A key component for ensuring high earthmoving productivity is identifying the right types of equipment to do the job. For many contractors, hydraulic excavators in the 45- to 90-metic ton classes are ideal, high-production earthmoving machines. But while excavators excel at digging earth, they are not suited for removal of spoil material. When that becomes necessary, articulated dump trucks in the 25- to <35-metric-ton classes are the perfect match up.
ADTs are, essentially, highly specialized, off-road dump trucks. Models in these size classes typically feature six-wheel-drive powertrains optimized with either full differential or limited-slip differential locks to unitize all six axles in sloppy ground conditions. All ADTs have an articulating, oscillating center hinge that gives a high degree of lateral and vertical movement. Steering, unlike the sytems on on-highway trucks that feature traversing front axles, is by side-mounted hydraulic cylinders. All of these features combine to create a dedicated, high-capacity hauler that can travel even the sloppiest ground conditions.
Match up equipment,
attachments and cycle
times when planning ADT use
In addition to excavators, wheel loaders in the 250 to <600 net horsepower classes also match up well with ADTs in the 25- to <35-metric-ton classes. In either case, ensuring optimal production is easy, says Fred Casten, product manager, trucks and scrapers, Terex. "The rule of thumb is you want to load these trucks in four to six passes," he explains. "And once the truck is loaded, have another ADT waiting to pull into position and repeat the loading process."
"Pay attention to bucket sizes, whether you're using an excavator or a track or wheel loader," advises Michael Stec, ADT product specialist, Volvo Construction Equipment. "Oversized buckets on excavators slow digging and making an excavator tippy. You might be able to load the truck with fewer passes, but more likely, the unstable machine is going to slow your production down."
On the other end of the spectrum, Stec cautions against using too narrow a bucket on your excavator. "Narrow buckets cut through soil easier, but while the excavator might make quicker passes, you'll have to make more of them to fill the truck up," he says.
"Pay attention to the length of the arm on the excavator," says David Wolf, marketing manager, Case Construction Equipment. "Long arms are typically a good feature for applications that require reach, but in mass excavation applications, machines gain breakout force and lift capacity with shorter-length arms. The higher-performance specs can assist in matching excavator and bucket size to ADT for rapid load cycles."
Stec notes excavators are typically positioned above or on the same level as the ADTs they're loading. Regardless of which method you choose, he suggests keeping the machine's reach into the loading area roughly equal to the length of its dipper arm. To maintain maximum efficiency and quickness, use 45-degree swing angles during your loading passes.
Wheel and track loaders with 4 to 9.5-yard buckets match up best with ADTs in these two size classes, Stec says. "With these machines, loading height and tipping height are your key production factors," he says. "If the loader's tipping height is too low, the last few passes dumped into the truck bed will be pushed in and not heaped. Worse, a tippy loader is harder, and takes longer, to maneuver near the truck body. But if the machine's tipping height is reasonably higher, the last pass or two can be heaped – boosting the truck's productivity and keeping loading times quick and constant."
Calculating the correct number of ADTs to maintain peak productivity is straightforward as well. Since you want to maximize your excavator or wheel loader's productivity on the jobsite, the key is to keep that machine moving at all times. The easiest way to make sure that happens is to take your ADT's cycle time (the length of time it takes for the truck to leave the loading zone, travel to the dump site and return) and divide it by the excavator's load cycle time. So, if it takes an ADT 10 minutes to complete its travel circuit, and it takes your excavator 2 minutes to load that truck, divide 10 by 2, and you discover you need five ADTs on site to keep that machine digging at all times.
"I think 8 to15 minutes per hour is acceptable queue time for trucks waiting to be loaded by the excavator," Stec adds. "Remember an excavator operator will need to reposition the unit and clean his area before he's ready to drop a quick first pass into the hauler's body when it approaches the loading area. Factor those requirements into your calculations."
It's worth pointing out a jobsite is a dynamic place, and as a job evolves, the math might change on you. Haul road lengths may increase as excavation work advances. As more contractors and machines appear on a site, congestion can also increase haul times. So don't decide on how many ADTs to run and then consider the matter settled for the entire job.
Double-check both ADT and excavator cycle times periodically to make sure the math is still in your favor.
Training operators on all aspects of safe operation – including proper dumping procedures – should be a priority for you.
Plan ahead when
laying out haul roads
Another easy way to boost your ADTs’ productivity is to properly plan out and prepare haul roads for the trucks. Just because an ADT can slog its way through incredibly sloppy ground conditions, it’s not best for your bottom line to have them do so day in and day out. Well-graded, level and clean haul routes can do as much as anything to boost ADT performance in the field and keep a job on schedule and under budget.
Obviously, when you’re laying out haul roads, you want to keep distance, grade, rolling resistance and ground structure in mind. Keeping the distance between the loading and dumping sites as short as possible is a no-brainer. But remember to keep the route’s grades at a minimum too – not just to save time, but ADT fuel consumption as well.
Rolling resistance is tougher to get a handle on since it varies with the type of soil you’re working in, its moisture content as well as vehicle loads, and tire type and diameter. If you can improve any of these factors on a haul road, you’ll instantly see a marked increase in haul speeds and fuel economy.
Casten says he’s seeing a burgeoning interest in the use of oversized tires lately on ADTs. “Contractors are finding that wider, lower tires can give you better truck stability and flotation on many jobsites,” he notes. “And this is an area where OEMs aren’t as flexible as they need to be yet. You know, it’s only natural that guys like me want to sell trucks with the tires we’ve spec’d at the factory. But I’m seeing a lot more contractors order wider tires as standard equipment on their new trucks. It’s an option more contractors should investigate as a way to improve their trucks’ performance.”
Case’s Wolf points out that a wide ADT frame also allows for a wider dump body. “A wide frame lowers the center of gravity for the payload, while placing more of the weight to the outside of the vehicle,” he explains. “This improves overall machine stability and operator comfort, making it possible to move more tons of material per hour.” Wolf says a wide frame also allows the truck’s lift cylinders to be stored inside the frame. “It’s an easy way to protect those cylinders from impact damage caused by excavators and falling debris,” he adds.
Smooth haul roads are important too. “An ADT operator is only going to drive a truck as fast as his backside will allow,” quips Steve Moore, product manager, trucks, Komatsu. “If he’s getting bumped and jostled around in that cab, he’s going to slow down – and he doesn’t care who’s not making any money.” Good grade work aside, Moore says spec’ing a truck with a good suspension and a high-quality air ride seat can help boost productivity in uneven ground conditions.
It’s a good idea to lay your haul routes out in a circular or oval pattern. Excessive and unnecessary maneuvering is a productivity killer. So try to create a traffic flow pattern that’s logical and efficient. And don’t forget to err on the side of safety when planning haul routes. Try to maximize driver sight lines when confronted with crests or curves in the road. Minimize encroachment by other vehicles and equipment types, and maintain the roads as well as weather and working conditions will allow.
Consider lower, wider tire options to improve traction and flotation if you consistently work in poor ground conditions.
Take the time to train
Who drives the ADTs in your fleet? If you’re like many contractors, it’s the rookies on your jobsites – the youngster no one is sure can yet handle another machine. Moore finds this a little frustrating – although he understands. “It takes less time to train for an ADT than other pieces of equipment,” he notes. “And some unions classify truck drivers differently than they do equipment operators – so there can be some advantages for you there.”
The main mistake Moore sees contractors make is assuming because an ADT is simply a ‘big truck’, any training these novice operators require is minimal. “An artic is an expensive, sophisticated piece of machinery,” he says. “It doesn’t have to be babied – but a poorly trained operator can kill your production, cost you money or even destroy a truck in a worst-case scenario.”
To that end, you need to make sure your drivers understand all the ADT’s systems and controls. One of the least understood systems on an artic truck is its differential lock system. “On our trucks we place our differential within the output transfer gear where it divides drivetrain torque between the front and rear drives with lock-up capability,” explains Bob Todd, product manager, ADTs, Caterpillar. “When working in arduous conditions engagement of the inter-axle differential lock provides improved traction when climbing steep grades or when negotiating poor underfoot areas.”
Put simply, Todd says, the differential lock works by modifying torque distribution to deliver more power to the slowest turning wheel at any given moment. “That’s why you get excellent traction when it’s engaged,” he adds.
The problem for many drivers is that each manufacturer has a different control technique for engaging the system. But there are other, lower tech alternatives available. “You get higher numbers on a full, 100-percent lockup,” Moore notes, “but the automatic feature of a limited slip is worth a lot because it’s so easy: the operator doesn’t have to do a thing.”
Limited-slip differentials are a by-product of 1960s muscle car technology, intended to improve traction and zero-to-60 acceleration times. “They work by sensing wheel slip,” Moore explains. “When one wheel loses traction, those clutch plates and that differential will automatically engage, without any electronic input. With some differential locks, you’ve got to hit the button on the dash and then put your foot on the pedal. But if you’ve got a poorly trained operator behind the wheel… maybe he’ll remember to do that stuff, and maybe he won’t. So for my money, a differential that locks up automatically and takes the operator out of the picture is your best bet for consistent productivity.”
Forgetting to disengage a differential lock system is another common mistake rookie ADT operators make. Because differential locks unitize the powertrain for situations that require high torque, they can damage the engine or transmission if engaged at high speeds. If this is a problem, Todd says an engine over-speed inhibitor is a good safety feature to check into. “This is an automatic system where the electronic transmission control protects against engine over-speeding by shifting the transmission up one gear at specified rpm settings,” he explains. “If the transmission is in its highest gear, torque converter lock-up disengages, preventing engine, transmission or axle damage.”
Beyond competency with a truck’s operating systems, your operators need to fully understand the truck’s capabilities, as well as its limitations. And, of course, consistently safe ADT operation should be foremost in any training program. ADTs are incredibly stable machines. But beds – or even entire trucks – can roll over if operating on steep slopes. Dumping is another potential danger area: A truck’s center of gravity shifts significantly when a loaded bed is raised. If the truck isn’t stable or is on a slope as the bed is going up, a rollover can occur. And just like with any dump truck, operators should be aware of any overhead obstacles, particularly power lines, when dumping.
An ejector body truck – offered by Cat – is another ADT system that can increase safe truck operation. Unlike conventional trucks, an ejector model pushes material out the rear of the bed, instead of raising it and allowing gravity to do the rest. “Ejecting the load without raising the body increases truck stability,” Todd says. “It also allows load dispersal on inclines, side slopes and in very soft underfoot conditions.”
Terex’s Casten says other ADT safety and productivity enhancements can be relatively simple in nature: “If a jobsite is particularly soupy or running conditions are bad, the addition of a tailgate on the bed can boost productivity and reduce spillage,” he says. “The same is true with sideboards. And if you’re in a severe-duty application – hauling shot rock, for example – you should look into spec’ing steel bed and body liners. Onboard weighing systems can help you track production. And if you’re serious about safety on crowded jobsites, then a rear-view camera system is an excellent idea.”