After Bob Powers removed 10 bolts and two pins from beneath the cab of the Cat D7E, Wes Reetz used the tractor’s on-board hydraulic jack to tilt the cab sideways to a steep angle.
Tilting the cab (an impressive bit of technology in itself) exposed the D7E’s innards, revealing why this relatively new crawler dozer from Caterpillar is one of the industry’s most innovative machines.
Looking down inside the D7E’s frame, the engine and hydraulic pumps look familiar, but the rest of the drive train is somewhat mysterious. Gone are the torque converter, power-shift transmission and related drive shafts. In their place reside a large, engine-driven electrical generator and two other primary components—the power inverter and propulsion module—connected together with cables thicker than your thumb to create the D7E’s diesel/electric drive system.
Construction Equipment got this inside look at the D7E last fall in Wilmington, IL, where Local 150 (International Union of Operating Engineers) has its training facility. There, we met up with the two D7E experts from Caterpillar—Powers, senior product application specialist (and former operator with a current Local 150 card), and Reetz, D7E project engineer—along with Kevin “Zip” Ackert, Mike Evans and Troy Butler, all instructors for Local 150’s Apprenticeship and Skill Improvement Program.
The intent of our visit was twofold: to get a close look at the D7E’s design and to hear first-hand from some of the best dozer operators around what they think about the D7E’s features and performance.
Fresh design, fewer gears
Reetz told us that the generator (driven by a 235-nethorsepower Cat C9.3 engine) produces 480 volts of threephase AC power, which is directed to the power inverter, a gray box attached to the front of the fuel tank. The power inverter changes this input to 640 volts of DC power, some of which goes to an accessory-power converter that sends DC current at around 340 volts to the D7E’s electrically driven water pump and roof-mounted air-conditioner.
The power inverter also converts a portion of the DC power to “frequency controlled” AC current. This process is critical, says Reetz, in order to provide the proper type of current to a pair of technically advanced AC electric motors housed in the D7E’s propulsion module.
“One motor for each track?” Evans wanted to know. The motors actually are geared together, said Reetz, and drive into a conventional Cat differential-steering system, then out to double-reduction finals. All the motors are doing, he said, is providing forward or reverse movement.
“There’s a lot of voltage going to various parts of the machine,” said Butler. “Any danger when you’re in the cab—or when you’re working in water?”
“The short answer is ‘no,’” said Reetz. “All the high-voltage cables—the ones with orange braid—are military-spec armored and shielded. If they’re ever damaged, a sophisticated ground-fault-detection system will sense the problem and shut down the drive train before it ever gets hazardous. It’s all isolated from the frame, and from the machine’s 24-volt electrical system, and it’s completely sealed.”
Reetz anticipated the next likely question, and explained that the D7E also is a safe machine to service: “When the machine is switched off, any residual energy in the drive system is bled off through the water pump in several seconds. If there were a problem with the pump, then a bleed-down resistor in the power inverter would make the system safe in three to five minutes.”
Reetz then called our attention to an amber indicator lamp with an adjacent switch on the left side of the machine.
“Any time there’s potentially dangerous energy in the system,” he said, “the lamp is on; if it’s off, energy has been dissipated and it’s safe to work on the machine. The switch lets you check that the lamp is functional.”
“So what’s the advantage of diesel/electric drive?” asked Evans. “Seems as if it adds considerable complexity to the machine.”
“Comes back to the intent of the D7E design—to build the most efficient earthmover possible,” said Reetz. “Compared with the Cat D7R [with a conventional drive train and similar horsepower, weight and blade capacity], the D7E, on average, moves 10 percent more material per hour, burns 10 to 30 percent less fuel in the process, and delivers a fuel-efficiency advantage [material moved per gallon of fuel] on the order of 25 percent.”
On to more familiar features
As Reetz moved around the D7E noting its features, he made the point that the tractor is designed to work as efficiently at grading as at bulk earthmoving, having an extended undercarriage that provides a stable platform (and lower ground pressure). In fact, he said, the D7E is one of the best grading machines in the Cat lineup.
Reetz’s mention of undercarriage prompted Ackert to ask the question everyone was probably thinking: “Why the conventional oval undercarriage instead of the Cat elevated-sprocket design?”
“The low configuration gives better sight lines to the sides,” said Reetz, “and allows room for the tilt cab—a big plus for serviceability. But the machine still has a modular final-drive, and the component sizes were increased for durability—for instance, D7E spindle bearings are the size of those in the D10.”
In the engine compartment, Reetz pointed out that the D7E’s smaller 9.3-liter engine, compared with the D7R’s 10.8-liter engine, leaves ample room for a Tier-4 aftertreatment package, required for future emissions control.
He also noted the “belt-less” engine front, the result of driving the water pump and air-conditioner electrically. The machine’s cooling package, said Reetz, has three separate coolers side-by-side—the right for air-to-air cooling of turbocharger output and the center for engine coolant. The left section uses engine coolant from a separate water-pump circuit to cool the drive train’s major electrical components, as well as hydraulic oil. Adding to D7E’s cooling power, he said, is a hydraulically driven demand fan that draws less horsepower and saves fuel.
In the cab
Powers then invited the operators up to the cab for a quick review of controls. The D7E has a new steering tiller with three convenient controls built in: a rocker switch for forward/reverse shifting; a thumb roller for adjusting engine speed; and a yellow button that recalls a pre-set speed for carrying material once the blade is loaded.
A rotary-dial throttle has five positions—800, 1,100, 1,300, 1,550 and 1,800 rpm. Most operators dial the throttle all the way up and leave it there, said Powers, but added that the 1,550-rpm setting still provides 90 percent power to ground and saves significant fuel. A four-position dial next to the throttle lets the operator select from three pre-set forward/reverse operatingspeed combinations. (The fourth position is programmable for custom speeds.)
The thumb wheel allows the operator to adjust engine speed (and ground speed) within the range selected at the throttle. Speed is indicated on the monitor by a number that equates to power-shift gear selections, not to miles per hour, with numbers ranging from 0.1 to 3.0, in 0.1 increments.
“The operator selects a speed that’s comfortable,” says Powers, “and the tractor manages the rest by adjusting the frequency of the current. It’s easy to train operators, because you can’t select the wrong gear and overspeed the engine or overheat the converter. There’s an inclination sensor and sensors for ground speed and engine rpm, and in a potential over-speed situation, the system applies the brakes.”
On the floor is a wide travel-speed control pedal that proportionately reduces ground speed when depressed, and then, at full stroke, applies the brakes. Unlike a decelerator, however, the pedal does not reduce engine speed. It’s there, said Powers, to give the operator added control, but need not be used when making directional shifts.
In-the-dirt performance
With this basic introduction to the D7E, the operators each ran the machine, combining a mix of heavy dozing, grading and maneuvering. After a couple hours, we solicited their comments.
“I tried to stick the corner of the blade,” said Butler, “but it cut in smoothly and carried the dirt. The power just never stops. I’m definitely impressed. It’s quiet, and the visibility is excellent—the way the center-post in the cab lines up with the pre-cleaner, the muffler and the blade cylinder—those obstructions seem to disappear. I have to admit that I used the speed-control pedal before making directional changes—just habit.”
Butler also was quite taken by the D7E’s optional rear-view camera system.
“I found myself several times looking back when reversing, then would realize that the all I had to do is look at the monitor. The clear view that the camera gives is amazing. It gives an exceptionally wide view, and you can actually see more of the work area with the camera than you can visually by twisting in the seat.”
Evans echoed Butler’s comments: “It’s got power... that’s for sure. No matter how hard I tried to push it in the cut, it just picked up and went. I was impressed with the visibility to the blade and to the sides...it’s excellent. Also, the fingertip speed controller—it’s a very convenient feature, and I like the fact that Caterpillar left the foot pedal—it’s a safety factor and I like having it there.
Ackert’s first impressions were of the D7E’s controllability. “The control you have, not only with speed and steering, but also the hydraulics, is second to none.”
Reetz explained that the D7E has “distributed hydraulics,” a design that places the control valve for the blade at the very front of the tractor, and that for the ripper at the rear. Locating the valves close to the implements, he said, shortens hose runs, resulting in uncommonly fast, precise response. Also, by using a D8-size steering pump, said Reetz, the D7E is extremely maneuverable.
“The balance of the machine is excellent...makes for really precise grading,” said Ackert, “and even though I thought at first that having the cab post in the center would be a problem, after five minutes it disappeared.”
Ackert also picked up on a couple of additional details in the cab’s design: “I also like how the seat is angled [15 degrees] to make it easier to look back when you’re ripping. Less stretching. It’s quiet, too—I’d say that even if you had the machine at high idle, you could put your cell phone on speaker and carry on a conversation while you’re running.”
Ackert, as did his co-operators, said he used the speed-control pedal “a couple times out of habit,” then when he was “running up at 3.0—wide open—I slapped the reverse button and away she’d go...amazing how the system ramps into directional changes.”
Like his cohorts, Ackert, too, was impressed with the D7E’s pushing capability: “The power of this machine is outstanding—no matter how you come at the cut...from the side, straight up, straight down...doesn’t matter, it just settles in does what you ask it.”
Concludes Ackert: “This is a very well-thoughtout machine—very operator-friendly—a ‘tractor- hand’s tractor.’”
