At the heart of the system, a custom-designed swivel joint enables
different parts of the motor to rotate in different directions. To achieve that
sort of movement, Zulu designed annular grooves into the swivel joint that
transport pressurized fluid into the spinning motor. During operation, fluid
passes into the stationary part of the swivel joint and into concentric grooves
on a port plate. As the fluid fills the grooves, it is drawn into a separate set
of holes on a spinning pressure plate. From there, fluid flows into the motor,
where it spins the rotor, then exits via a separate path.
The design of the swivel joint is critical because it performs two key functions: By filling the concentric rings on the port plate, it forms a hydrodynamic bearing surface that lubricates the metal-to-metal interface between the port plate and pressure plate. And, more importantly, it supplies pressurized fluid through the spinning motor housing and into the motor's cylinder block. Without the annular groove configuration, the supply of pressurized fluid would be unable to enter the spinning motor housing. As a result, Zulu says, the unit operates "on the theory of a commutator on a motor, where you can supply power while the armature rotates."
Power from the two spinning sources is transmitted separately into the transmission's output shaft. Using these separate power sources, the unit can speed up, slow down, or reverse the rotation of its output shaft. In applications with an engine running at constant speed, the system can also enter a "powered neutral" state, in which both inputs spin at equal and opposite speeds, thus canceling one another out.
Key to the system is its ability to eliminate planetary gear sets. By doing so, it improves operating efficiency and reduces manufacturing costs. That's because planetary gear sets contain dozens of parts and cause efficiency losses of about two percent at each gear mesh.
In one steering application, engineers cut the number of gears from 17 to 6 and reduced the overall part count from 283 to 114. In the process they eliminated three gear meshes--for an efficiency improvement of approximately 6%--and cut manufacturing costs by an estimated 18% to 23%.
Zulu's gearless design offers greater compactness because it does away with the gear box. And it provides a continuously variable speed output. The unit's combination of size, performance, and cost open a broad vista of applications, even making it a potential candidate for automotive applications. "It's a universal component that can be used for any hydromechanical power application," Zulu says. "There's no limit to the ways it can be used."
Joshua Zulu earned a BSME from Purdue University and an MBA from St. Ambrose University. He is worldwide team leader for Caterpillar's concurrent design, development, and maintenance of lower powertrain components for medium track-type tractors. As a member of that team, he was encouraged by Caterpillar management to push the state of the art and take risks in the design of his award-winning gearless transmission. During his career, Zulu has also received patents for the design of power systems for locomotives.
Additional details...Contact Joshua Zulu, Caterpillar Industrial Products, Inc., 100 N.E. Adams, Peoria, IL 61629, (309) 675-5693.