sprag clutch failure

by Carlos Filgueiras in 912 / 914 Technical Questions 10 years ago

Hi,

I’ve read somewhere in these foruns that some plane designs are more prone to sprag clutch problems than others. Is this true?
My question is because I bought a LSA, 4 years old, w/ 120 hours flown. And I had already to change the clutch, due severe kickbacks in start.
The removed part shows erosion in the surface in contact with the shaft.

Re: sprag clutch failure

by Piero Radivo » 10 years ago

Piero Radivo

0 1 0

I had the same problem. I think it is a point which should be improved by Rotax

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Re: sprag clutch failure

by Roger Lee » 10 years ago

Roger Lee

126 5.14k 4.58k

Prop/engine kickback is the common cause for a sprag problem. This is owner caused and can easily be corrected. If anyone starts to have kickback it should be corrected immediately. A sprag should last a long long time.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: YEN NIEN YU

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Re: sprag clutch failure

by Andre El Ghawi » 10 years ago

Andre El Ghawi

6 23 38

Hello Roger,

What would you do and how would you treat and care of your clutch other than have a good battery for the initial start? Having said this, and assuming that the engine and clutch are pretty new. (Less than 200 Hours)

What would a reasonable torque (Friction Torque) be as these figures are subjective and different from one source to another. Would (30-60 Nm) be an correct torque range?

Thank you said by: Moshe Kedar

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Re: sprag clutch failure

by Roger Lee » 10 years ago

Roger Lee

126 5.14k 4.58k

Hi Andre,

What I typically see for the friction torque in the gearbox from the factory is between 50-55 NMR. (440-485 in/lbs). I have seen some higher and a few lower, but these is a pretty normal range. In this case the higher the better.

You can't really do much to the sprag itself for starting, but what is important is to make sure your engine turns over immediately when the key is turned and doesn't have excessive crank time or any kickback. The way to do this is make sure carbs are synced and your starting procedure is good. Like make sure your idle isn't too low or too high and proper use of the choke and throttle at start. When setup properly you should be able to choke the engine on first start of the day with either no throttle or if you have a low idle rpm just crack the throttle. Too much throttle may cause you more problems in starting.

As far as the battery goes some think just because the engine turns over with a battery it should be okay, but a worn out battery doesn't turn the engine fast enough to make it fire and the human ear can't hear the difference unless it is really slow turning. Anytime I get a call about an engine that cranks and won't start I always recommend to try a known good battery or jump it. At least half the time a good battery cures the problem by getting the engine to crank faster.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: YEN NIEN YU

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sprag clutch failure

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