Many
times I have heard people complain that these come loose (Control arms
are an example). I ask if they torqued to factory specs and I always get
the "Yes I did..", then I ask if they adjusted for the preload
(frictional drag torque) on the nut. "Huh???" There torque is
inevitably lower than factory because they forgot this point.
Stated
torque measurements factor in the frictional force of clean parts, so
make certain that everything is clean, and has no burrs. Measuring for
Pre-load (frictional drag torque) is fairly easy in theory. The textbook
way assumes that you are screwing the nut onto a stationary bolt or thread;
we all know this doesn't work, since our stuff is not stationary. Here
is the easiest way to do it. Jam the bolt head with a wrench against something
so that it won't move. Don't hold it in your hand, since you will have
some give there, and may get an inaccurate measurement. Take the nut and
screw it on until it stops (hand tight), then take the torque wrench and
turn the nut. Start with the lowest setting and work your way up if you
are using a micrometer torque, if you are using a beam style, just watch
the pointer, the point at which the nut turns is the pre-load (frictional
drag torque). You then just add this to the torque you want to go to.
i.e. 10ft/lb pre-load (frictional drag torque) + 100ft/lb torque setting
= 110ft/lb setting on the torque wrench. There is another pre-load computation
for bolts based upon tension/torsion/bolt stretch/ friction/ etc... I
really don't want to get into that side, since it really doesn't have
a purpose in our application. For those of us with torque wrenches, here
is something that I forget to do also. Something to keep in mind when
we use them.
Preloading a torque wrench is an important to
the overall accuracy. It must be performed each time the torque wrench
is used after periods of non-use or whenever torque direction is changed.
There are several reasons for preloading your torque wrench. First, it
will set internal components so that when force is applied, torque begins
immediately with no internal settling. Second, it distributes lubrication
to moving internal parts.
How to do it: 1. Set torque wrench between 50 percent and 100
percent of full scale. 2. Mount torque drive in a stationary fixture
(i.e. socket welded to bench, vise). 3. Exercise the torque wrench three to five times
in the direction you will be verifying. 4. Perform torque measurement.
Remember to store torque wrenches (click-type)
in the low setting. Otherwise calibration will be needed at shorter intervals.
You should have your torque wrenches calibrated yearly for accuracy, but
it can be costly, so most of us don't do it. I admit that mine have not been calibrated in a long while. Though if you ask my wife, replacing them after I accidentally run them over counts as calibration.
