Ques. Describe the apparatus used in making a brake test.
Fig. 428.—Prony brake for determining brake horse power. It consists of a friction band ring which may be placed around a pulley or fly wheel, and attached to a lever bearing upon the platform of a weighing scale in such a manner that the friction between the surfaces in contact will tend to rotate the arm in the direction in which the shaft revolves. This thrust is resisted and measured in pounds by the scale. In setting up the brake the distance between the center of the shaft and point of contact (knife edge) with the scales must be accurately measured, the knife edge being placed at the same elevation as the center of the shaft. An internal channel permits the circulation of water around the interior of the rim as shown, to prevent overheating.
Ans. Tests of this kind are usually made with a Prony brake as shown in [fig. 428]. It consists of a band of rope or strip iron—the latter is the arrangement shown—to which are fastened a number of wooden blocks, several carrying shoulders to prevent the contrivance from slipping off the wheel rim. The brake band is drawn tight, as shown, so that the blocks press against the surface all around. The brake thus formed is restrained from revolving with the pulley by two arms attached near the top and bottom centers of the wheels, and joined at the opposite ends to form a lever which bears upon an ordinary platform scale, a suitable leg or block being arranged to keep its end level with the center of the shaft. By this arrangement the amount of friction between the brake band and the revolving wheel is weighed upon the scales. Since the brake fits tightly enough to be carried around by the wheel, but for the arms bearing upon the scale, the amount of frictional power exerted by the wheel in turning free within the blocks may be transmitted and measured, just as would be the case were a machinery load attached, instead of a friction brake.
Ques. Why must the point of contact of the brake with the scales be level with the center of the shaft?
Ans. In order to determine the force acting at right angles to the line joining the point of contact and center of the shaft.
Ques. What is the distance between the center of the shaft and point of contact with the scales called?
Ans. The lever arm.
Ques. What three quantities must be determined in a test in order to calculate the brake horse power?
Ans. The lever arm, the force exerted on the scales, and the revolutions per minute.