Bell's 4206D/T Makes Tracks Anywhere

Sept. 28, 2010

Bell Equipment, a South African firm, has been selling its articulated dump trucks (ADTs) in North America since the late 1980s. Several years ago, the company began to take note of a growing interest among contractors for the economics and efficiencies of pull-type scrapers. Recognizing that its truck had the makings of a good prime mover, Bell used the basic platform of its 40-ton hauler to create the 4206D Construction Tractor—a stump-puller of a machine designed specifically for drawbar work.

Bell Equipment, a South African firm, has been selling its articulated dump trucks (ADTs) in North America since the late 1980s. Several years ago, the company began to take note of a growing interest among contractors for the economics and efficiencies of pull-type scrapers. Recognizing that its truck had the makings of a good prime mover, Bell used the basic platform of its 40-ton hauler to create the 4206D Construction Tractor—a stump-puller of a machine designed specifically for drawbar work. Now, Bell has teamed up with ATI (a builder of modular track systems) to give its latest Construction Tractor—the 4206D/T—even more power at the drawbar.

Construction Equipment editors recently saw the new 4206D/T, fitted with four ATI track modules in place of wheels, at work on an Allied Waste landfill near Pontiac, Ill. As a bonus, the new machine was pulling two Ashland scrapers, also equipped with ATI track modules. This innovative earthmoving train, especially when slogging through sticky wet clay or climbing the landfill's steep grades, looked as if it could work just about anywhere. Says Allied Waste operator Mike Cole, who runs the new Bell tractor, the train is easy to handle and works efficiently with the landfill's articulated-dump-truck fleet.

From ADT to drawbar tractor

When Bell developed the original wheeled version of the Construction Tractor, the 4206D, which remains a viable model in the product range, the company essentially created a new vehicle, even though its design utilized basic technology and components from the Bell 40-ton articulated dump truck.

Among the most significant of the 4206D's design departures is its shortened rear frame, purpose-built for drawbar work, and engineered to more efficiently accommodate drawbar forces by assuring proper weight distribution when loads from towed equipment are transferred forward, Bell says. The 4206D also uses a solid rear axle, again to better accommodate drawbar loads; while the ADT, which must handle vertical loads, uses a rear axle with a walking-beam design.

Moving forward through the Construction Tractor's design, the articulation/oscillation joint between the frames is the same as that for the truck. According to Bell, the design of the ADT joint is "overkill" for the Construction Tractor and, thus, should be "virtually indestructible."

Up front, the 4206D retains the truck's basic mainframe and semi-independent suspension system, which uses nitrogen-over-oil struts to enhance ride quality. Also retained is the truck's basic engine, a Mercedes Benz V-6, turbocharged, charge-air cooled and generating 422 net horsepower and 1,473 pounds-feet of torque. From the engine back, however, the Construction Tractor's design is focused on developing more drive-train torque than its ADT counterpart.

The 4206D's six-speed Allison automatic transmission (with an integral locking torque converter), although similar in design to the truck's transmission, uses lower gear ratios. The Construction Tractor's transmission, says Gerrit "Dutch" Viljoen, Bell's factory technical analyst, is designed to absorb full engine horsepower in all gear ranges. Both Allison models do, however, drive through the same transfer case/inter-axle differential (manufactured by Bell) that provides a 50/50 power split between front and rear axles when running in its lock-up mode.

The Construction Tractor's axles also have lower ratios in both their controlled-traction Eaton differentials and in their outboard planetary final drives. And to accommodate the added torque, more-robust shafts are used between the differential and finals. The axles have dry-disc brakes at each wheel, as does the truck, and the brakes are fully hydraulic and controlled by two accumulators in a dual-system configuration. Multi-disc wet brakes are optional.

Hydraulics and computers

The Construction Tractor's implement hydraulic system, of course, differs significantly from that of the ADT. While the truck has essentially one implement circuit (body raise), the Construction Tractor has six, which can be accessed via the 12 quick-connectors located on a panel at the back of the rear frame. The 4206D's implement hydraulic system operates at 2,900 psi and uses a load-sensing piston pump that produces 80 gpm.

Many tow-behind scrapers today can be operated on two hydraulic circuits—one to control bowl raise, and the other, working through a sequence valve at the scraper, to operate both the apron and the ejector gate. Thus, the 4206D's six circuits allow up to three scrapers in the train. If the apron and ejector need separate circuits, however, then three of the 4206D's circuits are required per unit, and only two scrapers can be towed. (According to Viljoen, though, a software change may permit older, three-circuit scrapers to operate on two circuits.)

At the operator's right hand in the 4206D cab is a compact control console with six, switch-like controllers ("paddles") for operating the six implement circuits. Positioned between the paddles and the electronic solenoids at the main hydraulic valve, says Viljoen, is a very powerful computer, which can be programmed, via an overhead panel in the cab, to tailor each hydraulic function to the job situation or to the operator's liking.

Computer assistance also simplifies the operator's task when the Construction Tractor enters the cut with scrapers in tow. Pushing the "scraper-mode" switch on the console activates the engine-retarding system, slowing the machine as the transmission shifts down to first gear (or second, if the operator chooses). The computer then adjusts the engine for full power, locks the inter-axle differential, locks the axle differentials and adjusts the torque converter for maximum pull. Once the scrapers are loaded, a second touch of the switch cancels the scraper mode.

From tires to tracks

The design of the 4206D Construction Tractor is carried over, intact, to the new 4206D/T. The only differences between the two units are the addition of a support bracket in each of the 4206D/T's fender wells and, of course, the track modules, which are mated to the final drives via an adapter.

According to Robin Pett, general manager for Bell Equipment North America, headquartered in Garden City, Ga. (near Savannah), the initial idea of substituting rubber-track assemblies for wheels on a Bell vehicle came from ATI, headquartered in Mount Vernon, Ind. ATI's thought was to install tracks on an articulated dump truck, says Pett, but after considering the idea, Bell suggested the Construction Tractor as a more suitable candidate.

Bell's first concern about installing the track modules, however, was durability, says Pett. Bell insisted on a thorough test program, along with a warranty that would match that of the Construction Tractor.

Part of the resulting testing to evaluate the suitability and performance of the track modules for the Construction Tractor was performed at the prestigious Southwest Research Institute (SwRI), an independent, non-profit research and development organization in San Antonio, Texas. ATI's Glenn W. Kahle, Ph.D., executive vice president and chief technical officer, initiated the SwRI test. He says it not only measured and documented the performance of the track module on the 4206D/T, under both laboratory and field conditions, but also investigated track-module design parameters for higher-horsepower machines.

The ATI design, says Ken Juncker, the company's president and chief executive officer, has been granted 14 patents in the United States, and a number of overseas patents as well. A number of patents are still pending. According to Kahle, the ATI track module is designed to overcome what the company considers potential drawbacks to rubber-track systems that are powered by friction drive only, or by a combination of friction-drive and sprocket-drive techniques.

ATI markets three versions of its modular track system, says Juncker, "arctic climate," "temperate climate" and "desert climate." Each is designed with rubber compounds, seals and lubricants that tailor the system to its environment, he says, and the modules are completely maintenance-free.

The Ashland connection

The lead scraper in the train on the Pontiac landfill, Ashland's new I-180TS Track Special, was fitted with the same 36-inch-wide ATI track modules as the Bell Construction Tractor. The trailing scraper had similar ATI modules, but with the slightly narrower tracks (30 inches) of an earlier design.

According to Randy Rust, Ashland's national sales manager, the new Track Special scraper, available since May, is designed as a heavy-duty construction unit that works efficiently in any earthmoving situation. But it's especially in its element, he says, when conditions are soft underfoot.

The new 18-cubic-yard Track Special has an empty weight of 33,600 pounds, and a permissible loaded weight of 74,100 pounds. According to Ashland, the Track Special's 126-inch cutting width allows fast loading, and its 54-inch (vertical) apron opening, in conjunction with hydraulic ejection, ensures fast, controlled unloading. And when working with articulated dump trucks, says Rust, the scraper can be efficiently top-loaded by an excavator if that method better fits the operation.

Because tow-behind scrapers (and their prime movers) are subjected to stress not only when loading, but also when transporting loads, the I-180 TS Track Special comes equipped with a ride-control system, similar to that of a wheel loader. The system essentially links the bowl-raise cylinders to a high-capacity nitrogen accumulator, which absorbs and dampens hydraulic pressure spikes (exceeding 2,800 psi), caused by bouncing on the haul road.

Bell cites benefits

The 4206D/T and its wheeled counterpart, the 4206D, says Bell, are flexible power sources for towing not only pull-type scrapers, but also rear- and bottom-dump trailers, water tankers, construction plows and compaction equipment.

Working with scrapers, these Construction Tractors, says Bell, can move 54 cubic yards of material in one pass by keeping three, heaped, 18-cubic-yard scrapers efficiently in tow. For the wheeled version, which has been in service long enough for Bell to make such assessments, this capability "allows far longer haul distances than previously economically feasible with any other type of scraper." To that, says Bell, can be added the potential of 20 percent gains in productivity and 20 percent less fuel consumption.

Could be that the wheeled version of the Construction Tractor will remain the long-haul champ in the Bell line. But that said, the company has no reservation about the new tracked version being equally capable on long pulls. What's for certain, says Bell, is that the 4206D/T, with track modules substituted for wheels, can extend the capabilities of its wheeled counterpart to jobsites that have extremely poor underfoot conditions, or that can otherwise benefit from the low ground pressure of this new model.

Each of the ATI track modules under the new Bell 4206D/T Construction Tractor weighs 6,000 pounds. Two-thirds of that weight is in the central frame, which is mounted on the center hub of the drive sprocket and houses the front and rear idler assemblies and the two center bogey assemblies, all of which use a pair of wheels.

Bonded to the circumference of the idler and bogey wheels is a thick rubber covering, which makes these assemblies, says ATI, almost tire-like in their ride quality. The dual-wheel idlers and bogeys also serve to retain the track, side-to-side, by forming a channel through which the thick rubber lugs on the inside of the track are guided. The track is allowed to float across the top of the sprocket, a feature aimed at reducing wear.

The frame-to-sprocket mounting uses two large (12-inch-diameter) tapered-roller bearings which, first, allow the sprocket to turn in the frame and, secondarily, allow the frame to "swing" on the sprocket hub to accommodate undulations in terrain. The sprocket is attached to, and driven by, the Construction Tractor's planetary hub via an adapter.

A key feature of the drive sprocket is the metal bar positioned at the root of each tooth. When the sprocket engages a rubber lug (on the inside of the track), the tip of the lug pushes against the bar, thus placing the lug in compression between the bar and the track carcass. Consequently, as the sprocket tooth pushes laterally against the lug, a shearing force is exerted at the base of the lug.

Were the lug simply cantilevered between sprocket teeth, says ATI's Glenn W. Kahle, the sprocket tooth pushing against the lug would exert a bending force, tending to twist the lug. According to Kahle, the magnitude of a shearing force can be far higher than that of a bending force before any potentially harmful stress is generated in the lug.

The metal bar at the root of the sprocket tooth also keeps the track elevated just enough to prevent the sprocket tooth from pushing into the track's carcass with any great force, thus possibly damaging the steel-cable reinforcement. In fact, a 1/8-inch gap exists between the tip of the sprocket tooth and the bottom of the track for about two-thirds of the tooth's loaded travel distance across the top of the sprocket.

The track module also uses a patented tensioning system, which uses dual nitrogen accumulators set at different pressures. The system, according to Kahle, reduces overall stress on the track, reduces rolling resistance and assists in shedding debris.