SpudFiles

While waiting for the shipment of HDPE for my piston, I decided to try and replace the pilot schrader valve with something more heavy duty.

This is what I had in mind...



Basically its a check valve (green) with an extended shaft to center it within the housing (black). The check valve shaft extends out of the housing so it can be struck with a hammer, causing the valve to unseat and open thus triggering the piston valve.
The housing will have 3/8 threads on the outside and a hexagonal portion so it can be easily held with pliers and screwed into a T piece. I originally thought about making both the housing and piston out of aluminum but then I remember someone said if the front of a HDPE piston is smooth and flat enough against the seat, it doesnt need a rubber gasket to seal.

So I was wondering if this small piston could be made out of HDPE, seal without a rubber gasket against polished aluminum seat and survive repeated strikes with the hammer. Actually... the hammer would be a zip gun mechanism.

The hole in the housing for the shaft would be 3mm and the shaft itself a bit smaller so air can pass... then the seat would be 3mm and the diameter of the piston face 6mm.

In this configuration I guess it would have more flow then a schrader valve but I'm worried about the forces involved. I'm not sure how to calculate how much force it would take to push open this valve if its holding 40 bar... and if HDPE would take it. Should I change the material of the piston? Or reduce the seat/piston diameter to lower the force needed to open it?

Sorry about the drawing... it was a long time since I had to draw something like this so I forgot some of the details

FighterAce wrote:someone said if the front of a HDPE piston is smooth and flat enough against the seat, it doesnt need a rubber gasket to seal.

I mentioned that UHMWPE could seal... so I'm going to assume that's what you meant.

UHMWPE should survive the force just fine, so long as the hammer strikes aren't too off-center and causing shear stress, in which case it might bend and no longer be hit correctly. However, if I understand what you mean by a "zip gun mechanism," this won't be an issue.

I don't think seat/piston diameter ratio has anything to do with a valve like this, so I'll pretend it doesn't. When sealed, the valve should experience about 25.4 pounds of force closing it at 40 bars. It's hard to say how easy this will be to open, since that depends on how heavy the "hammer" is, how fast it's moving, and it's impact time. I wouldn't worry about the dimensions, but somebody may have difficulty making a 3mm diameter piece of UHMWPE.

Drill rod for the "stem" piece...delrin or nylatron for the "hat". I'm using the ones I make with unregged co2, and 3000psi air.

On an msp (crosman 2289), I've punched the stem right through a teflon "hat". Made a new one with a teflon seat and brass head...no more problems.

You'll need a counterbore behind the seat to allow better passage of gas, and then port that to atmosphere.

Drill on an angle from the rear to hit the counterbore would be a passable idea...make sure to vent the area immediately behind the valve so that the striker doesn't become a projectile.

Tapered seat designs are not ideal...they wedge themselves together, and require more energy to open than "flat" or perimeter sealing designs.

My .02

saefroch wrote: I mentioned that UHMWPE could seal... so I'm going to assume that's what you meant.

Thats interesting... because tech said UHMWPE cant really be polished/machined on a lathe to be smooth... and I cant understand how a jagged surface can seal against anything.

saefroch wrote:When sealed, the valve should experience about 25.4 pounds of force closing it at 40 bars.

Do you mind sharing how you calculated that?

saefroch wrote:somebody may have difficulty making a 3mm diameter piece of UHMWPE.

Why would that be a problem? Does it bend that much if its thin?

Gippeto wrote:Drill rod for the "stem" piece...delrin or nylatron for the "hat". I'm using the ones I make with unregged co2, and 3000psi air.

On an msp (crosman 2289), I've punched the stem right through a teflon "hat". Made a new one with a teflon seat and brass head...no more problems.

You'll need a counterbore behind the seat to allow better passage of gas, and then port that to atmosphere.

Drill on an angle from the rear to hit the counterbore would be a passable idea...make sure to vent the area immediately behind the valve so that the striker doesn't become a projectile.

Tapered seat designs are not ideal...they wedge themselves together, and require more energy to open than "flat" or perimeter sealing designs.

My .02

I thought about making holes on the side connecting to the shaft hole... the striker mechanism will be attached to the back of the valve housing so it cant go anywhere.
I dont really understand what you meant with the "Drill rod for the "stem" piece...delrin or nylatron for the "hat"."

Thats interesting... because tech said UHMWPE cant really be polished/machined on a lathe to be smooth... and I cant understand how a jagged surface can seal against anything.

Yes. Bringing UHMWPE down to size with sandpaper and various multi point seperation tools is not going to work. The smoothest surface you'll get is from a single point cutting element.

Drill rod is what drill bits are made from. Delrin is a very low friction plastic that can be brought to a polish with conventional means. I assume nylarton is a varyation of nylon, and so it will be similar to delrin.

These are for a qb78/79 co2 rifle, but should clear things up. Brass one is stock.

The metal shaft is the stem...the black plastic part (nylatron) is the hat.

Most simply call the entire assembly a "valve stem", but we're trying to differentiate the parts at the moment.

As to the drill rod...ask your machinist friend about oil hardening steel rod...likely they call it something different in Croatia.

I found the many small concetric ridges on machined UHMWPE made a nice gasket as it provides a pressure gradient with low leakage. You don't always need a perfectly smooth surface for a good seal. Leather has been used in pumps for years.

Technician1002 wrote:I found the many small concetric ridges on machined UHMWPE made a nice gasket as it provides a pressure gradient with low leakage.

That's probably what I observed, since I was using very high pressure and didn't place the valve in water to look for bubbles, but there didn't seem to be a significant buildup of pressure down the barrel when I put a piece of gasket material over it. You don't necessarily need a smooth surface (weird, I know), the surface I made to seal or nearly so was cut with a hacksaw blade while it was chucked into a drill press.

I'd go with Gippeto's suggestion, and use steel for the stem, the sealing face could be a plastic.

Gippeto wrote:oil hardening steel rod

Wouldn't that just be tempered?

FighterAce wrote:Do you mind sharing how you calculated that?

Calculate the net area of the piston exposed to pressure, and multiply that by the pressure. I converted to cubic inches and psi, then just multiplied. You'd assume normally that the entire 6mm diameter seals, so the area of a 6mm circle multiplied by the pressure its subjected to gives force.

Very well, steel it is.

But I tell ya... my machinist wont be happy, drill bits are hard mo.os

Technician1002 wrote:I found the many small concetric ridges on machined UHMWPE made a nice gasket as it provides a pressure gradient with low leakage. You don't always need a perfectly smooth surface for a good seal. Leather has been used in pumps for years.

What do you consider low leakage? Few psi in a day or few bars in 10 mins?

Few psi in a day, probably. But again, it depends on what pressure you're operating at. If it's high enough the plastic will probably deform to make its own perfectly fitted sealing face (this I have observed on my piston and bumper).

I said its gonna be around 40 bar... maybe even higher if I can find a stronger replacement for my pump cylinder.

Is this a homemade pump we're talking? What's the cylinder currently?

Yes... its a copper pipe, 1mm wall, 13mm ID.

That's really close to my pump, but with a fractionally larger diameter. If it's all made of copper, you might have a lot of work replacing the cylinder, but you might be able to use 3/8" pipe for the cylinder. The ID is pretty darn close, and the o-ring might be able to deform and absorb the difference.

If you have a bike pump sitting around, I humbly suggest that you make a two-stage system. The benefit is massive.

The only copper piece is the cylinder... the rest is all metal. I've found a supplier of Al pipes around here so its all about finding the right alloy with the right ID.
Have you turned yours a two stage?