Understanding the forces within a clutch

[zachr]

Hi all,

I'm converting a 2002 Boxster S to use an electric motor. However, my question is about the mechanical operation of the clutch. I've got a large electric motor mounted to the original transmission using a custom adapter plate. It's out of the car and not attached to axles. I'm also using a custom aluminum flywheel and a pressure plate/friction disc from Kennedy Engineering (built for high torque applications). I wanted to test the basic function of the clutch/motor assembly, so I attached the clutch hydraulic line to transmission's slave cylinder, and did my best to bleed all the air out of (I think there might be a little air in there as the pedal is a little soft, so this might be a clue). With the transmission in gear, I can spin the motor shaft by hand and I can watch the axle output cups spinning as expected. It's even easier to spin when the transmission is in neutral. So far so good, but it's about to get a little weird... If I press the clutch pedal while spinning the motor shaft, it becomes much harder turn. The friction seems to INCREASE when I press the clutch pedal. I would have though just the opposite would happen, pushing clutch in should decrease the motor friction. I'd like to understand how/why this is happening.

One idea is that the friction is coming from the throw-out bearing, but the friction seems too high for just that.

Another test I did was to hook a small battery up to the motor. With nothing pushing the clutch pedal, it spins just fine. However, if I press on the clutch pedal while the motor is running. I can hear a little sliding sound (like the friction disk spinning on the friction plate), but then hear the motor slow down (I would have thought the motor would speed up) as it deals with more friction.

This friction might typical and just not noticed in a regular vehicle (since you don;t really turn the crank by hand). However, it is counter intuitive and now would be a great time to solve a problem if there is one, as the motor isn't fully mounted in the car yet.


Any ideas/suggestions?

[howe00]

My guess, the friction with the thro - out brg engaged would be > the internal friction of the box and drive train. In addition you will also have friction in the flywheel spigot bearing. From memory rolling resistance whether its ball or roller = .035 x load, in your case the load is the force on the thro-out bearing.

Hope this helps

[986ron]

Some thoughts: The clutch throw out bearing would add a great amount of end load to your electric motor which it is probably not meant for and that would add resistance. The engine is designed with thrust surfaces.
You could remove your clutch pressure plate and measure the load needed to release the clutch disc and that would give some idea of the load you need to design for.
Would be interested in seeing photos of your installation.
Ron