Sprag Clutches
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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)
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)
Last edited by John R81; 12th Jan 2012 at 16:30.
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R44 thread sprag clutch discussion starts around here: http://www.pprune.org/rotorheads/189...ml#post5596243
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!
Last edited by 76fan; 12th Jan 2012 at 17:29.
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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
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
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MamaPut
... 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?
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?
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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
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
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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!
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!
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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.
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.
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By the book ......
John R81
contradicts yourself:
Why did you quote me in a Chinese accent:
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 .... 
AND:
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
!)
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?
I do the exercise by the book
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
Err.... doing the exercise per the manual is "unsatisfactory executed"?
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.
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
!)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?
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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.
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.
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Fair dinkum....
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
)
Good wishes to all !
Survey:
Has anyone here ever had a genuine 'failed split' in an R44?
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
)Good wishes to all !
Survey:
Has anyone here ever had a genuine 'failed split' in an R44?
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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
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
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.
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.
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
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sprag clutch
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
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
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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)
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sprag clutch
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
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
