Thread: 912s Sprag Clutch

Roger,

Was looking through the BMAA forum in England and noticed this thread regarding issues with the Sprag clutches on 912s engines….

http://www.bmaa.org/forums/default.aspx?f=15&m=75044

The basic theme is that the 80HP 912 is immune to the problem but the 100HP 912s fitted with slipper clutch is prone to sprag failure and quote ‘is only a matter of when and not if it’s going to fail’.

Have you any history of the problem over there?

Mac

Hi Mac,

Issues with the sprag clutch is very rare unless you have a major prop strike.

Its not in the UK Roger, its quite a common problem, even some on low hours engine's and at about £600 for parts + labour its not a cheap repair would you have any idea why these are failing at an early stage ? and prop strike is not the problem in this case ..there is an idea that avgas lead is causing the problem but not so in all cases as some don't use it at all ...

Mike

While lead does negativly effect the health of the clutch, most failures are due to improper start up and shut down procedures, excceding the propeller mass moment of inertia is infact the most common problem. There is a simple test for testing the "health" of your clutch that most Certificated Rotax Repair Centres should be familure with. Soft Start Modules have proven to help with kickback on the prop and in turn increase the life of the clutch and reduce wear of the spring and cam lobesinside the clutch.

Hi Jeremy,

You haven't seen many sprags go have you? (other than physical damage)

Well we havent seen any related to design defects, as long as the Service Bulletin relating to the Circlip is addressed. The parts do however wear out from time to time but agian this isnt realted to the clutch it self. The guys who have been getting the best life out of the clutches are people who have lighter props , run lower amounts of leaded fuel and people turn of the engine off on one mag. Idle set too high, over 1900rpm, also have been experencing more kickback on the prop during shut down.

It seems that the Soft Start Module does help the 912s from leaping about the engine mounts and manufacturers have acknowledged this with service bulletins when the Icarus C42 engine mounts were cracking, although on their installation they use large angle brackets for the Rotax to sit on. But that starting movement is much larger and seems much more benign than the higher frequency shuddering/juddering that can occur when closing down the engine. I have a permanent jump starting socket fitted in the foot well on the PAX side and without fail if starting from my home base I always plug in a 2nd battery for the first start of the day and without any hesitation the engine starts instantly, even recently with minus 8C it wasn’t an issue. A really good starting amperage is a must to overcome the high compression of the engine and to date I haven’t experienced kick back on starting.
It does help to leave the engine idling for a minute on only one mag, as well as holding the throttle lever hard against the stop, to get the lowest possible revs before switching off. Whilst this procedure eliminates 99% of juddering on stopping it can still bite me in the --- , and when it does happen, every mechanical gene in my body has all the sympathy in the world for the poor old gearbox.

Jeremy, It’s interesting to hear you say that some props are heavier than others and might have an effect on engine life etc. any pattern developing regarding manufacturers or types?


Mac

I may be wrong, But I thought that there is a difference between the sprag clutch and the slipper clutch! The slipper tends to overcome the majority of the gearbox vibration at low RPM. I also believed that the slipper clutch is not fitted to the UK models to keep the weight down. Is it posiblethat the absence of aslipper clutch may be causing the problems with the sprag clutch???. Help me out on this point anyone.....................

There is a sprague clutch and there is also what Rotax calls a "overload" clutch (also called a "slipper" clutch). In the U.S., the CT's come standard with the sprague but withouth the "overload" clutch. I believe that the overload is offered as a factory option here by FD. Not sure about the European CT's? Check your engine "type". I believe that the "version 2" ULS is the standard engine with only the sprague and the "version 3" contains the sprague and overload clutch.

Two differents things here..
the sprag clutch is on the starter side of the engine and there are no 912 without it.
the overload (or slipper) clutch is in the gearbox and is standard on all 912 100hp and an option on the 80hp.
the BMAA forum thread is about the sprag