The useful horse power of a stationary engine may be readily and accurately obtained by means of a pair of scales, and a brake, as shown in [Fig. 3356], which is constructed and used as follows:
On the crank shaft of the engine is a pulley enveloped by a friction brake, which consists of an iron band, to which wooden blocks are fastened.
The ends of the iron band do not meet, but are secured together by a bolt as shown.
By screwing up the bolt the wood blocks are brought to press against the circumference of the wheel.
This forms a friction brake that would revolve with the wheel, were it not for two arms that are secured to the brake, and rest at the other end upon a block placed upon a pair of scales.
The principle of action of this device is that the amount of friction between the brake and the wheel is weighed upon the scales, and this amount, multiplied by the velocity of the wheel at its circumference and divided by 33,000, is the horse-power of the engine.
It is necessary, in arranging this brake, to have its end rest upon the scale at the same height from the floor as the centre of the crank shaft, so that the line marked 5′ 3′′ (5 feet 3 inches), which represents the length of the lever, shall stand parallel with the surface of the platform of the scale.
To test the horse-power, we proceed as follows:
Suppose the pressure of the end of the lever on the scale is found by the weight on the scale beam to be 540 lbs., the diameter on which the brake blocks act being 3 feet, the length of the leverage being 5 feet 3 inches, as marked, and the engine making 150 revolutions per minute, and the calculation is as follows:
| 540 | lbs. on scale. | |||
| 5.25 | leverage in feet. | |||
| 2700 | ||||
| 1 | 080 | |||
| 27 | 00 | |||
| radius of pulley in feet 1.5 ) | 28 | 35.00 | ( 1890 lbs. at pulley perimeter. | |
| 15 | ||||
| 13 | 3 | |||
| 12 | 0 | |||
| 1 | 35 | |||
| 1 | 35 | |||
| . | ..0 | |||