QUESTION 1

A pulley 150 mm diameter is driven directly by an electric motor at 250 revs min-1. A V-belt is used to transmit power from this pulley to a second pulley 400 mm diameter against a load of 200 Nm.

The distance between the centre of the pulleys is 600 mm, the included angle of the pulley groove = 40°, the coefficient of friction between the belt and pulley is 0.4 and the ultimate strength of the belt is 8 kN.

(a) Calculate the actual power transmitted to the second pulley.

(b) Calculate the power which can be transmitted if the maximum tension in the belt is limited to half of the ultimate strength of the belt.

(c) What would be the effect of the following factors on the maximum power which can be transmitted (give reasons for your answer):

(i) increasing the coefficient of friction

(ii) increasing the included angle of the pulley groove.

(d) What would be the effect on the following if the load torque is increased and the speed maintains constant (give reasons for your answer):

(i) the tension in the tight side of the belt

(ii) the tension in the slack side of the belt

(iii) the power transmitted.

QUESTION 2

A gear train is to be designed to transmit power from shaft 1 to shaft 2 with the following condition:

(i) Shaft 1 (driven by an electric motor) rotates at 100 revs min-1.

(ii) Shaft 2 is to rotate at 400 revs min-1 in the opposite direction to shaft 1 against a load of 200 Nm.

(iii) The centre of shaft 1 is 300 mm from shaft 2.

(iv) The minimum number of teeth on any gear is 15, all gears must have a multiple of 5 teeth and have a module of 2 mm.

(v) The maximum number of gears permissible is 4 gears and the diameter of the maximum gear must be minimized.

(vi) The centers of the gears should lie on a line which is as close to straight as possible.

(vii) All shafts have a frictional resistance of 5 Nm. Carry out the following:

(a) Design a gear train which satisfies the above criteria. Use a sketch to illustrate your design, label the gears A, B, C, etc. from the driver to the driven gear, and state the number of teeth on each gear.

(b) Determine the input power required at shaft 1.

(c) Specify the efficiency of the gear train as a percentage.

(d) Determine an equation for the efficiency of the gear train in terms of the load (torque) on shaft 2 (all other factors remaining constant).