Wheel loaders: 250 to 350 horsepower
| June 12, 2007 |
Wheel loaders in the 250- to 350-horsepower class are high-volume earthmoving machines. Common applications include load-and-carry work at concrete batch or asphalt plants. They are also increasingly seen in heavy-highway and utility construction jobs and in site prep work on large subdivisions where machine size is not a limiting factor.
Almost all wheel loaders in the 250- to 350-horsepower range use z-bar front-end linkage. The exception is Volvo, which uses its exclusive torque-parallel linkage on models in this class and indeed throughout its wheel loader product line. “There’s not really a requirement for parallel lift with wheel loaders this large,” explains Lee Haak, senior product manager, wheel loaders, Komatsu. “The parallel-lift geometry used on toolcarriers is typically for material handling. It’s particularly useful with palletized materials or any type of material that has to be held level throughout the lift cycle. Smaller-sized loaders and toolcarriers will lift anything that can go onto a pallet.”
And since wheel loaders in this class are optimized for mass earthmoving applications, z-bar front-end linkage, with it’s higher breakout and lift forces, is a natural fit, generally speaking.
To many, Volvo’s torque parallel linkage appears to be a hybrid design, taking the best traits of z-bar and parallel-lift linkages and combining them into one package. But Jack Bolton, wheel loader product manager, Volvo, is quick to point out that torque parallel linkage was designed with these features from the ground up – in 1954. “Torque parallel linkage is designed to get the benefits of parallel lift and high breakout force throughout the loader’s entire lifting range,” he notes. “It also gives you high breakout forces when bucket loading down low. We’ve got similar capacity with this linkage as that found on a z-bar machine combined with the visibility that you normally associate with parallel-lift linkage.”
Hydraulic attachment use on wheel loaders in this size class is limited, but not unheard of. The most common attachment (besides buckets) is forks, used mainly for unloading trucks or carrying pipes in utility applications. Powered attachments like hammers and cutters are typically better suited to smaller loaders. But that has little to do with the lack of parallel lift linkage in this class. Space is a more pressing concern, according to Bolton. “Most attachment use takes place in relatively confined surroundings,” he says. “And that is simply more difficult to do with these larger machines.”
Specification-wise, these size wheel loaders are generally ready to go to work, regardless of your intended application. But there are some considerations you need to make based on the type of work your machine will be primarily engaged in. “In a load-out application, these loaders typically work on relatively flat ground with excellent footing,” Haak notes. “If this is your main application, then you can spec these machines with standard differentials. But if you use your loaders in heavy construction applications – or even governmental roles like clearing roads – you need to spec limited-slip differentials to make sure they’ve got all the traction they need regardless of ground conditions.”
Remember to keep the bucket as low to the ground as possible in load-and-carry applications.
Fuel consumption and component wear two major productivity gauges
Gauging productivity with wheel loaders in this size category is usually a function of the amount of material moved relative to the quantity of fuel burned while doing so. “Most contractors say ‘yes’ when you ask them if fuel consumption is important to them,” Haak says. “But when you ask most of them how many gallons per hour their wheel loaders burn, they don’t have any idea. In reality, it seems that most North American contractors are more interested in productivity than fuel economy.”
But to get the most from the wheel loaders in your fleet, you should be keeping tabs on several key operating points, including operator efficiency, fuel consumption and component wear – particularly tires and ground engaging tools.
Never look at fuel economy or productivity as stand-alone statistics. They go hand-in-hand and can help you establish a cost-per-ton figure you can use as a baseline for your wheel loader operations. Modern wheel loaders have on-board computer systems that let you easily track this data. And once you know your baseline – how much material your loaders can move for an amount of fuel burned – you can start adding the other factors, such as tire and tool wear, into the equation.
“Fuel consumption isn’t set in stone,” Bolton says. “Any wheel loader will get better fuel consumption rates if its operator is well trained and smooth on the controls. You can tell a good operator from a bad operator by how much money it costs to keep that machine up and running. That includes everything, from component wear to fuel consumption.”
Finally, if you want better performance from your wheel loader operators, consider giving them some pride of ownership in the machines. “I’m a believer in one machine, one operator whenever possible,” Bolton says. “You’ll find that operators who take the best care of their machines are the ones who get a machine when it’s new and they’re able to maintain it and take some pride of ownership in the machine. Even new guys who are ‘given’ an older machine and told it’s theirs tend to take better care of it since they feel a sense of responsibility and a connection to the machine. And that pride of ownership creates operators who tend to do the best in terms of productivity and cost for you.
Properly trained operators a must
Operator technique is also something that should be examined if you want to ensure consistent productivity in your wheel loader fleet – or even with one machine. “You can put two operators on the same loader, and one will have horrible productivity and burn a lot of fuel for the same application compared to another operator,” notes Andrew Bonde, product marketing manager, John Deere. He says the acid test for wheel loader operators is how they act when the machine moves into a pile. “Looking at a loader’s entire production cycle, its greatest fuel burn occurs when the machine is crowding into a pile and you’re lugging the loader to fill the bucket,” he explains. “To minimize fuel consumption during this time, your operators need to optimize the speed in which the loader goes through the pile. At the same time, they need to coordinate the speed and time it takes to raise the boom when they’ve cleared the pile. Operators should strive for a good match relative to where they are in the production cycle. You don’t want to see a loader sitting idle waiting for the boom to come up so it can dump into a truck. But at the same time, you don’t want the boom to raise so quickly that it raises stability issues or causes spilling.”
Another sign of poor operator control is excessive tire slippage when the loader is crowding a pile. Many operators think they have to “charge” a pile in order to guarantee a full bucket. But according to Bolton, that method is actually counterproductive. “If your operators are running the machine at more than 1,600 rpms when they enter a pile, they’re not gaining any productivity,” he says. “But they are wasting time and fuel and wearing the tires out early since he’s probably spinning them. Wheel loaders today are designed with high-torque, low-rpm powertrains. That means they’re built to load efficiently at low rpms. So you don’t have to build up a head of steam before entering a pile or work them as hard once you’re in a pile. You’ll get better productivity if you go in and easily lift up on the boom and curl the bucket back, and come away in one continuous motion.”
Bolton admits that it’s initially difficult to instill good productivity habits in wheel loader operators. “If you’re serious about improving operator performance, there’s no substitute for manufacturer and dealer training,” he says. “If you can take advantage of those programs, you’ll get better operators who understand that an easier working machine is actually more productive in the long run.”
And remember that part of improving a wheel loader operator’s performance is empowering him on the jobsite. “In truck loading applications, it’s a good idea to put your wheel loader operators in charge of the jobsite loading configuration,” suggests Gary Bell, vice president and general manager, Kawasaki Construction Machinery. “It’s not up to the truck drivers to decide where they want to load. Your operators should be working in a ‘Y,’ ‘V’ or ‘T’ pattern as they exit the pile and turn toward the truck, since this method will usually deliver the quickest loader cycle times. And to ensure those fast cycle times, the operator needs to have the authority to establish and change a loading situation to meet his production needs.”
Also avoid traveling too far from the pile to the truck, says Tom Brady, wheel loader specialist, Caterpillar Training Group. “Keep the bucket as low as possible in load-and-carry applications. And try to position the trucks from 40 to 45 degrees from the pile. This will give the loader the best approach to the bed for quick loading.”
Material density is your number one concern when spec’ing a wheel loader bucket. Spec smaller buckets for denser materials.
Oversized buckets can penalize productivity
Bucket sizes for loaders in the 250- to 350-horsepower classes range from 5 1/2 yards cubed up to 7 1/2 yards cubed, with general-purpose designs the most common. Selecting the right bucket for your job is fundamental for good loader productivity. Since bucket choices vary widely according to job, it’s a good idea to spec them based on material densities. Teeth and cutting segments can (and should) be added for machines working in rocky ground conditions or if soil penetration is an issue.
“Most contractors think that a larger bucket is always going to be the most productive bucket,” Haak says. “That’s not always true. If a loader bucket is too wide, then the amount of force needed to break material out (per inch of cutting edge) goes down. Or if the bucket is too deep, it will decrease the total amount of breakout force at your disposal. If you’re digging a lighter material, like coal, then you can definitely oversize a bucket and retain, or even increase, your productivity.”
“Bigger buckets are more difficult to fill,” Bolton adds. “And that leads to longer cycle times. The next time you’re tempted to go with a larger-than-recommended bucket, remember that although that bucket’s capacity may be 10 or 15 percent more than the appropriately sized one, overall loader productivity can suffer up to 40 percent. Added to that is the significant rise in operating costs oversize buckets can cause as well in excessive fuel use and unnecessary component wear.”
“It takes about three passes to load a Class 8 dump trailer,” Bonde notes. “But if it’s suddenly taking you four passes to load a truck when it normally takes three, you need to re-evaluate the material density you’re working in. Odds are there’s nothing wrong with the loader or the operator. So I would start asking some basic questions: Have you had a lot of rain lately? If so, that can increase the weight of the material to the point that the loader can’t handle it as efficiently as it normally would. Are you working in a different area than normal? Maybe there’s more rock in the soil and it’s hurting your bucket capacities. Try to pinpoint anything out of the ordinary that would cause the density of the material to increase, particularly if nothing else has changed.”
When it comes to productivity and the number of passes required to load a truck, machine size is crucial. But so is the number of machines being used, Bolton says. “I’ve seen cases where a contractor buys a 320 horsepower loader when what he really needs are two smaller models – say in the 250-horsepower range. Even though the loader capacity is 30 to 50 percent greater on the 320-horsepower machine than on the 250-horsepower model, the overall productivity generated by two machines combined with the reduced wait time for trucks will deliver much greater productivity.”
“Just remember,” Haak says, “when you’re spec’ing a loader for your applications, communicate your needs to the people you’re buying the machine from. The more that dealer knows about how you’re going to use the machine, the better it can be spec’d.”
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