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I would like to use a pair of hydraulic pistons to raise and lower a surface that will hold about 150 lbs, and be capable of remaining in either fully extended or fully contracted modes for long periods of time. Should I mount them so that
A: expansion pushes the surface upwards, causing gravity to act as a compressing force,
B: contraction pulls the surface upwards, causing gravity to act as a tensional force?


Why? Or am I worrying about nothing with such a small load?

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I moved your picture inline; you should be able to do this yourself in the future now that you have more than 10 rep. See diy.stackexchange.com/privileges for more info... –  Niall C. May 4 '11 at 18:54
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3 Answers

up vote 4 down vote accepted

If you are dealing with hydraulics there is always a chance of leakage with time or catastrophic failure. For this reason I would suggest the following:

  • You should have a back up for holding the load in place if the hydraulics fail. This is quite often a simple pin that locks the piston in place in specialised systems but provided it is done adequately simple timber cribbing or similar may be sufficient.
  • In terms of how to set up the system I would set it up in such a way that if there is creep in the system with time and the load locks onto the secondary supporting system , you can still re-set the hydraulic system so that you can lift the load back of the secondary and safely lower it back into it's original position. You may find that how you mount it could mean that if you have a problem the system is locked in place and you will need another form of lifting system to unlock the hydraulics.

One of the problem with going for a tension systemn is that should it fail elements loaded in tension are more prone to fly long distances and cause more damage than those loaded in compression. One of the problems with a compression system is that the system could buckle if they are not positioned and supported correctly so I would worry less about the hydraulics and more about the lifting system as a whole.

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They are still working in compression - the only difference is the extra weight of the base, which hopefully is negligible compared to the load you are lifting.

Edit: Assuming you have a dual acting system - where hydraulic fluid pushes against either side of the piston (rather than just an air jack where atmospheric pressure pushes it back) then there is no difference - other than the weight of the outer cylinder.


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That doesn't match what I was thinking. Am I understanding it wrong, or did you misread what I meant? I've updated my question with a picture, but it's not embedded, since I don't have sufficient reputation. –  Anonymous May 4 '11 at 18:43
@anon what he is saying is that the cylinders always work with compression,it's just the position. So this is just a decision on where you want the cylinders on top or underneath, so it's up to you. Are you looking for a table or a rack, that's your decision. –  allindal May 4 '11 at 21:10
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From a usable space perspective A: is the better choice. You have all of the surface available for the load.

From an installation perspective it depends. If the surface is nearer the floor then A: would again be the better option, if it's nearer the ceiling then B: looks better (from the information available).

I'd repeat what Ian Turner says too - make sure that you have a means of locking the pistons in what ever position you choose (which may limit the flexibility of the system) so that if a piston fails the shelf doesn't move.

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