Trucks Go on a Diet
Commercial vehicles are traditionally big and heavy. But, with diesel prices skyrocketing, the industry is hungry for fuel-efficient vehicles. Truck manufacturers are responding with aluminum, composites and other lightweight, corrosion-resistant materials. This trend is forcing assemblers to use new joining methods, such as structural adhesives.
Caterpillar Inc. is world-famous for its bulky construction equipment containing lots of cast-iron parts. But, when it launched a line of on-highway trucks last year in conjunction with Navistar, engineers turned to lightweight materials.
For instance, the Cat CT660 vocational truck features an aluminum cab and a composite hood. The cab, which is 250 pounds lighter than traditional welded steel construction, is riveted and bonded.
“The primary trends at the moment are in aluminum and various new composite sidewall materials and plastics,” says Dan Tuerk, ground transporation leader at Infastech. He predicts that other lightweight materials, such as magnesium, will be used by commercial vehicle manufacturers in the future, as the technology trickles down from the automotive industry.
Demand for lighter materials among truck and trailer manufacturers has slowly evolved over the last decade. “Thinner and lighter weight sidewalls are the primary challenge facing us today vs. 10 years ago,” he points out.
The typical Class 8 truck weighs about 17,000 pounds. According to a recent study conducted by the National Academy of Sciences, the powertrain accounts for 24 percent of the weight, while the body structure is 19 percent. Other heavy components on big rigs include drivetrain and suspension (17 percent) and chassis (12 percent).
“Lighter weight is not the challenge in and of itself, but thinner joints require fasteners to work in new conditions,” adds Tuerk. “Lighter materials can be more subject to deformation when placed in compression. These require innovative solutions like shouldered lockbolt pins combined with large flange collars to increase the footprint and avoid crushing soft materials.”
“Certain segments of the [trucking] industry have always been weight-sensitive, but the real push only began in the last few years, as diesel fuel prices rose dramatically,” explains Steve Tam, vice president of the commercial vehicle sector at ACT Research Co. “New fuel economy standards that will be phased in over the next five years will make [cutting vehicle weight] de riguer.”
“Meeting tough new federal fuel and emission standards on medium- and heavy-duty trucks will require them to be lighter, cleaner and more fuel efficient,” adds Randall Scheps, chairman of the Aluminum Association’s Aluminum Transportation Group. “If newly built Class 8 trucks and trailers [used more] aluminum, it could save 3,300 pounds for each unit, in turn saving 1 billion gallons of diesel fuel and 10 million tons of carbon annually across the fleet.”
The Environmental Protection Agency and the National Highway Traffic Safety Administration recently collaborated on a fuel and emission reduction mandate. Their proposed standards cover not only engines, but the complete vehicle, which Tam says allows greater latitude for vehicle manufacturers to achieve reductions.
The standards, which cover model years 2014 to 2018, will reduce greenhouse gas emissions by nearly 250 million metric tons and save approximately 500 million barrels of oil over the life of these vehicles.
“In an industry where less vehicle weight means more company profit, strategic weight reduction is a smart business tool that is rising in importance,” say Scheps. “It’s simple physics and basic economics. Heavier vehicles need more fuel to operate and [have less] payload capacity, both of which waste money. That’s why the average Class 8 truck today already uses more than 1,000 pounds of aluminum in the form of forged wheels, trailer structure, cabs, fuel tanks and other critical components.”
Aluminum Use Is Growing
Cutting weight “with aluminum is one of the fastest ways to improve fuel economy,” claims Scheps. “While a new powertrain program likely would take five years or more, aluminum content can be added much faster. As powertrain advances come online, reducing vehicle weight will complement those new technologies and further extend their benefits.
Switching to high-strength, low-weight aluminum in Class 8 trucks and trailers can save as much as 1,612 gallons of fuel and 17.9 tons of carbon dioxide per year, according to a report released by Ricardo Inc. By using aluminum, engineers can achieve some dramatic weight reductions. For example, aluminum frame rails typically weigh more than 400 pounds less than traditional steel alternatives.
“The biggest opportunities are heavy metal parts, such as fifth wheels and axles,” says Tam. An aluminum drive axle hub can shave up to 160 pounds from a big rig, while an aluminum fifth wheel could trim 100 pounds.
“Even a slight reduction in component weight can translate to thousands of dollars of savings in fuel consumption, while allowing fleets to increase their payloads and consequently their return on investment,” says Kumar Saha, an automotive industry analyst at Frost & Sullivan Inc. “As fleet operating budgets get tighter, there is increased pressure on truck OEMs to deliver vehicles that are affordable, resilient and lighter. [They require] weight savings to add other necessary components to their trucks, [such as emissions equipment].”
Constructing truck cabs out of aluminum makes a lot of sense, because doors, roofs, floors, rear walls and other large parts can be made considerably lighter. That’s what engineers at Kenworth Truck Co. did when developing the new T680.
“We wanted a world-class door that was larger, more robust and easier to assemble,” says Robert Culwell, manufacturing engineering manager. “Our engineers utilized advanced three-dimensional computer imaging to design a door frame and door that closes tightly with a degree of consistency and high precision.”
The T680’s stamped aluminum door is 30 percent larger, yet lightweight and extremely stiff, making for excellent seal integrity. A pressure relief valve equalizes interior and exterior air pressure to make the door easy to open and close with little effort.
Engineers at Dana Holding Corp. also turned to aluminum recently when they developed a new driveshaft for commercial vehicle applications. The Spicer Diamond Series shaft is assembled using a proprietary magnetic-pulse welding process that joins an aluminum tube with steel U-joints. The one-piece component weighs 40 percent less than a traditional two-piece steel assembly and offers improvements in noise, vibration and harshness.
Driveline packaging space has become a key consideration for truck manufacturers as more and more components are added to meet efficiency regulations. According to George Constand, Dana’s chief technical and quality officer, the design of the Diamond Series driveshaft addresses this challenge by reducing overall part count and by eliminating the center bearing.
Composites Catch On
Composite materials are traditionally used to produce boat hulls, wind turbine blades and aircraft fuselages, due to their unique combination of high strength and low mass. Until recently, these materials have been far too costly for use in truck and trailer manufacturing. But, new materials, joining technologies and lightweight-construction concepts will soon help engineers shed additional pounds from their vehicles.
Carbon-fiber-reinforced plastics are particularly promising. They are roughly 60 percent lighter than steel and around 30 percent lighter than aluminum. The material also never rusts and can be used to construct crash-relevant structures, such as body components.
These materials get their stability from carbon fibers embedded in the plastic matrix. Depending on the demands, the fibers can be superimposed over several layers and in varying directions.
Earlier this year, engineers at Daimler Trucks North America unveiled a concept vehicle that features an all-composite body, marking the first time this material has been used in heavy-duty truck construction. Toray Carbon Fibers America supplied the prepreg material that was used to produce the vehicle’s hood, roof cap, side walls and back wall.
“One of our main priorities in developing the Revolution Innovation truck was to showcase how we can help customers reduce weight to increase payload, enhance fuel economy and optimize overall performance,” says Justin Yee, manager of vehicle concepts for Daimler Trucks. “Carbon fiber’s strength-to-weight ratio makes it an ideal material for weight reduction.”
A special sandwich structure of
low-density honeycomb material and carbon fiber resulted in extremely lightweight components, which also simplified the truck’s inner support structure and maximized interior space. “This resulted in significantly more interior space and functionality than a day cab, while weighing less than a sleeper,” Yee points out.
Although aluminum and steel still dominate truck manufacturing, plastic components are becoming more common for under-the-hood applications. For instance, the charge-air duct on the DD13 and DD15 truck engines made by Detroit Diesel Corp., another Daimler subsidiary, is made from polyamide.
Compared to its aluminum predecessor, the new duct is 50 percent lighter. The oil intake modules on the diesel engines are also made from polyamide. They replace metal parts and result in a 50 percent reduction in weight.
Scania, a leading European truck manufacturer, also uses some plastic components in its new Euro 6 engines. For example, an oil sump molded from nylon resin replaced an aluminum part and enabled engineers to reduce weight by more than 50 percent. In addition to improved fuel economy, the lighter weight material in the oil sump dampens engine noise.
Traditionally, fasteners are the rule rather than the exception in truck manufacturing. But, that’s starting to change, as more engineers turn to structural adhesives.
They’re bonding the wall skin to the frame of truck cabs and bodies, in addition to bonding and sealing diamond plate trim pieces to the wall and frame. Other applications include bonding trailer beds to frames and bonding interior trim pieces.
“Structural adhesives provide a number of benefits over traditional mechanical fasteners,” claims Mike Williams, OEM market manager at Henkel Corp. “The benefits of building vehicles with adhesives and sealants instead of welds, fasteners or rivets include reduced manufacturing costs, increased throughput, uniform stress distribution and much faster assembly times.
“The production time involved is reduced significantly, since the adhesive is typically dispensed out of a pneumatic dispenser and fixtured in approximately 30 to 40 minutes,” adds Williams. “The time required to install a number of fasteners can be quite lengthy.
“Eliminating welds or mechanical fasteners, such as screws or rivets, significantly reduces the total weight of the vehicle, thus increasing the overall efficiency of the vehicle,” Williams points out. “Aesthetically, the overall finished look of the truck or trailer is enhanced, [because] graphics are not compromised with a number of fasteners.”
“Mechanical fasteners still offer the best joint when compared to adhesives and can be used to bond dissimilar materials,” argues Infastech’s Tuerk. “Adhesives struggle to keep up and performance over time is still a little unknown. Welding is less preferred in many manufacturing environments [due to environmental issues] and cannot be used with many of the lightweight materials chosen by commercial vehicle [engineers] today.
“We see an increasing rise in dual-method fastening, where mechanical fasteners are used with adhesives to reduce weight while still offering mechanical fastener joint integrity,” adds Tuerk. “Using mechanical fasteners with adhesives [also] helps eliminate the curing times that may be necessary when only adhesive is used.”
Many truck manufacturers still prefer using fasteners, because multiple joints are easier to visually inspect than bonding or welding. “In addition to products that require less inspection time, engineers are looking for fasteners that can be installed in less than a few seconds and achieve uniform clamping,” says Rocco DiRago, product manager for the Americas at Alcoa Fastening Systems.
“Truck manufacturers are also looking for ways to reduce the size of the fasteners they use,” DiRago points out. “For example, Volvo Trucks recently downsized from 16 millimeter to 14 millimeter fasteners used on frame construction. That eliminated more than 30 pounds.”
Despite recent inroads by aluminum and composites, steel still rules in truck manufacturing. “We are seeing an increase in the use of higher-strength steels, since these materials allow lighter gauges to be used, which can be an effective method of reducing weight,” says Marc Purslow, arc welding applications engineer at EWI. “[However, manufacturers encounter some problems] when using these steels. [For instance], they are more difficult to weld, and their decreased ductility must be taken into consideration when designing to maintain fatigue life.
“While the joining of lightweight materials can be a challenge, it is not an insurmountable one,” Purslow points out. “A greater challenge is the joining of dissimilar materials, since manufacturers must find a suitable method of incorporating these lightweight materials into the structure of the vehicle.”
Purslow and his colleagues at EWI are developing and refining cutting-edge processes for joining many dissimilar material combinations that will be used in future commercial vehicle applications. “One of the largest challenges associated with the use of dissimilar materials is encountered in downstream processing of components,” he explains. “The dissimilar material properties of these materials can lead to significant issues with distortion.”
For example, when a structure made of aluminum and steel components is subjected to an elevated temperature in downstream processing, the differing coefficients of thermal expansion will result in distortion and possibly plastic deformation of the materials. “This can result in permanent distortion of the structure, which can have a significant adverse effect on fit, finish and function,” warns Purslow.
“Minimizing the negative impact of distortion can be accomplished through a number of methods, ranging from improved fixturing design during processing to intelligent design of these structures,” claims Purslow. “[We have] developed significant expertise in the modeling of welded structures to predict residual stresses and resultant distortion. By using modeling to predict the residual stresses and distortion imparted by the welding processes, as well as downstream processing, truck manufacturers can plan for these issues.”