Took Old sprag apart today

[procharger]

Spring was broken in 6 different places one retaining ring cracked, hub 

looked badly worn, installed new flywheel, reassembled engine, doing

hoses Wed. oil tank cleaning, all new rubber mounts, hope to have it

running next couple days.

[Runtoeat]

Procharger, do you have any pictures of the sprag?  Curious what this looks like.

[Anticept]

Procharger, do you have any pictures of the sprag?  Curious what this looks like.

 

Probably looks something like this! (I jest, just a random image I found of broken engine parts)

[procharger]

Runtoeat sent pic to u email

[Roger Lee]

Rotax sprag.

 

Ernie, what happened to it.

[procharger]

After removing clips banged it on bench it all came out, the clip that was broke

had a severe crack I just opened it up for picture taking. Spring was in 6 different

parts, the hub was worn quite a bit, with 3 grooves on the inner hub that I could

feel. I couldn't believe that the long spring that's wrapped around rollers is what

makes this thing work, the rollers have quite a taper or kind of flat places on them

to lock up when starter is turning. I would think Rotax could use a weak clip of some

sort instead of the spring to hold rollers in place, I am sure there is a good reason

for the spring, just seems that something else would work better if the spring is

what gives up the ghost when engine tries to start or turn backwards a little. Question

for Roger if someone rotates engine by hand backwards does that do damage to spring?

Might be good for other people to know if that is the case. I can see a student or new

pilot doing this by mistake.

[Runtoeat]

Corey, don't see much difference from your picture and ProChargers!  The sprag looks wimpy and prone to break.  I'm not too happy about our Rotax engines having such a critical part being so wimpy.  What are you guys finding for life expectancy of this?  Is there a usual time when these go south when they're not abused by frequent kick back or hard to start engines?

[FastEddieB]

I may start a thread - "ROTAX Engines - quirky or poor design choices?"

 

Look for it.

[Runtoeat]

Eddie, Good plan.  Thanks.  Roger, Corey, Tom, others?  What do you think regarding the longevity of the spraq when unleaded MoGas us used and there isn't "kick-back" starts with the engine?

[Anticept]

Sprag clutch issues are extremely rare. It takes a hell of a kick back to bust it. Usually the spring just gets sprung...

 

The sprag design is very lightweight and is actually common. Other methods are bulky and heavy. I've messed with a fair share of starter bendix engagements and the shaft alone is quite heavy, nevermind the housing and motor.

 

This is what it looks like.

 

This is how it works. There's no reversing in Rotax's starter implementation. In Rotax's implementation, a free wheeling gear meshes with the starter. The gear has a race on it, that the sprag mounts on. This gear race engages with the inner race on the sprag. On the other side of the sprag, is the hub, which is shaped like a bowl. It is interference fitted to the crankshaft, and goes around the outer race of the sprag.

 

Parts catalog reference.

 

When the starter is engaged, the inner race briefly spins faster than the outer race, engaging the sprag, until the engine starts and the outer race spins faster.

 

My only beef is the location of the sprag, and the difficulty in reaching it to replace it. I don't see any alternative options though without a ground up redesign.

[Tom Baker]

I may start a thread - "ROTAX Engines - quirky or poor design choices?"

 

Look for it.

 

Who makes the determination that something is a poor design? The layperson who is operating the design in the field? If it truly was a poor design I would think that some governing authority would have stepped in to make them fix the problems. It is easy to sit back and make judgement about something, but to make that judgement without knowing all the details is foolish.

[Doug G.]

Who makes the determination that something is a poor design? The layperson who is operating the design in the field? If it truly was a poor design I would think that some governing authority would have stepped in to make them fix the problems. It is easy to sit back and make judgement about something, but to make that judgement without knowing all the details is foolish.

Yes, but discussing them is not.

[Runtoeat]

Tom, I'm probably the lay person you refer to but I am asking those, like yourself, who know best about this component to tell us lay people what an expert thinks about the design.  It appears that you feel the Rotax sprag is a durable component but I'm wondering if there might be a consensus guestimate what the longevity of this is?  Wondering, down the road, what one might expect as far as the need to replace this.  Same situation as some of us have with our ignition modules.  Is the spraq something, like the module, that will jump up and bite us without warning @ "X" hours?

 

FWIW, as a lay person, I think the idea for a category where repetitive squawks are aired regarding our Rotax 912ULS is a good idea.  Hopefully, those who are experts will enter into discussion on these issues and provide opinion as to recommended actions.

[Anticept]

We have three aircraft. One has 3000 hours, another around 3500, and the last is about 1000. Never had to replace a sprag. Being a flight school environment, those hours have a very aggressive amount of starting.

 

We've not had to replace any ignition modules either.

 

The voltage regulators, on the other hand, suck. I have two on the shelf ready to go, if that tells you how often I'm replacing those POS. I'm pushing for adoption of the silent hektik regulators because of how much these things suck.

[FlyingMonkey]

 

The voltage regulators, on the other hand, suck. I have two on the shelf ready to go, if that tells you how often I'm replacing those POS. I'm pushing for adoption of the silent hektik regulators because of how much these things suck.

 

Corey, what are the symptoms you see that tell you a voltage regulator is giving up the ghost?

[Anticept]

Corey, what are the symptoms you see that tell you a voltage regulator is giving up the ghost?

 

The voltmeter is the first thing that will tell you. Roughly speaking (since there's a lot more factors than this), if you can't run everything in flight without the voltage staying steady (slowly draining over minutes), your regulator is likely going out. Usually as they die, their capacity drops.

 

Also, sometimes when you first turn on the regulator, the voltage doesn't move, and you have to cycle it. That's been a sign too.

 

As always though, before replacing, check for clean connections.

[Tom Baker]

I can not answer on the sprag life, as I have never seen a failure.

[Roger Lee]

The sprag isn't a weak link. The owners are the destructive link. The sprag will go well past TBO. A sprag is used because of its simplicity and weight. Remember everything in this engine's design is geared toward weight reduction and HP. The sprag goes bad when you have kickback and fail to cease and desist right there until it gets fixed. They go bad because people turn the prop backwards too far and too many times. A weak battery with constant hard starting also beats on a clutch. Using 100LL all the time with no lead scavenger and failing to change oil every 25 hrs. Will kill the sprag. Anything that turns the prop backwards stretches the spring and then it will not engage due to the stretching.

 

All these scenarios are preventable and most people have heard do not allow kickback or turn the prop backwards many times.

 

p.s.

How about this: If we fix all the owners most Rotax issues would go away.     

[Anticept]

 

p.s.

How about this: If we fix all the owners most Rotax machine issues would go away.     

 

Fixed that for you.

[Runtoeat]

I take back those mean things I said about the spraq.....  now I'll be able to sleep at night.

[cdarza]

ok - for those of us less informed:    how would you know sprag clutch is bad ?  What would indicate this or is it something that should be inspected?

Oh  - and what does this sprag thing do?    thanks!!

Edit - i did some research and now have a better understanding  - a very simple explanation would still be appreciated. 

[Anticept]

It goes bad when you can't seem to get a good crank speed.

[Roger Lee]

When the sprag dies the starter just spins at a little over 6K rpm and it doesn't engage the engine so the engine sits there and does nothing. The sprag connects the starter and engine.. It engages when the starter kicks on and disengages when the engine hits approximately 850 rpm.

[cdarza]

Perfect - thanks for explanations