The mechanical advantage of a tackle is the term applied to the relationship between the load being lifted and the power required to lift the load. If a load of 10 pounds requires 10 pounds of power to lift it, the mechanical advantage is 1. If a load of 50 pounds requires only 10 pounds of power to lift it, then the mechanical advantage is 5 to I, or 5 units of weight lifted for each unit of power applied. The mechanical advantage of a simple tackle is determined by counting the number of parts of the falls at the movable block. A gun tackle has a mechanical advantage of 2 because there are 2 parts of the falls at the movable block. To know the amount of power to lift a given load by means of determine the weight of the load to be lifted and divide that by the mechanical advantage.
If you wanted to lift a 600-pound load by a single luff tackle determine the mechanical advantage gained using this type of tackle. We count the the falls at the movable block and fined that we have a mechanical advantage of 3. By dividing the weight to be lifted 600 pounds, by the mechanical advantage of 3, you find that 200 pounds of power are required to lift a weight of 600 pounds using a single luff tackle.
ALLOWANCE FOR FRICfION
A certain amount of the force applied to tackle is lost through friction. Friction in the tackle is the rubbing of ropes against each other or against the frame or shell of a block passing of the ropes over the sheaves, and the rubbing of the pin against the sheaves. This loss in efficiency of the block and tackle must be added to the weight being lifted. The power required to lift a load, roughly, 10% of the load must be added to the load for every sheave in the tackle. When lifting a load of 500 pounds with a twofold purchase: 10% of 500 pounds is 50 pounds at 4 sheaves= 200 pounds, which must be added to the load.
If the total load is 700 pounds divided by 4 which is the mechanical advantage of a twofold purchase, 175 pounds is required to lift the load.
