Commercial vehicles operating under the Federal Motor Carrier Safety Regulations (FMCSRs) must meet the Compliance Safety Accountability (CSA) program. Cracked wheels, rusty components, loose or missing wheel fasteners, and elongated or broken studs all represent potential violations under the driver safety measurement system. To avoid wheel fatigue, it’s essential to maintain the proper clamping force.
When properly maintained, wheels should last the life of the vehicle. Lack of inspection and routine maintenance, not following the manufacturer’s instructions, mismatched parts and loose or over-torqued nuts are the main causes of losing proper clamping force on wheels and wheel end components. Assuring that your wheels stay properly clamped is the key to avoiding early fatigue in wheel end components.
- The ideal clamping force is achieved and maintained through the use of new fastening components that are applied using the manufacturer’s specific torque and re-torque practice criteria.
- If any one of the stud/nut combinations loses its clamping force, the load forces are redistributed over the remaining studs/nuts. This causes the adjacent nuts to fatigue.
- Insufficient clamping force may create “play” in the wheel. This can increase stress loads on the wheel and other components that lead to fatigue cracks, deformed stud holes, stud damage and, potentially, wheel loss.
- A common misconception in the industry is to correct low clamping force by over torque. This practice can stretch the stud beyond its yield point, rendering it ineffective.
- The proper preventive maintenance practice is for drivers and mechanics to periodically inspect wheels and wheel end components. This makes it possible to identify and address issues at an early stage, before damage occurs.
- Inspecting a wheel’s back-up diameter during tire and brake changes may reveal a scalloped back-up diameter.
This indicates low clamping force within the wheel end. Ensure that all mounting surfaces, studs, and nuts are properly cleaned and oiled as described in the TMC’s Safety and Service manual before the wheel is reinstalled.
What Can Cause Loose Wheel Conditions?
- Excess paint, rust, scale, or dirt between mating areas of wheel end components. All components must be in good working condition.
- Incorrect use of wheel, nut, and stud components.
- Failure to follow the manufacturer’s instructions for specified torque, the correct tightening sequence, and routine in-service torque checks.
- Improper use of impact and non-calibrated tools. Studs stretched beyond their yield point, fractured or worn out nuts with deformed threads that result in insufficient clamping force.
Wheel Failures Related to Loss of Clamping Force:
- Bolt Hole Cracks
- Bolt Hole to Bolt Hole Cracks
- Bolt Hole to Center Hole Cracks
- Wallowed or Elongated Bolt Holes
- Circumferential Cracks on Mounting Area (for hub-piloted wheels)
- Worn Bolt Hole Ball Seats (for stud-piloted wheels)
- Distorted Bolt Hole Ball Seats (for stud-piloted wheels)
- Burrs Around Bolt Holes (for stud-piloted wheels)
- Broken Studs
- Cracked Flange Nuts
- Cracked or Broken Inner Cap Nuts (for stud-piloted wheels)
- Stripped or Deformed Threads on Studs and Nuts
- Scalloped Witness Mark
- Edge of Nut Crack Scalloped witness mark
ABOUT MCCARTHY TIRE SERVICE
Founded in 1926, McCarthy Tire Service is a family-owned and operated tire dealer headquartered in Wilkes-Barre, Pennsylvania. The company has more than 70 service locations and 11 Bandag retread tire manufacturing plants along the east coast of the United States. McCarthy Tire is the 5th largest independent commercial tire dealer and one of the top 50 retail tire dealers in the country. The company offers tire sales and service; automotive mechanical services and repairs; fleet services; truck mechanical services; 24-hour commercial roadside assistance; and retread tire manufacturing. For more information, visit www.McCarthyTire.com.
SAFETY CONTACT: Russ Devens