WAVE GENERATOR AND STRAIN WAVE GEARING

Abstract

A strain wave gearing has a wave generator provided with a rigid plug and a wave bearing. The wave bearing is of a deep groove bearing type and is mounted on an elliptical outer circumferential surface of the rigid plug, whereby outer race and inner race thereof are flexed into an elliptical shape. Where D is a ball diameter of the balls, Ro is a raceway-surface radius of the outer race, Ri is a raceway-surface radius of the inner race, Ro/D is an outer-race conformity, and Ri/D is an inner-race conformity, the outer-race conformity Ro/D is greater than the inner-race conformity Ri/D. The friction torque of the wave bearing can be reduced while maintaining the practical service life thereof to be the same level as in a conventional one.

Claims

1. A wave generator of a strain wave gearing, in which the wave generator is configured to flex a flexible externally toothed gear into an elliptical shape, partially mesh with an internally toothed gear and move meshing positions between the both gears in a circumferential direction, the wave generator comprising: a rigid plug and a wave bearing, wherein the wave bearing is provided with an annular outer race that is flexible in a radial direction thereof, an annular inner race that is flexible in a radial direction thereof, and a plurality of balls inserted between the outer race and the inner race; the wave bearing is mounted on an elliptical outer circumferential surface of the rigid plug, and the outer race and the inner race are flexed into an elliptical shape; and where D is a ball diameter of the balls, Ro is a raceway-surface radius of the outer race, Ri is a raceway-surface radius of the inner race Ro/D is an outer-race conformity, and Ri/D is an inner-race conformity, the outer-race conformity Ro/D is greater than the inner-race conformity Ri/D.

2. A wave generator of a strain wave gearing, in which the wave generator is configured to flex a flexible externally toothed gear into an elliptical shape, partially mesh with an internally toothed gear and move meshing positions between the both gears in a circumferential direction, the wave generator comprising: a rigid plug and a wave bearing, wherein the wave bearing is provided with an annular outer race that is flexible in a radial direction thereof, an inner-race raceway surface formed on an elliptical outer circumferential surface of the rigid plug, and a plurality of balls inserted between the inner-race raceway surface and an outer-race raceway surface formed on an inner circumferential surface of the outer race; the outer race is flexed into an elliptical shape in accordance with the elliptical outer circumferential surface; and where D is a ball diameter of the balls, Ro is a raceway-surface radius of the outer race, Ri is a raceway-surface radius of the inner race Ro/D is an outer-race conformity, and Ri/D is an inner-race conformity, the outer-race conformity Ro/D is greater than the inner-race conformity Ri/D.

3. The wave generator of the strain wave gearing according to claim 1, wherein the outer-race conformity is in a range from 52.6% to 58.1%, and the inner-race conformity is in a range from 51.1% to 52.3%.

4. A strain wave gearing comprising: the wave generator as set forth in claim 1.

5. The wave generator of the strain wave gearing according to claim 2, wherein the outer-race conformity is in a range from 52.6% to 58.1%, and the inner-race conformity is in a range from 51.1% to 52.3%.

6. A strain wave gearing comprising: the wave generator as set forth in claim 2.

7. A strain wave gearing comprising: the wave generator as set forth in claim 3.

8. A strain wave gearing comprising: the wave generator as set forth in claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic longitudinal cross-sectional view of a cup-type strain wave gearing according to the present invention;

[0023] FIG. 2 is an explanatory view illustrating a meshing state between an externally toothed gear and an internally toothed gear of the strain wave gearing of FIG. 1;

[0024] FIG. 3 is a front view illustrating a wave generator of FIG. 1;

[0025] FIG. 4 is a partial cross-sectional view illustrating a wave bearing of the wave generator of FIG. 1; and

[0026] FIGS. 5(a) and 5(b) include schematic views illustrating a flat-type strain wave gearing according to the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0027] An embodiment of a strain wave gearing according to the present invention will be described hereinbelow with reference to the drawings. The present invention is applied to a cup type strain wave gearing in the following embodiment. The present invention can also be applied to a wave generator of a top-hat type or a flat type strain wave gearing in a similar manner.

[0028] FIG. 1 is a schematic longitudinal cross-sectional view of a cup-type strain wave gearing according to the present embodiment, and FIG. 2 is an explanatory view illustrating a meshing state in the strain wave gearing. A strain wave gearing 1 has a rigid internally toothed gear 2, a cup-shaped flexible externally toothed gear 3 disposed inside the internally toothed gear, and a wave generator 4 that has an elliptical contour and is fitted into the externally toothed gear. The externally toothed gear 3 has a cylindrical portion where external teeth 3a are formed, the cylindrical portion being flexed elliptically by the wave generator 4. The portions of the external teeth 3a located on the both ends of the major axis Lmax of the elliptical shape are meshed with internal teeth 2a of the annular internally toothed gear 2.

[0029] The wave generator 4 is linked to a motor shaft or other high-speed rotation input shaft. When the wave generator 4 rotates, the meshing positions of the both gears 2 and 3 move in the circumferential direction, whereby a relative rotation between the both gears 2 and 3 is generated due to the difference in the number of teeth therebetween. For example, the internally toothed gear 2 is fixed not to rotate and the externally toothed gear 3 is linked to a member on the load side. A reduced-speed rotation is taken out from the externally toothed gear 3 and is transmitted to the member of the load side.

[0030] FIG. 3 is a front view illustrating the wave generator 4 and FIG. 4 is a partial cross-sectional view illustrating a wave bearing thereof. Explanation will be made with reference to these drawings. The wave generator 4 is provided with a rigid plug 5 and a wave bearing 7 mounted on an elliptical outer circumferential surface 6 thereof. The rigid plug 5 is fixed to an outer circumferential surface of a hub lined to a rotation input shaft (not shown). The wave bearing 7 is provided with a circular inner race 8 that is flexible in the radial direction, a circular outer race 9 that is flexible in the radial direction, and a plurality of balls 10 that are mounted between the races in a rollable state.

[0031] The wave bearing 7 is flexed into an elliptical shape by the rigid plug 5 and, in this state, is fitted into the externally toothed gear 3, whereby maintaining a state in which the externally toothed gear 3 is rotatable relative to the rigid plug 5 lined to a high-speed rotation input shaft. The inner race 8 is formed on the outer circumferential surface with an inner-race raceway surface 11 and the outer race 9 is formed on the inner circumferential surface with an outer-race raceway surface 12. The balls 10, which are inserted between the elliptically flexed inner and outer races 8 and 9, perform a rolling motion along the inner-race raceway surface 11 and the outer-race raceway surface 12, so that the rigid plug 5 and the externally toothed gear 3 can smoothly rotate to each other with a small torque.

[0032] More specifically, the wave bearing 7 is flexed into an elliptical shape by the rigid plug 5. Where Lmax is the major axis of the elliptical shape and Lmin is the minor axis thereof, one or a plurality of balls 10 located on both ends of the major axis Lmax are a tight ball 10a. The tight ball 10a performs a rolling motion while being maintained in a tight state. In the tight state, the ball is sandwiched with a certain pressing force between the inner-race raceway surface 11 and the outer-race raceway surface 12 and is in a point contact with the inner-race raceway surface 11 and the outer-race raceway surface 12. The remaining balls 10, which are located on the portions other than the both ends of the major axis Lmax, are a loose ball 10b. The loose ball 10b is in a loose state in which a gap is formed at least between the loose ball and the inner-race raceway surface 11 or between the loose ball and the outer-race raceway surface 12 so as to perform a free rolling motion.

[0033] As illustrated in FIG. 4, the wave bearing 7 has a basic structure that is the same as in a conventional wave bearing of a deep-groove-bearing type. The wave bearing 7 is provided with the outer race 9, the conformity of which (a ratio of the outer-race raceway-surface radius and the ball diameter) differs from the conventional product.

[0034] Here, D is a ball diameter, Ri is a raceway-surface radius of the inner-race raceway surface 11, and Ro is a raceway-surface radius of the outer-race raceway surface 12. The ratio Ri/D of the inner-race raceway-surface radius Ri and the ball diameter D is called as an inner-race conformity, and the ratio Ro/D of the outer-race raceway-surface radius Ro and the ball diameter D is called as an outer-race conformity. The outer-race conformity Ro/D is greater than the inner-race conformity Ri/D. In this example, these parameters are set as in Table 1.

TABLE-US-00001 TABLE 1 Inner-race conformity Outer-race conformity Conventional 51.1%~52.3% 51.5%~52.5% wave bearing Wave bearing 51.1%~52.3% 52.6%~58.1% of this example

[0035] According to experiments conducted by the present inventors et al, it was confirmed that the friction torque of the wave bearing 7 can be reduced while maintaining the practical service life thereof to be the same level as in a conventional one by setting the outer-race conformity to be a value within the above-mentioned range.

Another Embodiments

[0036] FIG. 5 illustrates an example of a flat-type strain wave gearing to which the present invention is applied, in which FIG. 5(a) is a schematic end view illustrating a flat-type strain wave gearing and FIG. 5(b) is a schematic longitudinal cross-sectional view thereof.

[0037] A strain wave gearing 20 is provided with a rigid internally toothed gear having a first internally toothed gear 21 and a second internally toothed gear 22. The first and second internally toothed gears 21 and 22 are coaxially disposed in parallel, and a cylindrical flexible externally toothed gear 23 is disposed inside these internally toothed gears. A wave generator 24 having an elliptical contour is fitted into the externally toothed gear 23. The externally toothed gear 23 is flexed into an elliptical shape by the wave generator 24 so that the external teeth 23a thereof are meshed with both the inner teeth 21a of the first internally toothed gear 21 and the internal teeth 22a of the second internally toothed gear 22 on both ends of the major axis Lmax of the elliptical shape. For example, the number of teeth of the first internally toothed gear 21 is 2n (n is a positive integer) greater than the number of teeth of the second internally toothed gear 22, and the number of teeth of the externally toothed gear 23 is the same as that of the second internally toothed gear 22. The external teeth 23a mesh with the internal teeth 21a and 22a on positions of the major axis Lmax of the elliptical shape of the externally toothed gear 23.

[0038] The wave generator 24 is provided with a rigid plug 25 and a wave bearing 27 mounted on the elliptical outer circumferential surface 26. The wave bearing 27 is provided with an inner-race raceway-surface 31 formed on the elliptical outer circumferential surface of the rigid plug 25, a circular outer race 29 that is flexible in the radial direction, and a plurality of balls 30. The balls 30 are mounted in a rollable state between the inner-race raceway surface 31 and an outer-race raceway surface 32 formed on the inner circumferential surface of the outer race 29.

[0039] The wave bearing 27 is flexed into an elliptical shape by the rigid plug 25 and, in this state, is fitted into the externally toothed gear 23. The wave bearing 27 holds the externally toothed gear 23 and the rigid plug linked to a high-speed rotation input shaft in a relatively rotatable state. The balls 30, which are inserted between the inner-race raceway surface 31 and the outer-race raceway surface 32, perform a rolling motion along these surfaces, which allows the rigid plug 25 and the externally toothed gear 23 can be rotated relative to each other with a small torque.

[0040] In this example as well, the wave bearing 27 has a basic structure that is the same as in a wave bearing of a deep-groove-bearing type but has the outer-race conformity larger than the inner-race conformity. For example, the inner-race conformity is set to be a value within a range from 51.1% to 52.3% and the outer-race conformity is set to be a value within a range from 52.6% to 58.1%.