Articulated Trucks: Growing into Their Rough-Rider Image?
July 29th 2008
Articulated dump trucks exist solely to carry a heavy load fast over rough terrain. They have been in widespread use less than 20 years, and manufacturers are finally beginning to build trucks that will take the punishment a hard-hat cowboy can dish out after he’s seen the brochure pictures of these six-by-six contortionists working axle-deep in mud.
“The first thing project managers do when they get articulated trucks is save money by cutting out maintenance on the haul roads,” says Todd Perrine, equipment manager with Kokosing Construction in Fredricktown, Ohio. “One of the highest incidences of back injuries among equipment operators come from guys in artic trucks. They’re getting thrown around in the cab.”
With the guys who live and die by job costs planning to move dirt on rough roads, Perrine says the trucks seldom last as long as Kokosing would like. When he shops for articulated dump trucks (ADTs), he looks for an overbuilt drive train. The selection has been improving.
The ADT market leaders, Volvo and Caterpillar, draw their durability ace from being integrated manufacturers, meaning they build their own engines, transmissions, and other driveline components. Their proposition: If you build the drive system for maximum efficiency and durability, you can do a better job than those who buy components from vendors and assemble a driveline. The reasoning seems sound, but there are still plenty of ADT users who rent trucks for enough hours to justify a purchase. They prefer to rent because they don’t know how long the machines will last.
Until Komatsu’s entrance with the HM400-1 in 2001, Volvo and Cat were the only integrated ADT manufacturers. Komatsu has marketed its trucks largely from the durability angle, and the tactic seems to be affecting the intensity of ADT competition.
Komatsu’s 40-ton truck rolled out on power-train components adapted from rock trucks and wheel loaders. And in a move that asserts its commitment to durability, Komatsu is the only manufacturer that puts the sealed, wet-disc brakes on their 30-ton truck that serious ADT contenders reserve for 40-tonners. (Caterpillar does include wet-discs on its 35-ton 735.) Relying on the sealed brakes as retarders, too, plays into the durability claim. Komatsu says it results in less stress on the drive train than transmission retarders, which transmit brake torque through the driveline out to the wheel ends.
Components improving
Of course, the ADT field is replete with stout trucks manufacturers have assembled from venerable off-the-shelf components to serve buyers who want to save some money up front. Comparing 30-ton models, Deere, Terex, the CNH brands (Case, New Holland, Link-Belt), and Moxy sell trucks whose list prices average nearly $70,000 less than the three integrated manufacturers. The hourly ownership cost of a $310,000 assembled machine is nearly $2.40 less than that of a $350,000 unit, even if the less expensive unit falls 10 percent (700 hours) short of the integrated machine’s lifetime usage.
Most of the assemblers buy ZF’s 6WG260 countershaft transmission, and all of them drive ZF axles with limited-slip differentials in their 30-ton and smaller models. The electronic controller in the new series of this ZF gearbox engages the oncoming gear’s clutch before the clutch being released is fully disengaged. The result is uninterrupted delivery of torque. Reduced shock loading and larger clutch discs have stretched the anticipated life of the transmission to 10,000 hours.
Deere and Terex have the only trucks in the 30-ton class with planetary transmissions (both are ZF models). With no countershaft to spin, planetaries transfer more input-shaft torque to the axles. They also tend to be a little lighter and more rugged than countershaft alternates. That’s why planetary transmissions are common in 35- and 40-ton ADTs. With the TA30 introduced last month, Terex upped the ante in the 30-ton class significantly by going to ZF’s model 310. The same gearbox is used in some 40-ton ADTs.
The key benefits of virtually all engine/ transmission pairings today result from integration of electronic controllers. Linking the two computers allows wonders like simultaneously disengaging one clutch and engaging another, and adapting the clutch performance to manage driveline torque. You have to watch the tach closely to discern when the machine is shifting in the higher ranges.
Traction-control controversy
Virtually all ADTs have a switch the operator can throw to lock up the transfer case that proportions transmission output torque between the front and rear axles. Flip the switch and the drivelines fore and aft lock up to deliver equal amounts of power¡ªit’s sometimes called longitudinal differential lock. Only Volvo and Caterpillar continue to offer locking differentials for each axle. That’s transverse diff lock.
Caterpillar challenged the long-established dog clutches that Volvo uses for transverse lock-up with wet-disc clutches. Discs allow the Cat operator to throw on 100 percent transverse lock even when the wheels are spinning.
All other truck manufacturers use limited-slip differentials for transverse traction control. The diff switch in their cabs is only for longitudinal lock. Limited-slip is always on, which makes the operator’s job easier. It also makes the axles a little less expensive than lock-up clutches. Limited-slip proponents are quick to point out that it saves tires and driveline damage, and is quite effective in wheel loaders and other off-road equipment. But there’s no guarantee they’ll get your hardhat cowboy out of every quagmire that 100-percent locked axles will pull.
Creature comforts
Articulated-truck designers seem to be using sport-utility vehicles as a benchmark for cab comfort. Lists of creature comforts include power windows, heated mirrors with remote control, air-ride seats with adjustable lumbar support, and beverage coolers. The instructor’s seat in the new Terex folds forward so you can use the lunch tray and cup holders mounted on its back (like in minivans). You get the impression the marketers would just love to add a six-disc CD changer and retractable running boards.
It’s easy to take shots at the cab refinements because friction clutches and tractor seats were common on equipment as recently as 30 years ago. But the features are clearly intended to make operators more productive by keeping them more comfortable.
Productivity continues to be the articulated dump truck’s overriding objective. It keeps material moving when hauls are wet or rough. Manufacturers are competing for share of this market (which has nearly tripled in size over the past 15 years) with more durable transmissions, axles and brakes. And that makes it easier to meet the objective.
30-Ton Articulated Trucks: Wheel-to-Wheel
Make/Model Payloads (lbs.) Top speed (mph) Dump cycle (seconds) Outside turn radium Vehicle weight (lbs) Engine Torque (ft.lbs.) List price
One impressive number doesn’t guarantee a productive ADT. For example, Deere’s lightweight 30-ton truck has the best horsepower-to-weight ratio in the class, even with the smallest engine. Less-measurable numbers such as how often they get stuck make a big difference in choosing the right one.
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