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.

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 tomove.

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.