For the past 17 years, the Tire Industry Association (TIA) and its predecessor, the International Tire and Rubber Association (ITRA), have been dedicated to educating technicians on the proper procedures for servicing truck tires and wheels.
The Commercial Tire Service (CTS) Program was designed to provide the industry with standard training that complies with Occupational Safety and Health Administration (OSHA) standard 29 CFR 1910.177, Servicing multi-piece and single piece rim wheels. It also identifies the hazards related to this type of work.
Another major goal of the CTS Program was, and continues to be, educating the industry on the correct methods for installing wheels and rims.
For decades, truck tire technicians relied solely on an impact wrench to tighten the lug nuts. The theory was, the tighter the better, so people didn’t pay any attention to torque.
On demountable rim assemblies, the measure of success was simply making sure the rims didn’t spin. For stud-piloted wheels, technicians would be careful when tightening the inner cap nuts, since they could break quite easily, and then hammer on the outers to keep everything together.
New Wheel System
With the introduction of hub-piloted wheels, things started to change. The fear of breaking studs was basically eliminated since the components are a higher grade of steel.
Everyone had a different procedure for installing the new wheel system so there was very little consistency from shop to shop. Once again, the measure of success was simply making sure the wheels didn’t become loose after they were installed.
Torque
During the first CTS train-the-trainer class back in 1997, we put a lot of emphasis on using the proper torque when installing wheels and rims. All of the major manufacturers had torque specifications, so it made sense to follow them at all times.
Some of our members jumped on board right away and took steps to install every assembly with the proper torque. Unfortunately, most of them experienced an increase in the number of loose wheels and rims because all of the efforts were focused on torque.
After investigating several of the incidents, it was evident that we needed to focus on more than torque when teaching wheel installation. Torque is simply a measure of twisting force that ultimately determines how far the fastener is turned.
Clamping Force
Bolt tension, or clamping force, is the physical property that actually pins the components together against the hub or drum. Since clamping force cannot be measured in the field, the only way to approximate the correct amount of force is to control the torque used to tighten the fastener.
But in reality, the correct torque doesn’t guarantee anything at all.
Torque is like the setting on the oven. Temperature is a measure of heat just like torque is a measure of twisting force.
Even something as simple as a frozen pizza requires a precise temperature for a set period of time if the end result is expected to be edible. Change the temperature or the time. and all bets are off. In order to ensure a perfectly cooked frozen pizza, one must follow the directions and/or the recipe.
RIST
Since clamping force is the key and the correct torque doesn’t guarantee anything, TIA came up with the acronym RIST to help technicians remember the recipe for installing wheels and rims.
R = Remove debris from mating surfaces.
I = Inspect components for damage or excessive wear.
S = Snug the fasteners in a star pattern.
T = Torque to specification.
- R – Removing debris from the mating surfaces reduces the chances of joint settling.
When dual wheels travel down the road they flex to a minute degree. If foreign material is trapped between the duals or between the inner wheel and the hub or drum, it will work itself free and leave tiny gaps. These gaps will reduce the amount of bolt tension and ultimately will result in loose wheels.
- I – Inspecting the components is the second key ingredient when installing wheels or rims. Worn fasteners do not deliver the same level of performance, which means the correct torque results in insufficient clamping force.
TIA conducted tests of hub-piloted flange nuts with and without oil to demonstrate this point. When the flange nuts were dry and torqued to 475 ft/lbs, they delivered approximately 25,000 lbs of clamping force. When oil was applied to the stud and between the washer and the nut body, the same flange nut delivered about 50,000 lbs of clamping force at the same torque.
- S – Snugging the nuts in a star pattern helps to ensure that the wheels or rims are evenly seated on the hub or drum.
Again, TIA conducted tests that showed when technicians snugged the fasteners in a circle pattern, the amount of torque on each lug nut varied. If the same circle pattern was used to torque the lug nuts, similar variations were often measured, which meant that some of the studs had to work harder than others.
This uneven distribution can lead to fatigue and eventual failure.
- T – Finally, when all of the mating surfaces have been cleaned, the components inspected and the fasteners snugged in a start pattern, technicians can apply the proper torque in star pattern to approximate the correct amount of clamping force.
Again, there are no guarantees when it comes to torque and clamping force. But when technicians follow the RIST procedure on a regular basis, success is the most likely outcome.
On the other hand, if one or more of the steps preceding torque are skipped or incomplete, the chances for loose wheels or a wheel-off incident start to increase.
The Tire Industry Association (TIA) is an international non-profit association representing all segments of the tire industry, including those that manufacture, repair, recycle, sell, service or use new or retreaded tires, and also those suppliers or individuals who furnish equipment, material or services to the industry. The leading authority in the tire industry for advocacy and training, TIA (www.tireindustry.org) has more than 7,000 members from all 50 states and around the globe.
