Power Transmission
• A system that transmits the power from one
mechanical element to another mechanical element is
know as power transmission
Selection of power transmission systems depends upon
• The distance between two shafts
• Operating speed
• Amount of power to be transmitted
when the distance between two shafts is large belt drives
and ropes are used
• For intermediate distance between shaft chains
are used
• For shorter distances gear drives are used
BELT DRIVE
1. A belt is a looped strip of flexible material, used to
mechanically link two or more rotating shafts.
2. They may be used as a source of motion, to
efficiently transmit power, or to track relative
movement. Belts are looped over pulleys.
3. In a two pulley system, the belt can either drive
the pulleys in the same direction, or the belt may
be crossed, so that the direction of the shafts is
opposite.
Open belt drive
Cross belt drive.
Advantages of belt drive
• They are simple. They are economical.
• Noise and vibration are damped out.
• They are lubrication-
lubrication-free. They require only low maintenance.
• They are highly efficient (90–
(90–98%, usually 95%). Some
misalignment is tolerable.
• They are very economical when shafts are separated by large
distances.
Disadvantages of belt drive
• The angular-
angular-velocity ratio is not necessarily constant or equal
to the ratio of pulley diameters, because of belt slip and
stretch.
• Heat buildup occurs. Speed is limited to usually 7000 feet per
minute (35 meters per second). Power transmission is limited
to 370 kilowatts (500 horsepower).
• Operating temperatures are usually restricted to –31 to 185°
185°F
(–35 to 85
85°°C).
• Some adjustment of center distance or use of an idler pulley is
necessary for wear and stretch compensation.
• A means of disassembly must be provided to install endless
belts.
FLAT BELT
The flat belt is rectangular in cross section
The pulleys are crowned to prevent slip of the
belt to one side
It utilizes the friction between pulley and belt
It is used in factories and workshops
It can transmit moderate amount of power
It is operated at high speed such as 22m/s
It is used when the two shafts are separated by
a distance of not more than 8 meters
Materials used for this belt is leather, cotton and
rubber
V BELT
•The V-belt is trapezoidal in section with an
angle of 30 to 40 degrees
It utilizes the friction between inclined sides of the
belt and pulley and it is a widely used in industrial
drives and vehicular application.
The V-shape causes the belt to wedge tightly into
the groove, increasing friction and allowing high
torques to be transmitted so it is used when more
power is to be transmitted
It is preferred when there is a shorter distance
between shaft i.e 4m
It is operated at a speed of 24m/s
The cords, made from natural fibers, synthetic
strands, or steel. are embedded in a firm rubber
compound
Advantages of the V – belts drive
1. Higher velocity ratio up to 7 – 10
2. Provide long life, 3 –5 years
3. Possibility of using small center distance
4. Transmit higher torsion moment at less width and tensions
Disadvantages
1. Cannot be used with large center distances
2. Subjected to a certain amount of creep
3. More complex design
4. Belt life above 82oC and below – 15o is shortened
5. Centrifugal force prevents the use at speed above 50m/s and at speed
below 5m/s are not economical
Rope drives or circular drives
• It is used for transmitting power when the distance
between the shafts is more than 30m
(where the belt drives are not used due heavy loss of
power)
• Circular drives are used in elevators ,cranes, mine
hoists
• It is operated at a speed of 3 m/s types of ropes are
fiber ropes and metallic ropes
• Fiber ropes are made up of cotton ,nylon and used for
transmitting power
• Metallic ropes are made up o steel ,aluminum alloys
and used in elevators ,cranes, mine hoists
ROUND BELT
Inroduction to chain drive
• Chain drive is a way of transmitting mechanical
power from one place to another. It is often used
to convey power to the wheels of a vehicle,
particularly bicycles and motorcycles. It is also
used in a wide variety of machines besides
vehicles.
• Most often, the power is conveyed by a roller
chain known as the drive chain or transmission
chain, passing over a sprocket gear, with the
teeth of the gear meshing with the holes in the
links of the chain.
Roller chain and sprocket
Advantages of chain drive
• As no slip takes place during chain drive, hence perfect velocity ratio is
obtained.
• Since the chains are made of metal, therefore they occupy less space in
width than a belt or
rope drive.
• It may be used for both long as well as short distances.
• It gives a high transmission efficiency (upto 98 percent).
• It gives less load on the shafts.
• It has the ability to transmit motion to several shafts by one chain only.
• It transmits more power than belts.
• It permits high speed ratio of 8 to 10 in one step.
• It can be operated under adverse temperature and atmospheric
conditions.
Limitations of chain drive
• The production cost of chains is relatively
high.
• The chain drive needs accurate mounting and
careful maintenance, particularly lubrication
and slack adjustment.
• The chain drive has velocity fluctuations
especially when unduly stretched.
Types of power transmission chain
• Roller chain
– Large reduction ratio (usually up to 1:7)
– Chains can be used with long shaft center distances (normally up to
4m). By changing the number of links it is possible to freely adjust the
shaft center distance.
– The chain can be used on both sides and drive multiple shafts at the
same time.
– Easy installation and replacement (easy to cut and connect chains).
– If the distance between shafts is short, it can be used vertically.
– The sprocket diameter of a chain drive could be smaller than a belt
pulley while transmitting the same torque.
– Transmitting power by meshing many teeth results in less wear of
sprocket teeth than gears.
– Chain drives have greater shock absorption than gear drives.
Roller chain drive
• Inverted tooth drive chains
– Inverted tooth drive chain are the optimal solution
for requirements exceeding those met by other
chains (e. g. roller chains). They allow for low-
backlash drives with exact positioning. They are
also highly effective, inured to vibration and have
minimal noise emission.
Inverted tooth chain drive
