Hydrostatic Transmission with Variable Speed Motors

As previously mentioned, it is often necessary to provide a road speed, so the machine can be moved on the highway between job sites. (Generally, these travel distances are only a few miles, so road speeds in the range of 12 to 17 mph are satisfactory.) Shifting a transmission to increase pump speed is one method to increase vehicle speed, and a variable displacement motor provides another method. Reducing the motor displacement increases wheel speed. Available wheel torque decreases as displacement decreases, but this is not generally a problem for road travel.

A two-speed variable displacement motor has a high displacement position to provide high torque at the work site and a low displacement position for road travel. An infinitely variable displacement motor can also be used, but the cost is higher. In this case, a range of speeds from work speed to road speed is available. Both the two-speed and infinitely variable motors must be sized to prevent driving the wheel at higher than rated speed when they are shifted to the minimum displacement position.

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