Struggling to find a decent source of info on this...

I am designing a compressed air powered motor to be machined entirely out of plastic. The bore is 18mm. If I want a reasonable seal on the piston but not too much friction what size should I make the piston? Was thinking something like a 0.05mm gap between the piston and the bore?

Any thoughts?

Dunno about an answer but could this be controlled easier with a groove in the piston and 'O' rings and trying different sizes?

Just a thought.

Cheers

Rich

Small compressed air motors are generally vane type..

Yup, I know that.

Trying to make a scale (ish) radial engine to run on compressed air..

More of an experiment to see how good rapid prototyped parts are..

Might just make them exactly the same size and polish the parts until they fit...hmmm




[Edited on 17/11/12 by tegwin]

How fast is it going to turn and any lube? Put a small groove or two in the skirt on load it with oil.

Seems like a reasonable starting point but you may find it a bit tight. What's the manufacturing tolerances on the part?

I actually don't know the manufacturing tolerances.... That would be useful to know... I also am not 100% sure of the surface finish either...so I think making it tighter than it needs to be would make sense...then polish until I get a good fit.

Try and find a used 'Air Hogs' toy. Those had plastic compressed air motors. Use it as a reference/study piece.

The real problem is making valve gear on a reciprocating compressed air motor compact enough.

Ahah, I have come across a very simple rotary sliding valve. Will post some photos of it in a few days.

How about something like this?

http://www.animatedengines.com/co2.html

Don't know how you're planning on rapid prototyping them, but I very much doubt SLS or SLA will give you a smooth enough finish for the bore (unless you've got access to an F1 team..)

We've got some pretty fancy equipment at work but even that for a a piston bore I fear would cause the whole thing to bind to the point where it won't run due to the surface roughness. You can always sand it smooth of course

Make it relatively loose and just feed oil/lubricant to it?

Surfaces tend to grow on most types of RP. The problem being that this can be anywhere from 0-0.1mm depending on shape, location, size etc. seriously limiting your tolerances.

Your best bet would be making them the same size then machining them to your required tolerance.