Friction and its applications
1) A load of 1.5 kN, resting on an inclined rough plane, can be moved up the plane by a force of 2
kN applied horizontally or by a force 1.25 kN applied parallel to the plane. Find the inclination
of the plane and the coefficient of friction.( 0.292)
2) Screw press is used to compress books. The thread is a double thread (square head) with a pitch
of 4 mm and a mean radius of 25 mm. The coefficient of the friction (µ) for the contact surface
of the thread is 0.3. Find the torque for a pressure of 500 N. (4450 N-mm)
3) The screw of a jack is square threaded with two threads in a centimeter. The outer diameter of
the screw is 5 cm. If the coefficient of friction is 0.1, calculate the force required to be applied at
the end of the lever, which is 70 cm long (a) to lift a load of 4 kN, and (ii) to lower it. (17.5 N;
257.9 N; 8.3 N)
4) In a screw jack, the helix angle is and the angle of friction is Ф. Show that its efficiency is
maximum, when 2 = 90° – Ф
5) What is a screw jack? Explain the principle, on which it works.
6) Establish a relation between the effort and load, when a square threaded screw is used for lifting
purposes, taking friction into account.
7) Derive a relation for the efficiency of a screw jack, taking friction into account
8) An engine shaft running at 120 r.p.m. is required to drive a machine shaft by means of a belt.
The pulley on the engine shaft is of 2 meters diameter and that of the machine shaft is of 1 meter
diameter. If the belt thickness is 5 mm, find the speed of the machine shaft when (i) there is no
slip, and (ii) there is a slip of 3%. (239·4 r.p.m., 232 r.p.m.)
9) Find the length of belt necessary to drive a pulley of 500 mm diameter running parallel at a
distance of 12 meters from the driving pulley of diameter 1600 mm.(for open and crossed)(
27·32 m, 27·39 m)
10) Find the power transmitted by a belt running over a pulley of 600 mm diameter at 200 r.p.m.
The coefficient of friction between the belt and pulley is 0·25, angle of lap 160° and maximum
tension in the belt is 2·5 kN.(7.96 kW)
11) A laminated belt 8 mm thick and 150 mm wide drives a pulley of 1·2 m diameter at 180 r.p.m.
The angle of lap is 190° and mass of the belt material is 1000 kg/m3. If the stress in the belt is
not to exceed 1·5 N/mm2 and the coefficient of friction between the belt and the pulley is 0·3,
determine the power transmitted when the centrifugal tension is (i) considered, and (ii)
neglected.( 11·74 kW, 12·84 kW)
12) A flat belt 7·5 mm thick and 100 mm wide transmits power between two pulleys, running at
1600 r.p.m. The mass of the belt is 0·9 kg/m length. The angle of lap on the smaller pulley is
165° and the coefficient of friction between the belt and pulleys is 0·3. If the maximum
permissible stress in the belt is 2 MN/m2, find (i) maximum power transmitted; and (ii) initial
tensions in the belt. [15·44 kW ; 710·5 N]
13) Friction clutches can function as overload protection devices. Explain the theoretical basis of
this statement.
14) Give two circumstances in which relative slip in friction clutches would be desirable.
15) A plate clutch with two pairs of contacting friction surfaces is designed to transmit a maximum
torque of 165 Nm. The ratio of the maximum transmitted torque to the applied axial torque is to
be 55mm. The maximum diameter of the friction linings is to be 300 mm. Given that the
limiting coefficient of friction is 0.25 and that the uniform wear theory applies to this clutch,
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1) A load of 1.5 kN, resting on an inclined rough plane, can be moved up the plane by a force of 2
kN applied horizontally or by a force 1.25 kN applied parallel to the plane. Find the inclination
of the plane and the coefficient of friction.( 0.292)
2) Screw press is used to compress books. The thread is a double thread (square head) with a pitch
of 4 mm and a mean radius of 25 mm. The coefficient of the friction (µ) for the contact surface
of the thread is 0.3. Find the torque for a pressure of 500 N. (4450 N-mm)
3) The screw of a jack is square threaded with two threads in a centimeter. The outer diameter of
the screw is 5 cm. If the coefficient of friction is 0.1, calculate the force required to be applied at
the end of the lever, which is 70 cm long (a) to lift a load of 4 kN, and (ii) to lower it. (17.5 N;
257.9 N; 8.3 N)
4) In a screw jack, the helix angle is and the angle of friction is Ф. Show that its efficiency is
maximum, when 2 = 90° – Ф
5) What is a screw jack? Explain the principle, on which it works.
6) Establish a relation between the effort and load, when a square threaded screw is used for lifting
purposes, taking friction into account.
7) Derive a relation for the efficiency of a screw jack, taking friction into account
8) An engine shaft running at 120 r.p.m. is required to drive a machine shaft by means of a belt.
The pulley on the engine shaft is of 2 meters diameter and that of the machine shaft is of 1 meter
diameter. If the belt thickness is 5 mm, find the speed of the machine shaft when (i) there is no
slip, and (ii) there is a slip of 3%. (239·4 r.p.m., 232 r.p.m.)
9) Find the length of belt necessary to drive a pulley of 500 mm diameter running parallel at a
distance of 12 meters from the driving pulley of diameter 1600 mm.(for open and crossed)(
27·32 m, 27·39 m)
10) Find the power transmitted by a belt running over a pulley of 600 mm diameter at 200 r.p.m.
The coefficient of friction between the belt and pulley is 0·25, angle of lap 160° and maximum
tension in the belt is 2·5 kN.(7.96 kW)
11) A laminated belt 8 mm thick and 150 mm wide drives a pulley of 1·2 m diameter at 180 r.p.m.
The angle of lap is 190° and mass of the belt material is 1000 kg/m3. If the stress in the belt is
not to exceed 1·5 N/mm2 and the coefficient of friction between the belt and the pulley is 0·3,
determine the power transmitted when the centrifugal tension is (i) considered, and (ii)
neglected.( 11·74 kW, 12·84 kW)
12) A flat belt 7·5 mm thick and 100 mm wide transmits power between two pulleys, running at
1600 r.p.m. The mass of the belt is 0·9 kg/m length. The angle of lap on the smaller pulley is
165° and the coefficient of friction between the belt and pulleys is 0·3. If the maximum
permissible stress in the belt is 2 MN/m2, find (i) maximum power transmitted; and (ii) initial
tensions in the belt. [15·44 kW ; 710·5 N]
13) Friction clutches can function as overload protection devices. Explain the theoretical basis of
this statement.
14) Give two circumstances in which relative slip in friction clutches would be desirable.
15) A plate clutch with two pairs of contacting friction surfaces is designed to transmit a maximum
torque of 165 Nm. The ratio of the maximum transmitted torque to the applied axial torque is to
be 55mm. The maximum diameter of the friction linings is to be 300 mm. Given that the
limiting coefficient of friction is 0.25 and that the uniform wear theory applies to this clutch,
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