O-Ring Gland Design - Tolerance Question

[cbrock]

I have a question regarding machining tolerances on o-ring glands.

When designing a plug style o-ring gland, the Parker O-Ring Handbook (as well as other o-ring manufacturers) base their static seal dimension tables for a pressure of 1500 psi & a 70 durometer o-ring.

The design charts give the nominal bore diameter (tolerance +.002"/-0.000") and the plug diameter (+0.0001"/-0.001"). Using the chart dimensions, you will get a diametral clearance of 0.002" to 0.005".

In my application, this is percisely what I am looking for. However, when the drawings are sent to our machine shop, the machinists keep complaining that we are specifying unreasonable tolerances, especially on the plug diameter.

I am not a machinist and can understand that a diameter tolerance of +0/-0.001" can be quite tight. Are the tolerances recommended by Parker unreasonable? I would imagine that an industry leader such as Parker would not recommend tolerances that no machinist could hit.

I do understand that the tolerances can be relaxed, giving larger diameteral clearances and compensating by using a harder o-ring and using the limits of extrusion charts for clearance. I would like to avoid this if possible.

Any advice would be appreciated.

 

[israelkk]

Replace the machinist. With programmable CNC lathe machines available today this is as easy as 1-2-3. A good machinist even with a manual lathe will have no problem.

The Parker tolerances are reasonable and well proved. Be aware for the surface finish requirements too.

When we are working with higher pressures than 1500psi we use even tighter tolerances based on our experience.

Despite everything above said it will depend on the fluid. Sealing hydraulic fluid is not as sealing air or nitrogen and sealing air or nitrogen is not as sealing helium.

 

[cbrock]

Thank you for your response. This is exactly what I was thinking especially since we are sealing against methane gas.

Does material make a difference in meeting the tolerances? We are using 17-4 PH stainless steel.

Also, I would imagine that on larger diameters (the 400 series o-rings can be larger than 24" diameter) it may be harder to meet the 0.001" tolerance (this is not my case, I am using a 159 oring).

 

[israelkk]

I do not see a problem with 17-4PH from the point of the dimension accuracy except that the machining should be after heat treatment. Heat treatment of the finished part will shrink the dimension 0.002" to 0.005" (depends on the aging heat treatment) on a 5" diameter part. Which is two to five times larger than the tolerance.

The larger the part the difficult to meet the 0.001" tolerance. This is not just the machining accuracy or the lathe or mill but distortion of the part due to internal stresses in the material bar or plate as came from the producer of the raw material.

 

[monkeydog]

I agree with iraelkk totally!
If you look a little deeper, you will see the Parker recommended o-ring gland design has been around for decades (that is before NC machines were standard tools)!