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,
or 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?
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
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.
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.
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.