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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