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Joint Name?

  1. Sep 3, 2010 #1
    I am trying to find the name of a particular type of joint--I assume it has a name since it is pretty common I think.

    Imagine two shafts (A & B) joined at 90 degrees to one another with a bevel gear. With both shafts in a constant x-y plane, rotating shaft A will cause shaft B to rotate. Now, suppose that I want to rotate shaft B through the x-z plane with the center of rotation at the bevel gear intersection. With shaft A stationary, shaft B will rotate about its axis as the shaft B assembly rotates around the bevel gear.

    I would like shaft B to not rotate around its axis even though the shaft B assembly rotates around the bevel gear. So, I want rotation of shaft A to be the only thing that effects shaft B rotation, even though the shaft B assembly is rotating around the bevel gear.

    Does this make sense?

    I assume this must be a common need and their may be a joint component one can buy to accomplish this.

    Thanks for your thoughts,
    Greg Laird
  2. jcsd
  3. Sep 3, 2010 #2


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    Welcome to PF, Glaird.
    If I understand your requirements correctly, the only thing that I can think of would be to have some kind of over-running clutch on shaft "B". I'm probably missing something, though.
  4. Sep 3, 2010 #3
    Thanks for your response Danger.

    I probably wasn't clear. Maybe this will help.

    Imagine a robot body with a robotic arm that has a shoulder joint and a shaft for an arm. I want to be able to rotate the arm as one would as if swinging a bowling ball--forward and backward. There would need to be a shoulder joint for this.

    I also want to be able to rotate the arm on the arms long axis. This is separate and from and perpendicular to the shoulder joint rotation. I want to have the arm long axis rotation motor in the robots body. Now, you can imagine if I had a simple bevel gear transmitting the arm long axis rotation force to the arm from the robot's body, the arm would rotate about its long axis as the whole arm assembly swings forward and backward on the body. That is NOT what I want. I want the arm to not rotate even though the arm assembly swings back and forth (or completely around for that matter).

    The reason for this requirement is that I want both the motor that will rotate the entire arm assembly and the motor that will rotate the arm about the arms long axis to be located in the robot's body. I am sure there is some sort of gearing assembly that will allow the whole arm assembly to rotate back and forth without rotating the arm about the arms long axis. I imagine this is a common problem in robotics or power transmission.

    Does this make sense now?

    Thanks again for your reply.
    Greg Laird
  5. Sep 4, 2010 #4
    actually ..
    i am a new user in here...
    and don'T know how to use it...

    mechanical engineering is a subject that can make this world to be simple..
  6. Sep 4, 2010 #5


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    Yes, it does, and you're welcome. I still don't' know whether or not I can help you, though. My personal approach to your situation would be to use flex-cable drives to the gears located in the joints. Those are pretty much immune to the effects of intermediate joint displacements. Again, though, I'm no expert.
  7. Sep 4, 2010 #6
    I did think of flex-cable drives but since the joint will always be 90 degrees I thought some sort of gear box would provide a more efficient coupling. And I thought this may be something that has been done many times before. But, maybe it is less common than I thought.
  8. Sep 5, 2010 #7


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    I expect that it's probably fairly common in robotics, but that isn't a field that I'm very familiar with. Perhaps some Googling in that subject might be of assistance.
  9. Sep 5, 2010 #8
    Well, I have been googling around for the functionality but I haven't found anything--it is hard to google for the concept. That was why I was looking for the name of the joint--I thought this would help me find something.

    The joint is some sort of differential gear assembly I would guess. I was going to try to draw this up but I am just not that familiar with all the sorts of gear train options. I have even looked around for a a handbook of gear trains to get an idea how one designs these things. But I have found nothing yet. Gear box design seems like quite an art--one would think there would be a book or two on this.
  10. Sep 6, 2010 #9


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    Now I'm going nuts because I'm pretty sure that I have a great site for gearing in my "favourites" folder, but I can't find it (the peril of having a couple of hundred favourites). I would suggest, in the meantime, that you consult the "gears" section of "How Stuff Works". The answer to your problem might be there, and if not you will at least be given several links to the subject.
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