Universal joint for downhole motor drive

A constant velocity universal joint is assembled as a crown ball meshed within a housing socket for rotation about respective rotational axes. The outside diameter of the crown ball is greater than the inside diameter of the socket. A plurality of channels, equally spaced around the crown ball perimeter are cut into the crown ball surface generally along or parallel with the crown ball drive axis. An arcuate cup is cut into each crown ball channel to confine a respective torque transfer element. A number, corresponding to the number of crown ball channels, of partial cylinder channels are cut into the inside surface of the housing socket. One of opposite side walls for each housing channel is given an arcuate radius corresponding to that of the force transfer elements. Ridges between adjacent crown ball channels mesh with ridges between adjacent housing socket channels. Torque transfer elements confined within said crown ball cups engage the partial cylinder wall of the housing channels to transfer drive forces between the crown ball and socket housing through a departure angle between the respective rotational axes.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to earthboring, in general, and specifically to constant velocity universal joints. Universal joints have general utility in the realm of power transmission as a mechanism for redirecting an axial line of rotary torque. The present invention has particular value to the industrial art of directional well drilling.

Discussion of Prior Art

One procedure for directional drilling of boreholes in the earth includes a downhole drilling motor (also called a mud motor) for rotatively driving the drill bit. Drilling motors are modern adaptions of the ancient Archimedes screw used for lifting or pumping water but is operated in reverse. To drill directionally, drilling fluid essential for rotary drilling is pumped down the central bore of a pipe string. Just prior to reaching the drill bit, the drilling fluid is directed through the drilling motor. At the uphole end of the drilling motor the Archimedes screw is used to convert fluid energy into rotating mechanical energy. The drilling fluid acts against a helically lobed shaft, known as a rotor, which rotates about its axis within a correspondingly lobed housing known as a stator. The stator along with the drill string above and drilling motor outer housings below remain stationary. Only the rotor, output drive shaft and drill bit rotate when drilling in this mode.

To directionally drill or generate arced curvature of the wellbore, the rotary drive axis of the drill bit must be deviated from the uphole axis of the drilling motor. The traditional means for changing the angular direction of the motor output drive shaft is with a mechanism characterized as a universal joint. All universal joints must transmit both compressive and torque load from the rotor/stator power section to the bearing assembly. One of the most popular universal joint mechanisms favored by the earthboring industry is that described as a “constant velocity” or CV joint.

Generally, CV joints comprise mirrored upper and lower ball and socket housing arrangements. To accommodate axial compression loads, most employ some method of spherical ball bearing or semi-spherical ball shape secured to the end of a drive shaft which fits within a socket housing having a mating, semi-spherical pocket.

To transmit torque loads, the CV joint ball and socket housing are mechanically linked by a plurality of torque transfer balls. The drive shaft ball, hereafter characterized as the “ball”, typically confines the torque transfer balls within mating cups. The cups are angularly spaced equally about the ball perimeter in the diametric plane transverse to the in-drive axis. The cup diameters are substantially the same as the torque transfer balls but less than half the hemisphere depth.

The socket housing pocket, hereafter characterized as the housing “socket”, typically contains a plurality of race channels parallel with the socket out-drive axis distributed about the internal surface of the pocket. The number of race channels must exactly match the circumferential location of the mating driveshaft ball cups and are angularly spaced equally about the pocket perimeter. The channel depth is less than a hemisphere of each ball.

As the in-drive and out-drive shafts rotate, torque transfer balls shift along the socket channels from one side of a transverse diameter plane to the other. The torque force is transferred through the torque transfer balls from the socket surface area to an axially moving arced line across a respective channel. Consequently, most of the mechanical wear on the joint occurs to the socket channels in the area of the shifting ball contact line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and “downwardly”, “upstream” and “downstream”; “above” and “below”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or other relationship as appropriate. Moreover, in the specification and appended claims, the terms “pipe”, “tube”, “tubular”, “casing”, “liner” and/or “other tubular goods” are to be interpreted and defined generically to mean any and all of such elements without limitation of industry usage.

The term “sub”, as used herein, is a drilling industry term of art for describing a segment of drill string usually having a length less than a full pipe joint and formed or constructed to perform a specific task in the drilling or well completion process.

FIG. 1provides an overall representation of the invention operating environment. The lower distal end of a deviated direction drill string traditionally comprises one or more drill collars10which are, approximately, 30 ft. lengths of pipe having an exceptionally thick annulus section. The drill collars10provide the end-biased weight upon the cutting bit at the distal end of the drill string. Theoretically, that portion of the drill string above the collars is under tensile stress.

Below the collars is a directional drilling motor12driven by a flow of circulating drilling fluid. Referring toFIG. 2, a directional drilling motor broadly comprises a power section14, a transmission assembly16, a bearing assembly18and a bit box20. Within the transmission assembly16, between the power section14and the bearing assembly18is a bent housing assembly17. Below the bent housing assembly17is a wear collar or stabilizer19.

With respect toFIG. 3, the power section14comprises a housing22and internal rotor24. The housing22has an axially developed internal bore profile that corresponds with the external helical profile of the internal rotor shaft24. Drilling fluid pumped through the housing bore between the housing and rotor shaft profiles drives rotation of the rotor shaft24about its axis of revolution. As the rotor shaft24rotates about its axis, the rotor axis also orbits about the central axis of the housing22.

The downhole end of the rotor shaft24is secured to the housing sub25of an uphole CV joint26. The uphole CV joint26transfers rotation of rotor shaft24to the transmission shaft29as it accommodates the orbit of the rotor shaft24. The downhole end of the transmission shaft29rotatively drives a second CV joint28, substantially identical to CV joint26, which transfers shaft29rotation to the bearing shaft30. The rotational axis of the bearing shaft30is determined by the bent housing17which may redirect the drive axis from the motor rotor shaft24axis by small angles up to about 3°, for example. Accordingly, both CV joints26and28accommodate an angular departure of an output rotational axis relative to the input rotational axis.

The bearing assembly18includes a bearing housing31and bearing shaft30for transfer of drilling torque and weight to the bit box20. The bearing shaft30delivers rotating torque to a drill bit (not shown) secured in the bit box20and accommodates the consequential drilling shock. The housing31secures radial alignment for the bearing shaft30and transfers the collar drilling weight to the bit.

With respect toFIG. 4, the CV joint26of the present invention broadly comprises a crown ball40and socket housing50. The crown ball40has a substantially spherical surface secured to the distal end of a transmission shaft29. The crown ball40may be an integrally forged portion of the transmission shaft29. A plurality, usually four to eight, of arced force transfer elements such as balls60mechanically link the crown ball40to the socket housing50. A thrust seat51transfers the axial thrust of the drilling fluid static and dynamic loads from the drilling motor rotor shaft24to the crown ball40.

The crown ball40, shown byFIGS. 7 through 10, is a partial sphere about a center point36that is intersected by the crown ball axis34. Normally, the crown ball axis34is coincident with the drive axis of transmission shaft29. A number of chord traversing channels41are cut into the spherical surface of crown ball40. In this example, the selected number of chord traversing channels41is six; each aligned in substantial parallelism with the axis34and distributed about the axis34in equal increments of 60°. With respect toFIG. 7and for the purpose of descriptive nomenclature, each channel41comprises a channel bottom43, a loaded side wall44and a back wall47. Between each loaded side wall44and adjacent channel back wall47is a ridge46. It is also appropriate to explain that the term “chord”, as used and intended herein, is not necessarily a linear or straight line segment between two points on the surface of a sphere. A preferred embodiment of the invention aligns the channels bottoms43substantially parallel with the crown ball axis34and consequently, parallel with the torque axis of transmission shaft29. However, the channels41may also be skewed with respect to the crown ball axis34or even arced following a substantially constant radius from the axis34. The term “chord” is used to encompass all appropriate channel configurations.

Centered in the transverse center plane (FIG. 8cutting plane IX-IX) of each crown ball40is an arced cup or pocket42cut into the bottom43and loaded side wall44of each channel41. The depressions of the cups42are cut to an arced inside radius corresponding to the outside radius61of force transfer elements60(FIGS. 4 and 6). The outside diameter45(FIG. 9) of the crown ball40as measured between diametrically opposite channel ridge crests46, is greater than the inside diameter52of the socket housing50as shown byFIG. 11. The crown ball ridge crest radius about axis34coincides with the outside diameter45. This important relationship will be further developed with respect toFIG. 6.

Referring toFIGS. 11 and 12, the joint socket housing50comprises a major inside cylindrical boring ID52about the housing axis53. Into the inside surface of the cylindrical boring, six partial-cylinder channels54are cut to an axial depth, parallel with the housing axis53, sufficient to accommodate the crown ball40OD. These partial cylinder channels54are formed to substantially the same inside arc radius as the outside arc radius61of the force transfer elements60. Those of ordinary skill will understand that there is a dimensional tolerance difference between the outside arc radius61of the force transfer elements60and the inside arc radius of the cups42(and cylinder channels54). The reference to the outside arc radius61of the force transfer elements60as being the inside arc radius of the cups42and cylinder channels54is a literary convenience. Usually, the two radii are not identical but differ dimensionally by a slight degree.

As a partial cylinder, each channel54has two opposing walls. One wall55of the radius61is the loading wall opposite from the cup42. The back wall56, diametrically opposite from the loading wall55, is a tangential expansion of the channel54for crown ball ridge46relief space58. Housing structure between the loading wall55and the back wall56forms a socket ridge57.

From the foregoing general assembly description, it will be understood that dimensions are coordinated to maximize the loading area of the arced cup42against the force transfer elements60. Similarly, the compressive line contact of the crown ball40against the cylindrical channel54is also increased. The detail ofFIG. 6illustrates a single force transfer element60and the approximate directions of load distribution as torque from the crown ball40to the socket housing50. By countless wear tests and measurements, it has been found that prior art CV joints transfer torque load by line contact between the force transfer elements and the socket housing channels by about 60° as represented by arc “B” ofFIG. 6. Although there is contact between the force transfer elements60and the cylinder loading wall54as represented by arc D, the load transferred over this arc is insignificant.

Comparatively, applicants' invention has an effective load transfer from the force transfer elements60to the housing channel loading walls54over an arc “A” of about 75° for an effective load arc increase of about 15°, or approximately 20%, as represented by arc “C”. Such an increase in the load transfer arc has resulted in a synergistic increase in operational life of the CV joint.

The meshed assembly of the crown ball ridge crests46into the socket housing relief space58allows a transfer vector between the crown ball40and the socket housing50that is more normal to the axes34and53of respective components. As a corollary to the foregoing result, the magnitude of an ineffective radial force vector (arc D) is reduced. Additionally, a greater load arc (arc A) between the crown ball60and loading wall54is made available for greater operating life.

An alternative embodiment of the invention is represented byFIGS. 13 through 17. This alternative embodiment differs from the previousFIG. 7embodiment mainly in the provision of a trough72cut into the juncture between the loaded wall44and the channel bottom43. As shown byFIG. 16, the trough profile is substantially cylindrical with a radius corresponding to that of the force transfer elements60. The trough72length should be sufficient to accommodate rolling displacement of the torque transfer ball as the crown ball70completes rotation about its axis34.

Another embodiment of the invention is represented byFIGS. 18 through 23. In a first configuration of this third embodiment, the force transfer elements are rollers84as shown byFIG. 21having circular surface formed about the axis82between relatively flat end-faces85. Distinctively, the force transfer element channels94in the crown ball that link the socket housing90to the crown ball80have relatively flat bottoms94to interface with relatively flat roller end-faces85. The rollers84are aligned in the crown ball channels92with the roller axis82normal to the crown ball torque axis34.

The third invention embodiment may also include a force transfer element in the form of a partial sphere86as illustrated byFIG. 22having spherical surfaces about axis82between relatively flat end-faces.FIG. 23illustrates a partially elliptical force transfer element.

The primary utility of the above described invention is envisioned to be as a drilling motor transmission joint. In that application reverse drive occasions are rare to non-existent. Consequently, the invention is normally expected to be designed for applications restricted to a single rotation direction. However, to a limited degree, the present CV joint is capable of reverse torque transmission. In such an event, a reverse rotation about the axes34and53will engage the back wall47of crown ball ridge46with the back wall56of housing channel54for transfer of torsional forces.

Although the invention disclosed herein has been described in terms of specified and presently preferred embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modification of the invention are contemplated which may be made without departing from the spirit of the claimed invention.