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[TowerTalk] Pulling vs. lifting

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Subject: [TowerTalk] Pulling vs. lifting
From: (Stephen Vinson)
Date: Thu, 11 Dec 1997 21:34:50 -0800
Chris R. Burger wrote:
> I saw a reference earlier on the Reflector that reminded me of
> something I'd wondered about--often.
> I've noticed that come-alongs and other hoisting gear are often rated
> differently for pulling and lifting.  For example, a particular model
> might be rated at 10 kN for pulling, and 7 kN for lifting.
> Why the difference?  In both cases, the tension in the cable is the
> same.  And, if you're pulling a cable that runs across a pulley, you
> could in fact be lifting something!
> The only reasonable explanation I can come up with is that things that
> are being lifted tend to regain ground if the cable slips, while
> things that are being pulled will presumably remain in place.  Thus,
> things that are being lifted could easily force the cable into
> dynamic friction, which could be inadequate.  Is that it?
> Chris R. Burger
> --

The difference is in "safety" rating.  The ability to lift an object is
its relative weight vs gravity so to speak.  Pulling is its weight
against friction.  They look at weight to failure lifting where there is
constant weight (gravity) acting in between pulls vs the equalization of
the weight (maybe not to zero) on an incline pull.  Friction will over
come some of gravities attempt to return the object to its original
possition.  You can drag several times the weight you can lift.  

I'm sure some one out there can produce the formulas (motion dynamics).

Its all a matter of safety and "Liability" ... HeHeHe


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