Power Transmission Devices(Belt)
Mohammad Nezam Uddin Chy
Power Transmission devices
Power Transmission devices are used to transmit power(rotational energy)
from one shaft to another shaft.
These devices are :
1. Belt
2. Chain
3. Rope
4. Gears
Types of Belt:
Belt drive is a mechanism that transmits rotational motion from one pulley
mounted on a shaft to another pulley by means of a belt.
Types of Flat Belt drive:
1. Open Belt Drive 2. Crossed Belt Drive
open belt drive is used in shaft arranged Cross belt drive is used in shaft arranged
parallel and rotating in the same direction. parallel and rotating in the opposite direction.
The quarter turn belt drive (also known as
right angle belt drive), is used with shafts
arranged at right angles and rotating in one
definite direction
3. Quarter Turn Belt Drive
Idler pulley is used
• to reduced slack or increase the frictional
contact between belt and pulley.
• to change the direction of motion. No power
is required to drive it.
4. Belt Drive with Idler Pulleys
Idle One driver shaft and number of driven
pulley shaft.
One main driver shaft and number of
driven shaft.
5. Compound Belt Drive
6. Stepped or Cone Pulley Drive
More than one pulley in driver and driven shaft
(chain sprocket in cycle)
Velocity Ratio of Belt Drive:
It is the ratio between the speed of driver and the follower.
Peripheral Velocity of belt on the Driver
pulley,
d1 N1
v1 =
60
Peripheral Velocity of belt on the Driven
pulley, d N
v2 = 2 2
60
When there is no slip, v1 = v2
d1 N1 d 2 N 2
=
60 60
N 2 d1
=
N1 d 2
Shaft pulley
Velocity Ratio of Compound Belt Drive:
Velocity ratio of pulleys 1 and 2, N 2 d1
=
N1 d 2
Velocity ratio of pulleys 3 and 4, N 4 d3
=
N3 d4
Therefore,
N 2 N 4 d1 d 3
=
N1 N 3 d 2 d 4
Since pulleys 2 and 3 are on the same shaft, N2 =N3
N 4 d1 d 3
=
N1 d 2 d 4
Slip of Belt:
Let, s1 = Percentage of slip between the driver pulley and the belt
s2 = Percentage of slip between the driven pulley and the belt
d N
Now, Peripheral Velocity of Driver pulley, v1 = 1 1
60 s Velocity of belt
Therefore, Velocity of the belt on driver pulley, v = v1 1 − 1 on driver and
100 driven will be
s2 same.
Again, Peripheral Velocity of Driven pulley, v2 = v 1 −
100
s s d N d N s s
v2 = v1 1 − 1 1 − 2 2 2 = 1 1 1 − 1 − 2
100 100 60 60 100 100
N d s s N d S S=s1+s
2 = 1 1 − 1 − 2 2 = 1 1 −
2
N1 d 2 100 100 N1 d 2 100
With slip
For 1st drive (between pulley 1 For 2nd drive (between 3 & For full drive( pulley 1 to 4
& 2) 4) (by multiplying two drives)
N 2 d1 S N 4 d3 S N 4 d 1d 3 S S
= 1 − = 1 − = 1 − 1 -
N1 d 2 100 N 3 d 4 100 N1 d 2 d 4 100 100
Creep
Power Transmitted by a Belt:
T2
A B
T1
Driven Pulley
Driver Pulley
The effective driving force on the belt, F = T1 –T2
Velocity of the belt = v
Therefore,
Work done per second, P = F × v
Power Transmitted, P = (T1 –T2 )×v
Length of the belt for open belt belt drive
𝑟1 − 𝑟2
where , sin 𝛼 =
𝑥
Length of the belt for cross belt belt drive
𝑟1 + 𝑟2
where , sin 𝛼 =
𝑥
Initial Tension in the Belt
When a belt is wound round the two pulleys (i.e. driver and follower), its two ends
are joined together ; so that the belt may continuously move over the pulleys, since
the motion of the belt from the driver and the follower is governed by a firm grip,
due to friction between the belt and the pulleys. In order to increase this grip, the belt
is tightened up. At this stage, even when the pulleys are stationary, the belt is
subjected to some tension, called initial tension.
When the driver starts rotating, it pulls the belt from one side (increasing tension in
the belt on this side) and delivers it to the other side (decreasing the tension in the
belt on that side). The increased tension in one side of the belt is called tension in
tight side and the decreasedT + tension in the other side of the belt is called tension in
2
the slack side. = ...(Considering centrifugal tension)
initial tension.
A casting weighing 9 kN hangs freely from a rope which makes 2.5 turns
round a drum of 300 mm diameter revolving at 20 r.p.m. The other end
of the rope is pulled by a man. The coefficient of friction is 0.25.
Determine 1. The force required by the man, and 2. The power to raise
The casting.
A pulley is driven by a flat belt, the angle of lap being 120°. The belt is 100 mm wide by
6 mm thick and density 1000 kg/m3. If the coefficient of friction is 0.30 and the
maximum stress in the belt is not to exceed 2 MPa, find the greatest power which the
belt can transmit and the corresponding speed of the belt.
In a flat belt drive the initial tension is 2000 N. The coefficient of friction between the
belt and the pulley is 0.3 and the angle of lap on the smaller pulley is 150°. The smaller
pulley has a radius of 200 mm and rotates at 500 r.p.m. Find the power in kW
transmitted by the belt
Rope Drive: 1. Fiber Ropes 2. Wire Ropes
T1
= e s cosec
T2
A belt drive consists of two V-belts in parallel, on grooved pulleys of the same size. The
angle of the groove is 30°. The cross-sectional area of each belt is 750 mm2 and µ. =
0.12. The density of the belt material is 1.2 Mg/m3 and the maximum safe stress in the
material is 7 MPa. Calculate the power that can be transmitted between pulleys 300
mm diameter rotating at 1500 r.p.m. Find also the shaft speed in r.p.m. at which the
power transmitted would be maximum
A pulley used to transmit power by means of ropes has a diameter of 3.6 meters and has
15 grooves of 45° angle. The angle of contact is 170° and the coefficient of friction
between the ropes and the groove sides is 0.28. The maximum possible tension in the
ropes is 960 N and the mass of the rope is 1.5 kg per meter length. What is the speed of
pulley in rpm and the power transmitted if the condition of maximum power prevail?
