Harmonic Gear Drive

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.

TECHNICAL FIELD/FIELD OF THE DISCLOSURE

The present disclosure relates generally to power transmission mechanisms and specifically to power transmission mechanisms in downhole tools.

BACKGROUND OF THE DISCLOSURE

In a wellbore, rotation of components relative to the rest of the drill string may be desired. Typically, rotation downhole is generated by a motor such as an electric motor or mud motor. However, the rotation rate of electric motors and mud motors may be too rapid for the desired rotation. When a relatively slow rotation relative to the rest of the drill string is desired, one or more transmission devices may be required.

SUMMARY

The present disclosure provides for a harmonic gear drive. The harmonic gear drive may include an input shaft. The input shaft may be generally tubular. The harmonic gear drive may include a wave generator mechanically coupled to the input shaft. The wave generator may have a varying diameter. The portion of the wave generator having the largest diameter may define a major diameter of the wave generator. The harmonic gear drive may include a flex spline. The flex spline may be generally tubular and may include external teeth. The flex spline may be adapted to be positioned about the wave generator and to be elastically flexed thereby as the wave generator is rotated. The harmonic gear drive may include a fixed spline. The fixed spline being annular in shape and including a first number of internal teeth adapted to engage the external teeth of the flex spline aligned with the major diameter of the wave generator. The harmonic gear drive may include an output spline, the output spline being annular in shape and 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 may be different from the first number of internal teeth.

DETAILED DESCRIPTION

As depicted inFIGS. 1, 2, harmonic gear drive100may mechanically couple between input shaft101and output sub103. Input shaft101may be mechanically coupled to the output shaft of a motor (not shown). The motor may, for example and without limitation, be a mud motor, electric motor, or any other motor suitable for use in a wellbore. In some embodiments, input shaft101may be mechanically coupled directly to an output shaft of the motor. In some embodiments, input shaft101may include one or more power transmission couplings that mechanically couple input shaft101to the output shaft of the motor. For example and without limitation, in some embodiments, the transmission coupling may include castellations105as depicted inFIGS. 1, 2. Castellations105may be formed in input shaft101and may link with castellations formed in the output shaft of the motor to allow rotational forces to be transmitted into input shaft101by interlocking the castellations105.

Input shaft101may be mechanically coupled to wave generator107. In some embodiments, input shaft101and wave generator107may be generally tubular, allowing a central bore to be formed therethrough. In some embodiments in which harmonic gear drive100is used as part of a downhole tool, the central bore may allow, for example and without limitation, the circulation of drilling fluid therethrough. Wave generator107may be formed as an integral part of input shaft101. In some embodiments, wave generator107may constitute an eccentric cam having a varying diameter. For example, as depicted inFIG. 3, wave generator107may be generally elliptical in cross section. As used herein, “major diameter” of wave generator107describes the portion or portions of wave generator107having the largest diameter, depicted inFIG. 3as DM. One having ordinary skill in the art with the benefit of this disclosure will understand that depending on the cross-sectional shape of wave generator107, one or more major diameters DMmay be formed. “Diameter” as used with respect to a point along the outer perimeter of wave generator107means a line measured from the point through the center to a point on the perimeter of wave generator107opposite the point.

As depicted inFIGS. 1, 2, wave generator107may be positioned within flex spline109. Flex spline109may be a generally tubular member having external teeth111. In some embodiments, flex spline109may engage with fixed spline113and output spline115. Fixed spline113and output spline115may be annular bodies. Fixed spline113may be mechanically coupled to fixed sub117such that it does not rotate relative to fixed sub117. Fixed spline113may include internal teeth114adapted to engage external teeth111of flex spline109. Output spline115may be rigidly mechanically coupled to output sub103. Output spline115may include internal teeth116adapted to engage external teeth111of flex spline109.

In some embodiments, flex spline109may elastically deform in response to the rotation of wave generator107. In some embodiments, external teeth111of flex spline109may engage internal teeth114of fixed spline113and internal teeth116output spline115where flex spline109is aligned with major diameter DMof wave generator107. As depicted inFIG. 3, external teeth111′ are engaged with internal teeth114of fixed spline113, whereas external teeth111″ are not. Wave generator107may slide within flex spline109as wave generator107is rotated. In some embodiments, needle bearing110may be positioned between wave generator107and flex spline109. Needle bearing110may include a plurality of rollers or needles positioned between the surfaces of flex spline109and wave generator107and to rotate between flex spline109and wave generator107. Needle bearing110may, for example and without limitation, reduce friction between wave generator107and flex spline109as flex spline rotates around wave generator107. As understood in the art, needle bearing110may, in some embodiments, include additional components such as races (not shown) without deviating from the scope of this disclosure. As understood in the art, the teeth of fixed spline113engaged with external teeth111of flex spline109may thus precess about internal teeth114of fixed spline113as wave generator107is rotated.

As the engaged external teeth111′ precess, flex spline109rotates relative to fixed spline113based on the difference in number of teeth between flex spline109and fixed spline113.

As described above with respect to fixed spline113, external teeth111′ (aligned with major diameter DMof wave generator107) are likewise engaged with internal teeth116of output spline115. In some embodiments, output spline115may have a different number of teeth than fixed spline113. In some embodiments, output spline115may have between 1 and 10 fewer teeth than fixed spline113. Because output spline115has a different number of teeth than fixed spline113, as flex spline109rotates within output spline115and the engaged external teeth111′ precess about the teeth of output spline115, output spline115is rotated relative to fixed spline113. The ratio between the speed at which output spline115rotates relative to fixed spline113and the speed at which input shaft101rotates may be determined by the ratio of the difference in number of teeth between output spline115and fixed spline113and the number of teeth in fixed spline113. For example, in an embodiment in which fixed spline113includes 160 teeth and output spline115includes 159, output spline115may rotate one tooth, or 1/160thof a rotation for each rotation of wave generator107. Thus, such a harmonic gear drive100may have a gear-reduction ration of 160:1 between input shaft101and output sub103. One having ordinary skill in the art with the benefit of this disclosure will understand that output spline115and fixed spline113may include any suitable number of teeth and may have any tooth differential without deviating from the scope of this disclosure.

In some embodiments, as depicted inFIGS. 1, 2, output sub103may be a generally tubular member that mechanically couples to additional equipment (not shown), allowing the additional equipment such as components of a bottom hole assembly to rotate relative to fixed sub117. In some embodiments, output sub103and fixed sub117may be adapted to support the rotation of input shaft101. In some embodiments, one or more bearings119may be positioned between input shaft101and output sub103and/or fixed sub117.

In some embodiments, fixed sub117may be mechanically coupled to fixed spline113by, for example and without limitation, one or more fasteners including linking pin121as depicted inFIG. 1. In some embodiments, output spline115may likewise be mechanically coupled to output sub103by one or more fasteners such as linking pin123.

As understood by one having ordinary skill in the art with the benefit of this disclosure, the difference in number of teeth between fixed spline113and output spline115may be limited by the need for the teeth to properly mesh with external teeth111of flex spline109. In some embodiments, flex spline109may include two sets of external teeth111, each adapted to mesh with one of the teeth of fixed spline113or output spline115. As understood in the art, external teeth111in such an embodiment may, for example and without limitation, include different tooth geometry, spacing, or numbers. In some embodiments in which different sets of external teeth111are used with fixed spline113and output spline115, fixed spline113and output spline115may have the same number of teeth, while each set of external teeth111of flex spline109includes a different number of external teeth.

In some embodiments, as depicted inFIG. 4, input shaft101′ may be formed as part of rotor201of electric motor200. Electric motor200may include outer housing203mechanically coupled to fixed sub117. Electric motor200may include stator205. Stator205, as understood in the art, may include windings207positioned to induce rotating electromagnetic fields into the interior of stator205. In some embodiments, electric motor200may be an induction motor. In such an embodiment, rotor201may include a plurality of windings adapted to cause rotation of rotor201in response to the rotating electromagnetic field induced by windings207. In some embodiments, electric motor200may be a permanent magnet motor. In such an embodiment, rotor201may include a plurality of permanent magnets positioned to cause rotation of rotor201in response to the rotating electromagnetic field induced by windings207.

In some embodiments, by forming input shaft101′ as a part of rotor201, backlash may be reduced or eliminated. In some embodiments, bearings119may be sufficient to support and/or stabilize the entire length of rotor201, allowing electric motor200to operate without additional bearings. Additionally, the overall length of harmonic gear drive100may be reduced.

In some embodiments, input shaft101and wave generator107may be formed as an integral unit. In some embodiments, input shaft101may have a wall thickness of between 3 mm and 20 mm at its narrowest point and between 5 mm and 50 mm at its widest, corresponding with the major diameter DMof wave generator107.

In some embodiments, harmonic gear drive100may be used in rotary steerable system (RSS)300, depicted schematically inFIG. 4. RSS300may include RSS housing301and other components as understood in the art. RSS housing301may be mechanically coupled to output sub103and may be rotated relative to the rest of drill string305. In some embodiments, driveshaft303may be passed through the interior of input shaft101. In such an embodiment, the diameter of driveshaft303able to be used with harmonic gear drive100may depend on the interior diameter of input shaft101. In some embodiments, by forming input shaft101as a generally thin-walled member, the diameter of driveshaft303may be maximized for a given outer diameter of harmonic gear drive100.