Among the numerous considerations when spec’ing a truck is the drive axle – a critical component. Making the time and effort to ensure that it is properly specified is spec’ing essential to a truck’s performance, reliability, durability and fuel economy.
This includes defining the truck’s intended vocation and application. This information enables truck dealers to recommend the optimum combination of engine, transmission, driveshaft and axles needed to deliver the required performance.
Axle configurations are engineered for a certain range of applications. It would not be prudent, for example, to specify an axle optimized for linehaul applications and then use it in an off-road or severe-service application, as it will not live its intended life. Conversely, if the appropriate information is not used when specifying axles, it may result in less than optimum performance and lower fuel economy.
A SYSTEM
“The entire powertrain system – from the engine and transmission through the driveshaft and axle to the wheels – works as a single, coordinated system,” explains Steve Slesinski, director of product planning, commercial vehicle market, Dana. “Our engineers work closely with vehicle, engine and transmission manufacturers to fine-tune specifications on our axles and driveshafts to achieve the optimal balance of performance and fuel economy that fleets demand.”
Dana (www.dana.com) is a world leader in the supply of highly engineered drivetrain, sealing and thermal-management technologies that improve the efficiency and performance of vehicles with both conventional and alternative-energy powertrains. It serves three primary markets: passenger vehicle, commercial truck and off-highway equipment.
“Providing the correct application information to the OEM or axle supplier is critical to ensuring vehicle uptime, reliability and performance,” adds Karl Mayer, director, product line management, North America rear axles, Meritor. “When a truck and powertrain – including axles – are specified, there are variables considered, such as the type of roads, engine horsepower and torque, percent grades, Gross Vehicle Weight Rating (GVWR), desired cruise speed, number of starts and stops and annual mileage. These are just a few that are taken into consideration to ensure the right sized axle is used and to ensure maximum performance and maximum life.”
Meritor (www.meritor.com) is a leading global supplier of drivetrain, mobility, braking and aftermarket solutions for commercial vehicle and industrial markets. The company serves commercial truck, trailer, off-highway, defense, specialty and aftermarket customers around the world.
FUNCTION
Dana’s Slesinski says the drive axle is “the final link in the chain that transfers energy and torque from the engine to the wheels. The correct drive axle in any given application delivers the optimal blend of performance and critical at low speeds and efficiency at higher speeds.
“The application for a truck definitely impacts the kind of axle that is used. As a general rule, axles with lower numeric ratios – typically from 2.26:1 to about 3.08:1 – are used with low-rpm engines for engine downspeeding. Engine downspeeding is usually employed with Class 8 linehaul trucks that spend the vast majority of their working lives at highway cruising speeds where the emphasis is on efficiency.”
On the other hand, he notes that axles with higher numeric ratios – from 3:90:1 and above – are used with trucks for which traction and low-speed performance are more important than efficiency. These include cement mixers, dump trucks, fire trucks, tow trucks and other vocational vehicles.
Keep in mind, axle ratios are not chosen in isolation, emphasizes Slesinski.
“Our engineers work very closely with truck manufacturers to make sure that the engine, transmission, driveshaft and axles work as a finely tuned, integrated unit to deliver the specific performance needed for any given application,” Slesinski says.
MAIN COMPONENTS
Mayer of Meritor adds that the function of the drive axle is twofold:
1. To support the weight of the vehicle and payload.
2. To deliver torque from the powertrain to the wheel ends which will power the vehicle.
The main components of a drive axle are the housing, differential and axle shafts, Mayer explains.
“The differential allows the outer wheel to turn faster than the inner wheel when turning. Within the differential or carrier, there is a ring gear that meshes with a smaller pinion gear. The ratio of the number of teeth on the ring versus the pinion gear equals the rear axle ratio.
“Attached to the ring gear is a differential case containing spider gears that allow the left and the right axle shafts to turn independently. This will allow the outer wheel to turn faster than the inner wheel when turning.”
The correct ratio, says Mayer, is dependent on the desired cruise speed, tire size and desired engine rpm to maximize fuel economy. The top gear will be used to ensure the engines turn at the lowest rpm to maximize fuel economy, while still providing gradeability for loads.
Another critical factor when specifying a ratio is to help provide startability, he says. If the wrong ratio is specified, there may not be enough power to launch when carrying heavier loads.
TRENDS
Of late, some axle manufacturers have been designing axles for lighter weight, greater durability and enhanced life and fuel economy.
“Lightweighting – the process of reducing the weight of a vehicle – has emerged as a viable strategy for helping truck manufacturers reach new fuel efficiency and emissions standards,” observes Slesinski of Dana. “At the same time, designs that provide a reduction in weight still have to supply exceptional durability.”
By way of example, he says Dana’s Spicer AdvanTEK 40 tandem drive axle increases axle torque capacity at lower numeric ratios and eliminates weight versus competitive axles by utilizing high power density design features and integrating key components to offer a lighter weight solution.
Dana is also employing lighter weight solutions in its Spicer D-Series steer axle. This axle features a lightweight, strong, steel-forged beam design with an integrated air disc brake to “maximize the system’s integrity while eliminating up to 76 lbs from the steer axle assembly. The features of this axle allow fleet buyers to increase payload capacity, maximize stopping performance, and improve efficiency.”
6X2
Another trend in recent years is the 6x2 axle configuration, which is a means to cut weight from a vehicle, plus save fuel and reduce operating costs. A 6x2 axle configuration has three axles and six wheel-ends, but only one of the rear tandem axles (two wheel-ends) is powered by the engine. The non-driving axle often is referred to as a “dead” axle.
For 6x2 applications, Slesinski of Dana observes that the lightweight tag axle used on the Spicer EconoTrek tandem axle is lighter than conventional trailer type axles typically used on competitive offerings, and it is 20 percent more rigid for less deflection and reduced tire wear.
DESIGN CHALLENGES
Trends also include using different materials and designing components that are more power dense – smaller in size but able to handle the same or more torque, notes Meritor’s Mayer. Another trend is aggressive downspeeding to extremely fast ratios, such as 2.28:1 all the way down to 2.0:1 and even faster.
“These create a unique set of design challenges to package the components, being mindful of weight impact, while ensuring durability and life expectations of the industry,” he points out.
For lightweight applications, especially for truck users that are weight sensitive like bulk haulers, there are numerous solutions available. By way of example, he says Meritor’s aluminum carriers are “perfect for applications as such, saving between 60 and 90 lbs per tandem, depending on which axle you are comparing it to in the industry.”
“We offer a FUELite tandem which has an appropriately-sized drive axle coupled with a unique tag axle optimized for weight reduction – saving customers about 390 lbs over a standard 6x4,” Mayer says.
The design of Meritor’s 14X tandem, which has a 40,000-lb axle rating, has been optimized for both weight reduction and best in class performance, Mayer says, and it has the most options and ratios available for a multitude of applications.
“One of the most unique and important design features is the inter-axle differential (IAD) which is 20 percent larger than previous designs, contains needle bearings instead of bushings and a case that holds lube at all times,” Mayer says. “Each of these design features ensure proper lube flow to critical components and have virtually eliminated spin-out failures, which can be an industry problem.”
LOWER RPM
Both Slesinski and Mayer advise keeping in mind that downspeeding impacts drive axle specifications.
An engine running at lower rpms requires faster – lower numeric – axle ratios to maintain the same vehicle speed and performance in all driving conditions, says Dana’s Slesinski, and it generates significantly higher torque stresses in the drivetrain.
“By decreasing an engine’s rpm at cruise speed from 1,450 rpm for the typical engine to 1,125 rpm for a downsped engine, torque loads in the entire driveline increase by 29 percent,” Skesinski explains. “These higher torques place added stress on the axle, driveshaft and inter-axle shaft, which can reduce the overall life of these components, especially U-joints, if they are not specified properly to meet the additional torque demand.”
In general, downspeeding is using a faster – numerically lower – ratio to enable the engine to operate at lower rpms while cruising, therefore saving fuel, adds Meritor’s Mayer.
Mayer notes two different dowspeeding categories for Class 8 linehaul applications:
- Close-coupled overdrive applications. Minimal to no effects on axle design/specs and would just require a slightly faster ratio.
- Direct-drive applications using faster ratios. Typical ratios used in direct-drive applications range from 2.47 to 2.64. With aggressive downspeeding, such as 2.28 and even faster, he says there are different design approaches required. “Certain components need to be upsized to accommodate the higher torques at startup and ensure reliability at cruise speeds.”
SINGLE OR TANDEM?
What needs to be considered when deciding upon a single or tandem drive axle? According to Mayer of Meritor, drive axle configurations will be dependent on:
- The weight on the “back” of the axle – which correspond to the Gross Axle Weight Rating (GAWR). “Federal bridge law ratings dictate the maximum amount of weight that can be distributed on each axle, so sometimes that will require a tandem axle to distribute the load over two axles. In very general terms, you will typically see single axles on medium duty trucks and tandem axles on Class 8 tractors.”
- The Gross Combination Weight (GCW); how much weight it is pulling. “Occasionally you will see single-axle Class 8 tractor trailers combinations with single axles,” he notes, “but that will be totally dependent on the application, terrain and loads being hauled.”
- The tractive effort required. In other words, will the vehicle be operating solely on highway, a percentage off road or mostly off road. “A tandem axle will provide much better traction if operating in inclement weather, hills and grades and pulling heavier loads.”
DUTY CYCLE AND COSTS
Dana recommends that fleets consider the expected duty cycle and lifetime use costs of a given truck before spec’ing a tandem axle with a single drive axle – called a 6x2 configuration – or a tandem axle with dual drive axles – called a 6x4, says the company’s Slesinski.
“Over-the-road applications in primarily dry weather conditions, when torque on all four wheel ends of the tandem axle is not critical, will reap the most fuel economy benefits from a 6x2 configuration,” he says. “A 6x4 configuration provides better efficiency in applications where more than 20 percent of time is spent off-highway, in areas where snow and ice are typical or if the main application is climbing grades versus running on flat ground.”
Further, he says driver satisfaction, driver retention, tire wear and re-sale value are also important factors to consider when specifying 6x2 versus 6x4 drive systems.
Truck users who specify 6x2 axles can take advantage of weight savings over a 6x4, adds Meritor’s Mayer. A 6x2 configuration is also much more efficient than a 6x4 tandem due to one less drive axle carrier, which reduces power losses. Fleets that have success with 6x2s are linehaul operations that operate on well-paved roads with little to moderate grades.
FINAL THOUGHTS
“The best piece of advice we can give fleets is to consider the long-term costs of the drive axles they’re evaluating,” concludes Dana’s Slesinski. “While the initial purchase cost is substantial and immediate, long-term owning and operating costs – such as maintenance, fuel efficiency, freight efficiency and resale value – should also be weighed when deciding which axle to specify.
“Axles with proven long-term reliable performance combined with readily accessible service and parts facilities across the country will help fleets maximize uptime for the long haul, Slesinski says. “To reiterate, it is extremely important to ensure that you understand and communicate the specifics of the application and vocation to ensure durability, reliability and performance.”
Meritor’s Mayer wraps up, “Make sure to utilize experienced professionals at the OEMs – along with engine, transmission and axle suppliers if needed.
“In addition, make sure to specify the right-sized axle because under-spec’ing to save money will negatively impact uptime and create unhappy customers down the road.”
