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Overload clutch and gearbox friction torque


Safety Officer

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it is true that it is never too late to learn something.

Each time I post a question, I am never disappointed.

I just have to find the tool (don't appear to be delivered with the CT in France) and test the torque.

thanks again

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Hi Marc,

 

All you need is the crankshaft locking pin that was supposed to come with your engine/plane. If you don't have one you can get one from a Rotax service center. Always make sure the pin is seated down in the "V" on the crankshaft lobe. Never bend this pin in place because the only way to remove it is to split the case. When seated in the "V" you will have about 3/4" still sticking out of the case and the crank will be locked.

 

http://www.cps-parts.com/catalog/rtxpages/cps15-00155.php

 

Put it in place and then measure from the center of the prop shaft out towards the tip 28"-30". Place a mark there. Then get a fish scale that reads in pounds. I use a digital one and they cost around $30. Once the pin is in place the prop will still move through its 30 degree arch. Put the scsale on the blade with some twine at your mark and slowly pull the prop through that 30 degree arch and watch the scale. take the average reading off the scale and multiply it times the distance you measured out from the prop center.

 

So if you measured 28" and the scale read 16.8 multiply that times 28" and you get 470 in/lbs. This number would now go on the Line Maint. Annual/100 hr. check list.

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Just a note.

When measuring out on the prop to do the friction check on the gearbox be careful not to use too short a measurement. It gets too hard to pull the prop and the results may be a little erratic due to all the force needed.  Usually 26"-30" is a pretty good place to be. The farther out the smoother the pull and more accurate you can usually be.

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  • 2 weeks later...

Hello,

I measured it and found beetween 23 and 25 N.m instead of 30-60 N.m.

The point is that once blocked, the gearbox turn no more than 10-15° whatever the torque I apply is.

The plane is only 400 hrs old. Am I supposed to put the gearbox down or wait for 600 hrs?

What are the consequences on the engine if I wait (as long as there is no schock on the propeller) ?

 

PS the engine run very smoothly when idle..

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Hi Marc,

 

Average is 48-55 nm (425-486 in/lbs) and you are down around 24 nm ( 212 in/lbs). This is way below specs and an average gearbox.

The degree of the arc when the crankshaft locking pin is engaged is 30 degrees. 

Assuming you did the check properly and only got around 25 nm you need to have that gearbox re-shimmed. Don't wait. Just yank it off and get it done. Sounds like a belleville washer lost its spring tension.

 

Do not continue to fly like this until 600 hrs. comes around. If you need to fly it to a mechanic fine. Continued use like this will cause excessive wear in the box over time and cost you more money.

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thanks Roger, I will do it. Just need to fly 1 hour to go to a mecanics.

Nevertheless, I dont reach 30°, barely 10 to 15°. I measured it twice with different propeller position so i believe the measurement is OK.

Friendly

Marc

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 The old gearbox from early 1990's had a 15 degree arc because of the Dog setup in the box. Probably darn few of those exist any longer as Rotax wanted those gone a longtime ago. There are marks on the gear set to disinguish the difference. The 80 hp has a different gear ratio and no over load clutch compared to the 100hp. You did have the option of installing a clutch in the 80hp, but few people ever did it. If you have a 100hp with an overload clutch I guarantee the prop arch when the crankshaft locking pin is in is 30 degrees as it would be on a compass. If you have an 80hp and no clutch which is the way 98% of the gearboxes are then there is no friction torque to check.

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Yes you can move it either direction with the pin in. The crank isn't turning and the sprag clutch isn't engaging backwards. It's the starter clutch that gets damaged with backward rotation. Even with the pin out if all you ever did was move it that short ways backward it won't hurt the sprag.

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I dio have a rotax 100 HP 2008.

I will do it again this WE but once blocked I couldn't reach more than 15% But it true that I tried only counterclockwise when I face the propeller.

Not so easy to find the hole to insert the pin !

Thanks again

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The crankshaft locking notch is easy to find. Just remove the screw and take a flashlight and look into the hole. Rotate the prop in the proper direction and watch for the smooth shinny surface to come around. Just before that shinny surface disappears you will see the notch come into view. It will look like it has a horizontal line through it. That is the exact center of the notch and TDC of #1 cyl. If you are correctly in the notch with the pin the pin will stick out about 3/4".

NEVER EVER do anything to bend this pin. The only way to get it out is remove the engine and split the case.

NEVER EVER put anything inside this hole that can break off like a piece of plastic or wood.

 

When I'm done with this check I put the screw back in place and do the compression check because your are at TDC for #1 and ready to go.

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Hi Corey,

 

Rotating the prop in either direction while the crankcase pin is in the lock position won't hurt anything because nothing in the engine is moving and it only moves in that 30 degree arc.

 

With the pin out rotating the prop in reverse (clockwise) will engage the starter sprag clutch in the wrong direction. This stretches the sprag spring out and now the dogs in the spring will not engage when the starter is energized. Once this is stretched out you need to remove the engine and remove the rear end of the engine to install a new sprag spring and dogs. You don't want to go there. What makes the sprag disengage during a normal start is the engine rpm over takes the starter rpm and the sprag releases.

 

Rotating the prop by hand backwards is as bad as kickback when starting. If you have kickback fix it NOW and not later.

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Hmm...so what are we doing when we "burp" the engine correctly. Aren't we pushing air past the rings, into the oil system?

 

I guess in a sense you are, but the air is going to a good place instead of a bad place. When turning forward you pump air past the rings into the crankcase pushing oil back to the tank, while the oil pump pulls oil from the tank directing it to the critical parts in the engine. When turned backwards the oil pump is pulling air from inside the crankcase for those same critical parts. As we know some of those critical parts (lifters) don't like air.

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Tom's right.

Normal rotation moves compression air into the case and forces oil back from the bottom of the case to the tank. The tank is vented to the atmosphere so the oil stays and the air vents. The oil vent tube on the neck of the tank is the only vent on the engine an must always be open. When rotating backward you are pulling oil away from the tappets and other parts leaving voids of air pockets. Not good for hydraulic lifters that need fluid to operate. As mentioned above backwards rotation will also ruin your starter sprag. 

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I had an owner once use the tip of a small wood paint brush to find the notch. Needless to say it broke off about a 3/4" piece of this wood handle. Now this piece of wood was inside he case. The only way to normally remove it is split the case. This was on a CTSW. I had him pull the muffler and remove the oil return line off the bottom of the engine. I had him run oil into the engine and told him to pray. Guess what popped out. The wood piece came floating out. I guess praying did the trick. He was one lucky person.

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