Axial load in the fastener will be higher when the threads are lubricated. The
basic equation for determining the axial load in a threaded fastener is T =
KxDxL, where T = torque, L is axial load (i.e., how "tight" the bolt is), D =
the basic diameter of the fastener and K is a friction factor. The value of K
is usually given as .20 for plated or coated threads and .15 for uncoated
threads - that's a 25% higher axial load for the same torque when plating or
lubrication reduces friction between the nut and the bolt in the threads. There
are other factors, like friction under the nut or the head of the bolt, that
come in to play - precise determination of torque is a complicated affair.
On wheel lugs I have always used a torque equal to MW x TLOTLW (My Weight x The
Length Of The Lug Wrench) - haven't had one come off yet. The problems come
when the guy at the tire shop raps 'em down hard with an air wrench - dry
threads can gall up and seize and MW x TLOTLW won't be enough to back them off,
which you'll usually need to do at the side of the road in the middle of
nowhere in the middle of the night ;^)
BTW - personal experience with lubricated threads and torque values: I
replaced a cracked exhaust manifold on my '79 Toyota pickup last year and
decided I would put a touch of antiseize on the exhaust manifold studs so the
nuts wouldn't seize up - just a precaution as I hadn't had any problems getting
them off. A couple of months down the line I developed an exhaust leak. As it
turned out, lubing the threads increased the axial load in the manifold studs
to the point where the threads in the aluminum head started to fail, to the
extent that I lost a couple of studs completely and a few more were loose.
After drilling, tapping and heli-coiling all but 2 of the manifold stud
locations and installing new studs, I put the sucker back together dry! That
was one of those "If It Ain't Broke, Don't Fix It" situations...
Gary McCormick
San jose, CA
Tom Phelan wrote:
> <snip>
>
> > Use antisieze on the lugnuts/studs and insist on the nuts being torqued to
> the proper
> > specification, not just run down hard with an air wrench set on
> "cold-weld" ;^)
>
> On a side note - what is the torque value when you use antisieze? It's my
> understanding that you use a lower torque value when using antisieze since
> it is so slippery? Using standard torque values will result in over
> tightening the nut. I had some head bolts fail/break on a two cycle engine
> once when I torqued to recommended torque using antisieze. I have heard that
> the torque value can be in the neighborhood of 80% recommended torque. Is
> this true?
>
> Tom P
>
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