On Thu, 14 Apr 1994, Bownes wrote:
> ->> On a theoretical level, the socket extension can act as a torsion bar,
> ->> reducing the torque getting through to the head of the nut or bolt.
> ->Ummm, I don't think that's too likely. Any torque you apply to one end
> ->of the extension is going to have to be opposed by the equal opposite
> ->torque at the other end. Sure, it twists some (not so as I could tell,
> ->but "theoretically"), but it will transmit all the torque...
> Theory or no, my beam type torque wrentch came with a formula for the
> correction factor. It's not much though.
Maybe everybody is right. I'm not sure, though--there must be a lot of
engineers in the group for whom this is duck soup. My reasoning, as far
as I can take it, is:
A beam-type torque wrench imposes a bendable handle between the nut and
the hand. Push on the handle, and the handle turns the nut. As the nut
gets tight, the handle bends, by an amount proportional to the torque of
the nut. A scale marked in foot pounds (or N-m) is attached to the
bendable handle, and moves with it. A pointer attached to the axis of
rotation does not bend, so the pointer points to the torque as the scale,
attached to the bending handle, moves by the stationary pointer.
It is obvious (I hope) that an extension added to the wrench parallel to
the bendable handle, which moved the axis of rotation away from original
axis would make the readings wrong. The lever arm would be increased but
the pointer end would fasten at the original axis of rotation. Maybe they
make such extensions to allow wrenches to be used in higher torque
applications--like VW beatle rear axle nuts--I have never seen them, but
if they are made, they would require corrections.
But more interestingly, what if you attached a conventional extension
between the torque wrench and the socket. The torque applied twists the
extension now, as well as bending the handle. The *real* torque for a
given amount of hand force has not changed one whit, so those who said the
extension would not change the torque are right--it wouldn't change the
*real* torque. The question I cannot answer to my satisfaction is whether
the *reading* of torque would be wrong. Part of me says, well, you have
changed the overall spring properties of the system, which has to affect
the pointer. Another part of me says, hey, after you get the extension
wound up, the fact it is there does not matter because the torque at each
end of the extension is the same and the wrench should read correctly.
So, engineers, which is it? It is hard to believe that the wrench company
that put the corrections in the instructions was crazy, but maybe....
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
firstname.lastname@example.org (802) 656-8910