 OIL COOLED PISTON jet-shaker coollng ere-

rmx

-

20%

.-

20 30 40 50 €0 70 80 90 100 110 120

46

Marine Engineering ~ & e

1 I

I

Decarbonisation of Pistoti The Cylinder head connections are removed, and Cylinder head nuts removed by hydraulic means. The Cylinder head is now lifted. Carbon deposits on the upper part of the liner are removed, prior to removal of Piston. Turn the engine to TDC and clean the lifting holes in the Piston Crown, and secure the lifting bracket. The Piston rod Palm nut is hydraulically removed. The weight of the Piston should now be taken on the Engine room crane, and the Engine is now turned on Turning gear, to lower the Crosshead clear of the Piston rod. In some engines the Stuffing box needs to be dismantled and comes out with the Piston. The Piston is now landed on to its place for cleaning, overhaul and inspection. External inspection should be carried out for wear, corrosion and cracks. Piston Crown should be gauged for burning, corrosion and fatigue cracks (if required). Piston ring should be calibrated, and ring grooves cleaned and examined. If cleaning of the internal cooling spaces, and subsequent pressure testing is required, the Piston needs to be dismantled. Liner to be properly cleaned and calibrated. Lubrication is checked by manually operating the lubricators. Stuffing box is over-hauled, if required. It should be ensured that all safety precautions are taken.

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WORN

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PISTON TUNCG

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900 MM

PIZR)I~I,E B I I W - A W A Y OF PISTONCKO\YI*' 900hCM BORE

N I W rWG I WORN CRCOVI HIWYIH(ilYL.COPIIXIIOHII)<+Y YT.

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HM

MU 900HMBORL I11 115.

Marine Engineering Practice.

PISTON OVERHAUL

Pistol! ring failure > Piston ring may fail due to the following reasons : > Insufficient piston ring to groove clearance, causing the ring to jam in the groove. > Insufficient lubrication causing abrasive wear. > Excessive wear 1 ovality of liner. > Excessive diametrical clearance between Piston and Cylinder. > Ring gap inadequate leading to breakage. Improvements Piston ring grooves are now protected with a galvanic chrome layer on both sides. This is called as Double chrome plating. The layer thickness has been increased suitably to give increased corrosion resistance.

Piston cleaning ring (PC ring) The development of the new standard of a high top-land and a lower height of the Cylinder liner was, to allow the Cylinder head to bear the brunt of the harsh conditions in the cylinder. This resulted in a lot of carbon formation on the upper part of the Piston crown, which lead to increased wear. The purpose of the Piston cleaning ring is to scrape off excessive ash and carbon formation on the piston top-land and thus prevent between the Cylinder liner and these deposits. which would remove part of the cylinder oil from the liner wall. 'Bore polish' may be a decisive factor in the deterioration of the cylinder condition, especially for high-rated large engines. PC rings are now standard on the most recent large bore MC engines. The Piston cleaning ring is located on the top edge of the Cylinder liner. just below the Cylinder head. With the increasing mean indicated pressure, the traditional angle-cut ring gap may result in increased thermal load on the cylinder liner. With the new R or Controlled Pressure Relief piston ring, the thermal load on the cylinder liner is significantly reduced as no gas will through the double lap t. The top ring is 30 % thicker. The relief grooves ensure an almost even distribution of the thermal load from the combustion gases over the circumference of the liner and, as a consequence, the thermal load on the cylinder liner as well as the second piston ring is reduced. This has been confirmed by temperature nleasurements. With the R piston ring, it is possible to choose the total area of the relief grooves and the optimum number of relief grooves, so as to provide the optimum pressure drop across the whole piston ring pack. A running-in layer of Aluminium Bronze has recently been introduced on the piston rings to facilitate running-in, so no special running-in procedure is now required, thus saving time and costs. ........ .....

... .. .... .. ........, .. ..,:. .. ..

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-

Upperpiston ringwith double-lap 5 seal and Controlled h s x u r e &lief p p s Euen heai distribution on eccondpishn rLy Other ring have oblique cut geps

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Marine Engineering R&e

SlPWA Wear Trend processing The Sulzer lntegrated Piston Wear Analysis is a means of on-line measurement of piston ring wear. It also gives an advance warning of any piston ring problems, which can then be corrected. WEDGE SHAPED WEAK BAND S.I.P.W.A. PISTON RING ITOPl WEAR { RADIAI.

1

(

1

DIGITAL m j PIUN'IER

, SENSOR

TN THE CYLINDER LrNER

Repairs of Engine Components Reconditioning of certain engine components such as Piston and grooves is carried out at authorised workshops, where product is identified via reference numbers and at the same time the base metals present. Latest welding materials with correct flux, and proper heat treatment, increase the quality of the reconditioned product. M e r welding process, all products must a full heat treatment process, which takes at least 24 hours. The actual preheating is steady for 4 hours and it is followed by controlled cooling down. Final machining is done on CNC units.

Murine Engineering Practice

Reconditioning of piston crowns is done with Inconel welding materials, which can be applied by using submerged-arc strip welding equipment. Areas around injector bores are protected with Nickel cladding. All products are subject to final inspection and Class approval. Corrosion occurs in the gas duct of the exhaust valve housing around the spindle guide boss and opposite the cooling water inlets. This is caused by the condensation of water which mixes with oxides of sulphur to form sulphuric acid. In order to carry out repairs, welding should be done, if a minimum wall thickness of 5 mm is present. The corroded area is built-up by special arc welding. Diamalloy 1005 coating is applied after grinding down the pertinent area to bare metal. On the exhaust valves of the latest engines, wall thickness has been increased by about 40 % to take care of this. Recently, a U-type seal has been introduced for the Exhaust valve bottom piece, instead of the o-ring on previous engines. The exhaust valve seat is of the new Wldesign, wherc it is claimed that the narrow paths will crush possible i cdke and squeeze it out, so no seat damage occurs. 'l'he material is steel and cannot bc wcldcd, and it is hardened to a depth of 6 - 8 mm. Burning of exhaust valve seats : This is caused by poor combustion, due to defective injectors, afterburning, valves not completely closing, insufficient water cooling or build-up of deposits. Once the leakage starts, age of hot gases causes distortion, which further increases the leakage. Gases at high velocities expand across the location of the leakage.

Untton~piece nlcshaust valve New W design

STUFFING BOX The Stuffing box seals the crankcase from the Scavenge space, in two stroke engines. Shown below is a typical construction for a large marine engine, This shows how the crankcase oil is prevented from getting canied over with the Piston rod during its motion. It also prevents accumulated deposits and excess cylinder oil from entering the Crankcase.

New designs incorporate the following : v Additional gas-tight top Scraper package, wit11 a large drain area rr Stronger springs. u Modified position for the neutral space. rr Modified channel drains. u Ring of Bronze. w Hardened piston rods (in way of stuffing box). Two way dismantling (upwards and downwards).

-

Morine Engineering Ptaciice

RELIEF VALVE

The cylinder heads are fitted with relief valves, to relieve any abnormally high pressure. They are basically spring loaded valves. The pressure setting depends on the bore, type of engine and so on. A typical value for a 900 mm bore engine is 170 t 5 bar.

w OPENING PRESSURE 171ri-5 BAR

te

LOCK N W

ADJUSTING

UPRUT

Mnrine Engineering Prnclicc

Starting Air system This Starting air system is used for engines started on compressed air. The signal for opening is given by the Distributor. A i r is itted to the unit by the Distributor, as per the firing order. During operation, the starting air lines should be felt. I n case o f leakage, the line adjacent to the valve w i l l become abnormally hot. This can lead to a starting air line explosion. A leaky valve should be replaced as soon as practicable. This can also be temporarily blanked, however i t may lead to starting problems. The starting valve stem may also jam, with resulting problems for starting. Minimum number o f units for a propulsion engine is 3. There w i l l be an overlap provided, during which more than one air starting valve receives air at any instant. Mu TIP TI PIG VALVE

STARTING & REVERSING S U L Z E R RND

Marine Engineering Bodice

DISTRIBUTOR The air starting sequence of the engine is determined by the Distributor. This its air to one or more units, and is driven by a cam.

AIR TO CLOSE

CYLINDER AIR STARTING

SLIDE VALVE

v

m

OPENING

STARTING AIR DISTRIBUTOR SULZER

AIR IN

1

.'

AIK '1'0 OPEN

, .

AIRTOCLOSE

DISTRIBUTOR I C I

/ / / / I /

CYLINDER

AIR

I

STARTING VALVE

SHUTTING The new generation of marine engines have a system of controlling starting air to cylinders by individual solenoid valves. This will replace the mechanical Distributor (of conventional engines) shown above. Common faults include jamming of the shuttle valve, and leakage from pilot air connections.

Marine Enp'neering Practice

TURBOCHARGER CUTTING OFF

Running for longer periods with the Turbocharger out of operation If it is required to nm for a longer period, the rotor of the turbine needs to be locked. If there is any fault in the Turbocharger, it may be temporarily cutoff by insetting the blanks, as shown in the sketch below : EXHAUST OUT .+ I

BLANK

BIANK FLANGE

h\\

T

I \ I//

61-

FLANGE

TO SCAWNGE AIR COOLER

CUTTING-OFF TURBOCHARGER rr r

r rr

Lock the rotor, with the tool provided. If the engine has more than one turbocharger, insert orifice plate in the compressor outlet, and blanks in the turbine inlet and outlet sides, as shown in the sketch. There are restrictions on the load, depending on how many turbochargers are present. If there is only one, and it is locked, reduce to 15% of MCR power or less.

,

* * *

*

If one out of two turbochargers is to be locked, reduce to 50% of MCR power or less. If one out of three turbochargers is to be locked, reduce to 66% of MCR power or less. At this time, if one of the auxiliary blowers is out of action, reduce to 10% of MCR power. Temperature of the exhaust gases should not exceed 350'~.

Running for short periods without blanking the turbine If there are heavy vibrations, bearing failure or some other mechanical fault in the turbocharger, a short stoppage will be required, to lock the rotor of the defective turbocharger. If there are two or more turbochargers, one of which is to be cut-off, an orifice plate is inserted in the compressor outlet, so as to supply enough air flow to cool the impeller. As there will be a loss of exhaust gas through the damaged turbocharger, the maximum power will be limited to 15 % of MCR. Also check the exhaust gas temperatures, which should not exceed the maximum specified. Reduce the lubricating oil pressure for the damaged turbocharger and continue circulating the cooling water, as well as the sealing air for bearings. Improvements in Turbocharger oils I reduced maintenance requirements Turbocharger development has contributed to a progressive rise in the efficiency of marine engines, which can produce more power, but leading to higher stress levels. Modern turbocharger designs (ABB) have two bush bearings in the centre, with lube oil supply from the main engine lube system. This has lead to the use of special low friction synthetic oils, which increase the life of oil, leading to 5000 hours between oil changes. Oxidation resistance and thermal stability properties improve the performance at high operating temperatures. This oil also resists hydrolysis and acidic corrosion. Un-cooled Turbochargers are now in use, where the casing is un-cooled, while the cooling is only for the bearings, to prevent seizure. The reason that water jackets were used was to bring down the casing temperature. However, this had the disadvantage that the exhaust gases were cooled, which reduced the turbocharger efficiency and also the thermal efficiency. The presence of water also meant an increase in the possibility of corrosion, as well as water ingress into exhaust spaces, leading to turbocharger damage. The use of un-cooled casings means that ~lluch better materials are required, which can safely withstand the higher temperatures without distortion.

Marine Engineering Practice

GOVERNOR MAlNTENANCE The rnanual provides the best guide for Governor trouble-shooting. In addition to problems inside the Governor, the following external conditions can also affect it : b Tension of the chain drive (in case of engines having chain drive). > Oil pressure fluctuation (in case of separate lube oil supply). 3 Drop in control air pressure ( in case of pneumatic type). 3 Sluggishness in the regulating gear. It is most important to keep the Governor oil absolutely clean, since the complex moving parts inside can seize with dirt in the oil. Change the oil at the specified intervals, with the correct grade, as given in the manual. The oil usually has anti-foam and detergent additives. The viscosity index must be high and the viscosity is normally in the range of 22 to 68 Cst at 40 OC. In case of mechanical governors it is important to lubricate all linkages to keep from seizing and rusting. The pin holding the flyweights tends to wear and requires renewal. The compensating needle valve setting needs to be checked, if any hunting is observed, or if the governor operation has become excessively sluggish. Overspeed trip An over-speed trip is required to be separate from the Governor, and needs to be periodicalIy tested, to ensure that it is functioning correctly and that the engine has been brought to a stop. The electronic Overspeed Protection System monitors three individual magnetic pickups to provide over-speed protection. This system replaces mechanical over-speed devices, especially in high-speed applications when a mechanical over-speed device becomes unreliable. Ease of calibration and testing, accuracy, and repeatability are the benefits of this system. It uses digital speed-sensing technology to change the set point by programming the value 6om the touch on the front. To insure its accuracy and functionality, each of the three speed-sensing units can be individually tested while the prime mover is running, using the integral frequency generator. A key lock prevents programming and testing by unauthorized personnel. Accuracy is achieved through the three digital units which sense and indicate speed to within 0.1%. No mechanical over-speed device can match this accuracy level. The system's two-out-of-three voting scheme assures that the device will not shut down the prime mover because of transient conditions or conditions affecting only one of the three isolated units.

59

FUEL LNJECTION SYSTEM The fuel is injected by means of a fuel pump and one or more injectors. Conventional engines have mechanical fuel injection, as shown below :

The &el pump for MAN B&W engines is shown above. This has an arrangement for varying the timing of fuel injection, depending on the load (VIT). The changing of the timing is affected by movement of the pump barrel, with reference to the plunger. The fuel injector shown is of the re-circulating type, which' enables the engine to run on heavy fuel oil from pier-to-pier, without the need to change over to diesel oil. This also has a separate rlaccablc nozzlc tip, whicll can bc easily discardcd whcn thc holcs arc deformed, without having to ~eplacethe injector needle and guide. The fuel injection system for the latest generation engine features electronic fuel injection. This does away with the need for the conventional fuel cam. Electronic control allows precise control of rate, duration and shape of the injection pattern

Murire Engineering Practice

FUEL PUMP TIMING Setting of the fuel pump :

Accurate setting of the he1 injection timing will depend on precise adjustment of the following (M.A.N. B&W engines)k Fuel cam setting. h The angle at which injection is to commence, with respect to the TDC position of the piston. P The beginning of injection, i.e. the covering of the cut-off hole by the leading edge of the plunger. k The termination of injection, i.e. the uncovering of the cut-off hole by the helix of the plunger. P Zero ission setting of the pump. k Cut-out check.

Lead 'a' is defined as the distance between the top of the Plunger and the upper edge of the cut-off hole, when piston is at TDC.

.Marine Engineering Practice

Lead 'b' is defined as the distance between the top ofthe Plunger and the upper edge of the cut-off hole, when the Plunger is on the base circle. Lead 'c' is defined as the amount by which the Plunger is liAed from its bottom position on the base circle, to when the piston is at TDC. Lead 'c' is a combination of readings 'a' and 'b', with the proper signs. Measuring lead. s Shut-off the fuel oil in!et. s Unscrew the drain oil pipe from the fuel pump s Disconnect the air pipes for the Puncture valve and the safety system. s Dismount the protective cap and two plugs where the measuring tool is to be mounted. = Turn the engine till the piston is at TDC (Check from markings on the flywheel). = Placc the measuring tool on top of the top cover, push down the measuring Pin till it rests against the bottom of the threaded hole on the top of the plunger. The legs of the measuring tool should rest against the top of the barrel The distance between the top of the barrel, and the bottom of the threaded hole in the plunger, when the plunger is just covering the cut-off hole (i.e. the start of injection) is 'K', and is always constant for the pump. The tool is so designed, that when the plunger is just covering the cut-off hole (i.e. start of injection, the mark '0'can be read on the scale. Since the piston is now at TDC, the lead 'a' can be directly read-off from the scale, as the amount by which the reading is differing from 'O',and may be or (+) or (-) on the scale. Also note down the index of the WT rack for this position. Adjustment of the peak pressure can be done by moving the barrel with respect to the plunger by means ofthe VIT rack or by the cam ( for large adjustments.). Now measure the Iead of the fuel cam Dismount the cover ofthe fuel cam inspection hole in the housing. Turn the engine till the roller is on the base circle. Confirm this by observing the reading on the measuring pin, which will remain constant when the plunger is at its lowest position. Now take the reading 'b', which has to be read with the relevant (+) or (-) sign. E.g. suppose the reading (a) was + 12, when the piston was at TDC. Now, after lowering the Plunger to the base circle, if the reading (b) was -5, with the plunger in the lowest position, then, the fuel cam lead 'c' is got by rr

rr rr

Marine Engineering Ractice

c c

a - b + 17

= =

-

+I2

- (-9, or

If it is required to turn the cam, keep the measuring tool in place, and turn the cam till the desired change in lead is read directly on the measuring tool. To increase the lead and P,,,, turn the cam ahead or advance it . To decrease the lead and^,,, turn the cam astern or retard it. After making any changes, lead 'a' and 'b', along with the WT rack index at TDC, are to be noted. Now the puncture valve, the protective cap and pipes can be remounted Alternative method of measuring fuel pump-lead : The he1 pump and the fuel cam lead can be measured in the following manner. Fuel pump lead: 3

3

Check that the engine is in the AHEAD direction of rotation. Turn the engine in the AHEAD direction till the piston is at TDC (confirm from the marking on the flywheel ). Using a depth gauge, measure the distance X I from the upper side of the fuel pump top cover, through the centre hole, to the bottom of the threaded hole in the plunger.

The distance between the top of the barrel, and the bottom of the threaded hole in the plunger, when the plunger is just covering the cut-off hole (i.e. the start of injection) is 'K', and is always constant for the pump. Y = distance from the upper side of the fuel pump cover to the top of the pump barrel, which can be measured by means of a depth gauge. Now, the fuel pump lead can be calculated by, K + Y - XI Also, the index of the VIT rack should be noted, for this position of the piston at TDC. lead

=

Fuel cam lead :

Turn the engine in the AHEAD direction till the roller of the plunger is on the base circle or lowest position. Now measure distance X2,by means a depth gauge, from the top of the he1 pump cover, to the bottom of the threaded. hole in the plunger.

Murine Engineering fiuclice

Fuel cam lead

=

X2

- XI FUEL PUMPTlMlNG M h Y llaw

Replacing a servo cylinder of the fuel Pump : If a defective servo air-cylinder has to be replaced, the Pump barrel in the pump housing must be disconnected from the threaded guide, before replacing the servo cylinder. The control air pressure of 2.75 bar should correspond to the VIT rack index of 4.5, and if this is not the case, the relevant linkages must be adjusted, before the spare servo cylinder can be put into operation. Adjustment or Pilot valve : Spare pilot valve has been pre-adjusted to give from 0.5 bar to 5.5 bar output, when the pin has been depressed by 0.5 mm to 8 . 0 mm respectively. Check by connecting a pressure gauge, and connecting a pressure line, as shown in the sketch. The pressure in the gauge must correspond to the pressing of the pin, in accordance with the curve. If not, necessary adjustments can be made to the spring tension. For details of Fuel Pumps, and Variable Injection Timing (VIT), refer to 'Advanced Marine Engineering Knowledge - Volume I', Chapter 1.

Marine Enp'neering Ruciice

Z

~

J

~

~

C

O

~ I .~ PYO TDA ~SO N ~

OF CAM

LUO on P W S m WAX. loa shnl

COWLING

This is achieved by adjusting the relative angle between the Camshaft and the Crankshaft. Hydraulic pressure is applied as shown. lo rotation corresponds to 2.9 m i on the circumferen&of the camshaft. Thus, l o advance results in the fuel pump top-lift increasing by 2 mm. FUEL PUMP TIMING Sulzer Fuel pumps Fuel pumps must deliver fuel at the correct time, with respect to the TDC position. This is determined by means of the Dial gauges provided. The values are checked against the valve setting tables, which are recorded at the time of installation, arid any deviation is noted and adjusted by means of the push-rods.

Marine Enginaring Practice

A precise quantity of fuel must be injected by each pump, such that all units get equal quantity, in order to get balanced firing. Sulzer fuel pumps are controlled by a common shaft, called as the Fuel lever, which controls the quantity of fbel injected, by varying the position of the spill valve eccentric. The beginning of normal injection being fixed with reference to TDC, the angle at which injection starts must be checked. This is the position at which the suction valve just shuts. The angle of termination is checked, to confirm that the required quantity is delivered. This is the position at which the spill valve just opens. The position of Fuel cut-off, i.e. the position of zero ission, must be checked, to ensure that all pumps stop delivery, when the fuel rack is brought to the zero fuel position. For the cut-out check, the zero position of the Load indicator, the Cut-out Servo motor, and the Governor must coincide. The setting of the cam in relation to the plunger travel must be checked. Finally, all the linkages are examined to check for adequate clearances, and to ensure that the 'play' will not d e c t the correct functioning of the fbel pump. Worn-out bushes need to be replaced, before checking and adjusting the timing of the he1 pumps. DLAL GAUGES

Mnrinr Engineering Racticr

Checking ofFuel pump timing (Sulzer fuel pump) Pre-cl~ecks: Ensure that engine has been changed over to diesel oil before-hand, and the lines are thoroughly flushed. Carry out all the checks to ensure that enginc cannot be started Shut Starting air valves, engage Turning gear and keep the remote (of the turning gear) near the he1 pumps. The Tclcgraph is sct.for Ahead direction. The engine is turned in the Ahead direction, till the Running direction interlock changes over. It must be ensured that sufficient lubricating oil pressure is there, to keep the Servomotor in the Ahcad position, while the engine is rotated on turning gear, during the timing check. The Fuel cut-off Servomotor will have to be disabled, in order to allow the fuel lever to be moved, since the Trips would have operated, as the engine is not running. The zero position of the Load indicator, the Cut-out servomotor and the Governor must coincide, and this must be checked to coincide with the no-delivery position of the fuel pumps. All tools required for the job, including the special spanners for adjusting the push-rods, and the Dial gauges must be kept ready. Remove the suction, delivery and spill valves, and clean and inspect all components - especially the condition of thevalves and seats. Replace the suction and spill valves on their respective seats, but without the springs and covers. Turn the engine till the unit is at BDC and the fuel pump plunger roller is on the base circle of the cam. At this position, the plunger should be at its lowest position. Fit the Dial gauge on the plunger and set it to zero. At this position, the spill valve must be also shut. Fit the Dial gauge on the spill valve and set it to zero. Turn the engine till the unit is at TDC, at which time the suction valve should be in the shut position. Fit the Dial gauge on the suction valve and set it to zero. Now bring the unit to BDC, and confirm that the Dial gauges above the plunger and the spill valve are in the zero position. Slowly turn the engine, observing the dial gauge above the suction valve

Marine Enginaring Rudice

As the plunger rises, the suction valve starts closing, i.e. the Dial gauge reading starts reducing to zero. When the reading shows 0.02 mm, (i.e. nearly closed), stop turning the engine. Note the reading on the Dial gauge above the plunger as S1. Note the angle on the flywheel at this position, with respect to the TDC position, from the markings on the flywheel. This will give the angle of start of he1 injection. Now slowly continue to turn the engine in the ahead direction, till the Dial gauge of the Spill valve just starts increasing and shows 0.02 mm. Stop turning, and note the reading of the Dial gauge above the plunger as S2. Note the angle on the flywheel at this position, with respect to the TDC position, from the markings on the flywheel. This will give the angle of end of he1 injection. The difference in the readings ( S I and SZ ) of the Dial gauge above the plunger, give the effective stroke of the plunger. This value must be checked against the value in the Valve setting tables For the cut-out check, the zero positions of the Load indicator, the Cut-out servomotor and the Governor must coincide. In this position of zero he1 ission, the spill valve must remain open throughout the full revolution of the camshaft, i.e. the Dial gauge reading must not come to zero. Care should be taken in making any adjustments of the push-rod lengths, that the values should be compared and reset to the values in the valve setting tables, else the quantity of he1 injected will vary. and affect the balance of the engine, due to unequal firing. When the push-rod of the suction valve is lengthened, it will take longer for the valve to'shut, thus delaying the start of injection, while shortening the effective stroke and vice versa.

Fuel pump and exhaust valve actuator pumps

Marine Engineering Pructice

FUEL INJECTOR Fuel injector performs the task of atomisation of fbel. The nozzle tip is slightly lengthened to shield the Cap nut by the Cylinder cover, thereby not exposing it to the combustion space. The h e l injector is one of the most important components, since this directly affects the combustion of fuel, and hence the power produced as well as the he1 consumed.

Marine Engineering Radicc

Fuel Injector problems Excessive smoke and higher exhaust temperatures are due to faults in the Injector. Dripping : This occurs due to needle valve not sitting properly. Irregular spray : Nozzle holes blocked, needle sticking, nozzle holes enlarged. Partial or complctc sticking of nozzle : Due to insufficient 'play' in the y i d e , which results in inadequate lubrication. Damage to needle guide : Due to impurities in the fuel. Damage to needle seat : Due to impurities. Testing and Inspection of Fuel Injectors Prior testing, the injector must be cleaned with a suitable solvent such as Trichloro ethylene or kerosene, and a soft wire brush.

U

I

'

LNJI:CTION LWEI2

U

INJECTOR TESTING S T A N D

The function of the injector is tested in the stand shown above. Following must be checked : Valve spindle tightness, opening pressure, direction ofjet, atomisation and tightness of o-ring. If the above are not fulfilled, the needle and y i d e need to be replaced.

Marine Engineering Ractice

1

CYLINDER LINER The cylinder liner is inserted into the Cylinder block from above and retained by the Cylinder head. They are water-cooled, and have bore cooling in way of the piston running surface. The main problem of corrosion has been tackled in the new RTA 84 T by having two bands of Teflon insulation enclosed within sheet steel (Haramaki), as shown in the sketch below :

The wear rate of the Cylinder liner has now been reduced to less than 0.1 mm 1 1000 hrs. The load-controlled cooling system has ensured that the temperature of the liner is above the dew point, and has thus significantly reduced the problems due to corrosion. Life of the Liner is now increased to about 10 years, which has reduced maintenance costs. At the time of Overhaul of the unit, liners are cleaned, inspected and calibrated. They must be free from scoring, cracks and ridges. If the liner is chrome plated, the worn-off portion must be carehlly examined. The corresponding part of the Piston must be examined for gas blow-past. Cracked or Scored liner must be replaced. If the chrome plating is showing signs of wear, the liner needs to be re-plated. The Liner must be inspected at removal of piston (during annual surveys) or in case of seizure. The maximum wear of the Liner is around 0.6 to 0.8 % of the bore, which is the thumb rule for replacement. The readings should be carried out using the template, in both Fore-Aft and Port Starboard directions.

Marine Engineerbig Practice

MEASUREMENT TEMPLATE NTS

P

LINER

GAUGING

Marine Engineering Ractice

CAUSES OF LINER WEAR sulphvns url

-

Cylinder liner

'Salt Sea Ssa Air ,Cyl.

Cornion

in intake s i r l a t e r i n Fuel water i n c y l . o i l cooler leakage, o i l film deficiencies

-

Piston rings

'Condensation i n a i r cooler Water leakages "Cold" l i n e r wall Cleaning agents Cyl. o i l F i l m deficiencies

Too low cyl. o i J &sage Distributiw,

-

Film deficiencies

Water i n c y l . o i J Water leakage t o cylinder

deFiciencies.

Hicroseizure

-

Abrasion

-

-

Scratches

73 2

Hechanical load r h s r n s l load Deformation of piston rings (during F i t t i n g ) Hiaaligmnk

Impurities i n . fuel o i l I ~ u r i t i e si n intake a i r Weer p a r t i c l e s

Marine Engineering R a d i c e

Cloverleali~~g is a term used to describe the corrosive wear at points around the liner circumference, which are in between the lubricating oil entry points. The cylinder oil being alkaline, neutralises acids in the vicinity of the oil entry points. This leads to the Cloverleaf type wear, shown below :

'CLOVER LEAF'

WEAR

CYLINDER LINER

The lubricating oil grooves in the cylinder liner should be shaped and not end in the lubricating hole, but at a point slightly below it.

I

Marine Engineering Ractice

REMOVAL OF CYLINDER LINER The Cylinder liner may be removed in case of any defectldamage. The following needs to be carried out : J Drain the jacket water and remove the cylinder head. J Engage the Turning Gear and turn the unit to BDC. Grind away any ridges on the top of the Liner. J Remove the Piston, as explained under Decarbonisation. J Disconnect Lubricator fittings and remove. J Fit the lifting tool, as shown.

J J

J J J

J

Lift the liner complete, by means of the engine room Crane. Land the liner on suitable planks, to carry out inspection. While re-fitting, ensure calibration is done. New sealing rings are fitted, suitably coated with molybdenum disulphide. In case of new Liner, new Piston rings must be used. All running-in procedures to be carried out as per maker's instructions.

Marine Engineering Ractke

CYLINDER LUBRICATION Various methods are used to supply lubricating oil to the cylinder walls. In Trunk pistol1 e ~ ~ g i n ewith s forced bearing lubrication, the quantity of oil thrown from the bearings onto the cylinder walls is sufficient to lubricate the liner1 piston rings. Scraper rings are provided to prevent excessive consumption of the crankcase oil. In two stroke engines, the supply of cylinder lubricant is achieved through forced feed lubricators, using a different oil than that used in the crankcase. The oil is injected into each unit through Quills, which are basically non-return valves. These eliminate the pressure pulsations in the lubricator delivery pipe, and prevent air I producis of combustion from entering. They also keep the pipe full, when the engine is stationary. Modem Sulzer engines use multi-level lubrication. The lubricator pumps are driven by frequency-controlled electric motors, using load-dependant lubrication. The oil is distributed to oif accumulators by oil distributor, as shown in the sketch.

Exhaust valve spindle lubricaiion Quill with

I accumu'atOr Progressive distributor

Filler \

Cylinder oil module pump

Murine Engineering Ructice

The cylinder oil feed rate should be suficient to ensure a clean and wet liner, without excessive accumulation of lubricating oil. A rough rule-of-thumb is 0.6 g d BHP-hr. This feed rate should be adjusted in accordance with the actual load, so that it is proportional to the mean effective pressure. The feed rate per cylinder per 24 hrs should never be allowed to drop below 40 % of the recommended oil consumption. OUILL

-

CYLINDER LIIBRICATIXC OIL FEED RATE g/kWh. g/BHPh

Minlnum runnlnp-In feed r a t e (nonlnal

UCRJ

0.9 Nonlnal f e e d r a t e lnanlnrl MCRl

"a

I Recommended

0.L

runnlng-in

/

Hininlm feed rate ( n o n l ! ~ a l RCRI

Service hours

Marine Engineering Practice

THRUST BEARING The Thrust bearing clearance is checked with a Feeler gauge, as well as any special gauge, if supplied. For a new engine, this is 0.5 mm to 1.0 mm. For an engine in service, it must not exceed 2 mm.

Procedure Checking the clearances on the Thrust bearing is done as follows : Turn the engine so that the after most crank throw is at BDC and the thrust bearing collar bears on the foremost thrust bearing segments. @ = 0 mm).

The gauges arc to be insc~tcdbetween side of the after-most lower main bearing shell and tlie side of the afier-most crank throw If a feeler gauge of less than 2 mm minus (B+C) is qble to enter, the Thrust bearing clearance is correct, else the bearing is to be overnauled.

Murine Engineering Roclice

GEARS Gears are found on medium speed and high speed engines. There are various types of gears : Gears used for parallel shafts 9 Spur gears with external teeth. The line of the gear teeth is axial. 9 Helical gears. The tooth line follows a helix. Gears used for non-parallel shafts P Bevel gears. For changing the direction. Used in governors. 9 Spiral bevel gears. Similar to helical gears. Used in Indicator drives 9 Worm gears. Used in Turning gear for engine. Faults in gearing Noise is the most common indication of wear or damage. There may a variation of the tooth pitch of an individual wheel, which can cause this. Tooth damage takes place due to shock loading. Pitting indicates a deficiency in lubrication. Usually found in the tooth at mid-height. Scuffing, galling and undercutting are due to incorrect meshing or total lubrication failure.

The reduction gearing system shown above has a quill drive, which is made up of two parts : a pinion and a quill shaft, with a pneumatic clutch and brake. The torquc applicd to the coupling will be transmitted along the shaft to the pinion. The arrangement is very flexible due to the natural elasticity of the shaft. The rate of wear on the teeth is directly proportional to the misalignment.

INDICATOR CARDS Analysis of combustion can best be done by means of an instrument called the Indicator. This measures the variations of pressure during the cycle, in the form of cards. The angle a is a yardstick for the impact of the ignition pulse. Mainly on of the compressibility of the fuel, the commencement of fuel delivery by the fuel pump and the commencement of the fuel injection do not coincide in relation to the time. The result is a time-lag A - B. The dislance B - C represents the Ignilion lag. This is the time which elapses between commencement of he1 injection and the first perceivable increase of

Time

! I -1

*

Combustion

0

0

pressure..The Ignition lag is the characteristic of the fie]. Large lag indicates unsuitability of the file]. When the pressure rises rapidly, the combustion and expansion curves show vibrations, due to a fault in the spring. The diagram is then evaluated, as shown below, to obtain the actual maximum pressure. Prasrurr recorded by indicator Convont;onaI prsssurm curve with tangant at A and urn. radiva o f curvolure R