FIG. 17 exhibits a schematic view of a conventional planetary reduction gear. The reduction gear has a sun gear (11) mounted at an end of an input shaft (1), plural planetary gears (2) meshing with the sun gear (11), an internal gear (41) fixed in a casing (4) and a carrier (31) rotatably supporting the planetary gears (2). An output shaft (3) is fitted to the carrier (31). Such a planetary reduction gear has advantages, i.e. transmittance of high speed rotation, small size of the device and coaxial structure of the input shaft and the output shaft.
However, the conventional planetary reduction gear has some drawbacks. Backlashes between tooth faces of meshing gears often generate loud noise and an error of rotation angle. The error of transmission (rotation angle) especially appears when the rotation of gears is switched to a reverse direction. Bilateral rotation of gears is likely to induce bad effects of backlashes.
For example, when a conventional planetary gear assembly is used as a reduction gear of robots or machine tools which require high precision of transmission, the error of transmission will deteriorate the accuracy of the motion of parts and the accuracy of cutting objects. The error of transmission is born from backlashes of meshments of gears. In general, backlashes are allocated to gear teeth in order to facilitate the assembly of a reduction gear and the smooth rotation of gears. However, backlashes allow some amount of ambiguity with regard to the rotation angle of a driven gear, because the driven gear can slightly rotate by backlashes, even if the driving gear is rigorously at rest. Thus, backlashes will reduce the accuracy of displacement of parts of robots or machine tools which require high precision of displacement of parts in bilateral motion.
Japanese Patent Laying Open No. 63-13939 (13939/88) disclosed a reduction gear to solve the problems on backlashes. The prior art proposed planetary shafts which are eccentric to the center of the shaft holes of the carrier (31). The eccentric planetary shafts would be displaced within a little distance in angular direction. By adjusting the displacement of the center of the planetary gears with regard to the center of the shaft holes, backlashes could be suppressed effectively. The adjustment should be done for every planetary gear in order to regulate the meshment between the planetary gear and the sun gear as well as the meshment between the planetary gear and the internal gear at the same time. Since the adjustment succeeds in suppressing backlashes, the output shaft could be rotated clockwise and counterclockwise with a little error of transmission regarding the input shaft.
However, the prior art requires difficult, tedious adjustment of all planetary shafts for the holes of the carriers. The adjustment makes the assembly very difficult and raises the cost of manufacturing. Furthermore, long use of prior art would wear out the faces of gear teeth. The wear of gear teeth causes new backlashes which would lead to an error of transmission. In such a case, the displacement of the eccentric planetary shafts must be adjusted again.
The readjustment would be also a difficult, tedious operation. The difficulty of adjustment of the eccentric planetary shafts and the inconvenience of readjustment seriously depreciate the advantage of the prior art.
A purpose of the present invention is to provide a planetary reduction gear without backlashes at the meshments of gears in order to suppress the error of transmission and noise.
Second purpose of the invention is to provide a planetary reduction gear which requires no adjustment in assembly nor readjustment after long use. Third purpose of the invention is to provide a planetary reduction gear which can transmit strong torque. Fourth purpose of the invention is to provide a planetary reduction gear whose teeth are protected from an external shock.