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Hydraulics gives multi-articulated excavator wide range of motions

Hydraulics gives multi-articulated excavator wide range of motions

Menzi Muck AG, Widnau, Switzerland, makes all-terrain excavators named after a legendary elf, Mr. Muck, known as “the little man who does everything.” Menzi Mucks allow each leg or wheel to be raised, lowered, extended, retracted, or moved in or spread out so the operator can adjust the working base of the machine to conform to the terrain. This control provides extreme maneuverability and remarkable flexibility, stability, and safe operation.

The Menzi Muck A91, one of five different sized models, can climb and work up, down, and across steep slopes, in 7 ft of mud and water, in tight spots, or in bad weather that mires down other equipment. All the while, the operator sits comfortably upright, with a full 360° view and precise control, regardless of the terrain.

The Menzi Muck A91 uses electrohydraulic pump control that overcomes limitations of conventional load sensing and flow sharing systems

The basic version of the Model A91 features two-wheel drive, telescopic stabilizers, and full 360° rotation of the cab. Options include all-wheel drive, variable parallel wheel track widths, all-wheel steering, parallel or mountain stabilizers, and a host of others, all powered by hydraulics.

Pumps do it all
The A91 is powered by a John Deere 4045HF285 four-cylinder diesel engine that delivers 99 kW at 2000 rpm and is sized at 4.5 l. The engine transmits power to an over-under dual-station pump drive box connected to two variable-displacement axial-piston pumps. The pump drive box is geared to provide the most economical operation of the hydraulics. But the gearing can be shifted to provide higher torque at higher engine speeds for maximum power when needed. Electronic load limit control provides fast, smooth interaction between engine and hydraulics.

The upper mounted pump provides a maximum 220 lpm flow at 400 bar to power the A91’s hydrostatic transmission for propulsion. The operator controls this pump through a bidirectional, proportional foot pedal. The basic, two-wheel drive model has a speed range of 0 to 5 km/hr, with an optional two-stage version capable of 15 km/hr. Optional four-wheel drive models can attain 10 km/hr.

Tandem mounted to the rear of the upper pump is a gear pump rated for 36 lpm that powers the vehicle’s hydraulic fan drive. Compared to conventional belt driven cooling fans, the hydraulic fan drive reduces fuel consumption and emissions by providing only as much cooling power as needed. It also keeps the engine operating at optimum temperature. An optional directional control valve in the fan circuit reverses direction of the fan, which is useful for blowing debris or other contaminants from the radiator that would otherwise clog it.

A two-speed drive box powers a pair of axial-piston pumps. A gear pump tandem mounted off the upper pump provides power for a hydraulic fan drive.

The lower pump provides up to 220 lpm of flow at 280 bar of hydraulic power for all work functions. An optional axial-piston pump rated for maximum flow of 170 lpm at up to 340 bar may be tandem mounted to the rear of the lower pump. This optional pump has three settings for powering additional hydraulic implements through a joystick.

A standard two-wheel drive model has rear wheels powered through a hydraulic wheel-hub drive with integral gearing and spring-applied, hydraulically released parking brake. The wheel drives can be disengaged manually to allow towing of the vehicle. Four-wheel drive models have a variable gauge feature that allows adjusting the width between wheels, providing the operator with a choice between maximum stability or minimum vehicle width. Menzi Mucks are also equipped with hydraulically extendable stabilizers, and the front wheels can be pivoted upward to prevent impeding the stabilizers.

More hydraulics
The reservoir of the A91’s hydraulic system holds up to 180 l of fluid and comes factory filled with Panolin HLP 46 or Synth46 hydraulic fluid. The fluid runs through a 10 µm filter with bypass and an air-to-oil heat exchanger before returning to tank. Fluid temperature is monitored, and a warning light indicates if the oil temperature reaches 85°C. The light goes off once fluid temperature drops below 80°C.

The A91’s cab features continuous 360° swing rotation provided by a hydraulic motor and two-stage planetary gear box to reduce speed and multiply gross motor torque to 46,000 N-m. This swing drive can rotate at up to 10 rpm and has an integral brake for precise positioning and maximum safety. A proportional, demand-driven torque control regulates the turning force via a joystick.

Versatile and efficient control
All Menzi Mucks are designed for high productivity, so they work fast. But to provide finer control for delicate operations, maximum load can be scaled down to work more accurately by limiting overall hydraulic flow rate without reducing working pressure. The operator can toggle fine mode on and off via a simple switch.

To ensure that flow matches demand, conventional systems use control valves with pressure compensators that control flow rate and distribution. The two most common are load sensing, which uses an upstream pressure compensator, and, flow sharing, which has a downstream pressure compensator.

In both, the pump operates as a hydromechanical closed-loop pressure control to ensure that supply pressure exceeds the highest load pressure by a constant differential pressure (∆P). Because supply pressure constantly adjusts to the highest load pressure, hydromechanical load sensing and flow sharing save energy compared to open-center controls that divert some flow to tank.

Menzi Mucks also use the latest generation of load-independent flow distribution (LIFD) technology from Bosch Rexroth for highly sensitive control and quick response. LIFD controls overcome the limitations of hydromechanical load sensing control. Instead, they use electrohydraulic flow matching (EFM) to improve the hydraulic system’s efficiency, stability, and dynamic response. EFM systems use a pump with an electronic displacement control instead of the hydromechanical displacement control found in conventional load sensing and flow sharing circuits. As a result, improvements include:

• The pump modulates flow proportionally, so excess pressure between the pump and control valve can be set independent of the system’s maximum load pressure. In certain cases, EFM’s ∆P is lower than the predetermined ∆P of conventional load sensing systems, which saves energy.
• Instead of operating as a pressure controller, the pump in an EFM system operates as an electroproportional variable pump in an open control loop. Therefore, instead of responding to changes in load pressure, it operates independently of a pressure compensator.
• Operation of the pump and control valve are more closely synchronized. Therefore, EFM eliminates delays between joystick inputs and the load sense signal arriving at the pump. This, in turn, improves system response and makes the working hydraulics more agile and less susceptible to oscillations.
• Finally, EFM can use well-established components, such as variable pumps with electrohydraulic load-sensing valves. This limits development work to functional interactions between components in specific applications.

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