Can I cause damage to cylinders by pulling a vacuum?

[gregorybosma]

I'm making a hydraulic suspension system for a heavy truck (mining equipment hauler). Essentially, it's a trailing arm design with a hydraulic cylinder in the place of a usual spring/shock. As a safety feature, the cylinders have an on-off valve attached on the "extend" side of the cylinders to make sure hoses depressurize for servicing.

One of our techs shut this on-off valve and jacked up the frame of the truck. Magically, the cylinder extended without a way for fluid to fill the "extend" port! There's only one explanation- when you jack up the frame of the truck, the axle is hanging off the cylinder. The axle is so heavy we're pulling a vacuum in the cylinder (see attached picture). With the orientation I know for a fact the piston seal is wetted on both sides, but the O-rings and cartridge on the valve are dry and see a vacuum on one port.

Will this cavitation cause damage in a cylinder or a cartridge valve? If so, what failure modes can I expect from this?

 

[3DDave]

While cavitation is linked to damage, it isn't cavitation that causes it - it's the energetic collapse of the fluid into the cavity that does the damage. It's unlikely that any damage will be done unless the vehicle is allowed to fall from a great height, like off a cliff, to push the cylinder back

The usual concern with this case in hydraulics is that people who service them may have a mental model that doesn't include that a vacuum won't hold the cylinder up and they get crushed or mangled when they open the pressure-side thinking it won't move because the non-pressure side is closed off. Even in this case the hydraulics will usually be fine and just people or external machinery will take the damage.

Since your tech was surprised by this, I'd suggest finding them a hydraulics safety course to take before they are seriously injured by some other misunderstanding.

 

[hydtools]

I would expect no failure due to low pressure.

Ted

 

[LittleInch]

I can't workout how what looks like a hydraulic cyclinder is acting like suspension unless you've got some gas in the system somewhere, but that's not the issue here I suppose.

Issues for a pure vacuum are low, but the hydraulic oil will probably be vapourising a little and hence when the load goes back on you will have some gas in the system, or the valve or cylinder valves leak because they are not expecting negative pressure. Or air bypasses it and hence you end up with air in the system you don't expect.

If you have a small leak you could get some damage from high flowrate air passing small point on the valve or cylinder seal, but seems unlikely to be an issue unless you do it lots of times.

Oh and it's not magic - it's physics. ;0)

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