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CLAIM OF PRIORITY 
     This application is a U.S. National Stage of PCT/US2013/040544 filed on Jan. May 10, 2013. 
     TECHNICAL FIELD 
     The present disclosure relates to systems, assemblies, and methods for a gearbox for a tool string positionable in a wellbore. 
     BACKGROUND 
     In oil and gas exploration, it is important to provide rotational energy to downhole tools connected to a drill string. In some implementations, a drilling rig located at or above the surface rotates a drill string disposed in the wellbore below the surface. The surface equipment on the drilling rig rotates the drill string and the drill bit as it bores into the Earth&#39;s crust to form a wellbore. In some implementations, the drill string may include a downhole power section (e.g., positive displacement mud motor) that includes a stator and a rotor that are rotated and transfer torque to a drill bit or other downhole equipment (referred to generally as the “tool string”) disposed below the downhole power section. In some implementations, both surface and downhole sources of rotations are used (e.g., the surface equipment rotates the stator connected to the drill string, and the rotor of the positive displacement downhole motor is rotated due to a fluid pressure differential of the pumped drilling fluid flowing across the power section relative to the stator. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of a drilling rig and downhole equipment including a downhole power section and a gearbox disposed in a wellbore. 
         FIG. 2A  is a cutaway profile view of an example gearbox positionable in a wellbore. 
         FIG. 2B  is a partial perspective view of an example gearbox. 
         FIG. 3  is a perspective view of an example planetary gear system. 
         FIG. 4  is a flow chart of an example process for driving a gear system of a tool string positionable in a wellbore. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , in general, a drilling rig  10  located at or above the surface  12  rotates a drill string  20  disposed in a wellbore  60  below the surface. The drill string  20  typically includes a power section  22  of a downhole positive displacement motor (e.g., a Moineau type motor), which includes a stator  24  and a rotor  26  that are rotated and transfer torque down the borehole to a drill bit  50  or other downhole equipment  40  (referred to generally as the “tool string”) attached to a longitudinal output shaft  45  of the downhole positive displacement motor. The wellbore  60  is reinforced by a casing  34  and a cement sheath  32  in the annulus between the casing  34  and the borehole. During the normal operation, the surface equipment  14  rotates the stator  24  which is connected to the drill string  20 , and the rotor  26  of the power section is rotated relative to the stator  24  due to a pumped drilling fluid flowing through the power section  22  (e.g., positive displacement mud motor). Rotation of the rotor  26  rotates an output shaft  102 , which is used to energize components of the tool string  40  disposed below the power section. 
     Energy generated by a rotating shaft in a downhole power section can be used to drive a variety of downhole tool functions. Components of the tool string  40  may be energized by mechanical (e.g., rotational) energy, electrical power, fluid (e.g., hydraulic) power, or other energy that can be converted from the rotation of the rotor. However, the rotation rate of such a shaft is often either too fast or too slow to directly drive a given downhole function. By inserting a gearbox  100  between the output shaft  102  and an output shaft  45  which powers the function to be driven, the rate of rotation can be altered for the driven function, thereby improving overall performance of the function. In some cases, altering the driving rotation rate can allow for a reduction in component size. 
     In some downhole embodiments, the output shaft  102  rotates at a rate that is substantially higher than a desired rotation rate for a tool component. For example, the output shaft  102  may rotate at  120  revolutions per minute or RPM, while a desired rotation rate of a rotary steerable tool face control sleeve may be 2 RPM. In such embodiments, the gearbox  100  may include gearing adapted to provide reduced rotational speed relative to the output shaft  102  rotation rate. 
     In some downhole embodiments, the output shaft  102  rotates at a rate that is substantially slower than a desired rotation rate for a tool component. For example, the output shaft  102  may rotate at 120 RPM, while a hydraulic pump can operate at a much higher rate of rotation, such as 5000 RPM. In such embodiments, the gearbox  100  may include gearing adapted to provide increased rotational speed relative to the output shaft  102  rotation rate. 
       FIGS. 2A and 2B  are cutaway profile and partial perspective views of an example downhole gearbox  200 . In some implementations, the gearbox  200  can be the gearbox  100  of  FIG. 1 . The gearbox  200  includes a housing assembly  202  and a rotatable drive shaft  205  substantially oriented along a longitudinal axis  207 . In some implementations, the drive shaft  205  can be rotated by the output shaft  102  of the rotor  26  about the axis  207 . 
     About the periphery of the drive shaft  205  is a first magnetic coupler  210 . The first magnetic coupler  210  includes a collection of magnets arranged about the periphery of the drive shaft  205 . A second magnetic coupler  215 , located substantially adjacent to the first magnetic coupler  210 , also includes a collection of magnets arranged such that the second magnetic coupler  215  is magnetically coupled to the first magnetic coupler  210  across a substantially non-magnetic membrane  220 . Rotation of the drive shaft  205  rotates the first magnetic coupler  210 , which in turn urges rotation of the second magnetic coupler  215 . 
     In some implementations, the membrane  220  may provide protection from contaminants found downhole. For example, the drive shaft  205  and/or the first magnetic coupler  210  may be exposed to drilling fluid, water, formations cuttings, and/or other contaminants in the drilling fluid. The membrane  220  allows rotating magnetic flux from the first magnetic coupler  210  to pass and urge rotation of the second magnetic coupler  215  while preventing downhole contaminants from reaching the second magnetic coupler  215  or other components of the example gearbox  200  that will be described below. 
     The second magnetic coupler  215  is rotationally coupled to a drive sleeve  225 . The drive sleeve  225  is a substantially cylindrical assembly that can rotate about the axis  207  in a cavity  227  formed within the housing assembly  202  and protected from downhole contaminants by the membrane  220 . 
     The drive sleeve  225  includes a collection of gear teeth  230  radiating outward from the outer periphery of the drive sleeve  225 . The gear teeth  230  are formed to mate with a collection of gear teeth  237  and drive the rotation of a spur gear  235  located within a cavity  240  of the housing assembly  202 . The spur gear  235  includes an output shaft  239  that also rotates as the spur gear  235  is driven. 
     The example downhole gearbox  200  also includes a planetary gearbox assembly  260 , e.g., on epicyclic gear assembly, located within a cavity  270  of the housing assembly  202 . The planetary gearbox assembly  260  includes a housing  264  and an input shaft  262  is rotationally coupled to the output shaft  239  by a jointed drive shaft  250 . The jointed drive shaft  250  includes an input section  252  rotationally coupled to the output shaft  239 , a middle section  254 , an output section  256  rotationally coupled  10  the input shaft  262 , a universal joint  258   a  rotationally coupling the input section with the middle section, and a universal joint  258   b  rotationally coupling the middle section  254  with the output section  256 . In some embodiments, the jointed drive shaft  250  may rotationally couple the spur gear  235  to the planetary gearbox assembly  260  across a flexible joint. For example, the planetary gearbox assembly  260  may be installed in a steerable tool head, across an articulated joint from where the spur gear  235  is located. In use, the jointed drive shaft  250  can transmit rotational energy from the spur gear  235  to the planetary gear box assembly  260  as movement of the articulated joint causes the axes of the output shaft  239  and the input shaft  262  to become offset or angled relative to each other. 
     Referring now to  FIG. 3 , an example planetary gearbox  300  is illustrated. In some embodiments, the planetary gearbox  300  can be included in the planetary gearbox assembly  260 . The planetary gearboxes  300  include a sun gear  310 , a collection of planet gears  320  on a planet gear carrier  330 , and a ring gear  340 . The planet gear carrier  330  is rotationally coupled to and is driven by the input shaft  262 . The sun gear  310  is rotationally coupled to and drives an output shaft  350 . In general, when the planet gear carrier  330  is rotated by the input shaft  262  and the ring gear  340  is held stationary, the planet gears  320  will revolve around the sun gear  310 . The revolution of the planet gears  320  rotates the sun gear  310  and the output shaft  350  at a rotational speed that is higher than that of the input shaft  262  and the planet gear carrier  330 . In the example of the planetary gearbox assembly  260 , the ring gear  340  is formed about the interior of the housing  264 , which is held substantially stationary. When the input shaft  262  is rotated, the planetary gearbox  300  increases rotational speed at the output shaft  350 . In some embodiments, multiple stages of planetary gearboxes can be used for higher speed reduction. 
     Referring back to  FIGS. 2A and 2B , the rotation of the output shaft  350  (not visible in these views) drives the rotation of an output assembly  280 . In some implementations, the output assembly  280  can be a fluid pump. For example, rotation of the drive shaft  205  at a first speed, e.g., about 120 RPM, can be transferred through the magnetic couplers  210 ,  215 , the spur gear  235 , and the planetary gearbox assembly  260  to spin a fluid pump at a second, generally higher speed, e.g., 5000 RPM. The fluid pump can provide pressurized fluids for use by downhole fluid actuators, e.g., hydraulic actuators. 
     In some implementations, the output assembly  280  can be an electrical generator. For example, rotation of the drive shaft  205  at a first speed, e.g., about 120 RPM, can be transferred through the magnetic couplers  210 ,  215 , the spur gear  235 , and the planetary gearbox assembly  260  to spin a generator at a second, generally higher speed to produce electrical energy that can be used to drive downhole electronics and electrical components. 
     In some implementations, the planetary gearbox assembly  260  may be configured to reduce the speed of the input shaft  262  and provide the reduced rotational speed through the output shaft  350 . For example, the planetary gearbox  300  may accept rotational energy at the output shaft  350  to drive the sun gear  310  while the ring gear  340  is held substantially stationary. Rotation of the sun gear  310  drives the revolution of the planet gears  320  about the sun gear  310 , which in turn drives the rotation of the planet gear carrier  330 . Rotation of the planet gear carrier  330  at an input speed drives the rotation of the outputshaft  350  at a speed that is reduced compared to the input speed. In such implementations, a rotational speed can be reduced. For example, the planetary gearbox assembly  260  can be configured for speed reduction, and can be used as a speed reducer to manipulate downhole tool faces and offset magnitudes. 
     In another example, the planetary gearbox assembly  260  can used to increase the amount of torque being provided from a rotational input. For example the planetary gearbox assembly  260 , as depicted in  FIG. 2 , is indirectly coupled to the drive shaft  250  through the magnetic couplers  210  and  215 . In some example magnetic couplers, the amount of torque that can be transferred can be limited due to the non-contacting nature of magnetic couplers. Implementations of the planetary gearbox assembly  260  as a speed reducer can help by amplifying the torque provided through the magnetic coupler to a rotational load. In some embodiments, multiple stages can be used for higher speed reduction. 
       FIG. 4  is a flow chart of an example process  400  for driving a downhole tool with a downhole gearbox. In some implementations, the process  400  can be performed using the example downhole gearbox  200  of  FIGS. 2A and 2B . 
     At  410 , an input source of rotation is provided. For example, the rotor  26  of  FIG. 1  can be rotated to provide the input source of rotation. 
     At  420 , a spur gear is provided. For example, the spur gear  235  of  FIGS. 2A and 2B  of the example downhole gearbox  200  can be provided. 
     At  430 , the spur gear is driven with the source of input rotation at a first rotational speed. For example, the rotation of the rotor  26  can be connected, directly or indirectly, to the spur gear  235  to transfer rotation of the rotor  26  to the spur gear  235 . 
     In some implementations, the spur gear may be driven with a magnetic coupling driven by the source of input rotation. In some implementations, the magnetic coupling can include a first magnetic rotor, a second magnetic rotor having gear teeth formed to engage with and drive the spur gear, a gap between the first magnetic rotor and the second magnetic rotor, and a non-magnetic membrane partly filling the gap. In some implementations, driving the spur gear with the magnetic coupling can include driving a first magnetic rotor with the source of input rotation to rotate a field of magnetic flux, and transferring rotation of the first magnetic rotor to the second magnetic rotor across the gap by the field of magnetic flux. For example, the magnetic coupler  210  of the example gearbox  200  may be coupled to a source of input rotation, such as the rotor  26 . The magnetic coupler  210  will rotate a magnetic field which can penetrate the membrane  220  to urge rotation of the magnetic coupler  215 . 
     At  440 , a gearbox assembly is provided. The gearbox assembly includes a central sun gear, a collection of planet gears disposed in a planet gear carrier that is disposed about the central sun gear, and a ring gear disposed around the planet gears and the planet gear carrier. For example, the example planetary gearbox  300  of  FIG. 3  may be provided. In some implementations, the plurality of planet gears can include at least six planet gears. 
     At  450 , a downhole tool is driven with an output of the gearbox at a second rotational speed. For example, the spur gear  235  can drive the input shaft  262  of the gearbox  300 , and the output shaft  350  can drive the output assembly  280  at the same or a different speed relative to that of the rotor  26 . 
     In some implementations, driving the downhole tool can include driving a downhole fluid pump at a second rotational speed greater than a first rotational speed of the rotational input. In some implementations, driving the downhole tool can include driving a downhole electrical generator at a second rotational speed greater than a first rotational speed of the rotational input. 
     In some implementations, driving the gearbox with an output of the spur gear can include driving the central sun gear with the output of the spur gear, driving the plurality of planet gears about the ring gear with the sun gear, driving a planet gear carrier with the planet gears, and driving the output of the gearbox with an output of the planet gear carrier. For example, the input shaft  262  of the example gearbox  300  can be driven at a first speed, and the gearbox  300  can transfer the rotation to the output shaft  350  at another speed greater, less than, or equal to the input speed. 
     In some implementations, driving the gearbox can include rotating an output shaft connected at a first end to the spur gear. The shaft can be coupled to a rotatable attachment structure at a second end, rotating the rotatable attachment structure coupled an input shaft of the gearbox. In some implementations, the rotatable attachment structure can be a universal joint. For example, the universal joint  250  can transfer rotation of the spur gear  235  to the input shaft  262 . 
     Although a few implementations have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.

Summary:
The subject matter of this specification can be embodied in, among other things, a tool string positionable in a wellbore that includes a spur gear driven by a source of input rotation, a planetary gearbox having an attachment structure to connect to the spur gear. The gearbox includes a central sun gear, a collection of planet gears disposed in a planet gear carrier disposed around the central sun gear, and a ring gear disposed around the planet gears and planet gear carrier. The attachment structure drives a hydraulic pump or electrical generator at a rotational speed different than a rotational speed of the source of input rotation.