The patented HyVee Ox provides unmatched push-pull capability in comparison to its dimensional footprint and weight by virtue of its six-wheel hydrostatic drive. It’s not much bigger than a golf cart, yet it can tow more than 66,000 lb and carry a payload of 4000 lb — both at the same time. It can also scale a 15° slope while carrying or towing a full load. The Ox can do all this for indefinite periods without overheating, thanks to the innovative design of its powerful hydrostatic drive system.

Hy Vee Ox

A tactical ground-support vehicle, the Ox is manufactured by HyVee Equipment LLC, Clarksville, Tenn. Its small size and light weight mean it can easily fit inside a medium helicopter. Yet, it has more than enough power to tow a CH-47 helicopter up an incline.

The Ox is nearly 4-ft shorter and almost 2-ft narrower than an HMMWV (Humvee). The Ox also weighs less — 3000 lb, as opposed to the Humvee’s 5700 lb — and the Ox’s towing capacity far exceeds the 8510 lb of a Humvee. However, the Ox is not intended to replace the Humvee but, rather, to augment it by performing tasks where a smaller, lighter vehicle is needed.

Power and versatility
The Ox’s hydrostatic drive system consists of a variable-displacement, axial-piston pump with reverse-flow capability from Sauer Danfoss, Ames, Iowa. Each of the Ox’s six wheels is driven by a high-torque HB wheel motor from White Drive Products, Hopkinsville, Ky. The pump, capable of 50 gpm at 4500 psig, was chosen for its combination of performance and reliability for the money. The wheel motors, each sized at 25 in.3/ rev, were chosen because of their high torque capability, smooth rotational output at low speeds, and low internal leakage, which translates to high volumetric efficiency. Their low-speed, high-torque output eliminates the need for an auxiliary gearbox, which provides a lighter, more compact, and simpler wheel drive.

But what makes the Ox achieve its efficient and effective high-torque drive is its 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.

Achieving the improbable
The illustration shows a simplified schematic of the Ox’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 Ox 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 rotary flow divider is a Roller Stator flow divider from White Drive Products and is essentially made up of three 18-in.3 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, ensuring that all three motors rotate at the same speed.

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.

However, each wheel motor has full pump pressure available — at least theoretically — 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.

Switching modes
To switch the Ox into its highspeed, 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, lowtorque mode.