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  • Hollow shaft stepper?

    Discussion in 'The main mechanical design forum' started by thomas, Mar 4, 2013.

    1. thomas

      thomas Member

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      Hello.
      I discovered this forum a few days ago, and I thought it could be the place to get some help.

      (First of all, please forgive my english and my poor scketching skills)

      For a little project, we will build a stacking of 3 wooden box (10x10x6 inches). one at the base that will be ground or wall mounted, and two other on the top that will have to rotate independantly.
      (Their axis of rotation could not be concentric)

      We have a couple of steppers that could do the job.
      The thing is that we need to keep the wires hidden in the "sculpture" that is why I 'm trying to create some holow shaft rotations.

      Controling the steppers is not a problem but I'm really new to mechanics.
      Even if this sound to be basic things, I would need an external advice to know how to achieve this.

      Here is a simple sketch. where I'm showing you my starting point.
      I have heard about harmonic drive that could offer an hollow shaft but it is far to expensive for our use.

      Do think I could achieve this with hardware such as pulleys, belts and strong bearings that could handle a radial weight (if this is wall mounted)

      thanks for any of your advices.

      [​IMG][/IMG]
       
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    3. bluerover

      bluerover Member

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      I would suggest installing both steppers in the base and using simple concentric shafting and toothed belt drives to transfer the motion to the second and third boxes thereby enable infinite rotation of the sculpture without the problem of dealing with wires. The intermediate bearings could be a simple ball race such as used for office chairs or lazy susans.
       
    4. PierArg

      PierArg Well-Known Member

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      Hi Thomas,
      I know you need answers but I have to ask you some questions :D

      What is the rotation speed of the wooden box? And the weight of the woden box?
      Are you sure that you don't need to build a frame (for example made in steel or aluminium) inside the lower box?
      I suppose the lower box is not strong enough to support the weight of the two others and the forces of the transmission system.
      Furthermore you need to fix the stepper somewhere otherwise it starts to rotate due to reaction torque couple. :)
       
    5. thomas

      thomas Member

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      thanks for your replies.
      @bluerover.
      My rotation axis won't be concentric, so I can't use this solution unfortunately.
      @PierArg.
      We do not need infinite rotation. when rorated at 360° , the box will go in reverse and so on.
      Weight is a reall issue. our prototypes are in wood, but we can make it out aluminium or plastic to get something lighter.
      steppers will be fixed inside each box of course ( forgive my sketch which was really basic)
      I thought the main problem was to find strong bearings that could handle radial forces and support all the boxes weight. Once these bearings found, I think strong stepper could do the job.
      I found something called slewing rings that could handle large radial forces:
      http://www.ritbearing.com/images/products/slewing-ring-bearings.jpg
      have your ever heard of it?
       
    6. PierArg

      PierArg Well-Known Member

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      Hi Thomas...I believe I begin to understand what you are going to realize.
      Can you tell me what is the weight of each box?
      And what about the round speed? How long it takes to complete 360 degrees?

      Pay attention about the bearings!!! They don't absorb the reaction forces but they just transmit them to the seats in which they are installed. So the woden box, in the bearings seats, are subject to forces.

      I think you can create a frame hidden in the first (lower) box wich support the other boxes and the reaction forces of the transmission mechanism.

      How do you think to supply the steppers? By an internal battery?
      Do you want to use only one stepper for all the boxes?
       
    7. thomas

      thomas Member

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      HI PierArg.
      Happy to see you trying to help me.

      If we make the box a bit smaller and out of plastic, they should weight around 1/2 kg I guess (without stepper in it)
      The speed does not need to be higher than 90°/second.

      I,now, understand the use of the frame needed in the basis. the wood was very strongly assembled, but plastic will be weeker.
      the controller board of the stepper wil be fed with wall power, maybe hidden in the fixed base box .

      I think I cannot use one stepper for the 2 rotating boxes on the tob of the base because their speed and rotation direction will be different. So we will use 2 steppers

      do you want me to create something more detailed with my beginner solidworks skills?
       
    8. PierArg

      PierArg Well-Known Member

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      You're welcome.

      Ok, I understand more things!

      So you want to create a sort of robot-arm made by 3 pieces (box). What is its application? I suppose it is a sort of marketing "totem" for an advertisement application!

      If you can write down a mock-up of your device I'd be very happy to help you.
      It is not important you do a solidworks model...but is sufficient just a simple hand drawing.

      If I was not very busy here at the office, I would have done it for you!
       
    9. thomas

      thomas Member

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      It is not something commercial as we are still student but it is kind of a moving plate on which little mockups will be shown. we want to have something more attractive than simple mockups on a table. Ours will be in motion!
      As axis of rotation are not concentric, actually it could be compared to a robot arm but with 2 articulations only.
      I will make a new sketch.

      Are you involved in mechanical design?
       
    10. PierArg

      PierArg Well-Known Member

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      I'm a mechanical engineer (I'm 28 years old...so I am only 5 years experienced ) and I work for a company that manufactures high pressure pumps.
       
    11. thomas

      thomas Member

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      hello again, here is a new quick sketch to show you how boxes and rotation axis will be arranged
      [​IMG]
       

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