There is no good time for loose truck wheels; never was,don’t expect there will be. Modern hub-piloted wheel attachment systems havesimplified fastener inventories and have extended the durability of wheels, lugstuds, nuts and other attachment hardware. Because the majority ofover-the-road wheel end systems are now hub-piloted systems I will focus onthem. Many of the following points also apply to earlier stud-piloted doublecap nut systems, but it is recommended that users seek specific installationand maintenance guidelines.
The first step in avoiding loose wheels is to tighten themproperly on initial installation. Using correct fastener types from qualitysuppliers is mandatory. Industry engineers recommend using a 3/4-in. impactwrench for running the lug nuts snug, then using a calibrated torque wrench forfinal tightening to the specified setting (450-500 ft.-lb. for the popularM22X1.5 two-piece flange nut used on most Class 8 hub-piloted wheels). Impactwrenches are helpful to save time in removing wheels, as well as for snugginglug nuts when installing, but should NOT be used for final tightening.Variables in supply air systems, contamination from moisture or oil in supplylines, individual tool differences, maintenance and wear of internal componentsare just some of the issues that make these devices difficult or impossible totorque calibrate reliably.
Another concern is the use of large 1-in. impacts, whichalso have the ability to over-torque nuts when fastening, resulting in a stretchedstud that exceeds the material yield point, risking torque (and clamping force)degradation over time. Bottom line: excessive initial torque can lead to toolittle torque over time. Calibrated torque wrenches are the industryrecommended and optimum wheel tightening tools.
A variety of other conditions can also affect wheel torqueretention. A common procedure is to refinish steel wheels, usually when tiresare dismounted for retreading or replacement. Excessive paint thickness on theflat mating surfaces can cause torque loss as the wheels heat cycle and reducethe paint film thickness. This condition has been traced to wheel-offs, but canbe avoided by selecting a quality re-finish supplier and enforcing the industryrecommended 3.5-mil maximum paint thickness. Cleanliness of the mountingsurfaces is also very important. All corrosion, dirt, grease and othercontaminants must be removed before wheels are installed.
Emergency road service for tire replacements can also causeissues. Some quality vendors torque wheels properly, while others simplytighten (or overtighten) lug nuts with impact wrenches, creating uneven hub andstud loading, or worse yet, stretched studs that may loosen over time. Somefleets have a policy of checking lug torques on all road-serviced trucks assoon as practical following the emergency service. Although not necessarilyrelated to torque loss, another frequently ignored practice is hub indexingbefore placing the new tire/wheel assembly weight on the hub. Hub-pilotedwheels should always be installed with one of the tapered pilots positioned at“12 o’clock.” This practice correlates with reduced radial runout and lesslikelihood of ride complaints and irregular tire wear in later life. Observingthe proper sequence of lug nut tightening will also assist in optimum torqueretention and help reduce ride complaints.
While new two-piece flange nuts don’t normally requirelubrication, used nuts should receive two drops of 30-weight oil between thenut body and the flange. Also the first two threads of studs should receive twodrops of oil. No lubrication is recommended on stud-piloted systems. Anti-seizecompounds should never be used on lug nuts or studs, as many of these willaffect torque values, typically resulting in an overtorque condition.
The Technology and Maintenance Council has severalRecommended Practices (RPs) related to wheel maintenance. RP 237 is especiallyapplicable to this issue, as it addresses in detail a list of factors affectingtorque retention and includes a table of recommended mounting torques for mostpopular disc wheel types. Make certain that you have (and communicate) thecorrect information for the specific model and type of wheel system for yourequipment.