Properly spec’ing a large articulated hauler depends a great deal on the application at hand. Remember that ADTs are only one half of a team. In order to get the most out of your trucks, it’s vital that you correctly match the excavators to the truck’s rated tonnages. Effective loading in general construction applications is best carried out with excavators no smaller than 30 metric tons. Larger excavators, in the 90-to-150-metric-ton class, may be spec’d for 40-metric-ton ADTs in quarry applications.
When it comes to matching excavators and ADTs, bigger is usually considered better. That’s because optimum hauler productivity is attained with the fewest possible excavator passes to load the truck. “Ideally, most contractors would like to see their ADTs fully loaded in three passes,” notes Bob Todd, product and articulation specialist, Caterpillar. “Four passes are the norm. Some excavators take as many as 10 passes to load a truck. That’s not very productive, but might be a contractor’s only option in particularly dense soils.”
“An ADT should be loaded in three to five passes, in two minutes or less,” says Buddy Goodman, ADT product manager, Volvo. Part of that equation is the size of the truck. “If you’ve spec’d a truck that’s too big for your excavators, you’re just wasting time trying to fill it up. Always remember that money is made by moving material, not loading it.”
Beyond sizing an excavator, spec’ing the proper bucket goes a long way toward increasing ADT productivity. “It’s very important to match the bucket, not only to the excavator and the truck, but also to the excavation and soil type to ‘max out’ your productivity,” Todd says. Assuming that your crews are working to SAE 2:1 specs, Todd says ideal bucket sizes would be:
· 5 cubic yards for 30-metric-ton ADTs
· 6.3 cubic yards for 35-metric-ton ADTs
· 7.3 cubic yards for 40-metric-ton ADTs
· 7.8 cubic yards for ADTs over 40 metric tons
“The density of the material is key,” says Steve Moore, ADT product manager, Komatsu. “Don’t spec too big or too small a bucket. Watch your excavator performance closely. If a bucket’s too big, it’s going to affect the excavator’s performance and that’s where you’ll see production delays. Your fill factors aren’t going to be where they should be, or the excavator will be doing too much work to fill the bucket.”
“Your tons per hour required will determine the number of trucks you’ll need on the jobsite,” notes Fred Casten, product manager, Terex. “Start by figuring the estimated cycle time and then size the trucks accordingly.” As an example, Casten cites a job requiring the removal of 600 tons of material per hour with a cycle time of 15 minutes. “If you’re using a 30-ton truck, figure 30 tons per load at four loads per hour. That works out to 120 tons per truck, per hour, which in turn equals five ADTs to meet that production schedule.”
Well-trained operators equal better productivity
Many contractors place rookie operators in ADT cabs. Unfortunately, these drivers may receive only rudimentary instruction on operating the truck efficiently. The common reasoning seems to be if you can drive a car, you can drive an ADT. Moore says this is a mistake that can dramatically lessen productivity.
“Have your operators read the truck’s manual and understand all its systems,” Moore notes. “An operator needs to know how differential locks and interaxle locks work and when to use these systems. If they do, they’ll be able to get more performance out of the truck. Also impress upon them that they are an operator just like a guy on an excavator. They can’t just shift it in gear, put their headphones on, eat potato chips and sit there with their right foot down and let the truck do whatever it wants to do. That’s not the path to productivity.”
Moore says a good operator needs to be trained to anticipate with his eyes what terrain problems are coming up, and have an idea what he ought to do to meet and defeat those circumstances. “He needs to have the skills to control the truck’s transmission through a hold switch or put it in the proper gear instead of putting his foot down and letting the computer do all the work.”
Although ADTs today are equipped with computer-controlled automatic transmissions, Moore says good operators know how and when to switch over to manual control of the gears. “As good as modern automatic transmissions are, you can still get into situations where it’s better if the operator controls the truck. If you’re in mud, why let the truck shift up through a couple of gears then shift back down as it goes through the slop? Why not shift into the proper gear, hit the hold switch and plow through it in one gear? Once you clear the mud, hit the button again and shift back into automatic mode. That’s not only going to help you get faster cycle times from the truck, but it’s a lot easier on the transmission and drivetrain.”
Common sense pays dividends when laying out haul roads
An easy way to help keep ADT cycle times low is to pay attention to your haul roads. “Remember that the better the haul roads, the better the productivity,” stresses Ian Marshall, marketing manager, articulated dump trucks, John Deere. “Although ADTs are capable of near-extreme conditions, they are not very productive in those situations. An ADT’s extreme off-road capabilities should be used as an insurance against bad weather, unavoidable sections of bad haul road and soft conditions at the load and tip areas.”
Books can, and have, been written on setting up ADT haul roads. But there are some basic rules worth reviewing here. Use your graders and dozers to maintain haul roads as much as possible throughout the course of the job. “Haul road design and maintenance requirements should support sound industry practices,” Todd notes. “To maximize safe working, corners and crests must be designed to allow machine operators to see and avoid hazards when traveling at normal operating speeds. And when laying out your roads, all calculations should be completed using worst-case scenarios. Factor in variables such as the smallest obstacle the trucks will encounter, longest stopping distance, highest expected speeds, wet roads, etc., to best reduce any idle or ‘dead time.’ Remember that most ADT production delays due to waiting are because of poor organization.”
“Road maintenance leads to higher average truck speeds,” Goodman says. “Try to lay out dump areas so that a minimum of reversing is required when dumping and loading. Optimally, an operator should be able to drive his truck directly into the loading zone, fill his bed and continue on through to the dumpsite. A circular track pattern often is the best choice for continuous truck loading.”
“Don’t make your life miserable,” Moore says. “Use your head. There are a hundred different ways to lay out a road. But a good foreman will be willing to take a little different route and find the best path for your trucks. And that will do wonders for your bottom line.”
Alignment during loading dependent on application and equipment
Proper truck placement during loading operations is of the utmost importance. According to Moore, side-loading ADTs is the most common method when using excavators and wheel loaders. “You’ll see maybe 10 percent of contractors who load from the rear of the truck, but that’s usually dictated by jobsite or application.”
The advantages of side loading are obvious, Goodman says. “Loading perpendicular to the truck lets the ADT operator monitor the loading process in his rear-view mirrors and check it before he pulls off. So it’s a definite boost to safety on the jobsite. Also minimize the excavator’s tailswing to reduce loading time, and place the excavator above the truck if possible. This improves the excavator operator’s field of view and further decreases loading times.”
When using an excavator, Todd says it’s generally best to use the bench loading technique for over-the-rail, through-the-gate, or off-set loading. “Bench height should be equal to the excavator’s stick length or slightly less,” he explains. “The truck’s body top rail should be positioned below the boom stick hinge pin. Limit your work zone to about 15 degrees on each side of the excavator’s centerline to minimize swing. Position the ADT as close as possible to the excavator’s centerline to ensure no overstretching or cramping of the excavator is occurring.”
When loading with a wheel loader on a parallel dig, Todd suggests adopting a rightward loading methodology. “Try to position the truck in the pocket at a 45-degree angle to the loader for optimal loading. The truck’s body top rail should be below the wheel loader bucket stick hinge pin.”
When using a wheel loader on an angular dig, Todd recommends adopting a leftward loading methodology. “The truck should be in the loading area at a 15- to 20-degree angle to the dig for optimal loading,” he explains. “Position the truck with the body top rail below the wheel loader bucket stick hinge pin.” In both scenarios, Todd says, the loader’s wheels should only turn between one and one and a half revolutions during each load pass maneuver to ensure optimal productivity. “And always remember,” he adds, “that when loading with a front wheel loader, alignment of the top rail of the body with the bucket pin of the wheel loader is imperative.”
A final note on truck alignment deals with dumping situations where the bed is raised in the air. Always park the truck in the straight-ahead position in the tip area. This will help you avoid potential overturn possibilities due to sticky material held up in the body. Remember that the bed, when raised, gives the truck an extremely high center of gravity. An abundance of material in the bed can make a truck laterally unstable and increase the chances of the bed overturning. Also, keeping the truck in a straight-ahead position will reduce the chances of a turnover due to soft ground conditions.
On-board computers simplify ADT production management
As with any piece of construction equipment, gauging ADT productivity is crucial to ensuring high profitability. Happily, unlike some equipment types, ADT productivity is fairly easy to monitor. “The most basic method to calculate ADT productivity is to stand out there in the field and count the cycles it makes in a day,” Moore says. “If one truck is cycling 48 times a day, and another is only getting 42 trips in, you have a pretty good idea as to what’s working and what isn’t.”
Although that approach sounds simple enough, Moore says it can easily be worked out into more specific figures. “If you know how much the material weighs per cubic yard, you can calculate your tonnage,” he says. “If you know how much the truck is covering you can calculate your daily volume. So you can get as scientific as you want. But the bottom line is that you’ve got to know how many cycles a truck makes in a day.”
There are, of course, other methods for gauging ADT productivity, some of them quite high tech. Almost all trucks now come with on-board computer systems that can perform tasks as simple as ticking off the number of trips a truck makes in a predetermined period of time, to monitoring idle times, to relaying the tonnage hauled per trip. “If fuel is an overriding factor for you, then these systems can track tons per gallon and gallons per hour as a productivity measurement,” Marshall says. “It’s fairly easy to have a wireless communication system fitted to the truck. These systems can transmit any performance or productivity data to a PC, mobile phone or fax machine for equipment management reports.”
