By
Airpot
05 February 21
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Piston Leak and Dimensional Stability

One of the ways our air cylinders have such low friction is that the Airpot (and Airpel) pistons are fit to the cylinder with clearance and therefore, there are no true seals. However, this means that whenever there is a pressure difference that there will be a non-zero flow of the fluid from the high pressure side of the piston to the low pressure side. For most applications, we want to keep this leak to a minimum. The magnitude of this leak is a function of several factors, but the most important factor is that it is proportional to the third power of that clearance of the piston to the cylinder.

So, it makes sense to fit the piston as closely as possible using modern manufacturing methods. But there can be trouble as the fit starts to get too close. Materials change size with temperature, so you need materials that have good dimensional stability. This means that changes in temperature of humidity must not significantly change the size of the parts. Positive clearance between the piston and cylinder is obviously needed in order to maintain the low friction.

For temperature stability, materials are characterized by their coefficients of thermal expansion (CTE). Low CTE’s prevent significant size changes that could cause interference or too much clearance. It’s not enough just to have matching CTE’s as you could have a piston and cylinder made from the same material (equal CTE’s) but if they have high CTE’s, thermal gradients (say, the cylinder gets colder than the piston) could cause interference.

OK, so we can see that we need low CTE’s for the piston and cylinder material, but it’s also good to have the CTE’s match fairly close to minimize leak changes. The Airpot borosilicate glass cylinder and carbon graphite piston have both low and closely matching CTE’s. In addition, the piston material itself is self-lubricating so there is no need for oils or greases.

Dimensional stability also requires materials that change very little with changes in humidity.  The Airpot borosilicate glass cylinder and carbon graphite piston do not change measurably with humidity.

If used in a dashpot or pneumatic damper, the leak limits the maximum damping that you can get from the dashpot. Normally, this is not a problem unless the application calls for very, very slow motion where a very high damping coefficient is required.

When the piston and cylinder are used in a pneumatic actuator or an air cylinder, too much leak means there may be trouble when throttling the flow to get very slow speed control. Also, the pressure supply should be an active source of compressed air (not just a compressed air tank) because of the leak.

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