(^604) „„„„„ A Textbook of Engineering Mechanics
Fig. 30.4. Proney brake dynamometer.
When the brake is to be put in operation, the long end of the lever is loaded with some suitable
loads and the screws are tightened, until the engine shaft runs at a constant speed and the lever is in
horizontal position. Under these conditions, the frictional torque, due to the weight hung, must be
equal to the torque being transmitted by the engine.
Let W = Weight hung from the lever
L = Horizontal distance between the centre of the pulley and the
line of action of the weight (W),
N = Speed of the engine shaft in r.p.m.
∴ Frictional torque = W × L
and brake power absorption by the dynamometer
2( )
60
π×NW L

Notes. 1. In this dynamometer, it is not necessary to know the radius of pulley, coefficient
of friction between the wooden blocks and the pulley and, the pressure exerted by the tightening
screw.
2.The dynamometer is liable to severe oscillations, when the driving torque on the shaft is not
uniform.
Exmple 30.4. Following observations were recorded during the trial of a proney brake
dynamometer :
Weight hung from the lever = 100 N
Distance between weight and pulley = 1·2 m
Shaft speed = 150 r.p.m.
Find the brake power of the engine.
Solution. Given :Weight hung from the lever (W) = 100 N ; Distance between weight and
pulley (L) = 1·2 m and shaft speed (N) = 150 r.p.m.
We know that brake power of the engine
2 ( ) 2 150 (100 1·2)
= 1885 W
60 60
π× π×NW L ×

= 1·885 kW Ans.