Choosing between one axle model and the next can be deceptively simple. After all, the option of a 12,000-pound lift axle over its 9,000-pound counterpart will largely be dictated by the rules that govern weights and dimensions. What needs to be considered beyond that?
But when Charlie Allen is asked about the leading cause of today’s axle failures, he is quick to point to under-spec’ing as the main culprit.
“You can get out of the approved envelope for some of the componentry,” says Allen, the director of global service for ArvinMeritor, Inc., referring to the challenges that can emerge if an inappropriate axle is installed, either from the factory or through retrofit.
Indeed, equipment owners may be quick to add axles in a bid to increase potential payloads, but they also can fail to recognize the impact that such decisions may have on related suspension components, wheel ends and the life of the axles themselves. Owners are not alone in making such mistakes, either. Those who sell trucks have been known to ask body builders to modify stock equipment with extra axles, all in a bid to close a sale and give a customer what they want. Presto, it is ready for a new vocation.
But that overly simplistic thinking can be hazardous, and there are several factors that must considered when it comes to axle installations.
Brian Beheydt, director of the axle business unit at Hendrickson International, points to the misconceptions that can emerge over axle ratings and brake ratings as one example of things to consider. Even though an axle is rated at 25,000 pounds, the associated brake rating may only hover between 20,000 and 23,000 pounds.
“The Gross Axle Weight Rating (GAWR) is going to be based on what the entire system can do,” he says. A tractor originally spec’d to haul a tandem trailer may even require a larger compressor if it is suddenly asked to haul a multi-axle Michigan configuration and all the brakes that come with it.
When it comes time to choose the axle itself, buyers and sellers alike need to remember that each model is designed to perform in a specific type of operating environment, in conjunction with other carefully selected components and systems.
Those who need to run equipment off highway and through punishing job sites probably will want to lean toward housings with thicker walls. A 9.5-mm thickness may be fine for a standard highway tractor with an air-ride suspension, but the equipment that operates in an on/off-road environment might require the extra protection that comes with a 12.5-mm thickness, explains Steve Slesinski, Dana Commercial Vehicle Systems’ director of global product planning. The gearing itself also will need to be more robust.
Even the truck’s engine can lead to important axle-related decisions.
“There’s a lot more emphasis on making sure you build the drivetrain system to complement the engine,” Allen says of today’s spec’ing process.
Not too long ago, the users of on-highway tractors were requesting rear axle ratios around 3.90:1, but the introduction of today’s engines and their higher torques have seen the related numeric ratios drop as low as 3.36:1 in an effort to ensure that equipment cruises along in a fuel-efficient sweet spot.
Even with the smaller ratios, Slesinski assures that trucks equipped with “the bigger 16-litre or 500-horsepower engines, at 1,650 lb-ft, will still pull a grade.”
In contrast, larger ratios might best be left with stop-and-go operations and those that require a little extra pulling power.
MORE ROBUST, LESS WEIGHT
One source of good news is that axles themselves have evolved a great deal over the past few years, and they boast greater strengths in packages that are lighter than ever. Yesterday’s cast metal pieces have been replaced by fabricated options, and axle makers are designing products using high-tech tools such as Finite Element Analysis (FEA) to trim excess weight or fortify components.
“In the past, you didn’t have the visibility to say, ‘I can optimize the housing or optimize the gear set,'” Allen says, noting the importance of such technologies.
Beheydt refers to the weight savings in spiders as an example of the advances that are possible. In the last decade alone, manufacturers have been able to shed up to five pounds from this component, he says. “We still think there is an opportunity [for additional weight reduction] in the cam system itself, in the bushing design of the spider, and in the way the cam carrier bushing works.”
The weight savings hardly end there. Today’s weight-conscious bulk haulers regularly shed additional pounds by choosing related components such as aluminum carriers, hubs and super-single tires. In some cases, the choice of a 6×2 axle configuration also will remove further weight since the tag axle will have no gearing at all. But once again, these choices make a significant difference to the equipment’s overall performance.
INTEGRATED SYSTEMS
Given the impact that an axle can have on surrounding components, there should be little surprise that the evolution of integrated systems includes everything from preinstalled wheel ends to air-ride suspensions.
“Manufacturing the axle for the design of our suspension allows us to create an axle that is straighter than what you get traditionally,” Beheydt says. Rather than dealing with spindles that feature attitudes that point in two different directions, these axles always will run straight as an arrow from one side of the vehicle to the next, and the end result is straight-running equipment that promises to improve fuel-efficiency and help reduce irregular tire wear.
For service facilities, the biggest impact of integrated systems can be seen in the related installation times. Instead of spending six to eight hours struggling with the welds between a beam and a stand-alone axle, aftermarket installers can invest a mere quarter of the time dropping an integrated axle and suspension system into the appropriate hangers. Integrated wheel ends, meanwhile, help to protect bearings and seals from damage that can occur when they are installed in challenging surroundings.
“Seal technology overall, whether wheel ends or a cam bushing or really any type of radial seal, has gotten significantly better,” Beheydt says. Today’s offerings can last well beyond older single-lip designs that needed to be replaced every five years. The double or triple lips that are found on modern seals form an effective barrier to block most debris and retain the vital lubricants inside.
Even the lubricants have evolved. A synthetic lube inside a 40,000-pound tandem, for example, can last as long as 500,000 miles before needing to be changed, and that is double the life of petroleum-based lubes previously used.
“The synthetic lube offers advances in fuel economy, seal life and axle performance,” Slesinski says.
Of course, the advanced lubricants present their own buying issues for purchasing teams since no single grease will meet a shop’s every need. The lithium-based formulas used in many axles are well-equipped to handle high heats, but they cannot be mixed with calcium-, sodium- or barium-based compounds.
The maintenance considerations don’t end there. Slesinski actually balks at any suggestion that axles have become “spec-it-and-forget-it” components. “We don’t want people to forget to inspect the axles,” he says.
Indeed, service technicians and drivers alike must pay particular attention to any cracks that might emerge. Such damage that forms at the bearing journals can require an outright axle replacement, and issues with the spindle threads are another cause for concern.
Drivers will play their own important role in determining whether any bad situations become worse. Those who have axles equipped with a temperature gauge should be advised to pull off the road if the needle begins to climb above the usual 180
