wet clutch material

[RickyW]

Hi,
I'm looking for some advice on a wet clutch material for a limited slip differential. I've found it hard to find much useful information on metal/metal clutch materials.

My knowledge of materials is very basic but looking at a couple of other clutch packs i've seen some very basic metal/metal multi plate setups that look to be nothing more than stamped spring steel. The driven/drive disc appear to be made of the same material in some I have physically seen, but the dana ones posted below have some slight color difference?

Some suggestions have been hot rolled c1045, 65mn/1066. I've heard also heard nitriding would help prevent galling?

Any advice on material would be greatly appreciated. I'm also thinking of placing some holes or angled slots in one or both of the plates to aid in oil circulation, but would be interested in any suggestions.

 

[EdStainless]

The only disc packs that I recall seeing had every other disc in the pack that was treated.
I couldn't tell how but nitride or carbo-nitride would make sense.
I wouldn't add holes of slots.
These work because they grab each other.
Less friction means more slip.
That sort of defeats the purpose.

 

[RickyW]

Thanks for the reply. It does appear that the discs in the pack shown are slightly different shades, so what your saying makes sense.
As for the holes or slots they are basically to allow oil to circulate to prevent galling and to allow some cooling of the plates. I understand they would reduce friction somewhat but it might be a necessary compromise to stop the friction surface from overheating. I'm getting some plates cut and will give them a shot with and without slots. Literally shooting in the dark for material, but that's the nature of experimenting.

 

[3DDave]

The point of a clutch like this is to generate friction between the mating surfaces. Increasing the lubrication of the surfaces lowers that friction, which means the oil that is between them will be subject to high fluid shear for longer, increasing the oil temperature.

Making slots is like having an oiler on disk brakes in order to keep the brake rotor from getting worn by the pads. It sure will work, but maintaining an oil film for longer than necessary is adverse to the purpose.

 

[gruntguru]

OTOH slots allow oil to escape more readily during engagement. They also reduce the contact area, increasing the contact pressure and improving the rate of collapse of any oil film.

 

[3DDave]

Only if the slots go all the way to the edge, which would significantly weaken them.

 

[gruntguru]

Not full depth slots. Just shallow ones across the friction surface.

 

[RickyW]

The point of a clutch like this is to generate friction between the mating surfaces. Increasing the lubrication of the surfaces lowers that friction, which means the oil that is between them will be subject to high fluid shear for longer, increasing the oil temperature.

Making slots is like having an oiler on disk brakes in order to keep the brake rotor from getting worn by the pads. It sure will work, but maintaining an oil film for longer than necessary is adverse to the purpose.

I see your point. I'm basing my ideas off other wet clutch arrangements that I have seen. I would think if there is no oil and enough heat the clutch disc could friction weld together. I've seen a few abused differential clutch packs (that were slipped to excess under high load/speed) which welded themselves together and stripped out the splines. I understand that it's a failre

Not full depth slots. Just shallow ones across the friction surface.

I think you are correct. I have been doing some more research and have found some plates where one disc has spiral grooves, allowing oil to enter/leave, and have some cooling effect, and the other disc has straight line grooves from the id directly out to the OD. Would you have any idea on a material?

 

[BrianPetersen]

I have no experience with limited-slip differential clutches, but I've seen the insides of "wet" clutches in both motorcycle transmissions and automotive automatic transmissions a bazillion times. They all use alternating steel plates and friction plates, and the friction plates each have a steel core plate and then some sort of friction material (the composition of which, I don't know) bonded to it on both sides. They all have grooves or slots in the friction material, and they are all designed so that oil fed to the inside hub of the clutch has to work its way through between the plates to the outside, thus carrying away heat from slippage. The oil feed through the plates in the ones I've seen isn't particularly structured ... the bike clutches have a roller bearing in there between the clutch basket and clutch hub, which has oil fed to it from a hole in the central shaft, and some of the oil flung off that bearing works its way randomly through the plates.

The stuff that I'm familiar with, from a company whose products I've bought for my own bikes: https://www.ebcbrakes.com/products/motorcycle-clutches-and-clutch-tools/

Minor digging finds that there are companies that build this sort of thing for other applications. Examples (random from web search - zero experience with them)

Wet Friction Plates | ProTec Friction

www.protecfriction.com

Tribco Inc. - Wet clutch specifications for Clutchtex® Kevlar friction material

Tribco Inc. mfg. of Braketex and Clutchtex, the first 100% KEVLAR fibered textile composite friction materials for brakes, clutches, transmissions and other applications.

www.tribco.com

I suspect the formulations and means of construction are proprietary, which may make it hard to independently design ... but, any particular reason why you are re-inventing this, as opposed to buying something from a company that already makes this sort of thing?

How much slippage energy do you anticipate having to absorb? If you're driving the vehicle normally around a bend, the slippage speed between the left and right wheels is pretty small, and usually it's not transmitting full engine output torque in that situation. If you're doing a hard launch (or burnout) then if the diff is properly designed, it will lock the left and right outputs together, thus not requiring the clutch pack to absorb any power except in the fraction of a second that the mechanism takes to react. If you completely lock one output of ANY limited-slip diff and apply full drive power thus forcing the other output to turn such that the limited-slip mechanism has to absorb a large portion of drive power for a good length of time, you WILL smoke it, and that goes for any limited-slip that has ever been built. The "Detroit locker" types are incapable of dealing with this situation, although they have other problems.