Figure 13.7 shows a single fixed pulley arrangement which consists of a wheel with a groove around its circumference to receive a rope or chain. It is mounted on an axle about which it can rotate freely. The ends of the axle are supported by a frame called block. Block may be fixed or movable, if it is fixed the pulley is called fixed pulley and if it is movable the pulley is called movable pulley.
Figure 13.7 (a) Single Fixed Pulley Arrangement, (b) Single Movable Pulley, and (c) One Fixed and One Movable Pulley Arrangement
There are following assumptions made for pulley arrangement:
- Pulley weight is neglected since the pulley weight is considered very small compared to the weight to be tied.
- Pulley is smooth hence tensions in both side of the string tied on pulley is same.
Direction of effort P in Figure 13.7 is vertically downward; it may be upward or inclined which does not change the value of tension in the string. In all cases, P = W
Figure 13.7 (b) shows a single movable pulley arrangement. The block is movable. In this case, the effort may be applied in any convenient direction. The weight W will be equally shared by the two portions of the string. In this case, mechanical advantage is more than one. But, the force cannot be applied easily. For easy application of force P, a fixed pulley is introduced as shown in Figure 13.7 (c).
It is clear from Figure 13.7 (c) that to raise the load W through a certain distance, the effort will have to move double the distance.
Hence,
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