SpudFiles

Hi, just background the inner diameter of the outer tube is approx.19mm. The design is part of a blow forward design using either nylon piston but probably brass when it arrives.

Should the piston/cylinder be basically as close fit as possible and not bother with o rings, if not and l rings are used how much diameter difference should I have? Should the o ring be sized ao that when fitted in the 'U' om the cylinder the outer diamter of the o ring just touches the cylinder bore? Should the o rings groove be the same width as the o ring or should it be wider.


Any help gratefully received

Ps I realize there are documents etc explaining it all in great detail but that's all abit beyond me and my project.

If you have quite a strong return spring, or are using heavy ammo, you might find o rings can make quite a bit of difference, but it's worth noting that even if you decide to use o-rings you are going to have to fit the piston and cylinder very closely.

With regard to designing o rings, I'll give some more general rules so you are free to change your design etc, sorry if some of these seem to be quite common sense.

Firstly the cylinder and o-d of the piston should be as close as possible, as this helps reduce friction on the rings and is necessary for the rings to function correctly.

Secondly when designing the groove for the o ring, it should be able to fit into the groove, be tight against the bottom of it, but not so tight that it stretches the o-ring, (rule of thumb i-d of o-ring ~ i-d of groove) .
The depth of the groove will massively depend on your choice of o-ring dimensions, o-ring fits are commonly referred to by the amount of compression on the diameter of the o-ring,
An example would probably be the best way to demonstrate the principal:
A 10mm i-d o ring with a 1mm cross section (so an outside dia of about 12mm), if it were put in a piston with a groove with an inner diameter of 10mm and a cylinder with a bore of 11mm, then the cross section would be compressed by 50%. Hence we would refer to that as a '50% compression'
For smooth running fittings I usually go about as low as 5-10% compression.

With regard to choosing the cross section of your o-ring, larger o-rings are better for higher pressure but cause more friction, whereas smaller o-rings can't hold higher pressures, but cause a lot less friction. so again I would go towards a 1mm(ish) o-ring, dependant on what you have available to you.

Finally the dimensions of the groove, should be big enough so that when compressed the o-ring has enough area to expand into, but at the same time it should not be too wide as this would allow the o-ring to roll, and cause problems with tears or twists.

So in summary, you want a 1mm(ish) o-ring with about 10% compression, in a groove about 1.25-1.5mm wide.
But that is my opinion, there are members on the site who probably have more experience using o-rings than my rule of thumb style approach, but it's worked well for me on a few more recent non-spudding related projects.

It is fairly common to make the O ring excessively deep, creating intentional clearance (called a floating o-ring). In this case, the The OD of the o-ring should be slightly larger than the ID of the cylinder. The air forces the O-ring into the corner of the groove and the wall of the cylinder, sealing against both the piston and the cylinder.

http://www.spudfiles.com/pneumatic-cann ... 17899.html
http://www.spudfiles.com/pneumatic-cann ... 24317.html

Many thanks to you both, you have given me all the information I need.