Question on how to calculate assembly torque 1

[tcwong]

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?

 

[CoryPad]

Yes, others have had this same problem. In fact, selecting preload is the basic problem for any joint. You have not identified your problem correctly, though. Almost any joint can have excessive preload that causes damage to the clamped members. Your problem is that the plastic may creep, causing time-dependent preload loss. Independent of creep or other damage, you need to identify the preload required to keep your joint together. If you cannot measure or calculate the forces, then you need to estimate them. Then, you can select a fastener that can generate the required preload. Then, you can design the fastener and joint geometry to keep the compression stress below the critical level that causes excessive creep.

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.

 

[tcwong]

Indeed, the problem is to identify the preload required to keep the joint together... I have to admit that I don't have a clue on how to do it...

 

[tcwong]

I think I should clarify the problem a bit more... a product has been redesign to go from metal to plastic through a cost reduction effort. The designer who worked on the project didn't consider to add a boss to the mounting flanges of the product. I am new to this company; but from what I understand, it is highly unlikely that I can specify a new type/size of screw (or use loctite) to be use for this application.... I have never studied the relationship between preload and the "tighteness" of the joint for a product to be used in a earthquake prone country... for starter, I could use help or could use technical reference on preload and how it can prevent the joint from loosen under vibration.

 

[CoryPad]

tcwong,

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.

 

[tcwong]

Cory,

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

 

[CoryPad]

Thomas,

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.

 

[avjJOE]

not knowing the particulars of this application I can only sugest that you consider nylon screws. I worked on a probe housing that was continously presurized and normalized.The steel screws would eventualy eat into the plastic cover, However, the nylon would streach and contract with the assembly and in the event of failure were easy to remone and replace.