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The
Tire Doctor Responds:
It may not be worth
getting "torqued" over
lug nuts, but getting the right torque on
lug nuts is actually very important. And, like so many things
in life, you want just the right amount
not too much, not too little.
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What is "torque" anyway?
Torque is defined as "a turning or twisting force."
As opposed to a pushing or pulling force that acts
in a straight line, torque is the result of forces rotating
or twisting something. Like a wrench on a lug nut,
for example.
What does torque do?
On lug nuts, the twisting force of torque creates the
straight-line force of clamping. And it's clamp force
that holds the wheels on.
Is "clamp force" the same as torque?
Not at all. Clamp force is the result of tension created
by torque. It's very difficult to measure in the real
world, but it's expressed in units of force, like pounds.
Torque, on the other hand, is relatively easy to measure,
and it's expressed in special units (foot-pounds) that
are the result of multiplying distance by force.
Distance?
Sure. Anybody who's ever used a breaker bar knows that
the longer the handle on a wrench, the tighter the nut
you can remove or install. That's because in torque, force
and distance act together, multiplying the effect.
In what way?
In our illustration, we show a 2-foot long wrench. Gripping
the wrench at the end gives you more mechanical advantage
than gripping it in the middle.
The mathematics of torque bears this out. Apply a 10
pound force 2 feet from the nut, and you get:
2 ft. x 10 lbs. = 20 ft-lbs.
To get the same amount of torque with your hand
at the 1 foot position, you need to pull twice as hard:
1 ft. x 20 lbs. = 20 ft-lbs.
How does torque create clamping forces?
You might remember from science class that nuts and
bolts are basically compact versions of the old inclined
plane. And, an inclined plane is a fancy term for a
wedge. So, nuts and bolts act like wedges, forcing the
wheel against the drum or hub.
How much torque is enough?
Check the specifications provided by the manufacturer
of the wheel end, but in general, most lug nuts should
be torqued to about 450 to 500 ft-lbs.
Can you have too much torque?
Absolutely. Remember how we said that torquing causes
the studs to stretch slightly, and that's what creates
the clamping force? When you remove the lug nuts, this
tension is released, and the stud goes back to its original
dimensions.
But with too much torque, the stud will "yield"
(or stretch permanently), and never go back to its original
dimensions. Not only that, but clamping force can become
dangerously low, nuts can work themselves loose, studs
can break off. In short, the symptoms of too much torque
can be similar to those of too little.
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Shouldn't our impact wrenches produce the right torque?
Yes, they should. But that doesn't mean they will. There
are several factors that influence the amount of torque
an impact wrench applies.
Air: Pressure fluctuations can create torque
fluctuations. Follow the wrench manufacturer's directions,
and install regulators on air tool lines if necessary.
Air Quality: Air that isn't clean, dry
and properly lubricated can cause torque to vary, and
may cause premature wearout of impact wrenches.
Time: Most impact wrenches are designed
to apply the correct torque when applied to the nut for
3 to 5 seconds. Too short a time and torque may be too
low; too long, and torque can be too high.
Component Condition: Nuts, studs and wheels
that are worn, corroded, dirty or damaged can cause improper
torque values. Manufacturers recommend you keep all components
clean and lightly lubricated for best results.
How do we know we're getting the right torque?
Once all equipment and components are in good condition
and operating to manufacturer's specifications, you
can test torque using a calibrated torque wrench.
Torque the wheel nuts according to your normal
method, then apply a manual torque wrench, turning the
nuts in the same direction as you would to tighten them.
Note the torque at which the nut first begins
to move.
If the measured torque differs from specification,
check the impact wrench, air supply, wheels, hubs and
components again. It takes something like 30,000 pounds
of clamp force to properly fasten a
wheel to a truck. That means every lug nut needs to
be at the right torque.
Anything else?
Three final things, all very important: First, mixed
nuts are fine at parties, but not on trucks. Even though
most new trucks are spec'd with hub-piloted wheels,
there are still lots of stud-piloted wheels out there.
The nuts and wheels are not interchangeable.
Second, if you find a broken stud while inspecting
a wheel end, be sure to replace both it and the neighbors
on either side of it before returning the vehicle to
service.
If two or more studs are broken off, replace
all of them. And, you probably ought to find out why
they broke. Remember, stud breakage and damage can be
caused by both too little and too much torque.
Third, follow manufacturer's instructions on
criss-cross tightening and on final torque levels.
Keep torque where it belongs and your wheels
should stay where they belong too. 
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