Loss torque on gasketted joint.

[APH]

My application is a rubber gasket, aluminum plate, and plastic flange. These are my component on my gasket joint. My problem is after 24 hours of installation (not even subjective to any elements) I have to re-torque it. It was rounghly 1 in-lbf loss of torque.

Once it is subjected to the elements for 3 days, I loss roughly .5 in-lbf of torque. I would then re-torque it again and ran it for another 3 days. This time I didn't loose any torque. It seemed like the gasket joint become "Stable". My thought was that the rubber gasket was sort of "cured" during the last 3 days of testing. If that's the case then I need a different material. What type of mechanical property that I should look for?

Thanks
APH

 

[israelkk]

Rubber has a propery called permanent set. When it is compressed it loses part of the compression. For example in o-rings they are usually designed to be compressed 10-20% of there thickness knowing that they will lose approximately 5% of the compression, leaving 5% to 15%.

 

[APH]

But how do you know that it will lose 5% of the compression?. Is there any mechanical property that corelate with dat.

APH

 

[monkeydog]

I would also look at your plastic flange. Plastics have a tendency to "cold flow" under compression. It is not unusual due to the permanent set israelkk wrote about. I use Teflon seals that are under compression, and need to readdress the preload on the seals due to "cold flow" after 24 hours.

 

[Engr777]

But why u check torque again in 24 Hr. again and again. Leave it if it is not leaking and it will hold after some reduction in initial torque. Initially torque it once 10%above then required seating stress of the gasket.

 

[Onno]

I agree with Tahir: if it is not leaking, leave it. Consider also that some torque may be lost on thread friction on the flange bolts.