piston seal moving over cross hole

[BrianE22]

I'm rebuilding a gear hobbing machine at work. The air cylinders that clamp one of the slides was leaking badly so I took them apart, cleaned them out, chromed and ground the cylinders, made new pistons and installed new seals. Two of the four pistons are leaking air across the piston seal.

Seems strange to me, the piston seal overlaps the cross holes that deliver air to the cylinder. I always thought that was a no-no. I assume the design at one time worked so I guess it's an acceptable practice. I'm thinking I might need to round the corners of the cross holes so they don't eat the seal.

My question is, have you seen this practice before, allowing a seal to move across a cross hole?

 

[carnage3]

Yes, but I still dont like it. if you can smooth the transition arround the hole it should help it last longer.

 

[BrianE22]

Right now, I'm tumbling the block that holds the cylinders to try to get a rounder edge on the holes.

 

[hydtools]

I've had ID gooves machined where there are cross holes. The groove is shallow and has shallow tapered sides to give lead-in and lead-out for the seal.

Ted

 

[Cockroach]

You're correct, this is a textbook case of NO-NO. You can deburr the engress holes all you want, if you're lucky and with patience you may hit on enough fillet that you won't cut your o-rings.

Classically you would need to counterbore a step into that bore, thus removing the sharp holes from the contact surface with the Piston elastomer. Make the chamfers something like 20-30 degrees tapered off the main bore. The step is something like 1/16 inch on diameter. Check that by measure of the o-ring on the Piston and adding 1/32 inch to that maximum diameter.

You could be there for a long time deburring that cross hole. Gets even better when you have a small bore. I could say something ignorant here that only machine shop personnel would appreciate, so I'll bite my tongue.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[tbuelna]

BrianE22,

Depends on the type of seal and the size of the hole. It's acceptable to traverse small holes with piston rings. But with elastomer seals like o-rings, the seal will bulge as it passes over the hole and get pinched.

Good luck.
Terry

 

[Philrock]

Would it be practical to limit the stroke of the cylinder so that the seal never actually passes over the hole?

 

[BrianE22]

That's what I actually ended up doing Philrock. I tried tumbling the block and honing the cylinders but the hard chrome plating made it tough to get the sharp edge of the hole. I'm too chicken to use a Dremel grinder on it.

Hopefully I'll have enough stroke but I think I'm o.k. The spacer I used was .06 and the piston stroke was .50".

The cross holes are one thing. What's got me more confused is why the cylinder is double acting. It can only push on the clamping mechanism - it can't pull.

 

[gerhardl]

Doubleacting: probably original commercial reasons, single acting are more seldom used, and normally would cost more, thus doble acting is selected if performance and function is OK.