sprag clutch 2

[pontiacjack]

Hi- new member here. After stumbling across these forums and some very illuminating discussion of silent-chain drives, I figured I'd better join to get help with other questions.

I have an application for a sprag clutch, but never employed one before. From application data, I had selected a sprag to match my torque requirements (900 ft.lb.) and dimensional constraints (5" diameter), and general configuration (sprag with sealed bearings, center shaft can over-run). But the maximum driving RPM is 230 RPM, while I need up to 5,000 RPM. I need help understanding this severe speed limitation. And, to shortcut to the answer I need, is there such a thing as a "high speed sprag" design?
Thanks for listening.

 

[thundair]

pontiacjack
Usually a sprag clutch is used in a temporary drive as in a starter motor then is over ridden... is the rpm limit in the over run condition if so it doesn't sound right..

What is your application in detail?

Cheers

I don't know anything but the people that do.

 

[tbuelna]

There absolutely is such a thing as a high speed sprag. And they drive continuously. They're used on turbine engine helicopter gearboxes. They're located between the engine and gearbox and operate for thousands of hours at speed in excess of 15,000 rpm. They are used to make sure the rotor can continue to rotate if the engine stops for any reason.

 

[pontiacjack]

Thanks for the replies.
tbuelna- yes, rotary-wing drives are a good example. My application would "add" the torque of an auxilliary engine to a vehicle's driveline (forward only) whenever that engine is throttled up to exceed the pre-existing driveline speed. I was looking at the Marland RMI-26HP sprag unit when I noticed the "maximum driving RPM 230". What inherent property of this design gives it such a low limit? Does radial acceleration (centrifugal force) interfere with the normal movement of the sprag elements? If so, would exceeding 230 RPM cause the whole unit to "lock up" and not allow over-run of the driven element?
(All of the sprag clutch assemblies I looked at have very low "maximum driving RPM")

 

[GregLocock]

Also Saab cars used to have an override freewheel in the gerabox - so they must have been good for several thousand rpm, and long lives. I don't know how they worked, but some Saab enthusiast will probably be able to explain.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Philrock]

http://www.epilogics.com/md/index.htm

 

[Tmoose]

Lots of automatic trans have sprag clutches
Chrysler Hemi torqueflite auto trans sprag clutch

http://www.coperacingtrans.com/images/sprag.JPG

 

[MikeHalloran]

Industrial hardware is rated differently than automotive hardware.

One extreme example is roller chain. The stuff you find on the back of a 100HP motorcycle will have an industrial rating of ~5HP. Which means that it can carry 5HP essentially forever, but if you look at the individual elements, you will find that it can carry 100HP 'long enough' and 'often enough' for a finite life on a motorcycle.

For rolling element bearings, the manufacturers' data books will give you ratings and equations that help you assess the statistical life for any duty cycle that you define; of course, _you_ have to define that duty cycle.

The sprag clutch manufacturers may or may not be able to help as much. If you can convince them that there's some chance you'll be buying 10,000 units a day, they'll try pretty hard.







Mike Halloran
Pembroke Pines, FL, USA

 

[pontiacjack]

Thanks guys. 10,000 units per day? How about a quantity of one! This is merely a hobby project (or- potential project- just looking at feasibility now).
I'll be calling sprag manufacturers, to see if I'm understanding their application data correctly. I had assumed that "Maximum Driving RPM" referred to the maximum speed of the whole unit when driven with the sprags locked up. Perhaps it's actually the high-end limit for the speed at which lockup occurs?

 

[MikeHalloran]

For a hobby project, just buy something that looks about right, and if breaks, buy a bigger one.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

try here, size 500 (1175lb-ft & 2500rpm w/grease):

http://www.formsprag.com/PDF/P-956-FC_pg014-047.pdf

 

[RossABQ]

I had a Saab with the "freewheel" sprag clutch. By the time they had 50k miles on them, most were welded into the "locked" position. They were only about 2" diameter and had too few sprags.

Another industrial application that may be close to what's needed is on big fan shafts; called an anti-reverse-rotation clutch. They're used to keep fans from spinning backwards when they are off, from wind or leakage in their discharge dampers. I've seen several that were about 4" OD. Don't know what they would be rated for, HP-wise.

MikeH., interesting comment about chain on motorcycles! A personal beef of mine. They last at most 15k miles, and cost over a hundred bucks now. The packaging requirements of a bike drives use of sprockets that are smaller than recommended for long life, unsupported spans are too long. No protection from the elements.

 

[GregLocock]

There's also the anti-rollback bearings used on big uphill conveyors. I don't know what they are called or what their rating is.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[pontiacjack]

Thanks for the replies.
I'm awaiting an answer from Marland to help me understand their "230 Maximum Driving RPM" specification of their RMI-26HP sprag clutch.