Innovative Vehicle Breaks Rules in Size: Power Ratio

Reversing the order

Switching the Ox into reverse is accomplished through the pump’s displacement control. For the Ox, this is done by providing a lever that the operator shifts. Doing so essentially reverses the polarity of the signal from the accelerator pedal’s position transducer to the pump’s displacement control, which controls position of the pump’s swashplate.

Hydraulic flow in the reverse mode again can be seen in the schematic. If the Ox is in its low-speed, high-torque mode, valve A is not energized, so pilot pressure is not transmitted to valves B_L, B_R, and B_M. 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 B_M 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 B_L or B_R 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 in the high-speed, lowtorque mode in reverse, the operator activates the lever for high-speed operation, so valve A opens to route pilot pressure to valves B_L, B_R, and B_M. 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 B_M, then to the bottom motors through their respective valve B_M.

After exiting the bottom motors, fluid cannot flow into the flow dividers because valves B_L and B_R, have shifted to block flow from them. Instead, fluid exits the bottom motors and flows through valves B_L and B_R, 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 B_L and B_R. A larger, single valve could have been used. However, designer’s 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 B_L and B_R valves are the same as the B_M 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 HyVee Ox could’ve been fitted with, say, a multi-axis joystick for single-point control. However, because the Ox was developed for military applications, designers wanted it 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 Ox 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 throws the lever into the high-speed mode. An interlock is also provided to prevent the vehicle from starting while in the high-speed mode.

The Ox gives the operator a choice of two speed ranges — a low-speed, high-torque mode and a high-speed, low-torque mode for road speeds to 40 mph or higher. Each of the Ox’s two flow dividers is a Rotary Drive Roller Stator design from White Drive Products.

Another consideration was the 3:1 torque-speed range. If the HyVee Ox requires only a 2:1 range, it can be fitted with a regenerative circuit that uses one 4-outlet flow divider. A description of this circuit appears in the December 2007 issue of Hydraulics & Pneumatics; click here to view the article in a new window.

Other applications
This system was first applied, under license from HyVee, by Mark Perry, Fitzsimmons Hydraulics, Clarence, N. Y., in a crop harvester. The machine often must operate in deep mud, so high torque at low speed is necessary to propel the vehicle through this challenging terrain. In its low-speed mode, it runs at a harvesting speed of 300 ft/hr (1 in./sec). It can be shifted to a high-speed mode for traversing between fields at 4 mph. This keeps the pump and the motors operating efficiently in both speed ranges. The harvester uses Sauer-Danfoss OMS 12 in.³/rev hydraulic motors coupled with 30:1 Power Wheels.

But 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 the conveyor would run efficiently whether lightly loaded or fully loaded.

The Ox in action
The Ox, due to the combination of the Sauer-Danfoss variable-disare placement axial-piston pump with the White Drive’s HB motors, has exceptional control and smooth operation, pushing or pulling loads, without jerk, from dead stop all the way through its operating range, making it ideally suited to moving sensitive loads such as ammunition, explosives, and aircraft where precise movement control is required, especially when loading sensitive loads into an aircraft, up a ramp, or even towing an aircraft up a ship loading ramp.

Twenty five HyVee Ox vehicles are currently in use at the U. S. Army’s Aviation Center, Ft. Rucker, Ala. These are standard models, specifically designed for airfield use with a custom 28-V, 3000-A aircraft jumpstart system integrated within the vehicle. Later models have off-road capability with no loss of torque. The integral hydraulic power system can be used to power an integrated dump bed and accessories, such as auxiliary power, winches, power tools, lifts, booms, fire fighting and snow removal equipment, and construction accessories. Its 4000- to 6000-lb payload capacity accommodates water or fuel tanks.

The Ox also can be used to move, place, and displace towed howitzers, mortars, supporting ordinance where deployment of heavy prime movers is restricted. The Ox is a prime candidate for special purpose aviation units, such as carrying ordinance and offset heavy mortars, increasing safety for crew and aircraft.

Lastly, the Ox is suitable for natural disaster environments or an urban warfare environment where an easily deployable, durable, small wheel or track vehicle can be useful even in mundane tasks, such as placing and removing barricades, moving damaged vehicles and equipment, or moving debris from flood and earthquake zones. The powerful, easily deployable OX reduces manpower requirements for a wide variety of combat, and combat support tasks, including use as a litter carrier.

The HyVee Ox has performed successfully in Afghanistan, Iraq, and various locations by Special Operations tasking. It passed the test of operations in dusty conditions where temperatures reached 165° F. The Ox can easily fit inside medium lift helicopters as well as any fixed wing transport that would accommodate the discontinued M151 vehicle.

George Morgan, P. E., is a registered patent agent with Morgan & Associates. He can be reached at (812) 476-4065 or by e-mail at patagent@evansville.net . Bud Trinkel is a fluid power consultant . He can be reached at (812) 853-3234, or by e-mail at fluidpower1@hotmail.com.