Fleet Maintenance: Wheel Bearing Preload vs. Endplay
What’s the best way to ensure a safe, consistent setting for every wheel bearing in your fleet? The debate traditionally came down to bearing preload versus endplay: two different approaches to installing wheel ends while allowing the bearings to spin freely.
In real-world driving conditions, even a slight amount of endplay inevitably leads to a loss in fuel economy, as well as increased tire wear and reduced bearing life. Fleet operators who stick to outdated endplay settings are therefore leaving a lot of money on the table – both in terms of their diesel costs and wheel end maintenance.
On the other hand, the correct amount of preload virtually eliminates the wheel vibrations and excess friction causing the above issues. Proper preload settings will also ultimately create a more secure connection to the hub that minimizes the risk of wheel-off incidents.
Understanding Wheel Bearing Endplay:
Essentially, endplay is a tiny amount of axial movement in the wheel hub-bearing interface. The idea is to allow the bearing to spin as freely as possible while still securing the wheel tightly enough to minimize any vibrations or wobbling.
That’s where the challenge comes in: a tiny amount of endplay can have huge consequences on fleet performance and safety. Moreover, endplay settings tend to wander over time as components undergo thousands of heating/cooling cycles, wear out, and work their way out of tolerance.
What is deemed an acceptable amount of endplay?
Currently throughout the heavy duty trucking and bus industry, wheel bearing endplay is generally limited between 0.001” to 0.005” – a standard set by the Technology & Maintenance Council (TMC) in its Recommended Practice (RP) 618 service procedures.
However, it should be noted that this isn’t necessarily the most ideal or optimum setting for endplay – certainly not for every different type of bearing, hub, and wheel/tire setup. Rather, it’s an agreed-upon standard largely set by the bearing manufacturing sector and fleet industry based on limitations in both manufacturing and fleet maintenance tooling.
In other words – endplay is never ideal, it’s merely “acceptable” due to practicality.
What are the issues with bearing endplay?
Premature tire wear.
Premature bearing failure.
less fuel economy.
Wheel vibrations which can affect seal and brake system performance.
Bearing Preload vs. Endplay: A More Efficient Solution
Modern wheel end tapered roller bearings commonly found in fleets are engineered with preload conditions in mind because it offers numerous advantages over endplay. Once set to the proper preload rating, any looseness between components is eliminated, creating a more rigid bearing-hub interface. All bearing components still spin smoothly while virtually eliminating any internal play or vibration.
To properly understand wheel bearing preload for your fleet, there are two important factors to consider:
Preload Is Not the Same as Pre-Adjusted – “Pre-adjusted” is a term that refers to wheel ends featuring a cone spacer and higher-cost “close-tolerance bearings” which are preset to a minimum endplay. No matter how precisely adjusted, the preset amount of endplay will still reduce tire life and fleet efficiency. Moreover, these components cannot be adjusted to a better setting without performing a complex removal/reinstallation of the cone spacer.
Preload Is Not Interchangeable with Torque Settings – Torque settings shouldn’t be used to adjust bearings to a presumed preload condition because the final preload setting cannot be measured. Trying to use torque settings can create unbalanced preload settings from wheel to wheel, which is potentially unsafe and certainly not ideal for maximum bearing life.
The Advantages of Wheel Bearing Preload for Fleet Maintenance:
Better Wheel Alignment = Better Tire Life
Independent fleets have reported up to 30% better tire life after switching to a proper preloaded bearing system.
Less Bearing Wear for a Longer Maintenance Interval
Eliminating endplay and its accompanying vibrations maximizes the service life of wheel seals and bearings, and can even help improve brake system performance.
Easier Maintenance and Less Downtime
In addition to the performance benefits above, Doctor Preload bearing preload adjustment tools facilitate easier maintenance procedures and reduce equipment downtime.
Making Preload Settings Easier, More Convenient and Consistent with Doctor Preload
Doctor Preload® bearing adjustment tools allow your technicians to quickly and accurately adjust wheel end bearings to SAE standard light preload settings while measuring preload forces all the way through the adjustment process and at the final preload setting.
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