Tractive Performance From Axle Instrumentation

Abstract

The forces associated with the traction of a wheeled farm tractor can be measured in the axles driving the wheels. An instrumentation system was developed to measure the bending forces in an unreduced section of axle. Two four-gage, constant current, semiconductor strain gage bridge circuits were developed to measure the strains associated with the bending forces. The analog strain signals were converted to FM signals on the axle. A multi-channel capacitor with rotating and stationary plates was developed to couple the signals to stationary receiving and processing circuitry.

An axle simulator was constructed to provide strain levels in a shaft equivalent to those expected in tractor axles, yet at considerably lower levels of loading. Tests were devised to separate effects of the instrumentation system from effects due to the mechanical system used to load the shaft under test.

The transmission accuracy through the coupling capacitor was found to be independent of the shaft velocities tested, which spanned 0 to 140 rev/min. The instrumentation was found to be sensitive to mechanical vibrations which were due, in this case, to the apparatus used to load the shaft. Averaging 730 points collected over one and one-half shaft revolutions reduced the errors due to vibration variations. The average values at a 4.3 $\mu$strain level, which corresponds to a drawbar pull of only 2 kN for an Allis-Chalmers One-Ninety tractor, were measured with less than 6.5% error. The vertical load corresponding to this pull, 19.3 kN, which would cause 87.4 $\mu$strain in the sample tractor, was measured with less than 0.6% error. The angle of the vector sum of these two forces was measured with less than 0.3% error. Single point readings, however, contained errors equal to the minimum pull itself, up to 6.5% of the vertical load and up to 2$\sp\circ$ in the resultant angle.