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  • Clutch stiffness and hysteresis - Role in gearbox NVH

    Discussion in 'The main mechanical design forum' started by k.udhay, Nov 29, 2014.

    1. k.udhay

      k.udhay Member

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      Hi,
      There is a rattling noise in one of the automobile passenger car gearboxes that we designed. After few experiments and observations, it was found that the noise is because of incorrect clutch stiffness and hysteresis behavior. I have got some fundamental questions about both:
      1. Clutch stiffness Vs rattling:

      • I understand that higher the stiffness of clutch, lower the relative displacement between crank shaft and transmission shaft for a particular torque.
      • All infiltrated fluctuations to the clutch will be absorbed to an extent. And this is done by the springs in the clutch. But, won't the springs start vibrating (as it is an elastic body and not a dampening member) after they absorb this fluctuating forces? What is better for rattling - High stiffness clutches or low stiffness clutches?
      2. Clutch hysteresis Vs rattling:

      [​IMG][/SIZE][/IMG]

      • The attachment shows the hysteresis values of the clutch that I am discussing here about.
      • The unit is NM. What exactly does this hysteresis torque physically mean?
      • How is hysteresis responsible for gearbox rattle?
      Thanks a lot!
       
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    3. K.I.S.S.

      K.I.S.S. Well-Known Member

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      Hi,
      Your question is very wide and not expressed very well - Hysteritic values (in an instance such as this) are based upon previous loads and conditions placed upon a certain component that effect its current state - the values you provide make no mention of usage cycles at a specific torque for example, thus making it fairly meaningless.
      As you make no mention of the fact that this may be either a brand new gearbox or one that has experienced any form of specified load and duration testing, it's hard to provide a useful answer other than to say that you probably shouldn't blame the spring (if it's been correctly designed and made) in this instance (and I spend much of my life blaming springs for things that go wrong...)
      The problem will probably occur due to the combination of the friction materials used in the clutch relative to the radius and RPM at which the friction is applied. There is also the possibility that the design in itself is faulty, allowing for a unwanted vibration at a mid load torque.
       
    4. k.udhay

      k.udhay Member

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      Hi K.I.S.S.,

      This is the full hysteresis table that I find in the drawing.
      [​IMG]

      I am not so interested in discussing to solve the problems of this case. I would request you to take this as just an example to explain the effects of hysteresis and stiffness on gearbox NVH.

      First of all, I would like to know what this hysteresis means. It is given in N-m and hence I assume this to be a hysteresis torque. Does it mean the loss of torque transfer between Engine torque and clutch output torque? What is the reason for this loss, if so?

      And, does it become a reason for an oscillating clutch torque output?

      Similarly, will high clutch stiffness help me controlling rattling or otherwise?

      Thank you very much.
       
    5. CTH1313

      CTH1313 New Member

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      Vibrations are due to the fact that your system isn't tuned to minimized vibrations for the applied torques, which are resultants of the masses of the elements chosen to make up your gear box and their angular speed. You have a spring which forces (radially) two elements to push together. One rotating and one fixed (but able to rotate). These are the initial conditions of the system. You are concerned with steady state behavior, or what is happening once the clutch is engaged. The small differences in cog size and the imperfection of the gear's circular nature cause radial displacements given by the force of the clutch spring and the restorative forces of the gears themselves. These radial displacements are the source of your vibration (radial vibration, at least) and should be maximally amplified at some value of RPM for each gear. This is a two spring system with the spring rate, K, of the clutch spring being of importance and the stiffness, area and second inertial moment of the gear (in question), the flywheel, and the crank assembly. So the vibration is in essence, do to the lag between input and output of torque.

      Your hysteresis is due to mechanical lag and interfering he type is rate-dependent, because you have an input Torque, which results in an output vibration {say, Y(t)}. Y(t) depends on the latent (internal, can't be seen) vibration of the interconnecting elements of the system (gears, flywheel, springs) AS THEY INTERACT with the DELAYS caused by the mechanical lag itself. This is what creates the hysteresis - the mechanical lag between input Torque and Output Displacement of your gears. Think of the gears, clutch, flywheel and cam shaft being one equivalent spring value. This "total spring" exhibits standing wave vibrations WITHIN (in this case, actually between) itself. So you can think, the flywheel and the cam are vibrating at some value, (z(t), say), which affects the output of the vibration of the flywheel and the clutch elements (u(t), say) (springs, diaphragm spring, etc.), which affects the output of the vibration of the clutch elements and the gear in question (which is the original Y(t) that I was talking about earlier - the vibration that you "feel" "hear" and could "see" if you measured it with the correct devices (possibly even your eyeball haha). SO, in conclusion the displacement functions z(t) and u(t) depend on the initial position of the flywheel and clutch and the input torque. The displacement functions z(t) and u(t) operate differently because of the different equivalent stiffnesses of the cam-flywheel and flywheel-clutch link-assemblies. Therefore z(t) and u(t) constructively & destructively interfere with one another to produce fluctuating values of Y(t) that range between some minimum and maximum value, which corresponds to the range of your hysteresis. Get it? haha your system is just complicated in actuality.
       
    6. CTH1313

      CTH1313 New Member

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      If you increase the stiffness of the clutch it may increase the delay time between input torque and output vibration or decrease it. Increasing the stiffness will definitely limit the entire assemblies ability to "displace" from some initial positional state in any direction, which is why it would make sense "off the bat" to increase stiffness to decrease output vibration.

      Usually increases in delay times between input and output will create a lower freqeuncy of undesired vibrations. 0.1 mm of vibration (Y(t) = 0.1 mm) is OKAY, say, and Y(t) = 1 mm is NOT OKAY. Therefore, IF increasing the stiffness of your clutch spring INCREASES delay times of transfer of input torque to output vibration, and latent (internal) input vibration to output vibration, then you should see a smaller amplitude of vibration (due to K increase) and the maximum vibration amplitude should occur less frequently​.
       

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