I'm tasked with designing a lift system for a frame that goes around the outside of a 12" x 18" rectangular structure.  Due to budgetary constraints, we don't have many choices and a pneumatic system is what we tentatively settled on.

The preliminary design has four air cylinders, one at each corner, and they are plumbed through a common manifold to a single control valve.  Although the frame is guided with linear bearings at each corner, I'm still worried that one of the cylinders will get ahead of the others and cause the frame to seize.

After a few hours with Google and looking through several pneumatic handbooks, all I could come up with is using a pressure regulator or a flow control valve on each cylinder's inlet.

What is the accepted method for synchronizing air cylinders?

You shold look at hydraulic system, pneumatic will give you problems in this application, as you will have an "air spring" effect.
Synchronizing air cylinders is done by means of Solenoid operated valves (in this case 3/2 way),these will either allow air to flow into the cylinders at the same moment, or allow the air to exhaust at same moment (in neither case do you have the assurance that cylinders speeds will be exactly the same.
The same principle works for hydraulic (without the boucing up&down effect).

I agree strongly with you and Itascot both. I doubt that pneumatic will be that much cheaper, if at all, and it will probalby only contribute to higher risk and larger control problems.

You leave too sketchy details to advice (weight to be lifted, how many cylinders, time for movement or repeated or onetime movement), but for most purposes readily constructed, comercially available and cheap oil-hydraulic systems exists.

What you actually have to do is to control the speed of rising of each single cylinder, eg equal volume and equal pressure in each cylinder. With air as compressible mediom you will have a much larger effect of any unevenly distributed weight than for hydraulic oil. This will be minimized with hydraulic oil as incompressible medium.

It is unlikely that the weight will be evenly distributed, and very probably give, as you fear, tilting of the platform with air. This will lead to a necessity of measuring and compensating for each cylinder separately


One can, of course, hope that a presssure and volume equalizing and interconnection between cylinders will give the necessary accuracy.

As a purly academic exercise, absolutely not recommended (I doubt it will work properly) :

For air cylinders you will use double acting cylinders in all cases. For this you need 5/2 solenoid valves (ports 1 for air in, working port 2 to lift cylinder, port 4 to drop and return ports respectively 3 and 5 for return air from each side of the cylinder.)

If you use high-quality, or direct acting solenoid valves, you could try to equalize the pressure for all cylinders by controlling the air input to a common feed stock, and likewise control air return pressure out at a separately interconnected feed stock connected to all return ports. The solenoid valves must be constructed to allow this throttling balance of pressure in and out. (This could be a high risk for breakdown of lifting effect if not constructed for the purpose.)

(The upside chambers of the air cyllinders need to be prepressurised before you start lifting from downside chambers).

The other way is to feed each cylinder by proportional solenoide valves, electronically measure position and compensate electronically controlled for each single cylinder. I believe something like this has been done for theater stages, to speedily control lifting, sinking and all-angle tilting - but the cost!

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