To Stretch or Not to Stretch? (tech)

[fahrvergnugen at cox.net (David Utley)]

The truth is out of the bag, yes I confess, I am occasionally a conspiracy
theorist.  I stick to my original assessment on VWs reasoning...  

David

-----Original Message-----
From: Allyn [mailto:amalventano1@tds.net] 
Sent: Tuesday, January 02, 2007 1:58 PM
To: 'Dan Bubb'; 'Don Walter'; 'Scirocco List'
Subject: RE: To Stretch or Not to Stretch? (tech)

> Excellent explanation Allyn!

Thanks Dan.  I got pissed off at myself for not knowing the real deal about
this for so many years, and therefore researched it
myself over the past few weeks.

> I just have one comment. You reference lubrication and how it 
> can change the clamping force of a bolt. Does it not make 
> sense that torquing to a lower value, and therefore incurring 
> a much lower frictional resistance to turning,  then turning 
> an addition angle of rotation will give a more consistent 
> clamping force irrespective of lubrication?

I assume you are talking about a standard bolt.  If you apply the plus angle
idea to a standard bolt, you will end up with a fixed
'gain' in clamping force, where that gain would depend on the K-value, which
in turn would vary with lubrication, etc.  Wait a sec,
ok, I see what you're saying.  Starting at that lower value would introduce
other errors into the equation for several reasons:
1. starting at a lower value, and torque amplifies as bolt head rotated ->
small error in initial torque could translate to a large
error in the final torque.
2. very small angle - since the standard bolt doesn't stretch (at the
desired tensile strength), and therefore doesn't provide any
buffer, you would get a very rapid increase in tensile force for a given
rotation.  For a short bolt, a single degree of rotation
could translate to the equivalent of a several ft-lbf error.  5-10 degrees
off, and you'd be way out of range.

> Just because the tightening spec specifies a torque plus an 
> additional angle of rotation doesn't necessarily mean the 
> bolt is a stretch bolt. It's simply a way to get a more 
> accurate clamping force in a critical application.

This assumes there is absolutely no galling / fouling of the threads.
Anything that interferes with the no-load free running of the
threads could interfere with either method.

This also assumes a very 'solid' application, where there would be no
expected compliance issues during the additional angle.
Imagine trying this on lug bolts, where the wheel wasn't fully seated
against the hub when the initial torque was reached.  That
extra angle of rotation would not result in the intended outcome (and you'd
probably lose that wheel after a few miles, to boot).
:)

> As far as the crank bolt IIRC there are no reduced sections 
> or other geometric features typical of stretch bolts. 

You're talking about the ones we deal with all the time, right?  Don had
mentioned a stretch variant available, as if there was an
option.  I'd guess you and I have only ever seen the 'old' type.

Al


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