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MACHINE TOOL GEARBOX DESIGN.
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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 of the gearbox is in 4.2 (i.e.; four
different speeds produced intermediately between 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