A 50-percent reduction ratio is achieved with two stages utilizing four gears.
The Offset Compound Gear Drive is an in-line, discrete, two-speed device utilizing a special offset compound gear that has both an internal tooth configuration on the input end and external tooth configuration on the output end, thus allowing it to mesh in series, simultaneously, with both a smaller external tooth input gear and a larger internal tooth output gear. This unique geometry and offset axis permits the compound gear to mesh with the smaller diameter input gear and the larger diameter output gear, both of which are on the same central, or primary, centerline. This configuration results in a compact in-line reduction gear set consisting of fewer gears and bearings than a conventional planetary gear train. Switching between the two output ratios is accomplished through a main control clutch and sprag. Power flow to the above is transmitted through concentric power paths.
Low-speed operation is accomplished in two meshes. For the purpose of illustrating the low-speed output operation, the following example pitch diameters are given. A 5.0 pitch diameter (PD) input gear to 7.50 PD (internal tooth) intermediate gear (0.667 reduction mesh), and a 7.50 PD (external tooth) intermediate gear to a 10.00 PD output gear (0.750 reduction mesh). Note that it is not required that the intermediate gears on the offset axis be of the same diameter. For this example, the resultant low-speed ratio is 2:1 (output speed = 0.500; product of stage one 0.667 reduction and stage two 0.750 stage reduction). The design is not restricted to the example pitch diameters, or output ratio. From the output gear, power is transmitted through a hollow drive shaft, which, in turn, drives a sprag during which time the main clutch is disengaged.
High-speed operation is direct-drive (1:1) through the main clutch. During this mode of operation, the above gear train free-wheels the overrunning sprag. A slight reduction in input speed is required to overrun the sprag. The above gear train always spins.
The configuration was conceived to meet a rotorcraft drive design objective to provide a 50-percent reduction ratio. The configuration does so in two stages, or meshes, utilizing only three gears replacing multiple planet gears required in conventional planetary stages. A same-direction 50-percent reduction is not possible with a single-stage simple planetary gear configuration.
In addition, ratios other than 50 percent can be configured to meet specific design requirements. This configuration overcomes a technical design challenge of configuring a simple and robust two-speed/variable-speed drive-line transmission that is lightweight yet capable of transferring high power at high speed for next-generation rotary wing aircraft, which are forecast to require speed range variations on the order of 50 percent.
This work was done by Mark A. Stevens of Glenn Research Center, and Robert F. Handschuh and David G. Lewicki of U.S. Army Research Laboratory, Vehicle Technology Directorate, (located at Glenn Research Center). For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Mechanics/Machinery category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18340-1.