Shore A Hardness for sealing 1

[Garrett Green]

I am building a testing apparatus for hydraulic cylinder tubes. We want to test them with air immediately after welding to check for leaks in the welds. I have designed an apparatus to plug all ports and the open end of the tube but I am looking for a rubber that will seal well and hold pressure. The problem is as follows:

Temperature= ~400 deg F
Sealing Pressure = 1264.8 psi
Compression Force= 1000 lbf
sealing Surface Area = .7906 in^2
Tube O.D.= 2.34 in
Tube I.D.= 2.114 in
Internal air Pressure = 1000 psi

I have found a Fluoro Rubber that can withstand the heat and has a Shore A80 hardness. I believe this will work but would like input as I am a young engineer and do not have experience with rubber hardness for seals. Please let me know if you think the Fluoro Rubber is my best option or if there is something better. Also if there is a way to calculate how well the rubber will seal I would appreciate that knowledge as well!

 

[btrueblood]

Silicone would likely be cheaper.

Why 400F? Why not let the tube cool a bit more and use a cheaper rubber? Why not dunk the tube in a water bucket to check for bubbles? Why 1264.8 psi vs. say 80 psi shop air?

 

[Garrett Green]

Great Questions,
Why 400F- They don't want to stack them up and wait. If I can show significant savings on testing maybe they wont mind the slight inconvenience. First I will need to find the rubber I would use in both scenarios to show the cost difference.
dunking in water- The cylinder is not complete yet so we would have to spend time sealing all 3 ports and the end of the tube to then dunk it in water. we also want to keep these away from water as much as possible until they are powder coated because it can cause the tube to rust.
air pressure- the exact air pressure we will use to test is still being debated, however we have found when we tested with lower pressures in the past there were pin holes that we didn't catch, powder coat would then fill them for a time and we would be dealing with warranty calls a few months when fluid was slowly leaking at the welds. So we are trying to find the lowest air pressure that will reveal these small holes and I think it is something we will have to test after it is built (the air inlet will have an adjustable regulator).

I think you have a good point that letting them cool first would allow me to use cheaper rubber but either way I need to find a rubber that is soft enough to seal well and hard enough not to wear quickly. Do you think I am in the ballpark at Shore A80?

 

[TugboatEng]

80A is standard for that pressure range IF retained in a groove. Are you compression this cylinder against a rubber sheet to seal it or is there some support for the seal?

Air and soapy water are very good at finding leaks. If the holes are showing up much later are you certain the welds aren't cracking? Perhaps a hydro test should be performed instead of a leakage test until the process can be improved?

 

[Garrett Green]

Thank you for the information! My plan is to use a rubber sheet with a steel backing plate and then held in place by sandwiching it with another plate with a hole in the middle like this:

so the rubber will be fully supported.

We have been using soapy water sprayed on to find leaks but my company wants to find a way to detect leaks that doesn't require an employee to find it. The hope was to pressurize the vessel and leave it along while the next tube is welded and see if the pressure drops or remains the same. If they see a pressure drop then they would use the soapy water to try to find the location.

Some of the welds may be cracking, this is a major concern of mine especially after I found them quenching the part to cool it faster. That has since been stopped and I am trying to narrow down exactly what is causing the failures. It is just often difficult because we don't always get the failed cylinder back.

unfortunately in regards to the hydro test we run into the same issue as dunking it in water. The inside of the cylinder would rust, and this is testing that we do on every tube after welding because we have seen error with our welding robots over the years and it is much cheaper for us to find the issue right away than have to take a cylinder apart after it has been powder coated and assembled. Also we do a water dunk test after the cylinder has been powder coated and fully assembled it is just much more efficient for us to find the leak when it is a bare tube.

Thank you both for your knowledge! during this time I found a Silicone Rubber for less than 1/3 of the price that can handle 390 deg F. it is A70 Shore hardness, so my recommendation will be to use that and let the part cool for a few minutes before testing (probably leaving 1 "on deck" will be long enough).

 

[TugboatEng]

For your test apparatus, I would suggest bonding a rubber disk to the backing plate (inside the hole). This will give more support for the rubber without altering your design. Compressing the rubber between two plates will bulge the portion in the hole making it difficult to seal against.

 

[Garrett Green]

Okay I will try that.
Thank you again for your help!

 

[3DDave]

Good plan but for one thing. As the tube cools the air inside will cool and the pressure will drop.

If you are concerned with detecting cracks that are large enough to leak then you have a severe production problem as not all cracks will leak - at least not at first. They will when they finally fracture all the way through, likely after installation and during use. What you need is better control over the welding operation.

 

[NEWTOP]

In fact, silicone is not a good choice for your situation.

NEWTOP Medical Silicone Molding

https://lsrinjection.com/

 

[Tmoose]

I don't think we saw details about the cylinder materials being welded, nor the joint prep or the procedure.

I Agree that the welding process (or operator) likely needs a tune-up.

When the welded parts are being wheeled over to the pressure tester I'd be basting the welds on the pressure side with Loctite 290, or a higher temperature variant if available.

I think I've been using 290 for about 30 years, and 290's porosity sealing capabilities continue to impress me.