M9 Ace Transmission Output Shaft Failures
Author: Mentzer, Jr., R.D.
Source: Report No. AMSAA-TR-420
Doc ID: 1986007
Year of Publication: 1986
Abstract:
During the Follow on Evaluation of the M9 ACE conducted at Ft. Hood, TX in 1985, five transmission output shafts failed. These occurred during competitive side by side trials with the D7 bulldozer. During these trials, the M9 was repeatedly shifted from reverse to forward without stopping the vehicle as required in the operator's manual. It was determined by contractor testing and confirmed by the Physical Test Division, Engineering Directorate, Combat Systems Test Activity (CSTA), that the reversed torsional loading applied by this type of operation caused fatigue failure of the shafts. Failure occurred at approximately 30 percent of the required life of the shaft. The vehicle contractor and the transmission manufacturer, proposed modifications to eliminate the fatigue failures of the shafts. These modifications included: 1) Redesigning the shaft to increase the fatigue life by changing the shaft material, increasing the root fillet radius of the splines, and shot peening the shaft; 2) Changing the steer linkage to prevent the M9 from being driven in the geared steer guide mode in reverse to reduce the torque spikes when shifting from reverse to forward; and 3) Modifying the gear shift guide to increase the time to shift from reverse to forward allowing engine speed and torsional loading to drop.
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