Clutch material

[lucky-guesser]

I am working on a slip clutch mechanism where the central drive shaft has a shoulder sticking out, and I slide on a clutch pad, a gear, another clutch pad, then run a nut down on the shaft to clamp everything down against the shoulder (the shaft is threaded). The goal is that we can set the torque for the slip clutch by how much we torque down the nut. We have a different assembly using a similar concept that works well. The difference is, in our other assembly it is a dry system. In the new assembly we are hoping to be able to grease it, so we are in need of a clutch material that either doesn't have a change in friction when covered in grease (unlikely) or rather a pad that when squeezed together during the initial assembly, is not porous so that even if the OD gets covered, it will not absorb the grease and allow it to get down in-between the gear and the clutch pad. I might be totally out of my mind here but I have seen mechanisms that use steel clutch pads so I was hoping we could get something similar. We are hoping the pad material can be fairly thin, 1/16" maybe. Any material suggestions welcome.

 

[TugboatEng]

Recess the sides of the gear where the clutch rides. The gear teeth should extend past the plate that compresses the clutch and over the shoulder on the shaft. You can also use v-ring seals on the shaft and plate with the lip against the side of the gear.

Also consider using a spring between the but and clutch plate if you want any chance of repeatable friction forces.

 

[kjoiner]

Hello,

As TugboatEng Mentioned, adding a spring will provide more predictable force which you should be able to measure by deflection of the spring vs. torque on a nut. When I read your application, it reminded me of my lawn mower. I have a Tru Cut reel mower and it uses clutches to engage the reel and to engage the drive wheels. The reel is fully engaged when mowing, but the drive wheel clutch is allowed to slip so speed can be controlled. In the case of my mower, the system is dry and uses a friction material against a steel pressure plate. The engagement mechanisms have stamped cam levers that push on springs to put pressure on the pressure plates.

How much torque and speed are you working with?

Kyle

 

[lucky-guesser]

@TugboatEng
Unfortunately recessing the gear so that the clutch will sit "underneath" it as you describe and be semi sealed won't be doable on my prototype version, however I do like that idea for the production version (or a second prototype) so I will keep that in mind.

How much torque and speed are you working with?

I would ballpark 35-40 lbft and 2000RPM.

 

[btrueblood]

see discussion of wet vs. dry clutch here

https://en.wikipedia.org/wiki/Clutch

I learned to drive a bulldozer that had a multi-plate oil bath clutch, it could transfer a ridiculous amount of torque in very short amounts of slippage time, i.e. accelerate a multi-ton machine to 1 mph in less than a tenth of a second...repeated every 10 seconds or so...forever (well, for years).

 

[pmbrunelle]

Is this meant for regular slippage, or only intermittent duty as a "fuse"?

For intermittent duty, you may consider a tolerance ring. The tolerance ring uses grease on the friction surface, so you don't have to worry about grease contamination.

 

[lucky-guesser]

Is this meant for regular slippage, or only intermittent duty as a "fuse"?

For intermittent duty, you may consider a tolerance ring. The tolerance ring uses grease on the friction surface, so you don't have to worry about grease contamination.

It should be intermittent, only coming into play when the operator takes the machine to the ends of its range and we want it to stop without damaging anything. Which if the operator has more than a few brain cells to rub together should be pretty infrequent.

The tolerance ring idea will take some redesign and I'm a little nervous about the cost ($10ea on mcmaster and we will have fairly high qantities) but I really like how these work, at least in principle.