The Tat _ _ ham dynamometers are well-known forms of belt dynamometers but these are now seldom used since in most forms of belt dyna mometers there is a tendency of the belt to produce excessive vi bration at high speeds which causes very ir regular readings. Not withstanding this dis advantage, the belt dynamometer may be very satisfactorily used in certain cases.
It is quite feasible to measure the power transmitted to a ma chine or given out by a motor by supporting the machine upon trun nions and directly weighing its turning effect. This method of supporting a machine on its trunnions and determining its torque has been utilized by the Sprague Electric Works in the design of an electric dynamo meter, for testing auto mobile and other motors, which consists of an electric generator supported on its bearings and connected by a flexible coupling to the motor to be tested. The pull is weighed di rectly on a scale-beam which can be calibrated with great accuracy. By weakening the field of the dynamometer the speed of the engine under test and the dynamometer can be varied and in this way values obtained for plotting the power curve of the engine. With this form of dynamometer it is possible to return the gen erated energy into the shop line instead of wast ing the power as is usual with dynamometer work It is to be noted that in this machine the electrical readings have nothing to do with the measurement of power. In this respect it differs from an ordinary electric motor which may be used as a dynamometer with a very high degree of accuracy, where suitable current is available, provided the motor be properly calibrated and the efficiency factor thus obtained used in determining the power transmitted to the machine to be tested.
In the Brackett cradle dynamometer the torque is determined by suitably mounting the machine to be tested upon a swinging platform suspended from knife-edges and supplied with a scale-beam and sliding weight; the tendency of the driving belt to rotate the machine may be weighed on the scale-beam, and this will give a measure of the power when the speed is known. This dynamometer consists essentially of a strong stiff platform, furnished with two rigid uprights carrying steel knife-edges from which the platform is suspended. These knife edges rest upon firm supports bolted to the floor and so constructed that a slight swinging mo tion is allowed to the platform upon which the machine to be tested is mounted. To one of
the swinging uprights, near the knife-edge, is fixed a graduated horizontal lever which car ries a sliding weight. Adjusting screws are provided, by means of which the axis of rota tion of the armature of any given machine may be made to coincide with the axis of oscillation of the cradle, viz., the line which passes through the two knife-edges. In this way machines of various makes and sizes can readily be put in position and their data determined. It is obvi ous that the cradle dynamometer can be used to measure the work absorbed by any machine which can he conveniently mounted on the swinging platform.
Among other forms of dynamometer not already discussed is the Emerson power-scale — an instrument which is connected directly to the revolving shaft without the interposition of belts, except that used to drive the shaft itself. The machine in principle is a rotary scale, and its construction closely resembles the well known Fairbanks platform-scales. This dyna mometer is largely used in cotton-mills to deter mine the power consumed by the individual machines, and when used with care forms an excellent instrument for the purpose, being self contained and readily applied. In this machine the pulley which receives the power is loose on the shaft, and is connected with the latter by means of a spider which is keyed to the shaft, the hub of the spider forming one of the guides to the position of the pulley. Around this spider is a loose rim from which studs project and serve to connect it to the pulley. In trans mitting power from the shaft to the loose pul ley the tendency of the rim to rotate on the spider is resisted by a system of levers which communicate with a pendulum balance-weight.
Another form of shaft-dynamometer is the power meter devised by Franklin Van Winkle. This is a rotary transmitting dynamometer shown in Fig. 7, in which helical pull-springs are employed for weighing the amount of force transmitted from the driving to the driven por tion of the dynamometer. This instrument is especially adapted for adjustment to any shaft or pulley for measuring power transmitted by a shaft to a pulley, or vice versa, in this respect resembling the Emerson power-scale.