ETME360 S2014

Torque measurement

R. Larson

Larson
Engineering has plans to market an in-line torque monitor
for use with an electric drill. The prototype design concept
uses an instrumented hollow steel cylinder between the drill
output shaft and the drill chuck.
Power (i.e.Torsional moment) is transferred from the
motor to the drill chuck through the cylinder.
Circuit connections are made through slip rings, and
torque is displayed on a small digital display attached to
the drill body.

The
prototype unit is 2.000” long, 1.250” diameter X
0.030” wall thickness aluminum cylinder (E=10 X 10^{6}
psi) and has two strain gages (R = 350 ohms, F=2.05) mounted
at +/- 45° to the cylinder axis. Gage leads are connected
to a Wheatstone bridge circuit with input voltage supplied
from the drill battery so that ei = 14.4 Volts, DC. Fixed
resistors (Ro=350 ohms) complete the circuit.

A.)
Determine the torque sensitivity de_{o}/dT (milli-volts/inch-pound)
for this design. Assume torsion input only; no compressive
load or temperature changes exist.

B.)
What would be
the effect, in terms of circuit voltage output and in terms
of indicated Torque, if a compressive load of 25.0 lbf is
applied during torque application? (i.e. the operator
leans on the drill
during a drilling operation.)

**Solutions:**

**For part a), I chose to do a
parametric study by assuming a Moment value and then solving
for the eout that would result... plotted values and found
slope of the line via simple rise/run.**