Hamilton 751 Bearing replacement - Insulated?

Discussion in 'Jet Drives' started by ijw1234, Aug 28, 2009.

  1. ijw1234
    Joined: Jun 2006
    Posts: 3
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    Location: Dorset, UK

    ijw1234 New Member

    Hi all,

    I've finally got back to my project boat - 751 with a rover v8 engine :D
    I need to replace the main bearing(+housing) which i have purchased from Hamilton, at the princely sum of 450 pounds...However the inner-race of the bearing is not electrically isolated from the housing - according to the Hamilton workshop manual for the 750 this is BAD!

    Has thinking changed on this, as the laws of physics may have changed since the 70's when the jet was made - or is the bearing i have received defective?

    Cheers!
     
  2. anthony goodson
    Joined: Mar 2007
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    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    The outer ring of the bearing should be insulated from the casing ,and should arrive this way from the factory The electrical resistance measured between the shaft and the casing on an assembled jet with a DRY rear bearing should not be less than 1000 ohms
     
  3. ijw1234
    Joined: Jun 2006
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    Location: Dorset, UK

    ijw1234 New Member

    Got a reponse from hamilton saying that they no longer consider it neccesary to electrically isolate the shaft (as of 2004) - i have sent the bearing back and bought one from an automotive supplier (still a four point contact bearing!) for £36

    I have applied a film of epoxy to the outer face of the bearing and pressed it into the housing so it is electrically isolated - not sure what i think of hamiltons reponse...if components last 20% shorter lives, they sell more components...? why would they change their minds after 25 years?
     
  4. anthony goodson
    Joined: Mar 2007
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    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Yes I think yours is a common sense approach why take a chance on immersing a mixture of metals coupled together in salt water I would insulate every time.
     

  5. Noel Sans
    Joined: Mar 2010
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    Location: Madrid

    Noel Sans Junior Member

    Hamilton are correct that the shaft does not need to be isolated from the jet. Even on there new models of jets the shaft have brushes conecting to the bonding system.

    From experience if you isolate the jet shaft it can cause more problems. Remeber many engines are used from the car industry. The negative of senders, starter motors, etc run through the engine main body. This in turn can run down the shaft where it can cause problems as the electrons will try jumping to the stator vanes or isolated steel wear ring causing premature corrosion. Its much better for this current to run direct through the brushes down bonding system and to the anodes and battery.

    Also the 750 and 770 series were never designed to be in sea water continuosly although I have seen one where they mounted plenty of anodes on it and has never had a corrosion problem.
     
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