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  1. #31
    Join Date
    Jun 2010
    Posts
    948

    Default Re: Bow Thruster Problem Advice sought

    Quote Originally Posted by AD23 View Post
    The roll pin cannot be accessed without taking the boat out of the water and dismantling the bow thruster's gear box. My bow thruster has 2 roll pins (one on the prop shaft and one on the vertical shaft) and their job is to secure the pinion gears on the shafts. Because they are so difficult to access, they are definitely NOT the equivalent of the shear pins!!
    The 2 woodruff keys (I think they are identical in size, shape etc) play the same role as shear pins on other bow thrusters. One of them connects the prop to the prop shaft and is under water. The other one connects the motor to the vertical shaft and can be accessed from inside the boat without taking it out of the water. These woodruff keys should shear (hopefully the one inside the boat!) if something fouls the prop as you describe.
    Unfortunately I did not get any diagram of my bow thruster - such a diagram would have been very useful.
    I suggest using Google to get images and descriptions etc of "roll pins" and "woodruff keys".
    BTW I am a layman too and never heard of a "roll pin" until I opened the bow thruster gear box with the help of a more knowledgeable friend :-)
    Woodruff keys are not roll or shear pins and are not designed to fulfil the function of shearing as they are merely a method of attaching a parallel (or tapered) shaft to another bore of the same shape and transmitting the full power from the shaft to the bore, often with a lot of contingency.

    Woodruff keys seldom shear and it is mainly the shaft keyway into which the key sits which gives way and usually this is by wearing away the outer edges of the shaft.
    I am old and wise because i was young and stupid.

  2. #32
    Join Date
    May 2018
    Posts
    13

    Default Re: Bow Thruster Problem Advice sought

    Thank you for that info - I just assumed that the woodruff key would shear because I can see no other "shearing mechanism" on the bow thruster. Perhaps it doesn't have one?
    BTW the bow thruster has continued to function perfectly since I put the boat back in the water in April this year - happy days :-)

  3. #33
    Join Date
    Mar 2006
    Location
    Manchester
    Posts
    16,923

    Default Re: Bow Thruster Problem Advice sought

    Quote Originally Posted by Assassin View Post
    Woodruff keys are not roll or shear pins and are not designed to fulfil the function of shearing as they are merely a method of attaching a parallel (or tapered) shaft to another bore of the same shape and transmitting the full power from the shaft to the bore, often with a lot of contingency.

    Woodruff keys seldom shear and it is mainly the shaft keyway into which the key sits which gives way and usually this is by wearing away the outer edges of the shaft.
    Then very poor design if the pins that are designed to shear, or are likely to shear, require a boat to be lifted to replace them.

    I dont think this is the first time I have heard of this type of thing and, presumably, a bowthruster owner only discovers it the first time it happens.
    Narrowboating From Stretford!!

  4. #34
    Join Date
    May 2018
    Posts
    13

    Default Re: Bow Thruster Problem Advice sought

    Wikipedia suggests that woodruff keys may be used as shear keys - I am not sufficiently qualified to know whether or not this is true!!

  5. #35
    Join Date
    May 2009
    Location
    East Coast
    Posts
    362

    Default Re: Bow Thruster Problem Advice sought

    A Woodruff key is a shear key, in the sense that it transmits shaft torque by shear stresses across the key. On tapered shafts the interference of the tapers would probably transmit most of the torque, leaving the key to make sure the shaft is locked to the bore. I don't think one would normally be used as a torque limiting device by being designed to fail, as the precise form of the failure would be much harder to predict than that of a shear pin.

  6. #36
    Join Date
    Apr 2015
    Location
    Staffordshire / Torquay
    Posts
    50

    Default Re: Bow Thruster Problem Advice sought

    Don't believe everything you read on there, most times you shear a wood ruff key it wrecks the shaft and the boss/sleeve, shear pins are usually made of a softer material .

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