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[TowerTalk] Pulling vs. lifting

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Subject: [TowerTalk] Pulling vs. lifting
From: (Chris R. Burger)
Date: Thu, 11 Dec 1997 18:37:43 +200
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

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