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Re: torque settings with a crowsfoot

To: tboicey@brit.ca, mgs@autox.team.net
Subject: Re: torque settings with a crowsfoot
From: REwald9535@aol.com
Date: Sat, 10 Apr 1999 17:30:49 EDT
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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