Tolerances for sealless hydraulic cylinders

[Strostkovy]

I know that hydraulic spool valves have no seals, and just rely on tight tolerances and oil viscosity to seal. For very small cylinders, in the 1/4" range (say you need a row of them for a clamping application), would it be practical to use reamed holes with gauge pins or similar to make nonleaky single acting cylinders? Obviously you need to prevent them from extending all of the way out of the bore, but that's manageable.

Any idea on the tolerance requried? Does 2 tenths on diameter sound about right?

 

[DanielG]

We looked at this for a project and inquired of someone who knows about such things. He said the tolerances were so tight they use air gauging and matched parts rather than trying to grind/lap to a tight enough tolerance.

 

[Strostkovy]

We looked at this for a project and inquired of someone who knows about such things. He said the tolerances were so tight they use air gauging and matched parts rather than trying to grind/lap to a tight enough tolerance.

Ah, sadness. Probably cheaper to use a bunch of itty bitty seals then.

 

[Garwood]

What's wrong with seals? O-ring grooves are pretty simple to add. Even on small stuff.

Diesel fuel systems have some tight tolerances in them. And they still use seals to atmosphere.

And hydraulic spool valves leak externally if the seals go bad.

 

[ViktorS]

And, hydraulic spool valves still have an internal leakage, about 4-8 cc/min in most cases if we're talking NG06 size.

 

[eKretz]

Spools won't hold pressure either, so unless you have a hydraulic pump maintaining pressure the whole time, a spool will leak down. No seals = mess.

Edit: Oops, looks like Viktor posted while my screen was sitting unrefreshed.

 

[Strostkovy]

What's wrong with seals? O-ring grooves are pretty simple to add. Even on small stuff.

Diesel fuel systems have some tight tolerances in them. And they still use seals to atmosphere.

And hydraulic spool valves leak externally if the seals go bad.

I was just being hopeful that I could just drill and ream a bunch of holes in a thick wall tube and slip in some pins and have a hydraulic clamp assembly. Alas, it is not to be.

 

[TGTool]

Hydraulic systems use check valves if they really need to hold position.

When I was in a Case plant a few years ago, they had recently moved from selective assembly as the only way to get satisfactory fit, to diamond tooling. At least that's what they told us.

The company I was with was working on a test stand for spool valves. Case wanted automated testing with minimal operator setup. We weren't allowed to use face seals at the port connections because the force required for a leakproof seal would distort the valves. It was a nightmare project from which they finally canceled the contract.

 

[john.k]

Diesel injection equiptment has working fits that leak one or two drops per minute of thin fuel oil at 10,000-20,000 psi........The method of making the fits was invented by the Ludwig Loewe Co in 1916,and made submarine engines more reliable.........The British turned Berlin upside down looking for the Loewe machinery in 1919/1920 ........but by a clever trick ,Loewes showed the British officers opposite ends of the same factory ,reportedly after filling them up with cherry brandy.