Most all of a vehicle’s energy and the forces that act upon it while driving are ultimately transmitted through wheel studs, so we must ensure they continue to do their job by routine inspection and proper maintenance practices using the correct tools.
Routine inspection at the company level includes checking for loose lug nuts on the daily inspection and learning to recognize the signs of loose lug nuts. Loose nuts can be identified most often by streaks of rust or discoloration radiating outward on a wheel from the nut. Visible cracks in lugs nut, studs, wheels, or loose lugs are unacceptable and should be repaired immediately. Ultimately, a loose wheel is already in a state of failure and this is where the maintenance facility comes into play. Your maintenance department, person, or independent shop should include a torque verification of these wheels on a routine basis to minimize the risk of such failure. There are additional methods and products available to recognize or prevent problems early such as Zafety Lug Lock, Rollock, or the application of frangible lacquer.
Alcoa, a manufacturer of most commercial duty aluminum wheels provides very useful information on their website relating to proper installation of their wheels on your apparatus. They recommend tightening ball seat style lug nuts in the proper sequence between 450-500 foot-pounds with unlubricated threads or 350-400 foot-pounds if the threads are lubricated. Lubrication directly to the seating area of the nut is not recommended and can lead to over tightening and possible elongation and/or failure of the stud. For hub piloted wheel systems the torque sequence varies slightly by design; however, it is recommended to lubricate the hub pads, stud, and area between the washer and nut on a two piece nut. The torque of these hub piloted wheels following the aforementioned procedure should still be in the 450-500 foot-pound range. Ensuring correct installation of your department’s particular component combinations is an essential step in establishing or evaluating your procedures.
Following the specific torque sequence, cleaning, and inspection steps illustrated in most service literature affords the best opportunity to have an equal amount of pressure applied and a proper torque that can remain within specification during operation. Evenly torqueing the lug nuts can combat loosening of nuts after operation and prevent cracks in the wheels. Sequence varies slightly by mounting style and lug count so it is important to reference this material and verify the procedures are correct should a new style become standard in your fleet.
Torque sticks have made their way into heavy duty truck applications and originally started with high use in retail tire shops looking for a quick alternative to using a torque wrench. Although the efficiency may be a big motivator for their use, the fine print spells out a detailed list of checks that must be followed in order to ensure their accuracy. One manufacturer boasts a list of well over 16 factors that must be met to insure a 4-5% window of accuracy. The theory of operation is simple, install a device with a precision diameter, length, and metal composition then the twisting force or torque is limited to a specific and rapidly repeatable range. The down side to these devices is that if the input from the impact gun is too great or unacceptable then the output will not be within the designed and tested range. Tool manufacturer guidelines must all be met in order to guarantee this 4-5% accuracy while standard calibrated torque wrenches can provide an accuracy of 3% or better without most of these considerations. One maker of torque sticks requires the following:
- Shop line pressure to that device must be regulated at 90-100 PSI
- Impact guns must be calibrated or maintained to provide 650-1000 lb/ft of output (from one manufacturer) and 1500 lb/ft (another manufacturer)
- No adapters may be used, extensions, extended anvils, etc.
When all of these conditions are met including proper mounting techniques, the best you may be able to achieve is +/- 20 lb/ft of torque on a typical apparatus lug nut. We recommend that your department consider use of torque sticks as a temporary alternative only and that all wheels torqued using these tools be verified as soon as possible with a torque wrench or the use of these sticks be discontinued entirely.
Tightening wheels either by using uncalibrated tools with torque sticks, only using an impact gun, or not verifying the correct value or procedures can result in over tightening of the wheels. Lug nuts that are over tightened are just as unsafe as loose lug nuts, they create similar issues that can result in wheel loss and a potential incident. If over torqued, the stud may pass through the elastic range (that it may recover from) into the plastic range. Imagine a spring being pulled and released, when it is stretched within its designed limits it returns to its normal size and shape, when a fastener reaches its plastic range it is as if you were to pull on the spring too much and it remained elongated. This allows it to easily reach the ultimate tensile strength when tightened again. Even if this elongated stud falls within the proper torque specification, any additional forces exerted on this component can then allow it to reach the tensile or breaking point. Fasteners used either intentionally or unintentionally within the plastic range must be replaced.
When a wheel lug nut is tightened correctly to the proper specification, it is still recommended that it be rechecked within 50 miles of operation by most manufacturers. This is one of the most important steps that can be omitted as it can become difficult to schedule or track. Requiring a vehicle to return to the shop following a call would be the easiest way to facilitate this. Regardless of if this additional service requires the unit to be temporarily put out of service, the effort put forth could prevent a future incident or failure.
With everything that has been explained thus far, it is still important to mention that all wheel system combinations have not been highlighted and that none of this information should replace the actual and current service literature written for your vehicle. Understanding the importance of these practices and knowing where to locate the information should get the ball rolling in the right direction should you encounter stud/wheel problems in your fleet. Most installation and service literature for your wheel systems should be located in the maintenance manual or available online at the manufacturer’s website to verify or establish correct procedures. It is advisable if an outside tire repair service is utilized to request or review their procedures periodically to confirm that they are meeting the requirements of the components installed and in some cases it may be necessary to request that a note be placed in your file for the mechanic to follow any special procedures you may have.
Following the advice of the manufacturer, accepted practices, and routinely inspecting the effectiveness of your wheel and tire service or program is an excellent way to address the potential of wheel problems in your fleet and will keep the tires rolling down the road, along with the vehicle.