Organic facings wear well, if there is enough surface area. Coefficient of fricction is moderate. Around 400-450 deg. F, they will glaze just like brake shoes, and will not hold the torque after that. So. slipping them is a no-no. Keeping the temperature down means using lots of metal around them (like the old 13" clutch pressure ring had), and absolutely minimizing slippage time.
Ceramics, metallics, etc. have better coefficients of friction but tend to chatter terribly, meaning they are in-or-out. Some of them also tend to wear the flywheel. Wear quality varies with the individual material.
I am testing a carbon fiber/high temperature resin disk friction material for TST, and testing so far indicates it will take 700 deg. before losing holding power. After cooling, it regains all its holding power. It also wears about 1/10 as fast as organic facings. The one drawback up to now has been that some of them chatter a bit, due to uneven saturation of the carbon fiber mat with the resin. This seems to be fixed now, pending final testing. This material has a fantastic coefficient of friction as well.
A somewhat related question that should be asked is "does the clutch use a sprung hub and marcel?" The sprung hub cushions torque loading and unloading and dampens vibration. Transmission damage can result from vibration, even if you can put up with it. Marcel, or similar wave plates, will cushion engagement so the clutch does not chatter. This chatter can shake the truck, and in bad cases, you can see the front of the truck bouncing up and down! The wave plates are between the facings.
In general, holding power of the clutch is a direct function of clamping pressure and coefficient of friction for the friction materials. Holding power is inversely related to friction surface area. If area is reduced too much, wear escalates unacceptably. Cast iron, such as the stock flywheel and pressure plate, has a good coefficient of friction (better than steel, for example). Finish and flatness of the friction surfaces are also important. For example, never use sandpaper or a body grinder to finish your flywheel. Get an auto machine shop to use the proper stone, and take the minimum off the flywheel. Cummins and Dodge warn not to remove "heavy stock. " Remove only a couple thousandths of an inch if possible. A good surfacing machine can give you a clean, flat surface, remove the baked-in clutch disk material, and leave a swirl pattern that will help break in the new disk. Commercial friction disksa are not completely flat, and break-in over 500 or more city miles will be required to achieve full engagement. Frequent shifting is good, but never "burn-in" the clutch or deliberately slip it to hasten break-in. For example, I removed a 13" clutch with a few thousand miles on it, and still only about 50% of the friction material was worn (engaging the pressure plate or flywheel).
This break-in is very important with high-torque diesels such as hopped-up Cummins Rams. The clutch is on the small side for this application, and partial engagement of the friction surface on the disk, plus excessive slippage, can lead to glazing the facing and hot-spotting the iron surfaces.
[This message has been edited by Joseph Donnelly (edited 01-11-2000). ]
[This message has been edited by Joseph Donnelly (edited 01-11-2000). ]
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