Thank you for the explanation. Most of this wasn't news to me but still nice to reinforce/expand on existing knowledge. Question though: Why do clutches start to slip? Is it simply because the friction disc has worn down and thus less spring preload and less clamping? I believe this to be the case...but the thickness difference between a fresh disc and a worn out, slipping one is only about 0.050". It just doesn't seem like only 0.050" variance in thickness would make the difference between FULL clamping and slipping. As an aside, I just pulled the engine out of my subaru for head gaskets (of course). Clutch has 180k miles on it and is ALMOST down to the rivets. Wasn't slipping, but I'm guessing it would start soon. What's amazing to me is there is no discoloration, no heat cracks, no smearing, no damage at all on the flywheel or pressure ring. Both look almost new. Never seen a used flywheel or PP in such nice condition before.
Good question. The disc and steel plates are never supposed to touch. A thin film of oil protects them. I have seen transmission with 100K on them come apart and you can still read the numbering on the friction disks. A few things can shorten a clutch pack life and cause wear and slippage. Here are some that come to mind. The additives in the fluid are worn down and they can't protect the disc anymore. This can be from not changing it to the fluid being overheated and running too hot. Running to hot will destroy the fluid additives. I don't believe in lifetime fluid or even 100K mile fluid. In my book no more than 60K on AFT or 30K on CVT fluid. If fluid pressure to the clutch pack in inadequate or builds up too slow. There could be a pressure regulator of valve leakage problem, seal ring or piston seal problem. Low pressure = low clamping force = slippage. Slow buildup of pressure for example if the clutch pack is supposed to go from fully decoupled to fully coupled in 3/4 of a second but it takes 2 seconds the clutch will slip excessive during engagement. This will generate unwanted heat, damage the fluid and cause slippage and disc wear. The transmission is abused by either hard driving or vehicle overloading. Example might be a truck that is hauling more weight than it is designed for or a youngster who finds it fun to do a lot of brake stands or neutral drops to burn rubber. The clutch pack may have been poorly engineered. It could have too few disc. the lubrication circuit could be inadequate or the clutch piston return mechanism is inadequate and the clutch is experiencing centrifugal apply. The programing or design of some transmissions may have one clutch being applied as another is being released. This timing is super critical. If it's not perfect you have either unwanted slippage or unwanted binding. Binding would be when the transmission is partially in two gears at the same time. When the timing is right there is the smallest amount of overlap so slippage doesn't occur and binding is not noticed. Bottom line is that if the transmission is properly engineered, not abused and proper fluid and filter changes it should never need rebuilding. Ha, like that ever happens. I did get close to 160K on my 2010 Nissan CVT before the trans needed rebuilding. These transmissions were know to go at 60K to 90k.
That doesn't sound good. What year, make and model vehicle do you have. If you have an older vehicle where the clutch can still be adjusted you might need to adjust the cable or linkage. You might not have enough free play.
Took your class almost 10 years ago and I'm still learning till this day
Very cool. I hope your well.
Great animations and explanations! Love these tutorials.
Thank you for the explanation. Most of this wasn't news to me but still nice to reinforce/expand on existing knowledge. Question though:
Why do clutches start to slip? Is it simply because the friction disc has worn down and thus less spring preload and less clamping? I believe this to be the case...but the thickness difference between a fresh disc and a worn out, slipping one is only about 0.050". It just doesn't seem like only 0.050" variance in thickness would make the difference between FULL clamping and slipping.
As an aside, I just pulled the engine out of my subaru for head gaskets (of course). Clutch has 180k miles on it and is ALMOST down to the rivets. Wasn't slipping, but I'm guessing it would start soon. What's amazing to me is there is no discoloration, no heat cracks, no smearing, no damage at all on the flywheel or pressure ring. Both look almost new. Never seen a used flywheel or PP in such nice condition before.
Good question. The disc and steel plates are never supposed to touch. A thin film of oil protects them. I have seen transmission with 100K on them come apart and you can still read the numbering on the friction disks. A few things can shorten a clutch pack life and cause wear and slippage. Here are some that come to mind. The additives in the fluid are worn down and they can't protect the disc anymore. This can be from not changing it to the fluid being overheated and running too hot. Running to hot will destroy the fluid additives. I don't believe in lifetime fluid or even 100K mile fluid. In my book no more than 60K on AFT or 30K on CVT fluid. If fluid pressure to the clutch pack in inadequate or builds up too slow. There could be a pressure regulator of valve leakage problem, seal ring or piston seal problem. Low pressure = low clamping force = slippage. Slow buildup of pressure for example if the clutch pack is supposed to go from fully decoupled to fully coupled in 3/4 of a second but it takes 2 seconds the clutch will slip excessive during engagement. This will generate unwanted heat, damage the fluid and cause slippage and disc wear. The transmission is abused by either hard driving or vehicle overloading. Example might be a truck that is hauling more weight than it is designed for or a youngster who finds it fun to do a lot of brake stands or neutral drops to burn rubber. The clutch pack may have been poorly engineered. It could have too few disc. the lubrication circuit could be inadequate or the clutch piston return mechanism is inadequate and the clutch is experiencing centrifugal apply. The programing or design of some transmissions may have one clutch being applied as another is being released. This timing is super critical. If it's not perfect you have either unwanted slippage or unwanted binding. Binding would be when the transmission is partially in two gears at the same time. When the timing is right there is the smallest amount of overlap so slippage doesn't occur and binding is not noticed. Bottom line is that if the transmission is properly engineered, not abused and proper fluid and filter changes it should never need rebuilding. Ha, like that ever happens. I did get close to 160K on my 2010 Nissan CVT before the trans needed rebuilding. These transmissions were know to go at 60K to 90k.
Good Day.
My Clutch Pedal Vibrate Heavily When i accelerate After Changing Gears. What could be the problem with this?
That doesn't sound good. What year, make and model vehicle do you have. If you have an older vehicle where the clutch can still be adjusted you might need to adjust the cable or linkage. You might not have enough free play.