Difference between pneumatic and hydraulic cylinders 1

[Electrojim]

What is the fundamental difference between the construction of pneumatic and hydraulic cylinders? I sense that pneumatic systems respond quickly and hydraulic ones are capable of much greater thrust over a generally-longer actuating time. But pneumatic cylinders are very inexpensive and hydraulic ones are not. I have an positioning application requiring the precision of hydraulics; that is, a system where the fluid cannot be compressed, but a budget that favors a pneumatic cylinder. Can a pneumatic cylinder be used with hydraulic fluid (oil, brake fluid, water... whatever) as long as the pressure behind the fluid is not great? 50psi would do the job just fine; it's four inches of piston travel over the period of a half an hour or so, and I was going to pressurize one side of a cylinder and let the fluid from the other side leak out through a needle valve. Any tips would be most welcome.

 

[hydtools]

Fundamental difference? Design pressure. Pneumatic, 100psi; hydraulic, 2000psi.

Ted

 

[Electrojim]

Thanks, Ted,

That's kinda what I figured. My main worry is how the construction might differ that would make using a pneumatic cylinder in a hydraulic application a bad idea because of what these cylinders are like inside. My pressure would be under 100 psi, but would hydraulic fluid be bad news for whatever O-ring or other seal might be used in a pneumatic cylinder? Also, are pneumatic cylinders made to as rigid specifications? I mean, if an air cylinder leaks a bit, no big deal usually. But if hydraulic fluid seeps out around the seals, that would not be good.

 

[hydtools]

Now you are moving beyond fundamental. Your concerns will be fluid compatability of seal materials and preload for low pressure sealing of fluid.

Ted

 

[zdas04]

Oddly enough I've done this. We had (air-derivative) oil-flooded screw compressors with turn valves (unloaders). The skids came from the factory with pneumatic actuators to use compressor gas discharge pressure to position the turn valves. These never did work very well for a lot of reasons and we decided to change to screw oil actuation. What a mess. None of the seals lasted a week, compatible replacements were not readily available, supposedly compatible seals and o-rings weren't. After a couple of weeks we pulled the pneumatic cylinders and installed hydraulic cylinders. I still think it could have worked, but the price (i.e., improperly operating compressors that either wouldn't unload or would unload at inappropriate times) was just too high to continue the experiment.

For us the big deal was seal compatibility. For the next project it might have been pressure and so forth. If I was starting from scratch, I'd pony up the money for hydraulic cylinders out of the box.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[hydtools]

Electrojim, buna-n o-rings and PTFE seals are compatible with most oils.
There will be some oil carryover on the cylinder rod as most rod seals do not wipe the rod powder-dry. A film of oil lubricates the rod and seal.

Ted

 

[Electrojim]

Thanks very much, guys. I'm going to give this a try keeping pressures low. One more question, and again pardon my ignorance in this area. Is the oil used in hydraulic systems an important consideration when it's a rather bonehead, low pressure application? Because of the very slow action I'm seeking here, I was figuring on using a very viscous oil, perhaps even a heavyweight motor oil. Assuming buna-N O-rings and PTFE seals, would that be wise or compatible, or is there a viscous hydraulic fluid that would be a better choice and readily available?

 

[hydtools]

Since you have an adjustable needle valve, start with a relatively low viscosity oil like ATF, automatic transmission fluid.

Ted