After a recent thread discussion on the subject, I thought I'd put together a little writeup on the hydraulic clutch. Contrary to the confusion, it's really a simple design if you take it apart piece by piece. Here is a basic illustration, followed by an explanation (sizes and shapes are exaggerated and not accurate):
We start with the flywheel, which delivers the rotational power of the engine. In its normal position, the clutch disc is pressed against the flywheel by the spring loaded pressure plate. This pressure plate resides in the clutch housing along with the diaphragm, and all three rotate along with the flywheel. At the center of the clutch disc is a splined hub, which mates with the transmission's input shaft, delivering power to the gearbox. The front end of this shaft is supported and centered by the pilot bearing.
When you depress the clutch pedal, the master cylinder (not shown) forces brake fluid down to operate the slave cylinder, which in turn moves the clutch fork via a piston. The fork then slides the throw-out bearing along the guide tube to mesh with the diaphragm. The diaphragm 'dishes out' to pull the pressure plate backwards, releasing the pressure on the clutch disc. This frees the transmission from the engine so you can change gears. The whole process looks like this:
That's it, in its basic form, but there are other design details that I've left out so as not to get too confusing. Here's a typical clutch assembly:
Note the damper springs located near the center of the clutch disc. These act as a buffer against the shock forces induced when the clutch is engaged; the center hub and clutch lining actually rotate independent of each other to a certain degree, and the springs take up and resist this 'slack'. Clutch discs made for racing/performance applications often do not have these springs. Without them, shifting usually needs to be more precise, but power transfer is dramatically improved.
Now for a couple of things you may not know . . The throw-out bearing is not designed to handle constant loading. If you're coasting or stopped with the clutch pedal in, or maintaining a hill with the clutch, you're loading the bearing against the diaphragm, and the bearing will fail prematurely. You're also probably slipping the clutch disc unnecessarily, wearing out the linings.
Don't ride the pedal. Unlike cable actuated clutches, which have to be adjusted periodically as the friction lining wears down, hydraulic clutches are self-adjusting. The slack is taken up via the master & slave cylinders. There is a certain amount of 'play' on the pedal before the clutch actually disengages, which accounts for this adjustment. But if you ride the pedal, even barely keep your foot on top of it, this adjustment cannot take place. Incidentally, doing this with cable actuated clutches is just as bad, since there is less 'play'.
Aside from imperfections or defects, the life expectancy of a clutch depends a lot on the manufacturer, but its real life really depends on the driver. BMW's should typically last anywhere from 80,000 to 120,000mi, so if you're not even reaching 50,000 or so, you might want to examine your driving habits and make corrections as necessary.
Hope this helps someone out there