Smarter Grip, Less Slip

By Walt Moore, Senior Editor | September 28, 2010
Volvo D Series Truck

In an effort to reduce operating costs for its articulated haulers, Volvo Construction Equipment designed its current range of D-Series trucks to go easy on drive-train components, tires and fuel, says Buddy Goodman, Volvo's product marketing specialist and team leader for haulers and loaders. To that end, says Goodman, these six-wheel-drive trucks, left on their own, operate in a 6×4 configuration (front axle and first rear axle) with all of their differentials unlocked. The decision to power the third axle or to lock any of the differentials in order to increase traction has been left to the driver's discretion. Until now, that is.

Today, the two smallest Volvo haulers, the A25D and the A30D, with payload ratings of 26.5 and 30.9 tons, respectively, are equipped to think for themselves, so to speak, about when and how to apply extra traction. According to Gabriel Barsalini, product specialist, this new capability, called Automatic Traction Control (ATC), assists the driver and enhances truck efficiency by automatically and exactly applying extra traction. The new system, he says, "won't let the operator make a bad decision."

Understanding the ATC system might be a bit easier with a basic review of the Volvo truck's drive-train layout. Essentially, a driveshaft from the transmission powers a transfer case with a differential (often called a drop box). Drive shafts from the drop box power the front axle and first rear axle. Actually, the drive shaft that extends rearward connects to the power divider, which is basically a gear set housed in the upper section of the first rear axle's differential.

Within the power divider is an air-actuated dog clutch, which, when engaged, directs power to the rear-most axle. The dog clutch is a two-piece, gear-like assembly with teeth that inter-lock when the two halves are forced together — like lacing your fingers together. An air-actuated dog clutch also is used in each of the three axle differentials, and a spring-applied/air-released dog clutch is used in the drop box. Engaging a dog clutch ensures that power entering a component will be split 50/50 to its outputs, meaning that an axle or a wheel that loses traction can't rob all the drive train's power.

Controlling all this hardware in a truck without ATC are a green rocker switch, a yellow rocker switch and a floor-mounted button. Flip the green switch, and you lock the dog clutch in the drop box and in the power divider. (Volvo calls this the 6×6 longitudinal lock function.) Flip the yellow switch, and you lock the front-axle differential only. (Volvo calls this the front-axle transverse lock). Depress and hold the floor button, and you lock everything — the drop box, power divider and the differentials in all three axles. (This, says Volvo, is the 6×6, 100-percent lockup mode.)

But the potential problem with these manual controls, says Volvo, is that inexperienced drivers may use the green and yellow switches too frequently. Less skilled drivers often reason, says Volvo, that if the truck performs adequately in its default mode, that is, in its 6×4, open-differential configuration, then wouldn't it perform even better with the green and yellow switches on?

Volvo's answer to this let's-have-all-it's-got reasoning is that more traction is not always better. The logic behind running the truck in its default configuration, says Goodman, is that operating costs are reduced: fuel consumption drops, and since drive-train components and tires are running with reduced stress, they consequently incur less wear.

To illustrate the point, Volvo recently invited Construction Equipment to drive two of its articulated haulers, a conventionally controlled A40D and an A30D with the ATC system. Before first taking the A40D through the company's moderately tortuous and slippery test track at its Asheville, N.C., training center, Barsalini explained how to use the green and yellow switches, should they be needed.

On the test road, we did our best to anticipate where the A40D might need extra traction, and then to flip the appropriate switch at the appropriate time. You quickly learn, however, that this process is not an exact science. Anticipating the need for extra traction, and flipping a switch to engage it, is a judgment call; you don't know for sure whether the truck actually needs the help.

And even if it does, timing can be an issue; if you wait until a wheel is slipping before flipping the switch, the dog clutches can engage with enough protest to make you feel guilty about possibly abusing a half-million-dollar machine. Barsalini assured us that the components are designed to handle this potential mistreatment, but you begin to understand why a rookie operator might choose to keep the switches on.

Driving the A30D with the ATC system, however, was a different experience. We took note of how infrequently the truck needed to supplement its basic 6×4 drive; it pulled sure-footedly, with only four wheels driving, through some spots where we would have expected it to need added traction and, in fact, where we had chosen to give the A40D extra help.

ATC-equipped A25D and A30D models have the reasoning capacity to determine when they can work efficiently in their fuel-saving, gear-and-tire-saving, 6×4/open-differential default configuration — and when they need extra traction. They also tell you what they're thinking via a digital display (an icon on the panel in the outline of the drive train) that illuminates when the dog clutch in the power divider or in the drop box, or both, has been automatically engaged. The display blinks out when the clutches disengage.

A25D and A30D models with ATC have just one, operator-activated, traction-assist control — the floor-mounted button, which most operators realize is for only momentary use (it releases when you lift your foot) when backing up a slippery ramp to the dumpsite, for example, or when haul-road conditions are atrocious. The green and yellow switches are no longer on the A25D and A30D panel. The green switch is gone for good, while the yellow switch is actually hidden under the panel and can be reinstalled if an experienced driver prefers.

The switches have been removed, because these models now rely on multiple-sensor input to determine when the drop box or power divider should be locked — and in what sequence if both are required. When the ATC system decides that added traction is no longer required, the dog clutches disengage quickly, quietly and in the proper sequence. Clutch operation is smoother with ATC, says Volvo, thanks to a new clutch design that provides more tolerance between the reconfigured, interlocking teeth.

Basically, the ATC system does what the green switch did, except that the new system can control the drop box and the power divider independently. With both these functions engaged, says Volvo, the truck can handle 90 percent plus of the conditions it may encounter.

To accomplish this automatic action, the ATC system uses speed sensors to monitor input to the drop box; output from the drop-box to the front axle and to the power divider; and output from the power divider. This information, along with that from a steering sensor and an air-pressure sensor at the power divider, is electronically evaluated with proprietary software.

Based on this evaluation, the system decides, first, if extra traction is required, and if so, determines how best to apply it. We rode in the A30D, with Goodman driving, taking the truck nose first up a slippery ramp. In this situation, the drop box engaged first, then the power divider. When backing up the same ramp, the sequence of engagement reversed. (Actually, the system is designed to automatically select 6×6 drive when the truck reverses.) In both maneuvers, the A30D negotiated the grade without wheel spin.

We then observed as Goodman took the A40D through the same exercise. To illustrate a possible situation that an inattentive or inexperienced driver might cause, he allowed the A40D to remain in its default 6×4 drive mode as he backed up the ramp. When the left wheel of the second axle lost its grip, all the truck's power was dissipated through that spinning wheel. On a second similar pass, he flipped on the green switch at what he considered the optimum point, but the same wheel still slipped, momentarily, until the added traction took hold.

It was a convincing demonstration of the new ATC system's ability to react quickly and competently to changing conditions.

"The ATC system lets the operator focus on driving, instead of always trying to anticipate conditions," says Goodman. "And in the process, the truck owner can save on operating costs."

Goodman admits that skilled operators may resist the loss of manual control, but the ATC system is doing exactly what these good drivers would do, he says, but in a more precise and timely manner. He notes, too, that even skilled operators get tired as the workday wears on and may not be as diligent about shutting off extra traction that has been engaged.

To cover all eventualities, however, the new system, which adds around $3,000 to the truck's cost, is a "deletable standard," meaning that it is standard equipment, unless the buyer chooses to delete it.