On Thu, 12 Jun 2008 22:11:56 -0700 (PDT), John Martin
wrote:

On Jun 12, 10:23*pm, "Martin H. Eastburn"
wrote:
Take a wrench and hang it onto something.

One way you are putting the pressure on the top jaw, the other way
puts it on the bottom jaw.

You normally want it on the top one. *The bottom jaw isn't as strong,
being that of a sliding jaw that moves side to side and in and out.
The latter can break if the main force is placed there.

Martin


Martin:

I think you're way off the mark with your mention of the "main force".

As I see it, if you're pulling on a wrench with 100 pounds of force on
the end of the handle, then the far jaw should have on it 100 more
pounds of force than the near jaw. It's pretty much meaningless,
though, because it is dwarfed by comparison to the force on the jaws
coming from the nut's trying to wedge them open. That force is equal
on both jaws.

I wasn't trying to set you up with a trick question, though. I do
expect that most people will say as you did: if you are pulling a
wrench, the adjustable jaw should be toward you. I'm not sure that's
right - here's why.

If you're pulling on a wrench with the solid jaw away from you, the
force is at the base of the solid jaw and the tip of the sliding jaw.
The force on the sliding jaw is also to push it against the wrench
frame.

If you're puling on a wrench with the solid jaw toward you, the force
is against the base of the sliding jaw, and is also to pull it away
from the wrench frame.

I'd rather have the force against the base of the sliding jaw, even
though doing it "backwards" like that does try to pull the jaw out of
the frame. But I'd like to hear what some others think.

I agree with your conclusion, and I think for the same reasons. But I
don't agree that the "wrong" way applies the force on the moving jaw
in a different direction, i.e., pulls the jaw away from rather than
pushes it toward the frame.

To apply a torque to the nut there are two equal and opposite forces
(a couple, in statics jargon) applied to diagonally opposite corners
of the nut. (As you said, we can safely ignore the relatively small
force applied to the handle by your hand.) So depending on which way
you apply the wrench, the force on the adjustable jaw can be acting
either near the frame or near the tip of the jaw. In either case the
force applied to each jaw face is the same magnitude and direction,
and is resisted by the adjusting screw. What differs is the moment
produced by that force, which is trying to pry the jaw out of the
frame, and that moment increases as the point of application of the
force moves away from the frame.

Mark me down for pull with fixed jaw towards you.

--
Ned Simmons