Trevor,
Granted I got a little hosed up with the bolts inertia Vs static/dynamic
friction. However Enrique's question was about turning the crank in an
engine he built.
> Regarding the point you make of moving a bolt
> from rest, this is similar to what I found with my recently installed
> crank. Instructions state that after tightening the crank bolts the crank
> should move freely. Mine does but only after I have taken it out of rest
> with a wrench. Does this sound normal?
The crank does have a noticeable mass and interia, so wouldn't the situation
of turning a crank by hand would be a combination of Newtonian physics and
static/dynamic friction? (interia of trying to start 50 lbs of crank, and the
friction of the crank in the oiled bearing). I'm not trying to argue here I
just want to get a better understanding of the subject myself.
Rick
In a message dated 4/10/99 10:56:07 AM Pacific Daylight Time, tboicey@brit.ca
writes:
> > > 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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