TORQUE MEASUREMENT
·
Measurement of applied torques is of
fundamental importance in all rotating bodies to ensure that the design of the
rotating element is adequate to prevent failure under shear stresses.
·
Torque measurement is also a necessary
part of measuring the power transmitted by rotating shafts.
·
The four methods of measuring torque
consist of
·
Measuring the strain produced in a
rotating body due to an applied torque
·
An optical method
·
Measuring the reaction force in cradled
shaft bearings
·
Using equipment known as the Prony
brake.
Measurement
of Induced Strain
Measuring the strain induced in a shaft due to an
applied torque has been the most common method used for torque measurement in
recent years. The method involves bonding four strain gauges onto a shaft as
shown in Figure, where the strain gauges are arranged in a d.c. bridge circuit.
The output from the bridge circuit is a function of the strain in the shaft and
hence of the torque applied. It is very important that positioning of the
strain gauges on the shaft is precise, and the difficulty in achieving this
makes the instrument relatively expensive. This technique is ideal for
measuring the stalled torque in a shaft before rotation commences. However, a
problem is encountered in the case of rotating shafts because a suitable method
then has to be found for making the electrical connections to the strain
gauges. One solution to this problem found in many commercial instruments is to
use a system of slip rings and brushes for this, although this increases the
cost of the instrument still further.
Optical Torque
Measurement
Optical techniques for torque measurement have
become available recently with the development of laser diodes and fiber-optic
light transmission systems. One such system is shown in Figure. Two
black-and-white striped wheels are mounted at either end of the rotating shaft
and are in alignment when no torque is applied to the shaft. Light from a laser
diode light source is directed by a pair of fiber-optic cables onto the wheels.
The rotation of the wheels causes pulses of reflected light, which are
transmitted back to a receiver by a second pair of fiber-optic cables. Under
zero torque conditions, the two pulse trains of reflected light are in phase
with each other. If torque is now applied to the shaft, the reflected light is
modulated. Measurement by the receiver of the phase difference between the
reflected pulse trains therefore allows the magnitude of torque in the shaft to
be calculated. The cost of such instruments is relatively low, and an
additional advantage in many applications is their small physical size.
Reaction Forces in
Shaft Bearings
Any system involving torque transmission through a
shaft contains both a power source and a power absorber where the power is
dissipated. The magnitude of the transmitted torque can be measured by cradling
either the power source or the power absorber end of the shaft in bearings, and
then measuring the reaction force, F, and the arm length, L, as shown in
Figure. The torque is then calculated as the simple product, FL. Pendulum
scales are used very commonly for measuring the reaction force. Inherent errors
in the method are bearing friction and windage torques. This technique is no
longer in common use.
Prony Brake
The
Prony brake is
another torque- measuring system
that is now uncommon. It is used to measure the torque in a rotating shaft and
consists of a
rope wound round
the shaft, as illustrated in
Figure. One end
of the rope
is attached to a spring balance and the other end carries a load in the
form of a standard mass, m.
If the measured force in the spring balance is Fs,
then the effective force, Fe, exerted by the rope on the shaft is given by
Fe = mg - Fs
If the radius of the shaft is Rs and that of the
rope is Rr, then the effective radius, Re, of the rope and drum with respect to
the axis of rotation of the shaft is given by
Re = Rs + Rr
The torque in the shaft, T, can then be calculated
as
T= FeRe
While this is a well-known method of measuring shaft
torque, a lot of heat is generated because of friction between the rope and
shaft, and water cooling is usually necessary.
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