DYNAMOMETERS AND THE MEAS UREMENT OF POWER. In designing a modern industrial plant or manufactory and in 'estimating the amount and cost of power for its operation, an accurate knowledge of the amount of power absorbed by the different ma chines is not only desirable, but essential to economy and efficiency.
. If the power required is not known the engine or motor provided may prove incapable of doing the work; or, on the other hand, the motive power may be largely in excess of that required: in either case there is an unnecessary expense — in the first case, in remedying the deficiency; and, in the second, in the daily ex penditure for excess of power to overcome fric tion and other losses. If a more accurate knowledge of the power required to drive machine-tools were known, there would be a greater economy in running them. Moreover, the increasing use of electric motors and in ternal combustion engines for various purposes has created a demand for accurate determina tions of the power developed so that exact methods must be employed in measuring the power.
The attempt to determine the amount of horse power given up by a motor, or required to operate a machine, by considering the velocity and width of belt used in a similar machine is futile and very misleading results are ob tained; for while this method will give a close approximation to the amount of power a belt of a given width ought to transmit at a given velocity, there is nothing definitely known about the actual stress in the belt, therefore there cannot be anything definitely known about the power transmitted by it.
The only satisfactory method of ascertaining the amount of power is by the use of some form of dynamometer —by which is meant an in strument or machine for measuring the power exerted by a prime mover, or the amount of power consumed by a machine or group of machines. Among the many machines and de vices for measuring power one of the simplest is the Prony friction brake. Primarily this consists of a lever L, Fig. 1, connected to a revolving shaft or pulley in such a manner that the friction induced between the surfaces in contact will tend to rotate the arm in the direc tion in which the shaft revolves. This rotation
lever is maintained in its horizontal position, while the revolutions of shaft per minute re main constant. That this measure of the fric tion is equivalent to a measure of the work of the shaft will be seen when one considers that the entire driving power of the shaft is ex pended in producing this friction at the required number of revolutions per minute—and this driving power is equal to the mechanical effect of the shaft when running at the same speed in the performance of useful work.
A regulator or dash-pot attached to the end of the lever-arm or scale-beam may be used with the Prony brake — and other various forms of dynamometer in which the pressure is weighed, in order to maintain a more even balance and to prevent vibrations and sudden shocks due to momentary slip of the belt or inefficient lubrication of the brake.
Instead of hanging weights in a scale-pan, as in Fig. 1, the friction may be weighed on a platform-scale; in this case the direction of is balanced by weights P, hung in the scale pan at the end of the lever. A counterpoise attached to the brake-arm is often used in order to balance it before adding weights in the scale pan. If not balanced, the weight of the lever arm must be ascertained and its moment added to the total moment of the weight to obtain an accurate measure of the friction. In order rotation being the same, the lever-arm will be on the opposite side of the shaft as shown in Fig. 2.
This brake consists of a flexible metal strap to which are fitted blocks of wood forming the rubbing surface; the ends of the strap arc con nected by an adjustable bolt clamp, by means of which any desired tension may be obtained. A small stream of water constantly discharges to measure the power for a given number of revolutions of pulley, weights are added to the scale-pan and the bolts bb tightened, until the friction induced balances the weights and the into the pulley rim which is provided with in ternal flanges; a waste pipe p, with its end flattened, is so placed in the trough that it acts as a scoop, and removes all surplus water.