[s-cars] More on audi wheel bolts/studs/torque

Randy McCall redrandy at pacbell.net
Wed Oct 23 00:03:39 EDT 2002

Awright, Wolff throws in an equation for wet vs. dry torque!

So please allow me to throw my two relatively uneducated pennies into the
pond - torque is easy to get right, but correct bolt tension (thats the real
deal we're lookin' for!) for a given torque is a highly iffy thing with
contaminated or dirty threads, poor condition, corrosion, etc.  When I long
ago did high strength 7/8" galvanized (extrememly poor wrt clean threads)
windmill hardware we torgued to a low starting load (enough to pull the
joint together and take up slop) and then did turn of nut method.  Doesn't
matter whether its wet or dry, the same tension in the fastener is always
generated that way as the thread pitch (angle of the ramp if you will)
determines the amount of elastic stretch in the fastener per turn of the
nut. I'm not as up to speed on all of the metalurgy being bandied about, but
unless I forgot my basic engineering, for a bolt type fastener you want to
get up the stress/strain curve a bit, within the elastic limits of course,
and stand pat.  Torque is just an easy way to get to that point if you have
lots and lots of faith in correlations.

The caveat to that is of course the crux of the other half of this ongoing
debate, creep of the wheel allow around the bolt head interface, degree of
planar contact between hub and wheel and whatever other goodies you've
placed between the two(do they really touch each other, or does the bolt
preload get used drawing the two together...?), and other minutia that
probably assume a level huge importance if ignored!

So why not apply a good dose of A/S, torque to low level (for beginning
tension point) and then head on to the turn of nut method?  Yer crusty
studs/bolts will be at the correct tension regardless of the goop used, and
at least they come off when you want to swap those salt encrusted winter
tires for summer meat.

'91 20V 200TQW

> Message: 1
> Date: Tue, 22 Oct 2002 08:48:11 -0700
> From: wolff at turboquattro.com
> Subject: Re: [s-cars] More on audi wheel bolts/studs/torque
> To: urq at audifans.com, 200q20v <200q20v at audifans.com>
> Cc: mlp qwest <mlped at qwest.net>
> The first reference (DANotes) and the third reference (loctite) basically
> agree that the K number of a lubed thread is between .11 and .18 depending
> on the lubricant. The first reference lists .30 as the K number for an "as
> received, stainless on mild or alloy" thread and .20 as the K number for
> "as received mild or alloy on same" thread .   Loctite says:
> Where:
> T=torque (in-lb, ft-lb, N-m)
> K=torque coeficient or nut factor (determined experimentally for their
> of lubes and specifically not "friction coefficient" which is a different
> lower value)
> F=clamp load (lb,N)
> D=nominal diameter of bolt (in, ft, m)
> So if we keep F and D constant while changing K (the type of lube or lack
> lube and thread material), T will vary by as much as 63%.
> That's seems like a lot to me. If we use the lower .20 dry thread number
> take an AS K number of  .15 which looks like about an average value in
> loctite's chart then either F goes up 25% or you have to cut the torque
> to not raise your clamp load. What this tells me is that you can't just
> your lug bolts and torque them up to 82 ft-lbs or you are exceding the
> factory expected clamp load. Also, anytime torque is given for any
> fastener, it is critical to note what (or if any) thread lube is
> Will I lube and torque my lug bolts to 61 ft-lbs? Absolutely not and
> should you based on my uneducated ramblings and an unknown real K number
> Audi size and grade lug bolts. Locktite says "In critical applications it
> necessary to determine K values independently."
> Throwing more chaff out for the radar,
> Wolff
> "Nobody can forget the sound." - Michele Mouton

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