Innovators at NASA’s Glenn Research Center have developed a simple, lightweight, inline, two-speed drive that can be used either as an overall transmission, or as a supplemental add-on input transmission (i.e., overdrive or underdrive) for rotary wing aircraft and other applications benefiting from variable-speed transmission.
This Offset Compound Gear (OCG) is lightweight yet capable of transferring high power levels, and is capable of a 50% speed ratio change resulting in reduced external noise and increased performance. In addition, ratios other than 50% can be configured to meet specific design requirements. The NASA Glenn design overcomes the technical design challenge of configuring a simple and robust two-speed/variable-speed driveline transmission that is lightweight yet capable of transferring high power at high speed for next-generation rotary wing aircraft.
The NASA Glenn OCG innovation is a novel approach to offsetting the axis of rotations and embedding gear meshes. The concept is built upon the offset compound gear, which uses internal teeth on the input, and external teeth on the output end, thus allowing it to mesh with both a smaller external tooth input gear and a larger internal tooth output gear, in series. Within this geometry, the cluster gear rotates on a separate axis that is offset from the input gear and output gear, which are located on the machine axis. The OCG provides a 50% speed reduction in two stages, or meshes, utilizing only three gears (the cluster gear replacing the six planet gears required in a simple planetary stage). During high-range operation, the main clutch is engaged, and the output shaft spins at a 1:1 ratio. During low-range operation, the main clutch is disengaged, directing power through the OCG gear train and low-speed sprag (one-way clutch), resulting in an output ratio of 1:2, or a 50% speed reduction.
The OCG innovation offers the simplicity of a concentric shaft system along with the highly applicable design of a concentric device, like a planetary drive. Other ratios beyond the basic configuration of the OCG can be constructed, enabling application at higher or lower speeds.