Harmonic Gear Drive

Abstract

A harmonic gear drive includes an input shaft and an output shaft. The input shaft is mechanically coupled to a wave generator which has a varying diameter. The wave generator is positioned within a flex spline which includes external teeth. The external teeth of the flex spline engage with internal teeth of a fixed spline and an output spline when the major diameter of the wave generator is aligned therewith. The output spline has a different number of teeth from the input spline such that, as the wave generator is rotated and the flex spline rotates, the output spline is rotated relative to the fixed spline.

1 . A harmonic gear drive comprising:
 an input shaft;   a wave generator mechanically coupled to the input shaft, the wave generator having a varying diameter, the portion of the wave generator having the largest diameter defining a major diameter of the wave generator;   a flex spline, the flex spline being generally tubular and including external teeth, the flex spline positioned about the wave generator;   a fixed spline, the fixed spline being including a first number of internal teeth that engages the external teeth of the flex spline aligned with the major diameter of the wave generator; and   an output spline, the output spline including a second number of internal teeth adapted to engage the external teeth of the flex spline aligned with the major diameter of the wave generator, the second number of internal teeth being different from the first number of internal teeth.   
     
     
         2 . The harmonic gear drive of  claim 1 , further comprising an output sub mechanically coupled to the output spline. 
     
     
         3 . The harmonic gear drive of  claim 2 , further comprising at least one bearing between the output sub and the input shaft. 
     
     
         4 . The harmonic gear drive of  claim 1 , wherein the input shaft is mechanically coupled to an output shaft of a motor. 
     
     
         5 . The harmonic gear drive of  claim 4 , wherein the motor is a mud motor or electric motor. 
     
     
         6 . The harmonic gear drive of  claim 5 , wherein the input shaft is formed as part of a rotor of the motor. 
     
     
         7 . The harmonic gear drive of  claim 4 , wherein the input shaft further comprises a transmission coupling to mechanically couple the input shaft to the output shaft of the motor. 
     
     
         8 . The harmonic gear drive of  claim 7 , wherein the transmission coupling comprises a castellation formed on the input shaft and a corresponding castellation formed on the output shaft of the motor. 
     
     
         9 . The harmonic gear drive of  claim 4 , further comprising a fixed sub, the fixed sub mechanically coupled to the fixed spline and the motor. 
     
     
         10 . The harmonic gear drive of  claim 9 , further comprising at least one bearing between the fixed sub and the input shaft. 
     
     
         11 . The harmonic gear drive of  claim 1 , wherein the wave generator is generally elliptical in cross section. 
     
     
         12 . The harmonic gear drive of  claim 1 , wherein the difference between the first number of teeth and the second number of teeth is between 1 and 10. 
     
     
         13 . The harmonic gear drive of  claim 1 , wherein the external teeth of the flex spline comprise a first set of teeth and a second set of teeth, the first set of teeth engaging the internal teeth of the fixed spline and the second set of teeth engaging the internal teeth of the output spline. 
     
     
         14 . The harmonic gear drive of  claim 1 , further comprising a needle bearing positioned between the flex spline and the wave generator. 
     
     
         15 . The harmonic gear drive of  claim 1 , wherein the input shaft, wave generator, flex spline, fixed spline, and output spline are generally tubular having a central bore formed therethrough. 
     
     
         16 . The harmonic gear drive of  claim 15 , further comprising a drive shaft extending through the central bore. 
     
     
         17 . The harmonic gear drive of  claim 1 , wherein the output sub is coupled to an RSS housing of an RSS. 
     
     
         18 . The harmonic gear drive of  claim 1 , wherein the wave generator is an eccentric cam. 
     
     
         19 . The harmonic gear drive of  claim 1 , wherein the wave generator is generally elliptical in cross section. 
     
     
         20 . A method comprising:
 providing a harmonic gear drive including:
 an input shaft; 
 a wave generator mechanically coupled to the input shaft, the wave generator having a varying diameter, the portion of the wave generator having the largest diameter defining a major diameter of the wave generator; 
 a flex spline, the flex spline being generally tubular and including external teeth, the flex spline positioned about the wave generator; 
 a fixed spline, the fixed spline being including a first number of internal teeth that engages the external teeth of the flex spline aligned with the major diameter of the wave generator; and 
 an output spline, the output spline including a second number of internal teeth adapted to engage the external teeth of the flex spline aligned with the major diameter of the wave generator, the second number of internal teeth being different from the first number of internal teeth; 
   engaging the teeth of the flex spline with the internal teeth of the fixed spline and the output spline;   elastically deforming the flex spline with the wave generator such that at least one tooth of the flex spline engages the internal teeth of the fixed spline and the output spline;   rotating the wave generator such that the flex spline is elastically deformed while remaining engaged with the internal teeth of the fixed spline and the output spline such that the output spline rotates relative to the fixed spline.   
     
     
         21 . The method of  claim 20 , wherein the input shaft is coupled to an output shaft of a motor, and the method further comprises rotating the input shaft with the motor. 
     
     
         22 . The method of  claim 21 , wherein the input shaft is coupled to the output shaft of the motor by a transmission coupling, the transmission coupling comprising a castellation formed on the input shaft and a corresponding castellation formed on the output shaft of the motor, and the method further comprises interlocking the castellations. 
     
     
         23 . The method of  claim 20 , wherein the input shaft, wave generator, flex spline, fixed spline, and output spline are generally tubular having a central bore formed therethrough. 
     
     
         24 . The method of  claim 20 , wherein the wave generator is generally elliptical in cross section. 
     
     
         25 . The method of  claim 20 , further comprising:
 providing a needle bearing between the wave generator and the flex spline; and   reducing friction between the wave generator and the flex spline as the wave generator is rotated.   
     
     
         26 . The method of  claim 20 , wherein the output spline is rotated at a speed different than the input shaft. 
     
     
         27 . The method of  claim 26 , wherein the ratio between the speed at which the output spline rotates and the speed at which the input shaft rotates is determined by the ratio of the difference in number of teeth between the output spline and the fixed spline and the number of teeth on the flex spline. 
     
     
         28 . A rotary steerable system comprising:
 a harmonic gear drive comprising:
 an input shaft; 
 a wave generator mechanically coupled to the input shaft, the wave generator having a varying diameter, the portion of the wave generator having the largest diameter defining a major diameter of the wave generator; 
 a flex spline, the flex spline being generally tubular and including external teeth, the flex spline positioned about the wave generator; 
 a fixed spline, the fixed spline being including a first number of internal teeth that engages the external teeth of the flex spline aligned with the major diameter of the wave generator; and 
 an output spline, the output spline including a second number of internal teeth adapted to engage the external teeth of the flex spline aligned with the major diameter of the wave generator, the second number of internal teeth being different from the first number of internal teeth; 
   a motor having an output shaft mechanically coupled to the input shaft;   an RSS housing mechanically coupled to the output spline; and   a drive shaft extending through an interior of the harmonic gear drive and the RSS housing.