Caterpillar cam gears

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by petefan4000, Dec 28, 2020.

  1. petefan4000

    petefan4000 Light Load Member

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    Anyone know why, on many Cat engines, the camshaft drive gear has those metal cylindrical thingies that roll around in those holes? What purpose do those bores and rollers serve?
     
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  3. pushbroom

    pushbroom Road Train Member

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  4. petefan4000

    petefan4000 Light Load Member

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    It makes sense, as the injector lobes on the camshaft is what moves the injector plungers to generate the pressure needed to fire the injectors. For each injector that's a lot of force, which means a lot of torque variation, so I can see how torsional vibration becomes an issue.
     
  5. petefan4000

    petefan4000 Light Load Member

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    @pushbroom Also, where in SIS can you pull this info up? I'm talking about the stuff shown in your screenshot.
     
    Last edited: Jan 13, 2021
  6. pushbroom

    pushbroom Road Train Member

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    Documentation tab then Systems Operation. It has a run down of all the functions of parts.
     
  7. petefan4000

    petefan4000 Light Load Member

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    Sorry I'm bringing up a year old post, but I finally figured out what the rollers are for:

    The bores and rollers in the cam gears form what's called a centrifugal pendulum absorber.

    An absorber is a device that reduces torsional vibration at a particular frequency or range of frequencies, and in the case of a pendulum absorber, that resonant frequency is directly proportional to the rotational speed. The ratio between the absorber's natural (resonant) frequency and the shaft speed is known as the order of the tuning, so a third-order pendulum absorber would have a resonant frequency that is always three times the shaft speed. This is great because you don't dissipate energy as heat like a true damper does, but the absorber also works over a very wide range of speeds.

    In the case of a four-stroke even-firing six-cylinder engine, you have three cylinders firing per crankshaft revolution, and the dominant exciting component of the engine's torque signature is three times the engine speed. There are also other harmonics as well (6th order, 9th order, et cetera) but the 3 and 6 order vibrations are the most problematic.

    There are circular-path, cycloidal, and bifilar (two-point suspension) absorbers being used now. The rollers in the bores of Cat's camshaft gears form a circular path absorber.

    The mathematics behind the pendulum absorber are very clever. Put simply, if you want to attenuate an excitation on a shaft of order N, the distance between the pivot point and the shaft centreline needs to be N^2 + 1. So on an L6 truck engine, if you wanted to use pendulum absorbers to reduce the "lumpiness" of the engine's torque output, you need the absorbers' pivot radius to be 10 times smaller (3^2 + 1) than the distance between the pivots and the crank centreline.

    An absorber on the cam gear will have much smaller clearances because the camshaft's dominant excitation is of order 6, which means the distance between the rollers' centre of travel and the camshaft centreline needs to be 37 times that of the rollers' radius of travel.
     
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