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QUESTION:I'm working on a project that will have a pneumatically operated
piston. My concern is about wear on the sealing o-ring.
How is the high-polish on sealed shafts (automobile shock absorber
shafts for example) achieved?
1) polishing (mechanical removal of high spots)
2) plating (deposition of a smooth layer)
3) de-plating (electrically removing high spots)
4) other?
Is chrome plating a good idea to reduce scratching from sand or dirt?
The project is still in the early conceptual stage, so any thoughts
regarding sealing moving parts at high pressure (~300-500psi) would
be appreciated.
ANSWER: Funny you should ask about this. I'm a design draftsman & I design
hydraulic cylinders. Last night, I was quafing a few with some fellow
eng. techs, and we were discussing rods and rod finishes. We chrome plate
our rods, mostly in-house, although we do purchase some prechromed rod
for use in our small bore facility. Our standard chrome is .0005/.0007
thick. After we chrome, we grind and polish to 8/16 RMS. We use
centerless grinders to do both our prechrome G&P and our finish G&P. We
are currently fine-tuning a G&P machine made from a 1940's era lathe
that is being used to create superfinishes (it can easily produce a 1 RMS
& we are using it in production @ 2 RMS finishes.) The purpose of this
glass-like finish is to produce an extremely dry rod, with virtually no
stiction and no leakage past the gland U-cup and wiper seal. this is
important in some environments, as oil obviously atrracts dirt. Hope
this answers your finish question. As foraas sealing your piston at that
high PSI, you might need more than a run of the mill O-ring ( usually
a 70 durometer Buna-N material.) At the very le lleast you should use
Hytrel back-up washers on either side of the o-ring to prevent extrusion
of the o-ring in the clearance gap between piston & tube. However, this
can create pressure trapping & you might blow the seal. You might want to
try something like a capped t-seal or a teflon seal w/ a loader of
some sort. Your best bet is probably to use a crown seal. Good sources
for seals and info on seals are Parker-Hannifan, Macrotech, Shamban, etc.
Let me know, and I will forward more info to you.
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