An Engineer in Wonderland – Roller clutches
I am currently driving to work as my bicycle has a fault which is in danger of becoming a saga.
A long long time ago far far away, I bought a back wheel.
And within the back wheel was a novel freewheel mechanism based on a roller clutch rather than the standard pawl system that has served cycling well for a century.
A roller clutch is a bit like a roller bearing: It has two concentric cylinders, one inside the other, and between them are metal rollers.
The cunning bit is that, unlike a roller bearing, ramps cut into the surface of one of the cylinders – one ramp per roller- make the gap between the cylinders vary slightly above and below the diameter of a roller.
As such, in one direction of rotation the rollers are jammed by the ramps onto the other cylinder where they consequentially transmit torque and rotation.
In the other direction the rollers roll freely around transmitting no torque or rotation.
Sometimes springs help the rollers engage.
All this I have learned trying to make my bike work.
The advantages of the roller clutch compared with a pawl type are twofold: silent operation and instant take-up.
The disadvantage, as far as I know, is lower torque capability for a given size.
And without spring assistance, my bicycle experience tells me, they also seem to be a bit dependent on the lubricant used.
After years of faultless operation, mine started to drag when it should have been free.
I don’t have the special and unobtainable Shimano FH-40 tool, and without this they appear to be impossible to dismantle – I have two mangled home made tools as Exhibits A and B.
So in the first instance I dribbled oil into every little gap and the thing started to work – except that it occasionally and unpredictably slipped when it should have been driving.
Back to the drawing board. Thicker grease?
Through its narrow gaps I washed it out with white spirit. At this point it seemed to be working perfectly.
I then sprayed in motorcycle chain lubricant – a sticky grease with a lot of solvent that allows it to flow in, then stick.
Initial results were promising but, I assume once the all solvent had gone, it now slips all the time and transmits no torque at all in either direction. Luckily for me, the walk hone wasn’t a long one.
I now have three choices:
- Wash it out and try another penetrating lubricant.
- Have go three at making a dismantling tool.
- Cheat, follow Shimano’s advice, and replace it.
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