Hmmm. Makes good sense. I'm trying to overcome the problem that it's a
blind hole and you can't get the stud to bottom because there's always
some residual oil. You'd think the stud would only be in compression until
you torqued the nut on top of the pedestal, then it would transition to
tension. But I can see how it would be hard to say what was really going
on from top to bottom of the thread engagement. Might be compression at
the bottom, slack in the middle, and tension at the top, with perhaps even
less threads taking the load than if the bolt was slack in the hole.
-----Original Message-----
From: Michael D. Porter [mailto:mporter@zianet.com]
Sent: Friday, September 26, 2003 8:50 AM
To: Bill Babcock
Cc: 'Wes Dayton'; Friends of Triumph
Subject: Re: Pedestal Studs
Bill Babcock wrote:
> I've got billet pedestals and ARP bolts, haven't had a problem. Make
> sure that your bolts bottom out though--it's easy to have them only
> threaded in part way. Same for head bolts. I use a good stud extractor
> and lean on them after cleaning the hole.
Ding, ding, ding....
Wrong. If the bolt bottoms out and is torqued further, the bolt is in
compression, not tension. This actually _reduces_
the tensile load, because the compressive load is subtracted from the
tensile load. Increasing the thread engagement is
okay (although there's some evidence that only the first few threads carry
the total load), but bottoming and torquing
against that bottoming works the opposite of what is required of the bolt.
Tensile loading of the bolt is actually
reduced by doing so. If you want to test that theory, do it with the stock
hardware which has a far smaller margin of
safety, rather than the ARP hardware.
Proved in practice. Our idiot engineers spec'd a bolt for securing a
cradle mount to a ZF trans which bottomed out in
the mounting holes of the trans adapter. Broke `em all, at considerable
expense for fixes in the field.
It's just geometry. If the bottom of the bolt is forced against the bottom
of the thread bore, which side of the bore
thread does it engage? The lower side, because of compression. As the bolt
is tightened further, it's simultaneously
being loaded in compression and tension. Clamping force is therefore
reduced by the amount of the compressive load.
Always ensure a bit of space between the bottom of the bolt and the bottom
of the bolt boss.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto:mporter@zianet.com]
Never let anyone drive you crazy when you know it's within walking
distance.
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