Belt drive is a mechanism in which power is transmitted by the movement of a continuous flexible belt. It is used to tranmit roational motion from mechanical element to another.A belt is looped strip of flexible material which is used to mechanically link two or more rotating shafts. A belt drive is found in almost every modern engine. The amount of power transmitted between the two is dependent on the amount of friction between the two shafts.
Factors that are responsible for power transmission are velocity of belt, belt tension between the pulleys and the angle of contact between the pulleys.
Belt drives has two rotating shafts connected to each other using a belt. The belt that drives or the one that is connected to power source is called driving system and the other one which is driven or to which rotaional motion is transmitted is called driven system. Similarly, the pulley which transmit rotation is called driving pulley and the pulley to which rotation is transmitted is called driven pulley.
Types of belt drives :-
The belt drives are divided into two types based on the direction in which belt drives the pulley. They are open belt drive and cross belt drive. These two types of belt drives are discussed below.
1) Open Belt Drives :-
In open belt drive the driven pulley is in the same direction of driving pulley. Open belt drive is best suitable for those conditions is which driving and the driven pulleys are in horizontal position or a little bit inclined. The wrap angle which is the angle between the belt and the pulley is small i.e always less than 180º . In this belt drive length of belt needed is less but power transmission capacity is small due to smaller wrap angle.
2) Crossed Belt Drives :-
In crossed belt drive the driven pulley rotates in the opposite direction of the driving pulley. Unlike the open belt drive, it is not only suitable for horizontal positioned pulleys but also inclined and vertical positions of pulleys. In this belt drive the required length of cross belt is more than that of open drive.In this type of belt drive the wrap angle is always large i.e more than 180 º. In this belt drive the required length of cross belt is more than that of open drive . Other disadvantage of this belt drive is that in this belt drive the belt rubs with itself and the thus life of the belt reduces but it can transmit more power as wrap angle is more.
3) Stepped Cone Pulley Speed Cone Drive :-
This type of belt drive contains three or more number of pulleys of different sizes one adjacent to other. It is used when speed of the driven shaft needs to be changed very quickly. In this belt drive the speed of the driven shaft can be varied by shifting the belt from one pair of pulleys to the other. Examples of machines in which Speed cone drive is used are lathe machine and drilling machine.
4) Fast and Loose Pulley Drive :-
This type of belt drive is used when there is need to start or stop the driven shaft without interfering the driving shaft. In this type of belt drive, there are two types of pulleys attached to the driven shaft, that are fast pulley and loose pulley. Fast pulley is keyed to the driven shaft and rotates with the same speed as that of driven shaft. Loose Pulley rotates freely over the driven shaft and do not transmit any power. Whenever it is required to
stop the driven shaft,the belt is pushed to the loose pulley using a sliding bar having belt forks.
5) Jockey Pulley Drive :-
If in an open belt drive the driven pulley is very small, then the arc of contact of the belt with the driven pulley is very small. Due to very small arc of contact the tension in the belt become very less and to counter that an idle pulley is used which is called jockey pulley and the whole system is called Jockey Pulley Drive. Due to increase in tension of belt the power transmission is also increased.