As with most harvesting equipment, lettuce harvesters move slowly in the field to ensure maximum yield and to prevent damaging the crop. However, once a row has been harvested, the machine must quickly be repositioned to begin harvesting the next row for high productivity. Furthermore, lettuce often grows in multiple fields, so the harvester must also be able to quickly move from one field to another.

Six drive wheels

Another challenge for lettuce harvesters is traction. Fields are often muddy because growing lettuce requires using a lot of water, so even a four-wheel-drive vehicle can get bogged down. A track drive would offer maximum traction, but would cause too much damage to the field. As it turns out, six-wheel drive provides enough support to keep the vehicle from getting bogged down in mud and also reduces ground pressure compared to four-wheel drive. It also provides enough traction to keep the vehicle moving — even if wheels become caked with mud.

So the challenge is to provide a drive that can distribute power to six wheels, provide low-speed, high-torque power for harvesting, and high-speed drive for moving from one location to another. All of these requirements can easily be met with a hydrostatic transmission. However, an unconventional lettuce harvester uses a patented circuit to provide low-speed, high-torque or high-speed, low-torque operation simply by shifting a valve.

Power and versatility

As with most agricultural equipment, the lettuce harvester uses hydraulics for more than just propulsion. It also features hydraulic steering, braking, positioning of implements, and it also drives conveyors. However, the most unusual aspect of the lettuce harvester is its hydrostatic drive system, which consists of a variable-displacement, axial-piston pump with reverse-flow capability from Sauer Danfoss, Ames, Iowa.

Each of the harvester’s six wheels is driven through a 30:1 planetary gear wheel drive from Auburn Gear by a Sauer Danfoss Model OMS motor sized at 12 in.³/rev. The pump, capable of 50 gpm at 4500 psig, was chosen for its combination of high performance and reliability at a reasonable cost.

The harvester gives the operator a choice of two speed ranges — the low-speed, high-torque mode provides harvesting speed of 300 ft/hr (5 ft/min) and a high-speed, low-torque mode for speeds to 4 mph. Had the harvester required only a 2:1 range, it could have been fitted with a regenerative circuit that uses one 4-outlet flow divider.

For a description of this circuit, refer to the December 2007 issue of Hydraulics & Pneumatics, or view the article by clicking here.

But what makes the harvester achieve its efficient and effective high-torque drive is innovative hydraulic circuitry that switches the wheel drive motors between parallel flow (low-speed, high-torque mode) and series flow (high-speed, low-torque mode). This is accomplished using a pair of directional control valves and two rotary flow dividers, as shown in the schematic. This patent-pending design belongs to HyVee Equipment LLC, Clarksville, Tenn.

Achieving the improbable

The schematic shows a simplified view of the harvester’s hydraulic system. It is shown in its zero-state, at-rest mode. In this mode, the pump is at zero displacement, but once the pump’s displacement control moves off center, the vehicle will be in its low-speed, high torque mode.

In the low-speed forward mode, fluid flows from the left side of the bidirectional pump to junction 1, between valves BL and BR. Line pressure is also transmitted to solenoid valve A. However, because valve A is closed, pilot pressure is not transmitted to any downstream components.

From the junction, fluid flows to valves BL and BR. But because neither valve receives pilot pressure from valve A, they remain in their de-energized state, so they route fluid up to their respective rotary flow divider. Each of the two flow dividers is a Rotary Drive Roller Stator design from White Drive Products.

Flow divider circuits are not limited to mobile applications; similar circuits using flow dividers could also be applied to hydraulic conveyor drives. The flow dividers would ensure that each conveyor motor runs at the same speed, and each conveyor would run efficiently whether lightly loaded or fully loaded.

Each flow divider in the harvester is essentially three hydraulic motors with a common shaft. So fluid entering the flow divider assembly flows into all three motors and turns the common shaft, which causes all three to rotate at exactly the same speed. Therefore, flow from each of the three outlet ports of the flow divider is 13 that of the flow entering the single inlet port. This establishes the flow divider as a meter-in device to all three wheel motors.

From the flow divider, an equal amount of fluid flows into each wheel motor, so each rotates at the same speed. The flow dividers ensure that each motor receives 16 of pump flow — half of pump flow goes through valve BL, half goes through BR, and each flow divider sends 13 of the flow it receives (half of total flow) to each wheel motor.

Theoretically, though, each wheel motor has full pump pressure available, so it is capable of high torque. Fluid exiting each motor flows back to the pump either directly or through valve BM. This establishes a parallel flow circuit for each of the two sets of motors.