Propeller shaft bearings, whirling vibration

Discussion in 'Powerboats' started by christos, May 7, 2004.

  1. christos
    Joined: Jan 2002
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    Location: Athens

    christos Junior Member

    Any information or experience on propeller shaft bearings selection?
    This is a twin screw, 57 meter yacht, shaft diameter is 6inch, 600 shaft rpm.
    Shafts are about 13 meters each ( in 3 parts joined via SKF couplings), supported on a V bracket (aftermost) an intermediate bracket and the sterntube.

    We have two bearings (THORDON) on the sterntube, each of 300 mm length,
    one bearing on the intermediate bracket (305mm) and one on the V bracket (610mm length). Same arrangement for both shafts.

    We had 2 incidents of cracking of the intermediate bearing, extensive damaging of the sterntube inboard THORDON bearing.

    The whole system alignment is been re-assessed, whirling vibration calculations are being carried out and we intend to change the existing bearings with VESCONITE type.

    Any more suggestions for improvement? any recommendations for the proper selection of bearings' type? should we also consider the JOHNSON rubber bearings?

  2. MDV
    Joined: Feb 2002
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    Location: Singapore

    MDV Junior Member

    THORDON is recognised as a very good material, and doubt thats were the problem is. I'd say you have an alignment problem. Has any work been done to the shafts prior to these failures, l or is this a problem from the start?

    What about the propellers? any damage? are they properly balanced?

  3. christos
    Joined: Jan 2002
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    Location: Athens

    christos Junior Member

    Michael thanks for the reply,

    Propellers are checked and are well balanced, no damages. Shafts are in good order. Sure there is an alignment problem, which might get gets exagereted while the vessel is afloat, since the aft hull part (about 10 meters) was added in a later stage. We are now re-aligning the complete system (Aft V Bracket, the intermediate bracket and the stern tube, in situ boring will be carried out and final engine - gear box - shaft alignement will be carried out afloat.

    I do not focus the problem on the bearings, agree that the Thordon is a well known and very good bearing but I was thinking if anybody had experience with a type of bearing that may absorb/dampen some vibration. The shaft lines are too long, this is 3-part shaft system, 6-inch in diameter, shaft parts coupled together with SKF couplings. Due to the incresed shaft length, we should experience more effects due to whirling. Whirling calulations are now being carried out.

    This is an all aluminium vessel.

    Any more ideas? any similar problems?

  4. nevd
    Joined: Nov 2003
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    Location: Australia

    nevd Junior Member

    Bearing failures

    Hello Christos,

    Have you done any failure analysis of the bearing - if the housing failed it may be possible to determine direction and nature of the forces involved which could provide some leads. You could also use vibration analysis as this is now very powerful.

    I suspect you are doing the right thing in doing the final alignment with the hull floating under load as that should decrease the effect of any hull flexing. Is there any chance the engine and bearers are deflecting under driveline torque?

    You could also arrange for strain gauges to be placed on the shafts (and or bearing housings) and the data transmitted to remote recording equipment while the boat is underway - I have seen this reveal unexpected torsional harmonics in large drives. This will be costly, but is a certain way of finding the problem.


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