Machine Tool Gearbox Design. 5n2iy

UNTUK RUJUKAN SAHAJA. JANGAN MENIRU. JADILAH UMAT YANG SUKA BERFIKIR UNTUK KEBAIKAN DAN KEKUATAN ISLAM. KJM 561: Sessional Assignment 1 EM220 / 7M2 Date: 9/4/2005 UiTM Shah Alam

TITLE: MACHINE TOOL GEARBOX DESIGN.

This gearbox design is actually the interpretation of the available gearbox configuration of a cutting machine. The arrangement of the gearbox gears was taken from a catalogue at UiTM machine shop. For my case of study, I have chosen the eight speeds Cholchester 5 ½ in. (140 mm) Bantam Lathe. The diagram of the machine is shown in the Appendix. Figure

1

shows

the

gearbox

layout

of

the

lathe

machine. It consists of three major shafts with the input or driving shaft denoted by C. As shaft C act as the input shaft, it consists of four gears for speed changing mechanism with number of teeth of 12, 30, 14 and 24 respectively. On the intermediate shaft, denoted as D, four gears mounted on it. Each gears having number of teeth of 30, 13, 28, and 20 respectively. On the output shaft A, two major gears acting on it which have number of teeth of 44 and 51 respectively. Shaft B and E act as reversing shafts for the system. Furthermore the

arrangement

different

speeds

of

the

produced

gearbox

is

in

intermediately

4.2

(i.e.;

between

four

shaft

C

and D whilst two different speeds between shaft D and A). As a summary, number of teeth of the gears involve in the mechanism can be categorized as follows:

Shaft C: N11 = 12 (= a) N12 = 14 (= c) N13 = 24 (= e) N14 = 30 (= g) Shaft D: N21 = 30 (= b) N22 = 28 (= d) N23 = 20 (= f) N24 = 13 (= h) Shaft A: N31 = 51 (= i) N32 = 44 (= k)

From Figure 1, the first stage transmission ratio has four combinations:

uI1 = N11/ N21 (= 12/30) = 0.4 uI2 = N12/ N22 (= 14/28) = 0.5 uI3 = N13/ N23 (= 24/20) = 1.2 uI4 = N14/ N24 (= 30/13) = 2.3

Figure 1: Gearbox layout of 8 speeds Cholchester Bantam Lathe.

The

second

stage

transmission

ratio

then

has

two

combinations which are:

uII1 = N23/ N31 (= 20/51) = 0.39 uII2 = N22/ N32 (= 28/44) = 0.64

The speed layout or the Ray Diagram of the gearbox is shown in Figure 2.

Figure 2: Speed layout of eight speeds gearbox.

The step ratio of the gearbox can be calculated as follows: B = speed range φ

= step ratio

z

= number of speed

nH

= the highest speed

nL

= the lowest speed

B

= φ

(z-1)

= nH/ nL = 800/36 = 22.222 φ

=

22.2221/7

= 1.56

which give the speeds as follows : n1 = 36 rpm n2 = 36 (φ) (= 56.16 rpm) n3 = 36 (φ2) (= 87.61 rpm)

n4 = 36 (φ3) (= 136.67 rpm) n5 = 36 (φ4) (= 213.21 rpm) n6 = 36 (φ5) (= 332.60 rpm) n7 = 36 (φ6) (= 518.86 rpm) n8 = 36 (φ7) (= 800.00 rpm) Speeds of shaft D by each gear as referred to Figure 2 are: nII1 = n1 + (n2-n1)/2 = 46 rpm

nII2 = n3 + (n4-n5)/2 = 112 rpm

nII3 = n5 + (n6-n5)/2 = 273 rpm

nII4 = n7 + (n8-n7)/2 = 659 rpm

Speed of the input gear on shaft C as referred to Figure 2 is: nI1 = n4 + (n5-n4)/2 = 175 rpm

Because

of

the

space

requirement,

we

can

see

that

the

gearbox designed is very simple rather than because of its number of speeds is only eight. The gearbox also using the sliding gears as the type of speed changing mechanism.

From Figure 2, The overall transmission ratio at stage 1 = φ6 = 14.41

The overall transmission ratio at stage 2 = φ = 1.56

On

the

first

stage,

uI1 and

uI2 (i.e.

0.4

and

0.5)

are

relatively low, thus the differences between the torques which have to be transmitted is low but then uI3 and uI4 (i.e. 1.2 and 2.3) are relatively high, thus the required torque

is

also

high.

On

the

second

stage,

both

transmissions ratio are low (i.e. 0.39 and 0.64). The

first

speed

of

the

gearbox

is

provided

by

the

combination of gear a.b and gear f.i. When switching to the second speed, second stage meshing gear is then changed to gear d.k. To change to the third speed, first stage meshing gear changed to the combination of gear c.d and gear f.i on the second stage while fourth speed is the combination of gear c.d and gear d.k. The next speeds are following the same sequence. It can be said that, to change the speed, the gear on the output shaft has to be changed every time, thus this will

cause

fast

failure

because

of

fatigue

on

the

gear

teeth caused by the torque transmitted between gears on different shafts. This means that speed range is supplied by different set of gear set (i.e. to change the speed, gear set has to be changed at stage 1 and stage 2). The transmission ratio of the second stage is φ, and thus this is very preferable since it is low. The highest transmission ratio, i.e. the greatest speed change occurs in the first part drive. Therefore the gears for reduction uI1 must be strong enough.

Appendix

a. Eight speeds Cholchester 5 ½ in. Bantam lathe machine

b. The assembly of the eight speeds gearbox of Cholchester 5 ½ in. Bantam lathe

c. Separated parts drawing of eight speeds gearbox of Cholchester 5 ½ in. Bantam lathe machine