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Tension/compression spring thing

  1. Feb 10, 2017 #1

    MacLaddy

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    Gold Member

    Hello folks,

    I'm trying to find a spring that can act in both tension and compression, and return to a neutral (or zero) location when released.
    What I'm thinking of is basically a shock, but without the damper. It needs to be a nearly instantaneous return. Google search is giving me nothing but automotive shocks, and I can't find any type of combination spring.

    Anybody willing to throw me a few search terms for products? It's probably something simple, but I'm at a loss. It also needs to be fairly small. Maybe 1 to 1.5 inches of throw in each direction, and no more than 30lbf.

    Thanks,
    Mac
     
  2. jcsd
  3. Feb 10, 2017 #2

    Nidum

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    Two springs in series pre-tensioned . Fix at the ends and apply load at the centre connection .

    Actually a single spring will do what you want if you arrange a proper connection at the active end and put it in a tube to keep it stable .
     
    Last edited: Feb 10, 2017
  4. Feb 10, 2017 #3

    berkeman

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    What is the force application like? If the force is applied and then released, the spring will oscillate about its resting length. It seems like you would want some critical damping mechanism to engage when the force is released, and then disengage once the spring is back at its equilibrium length...
     
  5. Feb 11, 2017 #4

    MacLaddy

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    That isn't a bad idea. Perhaps just a piece of PVC tube? I'm going to assume that it would be nonlinear?

    The force ultimately is to return a leverset to the neutral position. The lever pushes and pulls, so I imagine it would oscillate some. There will be a motor attached in series with the spring resisting the motion, so I imagine it would act like a damper. However, if you know of any shock/damper...well...shocks, that can achieve what I am looking for, then I would love to hear about it.

    Thanks.
    Mac
     
  6. Feb 12, 2017 #5

    Baluncore

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    Here is a design for a bi-directional, linear, shock limiter.
    The force needed to start moving is set by the zero length of the compression spring compared with the distance between disks. That solidly fixes the zero position of the system. Forces are bi-directional and symmetrical.

    An outer tube has two internal walls, (or internal circlips in grooves), spaced by more than the required movement. An inner rod has two disks, ( or external circlips in grooves), attached with the same spacing as the disks above. Between the disks on the rod are two free floating cups that act as bushings to align the tube and rod. The ends of a compression spring rest in the cups. Those cups are limited in movement by either the tube internal walls or the disks on the rod.

    shock-limiter.png
     
  7. Feb 14, 2017 #6

    Baluncore

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    If the lever has two close knife edge or pin fulcrums, with a tension spring between them, then the lever system will return to the neutral position from either side when released. If the spring attachment point is off-centre, the return forces will have different magnitudes either side of centre.
     
  8. Feb 23, 2017 #7

    MacLaddy

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    This is great, thank you. That is exactly the solution we are looking for. Does this type of spring assembly have a name? I could probably manufacture one pretty easily, but purchasing one would be a much faster option.
     
  9. Feb 23, 2017 #8

    Baluncore

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    Unfortunately it is the type of mechanism that is simply built into a product rather than purchased off the shelf to be bolted on.
    I could give you a diagram of an easy to build unit if I knew better how your lever was mounted, actuated and connected.
     
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