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repeated breaking starters

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Hello all,

There is an engine that breaks the starters every 20 days in average

there has been a flushing in the RGB and a new start control valve.

Should we suspect engine vibration (RGB, Power Section, Turbine, Torquemeter shaft)?

 

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How does the starter break? Does it shear, seize, or otherwise fail to engage?

 

There's a couple simple possibilities, but you'll have to forgive the suggestions if they seem too accusatory. Your installation method may be incorrect, such as running the starters with shipping fluid instead of engine oil, or even running the starters dry. Some people don't realize that starters without dipsticks may still need to be drained and serviced if there is a fill port on the left side. Another simple possibility may be your supplier building defective starters. either could possibly cause the starters to sieze.

Certain part numbered starters use a friction seal that exerts resistive friction against a thrust assembly. If the friction seal is worn, damaged, or incorrect, it will allow the starter to freespin, and possibly shell out under high RPM. There is a procedure in USAF tech data to test the friction seal.

Another simple cause may be a clogged return line from the starter back to the starter control valve. This would cause the starters to shear excessively due to too much torque being exerted through the starter shaft shear point. Of course, a damaged propeller brake can cause the starter to shear as well. The key will be to check the engine start times. If the engine starts quickly and shears, it's getting too much air. If engine starts tend to be slow, the prop break and/or gearbox may be failing internally.

The worst possibility though would indeed be vibration. Since the starter is in the center of all the RGB harmonics, it tends to get quite a beating if the engine isn't quite right. If you run an engine signature and come up good, you may just have a run of defective starters, and I would recommend installing a starter you know to be reliable. I have seen an engine vibrate so much that it shook the starter nuts loose, and the starter ended up hanging on by just two nuts. The starter was still good though, so that might give you an indication of how much vibration one can take, but that of course may have more to do with the style of starter more than anything.

I wouldn't suspect torquemeter shaft unless either a signature run/torquemeter runout confirms it, or your torquemeter pickup has evidence of grinding on it.

You can test the power section for internal binding by releasing the prop brake using the air adapter and manually turning the engine over to listen for any abnormal noises. Rubbing blades would usually manifest as sparks shooting out of the tailpipe at night though, and internal engine failure should show up as significant oil quantity or pressure loss.

I'm sure I'm missing something if someone else would like to chime in.

 

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         Shear shaft may not be floating.. Check lube.. Wear.

 

 

                              Jack.......

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I can never remember which one it is, bur one of the starters requires the misting plug removed at the

bottom of the pad - check TO's

 

Vibration can be a bitch - couple years back the line guys came in with big eyes accusing each other of

not installing starter nuts properly as it had come adrift after replacement a few days before. It was tightened

tested and released to service. 2 days later it was dangling again. i stepped in and had the engine removed

and sent to test cell. During my hook-up I found all the control linkages to be excessively worn, and had to 

replace gimble assy before continuing. During start i found fuel flow to be unstable and 16% micro-switch

operating around 23%. Once the engine was on-speed RGB and compressor vibration were noted 

at 1.8mils. Once we got to cross-over both were running at about 2,5 mils with the turbine joining the party

at around 2mils. We changed out vibration transducers and even a known good propeller to see if the 

vibration numbers would change, but to no avail. Engine was torn down and sent to overhaul

 

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