clearance

[solid34works]

Hi,
I have to design a plastic reciprocating pump. Cylinder size is 3.126+/- 0.001. I will have to decide the piston size. I have already designed the O rings from the standard guides. I am not able to find any reference on piston cylinder clearance. Any help will be highly appreciated. I saw some of the IC engine piston cylinder clearance. Those clearances are very small.
Plus I am not sure if so small clearanc would work for plastic parts or not.
I would appreciate if someone could help me on this. I need to know the clearance between piston and cylinder.

Thanks,

 

[Unotec]

Depends on the plastic and its expansion/contraction characteristics with temperature, I assume.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[solid34works]

It will be used as a hand pump. So we can neglect the temperature effect of the plastic.
The material of construction is PVC.
Is there any reference on the clearance?

Thanks,

 

[Unotec]

I am not sure there is a standard in this case but if you have .001 (some units) tolerance, you have to give this at least. My guess, though

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[nickelkid]

With compressor PTFE blended piston rings and rider rings(bands) you do not want less than .010 diametrical clearance between the bore and the ring OD no matter what your expansion calculations tell you. This is for operating at elevated temperatures.

 

[hydtools]

Base your clearance on the o-ring material and your pressure. Make the piston-to-bore clearance just small enough to prevent extrusion of the o-ring.

Ted