Question on how to calculate assembly torque
Question on how to calculate assembly torque
(OP)
I have an application where I need to calculate the recommended assembly torque between a relatively thin plastic part - 0.1" thick (with clearance hole) to a metal frame with thread thru. hole using #12-24 screws.
My biggest concern is the selection of preload. While preload is ususally chosen as a certain percentage of the screw yield stress, but the weak link in my application is actually the relatively thin molded plastic part. If my preload is too high, my plastic part would yield or crack under the preload; if the preload is low, the assembly joint might come loose overtime....
Has anyone run into this type of problem before?
My biggest concern is the selection of preload. While preload is ususally chosen as a certain percentage of the screw yield stress, but the weak link in my application is actually the relatively thin molded plastic part. If my preload is too high, my plastic part would yield or crack under the preload; if the preload is low, the assembly joint might come loose overtime....
Has anyone run into this type of problem before?





RE: Question on how to calculate assembly torque
A beginner's tutorial is available from Textron Fastening Systems at:
http://www.fastenersforplastics.com/
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Question on how to calculate assembly torque
RE: Question on how to calculate assembly torque
RE: Question on how to calculate assembly torque
If the joint has the same external forces applied (a good assumption if the new product will do the same things as the old one), then you need the same preload. If you use the same preload but don't change the screw bearing area, then you may have unacceptable creep. You may need to change the screw design in conjunction with the material change.
In addition to the website I included above, here are suggestions for technical references:
FAQ725-600
http://www.boltscience.com/pages/info.htm
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Question on how to calculate assembly torque
The websites you have provided have been helpful so far, thanks.
I am 100% certain that no study was ever done for this product (otherwise, I wouldn't have the problem I have today)... they never considered it a problem until I came along...
I guess I haven't been entirely clear about the problem I am having. These joints are only loaded in shear, no tensile load. I know I would lose part of my preload due to time dependent creep, but would my joint loosen overtime due to shear?
Sincerely,
Thomas Wong
RE: Question on how to calculate assembly torque
I assume your joint is shear-friction, that is, it has oversized holes and the shear force is resisted by friction at the joint interface. This friction is the product of fastener preload and the interface friction coefficient. If you lose preload, you lose the friction force that keeps the joint together. However, this may not be a problem if the fastener can be used as a pin, and the cyclic stresses are kept below the fastener's fatigue limit.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Question on how to calculate assembly torque