Engineering Mechanics

Chapter 11 : Simple Lifting Machines „„„„„ 215

QUESTIONS

1. Distinguish clearly between a ‘simple wheel and axle’ and a ‘differential wheel and axle’.


2. Describe the working of a Weston’s differential pulley block.


3. In a differential pulley lifting tackle, the upper block consists of two pulleys of diameter D
   and d rotating on a fixed axis, and a movable pulley below, to which is attached the load.
   The tackle is operated by an endless chain and the effort is applied to lengths coming off
   the larger pulley. Find the expression for the velocity ratio, if D > d.


4. State the difference between a Weston’s differential pulley block and a geared pulley
   block.


5. Derive an equation for the velocity ratio of a worm and worm wheel when it is (a) single
   threaded and (ii) double threaded.


6. Distinguish clearly the difference between the working of a single purchase crab winch
   and a double purchase crab winch.


7. What is pulley? State the working of first system, second system and third system of
   pulleys. Derive relations for their respective velocity ratios.


8. What is a screw jack? On what principle does it work derive a relation for the velocity
   ratio of a simple screw jack and differential screw jack.

OBJECTIVE TYPE QUESTIONS

1. The velocity ratio of a simple wheel and axle with D and d as the diameters of effort wheel
   and load axle is :
   (a) D + d (b) D – d (c) D × d (d) D/d


2. The velocity ratio of a differential wheel and axle with D as the diameter effort wheel and
   d 1 and d 2 as the diameters of larger and smaller axles respectively is

(a)

12

2 D

dd+ (b) 12

2

D

dd (c) 12

D

dd+ (d) 12 –

D

dd

3. A differential pulley block has larger and smaller diameters of 100 mm and 80 mm
   respectively. Its velocity ratio is
   (a) 5 (b) 10 (c) 20 (d) 40


4. In a wormed geared pulley block, if we double the number of teeth on the worm wheel, its
   velocity ratio is
   (a) doubled (b) remains the same (c) halved


5. The velocity ratio of a single purchase crab winch can be increased by

(a) increasing the length of the handle

(b) increasing the radius of the load drum

(c) increasing the number of teeth on the pinion

(d) all of the above