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Snaproll
09-01-2009, 04:59 PM
Hi, it's the village idiot here... what is a slipper clutch? Is it like on a motorcycle? If you back off the throttle at speed, rather than the rear tire trying to wind up the engine the clutch allows the drivetrain to 'slip' rather than engine brake.

Is this how the slipper clutch works on the Rotax, basically the same concept except with wind resistance in a dive keeping the engine from over-reving? Something like that?

Dave S
09-01-2009, 06:23 PM
Jim,

Not the same thing as the cycle clutch.

Here is the story - best as I explain it. The earlier 912s did not have the slipper clutch. On this engine, without the slipper clutch, there was some opportunity to damage the gearset if the engine was idled at two low an RPM too often - Rotax has a demo video on their website where the demonstrator intentionally reduces the RPM of one of the non-slipper clutch engines below the minimum RPM and you can really hear the gears clatter. The engines without were simply operated at or above 1800 rpm at all times - that kept the gear bashing away. The slipper clutch is sort of a norwegian proofing deal (I can say that since my last name ends in ...son) on the newer engines where the prop is free to move over a 30 degree arc with a friction clutch holding a certain amount of pressure. If someone accidentally or periodically drops the RPM too low - no damage to the gearset because the slipper clutch takes up the motion. One of the periodic maintenance operations is to lock the crankshaft (rotax includes a pin with the engine to do this) and you measure the amount of torque it takes to slip the clutch - mine works out to something over 400 inch pounds.

When I spoke of the normal sounds of the gearbox - you will get a "squeek" out of the slipper clutch on shutdown under certain circumstances - a Rotax certified repairman told me this is the slipper clutch telling you it is still working correctly.

The gearbox also has a second device called a dog clutch in the housing (both versions of the engine have the dog clutch) - this device slips further but it takes something over 400 foot pounds to do it - a protection for the engine in case the prop is unintentionally used to mow down a telephone pole or similar.

Again, there may be others on the list who can explain it more precisely - but that is the jist of it as best as I can explain.

Sincerely,

Dave S
KF 7 Trigear
912ULS Warp Drive

eswallie
06-01-2021, 11:35 AM
Great explantion, I am amazed at the lack of information about the slipper clutch. Even the youtube videos go so far as to cut out the disassembly and reassembly of the clutch. I have a 400 hour gearbox that will not pass the pull test and twice, while starting it slipped the dog ears which caused a hell of a commotion. After cleaning when putting the clutch together (screwing the top down) I could find no information on how far and how hard it should be tightened. I think now that I was concerned about something that I shouldent be. Just screw it down all the way until it stops. That is the best I can come up with. If anyone has a better answer, I am all ears.
Ernie

napierm
06-02-2021, 06:24 AM
Great explantion, I am amazed at the lack of information about the slipper clutch. Even the youtube videos go so far as to cut out the disassembly and reassembly of the clutch. I have a 400 hour gearbox that will not pass the pull test and twice, while starting it slipped the dog ears which caused a hell of a commotion. After cleaning when putting the clutch together (screwing the top down) I could find no information on how far and how hard it should be tightened. I think now that I was concerned about something that I shouldent be. Just screw it down all the way until it stops. That is the best I can come up with. If anyone has a better answer, I am all ears.
Ernie
Seems it is hidden away in Rotax's magic manual. I found this on the Rotax Owner's Group forums:

I got a 912 with Overload clutch.I disassembled overload clutch but can't find proper tighten torque for reassembled it.
anyone knows where it mentioned?!

its mentioned in Overhaul manual
tight the collar nut to approximately 20 Nm then tight the nut 2 turns.
this setting gives a breakaway torque app. 600-800 Nm

For me, I would also ask Hal Stockton about it.

Av8r3400
06-02-2021, 05:14 PM
Seems it is hidden away in Rotax's magic manual. I found this on the Rotax Owner's Group forums:

I got a 912 with Overload clutch.I disassembled overload clutch but can't find proper tighten torque for reassembled it.
anyone knows where it mentioned?!

its mentioned in Overhaul manual
tight the collar nut to approximately 20 Nm then tight the nut 2 turns.
this setting gives a breakaway torque app. 600-800 Nm

For me, I would also ask Hal Stockton about it.

i was told it is assembled to a stack height, not a torque value.