Shaft fracturing 2

[RUFUS2K]

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 6 cyl piston 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??

 

[metman]

It appears to me from several of the fracture face photo's that there are beach marks near the surface and from side view machining marks in the transition radius all adding up to fatigue failure initiating from the surface.

Agreed -- get rid of the resulferized steel. polish the radius and shot peen. I would try 4340 steel not that it is needed for depth of hardening but readily available and much more fatigue resistant. If this does not answer, use a mararging steel which has about the best fatigue resistance you will find and very predictable properties and easy to achieve processingwise. Maraging steel is expensive but your part is small so should not impact expense appreciably. Machining cost will also increase as a trade-off for increased fatigue resistance but again it is a small part so that machining time cannot be extensive.

 

[swall]

Interestingly, the ASM Handbook on failure analysis (8th edition) has a case history on a similar shaft failure (pages 378-379). Corrective action was to eliminate the source of torsional excitation and shot peen the reduced section. Shaft material was 4340 rather than 1144.

 

[aviat]

RUFUS2K
Have you considered the effects of accelerating or decelerating your compressor? Because the drive is geared up it makes it more than steady running. Aircraft components are designed with the idea to keep the mass down, more than commercial and automotive components. Even if you do not have the compressor on when the engine is accelerated, it still has to accelerate the clutch. With an electric motor there is a more even acceleration, so that could be why shafts are not failing with electric.

I have seen components with similar design break and it often occurs after an engine speed problem. Usually they didn’t break when it happened, but occurs soon after. I agree that it is probably a torsional failure of a ductile material, because of the smooth surface with helical swirls, which should go in direction of rotation.

There is another item to consider. The engine manufacture lists in the TCDS the allowable torque that a drive can provide. I didn’t look up you particular engine but a similar one allows for max continuous of 90 inch pounds, max static of 600 inch pounds, with a max overhang moment of 65 inch pounds. Using these numbers I guess you could go up to 600 inch pounds, but with a compressor your overhang limit may be exceeded unless you can fit in a bracket.

It could still be a vibration problem, but the only way you will know for sure is if you do an analysis of complete airplane with your compressor. There was a case where one engine manufacture was having problems with the oil pump, with an engine in only one airframe. When they went back and re-evaluated, they found that the prop that was installed in that airframe was causing a resonance and when they changed it to another one, the problem went away.

If you have enough electrical power available, I think the best means to run the compressor would be to use a 24V electric motor. It would give you less headaches than trying to mount on the engine.

 

[DDRSS10]

A large part of the non breakage with electric/ breakage on the combustion is the rotating mass of the engine is much higher. The acceleration of the pump at clutch lockup is nearly instant when in service, being that the clutch is driven with a splined shaft. If you were to switch to a belt the slippage factor would reduce this greatly.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[MOB1]

Just a thought on this (I know it's a bit late). Has anyone experience with using a suitable steel and nitriding? This will give beneficial surface compressive stresses to resist the torsional fatigue loading but with only minor impact on the torsional overload properties.