Torque wrench types (was: Re: Tightening head bolts on enginestand?)

[treed2 at wsu.edu (T. Reed)]

> Ah, but the pointer type gets out calibration because of use, not improper
> storage...  :-)

I would argue that this applies to the clicker type, not the pointer
type.. I've never had mine get out of calibration. I think the biggest
risk factor for that is from accidentally bending the pointer.

I think I should make a clarification here. After a long time and/or a lot
of use, the clicker type is more precise, but less accurate. The pointer
type is more accurate, but less precise. This is all IMHO of course, but
its based on physics.

(precision means consistently getting the same result in identical
measurement trials, accuracy means getting close to the actual "God's
truth" value)

I chose "after a long time and/or a lot of use" because I think this
describes a typical scenario. If you have money to burn getting your
torque wrench calibrated on a monthly basis, go for it. I'd rather save
the hassle and have a little less precision.

Have a look at:
http://www.wildwestcycle.com/f_torque.html

(Scroll down to the "Torque Wrenches" heading.)

> Those pointer types get weaker and weaker with use, so therefore the more you
> use it, the likely you are to be under-torquing...

I disagree. The principle is simple.. I'm not an ME but I'll throw
out my explanation. Metals have an elastic region, and a plastic region.
Elastic means that it can be bent, but only so long as the force holds it
as such.. after the force is removed, it returns to its original shape.
Plastic means that it changes shape as a result of the force. I think
that's pretty clear. Anyway, the elastic region happens before the plastic
region. Kinda like how things get hot (which is undoable) before they burn
(permanent).

The (simplified) idea behind the pointer type is that you have a bar that
bends within the elastic region, and a bar that doesn't bend at all. The
relative difference between them is proportional (and hence, a scale can
be drawn) to the torque applied.

Now, if you exceed the elastic region and go in to the plastic region, the
torque wrench will look like a bent golf club. But the deformation will be
obvious..

> At least, this is what I have been told (by folks much smarter than I)...

I told you that?? :))

But seriously..

The clicker type works on friction and springs.. and is thus subject to
wearing of the friction surfaces and aging of the springs.

I've stripped and broken many bolts by trusting the clicker type. Since
then I've switched to using the pointer type almost exclusively because I
can see when I'm getting close and if I decide to compromise (not tighten
as hard as I had planned), I can see by how much.

To me, the clicker type is like having a "Engine overheated" led on your
dashboard versus having a coolant temperature gauge.

My 2 cents..

-Toby