|
|
 |
|
|
|
|||||||||||
| BASIC RULES THROUGH EXAMPLES |
|
||||||||||
THE ELEMENTARY HOISTING MECHANISM
A system consisting of a fixed (upper) pulley (sheave) and a traveling (lower) one is the minimum mechanism to multiply our hoisting capacity. The number of parts of lines that really supports the load - i.e. 2 in figure A - is the factor that makes the winch has to pull the cable with half the force.
If we add a second sheave to the upper block, we can fix the cable to the lower sheave, then having a factor of 3 (figure B).
As we have friction between the sheaves and their axis, we have losses for each part of line when the cable takes a 180º turn - that is, from 1% to 5%, depending on the material. (We could use i.e., 4% for bronze bushed sheaves and 2% for anti-friction bearing sheaves).
We have to take into account also the sheaves that usually are placed between the winding drum of the winch and the first sheave of the upper block. Those pulleys are for the deviation of the cable.
If N are the number of lines that takes a 180º turn and M are the intermediate pulleys that deviate the cable, then the necessary line pull of the winch T to lift a load P WITH NO ACCELERATION is: