Bolted Joint Design & Preload; Varying Number of Identical Bolts 1

[IronLotus]

Hello Everyone,

I have a question about the theory of bolted joints & preload. As a recent graduate I've done a far bit of personal research on bolted joint theory to help my project work & I have a decent understanding. However I think I'm missing something fundamental & wanted to check my basic understanding & tendency to overthink.

Let's say we have 2 plates/members bolted together with a SINGLE thru bolt & nut, the bolt is preloaded to say 1k lbf. There is a 1 kips compressive force on the 2 plates & therefore preload.

Now bolt the same 2 plates together with 4 of the same exact bolt (material & size), & preload all 4 bolts to the same 1 kips. Is the compressive force between the 2 plates now 4 kips? This is what my intuition from statics would assume, however I somehow tried to rationalize that by already preloading the plates with just 1 bolt the plates would be "precompressed" & adding 4 bolts would not increase the preload in this joint. I have found examples online that seem to assume what my intuition agrees with, but could not find any authoritative source.

Thank you for your help in clearing up my basic understanding, & how helpful the members of this forum have been over the years to an engineering student.
Cheers.

 

[3DDave]

What you are considering is why multiple bolts are typically tightened in a repeated sequence.

If only one bolt did the job most people would not use more than one.

It may clarify understanding to think of each bolt as a spring load. Would 1 spring support more or only as much as 4 springs identical to it?

 

[rb1957]

yes, 4 x 1 = 4.

look at it this way. In your 4 fastener pattern, install only one and preload. Sensibly the region around the fastener reacts the preload, feels compression, but away from the fastener there may even be gapping. Install the other 3 fasteners, now there is 4 kip of compression between the faces.



"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[IronLotus]

rb & Dave, thanks for the quick replies & all your forum posts over the years.