Hydraulic and Pneumatic Sealing in Shock Absorber

[Ryan95]

Good day,

I am a junior aeronautical design engineer working on an oleopneumatic shock absorber for aircraft landing gear. I've attached a simple diagram (omits a lot of detail so that only what I am referring to is shown) to the post to show the layout of the problem. The shock absorber consists of a cylinder, a piston, and a floating piston (all the other parts, like valves etc. are ignored here). A gas spring at the top - a chamber of highly pressurized nitrogen gas - is separated from light mineral oil by the floating separator piston. The gas spring has a maximum operating pressure (absolute) of 80 bar at its ultimate. The floating piston slides along a rod in the middle. The oil at the bottom provides damping by flowing through a stationary orifice not shown on the diagram.

Selecting a piston seal and guide rings for the piston (below the oil) is straightforward. However, I would appreciate some advice for selecting the piston and rod seals on the floating piston, which separates the oil from the gas. Is it appropriate to choose hydraulic piston and rod seals (rated to 80 bar), as if it were a hydraulic cylinder? For rod seals, would it be appropriate to select two single-acting hydraulic rod seals that are aligned in opposite directions to seal in both directions (also rated to 80 bar)? Would it be good design/maintenance practice, during assembly, to apply a thin film of the oil above the floating piston (in the gas spring area) to lubricate the seals properly?

Any advice is appreciated. Thanks.

 

[3DDave]

Since the seals will wipe over oil-covered surfaces then they might as well start off lubricated at installation.

There should be no pressure difference across the floating piston so the seals only need to act as oil scrapers to remove excess oil from the gas side back to the oil side.

 

[hydtools]

I would use a single rod seal.

Ted

 

[Ryan95]

Hi, Dave and Ted

Thank you very much for your feedback.

Would a better design for the floating separator piston be a simple disc with a single groove for a piston seal on the OD, and a single groove for the rod seal on the ID? This, instead of having 2 rod seals (sealing in both directions) and guide rings as well? I understand the rod the floating piston is on helps with its alignment, so it doesn't have to be thick or have multiple guide rings.

 

[BrianPetersen]

Take apart a remote-reservoir high-end automotive or motorcycle shock absorber (Ohlins is a good example brand name) and see how they do it. Might even be able to just order a seal and piston to have the parts in your hands. The inside surface finish of the cylinder is important.

 

[hydtools]

Guide rings are good. A thick disk will be more stable, less likely to tilt and wedge.

Ted

 

[EdStainless]

Surface finishes are critical, not too rough and not too smooth. And how the finish is achieved matters also.
There is a balancing act with the fit, surface conditions, and the surface finish.

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P.E. Metallurgy, consulting work welcomed