Sprag Clutches

[Brian Abraham]

I was interested to read on the thread 'Bell misleads 407 owners' Lu Zuckerman's account of the 205 incident where the engine oversped the rotor and Bell claimed the clutch had been damaged by compressor stalls.
On one occasion whilst waiting for the cool down time to elapse on a Huey I became aware that the rotor was turning at a lower than normal speed (read could count the blades). The tacho confirmed that the engine was overspeeding the rotor and an instant shut down was made. Have always wondered what may have been the result if the clutch suddenly decided to reengage. Maintenance was informed and they released it for ops without any maintenance being performed and the aircraft soldiered on without any further problem. Was told by Bell it was an uncommon event but not unknown.

[Cyclic Hotline]

Ask the Puma boys what happens when your freewheel suddenly engages!

Bit of a pain when you don't have an N2 indication, and its all under-speeding! Loud bang and varying amounts of damage/component replacement!:(

[Lu Zuckerman]

To: Brian Abraham

I was interested to read on the thread 'Bell misleads 407 owners' Lu Zuckerman's account of the 205 incident where the engine oversped the rotor and Bell claimed the clutch had been damaged by compressor stalls.

Actually, the Bell rep covered up the fact that the engine had experienced three compressor stalls. I found out about it when we pulled the unit out of the transmission. The rollers and the cams were destroyed. It was not until the accident report had been submitted and Bell signed off on pilot error that the Bell rep came forward and told me about the three compressor stalls.

What made matters worse there was no mention about the three compressor stalls in the maintenance records and the normal protocols initiated after a compressor stall had not been implemented.

Many years ago the US Army took delivery of a brand new H-34. They were checking out a pilot who had never flown in a helicopter with a fuel priming system. On helicopters that this pilot had flown the twist grip was rotated to pump fuel through the carburetor. The pilot was twisting the throttle but on this helicopter the throttle canceller had been misrigged and it allowed the helicopter to start with the throttle in the open position. The pilot cranked the engine and it caught on the first two turns and with the throttle wide open the engine spun up to 2800 RPM. This caused the blocker plate in the hydromechanical clutch to overstress to the point that it failed and the engine engaged into the rotor system at 2800 RPM. The helicopter, which had only about 16 hours on it, was written off and turned over to the training school to be used as a training aid.


:E :E

[Shawn Coyle]

Spragg clutches have failed and then worked perfectly afterwards. I know of at least two incidents of this on the Bell 212 / UH-1N.
Does anyone know of any statistics?
I also note it's not in any flight manuals as an emergency or abnormal procedure. Hmmmm.
You would think that someone would be educating pilots that if this happens they must write it up and submit it as an SDR, so that we can at least track statistics.

[bugdevheli]

Just wondered if anyone had experience of a spragg clutch that did not disengage, or one that failed to take up the drive.

[Gaseous]

There was a failure on an Enstrom in the late 1970s which failed in both modes due to unauthorised maintenance. (assorted sizes of rollers used): Clutch failed allowing major overspeed and then locked when the engine failed preventing autorotation. :eek:

Aircraft and crew lost.

Report is still on NTSB database I think.

[B Sousa]

Had one fail to disengage during a checkride in a Cobra. IP cut power and things started to get slow....fast. Appled power all was OK. Tried it again as the IP thought I was slow on collective reduction.......not even. Brought back in power did a precautionary landing on a Golf Course went in called Maintenance and had a Coke...........
Never did find out what was wrong. probably didnt want to know..........

[Lu Zuckerman]

The Italian Navy suffered a catastrophic failure on the left-hand freewheeling unit on an AS-61 (Sea king). The pilot put the helicopter into autorotation and landed in the water 22 seconds later. Upon removing the unit you could see where the unit fractured and it was totally destroyed by friction generated heat. The parts got so hot they actually melted.

:E :E

[imabell]

a non licenced engineer i new long ago did some moonlighting for a company out west and put the freewheel unit in upside down, twice.

a pilot friend in the territory had had a freeweel let go and he likened the noise to the sounds of a gut shot donkey, so if you know what that sounds like??:ugh: :ugh:

[HD 509]

I experienced a seized sprag clutch/freewheeling unit quite a few years ago in a 212. If my memory serves me well my first indication was when one of the engines was rolled back to idle the rotor rpm bled down more than usual, then on the same test flight I found the auto rpm were low.
The final clue was when I carried out the next start and cranking #2 engine first, #1 engine showed n1 rpm and oil pressure (this anomaly would not have been apparent if #1 had been started first)
All other c box indications were normal and no metal was located on the chip indicators or filters. The c box was removed and sent out of country to P and W, I never had a chance to read an engineering report so never found out why it seized.
I had'nt flown the aircraft in question for a month or so and no one else had noticed anything unusual so the sprag may have been US for quite a few hours. Things could have got a bit exciting thinking back on it

[Taff Missed]

How can you have n1 and oil pressure indicating when even with a stuffed clutch there's no connection between n1 and n2?

You might have had an indication on the triple tacho (n2) but not engine oil px as well.

Taff

[sprocket]

Taff Missed wrote: How can you have n1 and oil pressure indicating when even with a stuffed clutch there's no connection between n1 and n2?

If the N2 turbine was being directly driven via the FWU, then it might pull enough air through the engine to turn the compressor turbine.
The N1 has very little load and is very easy to turn.

[HD 509]

TAFF
Believe me we had an indication of n1, I demonstrated it several times to convince the engineer on site ( and other pilots ) that there was a problem.
He was finally satisfied when one oil cooler fan could be free wheeled by hand in one direction and the other side couldn't.
By the way, with ground power plugged in, the #2 starter didnt seem to be noticeably affected turning over more equipment than it should have been.

[The Nr Fairy]

I seem to recall a problem some years ago with R22 sprag clutches - some of the ears were breaking off and stopping it working properly.

That's why, if I don't get a satisfactory needle split during the run-up checks, I do it again.

[flyingscotty]

Can someone help me with a technical query ref the sprag clutch?

Is my thinking on how it operates correct? The clutch as we know transfers power from the engine to the transmission then the rotor shaft, & rotates one way. My question is, when practising an Auto or the engine actually does cease to work, the freewheel within the clutch wants to rotate the opposite way and the sprags within the outer & inner race apply friction to stop this and this enables the seperation of the low engine RPM from the rotorshaft allowing us to maintain RRPM!

Thanks.

[RVDT]

http://hrsbstaff.ednet.ns.ca/bvogel/K100/download/Starter_clutch/index_files/image003.gif

The graphic is a little misleading. In a helicopter the engine would be driving the outer part and the inner part would be the input to the MGB (On some designs shared with the TRDS.)

http://hrsbstaff.ednet.ns.ca/bvogel/K100/download/Starter_clutch/index_files/image005.jpg

[handysnaks]

Well, the sprag clutches used to start Royal Enfield 500 Bullets are notoriously suspect! Should I get my coat?

[AnFI]

That's why, if I don't get a satisfactory needle split during the run-up checks, I do it again.
What is an unsatisfactory needle split? and its ok the next time?

[hillberg]

The Bell 212 has two sprags, one for each power section, Some times the drag in the c-box can pull the unstarted side along.

[61 Lafite]

What is an unsatisfactory needle split? and its ok the next time?

I like to see a decent drop on the engine vs. rotor rpm, 15-20%, but a hot machine will often take quite a forceful drop of engine revs to achieve it, which is of itself not always good for the engine.

Sometimes it works best to roll the engine revs up from about 50% to 75% then immediately cut the throttle back, as the rotors are gaining momentum and it seems easier to split.

I'd say a minimum 5% split would be my go-decision. If I couldn't achieve that, I'd want to know why.

Lafite

[John R81]

AnFi

I suspect he means that he wants to see a very clear needle split (as do I).

When cold one of the R44s I regularly fly will not show much / any needle split when the test is performed from 70% as per the manual. In that case I increase revs to 80%+ and then chop the throttle to be sure that I can see the needles very definately splitting.

I know that R44 sprag clutches can fail but at least in some cases there are signs of the failure several days in advance of the thing letting go completely. Even though the needle split worked, there was a "bang" in flight (like someone dropped a bag of spanners in the back seat well) with no loss of power or other indication of a problem occurred. We continued our flight. It happened to another pilot, and then to a third who put the thing down in a field and called a recovery truck. He was right. It turned out (when inspected) that the sprag clutch was in the process of disintegration.

Take a look at the R44 thread - it is covered there with some pictures of a disintigrated sprag (I don't know how to link).

Edit to add thanks to Pandalet. Start where indicated and read forward a couple of pages for 3 examples (2 on that page and Bondu121 post 1250 onwards)

[Pandalet]

R44 thread sprag clutch discussion starts around here: http://www.pprune.org/rotorheads/189931-robinson-r44-62.html#post5596243

[76fan]

Long ago, whilst established in autorotation teaching engine-off landings in a Hiller, I had a clutch re-engage shortly after I had committed to the EOL, having closed the throttle fully at 600' .... the ERPM needle rose and snatched the RRPM needle down to about 2/3 normal autorotational RRPM and although I instinctively tried to open the throttle to regain powered flight the ground was approaching mighty quick at the now extremely high rate of descent. The flare did little but we landed level and straight but the crosstubes bent and the tailboom flexed. Until the engineers removed the freewheel unit nobody believed me when I said the freewheel had re-engaged but eventually the man sitting safely behind his mahogany desk proclaimed that had I waited long enough at full throttle flying RPM would have been regained ... I still don't think that there was that much time... So, I would presume that with a turbine the drag on the rotor would be relatively minimal but on a piston I would want to see a very definite and clean needle split during pre-takeoff freewheel checks!

[MamaPut]

One day when I'm a big boy and have saved up enough money for a pilot course, please don't let AnFI be my instructor :E

[Pilot DAR]

To answer Scotty's question, and with reference to the nice graphic, the sprags act individually, yet in combination, to roll on their own axis, and lock the inner shaft to the outer housing bore. Generally, each sprag is helped into the locked position by a little spring. Each sprag is shaped something like a figure eight in cross section, but sized such that the total height is greater than the space available for it between the shaft and the bore where it installed. Thus, they have to lie a little bit on their sides to fit in the space, with that little helper spring trying to stand them upright. The sprags are extruded steel, which are cut, hardened, and tumbled to a good finish. They are generally a little harder that the shaft and bore upon which they operate.

The engine drives the shaft (though could be the other way 'round). As it drives, the little springs bring each sprag into contact with both the shaft and the bore, and the friction caused by the engine drive torque keeps them there. This is the clutch locked up condition, and is what's happening the whole time the rotor is driven by the engine. Either the outer housing overruns, or they are locked together, nothing ever turns the opposite direction in there.

If the engine stops, the shaft stops. Assuming you lower collective promptly, the rotor keeps turning, so the clutch housing bore keeps turning. The instant that the the housing "overruns" the shaft (turns even the slightest amount faster [RPM]), the sprags lay over against the little springs just enough to no longer be in friction with the shaft. In the worst case, if the clutch did not overrun, and release, the stopped engine would drag the rotor to a stop very quickly.

Thus, the clutch is engaged for all normal operations. The clutch is never partly engaged, if it were, and it were slipping, it would be galled, and ruined very quickly. When you remove engine power, the engine driveshaft slows down relative to the rotor drive, and the sprags lay over, and disengage. As long as the rotor half of the clutch is turning at all faster than the engine input, the sprags are laid over, and no power is transmitted to the rotor, and it is not dragged down by the stopped engine.

When you close the throttle, and split the needles, you are assuring that the clutch does in fact release, and then re-engage, when power is applied again. Not releasing would be bad in the case of an engine failure, not engaging properly would be even worse, as it is an indication that not all of the engine power will get to the rotor. A slipping clutch will get hot, and wear very quickly. I've never heard of the engine overrunning the rotor, but if the sprag clutch slipped, I suppose it would be possible.

Similar clutches are used in places like escalators, chair lifts, and conveyor belts, so as to assure that if the motor stops, the escalator etc. does not back down, and pile people or product at the bottom. In that case, one half of the clutch is fixed to the structure, and the clutch overruns the whole time, until the escalator motor stops, and it tries to back down under the weight of the people. Then the sprag clutch locks up exactly helicopter like, and the people are left safely standing or sitting there - complaining about walking, or climbing down - unaware how safe they are! They're also used in some starters, so the running engine can overrun the starter motor drive, once the engine starts.

This company is a major manufacturer: Formsprag Clutch (http://www.formsprag.com/productsframe.htm)

[AnFI]

... that's unnecessarily rude... but then, since as you say you have not grown up yet - you could be forgiven...

my point was that an unsatisfactory needle split which is then 'ok' on a second attempt was really just an unsatisfactorily executed needle split (normally caused by trying to make a needle split from an RPM below (or at) the unloaded engine idle RPM - which would therefore not show a split) - if the split were in fact unsatisfactory due to a genuine Sprag clutch performance malfunction .... then a second 'successful' split would not make it ok to fly - would it?

Has anyone had a sprag clutch fail a sprag clutch check on an R44?
Failure to re-engage being the interesting case .....

Presumably nobody has had one that would not disengage if the test was conducted properly?

[Tony Mabelis]

From my personal experience on a Bell 47, should the sprags be fitted into the housing backwards, on engine start all you get is a lot of engine noise, and the rotors stay still!!
What follows is the removal of the unit, disassemble freewheel, from memory approx. 50 split pinned nuts and bolts, reverse the orientation of the sprags, fit 50 nuts/split pins and off you go.
Much easier said than done.....it was my first job on the 47, 40 years ago!!
Happy days.:)
Tony

[flyingscotty]

Pilot Dar, thanks for the explanation to my question. Had to read over a couple of times for my head to digest, but it makes sense, much appreciated.:ok:

[John R81]

AnFi

Err.... doing the exercise per the manual is "unsatisfactory executed"?

No, it is not. It is per the book so it is perfectly executed.

If your explanation is right then either (a) the exercise was not carried out by the book (Pilot error) or (b) the book is wrong (no doubt you will be writing to Robinson to correct them).


Without a clear split I am not going to leave the ground. I do the exercise by the book (works OK on later flights / same day). Increasing the rotor speed beyond the book number then repeating the exercise is how I satisfy myself that the sprag clutch is working - then I am happy to leave the ground.

Simples!

[topendtorque]

MamaPut,
I saw an artile in some newspaper recently which desribed how a leading economist had reshaped very well the financial structure of a country way back when, was i think the story line. The dude was asked in later life, "how did you work out to do it that way?"

His succint reply was to the point, he simply said, "Thinking."

I think, the best advice I can give you is to look forward to an instructor that makes you 'think'.

Back on thread, I always stress to others and practice myself needle splits in a manner of the least stress. Why? Simple, I want the machine to prove to me that under the least duress I will have control of the machine without the engine dragging it back.

Same same with the re-engagement check.

Any clown can force them to split by chopping the throttle at full RPM, thereby causing the engine to just about jump out of its blocks etc. always be gentle with all machines and all parts of them.

If it does not split at 70% like Frank asks, well you should check your laden idle engine RPM first and then your unladen idle. If you still have a problem the engineer is much closer than a six foot hole. Think Mamaput, think.

There is a reason why i do another only throttle chop at full RPM but only after a service and that is to check that the carburettor is set up right and doesn't cause a rush of poorly mixed fuel / air and thus an engine stoppage. However i always do that from a hover where more power is being applied, thus more reason to see a problem.

As a good father in law should gracefully advise a young suitor, 'Treat her like the thoroughbred that she is and she will never become an old nag.'

tet.

[AnFI]

John R81 I do the exercise by the bookcontradicts yourself:
Increasing the rotor speed beyond the book number then repeating the exercise is how I satisfy myself that the sprag clutch is working - then I am happyWhy did you quote me in a Chinese accent:
Err.... doing the exercise per the manual is "unsatisfactory executed"?Suggest if you don't understand why it didn't work and then did - all whilst doing by the book (apparently) but then making up your own procedure .... :zzz:

AND:
When cold one of the R44s I regularly fly will not show much / any needle split when the test is performed from 70% as per the manual.Doesn't make much sense either ..... "when cold" ? are you suggesting that the cold makes a needle split less pronounced (or even not occur)?
and
"70% as per the manual" ... doing it by the book?

If you misread the checklist properly it would be 75% - but you are misreading it in any case .... an RPM is not specified

Don't give me a hard time I'm just trying to help..... (Mama:yuk:!)


I think you will find part of your confusion arises from exactly what I said regarding the idle (which may be set deliberately high as is sometimes the case on training machines) - you cannot split from below the idle - ok now?

[John R81]

It doesn't come over to me that you are trying to help, it comes over to me that you are showing off.

I am sorry that you didn't jump to the conclusion that I wrote the post quickly and that I didn't have the Robinson manual in front of me at the time. Attempting what (I am left to assume) is meant to be a hurtful jibe based on a racial steriotype might be offensive to some and it certainly is helpful to no-one.

I assume that we are actually agreed that:

1. The Sprag clutch is important
2. Flying an aircraft with a malfunctioning Sprag clutch is not a good idea and it might even (in some circumstances) get you killed
3. The needle split test on start of a Robinson is important
4. If you perform the exercise from the warm-up RPM in line with its position in the start procedure and there is no split, try a higher RRPM because without seeing a good split, that Sprag clutch may be malfunctioning

If so then the rest of this exchange is a waste.

[AnFI]

Yup I agree with all you (now) say. Apologies if you thought me rude. I didn't occur to me for a moment YOU were Chinese - miss quoting me gave the impression that you were suggesting I was, not that I find that rude!! - Ni hao ma (?)

But it is quite important to give the right information here, especially when you are so insistent that you are doing the right thing... what you previously said was not correct... and gave the impression (to MamaPut (and TET)) you were doing the procedure correctly... I am sure you agree... anyone can slip up when in a rush... no worries ...

you've even got tet beleiving your version of the proceedure:
"If it does not split at 70% like Frank asks, well you should check....."

Thinking, mechanical empathy, understanding what you are doing and common sense are obviously a good idea.... (tet:rolleyes:)

Good wishes to all !

Survey:
Has anyone here ever had a genuine 'failed split' in an R44?

[riff_raff]

The graphic is a little misleading. In a helicopter the engine would be driving the outer part and the inner part would be the input to the MGBSorry about the tardy input. RVDT is correct, with sprag type overrun clutches the inner race is the part that overruns, while the sprag retainer assembly and outer race stop. The reason for this is that this arrangement keeps the surface sliding velocity between the sprags and race (PV) as low as possible, and it also allows a continued flow of oil to be delivered to the sliding interface via holes in the inner race shaft and centrifugal force.

As for a sprag clutch rapidly disengaging and re-engaging during operation, there are three design features that determine the characteristics of how a sprag clutch will re-engage. One is the cam profile of the sprag elements. Another is the mass CG of the sprag element. And the other is the radial force produced by the retainer garter springs.

Sprag clutches do not typically have much design margin for over-torque conditions. Sprag clutches work well with the relatively benign dynamic environment existing in turboshaft engine drivetrains. But with the harsher torsional conditions existing in recip piston engine drivetrains, sprag clutches have a harder time.

riff_raff

[Lonewolf_50]

I could swear that the video SASless posted was shown as a training film to us young, and impressionable, flight students in Whiting Field late 1981/early 1982 during one of those "weather is bad, do some training to keep the students busy" events.

IIRC, it was Lieutenant Warren who showed us the film, but man, that's over 30 years ago.

Good stuff, very sorry to hear about what happened to the crew. :(

For John D: Tom House (RIP) 1989, for other ways to get hurt by a cyclic stick (SH-60) during a mishap. :(

[n5296s]

If you perform the exercise from the warm-up RPM in line with its position in the start procedure and there is no split, try a higher RRPM because without seeing a good split, that Sprag clutch may be malfunctioning

Not sure I follow this. If the sprag clutch doesn't do its thing at the book RPM for testing, why would you give it another chance under easier conditions? Maybe I'm being over cautious (is there such a thing where helis are concerned?) but if I didn't see a clean split at 75% I'd scratch the flight right there. Of course as a recreational flyer that's an easier choice than if someone's life (or paycheck) depends on the flight.

[baobab72]

Hello folks

one question about the sprag clutch.

i do have an idea of what a sprag clutch is, a sort of roll bearing that when operated in one direction the roll ears roll freely, while when operated in the opposte direction they lock.

i also read that the sprug clutch in the r22, for instance, is mounted in the upper sheave of the clutch assembly but i do not quite understand how it relates to the whole autorotation deal.

Is the clutch mounted around the main rotor drive shaft coming out of the upper pulley?

if the engine quits and as the power to the upper pulley drops, how does it effect the clutch?

sorry if i couldn t make myself more clear but i just can not manage to get a solid and clear view of the whole picture about the sprag clutch.

Many thanks

Baobab72

[Soave_Pilot]

It has a freewheelling unit, if power is beig provided it will turn the drive shaft, if it's not the main rotor will turn on its own along with the drive shaft. (autorotation)

[Rotor George]

Have a look at this video.

R22 Sprag Clutch on Vimeo

[baobab72]

Folks thanks for the replies and for having posted the link to the video
however i am still having an hard time picturing the whole thing.

with reference to the video

Is that outside body the upper sheave cos i thought that the sprag clutch was housed in the upper pulley with the outer race driven by the engine through the belts and the main transmission shaft inserted inside the inner race? Is the shaft the main rotor transmission shaft? As the engine quits and the erpm’s drop below the rrpms and the sprags lock, does the outside body and thus the sheave spin in the opposite direction driven by the main rotor?

many thanks

baobab72