Equipment Type

Articulated Dump Trucks (ADTs) Produce For Those Who Are Prepared

Anticipating retarder need, gear changes and differential lock-up moves an all-terrain hauler’s loads at the fastest overall speeds

October 01, 2007
Loading Tips


For fast truck exchanges, approach the loading zone with minimal turning and be able to consistently back the truck into place under the stick or bucket-hinge pins of the loader's spot. Watch for opportunities to pull through into position. Production is best when an excavator can swing 45 degrees or less, and the loading target is biggest when it can swing through the truck's tailgate. Wheel loaders should be able to back away from the loading face no more than 1½ wheel revolutions, and reach dump position by advancing the same distance or less.


Dump Safely

Make sure the articulation joint is as straight as possible, and tires are on level ground. As the bed raises, rear tires may squat and any soft fill under them may compact. Operators who sense the wagon shifting should stop, reposition the truck, and try again. Confirm surface stability before dumping near an edge. The bed should be fully lowered and the hoist in the float position before accelerating to travel speed.


Getting Unstuck

To duck-walk a truck out of a sticky situation, engage all differential locks (if equipped) and accelerate slowly, trying to prevent wheel spin. Slowly steer to the left steering stop and back to the right three or four times. If you can't get free, try rocking the truck out of mud. If that fails, the next option is to dump the load and try again. The last resort should be to hook a tractor up to the towing points and pull the unloaded truck from the mud.

Their automatic transmissions might suggest that anyone who drove to the jobsite can operate an articulated dump truck, and it may be true. But in order to make the machine that's most like our cars produce reliably, operators need to know how to use the retarders, differential locks, and dump functions, and respect the limits of these outwardly Herculean haulers.

The obvious differences between an off-road haul truck and the vehicles with which people are most familiar are size and weight. A loaded ADT weighs something like 10 times more than a personal vehicle and typically operates on undeveloped roads. Operating it safely and efficiently calls for the foresight to properly engage things like retarders and differential locks in plenty of time to control the load.

Operators who regularly use a truck's axle or service brakes rather than retarders to slow the truck can cut brake life in half.

"Inexperienced operators like the positive reinforcement of service brakes — the instant feeling of stopping power," says Dan Snedecor, director of Volvo Construction Equipment's operator training program. "Using retarders is a bit more difficult because they need to be applied ahead of where the truck needs to slow down. They don't produce instant slowing power."

Truck features vary from brand to brand, but most have some automatic modes for retarders — either to retard speed as soon as the operator lets up on the throttle, or applies the retarder with the first travel of the brake pedal. Volvo articulated trucks have an independent pedal that allows the operator to vary the power of the hydraulic transmission retarder, but the most reliable way to use their systems is in their automatic modes.

"The only time you should use the hard service brakes, other than in an emergency stop, is when you bring the machine to a complete stop after the retarders have slowed it down," says Snedecor.

Anticipating the need to slow down, and applying the retarders early, gives the trucks' automatic transmissions a chance to find a gear and speed necessary to deal with an upcoming hill or turn.

Today's automatic transmissions are remarkably efficient, and manufacturers say most operators just select "D" with the transmission lever and go. But there are conditions that call for human gear selection.

"When an operator selects "D" for drive, machine operation is fully automatic — it is meant for ease of operation, and also an economy mode for light to medium applications," says Michael Stec, one of Volvo's training specialists. "Gear shifting runs in the range between 700 and 1,800 rpm, minimizing fuel consumption while negotiating haul roads with limited grades. The truck's not putting too much fuel through the engine, but it's also not suited for climbing steep grades or for muddy conditions.

"When haul conditions worsen with mud, grades more than 15 percent, diff locks needing engagement, and/or other unique challenges, an operator may want to pre-select gears. You do three things by pre-selecting a gear or switching on our shift inhibitor," Stec explains. "The engine can wind up to 2,200 rpm and provide maximum rimpull when climbing hills. Also, it provides maximum engine and transmission retarder force when traveling down steep slopes, which saves brake life. Lastly, it cuts down on gear hunting — when the transmission is continually shifting between two gears on a changing slope. Gear hunting wears out the drive train and you're losing momentum, going back and forth between gears, increasing cycle time."

Dump Safely
Dump Safely

The first rule of shifting gears in one of these big off-road trucks is to do it before you need a different gear. The grade and gear-selection labels inside the cabs of most trucks will tell you which gear is appropriate.

Getting Unstuck
Getting Unstuck

As a rule of thumb, Caterpillar recommends noting the gear and speed that the transmission picks when climbing a hill unloaded. Before starting down that same hill with a full load, choose one gear lower than what the truck used to climb it.

Keep an eye on brake temperature, especially when choosing gears.

"Heat build-up on brake discs dictates how fast a truck can safely come down a grade," says John Hymbaugh, senior marketing training consultant at Caterpillar. "If brake temperature is too high, you have to come down in a lower gear and slow down."

"You need to respect the grade chart (we put it in the upper right-hand corner of the windshield of every truck)," says Volvo's Stec. "The slope chart is designed to keep retarder-oil temperature in the green. If you use a bigger gear and overheat the retarder, then you're going to reduce retarder force and you're putting heat back into transmission."

Cat's artic trucks use an engine-compression retarder in lieu of a transmission device. Overheating is similarly hard on the engine.

It's also critical to know when to use differential locks on trucks equipped with the feature.

"The vast majority of the time operators of our trucks should use either 6×4 or 6×6 mode, occasionally pushing the button to lock all of the differentials when the going gets really tough," says Volvo's Snedecor. "But we see guys operating in the most aggressive traction mode all the time. It can slow the machine down, particularly if there's a lot of turning involved, and wear and tear on tires and drive train increases.

"The key is to get them switching the modes on and off at appropriate times based on the demands of the changing terrain," he adds, "because there's always a tradeoff with efficiency and component life."

Locked differentials actually work against the truck when it is operated on hard, dry surfaces. With very little wheel slippage, the truck wastes a lot of energy trying to twist drive shafts and strip gear teeth when cornering.

Cat's Hymbaugh points out that good operators learn to coax their trucks through some challenging underfoot conditions without locking up differentials at all (after all, more than half of the trucks on the market today don't even offer the option).

"If you can use technique and speed to get through some tough stuff without locking differentials, you increase fuel efficiency," says Hymbaugh, "and there's less wear and tear on the truck's gearing."

Volvo and Cat operating instruction agree, the logical progression for using differential locks is typically to start with longitudinal locks — those that lock power delivery from axle to axle — at the first suggestion of wheel slip-page. Only use full lockup — the button on the floor that adds transverse lockup across each axle — as necessary in really tough situations.

"The switch on the steering column which will engage the inter-axle differentials," says Ken Karpuleon, another of Cat's senior marketing training consultants. "If you think of it in terms of a four-wheel-drive pickup truck, that's like four-wheel high. The button on the floor, when you step on it, is like putting the truck into four-wheel low."

Haul roads are a controversial topic when ADTs are doing the hauling. Manufacturers recommend no change in haul-road maintenance from rigid-framed to articulated trucks, and in the same breath acknowledge that haul-road diligence drops off considerably with ADTs on site. Saving money on road maintenance comes with a cycle-time penalty, though.

Yes, ADTs can traverse less-manicured roads, but they can't cover the same distance nearly as quickly. To make matters worse, managers who can't slow operators down will likely incur high workers-compensation expenses when soft spots in the road deteriorate and their NASCAR hopefuls get bounced around the cab like a bean in a maraca.

Watch speed and ground conditions especially carefully when returning to the loading zone with an empty truck. Heavy-truck suspensions are tuned for carrying a load. They're very stiff when the box is empty, and the truck's center of gravity is high. It's a problem that Volvo's forthcoming Full Suspension, or FS, Series trucks are designed to combat. Suspension won't make the trucks invincible, but it will allow them to get back to the loader faster over rough terrain.

On the loading floor, truck operators should let the loader operator spot their wagon. Top operators get good at moving efficiently into place, using a minimum number of turns and backing, if necessary, one of their sideboards directly under the loader bucket's hinge pins or the excavator's boom-to-stick pin.

Quantity of each load should be the hauler-operator's responsibility. It can be a touchy call because it can cut across the loader and the foreman's orders and/or egos. But overloaded trucks are not only hard on the equipment and fuel efficiency, they're unsafe. Overloading reduces the truck's stability and exceeds its brake, steering, and tire capacity.

Side slopes are another challenge to ADT stability and safety. Volvo training materials say its OK to traverse side slopes of up to 15 percent, but the company's lawyers add that it's only OK under specific conditions established by jobsite management.

"If you can keep perpendicular to the slope, do it," says Cat's Hymbaugh. "You probably wouldn't want to dump on a cross slope. That's why we have an ejector model."

The truck's wheels should be near level when the loaded body is raised. If they're not, the truck's center of gravity will shift laterally as it rises and the wagon can easily roll onto its side. For the same reason, stay tuned to the wagon whenever engaging the hoist lever because the wheels on one side may settle as the full load is transferred to the tandems.

Position the truck to unload — wheels level and articulation joint as straight as possible — where the pit boss or the fill-tractor operator directs before moving the hoist lever. Put the transmission in neutral, set the parking brake, and make sure the area is clear of people.

It's OK to bring the engine speed up to medium or high idle for a faster dump cycle, but remember to slow down as the cylinders reach the top of their stroke. No good can come from testing their seals by repeatedly slamming the cylinders all the way to the top.

Dumping sticky materials, large items like boulders or tree stumps, or dumping downhill or with a tailgate should pique an operator's caution. If part of the load gets stuck in the bed as it is raised, the center of gravity can shift far enough to the rear to raise the tractor off the ground. Tractors are seldom perfectly balanced, and with nothing but an articulation joint hitching them to their wagons and tires lifted clear of the ground, they tend to roll on their sides. (Pay attention. You can get a wet, sticky load in otherwise non-threatening conditions by failing to dump a bed partially filled with rainwater.) In these situations, raise the bed more slowly and pay careful attention to the attitude of the body and wagon.

The bed should be fully lowered and the hoist in the float position before accelerating to travel speed.

"Those body-tilt cylinders are pretty long — they can get damaged, operating the truck with the bed up in the air," says Hymbaugh. "And the truck is top heavy with the bed raised. I've seen trailers tip over sideways with the bed raised, running over a huge stump or rock in the fill."

If conditions are smooth and stable, an articulated truck can dump on the roll to spread material or build a berm. But that's a call that should be made with caution, as the chances of a mistake tend to increase the longer an operator is working on a site.

"Operator fatigue and human arrogance can contribute to truck instability," says Stec. "Guys get too comfortable with a site after they've been working on it for a while, and they can make mistakes like going around curves too fast and test beyond the machine's limitations, or go too fast down-hill and overspeed the engine."

There's a time for speed, but a dedicated operator knows how to meter it out safely, and realizes that maximum productivity is not always the same as perpetual acceleration.

"The operator's goal isn't to achieve a high top speed for a route, but over the course of the day, the highest average speed — to keep production high and cycle times predictable," says Volvo's Snedecor. "The more you accelerate and have to brake, the more often you're going to have to stop for fuel, and the more likely the machine is to need maintenance. The best operators are ones that perform predictably, reliably and with a low amount of service needs."

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