Mark Rand wrote:
On Sat, 14 Jun 2008 16:34:36 GMT, "Leo Lichtman"
wrote:
The movable jaw transmits its moment into the slot in the handle, which
is likely to be weaker than the stationary jaw. So you want it to
receive less moment. Therefore, turn the wrench so the force on the
movable jaw is away from, the tip.
Count that five for those that understand moments. I could never
understand how people could convince themselves that the other way was
right.
Mark Rand
RTFM
It's not so hard to understand. There are a couple of extra dynamics at
work here you are not thinking about.
The handle of adjustable wrenches are angled at about 30 deg (half of the
hex nut 60 deg) to make it possible to flip the wrench over in tight places
and continue to turn a hex bolt with minimal movement of handle (allowing
it to be used in tighter locations than a wrench with a handle which was
parallel to the jaws).
This angle creates an issue however. If you push the handle perpendicular
to the rotation of the nut (the normal way you work any wrench) the wrench
will try to push off and slide off the nut if you turn it one way, and it
will tend to slide onto the nut if you push it the other.
Because the fixed head on standard adjustable wrenches is the one further
way from the handle, you must turn the wrench in the "right" way, in order
for the forces to cause the wrench to by pushed onto the nut, instead of
being pulled off the nut.
As the handle comes towards you when you are pulling, you start to apply a
force which is not perpendicular to the handle. When using the wrench the
wrong way, once the wrench gets to 30 deg away from pointing straight at
you, if you keep pulling towards you, the force will be in perfect line for
pulling the wrench off the nut. When using it the right way, it's sill
applying a lot of torque to the nut at that same point helping to keep the
wrench from slipping off. When using the right way, the forces are working
to your advantage to keep the wrench snug against the nut. When using it
the wrong way, the forces are working against you trying to pull the wrench
off the nut.
Adjustable wrenches are designed with jaws strong enough so they won't
break or bend if they are used by a normal strength human without extending
the length of the handle with a cheater bar. You don't need to worry about
the jaw bending or breaking.
What you need to worry about, is the wrench slipping off the nut and
rounding it off in the process (and making your knuckles bloody). This is
why there's a right way and a wrong way to use the wrench.
But then there's the question of why don't they swap the location of fixed
and adjustable jaw so it will both stay on the nut and reduce the force on
the movable jaw.
I think there's yet another dynamic at work here. When the pressure is
applied further out on the adjustable jaw, more pressure gets applied to
the ways, and less pressure gets applied to the adjusting screw. As a
result, there's more friction on the ways, and less force on the screw
working to make it slip open. So I strongly suspect that the two jaws are
located how they are, instead of switched, because it actually works better
to have the force applied further out on the movable jaw to keep it from
slipping open.
So again, if you use the wrench the wrong way, you create yet another risk.
You increase the risk that the pressure will cause the adjusting screw to
turn, allowing the wrench to open - again increasing the odds that you will
strip the nut, and skin your knuckles as well as potentially marring both
jaws as you try to torque down on a nut with the jaws open too far.
So even though you are right that using it the other way will put less
damaging force on the weaker movable jaw, I think that is not the risk you
need to worry about. Keeping the wrench on the nut, and keeping the jaw
from slipping open, is far more important than worry about damage to the
movable jaw. The movable jaw is designed to take the pressure without
damage.
The only exception to this I would see, is if you were using a cheater pipe
and trying to put far more pressure on the wrench than it was designed for,
it might be wise to turn it the other way to minimize the odds of damaging
the movable jaw because it will no doubt be the first part to bend when you
try testing the wrench to destruction like that. But great care must be
taken at the same time to not let the wrench slip off the nut, and to make
sure the jaws aren't slipping open as you stand on that 10 ft pipe.
--
Curt Welch
http://CurtWelch.Com/
http://NewsReader.Com/