Power Transmission Devices
•Belt drive,
•chain drive,
• gear drive
BELT DRIVES
The belts or ropes are used to transmitpower from one shaft to another by meansof pulleys which rotate at the same speedor at different speeds.
The amount of power transmitted dependsupon the following factors:
1.The velocity of the belt.
2.The tension under which the belt is placedon the pulley.
3.The arc of contact between the belt andthe smaller pulley
4.The condition under which the belt is used.
5.The material of the belt used.
Selection of the Belt Drive:
1. Speed of the driving and driven shafts.
2. Speed reduction ratio
3. Power to be transmitted
4. Centre distance between the shafts.
5. Positive drive requirements.
6. Shaft layout.
7. Space available.
8. Service conditions.
Types of Belt Drives
Light Drives: used to transmit small powers at belt speeds up to about 10 m/s in agricultural machines and small machine tools.
Medium Drives: Used to transmit medium power at belt speeds over 10 m/s but up to 22 m/s as in machine tools.
Heavy Drives: Used to transmit large powers at belt speed above 22 m/s as in compressor& generators.
Types of Belt [According to Shape]
Flat Belt : used in factoriesand workshops, where amoderate amount of power isto be transmitted, from onepulley to another when twopulleys are not more than 8 mapart.
V-Belt : used in factoriesand workshops, where amoderate amount of poweris to be transmitted, fromone pulley to another whentwo pulleys are very near toeach other.
Circular or Rope Belt :used in factories andworkshops, where a greateramount of power is to betransmitted, from one pulley toanother when two pulleys arenot more than 8 m apart.e.g. Crane
Timing Belt : They are toothed belts which transmitpower by means of teeth rather than friction. Hence there isno slip occurring in these types of belts. They need toothedwheels. e.g. CNC m/c, Automobiles
Types of Belt [According to Materials]
Leather Belt : Leather belts are made from1.2 m to 1.5 m long strip cut from either side ofthe back bone of the top grade steer hide.
The leather may be oak- tanned and Chrometanned. Leather belts are used in steam & oilcondition.
No. of layers: Single, Double, Triple ply belts.
Rubber belt : These are made fromlayers of fabric impregnated withrubber composition. They can notwithstand in hot, oily and greasyconditions. They are very flexible andcan be made endless. They are usedin saw mills, paper mills etc.
Cotton or fabric belt : These belts aremade from folding canvas or cotton duck.They are impregnated with linseed oil tomake water proof. They are suitable in warmclimates, in damp atmosphere. They areused in form machinery, belt conveyor etc.i.e. Flooring mills
Types of Belt Drive [According to its Arrangement]
The power from one pulley to another can be transmitted bydifferent arrangement of the belts.
Open Belt Drive :
It is used, when shafts are arrangedparallel and rotating in the samedirection, In this case, the driver A pullsthe belt from one side (i.e. lower side)and delivers it to the other side (j,e.upper side).
Thus the tension in the lower side beltwill be more than that in the upper sidebelt. Hence lower side belt is known astight side, whereas the upper belt isknown as slack side,
Cross Belt Drive :
This belt drive is used when shaftare arranged parallel and rotatingin the opposite direction. A crossedbelt drive can transmit more powerthan open belt drive, as the angleof wrap is more. However, the belthas to bend in two different planesand it wears out more.
Types of Belt Drive [According to its Arrangement]
Quarter Turn Belt Drive :
The quarter turn drive alwaysknown as right angle belt drive,is used with shafts arranged atright angles and rotating in onedefinite direction. In order toprevent the belt from leavingthe pulley, the width of the faceof the pulley should be greateror equal to 1.4 b, where b- isthe width of the belt. when thereversible motion is desired,then a quarter turn belt drivewith guide pulley.
Types of Belt Drive [According to its Arrangement]
Belt Drive with idler Pulleys :
A belt drive with an idler pulley, usedwith shafts arrangement parallel andwhen an open belt drive can not beused due to small angle of contact onthe smaller pulley. This type of drive isprovided to obtain in high velocity ratioand when the required belt tensioncannot be obtained by other means.
When it is desired to transmit motionfrom one shaft to several shafts, allarranged in parallel, a belt drive withmany ideal pulleys can be employed.
In conveyer system to avoid slacknessof the belt, idler pulleys are used.
Types of Belt Drive [According to its Arrangement]
Compound Belt Drive :
It is used to transmit power fromone shaft to another through anumber of pulleys.
Types of Belt Drive [According to its Arrangement]
Stepped or Cone Pulley Drive :
A stepped or cone pulley drive, usedfor changing the speed of the drivenshaft while the main or driving shaftruns at constant speed. This isaccomplished by shifting the belt fromone part of the steps to the another.
Types of Belt Drive [According to its Arrangement]
Velocity ratio
It is the ratio between the velocities of the driver and the follower or driven
