Too Good A Fit (Actually Wrong Material)

[John Hasler]

I'm making a hydraulic mechanism for my power hacksaw and need a small piston that needs to move freely and not leak too much but does not move often, far, or fast. It will be immersed in clean oil. Being cheap, I made both piston and cylinder of mild steel. Bad move. By luck and lapping I got a *very* good fit, better than I really need. Piston drops in freely under its own weight but settles slowly supported by air. Bob's my uncle, right? Nope. While test fitting other parts I found that the piston would suddenly start binding. A touch-up with 1500 grit would fix it, but it would happen again.

What's happening is that because the steel is mallable the tiniest scratch raises a ridge. Because the clearance is perhaps a tenth the ridge rubs, gets smeared out, and binds. This should not be problem once the thing is assembled and closed up but I'm worried that a single tiny bit of grit will be enough to make a scratch that will bind it up so I'm going to have to take perhaps .0005" off the piston.

This problem would not arise with hardened steel or cast iron.

[warmstrong1955]

I suggest you make the OD of the piston smaller, and add an o-ring groove. I would use back-up rings as well.
Example: For an 1/8" section o-ring, the piston should be .003"-.006" smaller than the cylinder bore.

https://www.sealingdevices.com/document ... ndbook.pdf

Look up dynamic sealing for size selection, and piston & groove dimensions.

Bill

[whateg0]

An o-ring seal is what I would suggest too. Also, what did you do to finish the bore? If it isn't almost polished/honed, it may tear up an o-ring. Some of the downfeed controls I've seen used a piece of copper pipe as the cylinder. Since it's already smooth, the o-ring works nicely.

Dave

[John Hasler]

I said I plan to reduce the piston diameter. I used O-rings elsewhere in the mechanism but I don't want one here: free movement is more important than low leakage. This is part of the lift-on-return mechanism and as long as it leaks down more slowly than the saw cuts I'm ok.

I just did the calculation that I should have done at the beginning. The leakage through a .001" gap will be quite acceptable.

[whateg0]

The o-ring also helps avoid metal-to-metal contact, not just control where the oil can flow. It's your deal, but I would prefer to rely more on a needle valve to control flow than designed-in-leakage. If you can get it to bind with a few thou clearance, I would expect more binding with more clearance as the piston will be able to rock more before it digs in. Taller aspect ratios and tighter clearance help prevent that rocking. If it's just digging in because of a flaw in the surface, that isn't really binding, in my books.

What are you doing to allow flow when you lift the saw?

Dave

[Russ Hanscom]

Might be a good application for a cup type seal, I think that is what our commercial saw has; sometimes can act as a check valve for the up stroke.

[John Hasler]

The o-ring also helps avoid metal-to-metal contact, not just control where the oil can flow. It's your deal, but I would prefer to rely more on a needle valve to control flow than designed-in-leakage. If you can get it to bind with a few thou clearance, I would expect more binding with more clearance as the piston will be able to rock more before it digs in. Taller aspect ratios and tighter clearance help prevent that rocking. If it's just digging in because of a flaw in the surface, that isn't really binding, in my books.

What are you doing to allow flow when you lift the saw?

Dave

I'm not relying on the leakage to control anything. The present gap is *much* less than a thou: I can't measure it. I'm going to open it up to a thou, having calculated that the resulting leakage won't interfere with operation. Yes, it only sticks when there is surface damage: it works fine when cleanly assembled with clean oil. My concern is that if it is this easy to scar the surface enough to cause sticking that after a few hours of running a microscopic bit of grit will find its way to the piston and I'll be taking it apart again.

A check valve allows oil to flow when the saw is lifted. This piston is part of a mechanism that opens that check valve as needed to allow the blade to come down as it cuts but to spring back up slightly when the downforce is released during the return. The downforce is controlled separately.

[Edit] Thank you for the suggestions. They are welcome, even if unused.

[Bill Shields]

hence the idea of a cup seal...which is what is inside my low-end power pivot type cut off band saw.

been running for years with zero maintenance

The only reason i know it is there is that when I was given the saw, the mechanism was dry and I took it all apart and cleaned it, filled with oil and have totally forgotten other than to use it.

[John Hasler]

My design is very different from the usual one. The main piston has an O-ring. The piston in question is part of a valve assembly. A cup seal wouldn't work.

[whateg0]

I guess a diagram would make it all make a lot more sense if you have 2 pistons.