Posted by Matt Wilson on Thursday, April 05, 2001 at 1:19AM :
In Reply to: Re: Rivets vs. Grade 2 Bolts posted by Jeff on Wednesday, April 04, 2001 at 12:58PM :
You are correct in stating that friction developed by large clamp-up is the primary load-transferring feature of many joints. However, the references I have in front of me state that this is the case only for bolted joints, since bolts can develop a very large axial pre-load.
Rivets apparently cannot be depended upon for such a large pre-load, even when heated, installed then allowed to cool and shrink. Therefore, rivets are only good for shear-type joints where friction is not considered.
This is not to say that you will be able to walk up to your Power Wagon, grab hold of a frame rivet and turn it by hand, but apparently the pre-load is nowhere near to being enough for a friction-type joint.
This brings up the question that Rob Rose asked above: Does the use of bolts under high pre-load in the frame, in place of rivets, cause any problems with frame flexibility? My gut instinct is that it probably would not be a problem.
When a beam made of two or more layers of material placed in bending, the beam's stiffness depends largely on whether or not the layers can slide relative to each other. If there is something connecting the layers (i.e., welds, bolts, rivets, glue, etc.) to prevent sliding, then the beam won't deflect much under a given load. However, if the layers are simply stacked but can slide independently of each other, the whole beam will deflect a lot more under the same bending load. This is demonstrated by taking a stack of paper (say 100 sheets) and holding it by one end of the stack. If the papers are glued together, the stack will be quite rigid. If they are not, then the stack will be flimsy and will fold right over, under its own weight.
In the case of a Power Wagon frame, the reinforcement channel is riveted to the main frame rails, thereby preventing slippage between the two. If bolts are used instead, I think this would be about the same situation, even with high-strength, high-preload bolts. The bolts provide a really large preload, and prevent slippage, but the rivets also prevent slippage by taking shear.
I would think that the only time a high-strength bolt might be a problem is in situations where the frame is being flexed a LOT. A high-strength bolt may be strong enough to resist breakage, but if it is forced to stretch too far, it will simply snap. A rivet, on the other hand, may be flexible enough to withstand the deflection, although it probably will have yielded. Not sure about all this, though. Sometimes strength is not everything.
Anyway, sorry for all my ramblings! This has been a learning experience for me, even if it is only from text books! I'm an engineer by trade, but I don't deal with these types of joints in much detail on the job. This is interesting!
Matt
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