[TheForge] rivets: chapter two and half

[David E. Smucker]

Tom,  Good discussion.

First you have more experience setting large rivets that anyone else I know 
and there is just nothing like first hand experience.  My guess is that 
today it is hard to find hardly anyone that know how to do this for large 
pressure vessels / boilers any more.  When I was in engineering school back 
in the 60's we were told not to worry about rivet design since no used them 
anymore.

None the less assuming your plates are tight to start with (bolted as you 
say) and that the final heading occurs as the rivets are reaching the black 
stage you should develop some preload.  It will not be equal to that preload 
you can get with a high strength grade 8 bolt today because it can't exceed 
the yield strength of the rivet (low carbon mild steel).  Older engineering 
design texts for example Design of Machine Elements by Virgil Moring Faires 
(new ones don't have much on heavy rivets) notes that "The lengthwise 
shrinkage upon cooling of a hot-heated rivet induces a tensile stress in it 
that may be close to the yield stress of the rivet.  Because of upsetting of 
the body hot driven rivets are slightly stronger than the original rivet." 
Note that it is assumed that rivets are very low carbon mild steel and as 
such have a rather low yield stress.

Also from my Dad's  Mark's Mechanical Engineering Handbook -- from the 30's 
it suggest that the clamping force of the of hot driven rivets will be high 
enough to prevent movement between the two plates.  This is important when 
considering the shear strength limitations of the joint design.  It also 
notes that it is important to use a safety factor of 5 in design because of 
many installation and material unknowns.  Also they noted as you did that on 
long body rivets that not all of the body will be upset and fill the hole as 
in your experience with rivets in the mud ring area.  Back in those days a 
lot of testing in mechanical engineering departments would have been done on 
these types of subjects -- in fact if you could find them there are mostly 
likely technical paper written on riveted joint design, failure etc.  One of 
the things I have learned is that by the end of the "steam era" that these 
folks damn well knew what they were doing.

To change the subject a little, my experience with cal rod heated high 
strength bolts for preload is that they are a pain to work with.  These high 
strength bolts had a center bore in which a cal rod heater was install to 
heat the bolt and length it.  Then the nut was run up tight and the cal rod 
heater removed.  As the bolt then cooled a high pre-stress could be 
obtained.  We used these on some very large drive shaft couplings, universal 
joints etc. to bolt of the flanges. Then the reverse process had to be used 
to remove the bolts.  The mechanics hate working with these in the grease 
and muck.  Mostly they have been replaced with a "Super Bolt or Super Nut" 
(Nothing to do with super quench -- smile)  These have a ring of simple cap 
screws around the outside dia. of the nut or bolt head and after the nut is 
made up to a light wrench tight then each of these dozen or so socket head 
cap screws is tighten X turns -- preloading the bolt.  Works very well and 
are much easier to use that the cal rod type units.  Even a blacksmith could 
install these "Super Bolts"  (blacksmith content for Mother.)

Dave Smucker
----- Original Message ----- 
From: "Thomas A. Troszak" <tom at tomtroszak.com>
To: <theforge at mailman.qth.net>
Sent: Saturday, March 05, 2005 12:06 AM
Subject: [TheForge] rivets: chapter two


>
> On Mar 4, 2005, at 8:13 AM, Dave Smucker wrote:
>
>> As you noted a correctly made riveted joint is as strong as a preloaded
>> bolted joint.  The hot applied rivets cools and develops a preload in the
>> rivet just as the torque applied to the bolt develops a preload in the 
>> bolt.
>>
>
> Dear Dave,
>
> I don't think that hot rivets really contract enough to create the proper 
> load, the joint has to be preloaded by bolting, and then if the rivets are 
> installed correctly, they will pretty much retain the preload of the 
> bolts. I think a lot of folks believe that hot rivets will suck a joint 
> closed, but it just ain't so.
>
> My gut feeling is that the hot rivets, being hot, stretch about as much as 
> they shrink while they are cooling (sounds weird, but think about it), you 
> only get the real clinch after the color has gone from the head, and only 
> if the joint is firmly bolted first. In other words, you have to keep 
> hammering and bucking until most of the color is gone. If you stop 
> hammering while the head is still bright, the rivets tend to be looser. 
> You have to to keep packing the hole full (upsetting) while the shank is 
> still shrinking. Also, if the rivet is fairly long (8-10 diameters or 
> more), like a mud ring rivet, you will find that the shank is really only 
> upset near the ends.
>
> Still, in order for the riveted joint to be effective at all, the plates 
> must to fit perfectly, and be bolted firmly before the rivets are applied. 
> I fit my boiler plates so that a strip of paper will not pass through the 
> joint between the bolts, and I bolt ALL the holes first.  Then when I am 
> ready to rivet, I remove the bolt(s) where I wish to install the rivet, 
> and stuff them in. You can avoid puckering the whole assembly by riveting 
> the ends and center first, then keep splitting the difference as you go; 
> if you start at one and and work along, you run the risk of cocking the 
> plate so badly that the bolts won't come out, which is then a lot of extra 
> work...
>
> I don't have hard data to back this up, but from my personal experience 
> with hand riveting ironwork (mandatory blacksmithing content here) I feel 
> I can often get a tighter clinch from riveting cold than hot. I think the 
> only reason that hot rivets are riveted hot is because they have to be 
> deformed so severely to create a decent sized head, as it takes about 
> three diameters of rivet to form a full half round head. it just would not 
> be possible to upset this much material cold by hand, especially when 
> balancing on a scaffold.
>
> Those babies are sticking WAY out when you start, and you really have to 
> hold your jaw right to keep the head centered. I draw chalk marks 
> perpendicularly across the plate through the center of each hole, so that 
> I can keep the heads centered more easily.
>
> I hope you don't think I'm being nit-picky, because I'm not. I'm simply 
> trying to be as clear in explaining my understanding of the things that I 
> do. I enjoy discussing engineering things with you. There may be others 
> lurking out there with more riveting experience than I who can explain 
> this better, and if so, please chime in.
>
> Tom Troszak
>
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