Universal joints & thrust bearings

Discussion in 'Propulsion' started by Wayne Kendall, Jun 30, 2013.

  1. Wayne Kendall
    Joined: Aug 2012
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    Wayne Kendall Junior Member

    I have a jack shaft with universal joints on each end, one end coupled on the shaft and the other coupled on the transmission. Do I need some type of a thrust bearing between the propeller and the UJ? Someone told me that I could use 2 pillow block bearings. I have 1 1/2 inch shaft with a 135hp engine. I desperately need advise.

  2. tunnels

    tunnels Previous Member

    Show us your set up and what you have

    Most all marine transmissions have a built in thrust bearing in the rear end so that will absorb the thrust from the prop shaft but all the push is being transferred onto your transmission and so onto the transmission and engine mountings because we cant see what's there we don't know exactly what the set up is !!
    For the jack shaft of any length pushing onto universal joints is never a good deal at all and who ever set this up is a absolute pee brain !!, so yes you definitely need a good thrust bearing on the prop shaft and a bracket out onto the hull or engine barers !! remember all the thrust from the propeller is pushing on to this so has to be really robust and don't forget it has to take ""PUSH AND PULL "" when going forward and in reverse ,AND make sure it can be lubricated with a grease nipple or if its hard to get to a hp flexible hose can be fitted with a grease nipple places some where easy to reach !!
    Sent some pictures we can assist you more . D:p:p
  3. keysdisease
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    keysdisease Senior Member

    Why? Sounds a lot more complex than it needs to be. Why not just one shaft from the gear to the prop? Is there an angle transition that needs to be there? Would one shaft be too long? If so an intermediate bearing would take care of that.

    :?: Steve
  4. Petros
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    Petros Senior Member

    in most installations on a drive shaft with a u-joint at each end there has to be a spline shaft or slip joint to allow some in-and-out acton (plunge-allows the shaft to slightly change length as it rotates) to prevent binding of the u-joints. If you actually have constant velocity joints (CV joints) instead of universal joints, they already have the plunge action designed into the joints.

    This means there has to be some other way to take up the thrust from the propeller shaft, either at the hull pass-through, or as noted above, in the transmission. If you have no thrust bearings and the prop thrust is indeed transmitted through a fixed length driveshaft with a u-joint at each end you will have very short u-joint life, a lot of vibrations and noise transmitted though the hull, and if you create any harmonics in the system you could do serious damage to the hard parts and even throw a shaft. I have seen it in similar installations.

    Typically this arrangement is done with a spline shaft to allow plunge to not just allow misalignment of the input and output shaft, but also to allow the engine to be mounted on soft mounts to attenuate the engine vibrations from hull (similar condition in the automobiles and other road vehicles).

    we need to know more about this installation to know for sure. Also, it is not normal to describe an off-set u-jointed mounted drive shaft as a "jack shaft". A jack shaft is usually sometime parallel to but out of line with the main power shaft used to reduce the gear ratio. So your choice of words/terminology is either not consistent with my experience as an engineer, or I may be completely misunderstanding the configuration of your installation.

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    FAST FRED Senior Member

    One universal joint in a shaft system can absorb thrust ,

    Two must have a thrust bearing , as two can not .
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