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[papasmurf]

[jefrs]

I've seen info indicating 200rpm to provide sufficient sprag grip. This may seem fast but it's only about 3 revolutions per second. Papasmurf's video demonstrates the principle but it needs stiction to engage the sprags and force to hold them out in position. When the outer ring speed exceeds the inner, the sprags are no longer driven by the inner and hence disengage. The sprags have to disengage or they would get excessive wear when the motor is running.

[papasmurf]

jefrs Subject:" Sprag????
I've seen info indicating 200rpm to provide sufficient sprag grip " ........ The type of sprag fitted to a Royal Enfield is instantaneous grip. That is why a backfire can mean catastrophic consequences up to and including scrapping the engine as being beyond economic repair. (Worst case scenario.)
Those sprags are widely for industrial applications where no rearward slippage is acceptable.
Personally I would NEVER use that kind of system on a starter motor.

[Dennis C]

Thack and Papasmurf are correct the Enfield sprag engages instantly if in good order, it is the engine's load which increases the the pressure of engagement and no reliance on speed is used. Look it up, there are plenty of YouTube clips etcetera showing this is the case.

[jefrs]

Whilst I've not had a RE sprag clutch apart in my hands I have seen others close up incl helicopter. Do we have close up pics of a RE sprag clutch, the workshop manual is not very useful. There are many pics online, the surfaces are smooth and it takes considerable force to hold them engaged. If the sprags were in constant contact with the motor ring they would wear down to a nubbin in no time flat. They are designed to tip out of the way just enough to disengage drive. Obviously the engine is far more powerful than the starter motor and if it tries to turn the mechanism backwards it will wreck it. There's instant grip and instant grip, it can't be permanently engaged or it would not work. There will be stiction provided by the oil in the primary case to drag them quickly into position to drive but this will not provide enough grip on its own, there has to be force provided by the starter (or the engine backwards) to make the sprags bite and fully engage the drive.



The point - if the starter is making a feeble buzzing noise, the sprag could be shot but it is more likely they are slipping, or the starter simply doesn't have enough power to turn the engine over. An electric motor will lack power when it doesn't have enough volts.

[Dennis C]

The actual sprags are always in contact but free wheeling, oil publication is used to prevent exessive wear. Take a look here. http://www.renold.com/products/freewhee ... ag-clutch/

[Dennis C]

Predictive text strikes again, oil lubrication.

[PeteF]

The sprags are in contact all the time and engage instantaneously (for all practical purposes)
They certainly don't need anything like 200 rpm to engage.
To say that they would "they would wear down to a nubbin in no time flat" if they were in contact is nonsense. That's what the lubrication is for.

[jefrs]

The, "The actual sprags are always in contact but free wheeling" is an oxymoron (contrary terms) - they cannot be in contact when free wheeling or it wouldn't free-wheel, which is what happens when the engine is running. A layer of oil between sprag and ring means the sprag is not in contact with the ring. That's all it takes to disengage them. It may be a small gap with a film of oil but that means they are not in contact.

[Dennis C]

Clutching at straws again Jeff?, pun intended. more info on the enfield sprag clutch here. https://www.youtube.com/watch?v=EAbRl8qWON4