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"Fretting" on a Sky Tec starter

donaziza

Well Known Member
There was a thread a couple of months ago with pictures showing a guy taking his Sky Tec starter apart and fixing it, AND, I realize there's a lot of you out there who probably know this, BUT, for those of you who don't, like myself, I pass this along in case it helps someone else.

About last summer, I began to notice what looked like "black grease" hanging on the bottom of my Sky Tec starter. I had two mechanics look at lt. No clue. I then called Sky Tec. Also no clue. A few days ago, there was a 3rd mechanic working on my plane, ( something unrelated), but anyway, he said he tightened up my starter. Not the mounting to the engine, but rather the starter halves themselves were loose. This allows the halves to grind on each other, making aluminum, and when mixed with a bit of engine oil from a small leak (right in the same place), looks exactly like "black grease".

Hope this helps someone.
 
Guilty

I am that "3rd" mechanic. It's the second SkyTEc starter I've noticed this on. Basically the starter is screwed together with 2 long bolts and they are held in place by the locking tabs that are bent up. Who knows why, but the starter halves get loose and any oil up on the front of the engine (what Lycoming engine doesn't have some oil leaks) mixes with the aluminum powder from the two halves rubbing against each other and it looks like the starter is leaking black grease.
Tightening the screws and rebending the tabs seems to stop it.

Vic
 
Just guessing here, but if the bolts have tab-locks, it seems that they wouldn't be loosening. I suspect they may be stretching over time and due to hot-cool cycles and maybe vibration. Long bolts stretch more than short bolts.

I would think periodic tightening would be better than loctite.

JMHO, and YMMV.
 
could be a thermal issue. If the case is aluminum and the bolts are steel the force won't be consistent as temperature changes.
 
Which model Vic? I have seen the described looseness and fretting on an NL inline.
 
I just replaced my starter with a new NL and it started "making grease" almost immediately. Perhaps there's a big initial stretch of the bolts? Anybody have any repeat events after tightening the bolts?
 
SAE threads in aluminum

I had the same issue, Skytec made me good on it but there is at least a small problem here.
SAE threads in the aluminum case is the problem IMO, if you see those screws loosen without the lock plates having opened then there is no doubt the threads are failing.
You usually will see thread-inserts when fine pitch screws are required in aluminum, in fact you won't often see fine pitch threads in cast iron either except the high dollar nickel alloys.
I mentioned my concern to Skytec, but they didn't feel there is a problem with the design. I'm watching mine closely for the aluminum oxide to reappear. If it does I'll install some Tineserts or equal.
One guys opinion....
Tim
 
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Checked my NL starter today. LOOSE! Tab washers were in place.

There is a problem here folks. Everyone should check theirs.

Erich
 
Failed again

Yeah mine has come loose again after 49 hrs, bolts pulled, lock tabs still in place. I'm not sure whether to send it in for warranty or just install some tine-serts myself, or buy a B&C.
Tim
 
Yeah mine has come loose again after 49 hrs, bolts pulled, lock tabs still in place. I'm not sure whether to send it in for warranty or just install some tine-serts myself, or buy a B&C.
Tim

If you don't send it for warranty how are they going to learn? It is not a flight risk. Put inserts in the new one.

Edit: Or . . Work with them to figure it out. Others might benefit too, and you won't have to experiment solo.
 
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Over the past 25 years of working on Lycoming engines I have seen many of the in line type starters with loose case bolts and weeping a grey "grease like" substance. A theory explained one day was that the starter motor house experiences small amounts of torque during starting and the long through bolts get twisted slightly, which eventually leads to fretting as the housing gets looser and looser. Not sure how true this is, but this is a fairly common issue.
 
Hmmm...I've absolutely noticed this repeatedly and repeadtedly tightened the bolts. I put torque seal on the heads and noticed it cracks so there definitely is some loosening going on. Mine did not come with any locking mechanism other than star style lock washers. I remove the phillips head bolts/screws, put new star washers under it, tighten and torque seal. At first it seemed like they loosened with fewer hours, as time has gone on it seems I have to do it less, though I don't know if it's associated with the number of start cycles. I haven't been able to fly as much recently:mad:

It would be great to have a more reliable solution...
 
Starter

Pulled the cowl to change the oil and give everything a good look . The Skytec Starter was making a mess , sent some pics to SkyTec . They want the starter back for warrantee . Guess they know about the issue and want to make it right .
 
Just back from the hangar, working on a condition inspection. Skytec NL with a few hundred hours on it completely loose between the halves. Full on "jiggle, jiggle 3/32" between the halves with locking tabs still in place. I'll be interested to hear Skytec's comment.
 
Just back from the hangar, working on a condition inspection. Skytec NL with a few hundred hours on it completely loose between the halves. Full on "jiggle, jiggle 3/32" between the halves with locking tabs still in place. I'll be interested to hear Skytec's comment.

I would not expect full disclosure, just warranty.

Someone should measure the bolt length each time it comes loose. It may be yielding instead of pulling out the threads. That would help those out of warranty narrow the failure mode, and effect a durable repair. These should be pretty high strength bolts, maybe they came from a non-heat treating country.

Edit: I guess this goes on the oil change check list?
 
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One trick might be to remove the bolts one at a time and reinstall them, preheating them first. Depending on the material of construction of the bolts, the bolt length difference due to thermal expansion can be very precisely calculated. Heat the bolts so they are maybe 0.0005"- 0.001" longer than cold, then install them.

Steam turbines are often assembled this way, to create a sealing pressure between the two halves that varies less when the casings and bolts heat up and expand.
 
Starter

Just spoke with SkyTec, they received my starter , they scrapped it .
Said the starter was defective and have sent me a new one .
If you have an issue with your starter ,might consider sending it in or send some photos to them as it looks like they acknowledge a problem
 
mine's OK after 6 years

As a result of this thread, I went right out to the airport, pulled the cowl, and inspected my NL style SkyTec starter. Its fine. Screws are tight, case is tight.

My starter is 6 years old, with 350 hrs.

Looking at the screws that hold the case together, they are threaded into the mating case, with quite a deep thread engagement. Enough so that even though normal custom would be to use a coarse thread, there is so much thread engagement that I can not imagine that thread failing. The screws look like 8-32, but I didn't measure. That is a moderately coarse thread. So it looks like the design is sound to me -- I wonder if they have changed the design on newer versions, or if the problem is in torque retention.
 
Fretting

recieved this from SkyTec , I asked what was different about my new starter ,

We changed the type of thread lock compound used on the bolts that hold the starter together and also our torque and locking tab procedures.
You may periodically spray LPS on the pinion gear to prevent corrosion, but we don?t recommend any other maintenance as the starter is sealed.
 
Typically, in highly loaded fasteners a tab makes the joint force retention worse. The required material to allow bending the tab, without failure, renders it soft, then it yields in loading. This is worse for shorter fasteners. It also reduces the applied torque to the fastener relative to the installation torque as it smears under shear load. Surely ST should know all of this, but maybe not. I learned it from 30 yrs of knowledge X 4000 engineers of history. Top fastener companies know this stuff very very well.

Factoid: Large diesel engines use hydraulic cylinders to stretch the head studs (all at once), then take a 1/4 dowel rod and turn the nut/sleeve down snug before releasing the stretch force.
 
Just a data point, I was performing my annual and noticed I had some small fretting/grease on the bottom of the starter, so I bent back the tabs and tightened the case bolts - well, I should say I tried to tighten. Once took a couple of partial swings of the socket wrench. The other, well, apparently took too many.

These bolts are pretty small #8 bolts and apparently they don't have much sheer strength as I wound up snapping the bold head off. So I had to remove my starter and am working to get ahold of SkyTec to return it for repair.

So diligence with inspecting is key as is keeping the housing snug. But be VERY careful of any torque you are putting on these bolts.

file_zpshh4y8jzp.jpeg
 
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You probably did not over torque the bolt, it was already failed.

What grade fastener is that? Your finding might be a clue to the issue, the bolt is yielding. I have had this happen, loose bolt, thought it was for another reason, retorqued, then happened again. Third time during retorque the bolt failed at the top thread. Replaced the bolt, never occurred again. It happens.

Not claiming this is the root cause, but is among the possibilities.
 
I was able to get my starter sent in and even though it was first installed over 3 years ago, they replaced it under warranty at no cost. I noticed they have made a few changes - see photos below. The bent tabs appear to have been replaced by internal teeth style lock washers. In addition, there appears to be a rubber gasket between the housing sections - where the fretting has been reported.

pic1_zpsmeo85pvj.jpg


pic2_zpsrvfthx2w.jpg


pic3_zps35i5ardl.jpg


pic4_zpskcoojqtr.jpg
 
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