Abstract:
A harmonic drive includes a circular spline, a flexspline meshed with the circular spline, and a wave generator abutted against the flexspline. Through a special parameter design to correct the perimeter curve of the wave generator, the meshing efficiency between the circular spline and the flexspline is increased, thereby improving the transmission accuracy and reducing the average load.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to speed reducing gear technology, and more particularly, to a harmonic drive that achieves a high meshing efficiency. 
         [0003]    2. Description of the Related Art 
         [0004]    Harmonic drive is a high-ratio speed reducer. A conventional harmonic drive generally comprises a circular spline, a flexspline rotatably mounted within the circular spline, and a wave generator rotatably mounted within the flexspline, wherein the wave generator is an elliptical member. When the wave generator is driven to rotate by a power source, the flexspline will be pushed to deform by the outer perimeter of the wave generator, causing the circular spline to mesh with the flexspline in the major axis of the wave generator and to be disengaged from the flexspline in the minor axis of the wave generator. Due to a difference in the number of teeth between the circular spline and the flexspline, a high speed reduction ratio will be achieved to provide a high torque output after the wave generator is been continuously rotated. 
         [0005]    Thus, the higher the meshing efficiency between the circular spline and the flexspline is, the better the overall transmission accuracy and the lower the average load of the teeth will be. However, the meshing efficiency between the circular spline and the flexspline depends on the change in curvature between the major axis and minor axis of the wave generator. In order to optimize the change in curvature between the major axis and minor axis of the wave generator, Japanese Patent Nos. 4067037 and 5256249 disclose a measure of correcting the curvatures of the major axis and minor axis of a wave generator. However, the correction equation used in the aforesaid prior art patents is complicated, further, the effect of the correction is not as good as expected. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention has been accomplished under the circumstances in view. It is the main object of the present invention to provide a harmonic drive, which uses a simple parameter design to achieve the effects of improving the meshing efficiency and transmission precision and reducing the average load of the teeth. 
         [0007]    To achieve this and other objects of the present invention, a harmonic drive comprises a circular spline, a flexspline, and a wave generator. The circular spline comprises an inner annular toothed portion. The flexspline is rotatably mounted within the circular spline, comprising an outer annular toothed portion meshed with the inner annular toothed portion of the circular spline. The wave generator is rotatably mounted within the flexspline, comprising an elliptical outer perimeter abutted against an inner perimeter of the flexspline. The radius of curvature of the elliptical outer perimeter of the wave generator is defined as r·r=+√x 2 +y 2 , the relationship between x and y satisfying the elliptical parametric equation: x={a +C a ×(sin (4θ−(π/2))+1)}×sin θ, y={b+C b ×(sin (4θ−(π/2))+1)}×sin θ, 0≦θ≦2π, wherein a is the semi-major axis of the elliptical outer perimeter of said wave generator; C a  is the semi-major axis correction factor; b is the semi-minor axis of the elliptical outer perimeter of said wave generator; C b  is the semi-minor axis correction factor; θ is the eccentric angle of the elliptical outer perimeter of said wave generator. 
         [0008]    Thus, during the operation of the wave generator to rotate the flexspline relative to the circular spline after the correction of the curvature of the outer perimeter of the wave generator, the number of teeth of mesh between the outer annular toothed portion of the flexspline and the inner annular toothed portion of the circular spline is increased to achieve a high meshing efficiency and a high level of transmission accuracy of the whole structure and to reduce the average load of the teeth. 
         [0009]    Other advantages and features of the present invention will be fully understood by reference to the following specification in conjunction with the accompanying drawings, in which like reference signs denote like components of structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic structural view of a harmonic drive in accordance with the present invention. 
           [0011]      FIG. 2  is a schematic drawing illustrating the correction of the curvature of the wave generator in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to  FIG. 1 , a harmonic drive  10  in accordance with the present invention comprises a circular spline  20 , a flexspline  30 , and a wave generator  40 . 
         [0013]    The circular spline  20  comprises an inner annular toothed portion  22 . The flexspline  30  is mounted within the circular spline  20 , comprising an outer annular toothed portion  32  facing toward the inner annular toothed portion  22  of the circular spline  20 . It is to be noted that the number of teeth of the inner annular toothed portion  22  of the circular spline  20  is 2 more than the number of teeth of the outer annular toothed portion  32  of the flexspline  30 . Further, the circular spline  20  and the flexspline  30  have a same modulus therebetween. The modulus referred to therein is the quotient obtained by dividing the gear pitch diameter by the number of teeth. 
         [0014]    The wave generator  40  is mounted within the flexspline  30 , comprising an elliptical outer perimeter  42 . When the wave generator  40  is driven to rotate by a power source (not shown), the flexspline  30  will be pushed and deformed by the outer perimeter  42  of the wave generator  40 , causing the inner annular toothed portion  22  of the circular spline  20  to be completely meshed with the outer annular toothed portion  32  of the flexspline  30  in the major axis direction of the wave generator  40  and completely disengaged from the outer annular toothed portion  32  of the flexspline  30  in the minor axis direction of the wave generator  40 . Thus, the circular spline  20  can be rotated by the flexspline  30  to achieve the effect of torque output. 
         [0015]    Referring to  FIG. 2 , before correcting the outer perimeter  42  of the wave generator  40 , obtain the initial radius of curvature r 0  of the outer perimeter  42  of the wave generator  40  by equation (1) r 0 =√(a sin θ) 2 +(b sin θ) 2 , 0≦θ≦2π in which a: the semi-major axis of the outer perimeter  42  of the wave generator  40 ; b: the semi-minor axis of the outer perimeter  42  of the wave generator  4 ; θ: the eccentric angle of the outer perimeter  42  of the wave generator  40 . Thereafter, obtain the initial perimeter S 0  of the outer perimeter  42  of the wave generator  4  by equation (2) S 0 =∫f 0   2π √φ θ (r 0 ) 2 +r 0   2    
         [0016]    In correction, obtain the corrected perimeter S of the outer perimeter  42  of the wave generator  4  by equation (3) E S =S−S 0 =0.1 m˜0.8 m, in which E s : the variable quantity of the outer perimeter  42  of the wave generator  40  before/after correction; m: modulus of the circular spline  20  or flexspline  30 . Thereafter, apply equation (4) S=∫ 0   2π √φ θ (r) 2 +r 2  to obtain the corrected radius of curvature r of the outer perimeter  42  of the wave generator  40 , and then apply equation (5) to obtain the relationship between x and y. The coordinate (x, y) of any point at the outer perimeter  42  of the wave generator  40  after the correction must satisfy the following elliptical parametric equation: x={a +C a ×(sin (4θ−(n/2))+1)}×sin θ, y={b+C b ×(sin (4θ−(n/2))+1)}×sin θ, 0≦θ≦2π, wherein C a  is the semi-major axis correction factor; C b  is the semi-minor axis correction factor. Thus, the relationship between C a  and C b  can be obtained through equation (5) and the aforesaid elliptical parametric equation, and then the relationship between C a  and C b  can be used to correct the outer perimeter  42  of the wave generator  40  to the optical elliptic curve. 
         [0017]    Thus, during the operation of the wave generator  40  to rotate the flexspline  30  relative to the circular spline  20  after the correction of the curvature of the outer perimeter  42  of the wave generator  40 , engaging and disengaging frequency between the outer annular toothed portion  32  of the flexspline  30  and the inner annular toothed portion  22  of the circular spline  20  is increased, thereby increasing the number of teeth in mesh, and thus, the harmonic drive can achieve a high meshing efficiency and a high level of transmission accuracy and can also reduce the average load of the teeth.