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bensen rotor head main bolt size

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  • bensen rotor head main bolt size

    Hi all ,as I was ready to mount the rotor head to the frame I discovered a problem which I better ask for help before going further ,the main bolt going true the bearing is 1/2 dia ,but it looks like it is too long ,my rear stop don't touch where it should and it touch at the bolt ,the rotor head has the square hollow tube where the yoke it self act as stop where it meets the solid insert in the pitch hollow bar ,this way I'm loosing almost 5+ degrees of travel ,I'm not sure what should be the right length of the bolt ,I was thinking to put there shorter bolt ,the one I removed measures 2 3/4 in grip length ,other option would be to cut the 1/8 of the bolt length ,I'm not sure if you guys get the idea what I'm talking about but ill try to post few pictures .
    I wonder if someone here had similar problem .
    When I'm at it I'm going to replace the main bearing ,I found this mrc on ebay ,it has no marks of any loading slot but as I found out here it would be probably ok to insert it either way .
    I'm also looking for the source of the big black plastic cover washer .
    Thank you
    Attached Files

  • #2
    If that is a Benson rotorhead it sure looks different then the one on my Benson. I will post pictures of mine here tomorrow along with bolt measurements


    • #3
      Did that head use to have a ring gear attached for a prerotator? If it did, the bolt would have had to be changed to a longer one to accommodate for the thickness of the ring gear. If the ring gear was later removed, the bolt would be too long.

      Most everybody I know has switched away from the hollow bar to a solid bar. I'd be Very leery of using a hollow one, especially one where the inside plug didn't extend well beyond the edges of the milled pocket in the hollow tube. I also wouldn't cut the bolt, I'd either get the correct length, Preferably, or use a 1/8" spacer washer instead of cutting it if I couldn't get one the right length. Getting a solid torque bar should be strongly considered, it may solve the bolt length problem at the same time.

      Lastly, the plastic cover washer needs to be carefully checked to insure it cannot, under any circumstances, touch the rotating part of the head. If it does, it will put a rotating force on the bolt that could potentially unscrew it.
      Last edited by Jazzenjohn; 01-11-2018, 07:58 AM.
      "Nothing screams poor workmanship like wrinkles in the duct tape!"
      All opinions are my own, I've been wrong before and I'll be wrong again. Feel free to correct me if I am.
      PRA# 40294


      • #4
        Bensen claimed that the hollow torque tube was adequate. The Bay State (Mass.) PRA chapter did a series of destructive tests decades ago that tend to prove that this claim is correct, as to ordinary flight loads. Their tests showed some vulnerability to fatigue failure in the face of (1) damage to the tube from rotors with excess vibration (whether from taxiing on rough terrain or maladjustment) or (2) groundstrike damage.

        Bensen claimed that the weaker hollow tube provided the gyro with the equivalent of "roll bar" protection, because the rotor would shear off the tube in a ground strike.

        This, IMHO, is hooey. We do not want rotors detaching during a rollover. The rotor is least likely to hit the pilot if it's still attached to the aircraft. Once it's free, it's more likely to strike the pilot or a bystander, which can turn a mere rollover into a fatality.

        I thought I'd bring up this old bit of lore, if only to debunk it.


        • #5
          I donít remember that plastic cover, most of the heads that I have seen had a black steel washer that is very important as it keeps the bearing from sliding up over the aluminum center piece leaving you to find your way to the planets surface without a rotor, you will still reach the surface but in an uncontrollable manner with the rotor arriving a bit later, you really need to have a knowledgeable carbon based life form check out the head for you befor flight.


          • #6
            Some heads (my old Air Command, for example) had a black plastic washer stuck onto the head of the bolt (above the structural washer that Phantom mentions). Its intended purpose was apparently to keep contamination out of the bearing, which was not sealed.

            A sealed bearing is advisable. though.