CBSE Book Class 12 Engineering Pulleys

PULLEYS

7.1 INTRODUCTION

In every machine or toy, we use power to operate and perform its function. This power is obtained mostly by the motor, run on electricity or battery. To transfer the power from the motor to the operational part of the machine, we use a combination of pulleys and belt (flexible connector). Pulleys are used in very small sizes to be fitted in wrist watches and tape recorders, as well as quite big in size as in ships.

The pulley used, with the motor shaft is called driver and with machine shaft is called driven. The  size of driver and driven pulleys define the ratio of speed transferred as reduced or increased. Ifboth the driver and driven pulley are of same diameter then the speed of the shaft / spindle will be same, if driver is of small diameter with respect to driven then the speed will be reduced at operating shaft and vice versa.

PULLEYS 

Outer cylindrical surface of the pulley used to hold the belt is called RIM, while the inner cylindrical part to be mounted on the shaft is called HUB. The RIM and the HUB are joined together with solid web or spokes or splines depending upon the sizeof the pulley. In the pulleys of diameter up to 200 mm a solid web is provided. The pulley is attached to the shaft either by the key or by a set screw of the suitable size and type.

The driver pulley and driven pulley are connected with different type of endless belts i.e. Flat Belt, Rope Belt, V-Belt etc. The material of the belt must be strong in tension yet flexible and relatively light in weight i.e. canvas, leather, rubber and so on.

Pulley - drive is very easy to install and require minimum maintenance. The power is transmitted from one shaft to another by means of friction between the belt and the rim. The losses in power transmission are negligible in V-belt pulley rather than flat-belt pulley. However power transmission capacity reaches its limit when the belt starts toslip.

Now we understand that pulleys allow us to

1. Lift loads up, down and sideways.

2. Rotate things at different speeds.

PULLEYS

So pulley is a simple machine used in our day to day life to complete the work with less efforts. In this class we will study Flat Belt and V-Belt pulleys, upto 200 mm diameter in detail. The rim of the flat belt pulley is not flat it is slightly convex and this is known as the crowning. Actually the rotating belt around the pulley

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