[s-cars] Re More on Wheel Torque

[Theodore Chen]

--- Ian Duff <iduff at rcn.com> wrote:
>
> Is there such a thing as a conversion from dry to wet torque? My guess is
> no, since there could be way too many variables. I'm guessing that applying
> a measured amount of torque to a fastener is an indirect way of measuring
> fastener stretch, which might be a direct way of measuring clamping force.
> Am I close yet? If so, might it be possible to install a lug bolt dry to the
> specified torque, measure the stretch, remove it, select and apply an
> antiseize compound, and re-install with a torque wrench until the same
> stretch was achieved? How does one measure fastener stretch? Micrometer? Are
> thousandths of an inch sufficiently accurate? Is measuring overall length of
> the same bolt dry vs. wet in the same hole good enough?

ian, there are equations for calculating tightening torque based on
desired clamping force.  those equations do take into account the
coefficient of friction of the thread surfaces and of the fastener head
surface.  generally, 0.16 is chosen as the coefficient for steel on steel.
there's probably a reference somewhere that indicates the coefficient of
friction for steel with oil on the surface, or with antiseize on the
surface.

with a dry fastener, all but about 10% of the tightening torque goes
into overcoming friction.

you could do a little experiment if you have a dial indicator.  measure
the bolt stretch of a bolt with clean dry threads under high torque.
lubricate the threads with antiseize and measure the stretch at the
same torque level.  lubricate the head with antiseize and measure the
stretch at the same torque level.  clean off bolt, use oil as the lubricant,
and measure again.

> Given that my track time has yet to occur, and is unlikely to do so in the
> near future, my need to torque after every run group is improbable.

get yourself another car to drive at the track.  it's cheaper than making
a 2-ton audi track-worthy.  how about a spec miata?

> I feel
> it necessary to check torque every now and then, just as I do to check tire
> pressures. However, when the time comes to change a wheel, the "graunching"
> as I tighten the lugs makes me think that the torque I'm applying, even thru
> a torque wrench, might be affected by whatever is causing that horrendous
> noise. BTW, how or where does one get their torque wrench calibrated?

the last time i did it, i connected the torque wrench to a breaker bar.
i attached a weight to the handle of the breaker bar, and calculated the
torque that it would produce.  my torque wrench was quite accurate.  it's
a craftsman digitork wrench, and has served me well.

-teddy


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