Dual-type strain wave gearing

Abstract

An externally toothed gear of a dual-type strain wave gearing is provided with first and second external teeth having different teeth numbers, and a gap formed between these teeth as a cutter clearance area for tooth cutters. The maximum width L1 of the gap is 0.1 to 0.3 times the width L of the externally toothed gear. The depth from the tooth top land of the first external teeth to the deepest part of the gap is 0.9 to 1.3 times the depth of the first external teeth, and the depth from the tooth top land of the second external teeth to the deepest part of the gap is 0.9 to 1.3 times the depth of the second external teeth. The tooth bottom fatigue strength of the externally toothed gear provided with differing numbers of first and second external teeth is increased.

Claims

1. A strain wave gearing comprising: a rigid first internally toothed gear formed with first internal teeth; a rigid second internally toothed gear formed with second internal teeth, the second internally toothed gear being disposed so as to be coaxially aligned in parallel with the first internally toothed gear; a flexible externally toothed gear in which first external teeth capable of meshing with the first internal teeth and second external teeth capable of meshing with the second internal teeth are formed in an outer-peripheral surface of a radially flexible cylindrical body, the second teeth differing in number from the first teeth, and the externally toothed gear being disposed coaxially inside the first and second internally toothed gears; and a wave generator for flexing the externally toothed gear in an ellipsoidal shape to cause the first external teeth to partially mesh with the first internal teeth and to cause the second external teeth to partially mesh with the second internal teeth, wherein a gap is formed between a tooth-trace-direction inner-end surface of the first external teeth and a tooth-trace-direction inner-end surface of the second external teeth, the gap having a prescribed width along a tooth trace direction, and the gap having a deepest part along a tooth depth direction at a tooth-trace-direction central portion; and wherein the wave generator has a first wave bearing comprising a ball bearing for supporting the first external teeth, and a second wave bearing comprising a ball bearing for supporting the second external teeth; 0.1L<L1<0.3L, where L is a width from a tooth-trace-direction outer end of the first external teeth to a tooth-trace-direction outer end of the second external teeth, and L1 is a maximum width of the gap along a tooth trace direction; and
0.9h1<t1<1.3h1
0.9h2<t2<1.3h2 and
t(1)<t(2), where h1 is a tooth depth of the first external teeth, h2 is a tooth depth of the second external teeth, t1 is a distance from a tooth top land of the first external teeth to the deepest part of the gap, t2 is a distance from a tooth top land of the second external teeth to the deepest part of the gap, t(1) is a rim wall thickness of the first external tooth, and t(2) is a rim wall thickness of the second external tooth, and bearing-ball centers of the first wave bearing and the second wave bearing are located at positions that are equidistant, along the tooth trace direction, from a tooth-trace-direction center of the gap; where an inter-ball-center distance Lo is a distance between the bearing-ball centers of the first and second wave bearings, and the inter-ball-center distance is set so as to increase correspondingly with an increase in the maximum width L1 of the gap, and to satisfy a relationship
0.35L<Lo<0.7L.

2. The strain wave gearing according to claim 1, wherein a number of the first external teeth differs from a number of the first internal teeth, and a number of second external teeth differs from a number of second internal teeth.

3. The strain wave gearing according to claim 1, wherein a number of first external teeth is less than a number of first internal teeth, and a number of first internal teeth and a number of second internal teeth are equal to each other.

4. The strain wave gearing according to claim 1 wherein the wave generator causes the externally toothed gear to flex into an ellipsoidal shape so that the first external teeth are caused to mesh with the first internal teeth at two positions along a circumferential direction and the second external teeth are caused to mesh with the second internal teeth at two positions along the circumferential direction; and a difference between a number of the first external teeth and a number of the second external teeth is 2n, where n is a positive integer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(a) and 1(b) are an end-surface views and a longitudinal cross-sectional view of a dual-type strain wave gearing to which the present invention is applied;

(2) FIG. 2 is a schematic diagram of the dual-type strain wave gearing shown in FIG. 1;

(3) FIG. 3 is a partial enlarged cross-sectional view of the strain wave gearing shown in FIG. 1;

MODE FOR CARRYING OUT THE INVENTION

(4) An embodiment of a dual-type strain wave gearing to which the present invention is applied is described below with reference to the attached drawings.

(5) FIG. 1 is an end-surface view and a longitudinal cross-sectional view showing a dual-type strain wave gearing (referred to below simply as “strain wave gearing”) according to an embodiment of the present invention, and FIG. 2 is a schematic diagram of the same. The strain wave gearing 1, which is used as, e.g., a gear reducer, has an annular rigid first internally toothed gear 2, an annular rigid second internally toothed gear 3, a cylindrical flexible externally toothed gear 4 comprising a radially flexible thin-walled elastic body, and an ellipsoidally contoured wave generator 5.

(6) The first and second internally toothed gears 2, 3 are disposed so as to be coaxially aligned in parallel with each other, with a prescribed gap therebetween, along the direction of a central axis 1a. In the present example, the first internally toothed gear 2 is a stationary-side internally toothed gear secured so as not to rotate, the number of first internal teeth 2a thereof being indicated by Zc1. The second internally toothed gear 3 is a rotatably supported drive-side internally toothed gear, the number of second internal teeth 3a thereof being indicated by Zc2. The second internally toothed gear 3 is the reduced-rotation-outputting element of the strain wave gearing 1.

(7) The cylindrical externally toothed gear 4 is disposed coaxially inside the first and second internally toothed gears 2, 3. The externally toothed gear 4 has a cylindrical body 6 that is a radially flexible thin-walled elastic body, first external teeth 7 and second external teeth 8 formed in the circular outer-peripheral surface of the cylindrical body 6, and a gap 9 (refer to FIG. 3) formed between the external teeth 7, 8 on either side, the gap 9 functioning as a cutter clearance area. The first external teeth 7 are formed on one side along the central axis 1a direction of the circular outer-peripheral surface of the cylindrical body 6, and the second external teeth 8 are formed on the other second-internal-teeth 3a side of the circular outer-peripheral surface. The first and second external teeth 7, 8 are formed such that the central-axis 1a direction is the tooth trace direction.

(8) Specifically, the first external teeth 7 are formed on the side opposing the first internal teeth 2a, and are capable of meshing with the first internal teeth 2a, the number of first external teeth 7 being indicated by Zf1. The second external teeth 8 are formed on the side opposing the second internal teeth 3a, and are capable of meshing with the second internal teeth 3a, the number of second external teeth 8 being indicated by Zf2. The numbers Zf1, Zf2 of teeth are different from each other. Further, the first external teeth 7 and the second external teeth 8 are apart from each other in the tooth-trace direction.

(9) The wave generator 5 has an ellipsoidally contoured rigid plug 11, and a first wave bearing 12 and second wave bearing 13, the first and second wave bearings being fitted to the ellipsoidal outer-peripheral surface of the rigid plug 11. The first and second wave bearings 12, 13 are formed from ball bearings.

(10) The wave generator 5 is inserted into the inner-peripheral surface of the cylindrical body 6 of the externally toothed gear 4, and causes the cylindrical body 6 to flex in an ellipsoidal shape. Therefore, the first and second external teeth 7, 8 are also flexed in an ellipsoidal shape. The ellipsoidally flexed externally toothed gear 4 meshes with the first and second internally toothed gears 2, 3 at both end positions along the major axis Lmax of the ellipsoidal shape. Specifically, the first external teeth 7 mesh with the first internal teeth 2a at both end positions along the major axis of the ellipsoidal shape, and the second external teeth 8 mesh with the second internal teeth 3a at both end positions along the major axis.

(11) The wave generator 5 is the rotation-input element of the strain wave gearing 1. The rigid plug 11 of the wave generator 5 has a shaft hole 11c, in which an input rotation shaft 10 (refer to FIG. 2) is securely connected in a coaxial arrangement. For example, a motor output shaft may be securely connected in a coaxial arrangement in the shaft hole 11c. When the wave generator 5 rotates, the positions at which the first external teeth 7 of the externally toothed gear 4 and the stationary-side first internal teeth 2a mesh, and the positions at which the second external teeth 8 of the externally toothed gear 4 and the drive-side second internal teeth 3a mesh, move along the circumferential direction.

(12) The number Zf1 of first external teeth 7 and the number Zf2 of second external teeth 8 differ from each other; in the present example, the number Zf2 of second external teeth is greater. The number Zc1 of first internal teeth 2a and the number Zf1 of first external teeth 7 also differ from each other; in the present example, the number Zc1 of first internal teeth 2a is greater. The number Zc2 of second internal teeth 3a and the number Zf2 of second external teeth 8 differ from each other; in the present example, the number Zc2 of second internal teeth 3a is less.

(13) In the present example, the externally toothed gear 4 is caused to flex in an ellipsoidal shape, and meshes with the internally toothed gears 2 and 3 at two locations along the circumferential direction. Therefore, the difference between the number Zc1 of first internal teeth 2a and the number Zf1 of first external teeth 7 is 2j, where j is a positive integer. The difference between the number Zc2 of second internal teeth 3a and the number Zf2 of second external teeth 8 is 2k, where k is a positive integer.
Zc1=Zf1+2j
Zc2=Zf2−2k

(14) In a specific example, the numbers of teeth are set as follows j=k=1:
Zc1=62
Zf1=60
Zc2=62
Zf2=64

(15) The speed ratio R1 between the first internally toothed gear 2 and the first external teeth 7, and the speed ratio R2 between the second internally toothed gear 3 and the second external teeth 8, are respectively defined as follows:
i1=1/R1=(Zf1−Zc1)/Zf1=(60−62)/60=−1/30
i2=1/R2=(Zf2−Zc2)/Zf2=(64−62)/64=1/32

(16) Therefore, R1=−30, and R2=32.

(17) The speed ratio R of the strain wave gearing 1 is represented by the following formula using the speed ratios R1, and R2. Thus, according to the present invention, a strain wave gearing having a dramatically low speed ratio (low reduction ratio) can be realized (a negative speed ratio indicates that output rotation progresses in the direction opposite that of input rotation).

(18) $\begin{array}{c}R=\left(R1\times R2-R1\right)/\left(-R1+R2\right)\\ =\left(-30\times 32+30\right)/\left(30+32\right)\\ =-930/62\\ =-15\end{array}$

(19) (Gap: Cutter Clearance Area)

(20) FIG. 3 is a partial enlarged cross sectional view of the strain wave gearing, which shows the externally toothed gear 4 as well as the first and second wave bearings 12 and 13 of the wave generator 5. The gap 9 formed between the first and second external teeth 7 and 8 functions as a cutter clearance area for tooth-cutting cutters used for cutting the first and second external teeth 7 and 8.

(21) The first and second external teeth 7 and 8 will be explained at first. Since the first and second internal teeth 2a and 3a has substantially the same tooth width, the first external teeth 7 and the second external teeth 8 having the same tooth width are formed in a symmetrical state with respect to the tooth-trace-direction central position 6a of the cylindrical body 6. When the first and second internal teeth differ in tooth width with each other, the first and second external teeth 7 and 8 will correspondingly differ in tooth width.

(22) The gap 9 has a prescribed width along the tooth trace direction; the deepest part, which is the part of the gap 9 that is formed deepest along the tooth depth direction, is formed in the tooth-trace-direction central portion. In the present example, the deepest part 9a is a portion at which the tooth-trace-direction central portion is defined by a straight line extending parallel to the tooth trace direction, as viewed from the tooth-thickness direction. At the two tooth-trace-direction ends of the deepest part 9a, a concave arcuate curve that defines the tooth-trace-direction inner-end surface 7a of the first external teeth 7 and a concave arcuate curve that defines the tooth-trace-direction inner-end surface 8a of the second external teeth 8 are smoothly connected. It is also possible to adopt a configuration in which the deepest part 9a is defined by a concave curved surface and the two inner-end surfaces 7a, 8a are defined by inclined straight lines. It is furthermore possible to adopt a configuration in which the deepest part 9a is defined by a straight line and the two inner-end surfaces 7a, 8a are defined by inclined straight lines.

(23) The tooth-trace-direction width of the gap 9 in the present example gradually increases from the deepest part 9a along the tooth depth direction. The maximum width L1 in the tooth trace direction is the distance, along the tooth trace direction, from the tooth-trace-direction inner end 7b of the addendum circle of the first external teeth 7 to the tooth-trace-direction inner end 8b of the addendum circle of the second external teeth 8.

(24) The relationship
0.1L<L1<0.3L

(25) is established, where L is the width from the tooth-trace-direction outer end 7c of the first external teeth 7 to the tooth-trace-direction outer end 8c of the second external teeth 8, and L1 is the tooth-trace-direction maximum width of the gap 9.

(26) The depth of the deepest part 9a of the gap 9 is set as follows. The relationships
0.9h1<t1<1.3h1 and
0.9h2<t2<1.3h2

(27) are established, where h1 is the tooth depth of the first external teeth 7, h2 is the tooth depth of the second external teeth 8, t1 is the tooth-depth-direction depth from the top land 7d of the first external teeth 7 to the deepest part 9a, and t2 is the tooth-depth-direction depth from the top land 8d of the second external teeth 8 to the deepest part 9a.

(28) [Distance Between Bearing-Ball Centers]

(29) The distance between the bearing-ball centers of the first and second wave bearings 12, 13 are described next with reference to FIG. 3.

(30) In the rigid plug 11 of the wave generator 5, an ellipsoidally contoured first outer-peripheral surface 11a of fixed width is formed on one central-axis-direction side, and an ellipsoidally contoured second outer-peripheral surface 11b of fixed width is formed on the other central-axis-direction side. The first outer-peripheral surface 11a and the second outer-peripheral surface 11b are ellipsoidal outer-peripheral surfaces having the same shape and the same phase. The first and second outer-peripheral surfaces 11a and 11b may be different ellipsoidal shapes in accordance with the difference in the amount of deflection between the first and second external teeth 7 and 8.

(31) The first wave bearing 12 is fitted to the first outer-peripheral surface 11a in a state of being flexed in an ellipsoidal shape, and the second wave bearing 13 is fitted to the second outer-peripheral surface 11b in a state of being flexed in an ellipsoidal shape. The first and second wave bearings 12, 13 are of the same size.

(32) The bearing centers 12a, 13a of the first wave bearing 12 and second wave bearing 13 are located at positions that are equidistant, along the tooth width direction, from the tooth-trace-direction central position 6a on the externally toothed gear 4. The distance between bearing-ball centers is set so as to increase correspondingly with an increase in the maximum width L1 of the gap 9. Furthermore, the inter-ball-center distance Lo is set so as to reach a value within the range indicated by the following formula, Lo being the distance between bearing-ball centers.
0.35L<Lo<0.7L

Other Embodiments

(33) In the example described above, the first internally toothed gear 2 is configured as a stationary-side internally toothed gear, and the second internally toothed gear 3 is configured as a drive-side internally toothed gear. It is possible to instead configure the first internally toothed gear 2 as a drive-side internally toothed gear, and configure the second internally toothed gear 3 as a stationary-side internally toothed gear.

(34) It is also possible to flex the externally toothed gear 4 into a non-circular shape other than an ellipsoidal shape, for example, into a non-circular shape such as a three-lobe shape. When h represents the number of meshing positions between the externally toothed gear flexed into a non-circular shape and the internally toothed gear, the difference in the number of teeth between the two gears may be set hp, where h is a positive integer equal to or more than 2, and p is a positive integer.