REwald9535@aol.com wrote:
> > place a crow foot on my torque wrench at 90 degrees wont the momentum
> > modify the torque settings?
>
> About the crank, yep that Mr. Newton and his law again, static friction is
> greater than sliding friction.
While for practical reasons all the effects are the same, it's
important to note that when applied to a nut on a bolt, this
is not a Newtonian situation.
ie: The concept of inertia is not the reason for the
bolt tightening situation.
As you correctly state, the static friction is greater
than the sliding (dynamic) friction, and that is the physics
in question.
The inertia of a nut spinning (or not spinning) around
it's axis is impossibly small, and a human would not even
notice the effort required to start or stop it. Even if
you have a nut spinning in an impact wrench at great
speed, stopping it can be done with one fingertip.
As you state, that effect is due to the differences in
the static and dynamic coefficients of friction.
The static and dynamic friction concepts basically have
to do with the very small surface interactions that happen
between two surfaces.
In a nutshell, the surfaces "lock" better when stopped
than they do with motion.
To use an example, imagine a very long set of stairs
and a cardboard box.
Attach a string to the cardboard box, and stand
at the top of the stairs and try to pull the box
up the stairs.
You can imagine that as long as you keep the box moving
rapidly up the stairs, it will skip and bounce along and
you will be able to keep it coming.
However, imagine that you let the box come to rest
on the stairs, then try to start pulling again. It's
quite likely that the box will find purchase in a nook
of the stairs and will require a great pull to break
it free.
This is what happens on a micro level with any
sliding surfaces such as the mating areas of a nut
and a bolt. As long as you keep the surfaces moving,
the surfaces will not mate to such as a degree as they
will if you let them stop.
Conversely, to start the nut moving, you have to
apply more force to break the bond than you do to
keep the nut turning once you have it turning.
In summary, everything that was said is perfect, and
all automotive applications are true. However, inertia
is not a meaningful factor, and is not a part of this
discussion.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
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