To switch the harvester into its high-speed, low-torque mode, the operator activates a switch that energizes the solenoid of valve A. The solenoid opens valve A, so pilot pressure now is transmitted to valves BL, BR, and each motor valve BM With valves BL and BR shifted, fluid now flows directly to the bottom motors in the schematic. However, because valve BM receives pilot pressure, it has shifted, so fluid no longer flows back to the hydrostatic pump. Instead, fluid flows from the bottom motors to the middle ones. Likewise, fluid exiting the middle motors flows into the top ones. Finally, fluid leaving the top motors returns to the pump.

This single flow stream through each pair of three motors establishes a series motor circuit. Each motor receives half of pump flow from valve BL or BR, which provides high-speed capability. Pressure capability, however, is now shared by three motors on either side of the machine. So the motors operate in a high-speed, low-torque mode.

Reversing the order

Switching the harvester into reverse is accomplished through the pump’s displacement control. For reverse operation, the pump’s swashplate moves over center, so fluid flows from the right side of the pump, as shown in the illustration.

 hydraulic circuit drives

Hydraulic flow in the reverse mode again can be seen in the schematic. If valve A is not energized, pilot pressure is not transmitted to valves BL, BR, and BM. Fluid flows from the right side of the pump to junction 2. Flow splits at this junction and travels around to the three motors on the left and three on the right. Fluid flows into the two lower motors through their respective valve BM and directly into the upper motors. Exiting the motors, fluid then flows into the respective flow divider.

Fluid from the lower motors cannot flow through valve BL or BR because they are closed, by virtue of no pilot pressure. From the flow divider, fluid combines at junction 1 and flows to the left side of the pump. In this configuration, each flow divider acts as a meter-out device — again ensuring that all three motors rotate at the same speed.

When the harvester is in the high-speed, low-torque mode in reverse, the operator activates the lever for high-speed operation, so valve A opens to route pilot pressure to valves BL, BR, and BM. The pilot pressure shifts the valves, so fluid can no longer flow through all six motors in parallel. Instead, fluid flows past the bottom and middle motors and enters the top motors. From there, fluid flows into the middle motors through valve BM, then to the bottom motors through their respective valve BM.

After exiting the bottom motors, fluid cannot flow into the flow dividers because valves BL and BR, have shifted to block flow from them. Instead, fluid exits the bottom motors and flows through valves BL and BR, and, finally, into the left side of the pump.

Design considerations

As with any system design, decisions had to be made based on reliability, performance, efficiency, cost, and practicality. For example, all valves could be solenoid operated instead of pilot operated. However, designers deemed pilot-operated valves more reliable for this application. Pilot actuated valves are more tolerant of fluid contamination than solenoid valves are, and electrical actuation is sensitive to corrosion and malfunctions from contact with water. Pilot lines are also less likely to be damaged from unpredictable environmental conditions.

Another consideration was the choice for valves BL and BR. A larger, single valve could have been used. However, designers found smaller valves to be more readily available — which would be important if a replacement was needed — and also less costly. Moreover, by using two smaller valves, the BL and BR valves are the same as the BM valves. This simplifies inventory and eases maintenance by consolidating part numbers.

The flow dividers also required careful consideration. The first choice was between rotary and valve-type flow dividers. However, because they split flow mechanically instead of using energy-wasting orifices, rotary flow dividers are much more energy efficient than their valve-type counterparts, so rotary flow dividers were chosen.

Another consideration was the number of flow dividers to use. Designers quickly settled on two 3-outlet flow dividers based on cost, practicality, and because the two flow-divider system provides a split power train, which operators and technicians would be familiar with.

Because speed, direction, and driving mode all lend themselves to electrohydraulic control, the harvester could have been fitted with a multi-axis joystick for single-point control. However, designers wanted to use conventional type controls. Therefore, speed is controlled with a foot pedal coupled with an electronic transducer that sends a signal to the pump’s displacement control. So as the operator presses farther on the accelerator pedal, pump displacement increases, which increases vehicle speed.

Forward and reverse is controlled through a lever similar to that used in conventional transmissions. Pivoting the lever to the reverse position changes the polarity of the signal from the accelerator pedal transducer. So pushing on the pedal increases speed in the reverse direction.

The harvester is also shifted out of and into low-speed, high-torque using a lever-activated switch. This switch is normally open (de-energized), so it does not send a signal to solenoid valve A unless the operator moves the lever into the high-speed mode. An interlock is also provided to prevent the vehicle from starting while in the high-speed mode.

Mark Perry is with Fitzsimmons Hydraulics, Clarence, N. Y., and George Morgan, P. E., is a registered patent agent with Morgan & Associates, Evansville, Ind. For more information on the lettuce harvester, e-mail For more information on the patent-pending flow divider circuit, e-mail George Morgan, P. E., at