On Tue, 09 Sep 2008 20:00:50 -0700, Larry Jaques
wrote:

On Tue, 09 Sep 2008 17:48:49 GMT, with neither quill nor qualm, "Ivan
Vegvary" quickly quoth:


"Larry Jaques" wrote in message
. ..
On Mon, 08 Sep 2008 23:48:48 -0700, with neither quill nor qualm, JR
North quickly quoth:

I don't kniow what the tensile rating is of the chuck joint, but I bet
it's nowhere near that of a properly swaged coupling.

2,680 pounds weight capacity. I wonder if you're supposed to divide
that by the 4mm opening, which would cut it to 422 lbs. Still, that'd
hold a couple large guys standing on one strand of the railing wire.

Actually, no. When applying force perpendicular to a tight wire, the force
multiplies by a large factor. In a pinch, you can winch a vehicle out of a
tight spot by tightly tying a rope between the vehicle and a tree and
applying force perpendicular to the middle of the rope. Needless to say you
have to take up the slack between each push on the rope.

I don't grok that, Ivan. A pull on the middle of the rope would be
equal on all 3 ends, wouldn't it? Even if it were a nice round pulley,
it would not give you any force increase. The individual snatch
blocks (pulley with a hook) we used on the tow trucks (at Flynn's
Frame and Collision eons ago) would only give us the -same- pull the
winch had, but it would be at a different angle, better for that
particular extraction. Multiple pulleys (forming a block and tackle)
would multiply the force.

If you still disagree, please cite the formula/law for us.

Ivan is right, and the easiest way to grok that is to make a vector
diagram of the forces involved.

Suppose you have a stretched, fairly straight rope and then hang a
weight at the middle of it. The rope stretches a bit, so you get an
angle. The downward force on the weight is counteracted by an upward
force, or you wouldn't have equilibrium.

That upward force is the resultant force of two force vectors, one
along each side of the rope.

Draw the diagram and you will see for yourself how huge the force
along the robe gets with even a little force downwards.

S.


For S&G, I asked the question to the guys at Feeney, the Cable Rail
people. I'll post their answer if/when I get one.