Date:         Wed, 3 Jun 2015 15:12:32 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Rear axle torque specs?
Comments: To: Frank Condelli <RAlanen@AOL.COM>
In-Reply-To:  <4363EC79-B099-4D87-B3E9-110965A275F6@aol.com>
Content-Type: text/plain; charset="iso-8859-1"; format=flowed

At 06:09 AM 6/3/2015, Frank Condelli wrote:
>in the Bentley, it is from my own 
>observations.  AND to add more to the confusion, 
>I have NEVER been able to understand why the 360 
>ft. lbs. of torques is needed on that axle 
>nut.  It’s just compressing the hub, bearing 
>inner races and spacer up against the circlip on the axle.

That's actually pretty simple, Frank.  Just as 
they did with the alternator pivot bolt they 
found that the six-slot castellated nut was 
exerting insufficient clamping force to keep the 
assembly from shifting and wearing under 
load.  It may be that they needed a nut of higher 
strength (more carbon, different heat treatment) 
or it may be that the variation in clamping force 
allowed by a sixth of a turn tolerance was too 
much, and the only difference in the nut is ten 
slots instead of six.  That could be determined 
from engineering specs for the nuts or from 
metallurgical testing of them.   Either way they 
ended up with a higher measured assembly torque 
and a smaller increment to get to the next slot 
lining up with the pin-hole in the axle.

I *think* the only issue here is of parts 
shifting and wearing against each other.  In the 
case of the alternator pivot bolt the issue was 
the original bolt breaking; and the new one 
breaking it not torqued enough.  In multiple 
attempts I have not yet been able to understand 
why a bolt becomes less subject to fatigue 
failure by being (with apologies to Phil 
Zimmerman) pre-loaded in tension,** but it is a 
fact.  And the stronger the bolt, the more tension it must be under.

**no, by Jove, I may have just gotten it, 
finally!  The idea is to keep all forces safely 
to one side of the stress/strain hysteresis 
curve, never approaching zero.  For some value of 
"safely".  Maximum force has to be below what 
would cause additional yield, and minimum has to 
be above the zone where the fatigue mechanism is 
invoked.  I may have just stated a truism rather 
than understanding what the ordinary explanations 
are trying to convey, but I suddenly feel a great 
deal clearer about fastener torque needs.

Yours,
David