Abstract:
A harmonic drive includes a flexible gear. Also included is a ring gear that meshes with the flexible gear. Further included is a rotor. Yet further included is a wave generator sleeve directly fitted over an outer surface of the rotor, wherein the wave generator sleeve is disposed radially within the flexible gear. Also included is a bearing assembly disposed between the wave generator sleeve and the flexible gear, wherein the wave generator rotates the flexible gear as the wave generator sleeve is rotated by the rotor.

Description:
BACKGROUND OF THE INVENTION 
     The embodiments herein relate to gear systems and, more particularly, to harmonic drive gear systems, as well as a method of assembling such systems. 
     Aircraft typically include flight control surfaces on aircraft wing structures that are moved and positioned in order to alter the lift characteristics of the wing structures. Actuators are coupled to the flight control surfaces and control and guide the movement of the flight control surfaces between positions. Generally, there are two types of actuators used in aircraft: linear actuators and rotary actuators. Conventionally, a rotary actuator uses an epicyclic-type reduction gear drive, commonly referred to as a planetary gear drive, to step down high speed rotation imputed by an electric drive motor. It is also common for a rotary actuator to use a planetary gear drive with multiple stages (multiple sets of planet gears) to increase the reduction ratio and torque-to-weight ratio of the planetary gear drive. While incorporating multiple stages into the planetary gear drive increases the reduction ratio and torque-to-weight ratio of the planetary gear drive, it also undesirably increases the size, weight, and complexity of the planetary gear drive. 
     Presently, the construction of aircraft wings is moving toward a thin-winged design, where the overall thickness of the wings is decreased from previous designs. Because the thickness of the wings is being decreased, it is becoming increasingly difficult to fit a conventional rotary actuator with a planetary gear drive within the cross-section of the wings, especially when the planetary gear drive incorporates multiple stages. The diameter of the planetary gear drive can be decreased in order to fit it within the reduced wing cross-sectional area, however, the size of the teeth must also be decreased in order to maintain the high reduction ratio. Reducing the size of the teeth is undesirable because it lowers the torque-to-weight ratio of the planetary gear drive while also increasing the manufacturing tolerances and cost of the planetary gear drive. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment, a harmonic drive includes a flexible gear. Also included is a ring gear that meshes with the flexible gear. Further included is a rotor. Yet further included is a wave generator sleeve directly fitted over an outer surface of the rotor, wherein the wave generator sleeve is disposed radially within the flexible gear. Also included is a bearing assembly disposed between the wave generator sleeve and the flexible gear, wherein the wave generator rotates the flexible gear as the wave generator sleeve is rotated by the rotor. 
     According to another embodiment, a method of assembling a harmonic drive is provided. The method includes positioning a wave generator sleeve radially within a flexible gear and a bearing assembly, the flexible gear comprising radially-outward-extending teeth. The method also includes positioning a ring gear around a portion of the flexible gear such that radially-inward-extending teeth of the ring gear mesh with the radially-outward-extending teeth of the flexible gear. The method further includes directly fitting the wave generator sleeve to an outer diameter of a rotor configured to rotate the wave generator and the flexible gear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is an end view of the harmonic drive assembly; 
         FIG. 2  is an end view of the harmonic drive assembly according to section A of  FIG. 1 ; and 
         FIG. 3  is a cross-sectional view of the harmonic drive assembly according to line A-A of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-3 , a schematic diagram illustrates a harmonic drive assembly  10 . As shown, an input  12 , an output  14 , and a ground  18  are connected to the harmonic drive assembly  10 . In addition, more than one output can come out of harmonic drive assembly  10 . The harmonic drive assembly  10  also includes a first flexible gear  20 , a second flexible gear  22 , a ring gear  24 , and a wave generator sleeve  30 . An embedded stator assembly  60  is disposed radially inwardly of the input (also referred to herein as a rotor)  12  and is configured to cause rotation of the input  12 . The harmonic drive assembly  10  can further include other components and features not specifically shown or discussed. 
     The second flexible gear  22  is larger in diameter than the first flexible gear  20  and is coaxial with the first flexible gear  20 . The first flexible gear  20  and the second flexible gear  22  are connected or integrated such that the first flexible gear  20  and the second flexible gear  22  may rotate together. The wave generator sleeve  30  is disposed radially inwardly of the first flexible gear  20  and the second flexible gear  22  and engages both the first flexible gear  20  and the second flexible gear  22 , directly or indirectly. The wave generator sleeve  30  typically includes an elliptical or otherwise noncircular geometry, such as the tri-lobular geometry illustrated. 
     The wave generator sleeve  30  is directly fitted to the input  12 . The input  12  can be an output shaft (referred to herein as a “rotor”) of an electric drive motor, a hydraulic rotary drive, or other suitable torque source, and rotates the wave generator sleeve  30 . The wave generator sleeve  30  is directly fitted to an outer surface  52  of the input  12  via any suitable process that establishes a tight, fitting securement between the wave generator sleeve  30  and the input  12 . In some embodiments, the wave generator sleeve  30  is thermally fitted to the shaft in a thermal process that hardens the wave generator sleeve  30 . In some embodiments, the wave generator sleeve  30  is directly fitted to the input  12  with a high-strength adhesive. Regardless of the precise manner in which the wave generator sleeve  30  is directly fitted onto the input  12 , the wave generator sleeve  30  comprises a hardenable material that is well-suited for a hardening process that allows the wave generator sleeve  30  to support a bearing assembly  31  located radially outwardly of the wave generator sleeve  30  and radially inwardly of an outer bearing race  33 . The material of the wave generator sleeve  30  may be steel, a steel alloy, a stainless steel, or any alternative having properties similar to any of the examples provided. In some embodiments, the wave generator sleeve  30  is formed of a material similar or identical to the material of the rotor. In other embodiments, the wave generator sleeve  30  is formed of a material different than that of the rotor. 
     As the input  12  rotates the wave generator sleeve  30 , the wave generator sleeve  30  causes the flexible gear  20  to rotate in the opposite direction from wave generator sleeve  30 . 
     Advantageously, by directly fitting the wave generator sleeve  30  on the input (i.e., shaft), additional fastening and intermediate components are eliminated, thereby reducing the volume, mass, and inertia of the overall system, while increasing power density. 
     In operation, the harmonic drive assembly  10  steps down the input  12  to the output  14 . The input  12  rotates the wave generator sleeve  30  at a first rate in a first direction. The wave generator sleeve  30  rotates the flexible gear  20  inside the ring gear  24  at a second rate that is slower than the first rate, and in a second direction opposite to the first direction. For example, if the flexible gear  20  has x teeth, and the ring gear  24  has x+1 teeth meshing with the x teeth of the flexible gear  20 , then the flexible gear  20  will rotate at 1/x the rate in the opposite direction as the rate of the input  12  and the wave generator sleeve  30 . The ring gear  24  is a rigid circular ring having a plurality of radially-inwardly oriented teeth. The flexible gear  20  and the wave generator  30  are placed inside the ring gear  24 , thereby meshing the radially-outwardly oriented teeth of the flexible gear  20  and the teeth of the ring gear  24 . Because the flexible gear  20  has an elliptical or lobular shape, its teeth only mesh with the teeth of the ring gear  24  in select regions of the flexible gear  20 . As noted above, there are fewer teeth on the flexible gear  20  than there are on the ring gear  24 , so that for every full rotation of the wave generator  30 , the flexible gear  20  would be required to rotate a slight amount backwards relative to the ring gear  24 . Thus, the rotation action of the wave generator  30  results in a much slower rotation of the flexible gear  20  in the opposite direction, and a higher reduction ratio is thereby achieved. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.