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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