Clutch slippage problem

[JLKDS]

Hi all,

I have been developing a pulley system which allows the user to lift a 50kg weight by turning a wheel. The pulley system has a mechanical advantage of 3 and there is also a 2:1 gear exchange along the line. There is a one way clutch bearing that stops the weight from falling (basically it's super easy to lift the weight). To lower the weight, there is a disc clutch that I have designed which when disengaged, allows the shaft to spin free of the one way clutch and the weight falls.

The clutch will hold the weight but when I turn the wheel to lift it, the clutch slips. Based on the uniform wear and uniform pressure methods of calculating clutch torque capacity, the theoretical values for the torque are 26Nm and 32Nm respectively. This should be enough as the torque exerted by the weight, after the pulley and gear reductions should be 7.5Nm.

THE CLUTCH: Two plates, one asbestos free moulded friction material one mild steel. The coefficient of friction from the friction material data sheet is 0.45. Four compressions springs exert 600N in total. The plates have internal diameter of 20mm and outside diamtere of 180mm.

Here is a link to the datasheet of the friction plate:
Link

Can anyone guess why the clutch is slipping or can anyone suggest how to improve the torque capacity of a clutch without increasing axial force?

Many thanks,

Pete

 

[Jboggs]

Two questions:
1) What did the friction material manufacturer say when you called them? You did call them didn't you?
2) Is there some reason you aren't using a commercially available pre-assembled clutch unit?

 

[JLKDS]

1) Yes, they said that I should try one of their higher friction coefficient materials, 0.5 or above. They are going to send me a sample.

2) It's a design installation, so it has to look nice

Cheers

 

[LittleInch]

My guesses,

1) your mild steel plate has a lower friction co efficient than the one chosen in the link (fine grained cast iron)
2) one or both of you plates is buckling hence your surface area is less than expected
3) you're starting the turning too hard and momentarily exceeding the torque and then moving into slip friction
4) Someone's lubricated / coated the steel surface with something (oil, varnish, wax, grease....)

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[desertfox]

Hi

A picture speaks a 1000 words so can we have one of your set up please.

desertfox

 

[tr1ntx]

Can anyone guess why the clutch is slipping? Inertia


Can anyone suggest how to improve the torque capacity of a clutch without increasing axial force? Increase contact area

 

[Tmoose]

Pictures would be nice.
Details of springs, and how they clamp the cluthc pieces would be interesting to me.

The material spec you provided claims it is laden with graphite, and one effect is to "narrow the gap" between static and dynamic friction coefficients. I wonder if higher static friction would make it

If evaluation shows no contamination, and the clamp force is indeed as designed, I'd put some radial stripes with magic marker every 45 degrees or so, and 3 concentric circles on all the metallic friction faces, then test again. If slippage occurs, I'd provoke plenty of it to wear off the MM stripes. If the MM wear shows various clamping faces are distorting ( circumferential waves, or coning ) there can be a lot less area than planned, and in odd cases the radius i suppose might be smaller too.

If MM stripe wear is even, adding another clutch disk with floater would double the capacity. I would not thin the main plates to make room for the twin disk setup.

Manufacturers of high performance automotive clutches used to talk about flywheel finishes that were 'too smooth" causing problems. I'm not sure I buy that, but it might not hurt to grit blast the metallic faces while you have it apart.

Are there ears or dogs driving the clutch plates ? If the mating surfaces are rough or have become notched then when under torque things can not slide, and can cause erratic engagement.

http://i600.photobucket.com/albums/tt81/chunkaracto/basketMedium.jpg

 

[Tmoose]

I said "The material spec you provided claims it is laden with graphite, and one effect is to "narrow the gap" between static and dynamic friction coefficients. I wonder if higher static friction would make it......"

Meant to say. If higher static friction is what you need, and overload in service is not likely to cause slippage and resulting runaway, maybe the "other" material is appropriate.