What Is Torque? How Is Torque Measured?

Torque is best measured with dynamometers, of which there are two classes: absorption and transmission. Absorption dynamometers absorb the total power delivered by the machine being tested, whereas transmission dynamometers absorb only that part represented by friction in the dynamometer itself.

Made in a wide variety of forms, typical forms are described in the following paragraphs.

The Prony brake is the most common type of absorption dynamometer. The torque developed by the machine to overcome the friction is determined from the product of force required to prevent rotation of the brake and the lever arm. The load is applied by tightening the brake band or adding weights.

The energy dissipated in the brake appears in the form of heat. In small brakes, natural cooling is sufficient, but in large brakes, special provisions have to be made to dissipate the heat. Water cooling is the usual method, one common scheme employing a pulley with flanges at the edges of the rim which project inward.

Water from a hose is played on the inside surface of the pulley and collected again by means of a suitable scooping arrangement. About 100 in2 of rubbing surface of brake should be allowed with air cooling or about 25 to 50 in2 with water cooling per horsepower.

The Westinghouse turbine brake employs the principle of the water turbine and is capable of absorbing several thousand horsepower at very high speeds.

In the magnetic brake, a metallic disk on the shaft of the machine being tested is rotated between the poles of magnets mounted on a yoke which is free to move. The pull due to the eddy currents induced in the disk is measured in the usual manner by counteracting the tendency of the yoke to revolve.

This form of brake can be made in very small sizes and is therefore convenient for very small motors.

The principal forms of transmission dynamometers are the torsion and the cradle types. In torsion dynamometers, the deflection of a shaft or spiral spring, which mechanically connects the driving and driven machines, is used to measure the torque.

The spring or shaft can be calibrated statically by noting the angular twist corresponding to a known weight at the end of a known lever arm perpendicular to the axis. When in use, the angle can be measured by various electrical and optical methods.

The cradle dynamometer is a convenient and accurate device which is extensively used for routine measurements of the order of 100 hp or less. An electric generator is mounted on a “cradle” supported on trunnions and mechanically connected to the machine being tested.

The pull exerted between the armature and field tends to rotate the field. This torque is counterbalanced and measured with weights moved along an arm in the usual manner.

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