Torque calculator for copper gaskets

[gearman1234]

I am lookng to Calculate torque for a gasketed connection on a gerabox with an oil under pressure no more than 8 psi.

This connection is a drain plug with plug size M20 and a copper washer to seal the joint. I have the cacualtion formulae for steel connections and bolts but not for copper washer. What preload is suitable for copper washer.

 

[iainuts]

I've designed a lot of copper gasketed joints, though they're all very high pressure and at cryogenic temperatures. An 8 psi plug should be relatively easy to seal using a copper washer. A few considerations:

Determine the contact stress (compressive stress) of the gasket when the bolt is torqued to a 'normal' load that some mechanic might apply. Annealed copper has a yield strength around 6000 psi, maybe a bit more depending on material. A mechanic is generally going to torque the bolt to that level regardless of what torque spec you put on something unless the assembly is carefully controlled procedurally by trained mechanics.

If the gasket compression you get using this calculation is much more than 6000 psi, the gasket will act more like a rubber gasket and will flow into all nooks and crannies. Yes, it will seal very well, but it may be very difficult to remove. If this is the case, design a gland for the seal so you know where it will flow to, or just have some other metal to metal surface prevent additional torque from forcing it to flow (ie: the metal bolt has a shoulder that ends up pressing on the threaded part and prevents additional torque from compressing the copper further).

If your gasketed joint is more like a flanged connection, you may also use the metal-to-metal technique, or you may contain the gasket in another way. Also, at such low pressure you might even consider designing it so that bolting doesn't produce in excess of a few thousand psi contact stress such that the copper doesn't flow.

 

[gearman1234]

Thanks for the information. I got the confirmation that what I was thinking is right. I have calculated the max.force on washer for that area and I have calculated the torque which will not exceed that preload. I have used 6000 psi (40 MPa) as the yield strength of copper.

 

[sprintcar]

Just remember that Mickey the Maintenance Man will just yank the wrench to tighten it after the first oil change. I've never seen anyone use a torque wrench on a drain plug, although I'm sure the Manual says what torque to use.

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein