Preventing a wheel separation from the tractor, trailer or coach is a very good reason to pay close attention to bearing adjustment on wheel ends. Improvements in vehicle performance, along with operating and maintenance cost savings, can also be had by properly assembling and maintaining wheel ends.
But do endplay settings on the bearings deliver satisfactory results? The answer may be: “No.”
Wheel end tapered roller bearings are designed to be set to preload. They are not designed to be “loose” or have endplay.
In a preload condition, all clearances (endplay) between components in the wheel end assembly are completely eliminated and the bearings are slightly compressed. Keeping a light preload force on the bearings keeps the tapered rollers lined up so they roll the way they should.
The rest of the wheel end components including seals, tires and brakes, are designed to work best under these preloaded bearing settings.
In the past, bearing companies took the position that even though light preload is the optimal setting, there was no reliable way to measure and control preload settings in wheel bearings. For example, the Timken Company - a leading global manufacturer of highly engineered bearings, alloy steels and related components and assemblies, in its Tech Tips Volume 6, Issue 3, written in 2001, pointed out: “Unfortunately, neither dial indicators nor any other standard tool will tell a technician the amount of preload in the wheel end. . . . To date, there isn’t anything available that will confirm that the fastener is providing the correct preload setting.”
Tools now are readily available to adjust and verify wheel bearings to preload settings.
For decades, the heavy duty truck industry has accepted a recommended wheel end bearing setting of 0.001” to 0.005” of endplay. This does not mean that loose bearings equals good wheel end performance. It just means that all techniques available for wheel end bearing adjustment have the potential to achieve measurable settings somewhere in this wide endplay range.
Bearing manufacturers, along with other relevant suppliers, led the way to the Technology & Maintenance Council (TMC) establishing its Recommended Practice (RP) 618, Wheel Bearing Adjustment Procedure. Its purpose was to achieve a verifiable wheel bearing endplay 0.001” to 0.005”. More precise settings were not possible with the available technology.
When bearings are loose at any endplay setting, the rollers can go out of line and the bearings will not rotate as they should, causing a wide range of problems in the wheel end system with the bearings, seals, tires and brakes.
When bearings are set to a measured light preload:
- Expenditures on tires are reduced because tires wear evenly and last longer.
- Seals don’t leak because preload provides truer motion between the hub and spindle, subjecting the seal to less movement geometry for sealing mechanisms to follow.
- Brake systems perform as designed because vibrations are reduced.
- Risk of catastrophic failure is reduced from the consequences of misadjusted bearings.
- Unscheduled and emergency maintenance goes down, resulting in less downtime and more uptime.
“If the people that make the bearings say that preload is okay, that is good enough for me,” says Greg Judy, the person in charge of maintenance for Coppercoin Transport, the delivery fleet for Fiber By-Products, a manufacturer of waste wood products. “Bearing manufacturers have published data that show optimum life of tapered roller bearings is always achieved from controlled preload settings.”
“Adjustment requires hair splitting accuracy in order to optimize performance of tapered bearings and wheel ends,” explains John Rode, CEO and chief engineer at Temper Axle Products Corporation. “With wheel ends, even one thousandths of an inch of endplay begins to deteriorate wheel end and vehicle performance. Preload settings deliver the best results.”