Cover Story/Productivity guide: Pull-behind scrapers and power units
| June 12, 2007 |
As earthmoving systems became increasingly mechanized in the 1930s and ’40s, pull-behind scrapers, usually coupled with dozer power units, were a highly popular and effective way of excavating and transporting large amounts of dirt. Eventually, pull-behind units were supplanted by motorized scrapers. By the 1990s, hydraulic excavator/articulated dump truck teams were considered the state-of-the-art way to quickly move large amounts of earth significant distances. Over the past 20 years or so though, a number of cost-conscious contractors have returned to the old pull-behind scraper concept as their preferred earthmoving system.
Many of these contractors opted to employ agricultural tractors as the primary power unit for towing these scrapers because of their much lower acquisition costs compared to “yellow iron” power units.
It was a trial-and-error approach: They learned horsepower on ag tractors was less important than on a dozer or mechanized scraper. Their operators had to adapt to new operating methods. A whole new life-cycle cost and timetable for ag tractors employed towing scrapers had to be developed. And some contractors found their productivity increased to such an extent they had to recalculate their cost/benefit analysis and purchase extra support equipment to keep up with the pull-behind scrapers.
Today, pull-behind scrapers are recognized by OEMs as a highly productive earthmoving method. Refined agricultural tractors, specialized scraper units and new “yellow iron” power units are all recent developments targeting contractors engaged in this earthmoving application.
Crunching the numbers
Acquisition costs are among the top reasons many contractors have moved to pull-behind scraper systems. By way of comparison, ag tractor and pull-behind scraper OEMs point out a new motorized scraper can cost a contractor anywhere from $600,000 to $1.2 million. “On the other hand,” says Greg Laudick, division marketing manager, John Deere agricultural, “you can buy a tractor with two pans for $400,000, and sometimes even less. And you’ll move more yards of earth in the bargain.”
Breaking those figures down, Roger Lewno, marketing and training manager, Case IH, says a typical four-wheel-drive agricultural tractor generally costs about $230,000 new. The scrapers themselves are about $60,000 apiece. “Tractors with pull-behind pans will move more material than a motorized unit in the same time frame unless the haul distance is over a mile,” Lewno notes. “Once the haul distance is extended to greater than a mile, the self-propelled units have faster travel speeds and the advantages of loading and unloading are taken away by the distance.”
In the past, says Mark Miskin, president, Miskin Scraper Works, motorized scrapers could out-perform pull scrapers in long-haul applications. “But not anymore,” he says. “With today’s high-speed tractors and heavier scrapers, haul length is no longer an issue. We have customers who are making long hauls of 5 miles and more and making more money than they ever could with motor-scrapers.”
A typical self-propelled unit will have a 21-cubic-yard-capacity bed, Lewno notes, carrying between 20 and 24 cubic yards of material per load. “But pull-behind scrapers can be tandem or even tripled,” he says. “If you use two 17-cubic-yard pans, we have the capability of moving 34 cubic yards of material with each pass. If the soil conditions allow, you can add a third scraper and move 51 cubic yards of earth in a single pass.”
A normal pull-behind scraper operation runs eight hours a day, five days a week. Some operations, on tight deadlines, have successfully run two, 10-hour shifts using pull-behind units. Generally, this translates into 1,200 to 1,600 hours of use for both scrapers and power units a year. Kent Stickler, product consultant, John Deere Construction and Forestry, says studies conducted at Deere and validated by its customers show contractors using pull-behind scrapers realize about half the labor costs they would using other earthmoving options. They also spend about one-third of the money they would to acquire their equipment and see operational cost savings from 30 to 50 cents a yard on a typical job.
That’s because ag tractors and scrapers aren’t just cheaper to purchase, Lewno says. They’re also cheaper to operate and maintain on most jobs. “First off, you’ve only got one engine per machine moving material,” he explains. “Some of the large self-propelled scrapers have two engines. Typically on fuel alone, a Case IH STX450 Steiger tractor may burn 16 gallons of fuel per hour, compared to approximately 25 gallons per hour on the self-propelled scraper.”
“A lot of self-propelled scraper operations will normally have a push dozer working along with each scraper unit,” Laudick adds. “So now you’ve got two laborers to pay and an additional piece of equipment burning fuel supporting each scraper. In addition, you’ve got the ownership costs of both machines, as well as their maintenance costs.”
Cycle times for ag tractor-scraper combos are also faster than motorized scrapers, says Tim Miller, general marketing manager for Challenger Tractor, a division of Agco. “Tractors with the pull-behind pans can load, travel, dump and come back quicker than some of the other types of equipment,” he says. “Most pull-behind operations run at speeds of 5 mph when loading pans. Some operations move faster than that. So you’re moving more dirt faster and you don’t need any support equipment, although you can put push blocks on the back of the scrapers, if need be.”
Pull-behind scrapers match almost any earthmoving application
Miskin says pull-behind pans have much higher flotation than motorized units, which allows them to ride higher over slick ground conditions while carrying heavier payloads. But they do have their limitations. Miskin says rocky soils, which he defines as material less than 50 percent dirt, and soils with high shale content are difficult for pull-behind scrapers to load. In addition, exceedingly slippery ground where a tractor can’t get good footing also makes pull-behind scraper use difficult.
But if those two criteria aren’t present, you should be able to use pull-behind scrapers and tractors, Miskin says. “You’ll also need to consider a jobsite’s topography, particularly any grades that must be climbed,” he notes. “And the haul distances involved and ground conditions should be studied to determine if you can use a single, double or triple train of pull scrapers.”
There are a wide range of pull scrapers on the market today – from light-duty farm scrapers to true “construction grade” units. “As you’d expect, the farm-grade units are cheaper to purchase,” Miskin says. “But they don’t seem to have the durability contractors demand from their equipment.”
The most popular pull scrapers used in construction applications are units in the 17- to 20-yard-capacity range, most often pulled in trains of two scrapers per power unit. “In the Miskin Construction line we have 9-, 17-, 19- and 26-cubic-yard scrapers, all of which can be pulled in trains of two or more units,” Miskin says. In addition, his company also manufactures a D-26 model that can be pulled in trains carrying 52 to 78 cubic yards of material per pass.
Another item you’ll have to consider when setting up a pull-behind scraper is the cutting edge style on the unit. In sand, a level cut works best, says Deere’s Stickler. In clay, a drop-center arrangement is the preferred choice. “If you’re excavating really heavy clay the cutting edge sections can be moved on Deere scrapers – alternate one up then one down at 18-inch intervals,” he notes. “This will break up the material during the loading cycle. If you’re running carry-all scrapers, hitch height becomes important because raising the hitch will make the scraper load better in sand. If you want the scraper to cut more aggressively, you can lower the hitch height.”
For his part, Miskin suggests serrated blades for penetrating extremely hard soil and mud slinger bars fitted between the tires to avoid clumping of muddy or rocky soil between the scraper’s tires. “Tires are also important,” he stresses. “You’ll need to spec large tires for flotation, and speed-rated tires if you’ll be doing long or high-speed hauls.”
Pull-behind units can also be combined with laser grade and slope control systems, Stickler adds. “If that’s your choice, we can also field-install a laser bracket on the scraper.”
Because many scraper teams routinely consist of two or three pans in tandem, OEMs are targeting specific improvements on their tractors to boost safety and productivity in scraper applications.
Miller says he’s seeing hydraulic trailer brakes becoming prevalent on tractors. “They’re now standard on Challenger tractors,” he notes. “Obviously when you have two or three pans behind the power unit, it’s a tremendous amount of weight back there and can be difficult to stop when the tractor is going fast.”
Likewise, Miskin says brake units for the scrapers themselves are available as optional equipment. “I’d say they’re a must-have for contractors working on steep grades or at high haul speeds,” he notes.
Depending on how often you expect to couple or uncouple tandem pans, Stickler says you may want to consider adding optional quick-attach hitches to your scrapers. “Deere Ejectors are designed to do this,” he notes, “and make it much easier to attach and detach the tractor from the pans and the pans from each other quickly and safely.”
Once you’ve got your scrapers properly set up, Miskin says be prepared for production increases. “A few triple trains of 57-cubic-yard scraper teams pulled by a high-speed tractor can bury a soil compactor. So you want to make sure your support equipment can keep up.”
There are several power unit types to choose from when pairing tractors with pull-behind scrapers. Regardless of the type you choose, OEMs stress that the term “ag tractor” is misapplied, since they now build power units specifically for earthmoving applications. “Take the Case IH STX Series Steiger scraper model,” Lewno says. “It’s not an ag tractor with add-on equipment. It’s specifically designed for this market. True, it’s red like the models that make up our agricultural product line. But we’ve found contractors don’t object to the red color over the yellow and in many cases use this to their advantage.”
Likewise, the Challenger tractor family was originally designed and manufactured by Caterpillar, before Agco acquired its rights. “So obviously this tractor was developed from a construction heritage,” Miller explains. “One of its core design features was pulling scrapers and Caterpillar developed an extensive earthmoving handbook specifically for this tractor that we still provide to our customers to ensure their productivity.”
Power units can broadly be broken down into three types: Rubber-tire models with varying wheel and drive configurations and two different track types – quad tracks (four independent track systems in place of wheels and rubber tires) like those found on Case IH Quadtrac tractors and full-suspension, “rubber-band” track systems like those on Challenger tractors. In addition, Bell Equipment North America offers its ADT-derived D Series tractors.
In most cases, terrain type and ground conditions dictate the type of tractor you’ll want to specify. Rubber-tire units will work well on straightforward jobs without any problems. If you’re working in exceedingly muddy terrain or jobsites with steep grades, then a track-type tractor is a more logical choice.
Despite the tweaking that many of these tractors have undergone to prepare them for earthmoving applications, OEMs warn that you should be aware there are significant operating and durability concerns when compared against traditional yellow iron equipment types. “When you buy a piece of construction equipment, you probably expect to get 8,000 to 10,000 hours worth of life out of it, depending on what kind of machine it is,” Laudick says. “Whether you’ll actually get that or not is a different story – but that’s the standard expectation. You won’t get that kind of life out of an ag tractor – I don’t care what color it is. They just weren’t designed to run those kind of hours.”
That’s one of the weaknesses Bell and Deere are exploiting with their D-Series tractors. Essentially the front (powered) half of an articulated dump truck, D-Series tractors offer construction-grade performance, durability and four-by-four drive power specifically for scraper applications. “Contractors should not be considering an ag tractor for construction-level pull scraping,” says Robin Pett, vice president, Bell Construction Equipment. “The Bell scraper tractor has been designed specifically for pull scraping on construction earthmoving jobsites. It offers the correct amount of horsepower and torque to pull construction-level scrapers. It offers the correct drivetrain features and computer controls to pull one scraper or a combination of scrapers, to move an optimum amount of material at the lowest possible cost per ton.”
Pett acknowledges that Bell’s tractor is more expensive to purchase than an ag tractor, but says contractors more than recoup this cost difference thanks to the long life the tractor offers. “In many cases, you’ll have to replace one tractor five times before a Bell tractor wears out,” he says. “You get what you pay for.”
Laudick doesn’t dispute this. “The thing you’ve got to keep in mind is that an ag tractor was designed to pull at a steady rate all day long,” he explains. “But now you’re putting it into applications where you’re pulling every ounce of horsepower out of it for a minute and a half to two minutes every 10 minutes of the hour,” he says. “It puts some totally different strains on that tractor that it wasn’t originally designed for.”
Does that mean these tractors simply aren’t up to the rigors of a construction jobsite? Not at all, says Lewno. “They can definitely handle those applications,” he says. “We will see some different types of failures compared to a tractor in a typical ag operation. But with good preventive maintenance, contractors are generally very pleased with the operating costs of the units. Some contractors have better serviceability out of this operation than self-propelled units. Still, most contractors will trade the power source two or three times before they switch scrapers.”
On the other hand, Laudick notes, the demands and stresses put on ag tractors are offset by a more substantial warranty than one gets with conventional construction equipment. “The standard warranty on a piece of construction equipment is, I believe, six months,” he says. When you buy an ag tractor, right now the warranty is two years or 2,000 hours. Now many of these machines pulling scrapers are running around the clock – so a lot of contractors will hit that 2,000 hour time in less than 15 months. But still, you’ve got warranty on a machine for 14, 15 months versus six months.”
Don’t depend on horsepower to get your scraper jobs done
If you decide to go with ag tractors to pull scrapers, then your operators will have to adjust to a new way of running equipment. Although ag tractors are classified by horsepower, Laudick says you shouldn’t get too hung up on engine output when selecting a tractor. “Construction guys are always looking at the top end when it comes to horsepower,” he says. “But with these tractors, you’re using horsepower in a completely different way.”
The key to making a tractor and pan combination work, Laudick says, is not to drop the blade into the ground and bulldoze it through the dirt. “You keep the speed of the machine up and use the weight and its massive momentum to throw the dirt into the pans,” he explains. “So you’re running at a faster ground speed and lugging isn’t nearly as important as it is with a self-propelled scraper.”
Laudick says he’s run scraper tractors for uncounted hours and can easily load two 18-yard pans at half throttle in most applications. “Horsepower isn’t needed for loading pans if the soil conditions are right,” he says. “The need for horsepower comes in when both pans are loaded. Now – with two loaded pans behind you – you’re pulling in the neighborhood of 100,000 to 120,000 pounds behind the tractor. That’s where you need horsepower – particularly in soft soil conditions.”
“These tractors ride a lot better than motorized scrapers, and their performance is better,” Lewno says, “But these tractors need to be operated at speeds typically used in the ag market. And they’re not equipped with torque converters, so the operator needs to learn to operate effectively since he can’t rely on a support vehicle to aid in loading.”
Ag tractors do come with powershift transmissions, which Laudick says is a bit of a concession to operators trained on yellow iron. “Back when this whole tractor and pan concept started, the scraper guys were saying ‘Oh, we gotta have a powershift transmission,'” Laudick explains. “What they were doing was putting the pan in the ground and never messing with its depth once they were moving.” The operators would just put it in an inch or three inches – or whatever the depth was – and then as the pan started loading up or the tractor started getting into a harder pull, they’d start downshifting the transmission to maintain speed and production. “And the powershift was the most logical choice for a transmission,” Laudick adds, “because you didn’t want to push the clutch in and shift the manual transmission while all this was going on.”
Today, Laudick says such an operating technique is not only outdated, but actually bad for the longevity of a tractor. “I’ve spent the last four years trying to retrain everybody on this,” he says. “I tell operators now to keep the tractor in one gear, whatever gear they want, it doesn’t matter. Pick your gear and leave the transmission alone. The worst thing that you could do to any transmission on any piece of equipment is to put it in hard pull under load and then start shifting. Doing so shortens drivetrain life considerably. So we teach everybody now to just put it in a certain gear and then use the hydraulics on the tractor to feather that cutting edge up and down to control the load on the tractor.”
Manipulating the blade will allow you to load the pan as fast as possible, but still get all the power out of the tractor and not burn the transmission up. Keep in mind though, Laudick says, a powershift transmission is still highly valuable on long-distance hauls. “Once that pan is loaded, if you’re going a half a mile or more, a powershift transmission is a lot better for transcending from the cut speed to the transport speed.”