A portable dynamometer capable of giving a continuous autographic record, designed by the writer with especial reference to the meas urement of power required to drive machine tools, is illustrated in Fig. 8.
In its latest form, this machine consists essentially of three pulleys carried by a hollow shaft which runs upon roller hearings mounted on a stiff frame as shown. The pulley on the right is fixed to the shaft and may he used for the belt which connects the dynamometer to the machine to be tested, or it may be used for a brake-band; the middle pulley is an idler and runs loose on the shaft; while the one to the left receives the driving belt and is free to turn only within given limits. This latter ley is connected to the shaft through the hydraulic cylinders and cylinder carrier. The cylinders and shaft are filled with oil, so that any resistance to turning produces a pressure in the cylinders. This pressure is transmitted by the oil through the hollow shaft, and is recorded by the movement of an indicator-piston which is fitted into the end of the shaft; the pres sure is also shown by the gauge at the opposite end. In addition to the force curve traced by the indicator pencil, the zero or datum line is traced by another pencil which is adjustable in position and may be located at pleasure on a line parallel with the axis of the shaft. Thus one is enabled to measure the total power required to run a given ma chine or any subdivided part of it. Springs of various intensities are used to transmit the pressure from the shaft to the indicator pencil, de pending upon the pressure to be recorded. These springs are changed as in any ordinary steam engine indicator. One of the interesting features about this machine is its adaptability to either transmit or absorb power. In either case an autographic record is traced upon a roll of paper which can be stopped or started at pleasure.
An examination of a diagram from this dynamometer shows that the power required to drive the machine empty can readily be determined if desired, for all that is necessary is to run the machine at the same speed without Pro. — Mather Transmission and Absorption Dynamometer.
any pressure on the cutting tool, and the sulting diagram will give a measure of the work. The same is true of the power required
to operate the feed mechanism of the machine, or the dynamometer itself. The actual value of the power is obtained by methods which will be obvious.
For many years the attention of engineers and power users has been directed to the practicability of using the twist or torsion in a shaft as a means of determining the horse power transmitted by it. When power is ap plied to one end of a shaft the amount of twist varies directly as its length, directly as the moment of the applied load, inversely as the rigidity of the material, and inversely as the fourth power of its diameter all of which is expressed in the formula: e= — G for hollow shafts 0— (32) Me L G(D4___d4) in which 0= the angular displacement in radians, M t= twist ing moment in inch-pounds, L = length of shaft in feet, G= the modulus of rigidity, or modulus of transverse elasticity, D = outside diameter of shaft and d=inside diameter if hollow, both taken in inches. Various methods have been developed for determining the magni tude of the twist in the shaft by means of tor sion metres or torsion indicators, most of which have been designed with especial reference to marine service. The rapidly increasing use of steam turbines in ships and the necessity of determining the power of the turbines— for which the steam engine indicator is not adapted —has created a demand for an accurate, easily operated torsion dynamometer, since the use of some form of torsion meter is the only known method of obtaining the desired in formation. The principle of operation in gen eral consists in the observation of the twist between two given points on the shaft and this has been frequently accomplished by means of beams of light in combination with rotating discs; in other cases electrical observations are used to determine the distortion: in any case the horse power is obtained from: H. P. = 0 CL ' i n which 0 is the angle of twist in dcgrecs, D= diameter of shaft in inches, N= revolutions per minute, L=length of shaft in feet between which e is determined, and C= a constant depending upon the modulus of transverse elasticity, and other numerical factors.