The Eco-Drive contains two transmissions, each driven through a combination gearbox/ pump drive. In its normal position, a clutch transmits power from the engine to a hydrostatic pump. When vehicle speed approaches 28 km/hr, the clutch disengages the pump and engages the geared transmission.
Versatile operation
The hydrostatic drive consists of a model A4VG56EP variable-displacement axial-piston pump supplied by Bosch Rexroth. The pump feeds a pair of Rexroth model A6VM107EP variable-displacement axial-piston motors. This setup provides a wide range of speed and torque because pump flow can be varied from minimum to maximum while motor displacement varies from maximum to minimum, respectively.
For example, with the machine idling, the pump and motors are at zero displacement. When the operator depresses the accelerator pedal, the motor displacement increases to maximum, and pump displacement remains at minimum. This produces low speed and high torque for accelerating the vehicle from rest. As the operator depresses the accelerator further, pump displacement increases and motor displacement decreases, causing vehicle speed to increase.
With the pump at maximum displacement and the motors near minimum, the vehicle can achieve 28 km/hr. To go faster, the control automatically shifts the vehicle into its mechanical drive mode. The control reduces engine speed to 2000 rpm, shifts the clutch to engage the geared transmission, and reduces pump and motor displacement to zero.
By monitoring speeds, the control synchronizes these events for quick and smooth shifting. Once the vehicle shifts to its high-speed mode, it can achieve 40 km/hr by increasing engine speed to 3000 rpm.
Of course, hydraulics also powers a variety of implements. For example, the sweeper attachment shown uses multiple hydraulic motors to spin brushes and cylinders to position them. Other optional implements include snow blowers, plows, mowers, etc. A second, open-loop pump is used for powering these implements.
Technical information for this article was provided by Heinz-Gerhard Essig, of Bosch Rexroth, Lohr, Germany. For more information, contact Terry Hershberger in the U. S. at [email protected].