Getting more from mobile machines

June 17, 2004
Hydraulic advances upgrade off-highway equipment

Managing Editor

Komatsu's WA200-5 wheel loader features a hydrostatic transmission that helps lower engine speed, emissions, and fuel consumption.

Construction-equipment operators ask a lot of today's machines, from competitive pricing and low operating costs to cleanrunning engines and rugged durability. But productivity ranks at the top of

Like most everyone, contractors are under the gun to do more in less time, while holding the line on costs. The obvious solution, building more-powerful machines and simply running them harder, is no longer a viable option.

"You can't really make the machines that much faster," says Bob Post, a product manager with Komatsu America Corp., Vernon Hills, Ill. ( "Excavators, for instance, are at about their limit in terms of speed because the operator really can't react much better or faster," without jeopardizing performance and safety, he explains. "So we've got to make the operator more efficient."

Thus, heavy-equipment manufacturers are increasingly relying on advanced controls and systems to enhance the operators' skills. Komatsu, for instance, has made sophisticated hydraulic systems and on-board computers an integral part of their machines. The ultimate goal is equipment that is easier and more intuitive to use and lets the operator handle wide-ranging tasks — whether it's deftly maneuvering around utility lines or brute jobs like crushing concrete. Here's a look at several performance-enhancing systems.


The new SK1020-5 skid-steer loader offers a standard, two-speed transmission with a working speed of 6.5 mph and a travel speed of 10 mph. Operators have the choice of three fully hydraulic control options, ranging from traditional hand and foot controls to two variations of the increasingly popular all-hand controls.

The company's new 22,872-lb WA200-5 wheel loader is powered by a 120-hp, Tier-2-compliant Komatsu engine and has bucket capacities from 2.2 to 3.1 yd3. Compared with previous models, says Post, it offers more digging power and responds quicker with less operator effort, making it suited for a wide range of construction, utility, and agricultural applications.

A key feature is an electronically controlled hydrostatic transmission (HST) that helps lower engine speed, emissions, and sound levels, and cuts fuel consumption by more than 15%. The HST replaces a conventional four-speed geared transmission with an engine-driven variable-displacement piston pump that provides flow to two variable-displacement hydraulic motors, Post explains. The motors, in turn, drive a geared drop-box that distributes power to the front and rear axles.

The foot throttle controls engine and pump speed. So when the operator pushes down on the throttle, the pump supplies more flow to the motors and the loader travels faster. Easing up decreases flow and slows the loader through " dynamic braking," so the operator can rely less on the service brakes.

While hydrostatic transmissions are fairly routine in small loaders, the real innovation is that Komatsu extends their use to the upper power classes, says Carsten Vogt of the Mobile Hydraulics business unit of Bosch Rexroth AG, Elchingen, Germany ( Bosch Rexroth supplies the A4VG axial-piston pump and A6VM axial-piston motors that drive the WA200-5.

Component size and power limitations of HSTs used to favor traditional geared transmissions in larger loaders. But mating hydraulic motors to Komatsu's switchable gearboxes overcomes these limitations. This general trend is due to the hydrostatic system's inherent advantages, says Vogt. These include:

  • Only a few or no switching stages, allowing inexpensive gearboxes or eliminating them altogether.
  • Full torque in both directions without a reversing gear.
  • Full power at low diesel rpm for better "bite" when digging into a pile.
  • Energy savings up to 30% because hydrostatic transmissions can often run load cycles at lower engine speeds.
  • Optimum power distribution between drive and working hydraulics. Power not needed by one system is easily transferred to the other.
  • Good efficiency at partial load.
  • Low heat generation at high tractive efforts and low travel speeds.
  • Full freedom of installation: hydrostatics can mount wherever there is space since hoses are much easier to install then mechanical shafts.
  • The combination of the pump and motor-integrated controls, such as the Rexroth DA-control, allows advanced drive and antistall control without electronics, which is still an important sales feature in some areas of the world. (See more information.)

The two high-speed motors respectively drive low-speed and high-speed gearing. In typical use, the loader requires lots of tractive effort to dig into a pile, so the HST supplies flow to both motors. This gives more than enough power to climb a pile or travel up grades when loaded, according to Post.

As tractive demand decreases and speed increases, such as when transporting a load, a clutch disengages the low-speed side and flow increases to the high-speed motor. All this is transparent to the user. The operator sets maximum ground speed (1st through 4th) and the HST runs like an automatic transmission.

A Variable Shift Control feature matches ground speed to hydraulic speed. It comes in handy in short V-cycles, where the unit loads, turns, and dumps into a truck in short order. It usually takes longer to raise the bucket over the truck bed than to move the loader into position. On older loaders, this forced the operator to apply more throttle to speed the lift hydraulics while riding the brakes to slow the loader.

Variable control lets the operator limit ground speed yet keep the throttle high to quickly raise the boom, saving on brakes and making the job less tedious. Top speed on the WA200-5 is 23.6 mph, but with variable control the operator can set it anywhere from 2.5 to 8.1 mph.

HST loaders also come equipped with a traction-control system that reduces flow to the low-speed drive and cuts wheel torque by about 50%. It controls wheel spin when working in slick or soft conditions, such as on wet concrete or deep sand.

Multifunction joystick hydraulic controls give the operator minimal, consistent resistance while providing easy and accurate command of the machine. One lever controls the boom and bucket while a toggle switch changes direction of the loader (forward or reverse) with a flick of the thumb. Compared with older versions that had separate levers for the boom, bucket, and transmission, the operator's hands never have to leave the steering wheel and joystick. "It's much easier and more intuitive," says Post.

According to Bosch Rexroth, in hydrostatic drives equipped with a summation gearbox, both hydraulic motors provide power in low-speed, high-demand conditions. As speed increases, a clutch built into the gearbox disengages one motor from the power train and full flow travels to the high-speed motor.

Komatsu's new SK1020-5 skid steer is a 7,525-lb unit equipped with a 70 or 83-hp diesel engine and has a rated operating load of 2,000 lb. The unit can load trucks 10-ft high and maximum reach exceeds 23 in.

An important enhancement on the skid-steer line is a hydraulic load-management system, a version of which has been successfully used on the company's excavators, says Product Manager Bob Lessner. The HydrauMind system uses load-sensing and pressure-compensated valves that automatically adjust to working conditions, letting operators simultaneously work the arms, bucket, and attachments without sacrificing power or speed in any circuit.

It senses pressures and control-lever movements and adjusts engine horsepower and pump output to instantaneously meet demand. The closed-center hydraulic system does not let the load determine actuator speed, says Lessner. Rather, the control-lever stroke determines pump flow and that determines the actuator speed, regardless of load.

In a conventional open-center hydraulic system, as load increases oil escapes via the neutral circuit, which slows the actuator, he explains. And when load changes, actuator speed can change even if the operator is holding the control lever in the same position. With HydrauMind, there is no neutral circuit so the pump delivers the full amount of oil the actuators require. Yet it delivers only the precise flow needed, which conserves energy and saves fuel.

"The user can perform simultaneous functions such as raising the loader arm and rolling the bucket back at the same time with no loss of speed or power to either circuit," says Tom White, a Komatsu district sales manager. "Quite often, in other machines, activating one circuit significantly slows another," he says. "We force full flow of oil to both circuits simultaneously, so there is no loss of power or speed. For the operator, it increases productivity and lets him work faster," says White, because he can concentrate on the job at hand instead of compensating for deficiencies in the hydraulics.

The skid-steer loaders also feature Automatic Power Control (APC), which lets the operator work at full power during any phase of the job — from digging to loading and carrying — without overloading the engine.

"It's an engine antistall feature designed within our hydraulic system," explains Komatsu's Edward Warner, Jr. "The pump swashplate will back off from maximum angle depending on the torque requirement of the engine." This prevents the hydraulic system from overtaxing the engine. Without this type of system, an inexperienced operator working too aggressively under load will stall the engine, he explains. Frequent stalling increases starter use and hampers productivity. "An experienced operator will likely catch the engine before it dies," he adds, "but it still slows down the process and requires more concentration. We've taken all that out of the equation. The machine compensates for the work that's being done so the engine is always on the right side of the power curve," says Warner.


The D39EX-21 dozer features a KomStat II hydrostatic transmission. It lets the operator precisely match dozing speed to job-site conditions, offers good control in side-cut operations, and provides excellent traction on soft ground.

The newest additions to the Komatsu line are the D31, D37, and D39 dozers, suited for construction, forestry, and landscaping work. Outfitted with Tier-2-compliant Komatsu engines, the dozers are equipped with the company's KomStat II Electronic Control Hydrostatic Transmission (HST) and Palm Command Control System (PCCS), which are said to offer advanced grading and precise operator control.

The HST is a dual-path, closed-circuit system with two variable-displacement piston pumps and two three-speed variable-displacement travel motors. It offers 20 increments of speed adjustment within three distinct speed ranges, letting the operator precisely match dozing speed to job-site conditions. The operator can also set reverse speed either slower or faster than the forward dozing speed.

Benefits of the HST, according to the company, include:

  • Easy turning, because power is distributed to both tracks without interruption on the inside track. The dozers are said to be highly maneuverable with turning radii as tight as 7 ft, 5 in. on the D31.
  • Precise control even when loaded on just one side of the blade, such as in side-cut operations.
  • Selectable travel speed makes grading operations more efficient.
  • Excellent traction on soft ground.

The D31, D37 and D39 are equipped with the ergonomic Palm Command Control System. The electronic PCCS provides single-lever control of speed and direction. It requires light operating effort and reduces operator fatigue because short lever strokes provide quick responses. A Proportional Pressure Control joystick controls hydraulic output to the blade in direct proportion to lever movement. To enhance operator comfort, power-train components and hydraulic control valves mount to the frame with rubber pads to soften vibration and minimize noise. The cab sound levels, at 76 dB(A), are reportedly the lowest in their class. An onboard diagnostic monitoring system tracks maintenance items such as oil and filters, checks pressures and temperatures, and lets the operator view machine fault codes through a monitor panel without the need for special software or laptop computers.

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