SpudFiles

I want to make a 3/4" cup seal similar to the seal on a bicycle pump piston.

Hints and suggestions appreciated.

BoyntonStu

what are you using it for there may be easier ways to do it besides a cup piston since the are not as easy as most to make. if it absolutely has to be a cup piston use this design and scale it down to 3/4 inch http://www.lifewater.ca/Appendix_L.htm this is leather but it should work just as well for what you want

spudtyrrant wrote:what are you using it for there may be easier ways to do it besides a cup piston since the are not as easy as most to make. if it absolutely has to be a cup piston use this design and scale it down to 3/4 inch http://www.lifewater.ca/Appendix_L.htm this is leather but it should work just as well for what you want

Thanks,

That is a good find.

BTW How well does leather seal?

IOW What material is capable of sealing to a higher pressure?

A cup piston is quite easy to make actually....

I use a .25" thick rubber disc that I've punched used a piece of the pipe that it's going to fit into, It's sandwiched between a washer just small enough to fit in the pipe and another that is between 50%-75% of the pipe diameter...

When you tighten down the washers the rubber disc will squish out and forward towards the small washer side...the diameter you will find will now be slightly larger than the pipe i.d., but that's ok, that's what you're looking for.... now it will take a little effort to get it into the pipe and you'll want to lube it as well with silicone or lithium grease...

once it's in the pipe it will allow filling from the pilot side but will not allow air to escape from the chamber to the pilot when fired.. It basically acts as a check valve.... Mine handles unregged co2, I've actually capped the barrel port of the cross and "pulled the trigger" and it held 100% of the chamber pressure...


look at the left end of the decimater piston and you will see what I'm talking about(the larger rubber bit is actually one of the bumpstops that is stuck on the rearmost nut...)

jeepkahn wrote:A cup piston is quite easy to make actually....

I use a .25" thick rubber disc that I've punched used a piece of the pipe that it's going to fit into, It's sandwiched between a washer just small enough to fit in the pipe and another that is between 50%-75% of the pipe diameter...

When you tighten down the washers the rubber disc will squish out and forward towards the small washer side...the diameter you will find will now be slightly larger than the pipe i.d., but that's ok, that's what you're looking for.... now it will take a little effort to get it into the pipe and you'll want to lube it as well with silicone or lithium grease...

once it's in the pipe it will allow filling from the pilot side but will not allow air to escape from the chamber to the pilot when fired.. It basically acts as a check valve.... Mine handles unregged co2, I've actually capped the barrel port of the cross and "pulled the trigger" and it held 100% of the chamber pressure...


look at the left end of the decimater piston and you will see what I'm talking about(the larger rubber bit is actually one of the bumpstops that is stuck on the rearmost nut...)

Thanks for a very good solution.

BTW How do you locate the center of the rubber to drill for the bolt?

(eyeball or what?)

well leather was used on luis and clarks 800psi stirrup pump so you tell me but i would go with jeepkahns design the parts are easiest to make and to get a hold of.

rubber just has to have outside forces to hold its shape rubber won't hold its shape by itself very well

boyntonstu wrote:Thanks for a very good solution.

BTW How do you locate the center of the rubber to drill for the bolt?

(eyeball or what?)

I usually eyeball it... the bolt hole is 1/2 of whatever the bolt size is so that it helps to seal against leaks around the shaft...

jeepkahn wrote:

boyntonstu wrote:Thanks for a very good solution.

BTW How do you locate the center of the rubber to drill for the bolt?

(eyeball or what?)

I usually eyeball it... the bolt hole is 1/2 of whatever the bolt size is so that it helps to seal against leaks around the shaft...

For thinner rubber I use a hand held single hole paper punch.

I have some 0.050" rubber, would 3 or 4 washers be as affective as a 0.125" one.

BoyntonStu

Thicker is better, if the rubber is too thin it will try to fold instead of flexing to the cup shape... if you use thin rubber you will prolly need the smaller washer to be 75% to 90% of the larger washers diameter to prevent that...