Rotating Shaft Torque
Measurement
Mark Klein – Accumetrics, Inc
(A PCB Group Company)
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Measuring Rotating Torque
•  Common auto applications
–  Driveshaft/halfshaft testing
–  Dynamometers
–  Drivetrain testing and monitoring
•  Primary parameters of interest
–  Shaft torque, but also shaft bending and thrust, rotating part
temperatures, internal pressures, component stresses, and
specialty measurements
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Solutions for Measuring Rotating Torque
•  Slip rings
–  Can have issues with physical fit, wipers wear out over time
–  The sliding motion produces noise, making data reduction difficult
•  Torque Flanges
–  Work well, and are calibrated, but may not be appropriate for insertion
into a driveshaft or other demanding applications
•  Telemetry
•  More adaptable to varied applications
•  Must be powered (battery or induction) and needs calibration
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Telemetry
•  Telemetry allows you to make good measurements on the
shaft, and to send a robust signal-conditioned measurement
to the stationary world
•  Think of telemetry as a black box amplifier that is rotating on
one end, stationary at the other
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Telemetry System Examples
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Shaft Torque to Strain
•  Applying torque results in shaft twist, measured in
microstrain. A twist of 0.1% is 1000 microstrain. (This is
an easy signal to measure with a strain gage!)
•  The strain level depends on shaft diameter (OD and ID),
shaft material and the torque
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Calibrate or Calculate
To find the specific effects torque has on strain, either:
•  Apply a known torque (NIST traceable), and measure the
output from the strain gage (or subsequently from the telemetry/
data acquisition system.)
-or•  A calculation is performed, using the diameters (OD and ID),
the torque, and either the modulus of elasticity (E), or the shear
modulus (G)
Strain / gage element = (8 x T x OD x 100,000)/(
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𝜋 x G x (OD4 - ID4))
Convert Strain to an Electrical Signal
•  A full bridge (4 resistor-element Wheatstone
Bridge) strain gage is used as the transducer.
•  When the shaft twists (on 45 degree angles)
–  2 opposite resistors physically expand (causing
higher resistance)
–  The other 2 resistors contract (lower resistance)
•
A small electrical signal can then be measured
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Full Bridge Strain Gage
A Better Approach:
Use 2 half bridges to cancel
shaft bending error. Wire them
as a Full Bridge configuration.
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Wheatstone
Gage Factor
•  Metal-foil resistive strain gages often have a “gage
factor” of approximately 2 (see data sheet that
comes with the strain gage)
•  If the shaft twists 1000 microstrain, then a full bridge
will output 2mV/Vexcitation
Note: Vexcitation is the voltage supplied by the telemetry transmitter to the gage. The
value isn’t important if the transmitter compensates for it (ratiometric measurement.)
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Transmitter Input Range
•  Specify the input range to be safely larger than the
signal of interest to prevent any possible signal
clipping
–  Perhaps use 2.78mV/V instead of 1.51mV/v if you expect
your signal is going to be above 1.1 or so
•  However, if the input range is too large then a very
small signal output may occur from the telemetry
receiver
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Input to Output:
Getting from Torque to Voltage
Example: Our AT-4500 EasyApp is used with a transmitter full scale input range of
2.78mV/V, which results in a positive full scale output of 10.0 volts from the receiver.
•  It is known from a physical calibration that at 600 Nm, the strain gage
outputs 1.29 mV/Vexcitation.
•  At this signal level, you will get a (1.29)/(2.778) ratio to the full scale input
range, or 0.464. Applying this ratio to the receiver’s 10 Volts full scale
output, you get 4.64 volts as the output. In other words, 1 volt = 129.3Nm
•  This provides lots of headroom for unexpected (and therefore interesting)
torque spikes that can be still be measured by this system.
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Conclusion
Strain gage/ torque telemetry combinations provide
direct measurement of rotating shaft torsional
dynamics, providing useful alternative tools to slip
rings and inline torque sensors.
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Thanks for attending!
Questions or Comments?
Visit us at Booth# 11022 to learn more
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Additional Information
•  Telemetry
–  Accumetrics 518-393-2200
–  Mark Klein, Ext:3403, mklein@pcb.com
–  See www.pcb.com/telemetry
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