RUFUS2K
Mechanical
- May 21, 2002
- 28
A couple of years ago I worked on a design of a small (approx .5" dia) splined shaft that had a reduced diameter section that was intended to shear in the event that the driven load increased to some unacceptable level. Through some calculation and some experimentation, I arrived on the correct diameter for the shear section. Under static tortion testing, the shafts "twist off" at a torque of approx 350 in-lb. The shaft is driving an automotive air conditioning compressor with a clutch. I set up a test that drove the shaft at the right RPM under the operating load, and cycled the clutch approximately 30K times. (about 2 minutes on, and 5 sec off). All the lab testing indicated that the shaft should perform as designed under normal operating conditions. However, when installed in the field, the shafts began failing with very few hours of operation. The system is installed on an aircraft powered by a IO-550 engine and the shaft is driven by one of the rear spline drives. The shaft is 1144 steel. The splines are induction hardened to 56C. The shear section is adjacent to the spline, but not induction hardened. We have made some design changes tht have eliminated the possibility of shaft/drive missallignment. The lab testing was done with an electric motor, could the piston pulses be causing a vibration that is causing the failures. Or is this a material problem??
