Engineering Mechanics

(^170) „„„„„ A Textbook of Engineering Mechanics
EXERCISE 9.3

1. A square threaded screw jack of mean diameter 25 mm and a pitch of 6 mm is used to lift
   the load of 1500 N. Find the force required at the mean circumference if the coefficient
   of friction between the screw and nut is 0.02. [Ans. 144.2 N]


2. A square threaded screw jack of mean diameter 50 mm has 3° angle of inclination of the
   thread and coefficient of friction 0.06. Find the effort required at the end of handle 450
   mm long (i) to raise a load of 20 kN, and (ii) to lower the same load.
   [Ans. 126 N; 8.4 N]

QUESTIONS

1. What is a wedge? State its uses and the method of solving the problems on wedge friction.


2. What is a screw jack? Explain the principle, on which it works.


3. Establish a relation between the effort and load, when a square threaded screw is used for
   lifting purposes, taking friction into account.


4. Derive a relation for the efficiency of a screw jack, taking friction into account.


5. In a screw jack, the helix angle is α and the angle of friction is φ. Show that its efficiency
   is maximum, when
   2 α = 90° – φ.

OBJECTIVE TYPE QUESTIONS

1. If a ladder is not in equilibrium against a smooth vertical wall, then it can be made in
   equilibrium by
   (a) increasing the angle of inclination.
   (b) decreasing the angle of inclination.
   (c) increasing the length of the ladder.
   (d) decreasing the length of the ladder.


2. The efficiency of a screw jack may be increased by
   (a) increasing its pitch.
   (b) decreasing its pitch.
   (c) increasing the load to be lifted.
   (d) decreasing the load to be lifted.


3. The efficiency of a screw jack is maximum when the helix angle is equal to

(a) 45

2

φ

°+ (b) 45 –

2

φ

° (c) 30

2

φ

+° (d) –30

2

φ

°

ANSWERS

1. (a) 2. (a) 3. (b)

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