Thanks all for the replies.
What's your voltage regulator set to?
Not sure to be honest. From what I remember the voltmeter reads ~28v when the gen is on just like the other ships we run. Haven't flown this particular aircraft in a while.
Are you just replacing the shafts in same started gen or are these freshly overhauled complete units ?
If just replacing the shaft then it sounds like the bearings the shaft runs on are u/s .
After the first shaft sheared we just figured it was due to break because like I said we go through shafts every so often due the work they are doing. After the second shaft broke, we swapped in a different starter/generator, and it broke the shaft either at the end of the day when I shut down, or the next morning when I first hit the starter button because it worked fine the entire day and the next morning when I hit the starter it wouldn't turn the engine, just made a "whirrrrr" noise because the shaft was sheared and not engaging.
Are you switching the generator on at less than 70% NG? This can cause shearing too.
No, I always roll up to 70% N1 before engaging the generator. And I am the only person flying this aircraft so I know that is not the issue.
Are all your electrical switches (radio master, lights) in the ON position, waiting for the generator? The sudden load-up might be the problem. Try turning on the loads one at a time.
No. Everything is off minus the fuel boost when the generator is engaged.
Where is the failure occurring in the shaft? In the same place every time? At the end of the splines?
There are a number of things that can contribute to or be responsible for, any shaft failure under load.
1. Poor quality material selection for the shaft. Upgrading to a superior grade of steel alloy with constituents such as nickel, molydenum, manganese and chrome, provides a material that has a much higher tensile strength and ability to absorb peak loads.
2. Poor machining. If the break is consistently in the same area, inspect the machining/spline cutting, for poor finish or wayward cuts by a machine tool, that are creating stress zones and fracture initiation.
3. Shaft overloading by applying additional high torque stresses when the shaft is already under load. Shock loading is also possible where a high load is applied rapidly, resulting in a sharp peak in torque. Overloading or shock loading creates stress zones that initiate shaft fractures.
4. Shaft misalignment. Misalignment will create bending stresses in the shaft, creating stress zones that initiate fractures. Ensure the shaft is running true by checking alignment of components.
The shaft always breaks in the same spot. in the tapered weak spot that was designed to shear to keep the gen from locking up the gearbox.
http://i.imgur.com/rgsWsgJ.jpg
We do apply load rapidly to the generator. We are running electric pumps (we do agricultural spraying). We do not cycle the pumps every pass, only once per load. But the gen still sees a 50amp spike probably 40 times a day. And we break them from time to time, but never this frequently.
When the first shaft broke, it broke at an angle in such a way that it was still actually turning. (the one with the large disc/plate in the picture) When it broke initially, I heard a soft "whack" noise and got an immediate vibration in the aircraft. Mostly in the pedals. No generator m/c light. I flew to base immediately and when I turned the generator off at shut down the vibration stopped immediately. I then tried to turn the generator back on, but it did nothing, and once the generator switch was placed to "on" the m/c light came on. The vibration did not return. We found that the clamping band had rotated all the way around and the locknut was against the fuel control and inaccessible. Which is why we had to cut it off.