Looking For Trouble
Without a Torque Wrench, You’re Just Under the Gun
On Sept. 27, 2000, a tractor-trailer on I-70 outside of Columbus, Ohio, lost two wheels on the trailer. One of the wheels bounded across the interstate and struck another semi traveling in the opposite direction, causing the vehicle to lose control and jump the concrete median barrier.
The resulting head-on collision between the second rig and a passenger van ended the life of the van’s driver.
At a truckstop a few miles up the road, the truck driver who lost the set of duals on his rig had no idea how long the tires were missing. He was about to go under the microscope.
But the truck driver wasn’t going there alone. Somewhere, the last tire technician who installed the now missing tires and wheels was returning another tractor-trailer to service. He certainly had no idea what happened on I-70, and surely didn’t intend to cause a fatality.
At some point, someone taught him how to change tires and mount tire/wheel assemblies on trucks. They told him, "This is the way you do it." The technician probably did it the same way he was taught, the same way every time he touched a vehicle.
But if that method didn’t include a torque wrench, there was going to be a serious problem.
Torque Wrench a Must
Tire dealers love the bottom line, so we’ll get right to the point. Any facility that installs tire and wheel assemblies must have – and use – a torque wrench. Whether it’s a small crackerbox car or a giant 80,000-pound truck.
A torque wrench is used to apply specific amounts of twisting force to a fastener (ie. lug nut). It is the only effective method for ensuring proper installation torque on a consistent and exact basis.
The one thing that might have saved the company employing the technician who installed the tire/wheel assembly that ended that van driver’s life would have been that a torque wrench was used to secure the assembly. If not, all bets were off.
Of course, a torque wrench that’s never used is as bad as not having one. In theory, a torque wrench should be used on every fastener on every vehicle without exception. It requires a technician who pays attention to details and follows the correct procedures for inspecting and installing tire/wheel assemblies.
To do their job properly, torque wrenches require periodic calibration and cannot be abused (dropped, tossed, thrown, slammed) or exposed to the elements for extended periods. Again, an uncalibrated torque wrench is as bad as not having one.
Unfortunately, too many people currently installing tire/wheel assemblies have never seen a torque wrench. And they have no idea how much torque is required on the fasteners, mostly because they don’t know how to check or where to get that information.
Instead they rely on the good old impact wrench. Now, I admit impact wrenches are a great time- and muscle- saver – when removing tire/wheel assemblies. But too many are using them to remount the assemblies, and have no clue how much torque is actually being applied to the fasteners. They just run them down until they’re tight and move on. If more of technicans would pick up a torque wrench and make an attempt to calibrate their impact wrenches, lives could be saved.
Calibrate an Impact Wrench?
But how do you calibrate an impact wrench? Well, you can’t. A standard impact wrench cannot by itself control the amount of torque applied to a fastener. Some new products on the market, such as a system by Ingersoll Rand, use computer technology to apply precise amounts of torque. But, for now, most shops use the standard wrench.
Impact wrenches can only be measured for efficiency and consistency. They are designed to operate at a 90 psi running pressure. This means a gauge mounted at the tool will read 90 psi with the impact wrench running wide open. When the wrench trigger is released, the static pressure – generally around 135 psi – can be read.
An impact wrench that consistently delivers 90 psi running pressure at the same static pressure is probably oiled daily and regularly maintained. It can never be a precision instrument, but it does need to be as good as it can.
Once a consistent amount of force is established, a technician can more effectively guess when the torque is correct. It’s still a guess, but it’s definitely better than the alternative. After tightening a fastener with an impact wrench, the technician still needs to use a torque wrench to measure the actual amount of torque applied.
On beam and dial torque wrenches, the first movement in a tightening direction should be recorded with the memory needle. On "clicker" or "break neck" type torque wrenches, the amount of torque must be gradually increased until the fastener moves.
In a perfect world, all torque wrenches should be set at 475 ft. lbs. for disc wheel systems, and every fastener would move a little bit before the torque wrench clicks, breaks or reaches 475 ft. lbs.
Check the Wrench
If the torque wrench is not used to properly apply the final torque, practically every guideline and recommended practice for installing tire/wheel assemblies has been violated.
Hub- and stud-piloted wheel systems have a recommended installation torque of 450-500 ft. lbs. Standard operating procedures require a specific target torque value to account for the natural variance of the torque wrench. In most instances, a beam torque wrench is accurate to around plus/minus 2%, a dial wrench to around plus/minus 3%, and a clicker to around plus/minus 4%.
If you do the math, the actual installation torque will still be within the recommended range (see chart).
Time No Substitute
As we said earlier, impact wrenches cannot be calibrated. A technician might think he has a pretty good "clock" in his head, using time as a means to guesstimate torque, but he can’t be specific. He may also think the impact wrench will deliver consistent results simply because the static and running pressures on the wrench haven’t changed recently. But he still can’t be sure.
What happens if he tightens a nut to "one Mississippi" (one second), but the torque wrench he has set for 475 ft. lbs. doesn’t move the nut? After increasing the amount of torque on the wrench to 500 ft. lbs., the nut still doesn’t move? At that point, the technician will feel assured that the fastener is not loose, but it’s probably overtorqued – just not as overtorqued as a nut that’s tightened with an impact wrench until it stops moving.
Fact is, it doesn’t take more than a second for most impact wrenches to apply 475 ft. lbs. of torque to a hub- or stud-piloted wheel fastener. Under certain conditions (like too high a running pressure), two seconds of impact wrench use can result in over 1,000 ft. lbs. of torque. At that level, stud replacement becomes inevitable.
Eliminate The Guessing
You can check an impact wrench’s consistency (static and running pressures) and use a torque wrench to become a better guesser. But you’re still going to be under the microscope if an impact wrench is used to establish the final installation torque.
An impact wrench torque audit simply shows some degree of care takes place when fasteners are tightened. This degree of care may result in some degree of reduced liability should a tire/wheel assembly come off a vehicle. When there is no care, disaster usually occurs.
If a torque wrench never enters the care system, the technician has no idea if a fastener is properly tightened. When the wheels come off a vehicle, some poor guy will probably be put under the microscope.
If you know anyone like this, do us all a favor and show him how to use a torque wrench – before someone else gets hurt.
Kevin Rohlwing is director of training for the International Tire & Rubber Association (ITRA), and this article originally appeared in ITRA’s Commercial Tire Service publication.