Jump to content

A clutch. Who says it's worn out and is it "designed in"?

Recommended Posts

So, how does a clutch wear out?  I know funny question, the friction plates wear out. BUT, when those plates wear out, is it spring pressure that decreases to nothing as they get thinner?  And why does spring pressure go to near zero.  In looking at some clutches, depending on the design, I can see a couple of things causing it.  For one, as the clutch pack gets thinner, the spring pockets in the pressure plate could bottom out on the clutch center or inner hub.  Also, the pressure plate has some kind of teeth in it which mesh with the inner hub and the inner hub could bottom out on those teeth, causing the loss of clutch pack pressure.  Which one of these happen first as the clutch pack gets thinner depend on the mechanical design(er).  Since some clutches are ALL steel plates, the fact that fiber friction plates wear down to the steel and the clutch starts slipping, it's because the pack got thinner, not that it was steel on steel.  You gain a working clutch again by putting in new THICKER plates to give the stack the correct dimension. So, theoretically, (no, I'm not going to try it), could a shim, similar in dimensions to one of the metal plates, but maybe thinner, be slipped in at either end of the clutch pack to give the pack the correct dimension again and work?  So, depending on the mechanical design, I could see a clutch being WAY more fussy about clutch pack height, probably mostly by chance because  one of those  2 areas above wasn't really a worry for designers. 

  • Like 1

Share this post


Link to post
Share on other sites

the springs are not part of the equation. they have plenty of travel, if they are installed properly, to deal with 1/4" or so of  "slack" from the fiders wearing out.

the fibers wear, or glaze, then they heat up the steels (blueing)   and or take the temper from the springs.

then wear accelerates rapidly, until it's unusable, even with much stiffer springs. it's a pretty simple design that works very very well, as long as it is adjusted properly and there is oil to cool and lubricate it. 

  • Like 1

Share this post


Link to post
Share on other sites

 I know the springs themselves have plenty of travel, but with the clutches I'm looking at right now (an old Honda and Suzuki) the pressure plate can bottom out in the 2 places I mentioned. Loosely piece a clutch together without any plates and you'll see the pressure plate doesn't move much at all before you get a mechanical stop.

Share this post


Link to post
Share on other sites
5 hours ago, ohgood said:

the springs are not part of the equation. they have plenty of travel, if they are installed properly, to deal with 1/4" or so of  "slack" from the fiders wearing out.

the fibers wear, or glaze, then they heat up the steels (blueing)   and or take the temper from the springs.

then wear accelerates rapidly, until it's unusable, even with much stiffer springs. it's a pretty simple design that works very very well, as long as it is adjusted properly and there is oil to cool and lubricate it. 

Hmmm. Rotate the pressure plate one bolt position in either direction and try again. Did things change?  Some clutch packs only go together one way. This is usually indicated by an alignment arrow on the pressure plate that aligns with a dot on the hub. In a 6 bolt system you may have 3 right and 3 wrong ways to assemble.

Another possibility is the hub or pressure plate has worn beyond minimum. Look to the manual for guidance. Often you can face on a lathe. I destroy a lot of clutches. You can usually run a stock clutch to stock HP plus 25%. HP and the heat it creates is the killer of clutches and you choices grow thin once you hit the limit of design. Once slipping starts it is time to replace the cultch in total.

If you don't already do you can drill the hub with 2-3 # 40 drill holes per spline to increase oil flow through the pack. It helps to cool the clutch pack.

Edited by pcnsd
  • Like 2

Share this post


Link to post
Share on other sites

Just testing out my theory again, I just set a Honda pressure plate on it's center hub or boss - no plates involved.  I can tell it can't go down very far at all from plates wearing and the pressure plate will bottom itself on the boss.  By then, you have NO clutch pressure on the plates.  All the springs are doing is pushing the pressure plate and boss together.

Share this post


Link to post
Share on other sites

Three things make a clutch work, or not:
1. friction area
2. clamp pressure
3. coefficient of friction between the plates.

So if the frictions wear there will be less preload on the springs and less clamping pressure. Shop manual will spec the min thickness for the frictions.

On XRs we usually shim the springs to increase preload and clamping pressure. The spring preload is typically 0.5" so a 0.050 shim under each spring will increase preload 10%. That is the equivalent of 0.010" wear on 5 frictions.

Wrong oil will affect coefficient of friction and can cause clutch slippage.

As a side note; several different friction materials are available, and discs are available in aluminum, steel, and dimpled steel.  Each affect clutch operation.

Edited by Chuck.
  • Like 2

Share this post


Link to post
Share on other sites

I agree with everything you say, Chuck.  What I was pointing out that CLAMP PRESSURE can be dictated, AND limited greatly, by it's mechanical designs & pressure could very rapidly go to zero because of mechanical interference.  Not sure a mechanical design engineer would even worry about it because he'd say "Plates are worn, just put new plates in" which WOULD be true but there's more involved here, at least with the clutches I've been messing with lately.

Share this post


Link to post
Share on other sites

Motoxvet, you are being completely oblivious to the pushrod and bearing assembly that disengages the clutch. This is the first thing that contacts the pressure plate and causes massive pressure drop on a worn clutch pack. You can simply not achieve any free play between the release bearing and the pressure plate once the pack is worn too thin, even though the cable may have free play. None of the interference points you describe come into contact. 

Spring pressure will diminish slightly as the pack wears thin. A common trick is to install 2 steel plates against each other in about the middle of the pack to regain pack thickness. Dotards, as Kim would say, will argue you can't do that, but the steels turn in unison and do not produce friction on each other. Your only concern is that the pack can't be excessively thick, or setting the free play and disengagement becomes impossible. 

For a very high powered unit, the posts on the clutch hub that the spring bolts screw into can also be ground off and shortened slightly for a stiffer more robust clutch that won't slip, but you still need the pack to be thick. That thought will make some folks on here blow a head gasket.
.

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×