Wave generator for harmonic reducer capable of withstanding axial loads

Abstract

Regarding the technical aspects of wave generators for harmonic reducers, the present invention provides a wave generator for a harmonic reducer that can withstand axial loads. [Solution] The system comprises a circular spline 1, a flexible spline 2, and a wave generator 3, wherein the wave generator is located inside the flexible spline, and the flexible spline is located inside the circular spline. The inner ring of the circular spline meshes with the outer ring of the flexible spline via teeth, the long axis portion of the flexible spline contacts the circular spline, and the short axis portion of the flexible spline does not participate in the operation. The wave generator includes an elliptical cam 31 and a flexible bearing 32, wherein the elliptical cam is located inside the flexible bearing, and the flexible bearing includes a bearing inner ring, a bearing outer ring, and balls. Both the bearing inner ring and bearing outer ring are provided with double grooves, and the distance between the two grooves of the inner ring is smaller than the distance between the two grooves of the outer ring. When multiple balls are provided in corresponding grooves on the bearing inner ring and bearing outer ring, a line passing through the center of the balls and connecting the contact points between the balls and the outer ring and the contact points on the inner ring forms a predetermined angle θ with the radial direction of the bearing, which is generally about 15°. Flexible bearings can withstand a constant axial load, significantly improving the load-bearing capacity, accuracy, and service life of speed reducers, expanding their application fields and scope, and they have a simple structure and are easy to assemble.

Description

Technical Field

[0001] The present invention relates to the technical field of wave generators of harmonic speed reducers, and particularly to wave generators of harmonic speed reducers capable of withstanding axial loads.

Background Art

[0002] Harmonic drive is a device for transmitting motion or power by utilizing the elastic deformation wave of a flexible member, and has advantages such as a simple structure, a large transmission ratio, smooth transmission, high precision, low noise, and high transmission efficiency. It is widely applied in fields such as aerospace technology, radar communication, actuating machinery, measuring instruments, and precision optical instruments.

[0003] Referring to FIGS. 1 and 2 of the specification, a harmonic gear speed reducer according to the prior art mainly includes a wave generator 113, a flexible spline 112, and a circular spline 111. The wave generator 113 has various structures, and the flexible bearing cam type wave generator is an example thereof, and can comprehensively control the deformation of the flexible spline. The flexible bearing cam type wave generator according to the prior art is formed by disposing one elliptical cam 1131 on a flexible bearing 1132. The wave generator 113 is disposed inside the flexible spline 112. Due to the action of the elliptical cam 1131, the flexible spline 112 forms an ellipse following the flexible bearing 1132. When disposed inside the circular spline 111, only the long axis portion contacts the circular spline, and the short axis portion does not participate in the operation. Since the number of teeth of the circular spline is more than that of the flexible spline, when the cam moves and rotates the flexible bearing, the circular spline and the flexible spline mesh with each other through teeth once per rotation, and the flexible spline generates a rotational displacement with respect to the circular spline by a certain number of tooth differences, thereby realizing deceleration.

[0004] Conventional flexible bearing cam-type wave generators have the advantages of a high transmission ratio, high transmission accuracy, and low error. On the other hand, due to the thinness of the flexible bearing, the contact angle becomes zero because the structure eliminates the gap in the radial direction, resulting in a disadvantage in that they cannot withstand large loads during use. Therefore, this type of product is mainly applied when the size is small and the rated load is small, and when the load is large, RV reducers are generally selected.

[0005] The flexible bearings in wave generators of currently widely used harmonic reducers are primarily designed based on the deep groove ball bearing design. However, deep groove ball bearings are designed, manufactured, and used under the assumption that the inner ring, outer ring, and rolling elements of conventional rigid bearings are all rigid and do not undergo plastic deformation. A detailed explanation is as follows.

[0006] Referring to Figure 3(a) of the specification, a radial clearance is ensured before the bearing is assembled due to manufacturing process requirements. Referring to Figure 3(b) of the specification, the clearance is eliminated during the assembly process by the axial displacement of the inner and outer rings, respectively. This ensures that the rolling elements contact the raceways and that the bearing operates smoothly by applying a predetermined load (preload, clearance) in advance. Due to the presence of the clearance, after assembly is complete, the bearing's force-bearing direction (the line passing through the center of the steel balls and connecting the contact points between the steel balls and the outer and inner raceways) forms a predetermined angle (generally 5-7°) with respect to the radial direction of the bearing. In this way, the bearing can withstand mainly radial forces while also being able to withstand appropriate axial forces.

[0007] Referring to Figure 3(c) of the specification, the flexible bearing used in the wave generator of a conventional harmonic reducer eliminates the clearance in the radial direction due to the elliptical major axis of the cam (when assembling the cam, the elliptical major axis becomes the interference fit). Because the size of the major axis of the cam is larger than the inner diameter of the bearing, when the cam is pressed into the inner ring of the bearing, the inner ring of the bearing expands and deforms radially, thereby eliminating the assembly clearance of the bearing in the major axis direction. Due to the preload requirement, the actual clearance becomes negative, and as a result, the actual contact angle of the bearing becomes zero.

[0008] The relationship between the axial force F1, radial force F2, resultant force F, and contact angle θ is given by: radial force F2 = axial force F1 / tanθ.

[0009] When the contact angle θ is very small and close to zero, even a small axial force can cause the radial force to increase infinitely.

[0010] The inventors of the harmonic speed reducer also considered this problem and added cross-roller bearings in the axial direction to eliminate axial displacement and prevent the generation of axial forces. In reality, due to errors in the machining and assembly accuracy of the parts, it is difficult to completely eliminate axial displacement. As a result, small axial forces are generated, inducing large radial forces. This is currently a major challenge in the practical application of harmonic speed reducers. In mild cases, accuracy decreases, and in severe cases, it leads to malfunction and ultimately to fracture of the flexible bearing outer ring. This is one of the reasons why harmonic speed reducers are generally not applicable in environments with large operating loads. The magnitude of the rated load of the harmonic speed reducer is also one of the important indicators for evaluating product quality.

[0011] Over the past decade or so, China has made significant progress in the design and manufacture of harmonic drive reducers. However, there are still gaps when compared to similar products from overseas (mainly Japan Harmonic Drive HD products), mainly in terms of accuracy, service life, and load capacity. Generally, the load capacity of similar products made in China remains at a level approximately 20% lower than that of overseas products. [Overview of the project] [Problems that the invention aims to solve]

[0012] To overcome the shortcomings of conventional technology, the present invention aims to provide a wave generator for a harmonic reducer that can withstand axial loads, where the flexible bearing can withstand a certain axial load, significantly improving the load-bearing capacity and accuracy service life of the reducer, expanding the fields and scope of application, and having a simple structure and easy assembly. [Means for solving the problem]

[0013] The present invention is realized by the following technical solution. The wave generator of a harmonic reducer capable of withstanding axial loads comprises a circular spline, a flexible spline, and a wave generator, wherein the wave generator is located inside the flexible spline, the flexible spline is located inside the circular spline, the inner ring of the circular spline meshes with the outer ring of the flexible spline via teeth, the long axis portion of the flexible spline contacts the circular spline, and the short axis portion of the flexible spline does not participate in the operation. The wave generator includes an elliptical cam and a flexible bearing, wherein the elliptical cam is located inside the flexible bearing, the flexible bearing includes a bearing inner ring, a bearing outer ring, and balls, both of which are provided with double grooves, and a plurality of balls are provided in corresponding grooves on the bearing inner ring and bearing outer ring, the contact angle between the direction of the force the balls receive in the groove and the direction of the radial force is 10° to 20°, and the two contact angles are equal in value and opposite in direction.

[0014] Furthermore, both the inner ring and outer ring of the bearing are integrally constructed.

[0015] Furthermore, the contact angle between the direction of the force acting on the ball within the groove and the direction of the radial force is 15°.

[0016] Furthermore, the ball eliminates gaps due to radial displacement within the groove.

[0017] Furthermore, by positioning the elliptical cam within the inner diameter of the bearing's inner ring, a radial force is applied, thereby eliminating the radial and axial gaps in the ball grooves.

[0018] Furthermore, by controlling the positional difference, groove shape size, and gap between the inner and outer rings of the bearing, the balls form a constant contact angle within the grooves. [Effects of the Invention]

[0019] Compared to conventional technology, the beneficial effects of this invention are as follows: 1. In the wave generator of a harmonic reducer that can withstand axial loads according to the present invention, the contact angle between the direction of the force acting on the ball in the groove and the direction of the radial force is approximately 15°. As a result, the flexible bearing can withstand a constant axial load, significantly improving the load-bearing capacity and accuracy service life of the reducer under the same manufacturing process level, and expanding the fields and scope of application. 2. In the wave generator of the harmonic reducer that can withstand axial load according to the present invention, both the inner ring and outer ring of the bearing are designed as a single integrated structure. This differs from conventional double-row angular contact ball bearings in which the outer ring is a single integrated structure, the inner ring is a two-part structure, and spacers are added. As a result, the wave generator according to the present invention has a simpler structure and is even easier to assemble. 3. In the wave generator of a harmonic reducer that can withstand axial load according to the present invention, the present invention controls the positional difference, groove shape size, and gap between the grooves of the inner ring and outer ring of the bearing, so that the balls form a constant contact angle within the grooves, and by positioning the elliptical cam in the inner diameter of the bearing of the inner ring of the bearing, a force is applied in the radial direction, eliminating the radial and axial gaps in the grooves of the balls, and the overall structure is simple. [Brief explanation of the drawing]

[0020] [Figure 1] This is a schematic diagram of the structure of a harmonic gear reducer employing a flexible bearing cam-type wave generator in conventional technology. [Figure 2] This is a cross-sectional view of a harmonic gear reducer employing a flexible bearing cam-type wave generator, as in conventional technology. [Figure 3] This is a schematic diagram showing three states of a deep groove ball bearing. 3(a) is a cross-sectional view of the deep groove ball bearing in its free state. 3(b) is a cross-sectional view of a general-purpose deep groove ball bearing in its operating state. 3(c) is a cross-sectional view of a conventional flexible bearing for wave generators when the radial clearance is eliminated. [Figure 4] The structural schematic diagram of the wave generator of the harmonic reducer capable of withstanding axial loads according to the present invention. [Figure 5] The upper structural schematic diagram of the wave generator of the harmonic reducer capable of withstanding axial loads according to the present invention. [Figure 6] The figure showing the positional difference between the grooves of the inner ring and outer ring of the bearing when using a combination of two sets of bearings. [Figure 7] The figure showing the positional difference between the grooves of the inner ring and outer ring of the bearing when the bearing according to the present invention is used. [Figure 8] The figure showing the protrusion amount or sinking amount of the inner ring and outer ring in a single bearing.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described by citing specific specific examples. Those skilled in the art can easily grasp other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied by other different specific embodiments, and for each detailed matter in this specification, various modifications or changes can be made according to various viewpoints and applications without departing from the gist of the present invention. Incidentally, as long as there is no contradiction, the following examples and the features included in the examples can be combined with each other.

[0022] An object of the present invention is to provide a wave generator of a harmonic reducer capable of withstanding axial loads in view of the drawbacks of the prior art.

[0023] Example 1

[0024] The wave generator of the harmonic reducer that can withstand axial loads provided in this embodiment, referring to Figure 4, comprises a circular spline 1, a flexible spline 2, and a wave generator 3, wherein the wave generator 3 is located inside the flexible spline 2, and the flexible spline 2 is located inside the circular spline 1, the inner ring of the circular spline 1 meshes with the outer ring of the flexible spline 2 via its teeth, the long axis portion of the flexible spline 2 is in contact with the circular spline 1, and the short axis portion of the flexible spline 2 is not involved in the operation. The wave generator 3 is a flexible bearing cam type wave generator, which includes an elliptical cam 31 and a flexible bearing 32, the elliptical cam 31 is located inside the flexible bearing 32.

[0025] Referring to Figure 5, the flexible bearing 32 includes an inner ring 321, an outer ring 322, and balls 323. Both the inner ring 321 and the outer ring 322 are provided with double grooves 324, and the multiple balls 323 are arranged in the corresponding grooves 324 on the inner ring 321 and the outer ring 322. The contact angle between the direction of the force the balls 323 receive in the grooves 324 and the direction of the radial force is 10° to 20°, and the directions of the two contact angles are opposite. Specifically, in this embodiment, the contact angle between the direction of the force the balls 323 receive in the grooves 324 and the direction of the radial force is 15°. As is clear from the calculation relationship between the axial force F1, radial force F2, resultant force F, and contact angle θ, the flexible bearing 32 can withstand approximately 27% of the axial force and can withstand a predetermined axial load. Under the same manufacturing process level, the load-bearing capacity and accuracy service life of the reducer can be significantly improved, and its application fields and scope can be expanded.

[0026] Referring to Figure 5, in this embodiment, both the inner ring 321 and outer ring 322 of the bearing are integral structures. The contact angle of a typical double-row angular contact ball bearing (double groove) is determined by the difference between the distance between the two grooves of the inner ring and the distance between the two grooves of the outer ring, the shape of the inner and outer ring grooves, and the clearance of the bearing. With general machining accuracy, it is difficult to ensure uniformity of part size errors, especially groove spacing and groove shape. Therefore, double-row angular contact ball bearings, which require a certain level of precision, are designed with an integral outer ring and a two-part inner ring. A spacer is usually placed between the two inner rings, and any errors that occur after the inner rings eliminate the clearance due to axial displacement during the assembly process are mainly distance errors between the positions of the two inner ring grooves of the bearing, which are adjusted by correcting the height of the spacer. In the flexible bearing 32 of this embodiment, the balls 323 eliminate the gap in the groove 324 by radial displacement, and therefore there is no need to provide space for movement in the axial direction, and both the inner ring 321 and the outer ring 322 of the bearing are designed as a single unit. By positioning the elliptical cam 31 in the inner diameter of the inner ring 321 of the bearing, a radial force is applied, eliminating the radial and axial gaps in the groove 324 of the balls 323. By controlling the positional difference, groove shape size, and gap of the grooves 324 of the inner ring 321 and the outer ring 322 of the bearing, the balls 323 form a constant contact angle (10-20°) within the groove 324.

[0027] The ultimate purpose of using two sets of bearings in combination is to ensure there are no gaps in either the axial or radial direction between the two sets of bearings, and that the steel balls make close contact with the inner and outer ring raceways, thereby integrating the two sets of bearings. Furthermore, a line passing through the center of the steel balls and connecting the contact points between the steel balls and the outer ring raceways, and between the steel balls and the inner ring, forms a predetermined angle with respect to the radial direction of the bearing. This allows the bearing to withstand primarily radial forces while simultaneously being able to withstand predetermined axial forces (the directions of the two forces are opposite). This is shown in Figure 6 (in the figures, the upper sizes represent the distance between the outer ring grooves of the two sets of bearings, the lower sizes represent the distance between the inner ring grooves of the two sets of bearings, and the rightmost size in the lower size chain is half the difference between the distance between the outer ring grooves of the two sets of bearings and the distance between the inner ring grooves of the two sets of bearings).

[0028] To achieve the above objective, as can be seen from the mechanical geometry, the hypotenuse corresponds to the diameter of the steel ball, is fixed, and is known. Therefore, the key is the difference between opposite sides, i.e., the distance between the positions of the two sets of bearing grooves on the outer ring and the distance between the positions of the two sets of bearing grooves on the inner ring.

[0029] In the case of a single bearing, this positional difference becomes the axial height difference between the outer ring plane and the inner ring plane of the bearing after axial displacement occurs in the bearing (i.e., the outer and inner rings move in opposite axial directions). (This can also be interpreted as the axial clearance of the bearing.) In bearing terminology, this is called the overhang or recess; if the inner ring is higher than the outer ring, it is called the overhang, and if the inner ring is lower than the outer ring, it is called the recess.

[0030] Many factors affect the amount of protrusion and recess, including groove shape errors, groove radius of curvature errors, groove diameter size errors, groove position errors, and clearance errors. Due to limitations in process technology and equipment levels, strict control is difficult. In the bearing industry, to address this challenge, methods are employed to control these errors as strictly as possible during the manufacturing process, keeping them within acceptable limits. Finally, a dedicated measuring instrument is used to inspect the protrusion or recess of each set of bearings in a single completed bearing. As shown in Figure 8, two sets of selected bearings are chosen and used in combination, with the algebraic sum of the protrusion or recess being zero (the sizes shown in Figure 8 are the protrusion or recess). For example, if one set of bearings has a protrusion of 0.003 mm and the other set has a recess of 0.003 mm, using these two sets of bearings together will result in the outer ring's plane contacting the inner ring's plane simultaneously, and eliminating gaps in both the radial and axial directions, achieving the effect of using one set (two) of bearings.

[0031] The bearings targeted by this invention have extremely thin walls, making it impossible to construct them in a two-part structure. Consequently, conventional harmonic generators could only employ single-row ball bearings. However, improvements in processing technology, advancements in equipment, and the emergence of new processing inspection methods have made it possible to manufacture both the inner and outer rings of the bearing as a single integrated structure. This is shown in Figure 7 (for the sizes shown in the figure, the upper size is the distance between the two grooves on the outer ring of the bearing, the lower size is the distance between the two grooves on the inner ring of the bearing, and the rightmost size in the lower size chain is half the difference between the groove spacing of the outer ring and the groove spacing of the inner ring).

[0032] The means of realizing this invention include the following: Regarding groove shape errors, diamond rollers can currently be used for dressing grinding wheels, and the grinding shape of the groove is determined by the shape of the diamond roller, thereby ensuring a general degree of groove shape uniformity. Regarding the positional error between two grooves on the inner or outer ring, servo control technology is now mature. Existing equipment can be modified to add a set of servo control systems in the axial direction. After grinding one groove, the equipment is moved axially to the position required by the design before grinding the other groove. Current servo control accuracy can reach 0.001 mm, thus perfectly controlling the mutual difference in groove positions within 0.003 mm during batch production. Regarding the difference between the diameters of two grooves on the same wheel, an active measuring system is added to the equipment, allowing for random measurement of groove diameter size simultaneously with grinding. This technology has already been widely put into practical use in bearing machining, mainly for grinding the inner diameter of bearings. With groove grinding using this technology, it is not difficult to control the size variation to within 0.003 mm at the current process level.

[0033] Based on the technical specifications described above, by selecting the inner and outer ring groove diameters, the clearance error can be controlled to within 0.003 mm, thereby limiting the angular error of the bearing in both directions to within ±1°. This fully meets the usage standards for bearings used in combination (according to international standards, the required angular error for angular contact ball bearings used in combination is typically ±3° or less, and in the industry, it is currently generally controlled to within ±1°).

[0034] To achieve better performance of the wave generator of the harmonic reducer according to this invention, the distance error between the double grooves 324 provided in the inner ring 321 of the bearing and the distance error between the double grooves 324 provided in the outer ring 322 of the bearing must both be kept to ±1 μm or less. The groove shape error of the grooves 324 provided in the inner ring 321 and the outer ring 322 of the bearing, i.e., the groove radius of curvature error, must be less than 3 μm, and the error between the diameter sizes of the two grooves 324 provided in the inner and outer rings of the bearing must be less than 3 μm. This is because it is necessary to control the radial clearance by the combination of the groove diameter size (size from the top bottom to the bottom bottom of the groove) and the size of the ball 323, and the error in the radial clearance determines the bearing angle error.

[0035] In the wave generator of a harmonic reducer that can withstand axial loads according to the present invention, the contact angle between the direction of the force received by the ball 323 in the groove 324 and the direction of the radial force is approximately 15°. As a result, the flexible bearing 32 can withstand approximately 27% of the axial force, and at the same level of manufacturing process, it can withstand a predetermined axial load. This significantly improves the load-bearing capacity and accuracy service life of the reducer, and expands its application fields and scope. Both the inner ring 321 and the outer ring 322 of the bearing are designed as a single integrated structure. This differs from conventional double-row angular contact ball bearings, where the outer ring is integrated, the inner ring has a two-part structure, and spacers are added. As a result, the wave generator of the present invention has a simpler structure and is easier to assemble.

[0036] In this invention, by controlling the positional difference, groove shape size, and gap of the grooves 324 of the inner ring 321 and outer ring 322 of the bearing, the balls 323 form a constant contact angle within the grooves 324. By positioning the elliptical cam 31 in the inner diameter of the bearing inner ring 321, a radial force is applied, eliminating the radial and axial gaps in the grooves 324 of the balls 323, resulting in a simple overall structure.

[0037] Furthermore, although this specification is described in sections of embodiments, it should be noted that each embodiment does not necessarily comprise only one standalone technical solution. The format of this specification is merely for the purpose of concise explanation. Those skilled in the art may understand this specification as a whole and, by appropriately combining the technical solutions of each embodiment, form other embodiments that they will understand. [Explanation of Symbols]

[0038] 1-Circular spline 2-Flexible Spline 3-Wave Generator 31-Elliptical Cam 32-Flexible bearing 321 - Inner bearing ring 322 - Bearing outer ring 323-Ball 324-Groove

Claims

1. A wave generator for a harmonic reducer capable of withstanding axial loads, comprising a circular spline (1), a flexible spline (2), and a wave generator (3), wherein the wave generator (3) is located inside the flexible spline (2), the flexible spline (2) is located inside the circular spline (1), the inner ring of the circular spline (1) meshes with the outer ring of the flexible spline (2) via teeth, and the long axis portion of the flexible spline (2) is in contact with the circular spline (1), The wave generator (3) includes an elliptical cam (31) and a flexible bearing (32), the elliptical cam (31) being positioned inside the flexible bearing (32), the flexible bearing (32) including a bearing inner ring (321), a bearing outer ring (322), and balls (323), both of which are provided with double grooves (324), the plurality of balls (323) being positioned in corresponding grooves (324) on the bearing inner ring (321) and bearing outer ring (322), the contact angle between the direction of the force the balls (323) receive in the grooves (324) and the direction of the radial force is 10° to 20°, and the two contact angles are equal in value and opposite in direction, characterized in that it is a wave generator for a harmonic reducer that can withstand axial loads.

2. The wave generator of a harmonic reducer capable of withstanding an axial load, as described in claim 1, characterized in that the inner ring (321) and outer ring (322) of the bearing are both integral structures, and both the inner ring and the outer ring are provided with double grooves.

3. The wave generator of the harmonic reducer that can withstand an axial load, as described in claim 2, characterized in that the contact angle between the direction of the force acting on the ball (323) in the groove (324) and the direction of the radial force is 15°, because the spacing between the inner ring grooves and the spacing between the outer ring grooves are different.

4. The wave generator of a harmonic reducer capable of withstanding an axial load, as described in claim 3, characterized in that the ball (323) eliminates the gap in the groove (324) by radial displacement, leaving no space for axial movement.

5. The wave generator of a harmonic reducer that can withstand an axial load, as described in claim 4, is characterized in that an elliptical cam (31) is positioned in the inner diameter of the bearing inner ring (321), thereby applying a force in the radial direction, and thereby eliminating the radial and axial gaps in the groove (324) of the ball (323).

6. The wave generator of a harmonic reducer capable of withstanding an axial load, as described in 4, characterized in that the ball (323) forms a constant contact angle within the groove (324) by controlling the positional difference, groove shape size, and gap between the groove (324) of the bearing inner ring (321) and the bearing outer ring (322).

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