Dressta's 32-ton, 315-hp, rubber-tired 555B wheel loader builds on more than 50 years of design experience to produce a machine with the features that customers need to meet today's high-production requirements. Multiple hydraulic systems are incorporated into this innovative loader: the main operating system for digging, a hydrostatic fan drive, and hydrostatic steering. Hydraulics also provide a soft-shift arrangement for the 4-speed mechanical transmission with a torque converter.

Main system

The flow-controlled main hydraulic system matches pump capacity to engine power for responsive loader operation - providing the combination of speed and power needed to meet digging conditions. Flow-controlled hydraulics also conserve horsepower and improve fuel economy. Two Vickers VQ fixed-displacement, single-vane pumps - from Eaton Corp. - run off the engine's accessory drive through helical gears to supply the main hydraulic system. System relief is set for 3000 psi. The vane pumps produce high flow under high pressure - 128 gpm at 2100 rpm and 1000 psi - with much less noise than gear pumps, for example. In addition, the helical gears are quieter than often-used spur gears. Noise restrictions for construction equipment are a growing design consideration today. (These pumps have replaceable cartridge elements that simplify service when necessary.)

A 70-gal reservoir is mounted high on the rear bulkhead above the pumps. Gravity flow floods their suction sides and thus prevents cavitation. The cylindrical tank's exposed position also allows ambient cooling air to circulate over its surface. The closed reservoir has 30-psi pressure relief and vacuum relief located in its breather. The tank is fitted with a 10-µm full-flow filter as well as a suction screen.

The main valves have two spools, normally spring-centered in a body with end covers. A variable pressure signal applied to either end cover shifts the spools to stroke the working cylinders. A single joystick in the cab operates a small hydraulic valve that produces the pilot signals to the main valve. Pilot operation provides low-effort actuation that makes it easier to feather the loader's functions. The control console contains a hydraulic lock-out that the operator can set to prevent unintentional movement of the joystick.

An unloading valve is the key to the flow-controlled hydraulics. If high flow is needed for speed, the valve combines output flow from both pumps. When the job calls for high pressure - such as during breakout - the unloading valve diverts one pump's output flow directly to the reservoir. Using one pump at high pressure generates less heat in the hydraulic-system and leaves more power to be transmitted to the drive wheels.

Hydrostatic steering

One of the vane pumps supplies priority flow to the hydrostatic steering system, with any remaining flow directed to the main system. There is no mechanical link between the steering wheel and steering cylinders. (The steering column has tilt and telescoping capability.) When the steering wheel turns, it operates a small, low-effort Sauer-Danfoss pump that generates primary flow to the steering cylinders. A flow-amplifier valve increases the flow by a pre-determined multiplier and the combined flow strokes the cylinders.

Cooling system

A large 3-in-1 on-demand cooling assembly includes the engine radiator and coolers for transmission oil and hydraulic fluid. Temperature sensors in all three systems actuate a flow-control valve between the engine-driven gear pump and the gear motor that make up the hydrostatic fan drive. The valve has two settings: high speed for ambient temperature of 115° F and lower speed for 100° F temperature. The lower setting, which runs most of the time, consumes about half as much power as the higher. By tailoring fan speed to fluid temperatures, there is less noise, and fuel consumption is reduced. The operator can depress and hold a button to stop the fan for quick machine warm-up in cold weather.

Phone Dressta North American, Ltd. in Buffalo Grove, Ill., at 877 / DNA-2001 or visit www.dressta.com.

Probir Chatterjea, proDESIGNER, Inc., Sleepy Hollow, Ill., provided details about the design of the 555B wheel loader hydraulics.