Articulated-frame steering and in-drum exciters—these are the two distinctive features that Wacker has always emphasized when talking about its trench rollers. The former helps the roller snake through narrow trenches without having to pause when steering, says Wacker, while the latter places the vibratory source close to the soil for optimal compaction efficiency.
So, when the company decided to redesign its popular RT 560 and RT 820 rollers, these two hallmark features obviously were retained. Truth be told, though, they were among the very few features carried over into the design of the new models, the RT 56-SC and RT 82-SC.
Construction Equipment recently visited Wacker at its Menomonee Falls, Wis., headquarters, where we had the opportunity to place the old and new rollers side by side and talk about design differences with Jay Baudhuin, lead engineer for the redesign project; Jim Bodis, compaction product manager; Mark Conrardy, sales engineering manager; and Jim Layton, marketing communications manager.
The former models, the RT 560 and RT 820, were identical to each other in design, but used drums of different widths—560 mm (22 inches) and 820 mm (32 inches), respectively. Replacing these machines (in early 2004) are the RT 56-SC and RT 82-SC which, again, are identical to each other in design and offer the same choice of drum widths as their predecessors. The "SC" in the new-model designation is for "smart control" (an apt way to describe their electronic intelligence). But, considering how fundamentally the design of the new models differs from that of their predecessors, "SC" could as well stand for "significantly changed."
"Even though the new models deliver more force per drum and have the potential to be more productive," says Conrardy, "the important news is not really about production, but about controllability, safety, reliability and serviceability."
When we had the chance to run an RT 820 (we'll call it the "RT") and an RT 82-SC (we'll call it the "RT-SC") one after the other in Wacker's indoor test area, a couple of the new model's improvements became quickly apparent. First, it's considerably quieter than its predecessor—the result, basically, of replacing the RT's air-cooled Lombardini diesel engine with a liquid-cooled version of the same make.
It's also easier to control, thanks to a larger remote-control transmitter that has two joysticks, compared with the RT's smaller transmitter with only a single joystick. Because the RT's solo joystick controlled all travel functions—direction (forward/reverse), steering and speed, it seemed always to need just a bit of tweaking to keep the roller on course. It's not difficult, but not as intuitive as the RT-SC's new two-lever design—the right for direction and speed, the left for steering only. (The transmitter, of course, is positioned at the operator's waist via a breakaway neck strap.)
The transmitters for both the RT and RT-SC use a wireless, infrared system to control machine movement and drum vibration. In addition, though, the RT can be operated with the transmitter "hard-wired" to the machine with a flexible power cord. By contrast, the RT-SC can be operated only by the infrared system, which requires that the transmitter and machine be in a direct line-of-sight attitude at all times. If you turn your back on the RT-SC, it stops moving and stops vibrating.
The "infrared-only" operation of the new model, says Baudhuin, is a design change in the interest of safety. Although the RT had a back-up bar that would stop the machine if it touched an obstacle, the machine would continue to travel and vibrate if it were receiving signals via the cord, even if the operator momentarily turned away, say, to converse with a fellow worker.
The only time the RT-SC's transmitter is hard-wired is when the transmitter's battery is discharged or missing. And, when so wired, the cord carries only electric power (from the machine) to enable the transmitter's infrared operation and, secondarily, to charge the battery. No operational signals are transmitted via the cord. Not only will the RT-SC cease operation when the infrared signal is broken, but it also will stop if the operator enters the 1-meter-radius safe zone (centered on the infrared eyes) that surrounds the unit.
As we said, the new liquid-cooled Lombardini diesel (LDW 903) is significantly quieter than its air-cooled counterpart used in the RT models. The new engine is a three-cylinder model, versus the air-cooled version's two-cylinder design, but displacement and horsepower are virtually the same. The RT engine was rated at 20.9 horsepower, and the RT-SC engine is rated at 21.2, both at @ 2,600 rpm.
According to Baudhuin, the primary reasons for using an air-cooled engine in the predecessor models were design simplicity and reliability—reliability from the standpoint of not having a radiator that could be punctured by jobsite hazards. The advantages of using a liquid-cooled engine, however, are significant sound reduction, more efficient cooling and, generally, lower emissions.
So, to reduce the liability of having to use a radiator in the new models to capitalize on these liquid-cooled advantages, Wacker engineers first "shock-mounted" the radiator in a protected location—at the rear of the front frame, just forward of the articulation point. They then surrounded the radiator with a grille, side panels and hood made of 1/2-inch-thick Metton, an "olefinic thermoset resin" (a really tough plastic).
"During testing, we dumped a load of boulders from about 10 feet onto both old and new rollers," says Bodis. "The Metton panels on the new machine were basically unharmed, except for a few surface abrasions. But the sheet metal on the old machine took a pounding."
According to Baudhuin, the all-aluminum radiator in the RT-SC also is substantially oversized for the application. During testing, he says, about 90 percent of the radiator's surface was blocked with cardboard, yet the engine did not overheat.
"The machine will be used in hostile environments that contribute to radiator plugging," says Baudhuin, "and we want to assure adequate cooling."
Along with liquid cooling, another major change in the RT-SC's engine compartment is the use of an electronic throttle, versus the mechanical throttle cable in the RT. Throttle cables can collect water and freeze, says Conrardy, and they can get out of adjustment. Electronic throttles have no such problems, he says, and they guarantee that the engine will run at full speed to power hydraulic functions efficiently. The new throttle, incorporating electronic-solenoid operation, has only two settings: idle or full speed.
The new RT-SC models deliver about 10 percent more total dynamic (centrifugal) force per drum than their predecessors. This gives them, as Conrardy says, the potential to be more productive, either by compacting thicker lifts at the same speed or similar lifts in fewer passes. The increase in force results from the new models' redesigned, more-efficient vibration system that uses heavier eccentric weights, and from the added 250 pounds the new models have, on average, over their former counterparts.
The vibratory system in the RT models had two dynamic force settings (7,000 and 14,000 pounds), which were selected by the operator at the transmitter. The settings were dependant on the direction of rotation of the drums' eccentric weights: In one direction, all the weights rotated as a single mass to provide maximum force; in the other direction, the center eccentric remained stationary, and force was reduced by half.
The benefit of dual settings, says Bodis, is that the lower force protects newly installed pipe when the fill is shallow. But as the fill builds, he says, force can be turned up for increased production.
Although the RT-SC models incorporate the same basic dual-force system, the vibratory mechanism in the new models has been substantially redesigned. First, the exciter housing, which serves as a mounting base for the eccentric-weight system, is now cast into the drum support, not bolted in as on RT models. It's a stronger design, says Wacker, and it eliminates a gasket joint and a potential leak.
The eccentric-weight system itself also has been redesigned, now using heavier weights and only two shafts, instead of three as in the former design. It's an arrangement that eliminates a number of mechanical parts and simplifies hydraulic plumbing, says Wacker, while also providing easier access to both the exciter and drum-drive hydraulic motors by placing them behind the same drum cover.
Review the design of Wacker's new trench rollers, compared to that of their predecessors, and it becomes apparent that the company gave considerable thought to how the new models could be enhanced in terms of overall performance, control, operator safety, serviceability and reliability. We think the new RT 56-SC and RT 82-SC take a giant step forward from their predecessor in all these departments.