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Why does the clutch disc disengages the flywheel?

  1. Sep 6, 2014 #1

    I am creating this thread to ask what is the physical phenomena that drive the smooth disengagement of clutch discs from flywheels. I am thinking about car clutches, specifically a longitudinal front engine, rear wheel drive clutch type.

    I've been reading some books that describe the components of the clutch system, their operation, the proper procedure for maintenance and repair, but I haven't been successful in finding a book that describes why the clutch disc disengages the flywheel (the physical and perhaps mathematical models of the operation). I understand how it engages and why it stays engaged, but after the pressure plate stops exerting pressure, why does the clutch disc backs away from the flywheel?

    I've read about the cushion (marcel) springs and about the grooves in the face of friction facings and, well, even though I can somehow get an intuitive feeling that it might help disengage the clutch disc from the flywheel in a straight road (with no slope), I can't understand how a clutch disc is able to disengage when the car is on a downhill road (going down), where gravity will try to pull the clutch disc towards the flywheel.

    My thought process so far has been like this: well I know that every real surface has some roughness that translates to microscopical (sometimes even macro!) peaks and valleys on said surface. When the pressure is removed from the clutch disc it "just contacts" the flywheel and both can slip past each other. This slip will allow the peaks of the flywheel to exert some force on the peaks of the clutch disc, and this force will cause the clutch disc to gain a (very) small acceleration, which will make it back away from the flywheel. Of course the friction in the clutch disc splines and the gearbox input shaft will bring the clutch disc to a halt after the clutch disc is like 2 ou 3 mm (completely random numbers due to my ignorance of the real ones) away from the flywheel.

    When the car is on a straight road the clutch disc will not try to move towards the flywheel until the pressure plate forces it. When the car is on a downhill road the clutch disc will move towards the flywheel, but the peaks/peaks interference process will occur once again, and since it would require a lot more energy to rotate the clutch disc and the gears in the gearbox, the system will try, as per the minimum energy principle, to achieve a configuration that minimizes the energy and that configuration will most likely be the separation of the clutch disc from the flywheel.

    My questions are: is this thought process anyhow conceivable? Does anyone know if there are good references/papers on the subject?

    I am not doing research work on the subject, nor am I trying to model the system, and this is not college homework. This is just some curiosity of mine that I would really like to satisfy.
  2. jcsd
  3. Sep 6, 2014 #2


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    There is so much verbiage in your question that I am not real sure what your question is!

    Have you ever driven a manual transmission? When you step on the clutch a mechanical linkage results in a fork connected to the throw out bearing physically moving the clutch plate away from the flywheel. This removes tension from the drive train allowing you to change gears.
  4. Sep 6, 2014 #3


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    Integral is right: you speak about clutches as if they move to engage and disengage by themselves.

    The clutch is very simple: there is one plate attached to the flywheel on the engine, and a matching plate attached to the transmission. Both plates are coated with a material with enhanced friction properties, so that when the car is starting to move initially, the moving plate attached to the engine is prevented from slipping when it engages the the stationary one attached to the transmission.


    The two clutch plates move together or apart because the operator of the car controls them with a foot pedal, not because there is some clutch genie living in the transmission.
  5. Sep 6, 2014 #4
    Integral, my questions are indicated as such in the line that starts with "My questions are: (...)". I am afraid you will have to read the entire post to understand my questions.

    Yes I have. In fact I have never driven any vehicle with an automatic transmission.
    I know "what" happens when I press the clutch pedal in my car, and in fact to correct your post I have to say that the throw out bearing does not physically move the clutch plate. It does physically moves the diaphragm spring which moves the pressure plate, which stops exerting a pressure on the clutch plate (I kept calling it clutch disc to try and keep things clearer). When this happens, then, yes, the clutch plate (aka clutch disc) moves away from the flywheel and stops spinning which makes the gearbox input shaft stop spinning aswell and the clutch is said to be disengaged.

    Perhaps an image would make you understand what components make up a clutch:

    http://www.ululu.in/first-year/elements-of-mechanical-engineering/img/single-plate-clutch.jpg [Broken]

    So, in short my questions are: "why" does it happens? What is the explanation of the behavior of the clutch plate after I stop exerting a pressure on it?

    SteamKing, Integral in neither right nor wrong. He expressed an opinion which in not a fact. An opinion will never be right or wrong. You may agree with him, but it does not make you both right. It so happens that I know people who have read my post and do not agree with you both, but it doesn't make my acquaintances right neither.

    The rest of your post just shows that you couldn't be bothered to read my entire post, because if you did you would have noticed that I know that the clutch can only be disengaged after the pressure plate is moved away from the flywheel. How it is physically moved away from the flywheel is irrelevant.
    Last edited by a moderator: May 6, 2017
  6. Sep 6, 2014 #5
    First off, your posts are needlessly long and very difficult to read and comprehend. I can appreciate that you may be using English as a second language, but you should know that the above quote comes across as rude.

    Now on to your question as I could decipher it:

    Once the pressure plate has released; the friction plate is free to float up and down on the input shaft spline. So it may be in contact with the flywheel, it may not. Your thought process of how the friction plate would move seems right, high spots on the flywheel will thrust it away, gravity will make it fall on to the flywheel. It seems slightly convoluted to view it in terms of 'minimum energy', though I suppose it's correct.

    My question is, why do you care? If you have the clutch depressed, it can't transmit any torque, so why is the exact position of the friction plate of any interest to you?

    If you car is in gear, the friction plate is spinning according to road speed when the clutch is depressed. If in neutral, it will slow to rest.
    Last edited: Sep 6, 2014
  7. Oct 9, 2015 #6
    I really understand the original question - I do not know why other responders did not see the subtle question there. The issue is, there is no spring that actually pushes the friction plate away from the flywheel. Why do I care? Because I wonder if the slightest contact could cause the friction plate to spin - with almost zero torque - and then cause the imput shaft on the transmission to spin. I suppose the oil in the transmission would then cause the input shaft to spin down to a stop. This whole issue comes about, in my scientific mind (I am an engineer) because my reverse non-syncro gear does not' wind down, yet it looked like my clutch was disengaging pretty well. I did not see clutch drag, but it acts like that...
  8. Oct 9, 2015 #7
    Scout, in your question I think I understand he conundrum and the answer mostly lies in the clutch plate friction material. The surface pads are created with an enhanced friction characteristics. This same material operates in a reduced friction mode when under low compression. The pressure plate applies compression and it is the variation in the pressure that regulates the amount of torque transmitted. When the pressure plate is fully released the clutch plate can and often does touch or rattle against one of the two other surfaces. In larger multiple disc clutches this can lead to audible noise coming from the bell housing.
    I am not positive as I have not researched this particular event but I believe that the reduced friction is a variation of the off gassing that occurs when brake pads are pressed against their rotor. In the case of clutches it is engineered to occur at a different point in the cycle.
  9. Oct 12, 2015 #8


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    Now I see your basic concern, most of the above responses correctly address the functional elements of your question; but, as to your specific concern, if you have a sufficient separation of the pressure plate and the flywheel then the position of the intervening clutch plate is not really determinable; but, it may well have light contact with the spinning flywheel or pressure plate.

    Apart from that, be aware that after disengaging the clutch, the clutch plate and input gear assembly will continue to spin due simply to their rotational inertia; and, the duration of the that spinning is (assuming a given assembly mass) in direct proportion to the engine idle speed at the time of clutch disengagement vs. the gear bearings' drag. As a result, if you try to engage the reverse gear very soon after depressing the clutch pedal then you will always experience some bumping of the reverse rear during engagement.

    A simple way to determine if the inertia effect is the issue is whether or not the gear bumping occurs after starting the engine with the clutch disengaged. If you experience bumping of the gears under that condition then there is some contact between the clutch plate and, probably, the flywheel due to friction between the clutch plate and gearbox shaft splines; but, the actual amount of drag from this contact will be very low and, although irritating, of no real concern.

    Alternatively, if you have high mileage on your vehicle, then this can be an indication that your clutch assembly is not fully disengaging due to wearing of the idler bearing and/or pressure plate contact arm ends; and, the reverse gear engagement bumping will the first indication of the extent of that wear because the synchronizers on your other gears tend to mask this ensuing problem.

    A method of getting some indication of whether or not wearing is the problem is to check the free travel of your clutch pedal between the initial engagement point of your clutch and the top clutch arm stop point. If you find this distance is very short; then, depending upon your vehicle, you may only need a clutch adjustment; or, it could mean you are approaching the point for a clutch assembly replacement. Your best move in that case is to have a qualified mechanic due a quick evaluation of the issue.
  10. Nov 12, 2015 #9


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    Lots of craziness here by armchair mechanics, no doubt. There is no clutch plate attached to the flywheel. The flywheel IS the other contact surface. And I never heard of any coating on a clutch plate. It's just raw cast iron. rz
  11. Nov 12, 2015 #10


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    The asker's native language is obviously not English, so the question and explanation is rather unfocused. The clutch has two positions, engaged and disengaged.
  12. Nov 12, 2015 #11


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    The engaged position allows torque to be transferred or coupled. In the disengaged position, the clutch disc likely touches the flywheel and/or pressure plate, but can not transfer torque. In actuality, it's just a minuscule drag.
  13. Nov 12, 2015 #12


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    P.S. I think the askers question might have been better asked as: HOW does the clutch disc become disengaged from the flywheel/pressure plate? The simple answer is the clutch disc becomes "unclamped" when the pressure plate is disengaged.
  14. Nov 12, 2015 #13


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    The diaphragm spring provides clamping pressures which keeps the clutch engaged. Your foot pressing on the pedal moves a fork which compresses the spring removing clamping pressure. The clutch plate doesn't have to move much, remember that friction is proportional to the Normal force not "displacement".
  15. Nov 13, 2015 #14


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    The typical arrangement in a car is a clutch shell with pressure plate, which is physically attached to the flywheel by bolts, and a friction plate, which is attached to the transmission on a splined shaft. The clutch shell has a bunch of stiff springs that press against the steel pressure plate, clamping the friction disk to the flywheel. I have taken apart many clutches on cars, but have never seen one that is raw cast iron. They are typically steel inside, with friction material riveted onto a thin steel disk.

    When you press the clutch pedal down, there is linkage that presses against the throwout bearing, releasing most of the pressure exerted by the pressure plate on the friction disk. This allows the flywheel to turn independent of the transmission input shaft, so you can come to a stop or shift gears.

    Motorcycle clutches are somewhat similar, but they usually have a clutch pack, a stack of interleaved friction disks and clutch disks (or "steels"). The steels have tabs or splines around their outer circumference, and spin as long as the motor is running. The friction disks have tabs on their inner circumferences, connecting them to the tranmission input shaft. When the clutch is engaged, a diaphragm or springs exert pressure on the whole stack, and the motorcycle moves forward.
  16. Nov 13, 2015 #15


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    I don't know if we are having a translation issue here or not; but, in English language and standard USA automotive terminology, the Flywheel and the diaphragm spring loaded Pressure Plate both have a metal contact face. The Clutch Plate is the floating center plate between those two components and always has a friction material (similar to that used on brake pads) bonded or riveted to both of its contact faces.

    If you question this, simply access any online automotive parts supplier website; or, automotive maintenance manual.
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