The drive for maneuverability

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All three Bosch Rexroth A10VG adjustable pumps used in the LinsiTrak are at a common location in the rear of the vehicle.

“When we launched development for the LinsiTrak, the target was a versatile tractor for landscape gardening that maintains its maneuverability even on steep grades,” said LinsiTrak company owner and engineer Werner Müller. “It was clear from the beginning that only a hydraulic drive concept could master this challenge.”

But initial trials came to nothing, said Müller. The major challenge was to apply the appropriate amount of driving power to each of the four wheels at all times, which he says was “simply impossible using conventional differentials, since on a slope the wheels that are not in contact with the ground will always spin.”

Locking differential offered no real help, either. “The wheels would rotate at identical speeds in every curve, thus causing unnecessary slip that would ultimately plow up the ground. Traveling in circles or evading obstacles would be very critical,” he said.

Müller ultimately brought in Bosch Rexroth as a partner. Rexroth suggested its hydrostatic drive concept with its High-Efficiency Traction Control (HET), but that was not a drop-in solution for the requirements for the LinsiTrak either.

“In the normal case a HET drive, with one adjustable-displacement pump each in two circuits, will drive two hydraulic motors connected in series,” said Müller. “Power distribution in this concept is entirely dependent on the torque, which means that pressure splitting depends on the traction at each wheel. The consequence is that the uphill wheels would always spin.”

To modify the HET concept for operation at an angle, the companies decided to use two separate HET circuits in the vehicle, comprising one each A10VG adjustable-displacement pump, two MCR wheel motors, and an HET control block.

“One circuit drives the left side, one the right side of the vehicle,” said Müller. “What might at first glance seem like a step backwards is in fact the decisive feature. In this way the differential can balance power distribution on the two sides of the vehicle.”

Rexroth’s BODAS RC 36-20 control unit automatically regulates the outputs of both pumps to deliver the exact volume of fluid required by the hydraulic motors.

“When traveling around a curve, the pump for the motors at the outside of the curve will always provide proportionately more fluid than to the inside wheels and thus forms a perfect basis for the motion,” said Müller. “The electronics control not only these features, but all the vehicle’s other functions, too. The LinsiTrak has no mechanical levers, no clutch, and no chain.”

The LinsiTrak runs for ten hours with a single tank of fuel, allowing large areas to be worked without interruption. Although its power output exceeds that of comparable vehicles, Müller says it is more efficient in operation. On the road, with the series connection active, fuel consumption would jump when speed tops the 40 km/h (25 mph) mark because each wheel traces a slightly different curve during travel, which results in continuous application of pressure without a load actually being applied.

“To avoid this needless consumption, the user can select from several operating modes for the hydrostatic drive system,” said Müller. “In on-road operations the series connection is disabled. Only one wheel motor in each circuit is involved in movement. Just like a rear-wheel-drive passenger car, in this mode the drive power for forward travel is applied entirely at the rear wheels, making for optimal traction during acceleration.”

By contrast, the hydrostatic braking acts almost exclusively on the front wheels. This is all the more important when traveling downhill, since the weight shift to the front axle enhances the braking effect at the front wheels. When traveling forward on flat surfaces, those wheels simply “idle” in the hydraulic circuit.

“Driving wheels individually in this way saves fuel,” said Müller. “A special HET valve made by Rexroth thus disables the series connections and applies only the required pressure level to the front wheels.”

A third mode, “offset mode,” is useful when traveling over soils sensitive to compaction. In this mode, the tracks of the front and rear wheels of the LinsiTrak are offset and thus help to protect the substrate.

Adjacent to the two adjustable-displacement pumps for the drive motors at the rear of the vehicle is an additional axial piston pump to power implements such as a lawn mower, mulcher, or sweeper. Several attachment points on the vehicle, between the axles at the front and rear, match a variety of implements, which can be attached without tools.

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