Question

What is the efficiency of differential screws when pitch of the two screws is 12mm and 8mm? The nut is rotated by applying a force of 120N at a radius of 300mm and the two screws remain stationary. The torque of raising and lowering for the two screws is 5k N-mm and 2.5k N-mm where k is the effective axial weight on the screw.

(a) 6.48%

(b) 8.48%

(c) 23.1%

(d) 42.8%

The question was posed to me in a scholarship test.

This problem appears in the Screw Jack topic in section Threaded Joints of Machine Design

Answer 1

The accurate answer is (b) 8.48%

Paint a vivid picture of understanding with: 120 x 300=5k+2.5k or k=4800N. Output=k x (12-8) or output=19200N-mm. Efficiency=output/2πx120x300.