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Needing Basic Engineering Help

  1. Jan 5, 2008 #1
    So the general idea is this:
    I am in the process of working out a contraption that can extend and retract a blade in and out of its sheath. It is very similar to a switchblade knive, but on a larger scale. I have come up with a design (which for patent and invention reasons I won't mention), however this proves to be quite a meticulous process.
    I project the final project to have 1 to 1 1/2 foot blades protruding from their sheaths, and while I can manage on having them extend (but any ideas to better the product are appreciated), I am in need of a way to, with equal force, have the blade return to its sheath.
    I do not know if I've made myself clear as to what it is that I am trying to achieve. If you have any questions concerning this, I would be more than happy to help in any way I can. That you to all who help make this possible.
  2. jcsd
  3. Jan 5, 2008 #2


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    What are these things attached to? Do you have electrical-pneumatic-hydraulic power available? How large are these sheaths? How fast do they need to extend/retract?

    It is pretty tough to come up with ideas when you don't know the constraints that you have to work with.
  4. Jan 5, 2008 #3


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    Use a hydraulic cylinder. If you want the same force, then make sure the rod end annulus has more pressure (it will depend upon the reduction in area due to the rod).

    I'm sure there are other ways but that's the first that comes to mind.

  5. Jan 5, 2008 #4


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    I was thinking either hydraulic or a plain mechanical linkage. However, there is no way to know what would be acceptable until we find out what the constraints of the design are.
  6. Jan 5, 2008 #5


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    A “switchblade knife” works because there is a storage of energy in the form of a compressed spring which is released by a latching mechanism. The stored energy in the spring is then converted to kinetic energy and heat due to friction as the blade extends, then converted to heat and sound as the blade stops its motion at the end of its travel. To get the blade to retract, the blade can either be pushed closed by the user, or the button can be used to aid in compressing the spring, reversing the blade. In this case, the energy required to recompress the spring is obtained by the motion of the button. The point here is that there are no mechanisms which don’t require energy input. Such a mechanism is also called a perpetual motion machine which obviously is an impossibility.

    In designing a mechanism which will extend and contract, you have to consider what amount of energy is needed to perform this task. In the case of a reciprocating piston pump for example, the amount of energy is not negligable because the energy goes into increasing a fluid’s pressure. On the other hand, the extension and retraction of something like a switchblade only requires a very small amount of energy to overcome the loss of energy due to friction and the impact as it ends the stroke.

    If you can explain what your mechanism is intended to do, it would be easier to understand the amount of energy and force needed to make this blade go in and out.
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