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3 Alternatives to the Double Nut System
Like so many other truck components, the wheel end assembly — consisting of the spindle, hub, bearings, drums, rims, tires and all mounting hardware — has undergone changes in the past few years.
Manufacturers found ways to improve quality and enhance life expectancy, which is always good news for drivers. But many in the industry remain unaware of the different maintenance practices and procedures these revamped components require.
One particular area of confusion concerns the various types of hubs and bearings systems. For years the majority of hubs and bearings were secured with a double nut system and a locking device. The inner nut on these systems sets the proper amount of play in the bearings in order to maximize their life. The outer nut secures the entire assembly, both by preventing changes to its adjustment and by keeping the assembly attached to the vehicle. But when the outer nut is torqued to manufacturers’ specifications during standard maintenance, it can cause the inner nut and bearings to get too tight. That can result in an overload, which creates heat that could be detrimental to the assembly.
A number of alternatives to the double nut system are now available.
Unitized hubs have grown in popularity due to the reduced maintenance required and to a very important principle called preloading. When a bearing is preloaded at the correct level, optimal life will be achieved. Since it is nearly impossible for a technician to measure the difference between “exactly right” tight and too tight, the industry standard, set by the Technology & Maintenance Council (TMC) is to leave .001 to .005 end play in a bearing.
In a unitized hub system the preload is set for the life of the assembly and does not need to be measured. The downside of this kind of system becomes apparent in the event of a seal failure that results in bearing damage or in the case of bearing failure. If that happens, the entire assembly must be replaced in some of the unitized hub systems.
For a number of years, many truck manufacturers have used a nut called the Axilok. This eliminates the need for a second nut, reducing the chance of overloading the bearings. It also lessens the time of hub assembly. Due to the reduced possibility of too much preload, the life of the bearings can be extended. However, the integrated locking tabs that make an Axilok system work can be easily damaged without any obvious way to detect that damage. That results in bearings that run with too much end play, which wears them out faster than normal.
The ProTorq nut system is rapidly growing in popularity due to its simplicity, dependability and ease of service. An orange clip holds the nut in place, resulting in more uptime for the vehicle due to the reduced amount of time it takes to service the assembly and the reduction in risk of bearing failure. This nut system is similar to the Axilok system because it only requires one nut to set the end play and to keep the bearings and hub on the vehicle.
A different approach
Another system slowly being introduced in the market utilizes a new nut configuration that requires very little maintenance, but requires a special tool to preload the wheel end assembly. This very expensive tool only works on this wheel end nut, and is not in widespread use. Since very few shops have the tool, it is more likely to raise maintenance costs than lower them. Perhaps in the future this new design will be used more, making wheel end maintenance less of a concern. Until then understanding the more commonly used systems, allows us to keep our vehicles as safe as possible.
Homer Hogg, Technical Training Manager for TA and Petro, has worked as a truck technician for more than 30 years. He is ASE Master-certified, a Daimler Certified Trainer and a member of the Nashville Auto Diesel College Hall of Fame.