turning plastic

Waxy plastic means UHMW (ultra-high molecular weight) polyethylene to me. It's softer, slipperier, and more abrasion resistant than nylon.

Ned Simmons

The quickest way to confirm UHMW is to burn a tiny bit with a lighter. If it smells like a candle, it's likely polyethylene. If it feels dense and heavy, it's probably UHMW polyethylene. UHMW is basically the same as parafin wax except the molecular chains are looooooong (4 to 6 million molecules long instead of about 30).

If you make a new part out of UHMW, be sure tho keep it spotlessly clean during fabrication and assembly. UHMW has a large tendency to pick up dust (especially grinding dust) which settles intot he surface and makes at into a file. Wipe the parts with an oily (mineral oil or similar) rag before assembly to get and residual dust off.

Your original question referenced how to attach the piston to the shaft. UHMW will cold flow over time so if you press fit (even a TIGHT press fit) the bore is likely to relax a bit and may come loose from a steel shaft. I'd mechanically fasten it with roll pin and then turn the piston on the shaft to get it truly round and centered to the shafting. Of course I'm just guessing at the existing set-up and I may be way off.

Koz

I confirmed that the original part was made of nylon. And the press fit on that one had held for decades. I have something in my shop that looks like nylon, I'll try that. I'm still looking for suggestions as to how much undersize to bore the plastic part. The piston is shaped like a very fat washer. OD is 1.250", ID is a press fit on a 0.625" shaft, thickness is maybe 3/8" (haven't measured it). I measure the ID as "about" 0.622" but it's pretty scored up. I was completely unable to find any information as to how much to undersize nylon holes for a successful press fit, either in Machinery's Handbook or by googling.

If I get desperate I can always order a replacement from Keller. They're still available. But I'm cheap nowadays ..

Grant

Koz wrote:

The info you need is usually in the design guide from the plastic mfg. But of course you need to know the type and grade of plastic you have. Otherwise we are all guessing. I think the basic guide is that the press fit should be less than the creep limit of the plastic. There are hundreds of types of nylon with very different properties. I couldn't find a design guide for nylon, but for a lexan for instance, the bore would be about .620" for a .625" shaft.

Put in a ring grove for a rubber O-ring....

Need some nylon or HDDP? Email me your snail mail addy and the dimensions. Acetal or delrin? Ive got a bit. Btw..Delrin with an o ring might be a good replacement.

Gunner

"In my humble opinion, the petty carping levied against Bush by the Democrats proves again, it is better to have your eye plucked out by an eagle than to be nibbled to death by ducks." - Norman Liebmann