Fixed Pulley System Question

Question

What would be the mechanical advantage of a fixed pulley system (made up of 6 pulleys)? E.g in the case of an elevator

Answer 1

If pulleys are the same size then for each 6ft of cable pulled the car should rise 1 ft so it would be a 6/1 advantage ie for  2 pulleys the travel of the lower pulley is 1/2 the length of the cable travel

Answer 2

http://findarticles.com/p/articles/mi_gx...

Answer 3

Maximum mechanical advantage is attained if the bitter end of the wire rope is attached to the pulley block of the elevator cab while the other end is attached to the winch drum. In this arrangement there are seven lines carrying the elevator cab. Assuming a coefficient of friction of 0.02 on the pulleys the tension force on the line going to the winch would be:

T = (P/7)(1.02)^6 = 0.20265P

Where P is the weight of the elevator. Hence the net mechanical advantage of the pulley system is:

MA = P/0.20265P = 4.935

Ordinarily the mechanical advantage is taken as the load divided by the number of lines carrying the load, but in the actual, friction makes a considerable reduction in the mechanical advantage.