Plastics thread seating torque 3

[mechintern]

I am trying to find out how much torque I should apply to a plastic nut which is fastened to plastic threads. Both are made of PC/ABS plastic. I have done some failure testing of 100 parts, and they failed at an average of 150 inch pounds. I have found sources that suggest using 75% of the failure torque. I assume this is for metal fasteners. Should I use this for my situation? I realize that there are issues with plastics and their tendency to creep over time, so I need to find a happy medium that wont cause excessive creep yet will be enough force so the nut will stay tight. Any suggestions are greatly appreciated.

 

[desertfox]

Hi mechintern

The torque or pre-load you apply to fasteners is usually based on the service loads and enviromental conditions of which you give no information. If these fasteners are not seeing any loads or enviroments which are significant you may be able to use less torque than 75%.

regards desertfox

 

[mechintern]

Thanks for the quick response desertfox. This unit will actually be in a carwash, so it will most likely see large temperature ranges which is why we will thermally shock the units before sending them out. We have also seen issues with the solvents found in the car wash causing the units to leak. Would you still recomend the 75% of maximum torque?

 

[CoryPad]

Do your test results have torque, tension, and angle recorded? If so, you can determine a proper tightening point. If not, 50% to 75% of the fracture point is a good estimate, but you should try to test torqued assemblies under the expected environmental conditions (forces, temperatures, fluids, etc.) before making a final decision.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[desertfox]

Hi mechintern

I echo the advice given by Corypad you need to do practical tests.
In addition it would be useful if you could obtain some Creep Data curves for your plastic as this would be a useful guide in determining your maximum stress allowable in the plastic, you could then base your initial torque for the fasteners on this stress prior to testing.

regards desertfox

 

[Tmoose]

Is your concern maintaining a liquid tight seal? Are the threads doing the sealing, like a tapered pipe thread?
Are you using the nut to compress an O-ring or gasket?
Or is the nut holding some part as part of a light duty structure? Or ..... ??

 

[mechintern]

The nut is used to hold the unit on a bracket, so it has nothing to do with maintaning a liquid tight seal. Yes, it is a rather light duty structure. It weighs 5 ounces at max. It should not see any major vibration or jarring, just mainly temperature variation which could cause a problem leading to loosening of the threads.

 

[Cockroach]

The physical characteristics of the plastic may change in your temperature range. Both DesertFox and CoryPad have indicated field tests which would probably be the best method of determining functionality in your environment.

Obviously you have already recieved good advice. I would use extremely conservative loading factors in order to account for thermal property variation of the plastic material itself. Creep is extremely difficult to design against, possibly cracking from the roots of the threads with your temperature fluxuations.

Take a look at your MTR, the manufacturer may have already done his homework! At least this would offer some indication of limitations to material application.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada