Gear Bearings and Gear-Bearing Transmissions
- Friday, 23 October 2009
Planetary transmissions could be simpler, cheaper, and more rigid.
Gear bearings are conceptual mechanical components so named because they function as gears and as roller and/or thrust bearings. Gear bearings will be essential components of the next generation of compact, large-mechanical- advantage gear drives.
Figure 1 shows a working gear-bearing transmission with 70/1 gain in a 1.25-in.- diameter-by-0.75-in. (3.18-cm-diameterby- 1.91-cm) package. Gear bearings exist as spur- or helical-gear variants of two basic types: roller gear bearings and phase-shifted gear bearings. For the sake of brevity, only spur-gear variants are described here; suffice it to say that the principles of operation of the helicalgear variants are similar.
Figure 2(a) depicts selected aspects of roller spur-gear bearings and a partial planetary assembly containing roller spurgear bearings. The diameter of the roller portion of each roller spur-gear bearing equals the pitch diameter of the spur-gear portion. The gear teeth are crowned at the roller end, such that the apogee of the crown on each tooth lies at the roller diameter/ pitch diameter: This arrangement provides thrust-bearing strength as well as matching of speeds and, hence, efficiency.
The planetary assembly would inherently hold itself together, without bracing by other structures. For example, if a planetary roller gear bearing shown in the cross-sectional view were pushed downward, the axial sliding of its teeth with respect to the ring- and sun-gear teeth is stopped by the abutment of the planetary roller against the crowns of the ring-gear teeth. When the same planetary gear is pushed upward, its axial sliding is blocked by abutment of the planetary teeth against the ring roller. Similar interactions prevent axial sliding of the sun roller gear bearing beyond the limits imposed by tooth/roller contacts between the sun and planetary roller gear bearings.
An additional advantage is that by enforcing the desired relative locations of gears more precisely, the incorporation of the rollers increases (relative to simple spur gears) the accuracy of meshing of spur-gear teeth. At the same time, the gears act as highly efficient and precise cages and carriers for the rollers. The net result is a superior, simpler, and relatively inexpensive assembly.
Figure 2(b) presents a simple example of a phase-shifted spur-gear bearing. The teeth in the upper and lower halves are shifted angularly, relative to each other, by precisely a half-tooth interval. This gear bearing meshes with a copy of itself. The upper and lower teeth are beveled and partly interdigitated where they meet. The contact between the beveled surfaces of the upper and lower teeth provide a thrust-bearing capability in a manner similar to that of the contact between the crowns and rollers of the roller gear bearings. Moreover, a planetary assembly containing phaseshifted gears holds itself together in a manner similar to that of an assembly containing roller gear bearings as described above.
This work was done by John M. Vranish of Goddard Space Flight Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Goddard Space Flight Center ; (301) 286-7351. Refer to GSC-14207.
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