Tapered Transitional Radial Support for Drilling Tools

Abstract

A bearing assembly for a downhole mud motor has a transitional radial support in the form of a transitionally tapered bearing mandrel. The new radial support includes a generally cylindrical bearing housing having an upper end facing upstream in the direction of the drill string and a lower end, an outer cylindrical surface and an inner cylindrical surface, and wherein a portion of the inner cylindrical surface surrounds and contains a plurality of axial bearings. The bearing mandrel underlies the bearing housing and has an outer surface which defines a lower bearing region. The lower bearing region including a series of stepped outer diameter regions that also define a transitional reduction in cross-section area for the lower bearing region of the mandrel which, in turn, allows the bearing mandrel to have a progressive reduction in bending strength and to become a more active element in bending strength of the mud motor, instead of relying primarily upon the bearing housing.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to earth drilling operations and, in particular, to an improved radial support which forms a part of the bearing section of a mud motor used for such earth drilling operations.

2. Description of the Prior Art

Mud motors are used for various drilling tasks such straight hole, horizontal and directional oilfield drilling as well as other diverse uses such as river crossings and underground utility drilling. Drilling motors utilized in drilling oil and gas wells are typically comprised of three main components:

• • 1. A power section that converts the hydraulic power from the drilling mud being pumped downhole to mechanical power in the form of rotational torque.
  • 2. A transmission that transfers the rotation that is generated in the power section, usually in the form of eccentric rotation to the bearing section.
  • 3. A bearing section which transfers the rotation generated in the power section to the drill bit or any downhole equipment installed bellow the motor. The rotation in the bearing section is concentric rather than eccentric.

Hunting Energy Services, Houston, Texas, is a leader in progressive-cavity, positive displacement mud motors. Typical sizes range from one and eleven sixteenths to nine and five eighths inches, for various applications, be it coil tubing or large hole applications. These include both straight hole and directional applications. Power sections are available in a wide range of speeds.

In a typical oil well drilling application, surface pumps are used to circulate drilling fluid to flush rock cuttings to the surface for disposal. The drilling fluid flows down through the bore of the drill string, exiting into the annulus of the well through the jets in the drill bit. The cuttings are flushed up the annulus of the well by the returning drilling fluid. The annulus pressure is substantially lower than the drill string bore pressure due to the pressure drop that occurs as the drilling fluid passes through the drill bit jets.

The typical mud motor used in these types of drilling operations is a progressive cavity positive displacement pump (PCPD). As briefly mentioned, the PCPD pump consists of a top sub, which connects the mud motor to the drill string; the power section, which consists of a rotor and stator; the transmission section, where the eccentric power from the rotor is transmitted as concentric power to the bit; the bearing assembly which carries the axial load and radial loads created on the drilling operations from the bottom sub to the outer housings of the motor; and the bottom sub which connects the mud motor to the bit or other downhole tool. These mud motors use drilling fluid (mud) to create eccentric motion in the power section of the motor which is transferred as concentric power to the drill bit. The mud motor uses different rotor and stator configurations to provide optimum performance for the desired drilling operation.

A mud motor may also be described in terms of its number of stages, lobe ratio and external diameter. Stages are the number of full twists that the stator makes from one end to the other and the lobe ratio is the number of lobes on the stator, to the number of lobes on the rotor (the stator always has one more lobe than the rotor). The operating parameters include flow rate, bit rpm and torque. The relationship between the rotor and the stator geometry determines the rotational speed and torque. The rotational speed is proportional to the flow rate and torque is proportional to the pressure drop in the fluid as it flows through the motor. These principles will be familiar to those skilled in the relevant mud motor arts.

The particular improvement toward which the present invention is directed is concerned with the previously mentioned bearing section of the assembly. The bearing section is responsible for the axial and lateral control of the assembly. Axial load is usually the weight applied to the bit (WOB) and sideloads are the result of the well tortuosity and the bend on the motor itself. The bearing section can be “sealed” or “mud lubricated.” The main difference between the two is that the sealed motors have seals to keep the environment surrounding the motor from invading the bearing region which is lubricated with oil or grease. A mud lubricated motor uses the drilling mud to lubricate and cool the bearings. Since the lubricating media is not optimal, the bearings themselves may be oversized. The present invention is concerned solely with the mud lubricated type of bearing assembly.

As will be discussed in more detail in the Detailed Description which follows, there are some shortcomings in the present design of the mud lubricated bearing sections of presently available mud motors. One aspect, in particular, relates to the shape of the bearing mandrel which supports the lower female and male bearings. The shape of the bearing mandrel is determined by the configuration of the radial support employed and by the limitations in packaging all of the required components inside a housing which has an outside diameter (OD) that is determined by the Tool Nominal Size. These constraints have resulted in bearing mandrel designs which have often had a large reduction in cross-sectional area in a region of the mandrel between the point of main load application and the point of radial support. This reduction in cross-sectional area presents a weak point in the prior designs.

Thus, despite the advances that have been made in the art of mud motors, there continues to exist a need for improvements in the component parts of such systems, such as the bearing sections of such drilling motors.

SUMMARY OF THE INVENTION

The improved bearing assembly of the invention features a “tapered transitional radial support” in the form of a bearing mandrel which provides a more transitional reduction in outside diameter (OD) and in cross-sectional area than the traditional mandrels employed in the past in the industry. The result is an improved bearing assembly which reduces the bending stress on the assembly and spreads the radial loads in a more effective manner. In addition to improving the transition of the bending moment, the improved bearing mandrel of the invention becomes a more active element on the bending strength of the assembly, rather than relying solely on the lower bearing housing. This modification also allows the use of larger diameter bearings, increasing the friction surface and reducing the surface pressure on the bearings themselves.

In one preferred form, a tapered, transitional radial support for a bearing section of a mud motor is shown where the mud motor has a top sub at one end for connection to a drill string and a bottom sub at an opposite end for connection to a drill bit section, a power section, a transmission section and a bearing section which transfers rotational power from the power section to the drill bit. The improved radial support has a generally cylindrical bearing housing having an upper end facing upstream in the direction of the drill string and a lower end, an outer cylindrical surface and an inner cylindrical surface, and wherein a portion of the inner cylindrical surface surrounds and contains a plurality of axial bearings.

An improved bearing mandrel makes up a portion of the bearing section of the assembly. A portion of the bearing mandrel underlies the plurality of axial bearings and the bearing housing. The bearing mandrel has an upper end for connection with upstream components of the mud motor , a lower end for connection to the drill bit section and an open bore therethrough. The bearing mandrel has an outer surface defined between the upper and lower ends thereof which includes a series of stepped outer diameter regions that define a transitional reduction in cross-section area and a transitional reduction in outer diameter for the mandrel. As mentioned, these features allow the bearing mandrel to become a more active element on bending strength of the mud motor, instead of relying upon the bearing housing.

A number of other features result from the changes in design of the new bearing section. The transitional reduction in cross-section of the bearing mandrel allows the use of larger diameter bearings, providing an increased friction surface and reducing surface pressure on the bearings, providing more uniform bearing wear.

In its most preferred form, the new radial support for the bearing section of a mud motor is provided with a bearing mandrel which has a given overall length which can be divided into an upper half and a lower half, and wherein there are at least two stepped outer diameter regions on the outer surface of the bearing mandrel, both of which are located in the lower half of the outer surface of the bearing mandrel. Most preferably, there is a first region of stepped outer diameter which is located above a second region of stepped outer diameter, the first region constituting a greater relative reduction in outer diameter than the second region. The second region of stepped outer diameter then continues as a constant region of cross-sectional diameter to the upper end of the bearing mandrel.

The improved radial support for the bearing section of a mud motor features a bearing assembly which includes a redesigned lower female bearing which threadedly engages a lower end of the bearing housing at one extent. The newly designed lower female bearing is intentionally designed with decreased wall thickness to accommodate a set of oversize radial bearings. Preferably, the lower female bearing is a ring-shaped body with a first cylindrically shaped outer region which steps down to a second outer cylindrical threaded region. The modified shape of the lower female bearing is designed with an internal diameter which takes advantage of the increase in diameter of the bearing mandrel, which allows a more uniform spread of load across the length of the entire lower female bearing.

In one preferred arrangement, the first cylindrically shaped outer region of the lower female bearing overlies a lower male bearing and the second outer cylindrical threaded region overlies an intermediate male bearing. The lower male bearing is of greater relative outer diameter than the outer diameter of the intermediate male bearing. Also, the first region of stepped outer diameter of the bearing mandrel forms an external shoulder on the outer surface of the bearing mandrel.

Additional objects, features and advantages will be apparent in the written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the improved bearing section of a mud motor employing the principles of the invention.

FIG. 2 is a side, cross-sectional view of a prior art bearing section of a mud motor, showing the principle components thereof and illustrating the areas which are improved by the design modifications of the invention.

FIG. 3 is a side, cross-sectional view of the bearing mandrel used in the prior art bearing assembly of FIG. 2.

FIG. 4 is a side, cross-sectional view of the improved bearing section of the invention, showing the relevant components thereof.

FIG. 5 is another side, cross-sectional view, similar to FIG. 3, but illustrating the improved bearing mandrel of the invention.

FIG. 6 is a side cross-sectional view of the female bearing which is used with the improved bearing assembly of the invention.

FIG. 7 is a perspective view, partially broken away, of a prior art mud motor of the type toward which the improvements of the present invention are directed, showing the location of the principle operative sections thereof.

DETAILED DESCRIPTION OF THE INVENTION

The preferred version of the invention presented in the following written description and the various features and advantageous details thereof are explained more fully with reference to the non-limiting examples included and as detailed in the description which follows. Descriptions of well-known components and processes and manufacturing techniques are omitted so as to not unnecessarily obscure the principal features of the invention as described herein. The examples used in the description which follows are intended merely to facilitate an understanding of ways in which the invention may be practiced and to further enable those skilled in the art to practice the invention. Accordingly, the examples should not be construed as limiting the scope of the claimed invention.

Turning first to FIG. 7, there is shown a prior art drilling mud motor, designated generally as 11, of the type toward which the improvements of the present invention are directed. There are many varieties of such tools in use in the drilling industries today, and FIG. 7 is merely intended to be generally illustrative thereof and to assist in defining the general environment of the invention. As explained briefly in the Background discussion, mud motors of this general type are used for various drilling tasks such straight hole, horizontal and directional oilfield drilling, as well as other utility drilling and related tasks. They are connected to the drill string (not shown) to rotate and steer the drill bit 13. While the description which follows will be centered around a “drill bit” such as that used for oil well drilling, it will be understood that other types of downhole tools could be attached to the mud motor and be powered by the mud motor.

As will be familiar to those skilled in the relevant arts, the typical mud motor of the type under consideration has a top sub (17 in FIG. 7) which connects the mud motor to the drill string, a power section (generally at 15 which is made up of the rotor 19 and a stator 21), a transmission section (generally at 23) where eccentric power from the rotor is transmitted as concentric power to the drill bit, a bearing assembly (generally at 25) carries the axial and radial loads created on the drilling operations from the bottom sub to the outer housings of the motor, and the bottom sub (generally at 27) which connects the drilling mud motor to the drill bit. Rotation is provided by the power section, which in this case, is a positive displacement motor that is driven by drilling fluid circulation. Axial and radial drilling loads are directed to the drill string by the bearings within the bearing assembly.

As briefly discussed, in a typical drilling application, surface pumps are used to circulate the drilling fluid to flush rock cuttings to the surface for disposal. The drilling fluid flows down through the bore of the drill string, exiting into the annulus of the well through the jets in the drill bit. The cuttings are flushed up the annulus of the well by the returning drilling fluid. A mud lubricated motor, of the type under consideration, uses the drilling mud to lubricate and cool the bearings. The present invention is concerned solely with the mud lubricated type of bearing assembly. Such an improved assembly is shown in perspective, with portions broken away, in FIG. 1 of the drawings.

In order to explain the principles involved in the improved bearing assembly of the invention illustrated in FIG. 1, it may be helpful to first review the standard bearing assembly of the type used in the prior art mud motors. Turning first to FIG. 2, a prior art bearing assembly is illustrated. In a typical prior art design, axial bearings 31 are carried on a bearing mandrel 32. The axial bearings 31are usually held under compression inside a main housing 33 which also surrounds the bearing mandrel 32. An internal shoulder 35 provides a stop and a supplementary housing 37 is threaded onto the main housing 33, pressing the bearing stack and keeping it from spinning. Lower radial support is provided by a lower female bearing or bushing 39, usually made of tungsten carbide “Tiles” or fused powder.

As has been mentioned, the shape of the bearing mandrel 32 is determined by the configuration of the radial support and the limitations that result from the requirement of packaging all the required component inside a housing which outside diameter (OD) is determined by the tool nominal size. The main disadvantage of this approach is a significant reduction in cross-sectional area of the prior art bearing mandrel 32 between the point of load application (arrow 41 in FIG. 3) i.e., where the bit is threaded or the remainder of the bore hole assembly is connected) and the point of radial support (arrows 43, 45, in FIG.3).

With further reference to FIG. 3, it will be noted that the cross-sectional area of the box end 47 decreases dramatically to a single, constant area along the remainder of the length of the bearing mandrel 32. A single lower male bearing (sleeve 39 in FIG. 2) would typically surround the bearing mandrel and sit in the region of the arrows 34, 35. In an actual example, the difference in OD between the box end 47 and the region of the arrows 43, 35, for an 8.00 inch bearing mandrel is 8.00-5.630 inches or 2.370 inches.

The advantages of the invention will now be described, primarily with respect to FIGS. 4-6. Instead of using a bearing mandrel with a dramatic reduction in cross-sectional area, as has been discussed with respect to the prior art tool of FIGS. 2-3, the improved bearing mandrel of the invention (49 in FIG. 4) presents a tapered transition radial support for the bearing assembly. In other words, there is a transitional reduction in cross-sectional area of the mandrel 49 along the length of the radial support area.. As a result, in addition to improving the transition of the bending moment, the new bearing mandrel 49 becomes a more active element on the bending strength of the assembly, instead of relying on the lower bearing housing 51. This modification allows the use of larger diameter bearings (see the lower male bearing 53 and the intermediate bearing 55 in FIG. 4), increasing its friction surface and reducing the surface pressure on the bearings. The new shape of the mandrel 49, provides a more transitional reduction in diameter, reducing the bending stress and spreading the radial loads in a more effective way. Note the arrows 75, 77, illustrating the areas where radial loads are applied in FIG. 5. As seen in empirical results, bearing wear has been more uniform with the new configuration than with the traditional geometry.

As illustrated in FIG. 4, the new bearing mandrel 49 has a given overall length “L1” which can be divided into an upper half and a lower half, and wherein there are at least two stepped outer diameter regions 67, 69, on the outer surface of the bearing mandrel. Both of these regions are located in the lower half of the outer surface of the bearing mandrel below the bisecting line 68. Most preferably, there is a first region of stepped outer diameter which is located above a second region of stepped outer diameter, the first region constituting a greater relative reduction in outer diameter than the second region. The second region of stepped outer diameter then continues as a constant region of cross-sectional diameter to the upper end of the bearing mandrel.

The lower male bearing 53 and the intermediate bearing 55 are sleeve like cylinders which surround a portion of the outer surface 57 of the new bearing mandrel. They are separated by a cross-over piece 59. The intermediate bearing 55 is situated between the cross-over piece 59 and a catch ring 61. The upper male bearing 53 is situated between the cross-over piece 59 and a thrust ring 63 which abuts an external shoulder 65 of the bearing mandrel. It will be appreciated that there are now two lower male bearings, rather than the single male bearing 39 of the prior art device. The tapered transitional nature of the new radial support made up by the new bearing mandrel can be appreciated with respect to the isolated view of the mandrel 49 shown in FIG. 5.

Note the stepped outer surface regions 67, 69 located below the shoulder 71 of the box connection 73, which in this case is a 6.625 API Regulation Box having an outer diameter of 8.000 inches. In an actual example, the difference in OD between the box end 73 and the region of the first arrows 75 for an 8.50 inch bearing mandrel is 8.000-5.893 inches or 2.107 inches, as compared to the difference in OD of the prior art example, which was 2.370 inches. The difference in OD between the region of the first arrows 75 and the region of the second arrows 77 is 5.893-5.003 inches, or 0.890 inches.

The lower male bearing 53 and the intermediate bearing 55 are generally cylindrical sleeve like bodies that are mode of metal carbides, typically tungsten carbide. Since tools used in conjunction with the drilling of oil and gas wells are subject to considerable abrasion and wear during use, metal carbides are used to form a bearing or wear surface for downhole tools because of their desirable properties of hardness, toughness and wear resistance. For example, these radial bearings may have a wear surface which is comprised of a steel support which is inlaid with a layer of solid rectangular tungsten carbide “Tiles”, surrounded by a powered tungsten carbide matrix. This type bearing surface has been found to provide maximum protection against wear while providing the superior durability necessary for extreme applications.

As will be appreciated from FIGS. 4, 5 and 6, to accommodate the oversized radial bearings (53 and 55 in FIG. 4) and the new shape of the bearing mandrel 49 itself, a specially designed female bearing 79 was required. The female bearing 79, as shown in FIG. 6 has a generally cylindrical outer surface which steps down to an externally threaded end region 83. The end region 83 threadedly engages a mating internally threaded surface of the outer housing 51 (see FIG. 4). The internal profile of the female bearing 79 also varies in diameter between a region of greater relative diameter 85 and a tapered region of lesser relative internal diameter 87. The newly designed female bearing 49 overlies the lower male bearing 53 at one end and overlies the intermediate male bearing 55 at the opposite end (see FIG. 4).

The new profile of the female bearing 79 takes advantage of the available space to increase the diameter of the lower radial support. The decrease in wall thickness and mechanical strength of this part itself is compensated by the increase in strength of the bearing mandrel 49 that allows a more uniform spread of the load across the length of the female bearing 79.

The remaining components of the bearing assembly are of conventional design. The bearing housing 51 overlays the axial bearing assembly 89 which, in this case, is a series of ball bearings. The assembly terminates in a flow restrictor/upper female bearing 91 and a flow diverter/upper radial bearing 93, in the example shown.

An invention has been provided with several advantages. The improved bearing assembly features oversized radial (male) bearings which can be formed with tungsten carbide Tiles. There are two sets of lower radial bearings instead of one. The maximized diameter of lower radial support allows for a progressive mandrel cross-sectional diameter transition. The dual radial bearing design improves the mitigation of bending stress moments through the bearing pack. The new bearing mandrel design allows the mandrel to become a more active element on the bending strength of the assembly. The modification in design allows the use of larger diameter bearings, increasing its friction surface and reducing the surface pressure on the bearings.

While the invention has been shown in only one of its forms, it will be appreciated that it is not thus limited, but is susceptible to various changes and modifications without departing from the spirit thereof.

Claims

1. A tapered, transitional lower radial support for a bearing section of a mud motor, where the mud motor has a top sub at one end for connection to a drill string and a bottom sub at an opposite end for connection to a downhole tool such as a drill bit, the mud motor also having a power section, a transmission section and a bearing section which transfers rotational power from the power section to the drill bit, the radial support comprising:

a generally cylindrical bearing housing having an upper end facing upstream in the direction of the drill string and a lower end, an outer generally cylindrical surface and an inner generally cylindrical surface, and wherein a portion of the inner generally cylindrical surface surrounds and contains a plurality of axial bearings;

a bearing mandrel, a portion of which underlies the axial bearings and the bearing housing, the bearing mandrel having an upper end for connection with upstream components of the mud motor, a lower end for connection to the drill bit and an open bore therethrough, the bearing mandrel having an outer surface defined between the upper and lower ends thereof which defines a lower bearing region for the bearing mandrel, the lower bearing region including a series of stepped outer diameter regions that also define a transitional reduction in cross-section area for the lower bearing region of the mandrel which, in turn, allows the bearing mandrel to become a more active element in bending strength of the mud motor, instead of relying primarily upon the bearing housing.

2. The lower radial support for the bearing section of a mud motor of claim 1, wherein there are multiple regions of transitional reduction in cross-section area in the lower bearing region of the bearing mandrel, each of which carries a spaced-apart lower male radial bearing, the transitional reduction in cross-sectional area allowing for the use of larger diameter bearings, providing an increased friction surface and reducing surface pressure on the bearings, providing more uniform bearing wear.

3. The radial support for the bearing section of a mud motor of claim 1, wherein the bearing mandrel has a given overall length which can be divided into an upper half and a lower half, and wherein there are multiple regions of transitional reduction in cross-sectional area in the lower half of the bearing mandrel.

4. The radial support for the bearing section of a mud motor of claim 3, wherein the lower bearing region of the bearing mandrel includes a lower female bearing with both a lower male bearing and a spaced-apart intermediate male bearing being received within the lower female bearing.

5. The radial support for the bearing section of a mud motor of claim 4, wherein there is a first region of stepped outer diameter on the bearing mandrel which is located above a second region of stepped outer diameter, the first region allowing a progressive reduction in diameter to the second region.

6. The radial support for the bearing section of a mud motor of claim 5, wherein the second region of stepped outer diameter may or may not continue as a generally constant region of cross-sectional diameter to the upper end of the bearing mandrel.

7. The radial support for the bearing section of a mud motor of claim 1, wherein the bearing assembly includes a lower female bearing which threadedly engages a lower end of the bearing housing, the lower female bearing being designed with a relatively constant wall thickness to accommodate oversize radial bearings following an outer profile dictated by the lower female bearing's outer profile.

8. The radial support for the bearing section of a mud motor of claim 7, wherein the lower female bearing is a ring-shaped body with a first cylindrically shaped outer region which steps down to a second outer cylindrical threaded region.

9. The radial support for the bearing section of a mud motor of claim 8, wherein the modified shape of the lower female bearing is designed with an internal diameter which takes advantage of the increase in diameter of the bearing mandrel, which allows a more uniform spread of load across the length of the entire lower female bearing.

10. The radial support for the bearing section of a mud motor of claim 9, wherein the first cylindrically shaped outer region of the lower female bearing overlies a lower male bearing and the second outer cylindrical threaded region overlies an intermediate male bearing.

11. The radial support for the bearing section of a mud motor of claim 10, wherein the lower male bearing is of greater relative outer diameter than the outer diameter of the intermediate male bearing.

12. A radial support for a bearing section of a mud motor, where the mud motor has a top sub at one end for connection to a drill string and a bottom sub for connection to a drill bit, a power section including a rotor and a stator, a transmission section where eccentric power from the rotor is converted and transmitted as concentric power and a bearing section which transfers rotational power from the transmission to the drill bit, the radial support comprising:

a generally cylindrical bearing housing having an upper end facing upstream in the direction of the drill string and a lower end, an outer generally cylindrical surface and an inner generally cylindrical surface, and wherein a portion of the inner generally cylindrical surface surrounds and contains a plurality of axial bearings;

a bearing mandrel, a portion of which underlies the axial bearings and the bearing housing, the bearing mandrel having an upper end for connection with upstream components of the mud motor, a lower end for connection to the drill bit and an open bore therethrough, the bearing mandrel having an outer surface defined between the upper and lower ends thereof which defines a lower bearing region for the bearing mandrel, the lower bearing region including a series of stepped outer diameter regions that also define a transitional reduction in cross-section area for the lower bearing region of the mandrel which, in turn, allows the bearing mandrel to become a more active element in bending strength of the mud motor, instead of relying primarily upon the bearing housing;

wherein the outer surface of the bearing mandrel has a given overall length which can be divided into an upper half and a lower half, and wherein there are two stepped outer diameter regions of the outer surface of the bearing mandrel, both of which are located in the lower half of the outer surface of the bearing mandrel, wherein there is a first region of stepped outer diameter which is located above a second region of stepped outer diameter, the first region constituting a greater relative reduction in outer diameter than the second region; and

wherein a set of lower male bearings are carried within the lower bearing region on the respective stepped outer diameter regions.

13. A radial support for a bearing section of a mud motor, where the mud motor has a top sub at one end for connection to a drill string and a bottom sub for connection to a drill bit, a power section including a rotor and a stator, a transmission section where eccentric power from the rotor is converted and transmitted as concentric power and a bearing section which transfers rotational power from the transmission to the drill bit, the radial support comprising:

a generally cylindrical bearing housing having an upper end facing upstream in the direction of the drill string and a lower end, an outer cylindrical surface and an inner cylindrical surface, and wherein a portion of the inner cylindrical surface surrounds and contains a plurality of axial bearings;

a bearing mandrel, a portion of which underlies the axial bearings and the bearing housing, the bearing mandrel having an upper end for connection with upstream components of the mud motor, a lower end for connection to the drill bit and an open bore therethrough, the bearing mandrel having an outer surface defined between the upper and lower ends thereof which defines a lower bearing region for the bearing mandrel, the lower bearing region including a series of stepped outer diameter regions that also define a transitional reduction in cross-section area for the lower bearing region of the mandrel which, in turn, allows the bearing mandrel to become a more active element in bending strength of the mud motor, instead of relying primarily upon the bearing housing;

wherein the outer surface of the bearing mandrel has a given overall length which can be divided into an upper half and a lower half, and wherein there are stepped outer diameter regions of the outer surface of the bearing mandrel, all of which are located in the lower half of the outer surface of the bearing mandrel;

wherein a set of lower male bearings are carried within the lower bearing region on the respective stepped outer diameter regions; and

wherein the bearing assembly includes a lower female bearing which threadedly engages a lower end of the bearing housing, the lower female bearing being designed with a wall thickness designed to accommodate oversize radial bearings.

14. The radial support for the bearing section of a mud motor of claim 13, wherein the lower female bearing is a ring-shaped body with a first cylindrically shaped outer region which steps down to a second outer cylindrical threaded region.

15. The radial support for the bearing section of a mud motor of claim 14, wherein the modified shape of the lower female bearing is designed with an internal diameter which takes advantage of the increase in diameter of the bearing mandrel, which allows a more uniform spread of load across the length of the entire lower female bearing and a more uniform bending stress distribution on the mandrel.

16. The radial support for the bearing section of a mud motor of claim 15, wherein the first cylindrically shaped outer region of the lower female bearing overlies a lower male bearing and the second outer cylindrical threaded region overlies an intermediate male bearing.

17. The radial support for the bearing section of a mud motor of claim 16, wherein the lower male bearing is of greater relative outer diameter than the outer diameter of the intermediate male bearing.