self stress relief?

[RalliartQA]

hey guys,

i am new to this so i will try to be brief. the company that i work for has a product that is assembled with a hot riveting method. all sides get this treatment. we measure the product before, during and after this method to ensure a very tight fit. however, about 3 days after the final inspection we find that the areas that are being riveted have moved sometimes .005 to .020 out of tolerance. what would cause this to happen after such a period of time without them being touched?

 

[btrueblood]

What material(s) is (are) the assembly?

 

[RalliartQA]

4140 approx. .002 thick

 

[CoryPad]

That is thin foil. How was it produced? If it wasn't fully annealed, then it has large residual stresses. These can be relieved by application of heat, which can lead to dimensional distortion. Even without residual stress, uneven heat plus uneven constraint can lead to dimensional distortion.

Regards,

Cory

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

 

[RalliartQA]

they actually are fully annealed and then hot riveted. but when they sit over time after assembled they change in dimension. they are annealed from a week to a month before being assembled and then hot riveted then inspected within the following couple of days

 

[btrueblood]

Unless the rivets are another material that expands (contracts) more than steel with temperature change, I doubt you are getting the rivet set that you were counting on with the hot riveting; i.e. the foil quickly heats through to nearly the same temp. as the rivets, and no contraction on cooling occurs within the rivet joint.

Could you not spot weld the material?

 

[RalliartQA]

I wish. For reasons i cant explain (confidentiality aggrement)the part cant be spot welded. To be honest, the part is under so much pressure that if not hot riveted it would basically explode with the chance of very serious injury to the operator. the other unfortunate factor is that the print was made in the 1940s. Not many changes have been made since then and noone that worked on it then is still alive. but the tolerences have become more intense over the years. Hence the problem.......

 

[unclesyd]

These are some alternate methods of rivet setting to your hot riveting that you may want to look at.

http://www.orbitalriveter.com/products.htm

 

[march1971]

I think that the problem with this has to do with expansion and contraction that accompany temprature changes in a material. I agree that the material is thin but one have to understand that the rivet is a type of joint whereby two separate parts are held together by a clamp-like component or bolted together. If there is a change in temperature (increase or decrease), the rivets themselves may expand or contract without a change in the dimensions of the foil. Where the rivet expands, there is bound to be an increase in joint opening (depending on the expansivity of the rivet material and the temperature difference from normal temperature)