Question

Five pulleys are arranged in the second system of pulleys. When not loadwd the effort required to raise the movable block is 35N. Further wastage in friction increases the pull at the rate of 3% of the load lifted. What is the efficiency of the system at 2kN?

(a) 80%

(b) 80.81%

(c) 80.50%

(d) none of the mentioned

I have been asked this question in class test.

I need to ask this question from Pulleys topic in chapter Belt, Rope and Chain Drives of Machine Kinematics

Answer 1

The correct answer is (b) 80.81%

To explain I would say: n = no. of pulleys = 5

Frictional effort at zero loading = 35N

Frictional effort at 2 kN loading = 35 + 2000 x 3/100 = 95 N

When the system is considered frictionless nP = W

                                           5P = 2000

                                            P = 400 N

Hence total effort = 400 + 95 = 495 N

Efficiency at this load = effort without friction/effort with friction

                        = 400/495 x 100

                        = 80.81%.