Patent Publication Number: US-2019186599-A1

Title: Circular spline for harmonic reducer, harmonic reducer and robot

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase entry of PCT Application No. PCT/CN2017/098460, filed Aug. 22, 2017, which is based upon and claims priority to Chinese Patent Application No. 201610799174.8, filed Aug. 31, 2016, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a harmonic reducer, in particular to a circular spline for a harmonic reducer, a harmonic reducer and a robot which incorporates the harmonic reducer. 
     BACKGROUND 
     Harmonic gear driving is a new type of driving which emerged with the development of space technology in the late 1950s, and belongs to generally in the family of planetary gear driving with small tooth difference. A harmonic reducer mainly consists of three basic components: a circular spline with an internal gear ring, a flexspline with an external gear ring and a wave generator. As a reducer, generally, the wave generator is configured as an input (active), the circular spline is fixed, and the flexspline is configured as an output. The wave generator is a rod-shaped component, and rolling bearings are arranged at both ends of the wave generator to form rolling wheels, which are mutually pressed with an inner wall of the flexspline through a flexible bearing. The flexspline is a thin-walled gear which can produce large elastic deformation. The wave generator is a component which can make the flexspline produce controllable elastic deformation. After the wave generator is installed in the flexspline, a section of the flexspline is forced to change from an original circular shape to an irregular curved surface through the flexible bearing. The teeth near the two ends of a major axis of the flexspline fully engage with the teeth of the circular spline, while the teeth near the two ends of a minor axis of the flexspline completely disengage from the circular spline. The teeth on other sections of the circumference of the flexspline are in a transitional state of engagement and disengagement. When the wave generator rotates continuously, the deformation of the flexspline changes constantly, so that the engaging state of the flexspline and the circular spline also changes constantly, from engaging-in to engaging to engaging-out to disengaging to engaging-in again, in cycles, thus allowing the slow rotation of the flexspline relative to the circular spline opposite to the wave generator. 
     Compared with common gear driving methods, harmonic gear driving has the advantages of high carrying capacity, large drive ratio, small size, light weight, high precision, low noise, etc., is also characterized in simple structure, small number of parts and easy installation, and is capable of transmitting motion and power to confined spaces, thus being widely used in electronics, aerospace, robots and other industries. 
     Although harmonic reducers have the plurality of advantages mentioned above, a traditional harmonic reducer still has the following defects: first, a circular spline of the traditional harmonic reducer is usually thick on the whole, has good rigidity and does not deform easily, which makes only the gear teeth at a major axis of a wave generator in an engaging state, resulting in low driving precision; and second, the gear teeth of the traditional harmonic reducer are made of metal materials, in order to meet the requirements of assembly and lubrication, a certain side clearance is left between circular spline teeth and flexspline teeth, and with the aggravation of gear wear, the side clearance gradually increases, causing a backlash of the harmonic reducer to increase, affecting the driving precision and driving stability of a harmonic gear, and even causing the harmonic reducer to fail. 
     SUMMARY 
     One aspect of the present disclosure is to provide a circular spline for a harmonic reducer. The circular spline includes a flange, a rigid ring and a flexible ring which are coaxially arranged, where the rigid ring is axially positioned between the flange and the flexible ring, and an inner peripheral surface of the flexible ring is provided with first gear teeth. 
     According to one exemplary embodiment, the outer diameter of the rigid ring is larger than the outer diameter of the flexible ring. 
     According to one exemplary embodiment, the outer diameter of the rigid ring is larger than the maximum outer diameter of the deformed flexible ring. 
     According to one exemplary embodiment, the rigid ring and the flexible ring are made of different materials. 
     According to one exemplary embodiment, the rigid ring and the flexible ring are made of the same material. 
     According to one exemplary embodiment, the wall thickness of the rigid ring is larger than the wall thickness of the flexible ring. 
     Another aspect of exemplary embodiments is to provide a harmonic reducer with higher driving precision. 
     In order to achieve the above objective, an exemplary embodiment provides a harmonic reducer including a wave generator, the circular spline as described above and a flexspline, where the wave generator is arranged on an inner peripheral surface of the flexspline, second gear teeth are arranged on an outer peripheral surface of the flexspline, and the first gear teeth engage with the second gear teeth. 
     According to one exemplary embodiment, a circular pitch of the first gear teeth is smaller than a circular pitch of the second gear teeth. 
     According to one exemplary embodiment, at least one of the first gear teeth and the second gear teeth is made of engineering plastic. 
     According to one exemplary embodiment, the engineering plastic is polyoxymethylene (POM), polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) or polyamide (PA). 
     According to one exemplary embodiment, the first gear teeth and the second gear teeth are both trapezoidal teeth. 
     According to one exemplary embodiment, the tooth addendum of the trapezoidal teeth is ha=(0.75-0.85)*m, the tooth dedendum is hf=(0.9-1.1)*m, and the tooth profile angle Φ is 20°-30°, where m is the modulus of the trapezoidal teeth. 
     According to one exemplary embodiment, the first gear teeth have a first tooth crest fillet and a first tooth root fillet, and the second gear teeth has a second tooth crest fillet and a second tooth root fillet. 
     According to one exemplary embodiment, the wave generator includes an oval cam and a flexible bearing, where the flexible bearing is arranged along a contour line of the cam. 
     In the harmonic reducer disclosed by an exemplary embodiment, the circular spline has a rigid structure except for a flexible and easily deformable structure at the part where the gear teeth are arranged. Compared with a “circular spline” (hereinafter referred to as “flexible circular spline”) in which the whole cylinder is flexible, the cylinder torsional deformation of the circular spline adopted by the exemplary embodiment is smaller, and no backlash occurs during torsion like the “flexible circular spline”. The backlash of the harmonic reducer disclosed by the present disclosure only comes from the backlash generated by the torsional deformation of a cylinder of the flexspline, thus having high driving precision. 
     An exemplary embodiment also provides a robot including the harmonic reducer as described above. 
     Other features and advantages of the present disclosure will be described in detail in the detailed description section that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are used to provide a further understanding of the described exemplary embodiments and constitute a part of this specification, and are used, together with the following specific implementations, to explain the present disclosure, but do not constitute limitations to the present disclosure. In the accompanying drawings: 
         FIG. 1  is a cutaway view of a harmonic reducer of an exemplary embodiment; 
         FIG. 2  is a sectional view of trapezoidal teeth used in an exemplary embodiment; 
         FIG. 3  and  FIG. 4  are schematic diagrams of engagement of gear teeth before and after deformation of a flexible ring respectively according to an exemplary embodiment; 
         FIG. 5  is an enlarged view of part A in  FIG. 3 ; and 
         FIG. 6  is an enlarged view of part B in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments are described in detail below with reference to the accompanying drawings. It should be understood that the specific implementations described herein are merely used to describe and explain the present disclosure rather than limiting the present disclosure. 
     As shown in  FIG. 1 , according to an exemplary embodiment, a circular spline  12  for a harmonic reducer is provided which comprises three parts which are coaxially arranged: a flange  122 , a rigid ring  123  and a flexible ring  124 . The rigid ring  123  is positioned between the flange  122  and the flexible ring  124  in an axial direction of the circular spline  12 . The rigid ring  123  is disposed on an end face of the flange  122  for installation and positioning of the harmonic reducer. The flexible ring  124  is disposed on an end face of the rigid ring  123 , and an inner peripheral surface of the flexible ring  124  is provided with first gear teeth  121 . 
     The “ring” in the above-mentioned rigid ring and flexible ring refers to the ring described below, and the length of a wall of the ring in an axial direction of the ring is greater than the thickness of the wall of the ring in a radial direction of the ring. However, the present disclosure is not limited thereto, and the length of the wall of the ring may be less than or equal to the thickness of the wall of the ring. 
     In an exemplary embodiment, the circular spline  12  is flexible and easily deformable only at the part where the gear teeth are arranged, and the rest (i.e., the flange  122  and the rigid ring  123 ) is rigid. The outer diameter of the rigid ring  123  is larger than the outer diameter of the flexible ring  124 , and in order to avoid interference with the deformation of the flexible ring  124 , the outer diameter of the rigid ring  123  is larger than the maximum size of the deformed flexible ring  124 . 
     In an exemplary embodiment, the rigid ring  123  and the flexible ring  124  may be integrally formed of the same material or may be made of two different materials respectively, as long as the two have different rigidities. When the rigid ring  123  and the flexible ring  124  are made of the same material, as shown in  FIG. 1 , the wall thickness of the flexible ring  124  may be smaller than that of the rigid ring  123 . The flexible ring  124  has a small wall thickness, thus tending to deform in the radial direction. The rigid ring  123  has a large wall thickness which ensures torsional deformation thereof does not happen. 
     As shown in  FIG. 1 , according to another exemplary embodiment, a harmonic reducer is provided, including a wave generator  11 , a circular spline  12 , and a flexspline  13 . The wave generator  11  includes an oval cam  111  and a flexible bearing  112 , where the flexible bearing  112  is arranged along a contour line of the cam  111 . The circular spline  12  includes three parts which are coaxially arranged: a flange  122 , a rigid ring  123  and a flexible ring  124 . The rigid ring  123  is positioned between the flange  122  and the flexible ring  124  in an axial direction of the circular spline  12 . The rigid ring  123  is disposed on an end face of the flange  122  for installation and positioning of the harmonic reducer. The flexible ring  124  is disposed on an end face of the rigid ring  123 , and an inner peripheral surface of the flexible ring  124  is provided with first gear teeth  121 . An outer peripheral surface of the flexspline  13  is provided with second gear teeth  131 , and the first gear teeth  121  and the second gear teeth  131  engage with each other. The flexible bearing  112  is disposed on an inner peripheral surface of the flexspline  13 . 
     In the harmonic reducer of an exemplary embodiment, a circular pitch of the first gear teeth  121  is slightly smaller than a circular pitch of the second gear teeth  131 , therefore, when the flexible ring  124  of the circular spline  12  of the present disclosure is in an assembled state, the first gear teeth  121  thereon are pressed by the second gear teeth  131  of the flexspline  13  in a major axis direction of the wave generator  11 , so that the flexible ring  124  extends outwards along with the flexspline  13  in the major axis direction of the wave generator  11 , and shortens inwards in a minor axis direction of the wave generator  11 , thereby forming an approximately oval ring shape (as shown in  FIG. 4 ). As a result, not only the gear teeth at the major axis of the oval but also some gear teeth between the minor axis and the major axis of the oval will engage, so that more gear teeth engage, and more teeth can bear load, making engagement more robust and driving precision higher. 
     In an exemplary embodiment of the harmonic reducer the circular spline  12  is flexible and easily deformable only at the part where the gear teeth are arranged, and the rest (i.e., the flange  122  and the rigid ring  123 ) is rigid. Compared with a “circular spline” (hereinafter referred to as “flexible circular spline”) in which the whole cylinder is flexible, the cylinder torsional deformation of the circular spline  12  in an exemplary embodiment is smaller, and no backlash occurs during torsion like the “flexible circular spline”. The backlash of the harmonic reducer disclosed by the exemplary embodiment only comes from the backlash generated by the torsional deformation of a cylinder of the flexspline  13 , thus having higher driving precision. 
     In the circular spline  12  of an exemplary embodiment, the rigid ring  123  is used for positioning during installation of the harmonic reducer. Specifically, for example, when the harmonic reducer is installed on a robot body, an outer peripheral surface of the rigid ring  123  is matched with a wall of a mounting hole in the robot body, so as to realize radial positioning of the harmonic reducer. Therefore, in an exemplary embodiment, an outer diameter of the rigid ring  123  is larger than an outer diameter of the flexible ring  124 . In order to avoid interference with the deformation of the flexible ring  124 , the outer diameter of the rigid ring  123  is larger than the maximum size of the deformed flexible ring  124 , that is, the outer diameter of the rigid ring  123  is larger than the outer diameter at the major axis of the approximately oval ring formed by deformation of the flexible ring  124 . 
     In the harmonic reducer of an exemplary embodiment, the rigid ring  123  and the flexible ring  124  may be integrally formed of the same material or may be made of two different materials respectively, as long as the two have different rigidities. When the rigid ring  123  and the flexible ring  124  are made of the same material, as shown in  FIG. 1 , the wall thickness of the flexible ring  124  may be smaller than that of the rigid ring  123 . For example, the flexible ring  124  has a small wall thickness, thus tending to deform in the radial direction and the rigid ring  123  has a large wall thickness which ensures torsional deformation does not happen. 
     In an exemplary embodiment, at least one of the first gear teeth  121  and the second gear teeth  131  may be made of engineering plastic with a self-lubricating property, so that there is no need to leave a side clearance between the first gear teeth  121  and the second gear teeth  131  for lubrication, and the driving precision and the driving stability of the harmonic reducer are improved. The first gear teeth  121  and the second gear teeth  131  may both be made of engineering plastic. In exemplary embodiments, the engineering plastic may be selected from polyoxymethylene (POM), polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) or polyamide (PA). 
     As shown in  FIG. 2 , in an exemplary embodiment, the first gear teeth  121  and the second gear teeth  131  are both trapezoidal teeth. As shown in  FIG. 3  to  FIG. 6 , since the circular pitch of the first gear teeth  121  is slightly smaller than the circular pitch of the second gear teeth  131 , theoretically there is interference between the first gear teeth  121  and the second gear teeth  131  when the flexible ring  124  having the first gear teeth  121  is not deformed (as shown in  FIG. 5 ). However, in the illustrated exemplary embodiment, the flexible ring  124  of the circular spline  12  is deformable. When the first gear teeth  121  are pressed by the second gear teeth  131 , the flexible ring  124  changes from a circular shape to an approximately oval shape (as shown in  FIG. 4 ), so that the first gear teeth  121  and the second gear teeth  131  are in a zero-side clearance engaging state (as shown in  FIG. 6 ). Since the initial state of the flexible ring  124  is circular, there is a resilience force for the flexible ring  124  which results in a tendency to return to a circular shape. Therefore, when the first gear teeth  121  or the second gear teeth  131  are worn, the flexible ring  124  rebounds to make the first gear teeth  121  thereon always attached to the second gear teeth  131 . However, in the above exemplary embodiment, the first gear teeth  121  and the second gear teeth  131  both adopt the same isosceles trapezoid cross section which gradually expands from the top to the root, so that the first gear teeth  121  and the second gear teeth  131  form bilateral engagement, and the resilience force of the flexible ring  124  can ensure that the side clearance between the gear teeth is zero at all times, thereby further reducing the backlash of the harmonic reducer and improving driving precision. 
     In the exemplary embodiment shown in  FIG. 2 , the tooth addendum of the trapezoidal teeth is ha=(0.75-0.85)*m, the tooth dedendum is hf=(0.9-1.1)*m, and the tooth profile angle Φ is 20°-30°, where m is the modulus of the trapezoidal teeth. The tooth addendum ha is the radial distance between a reference circle d and a tooth crest circle, and the tooth dedendum hf is the radial distance between the reference circle d and a tooth root circle. By using trapezoidal teeth within the above parameter scope as the first gear teeth  121  and the second gear teeth  131 , the side clearance between teeth can be reduced. 
     As shown in  FIG. 2 , the first gear tooth  121  has a first tooth crest fillet  1211  and a first tooth root fillet  1212 , and the second gear tooth  131  has a second tooth crest fillet  1311  and a second tooth root fillet  1312 . The first tooth crest fillet  1211  and the second tooth crest fillet  1311  can ensure smooth engaging-in and engaging-out of the gear tooth. The first tooth root fillet  1212  and the second tooth root fillet  1312  can improve the stress condition received by the gear teeth and reduce stress concentration. 
     To sum up, a part of the circular spline  12  of an exemplary embodiment is the deformable flexible ring  124  with the gear teeth. Combined with the characteristics of trapezoidal gear teeth, it can be ensured that the side clearance between the gear teeth can always be zero even after the gear teeth are worn. The rigid ring  123  of the circular spline  12  for installation and positioning has sufficient rigidity and will not generate backlash during torsion like the “flexible circular spline”. Compared with the prior art, the harmonic reducer disclosed by the present disclosure can automatically adjust the side clearance between the teeth and ensure that the side clearance between the teeth is zero at all times, thereby reducing the backlash of the harmonic reducer and improving driving precision. 
     According to yet another exemplary embodiment, a robot is provided, and the robot adopts the harmonic reducer as described above. 
     Although exemplary embodiments of the present disclosure are described in detail above with reference to the accompanying drawings, the present disclosure is not limited to specific details in the foregoing implementations. Various simple variations can be made to the technical solutions of the present disclosure within the scope of the technical idea of the present invention, and such simple variations all fall within the protection scope of the present disclosure. 
     It should be further noted that the specific technical features described in the foregoing exemplary embodiments can be combined in any appropriate manner provided that no conflict occurs. To avoid unnecessary repetition, various possible combination manners are not further described in the present disclosure. 
     In addition, various different implementations of the present disclosure may alternatively be combined randomly. Such combinations should also be considered as the content disclosed in the present disclosure provided that these combinations do not depart from the concept of the present disclosure.