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CROSS-REFERENCE TO RELATED APPLICATION 
   The present invention is an improvement over the invention disclosed and claimed in my prior U.S. Pat. No. 5,941,321, issued Aug. 24, 1999 on a “METHOD AND APPARATUS FOR SHORT RADIUS DRILLING OF CURVED BOREHOLES.” 
   FIELD OF THE INVENTION 
   This invention is related to a method and apparatus for boring a hole in the earth, or in material having similar characteristics. More particularly, this invention relates to an apparatus for boring a hole having at least one non-linear segment. 
   BACKGROUND OF THE INVENTION 
   Horizontal drilling technology has come a long way in the past 20 years, and is now an accepted drilling method that has numerous benefits for the recovery of hydrocarbons. Horizontal drilling can be used as both an exploration tool and as a completion technique. The benefits of horizontal drilling when used as part of a completion method include increased drainage area, connecting fracture permeability to the well bore, and reducing drawdown pressures. There also is a strong desire in the industry to reduce the surface foot print caused by drilling activities, and horizontal drilling has proven to be an effective means of reducing the number of wells required to develop a field. 
   Horizontal drilling is critical for exploiting reservoirs that have little to no primary permeability. To achieve maximum productivity, a horizontal well can be oriented in a particular direction to maximize the number of fractures that the well intersects. By connecting fractures to a well bore, horizontal drilling has been able to turn economically unproductive reservoirs into economic successes. Vertical wells have a much lower probability than horizontal wells of repeatedly intersecting fractures, because nearly all fractures are vertically oriented. A properly placed horizontal well also has been shown to dramatically lower the drawdown pressure across the face of the well bore, and, thus, horizontal drilling also can be applied to water drive reservoirs to eliminate coning. 
   Generally, a horizontal well comprises at least three distinct segments. First, a vertical borehole extends from the surface to a desired depth beneath the surface, at which point a second, non-linear (i.e. “curved”) borehole transitions the vertical borehole to a third borehole segment (i.e. the “horizontal” segment). The orientation of the third borehole segment, though, depends upon the curvature of the second segment. Thus, the third segment is not necessarily horizontal. In principle, the curvature of the second segment can be adjusted to drill a hole to any desired subsurface location or strata. In practice, though, steering a drill bit with sufficient precision to create the desired curvature has proven difficult. 
   Typical motor-driven, bottom-hole assemblies have a bent housing that tilts the axis of the drill bit to drill a curved borehole. The orientation of the obtuse angle created by the fixed bend is known as “tool face.” The rigid bend in the drill string points the face of the drill bit in a direction that is tangential to the longitudinal axis of the drill string. But because the bent housing is a fixed part of the drill string, a curved hole can be drilled with these conventional devices only when the drill string is not rotating. Consequently, the technique that uses this type of device is commonly referred to as “slide drilling.” 
   U.S. Pat. No. 5,941,321 (issued Aug. 24, 1999) describes a “rotary steerable” drilling tool that overcomes some of the disadvantages associated with the conventional slide drilling tools, and permits significantly faster penetration rates because of better hole cleaning. The rotary steerable tool is an apparatus for drilling curved boreholes, particularly in applications that require short radius curvatures, commonly referred to in the art as an “aggressive build rate.” The rotary steerable tool of the &#39;321 patent includes a sliding tube mounted for sliding movement within the central bore of the drill pipe sub-assembly. The upper end of the sliding tube is provided with a tapered throat that makes the sliding tube responsive to pressure from fluid flowing through the drill string. Fluid pressure pushes a deflection device against the side of the borehole, urging the lower end of the drill string to be tilted away from the longitudinal axis of the borehole above the drill bit such that the drill bit will tend to drill in a direction away from the longitudinal axis of the borehole. A knuckle joint also can be included in the drill string between the rotary steerable tool and the drill bit, which can decrease the radius of curvature of a non-linear borehole. 
   While the rotary steerable tool disclosed in the &#39;321 patent overcomes many disadvantages of the conventional slide drilling procedures, there still remains room for improvement. In particular, the tapered throat on the upper end of the sliding tube restricts the flow of drilling fluid as it passes through the drill string. Such a fluid restriction can increase the pressure above the tool and adversely affect the bit hydraulics, requiring more powerful and more expensive fluid pumps to compensate for the restriction. Additionally, the rotation of the drill pipe tends to cause the eccentric sleeve of the tool to rotate within the borehole, which can cause the deflection device to collapse or steer the drill bit in an undesired direction. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a rotary steerable tool that improves the flow characteristics of drilling fluid within the tool, and improves the isolation of the tool from the rotational forces of the drill string. 
   The invention described in detail below is an improved rotary steerable tool for steering an earth-penetrating drill bit. The improved rotary steerable tool comprises an eccentric sleeve having a cylindrical bore and a piston chamber; a piston spring positioned within the piston chamber so that one end of the piston spring engages the piston chamber; a piston that engages the piston spring; a deflection pad mounted to the piston through a port in the piston chamber; a mandrel positioned in the eccentric sleeve, the mandrel having a slot that exposes a bore in the mandrel to the mandrel&#39;s external surface; a control spring positioned in the mandrel; and a control tube positioned in the coiled control spring and the mandrel so that the control spring engages the tube and exerts a force on the control tube that urges the control tube vertically downward. In response to increasing pressure of drilling fluid in the mandrel, the control tube moves upward against the force of the control spring and exposes the piston to the drilling fluid through the slot in the mandrel. In turn, the piston responds to the pressure of the drilling fluid and causes the deflection pad to move outward and engage the borehole wall. Internal bearings isolate the eccentric sleeve and the deflection pad from the mandrel, thus allowing the mandrel to rotate freely without exerting any rotational force on the eccentric sleeve. External bearing assemblies strategically placed above and below the eccentric sleeve further isolate the mandrel and the eccentric sleeve from the borehole surfaces. 
   Additionally, a guide lug fixed to the control tube engages the slot in the mandrel and an alignment sleeve mounted to the eccentric sleeve. In response to increasing pressure of drilling fluid in the mandrel, the guide lug, so fixed to the control tube, moves upwardly in the slot to a position above the tip of the alignment sleeve, so that the mandrel rotates freely. In response to subsequent decreasing pressure of drilling fluid in the mandrel, the guide lug moves downwardly and engages the alignment sleeve, so that the eccentric sleeve—mounted to the alignment sleeve—rotates to a known position with respect to the mandrel. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will be understood best by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  depicts a drill string employing the present invention on the lower end thereof; 
       FIG. 2  depicts a longitudinal view of a portion of a drill string embodying the present invention; 
       FIG. 3A  depicts a longitudinal sectional view taken along line  3 A- 3 A of  FIG. 2 ; 
       FIG. 3B  depicts a longitudinal sectional view taken along line  3 B- 3 B of  FIG. 2 ; 
       FIG. 3C  depicts a longitudinal sectional view taken along line  3 C- 3 C of  FIG. 2 ; 
     FIG.  3 A′ depicts a view similar to  FIG. 3A  showing the changed positions of certain elements as a result of an increased fluid pressure in the drill string; 
     FIG.  3 B′ depicts a view similar to  FIG. 3B  showing the changed positions of certain elements as a result of an increased fluid pressure in the drill string; 
       FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 3A ; 
       FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 3B ; 
       FIG. 6A  is a cross-sectional view taken along line  6 A- 6 A of  FIG. 3B ; 
       FIG. 6B  is a cross-sectional view taken along line  6 B- 6 B of FIG.  3 B′; 
       FIG. 7A  is a top perspective exploded view of the upper mandrel and sliding tube associated with the present invention; 
       FIG. 7B  is a top perspective exploded view of the upper external bearing assembly associated with the present invention; 
       FIG. 7C  is a top perspective exploded view of the eccentric sleeve, deflection device, and alignment mechanism associated with the present invention; and 
       FIG. 7D  is a top perspective exploded view of the lower mandrel and lower external bearing assembly associated with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As used herein, “fluid” means a source or means of supplying pressure and shall include without limitation hydraulic fluid, water, high-pressure compressed air, and similar sources of pressure. 
   Referring now to  FIG. 1 , there is shown well bore  1  comprising the vertical borehole  2 , non-linear borehole  3 , and horizontal borehole  4 , described above. Well bore  1  extends downwardly beneath the surface of the ground through numerous and varied subterranean strata, some of which may be oil-bearing. Drill string  5  extends vertically downward in well bore  1  and connects with drill pipe  16 . Drill pipe  16 , in turn, connects to the improved rotary steerable tool  10  of the present invention. 
     FIG. 2  depicts the improved rotary steerable tool  10  of the present invention, which has been modified and ported in a manner later to be described. Rotary steerable tool  10  has upper mandrel  20  with female threads  12  on one end that mate with male threads  14  on the end of a drill pipe, such as drill pipe  16 . Rotary steerable tool  10  further comprises lower mandrel  50  with male threads  52  on one end that mate with female threads  42  on the end of a second piece of drill pipe, such as drill pipe  54 . Upper mandrel  20  and lower mandrel  50  have an outer cylindrical surface that receives eccentric sleeve  32 . Drill pipes  16  and  54  (not shown in further detail) are a portion of a plurality of vertical drill pipes that have been connected together to make a semi-rigid drill string, familiar to those of ordinary skill in the art. Alternatively, drill pipe  54  can be another drill pipe sub-assembly, or a drill motor, including air-driven hammer motors and fluid-driven progressive cavity pumps (commonly known as “mud motors”). Rotary steerable tool  10  is depicted in use within borehole  1  in earth  22 , and external bearing assemblies  27  and  45  (described in detail below) isolate drill pipe  54  and rotary steerable tool  10  from borehole  1 . 
   The interior of upper mandrel  20  is hollow, forming an upper bore  24 . The end of upper bore  24  adjacent to female threads  12  is funnel shaped in the current embodiment. Alignment lug  26  is inserted into hole  56  (not shown), which communicates with upper bore  24 . Upper external bearing assembly  27  encircles upper mandrel  20 . Eccentric sleeve  32  encircles the lower end of upper mandrel  20  and the upper end of lower mandrel  50 . Deflection pad  36  rests in recess  100 . Retaining bolts  38  attach pistons  40  to the underside of deflection device  36 . The upper end of lower mandrel  50  directly below eccentric sleeve  32  has two holes  44  (only one of which is visible). Lower external bearing assembly  45  encircles lower mandrel  50 . 
     FIG. 3A  depicts the upper portion of rotary steerable tool  10 . The hollow interior of upper mandrel  20  forms part of mandrel channel  25 , which comprises upper bore  24  and lower bore  158 . The diameter of upper bore  24  is less than the diameter of lower bore  158 , so that mandrel shoulder  160  is formed where upper bore  24  meets lower bore  158  in mandrel channel  25 . Alignment lug  56  is located near the joint between rotary steerable tool  10  and upper mandrel  20 . Alignment lug  56  extends into mandrel channel  25  and is aligned vertically with deflection pad  36  (see  FIG. 2 ) so that an alignment tool lowered from the surface can engage alignment lug  56  and determine the orientation of deflection pad  36 . Control tube  60  is mounted for sliding movement within mandrel channel  25  of upper mandrel  20 , making control tube  60  responsive to pressure from fluid flowing through the drill string, as will be described hereinafter. Control tube  60  is hollow, having tube channel  63  that allows fluid to flow freely through control tube  60 . Upper portion  152  of control tube  60  is shown, with control spring  62  encircling it within lower bore  158 . One end of control spring  62  rests against mandrel shoulder  160 . O-ring  58  prevents leakage between upper portion  152  and upper mandrel  20 . In comparison to prior art devices such as the rotary steerable tool described in the &#39;321 patent, the orientation of control tube  60  improves the fluid dynamics of drilling fluid as it flows from mandrel channel  25  into tube channel  63  because the diameter of tube channel  63  is substantially the same as that of mandrel channel  25 , as seen in  FIG. 3A . There is no measurable restriction in the flow of fluid through rotary steerable tool  10 . 
     FIG. 3B  depicts the middle portion of rotary steerable tool  10 , including eccentric sleeve  32 , upper external bearing assembly  27 , and lower portion  154  of control tube  60 . Also seen in  FIG. 3B  is alignment sleeve  112 , which is fixed rigidly to the inside surface of eccentric sleeve  32 . Upper portion  152  of control tube  60  is attached to lower portion  154 , which has a larger outer diameter than upper portion  152 . The opposing end of control spring  62  rests against tube shoulder  61 , which is formed where lower portion  154  meets upper portion  152 . O-ring  96  prevents leakage between mandrel channel  25  and lower bore  158 . Lower portion  154  has hole  120  in its sidewall. Guide lug  122  is connected to hole  120  through slot  102 . Slot  102  is present in the middle portion of the sidewall of upper mandrel  20 . In the position shown in  FIG. 3B , guide lug  122  also is engaged to alignment sleeve  112  so that control tube  60 , upper mandrel  20 , lower mandrel  50 , alignment sleeve  112 , and eccentric sleeve  32  rotate as a single unit with drill pipe  16 . Slot  102  is essentially equal in width to the diameter of guide lug  122 . The outer end of guide lug  122  terminates at or near the inner surface of eccentric sleeve  32 . 
     FIG. 3B  also illustrates components of upper external bearing assembly  27 , which includes first collar  28 , first sleeve  30 , first bearing ring  68 , and second bearing ring  74 . First spacer  72  separates first bearing ring  68  from second bearing ring  74 , and all three components encircle upper mandrel  20  and are enclosed in first sleeve  30 . First collar  28  is engaged to first sleeve  30 . Second bearing ring  74  rests on retaining clip  78 . O-rings  86 ,  88 , and  90  prevent leakage between borehole  1  and the internal components of upper external bearing assembly  27 . O-rings used in rotary steerable tool  10 , including upper external bearing assembly  27 , create a substantially frictionless seal. Low-friction O-rings are available from manufacturers such as Bal Seal Engineering Co. of California. Bearing rings  68  and  74  permit upper mandrel  20  to rotate freely with respect to upper external bearing assembly  27 , thereby isolating upper mandrel  20  from borehole  1 . 
   Referring again to  FIG. 3B  for illustration, eccentric sleeve  32 , which has thick wall  34  and thin wall  98 , encircles the lower portion of upper mandrel  20  below upper external bearing assembly  27 . Eccentric sleeve  32  also encircles second spacer  84 , which is positioned between eccentric sleeve  32  and upper mandrel  20 . Bearing ring  80  also is positioned between eccentric sleeve  32  and upper mandrel  20 , above second spacer  84 . Together with bearing ring  114 , which is positioned between eccentric sleeve  32  and upper mandrel  20  below alignment sleeve  112 , bearing ring  80  provides a low-friction surface that permits upper mandrel  20  to rotate freely with respect to eccentric sleeve  32 . O-ring  92  prevents leakage between borehole  1  and bearing ring  80 , and O-ring  94  prevents leakage between mandrel channel  25  and bearing ring  80 . Thick wall  34  of eccentric sleeve  32  defines recess  100 , which could be rectangular or circular in cross-section. Deflection device  36  rests within recess  100  and is attached to pistons  40  by retaining bolts  38 , each of which pass through piston chambers in eccentric sleeve  32 . The ends of pistons  40  opposing retaining bolts  38  have a slightly larger diameter than the diameter of the body of pistons  40  themselves, thereby creating a shoulder against which piston springs  104  engage pistons  40 . O-rings  106  encircle the opposing end of pistons  40 , preventing leakage between mandrel channel  25  and the piston chambers. Piston springs  104  encircle pistons  40 , with one end resting against washers  108 , and urge pistons  40  inwardly. Retaining ring  110  secures washer  108  against piston spring  104 . 
   Alignment sleeve  112  is hollow and has sloped surface  156  encircling the lower portion of upper mandrel  20  and lower portion  154  of control tube  60 . Sloped surface  156  terminates in a tip or point, and in side elevation, appears to be generally elliptical in shape (see  FIG. 7C ). O-ring  124  prevents leakage between mandrel channel  25  and bearing ring  114 , and O-ring  126  prevents leakage between borehole  1  and bearing ring  114 . The upper portion of lower mandrel  50  has two holes  44  in its sidewall 180° apart. Holes  44  provide access to recesses  118  present in the lower portion of upper mandrel  20 . 
     FIG. 3C  depicts the lower portion of rotary steerable tool  10 . Lower mandrel  50  is hollow with its upper portion joined to the lower portion of upper mandrel  20  by male threads  142  on upper mandrel  20  and female threads  144  within lower mandrel  50 . O-ring  164  prevents leakage between borehole  1  and mandrel channel  25 . Lower external bearing assembly  45  encircles lower mandrel  50  near the joint between lower mandrel  50  and upper mandrel  20 . Lower external bearing assembly  45  is comprised of components similar to the components of upper external bearing assembly. Lower external bearing assembly includes second collar  46 , second sleeve  48 , third bearing ring  130 , and fourth bearing ring  136 . Second spacer  134  separates third bearing ring  130  from fourth bearing ring  136 , and all three components encircle lower mandrel  50  and are enclosed in second sleeve  48 . Second collar  46  is engaged to second sleeve  48 . Fourth bearing ring  136  rests on retaining clip  140 . O-ring  162  and O-ring  128  prevent leakage between borehole  1  and third bearing ring  130 . O-ring  166  prevents leakage between borehole  1  and fourth bearing ring  136 . Like bearing rings  68  and  74 , bearing rings  130  and  136  permit lower mandrel  50  to rotate with respect to lower external bearing assembly  45 , thereby isolating lower mandrel  50  from borehole  1 . Threads  52  are present on the lower portion of lower mandrel  50  to connect lower mandrel  50  to the upper portion of drill pipe  54 . 
   FIG.  3 A′ depicts the upper portion of rotary steerable tool  10  in a pressurized state. As used herein, the term “pressurized state” refers to any state in which the pressure of the fluid flowing through mandrel channel  25  is greater than the pressure that control spring  62  exerts on control tube  60 . In operation, fluid is introduced into upper bore  24  of upper mandrel  20  by drill pipe  16 . Once sufficient pressure accumulates to overcome control spring  62 , control tube  60  is pushed towards the upper portion of upper mandrel  20 , compressing control spring  62 . 
   FIG.  3 B′ also depicts a portion of rotary steerable tool  10  in a pressurized state. As lower portion  154  of control tube  60  translates upward in upper mandrel  20 , guide lug  122  in hole  120  also translates from the lower end of slot  102  to the upper end of slot  102 , and guide lug  122  disengages from alignment sleeve  112 . Moreover, as depicted in FIG.  3 B′, guide lug  122  translates beyond alignment sleeve  112  so that upper mandrel  20  and lower mandrel  50  rotate freely within alignment sleeve  112  and eccentric sleeve  32 . The upward movement of control tube  60  permits pressurized fluid to flow through slot  102  and exert pressure on pistons  40 . Once sufficient pressure is exerted on pistons  40  to overcome the resistance of piston springs  104 , piston springs  104  are compressed between the shoulders of pistons  40  and washers  108 , and deflection pad  36  is pushed out from recess  100  in thick wall  34  of eccentric sleeve  32 . At this point, deflection pad  36  will bear against the side of borehole  1 , locking eccentric sleeve  32  in a fixed lateral position against the side of borehole  1 . Deflection pad  36  pushes thin wall  98  of eccentric sleeve  32  toward the side of borehole  1  opposite deflection pad  36 , thereby causing the lower end of the drill string to tilt away from the longitudinal axis of borehole  1  above rotary steerable tool  10 . Deflection pad  36  also forces external bearing assemblies  27  (see  FIG. 3B) and 45  (see  FIG. 3C ) toward the side of borehole  1  opposite deflection pad  36 . Since external bearing assemblies  27  and  45  minimize the contact of borehole  1  with drill pipe  16  and the other components of rotary steerable tool  10 , the propensity of rotation forces collapsing deflection pad  36  is reduced in this pressurized state. Moreover, the outer surface of deflection pad  36  can be smooth or grooved, but does not require grooves to keep rotary steerable tool  10  from rotating as the drilling operation proceeds. 
   Once the back pressure dissipates, control spring  62  returns control tube  60  and guide lug  122  to the positions depicted in  FIG. 3B . Alignment sleeve  112  realigns deflection pad  36  and eccentric sleeve  32  into the positions depicted in  FIG. 3B  as well. Likewise, piston springs  104  return pistons  40  and deflection device  36  to the positions within recess  100  depicted in  FIG. 3B . The position of the components depicted in  FIG. 3C  are unaffected by the presence or absence of back pressure exerted by a fluid within upper bore  24  and lower bore  34  of rotary steerable tool  10 . 
     FIG. 4  is a cross-sectional view of the upper portion of rotary steerable tool  10  (see  FIG. 3A ) in an un-pressurized state. Deflection pad  36  resides within thick wall  34  of eccentric sleeve  32 . Eccentric sleeve  32  and first sleeve  30  isolate upper mandrel  20  from borehole  1  in earth  22 . First collar  28  is attached to first sleeve  30 . Control spring  62  encircles upper portion  152  of control tube  60 . 
     FIG. 5  is a cross-sectional view of upper mandrel  20  encircled by first sleeve  30  in an un-pressurized state. Deflection pad  36  resides within thick wall  34  of eccentric sleeve  32 . Eccentric sleeve  32  and first sleeve  30  isolate upper mandrel  20  from borehole  1  in earth  22 . Bearings  76  within second bearing ring  74  permit upper mandrel  20  to rotate with respect to first sleeve  30 . First spacer  72  separates second bearing ring  74  from first bearing ring  68  (not shown). Control spring  62  encircles upper portion  152  of control tube  60 . 
     FIG. 6A  is a cross-section of upper mandrel  20  encircled by eccentric sleeve  32  in an un-pressurized state. Deflection pad  36  resides within thick wall  34  of eccentric sleeve  32 . Retaining bolt  38  attaches deflection pad  36  to piston  40 . Piston spring  104  encircles piston  40  and has one end resting against washer  108 . Retaining ring  110  secures washer  108  against piston spring  104 , and O-ring  106  prevents leakage between piston  40  and eccentric sleeve  32 . Eccentric sleeve  32  and second sleeve  48  isolate upper mandrel  20  from borehole  1  in earth  22 . Upper mandrel  20  has slot  102  in its sidewall, which is isolated from mandrel channel  25  by control tube  60 . 
     FIG. 6B  is a cross-section of upper mandrel  20  encircled by eccentric sleeve  32  in a pressurized state. Lower portion  154  of control tube  60  (not shown) has been displaced by fluid pressure, exposing fluid in mandrel channel  25  to slot  102  and sleeve channel  33 . The fluid then exerts pressure on piston  40 , which pushes deflection pad  36  out from recess  100  in thick wall  34  of eccentric sleeve  32 . Deflection pad  36  engages one side of borehole  1  in earth  22  and urges thin wall  98  against the opposite side of borehole  1 , thereby tilting the drill string away from the longitudinal axis of borehole  1 . Retaining bolt  38  attaches deflection pad  36  to piston  40 . Piston spring  104  encircles piston  40  and has one end resting against washer  108 . O-ring  106  prevents leakage between the piston chamber and mandrel channel  25 . Eccentric sleeve  32  and second sleeve  48  isolate upper mandrel  20  from borehole  1  in earth  22 . Alignment sleeve  112  is shown partially encircling upper mandrel  20 . The translation of lower portion  154  of control tube  60  (not visible) has lifted guide lug  122  above tapered end  156  (see FIG.  3 B′), thereby permitting upper mandrel  20  to rotate freely within alignment sleeve  112  and eccentric sleeve  32 . 
     FIG. 7A  is an exploded view of upper mandrel  20  and control tube  60  associated with the present invention. Upper mandrel  20  is hollow with female threads  12  in its interior at one end and male threads  142  on the exterior of the opposing end. Hole  56  is present in its sidewall below female threads  12  for receiving alignment lug  26  (not shown), and slot  102  is present in its sidewall above male threads  142 . Hole  56  and slot  102  are vertically aligned with each other. Control tube  60  has control spring  62  encircling upper portion  152 . One end of control spring  62  rests against tube shoulder  61  on lower portion  154 , which has a larger outer diameter than upper portion  152 . Lower portion  154  has hole  120  in its sidewall for receiving guide lug  122  (not shown). Upper portion  152  is inserted into upper mandrel  20  when rotary steerable tool  10  is assembled. 
     FIG. 7B  is an exploded view of upper external bearing assembly  27  associated with the present invention. First collar  28  has male threads  66  on one end that attach to female threads  64  in one end of first sleeve  30  when rotary steerable tool  10  is assembled. First bearing ring  68  fits below first collar  28  and is separated from second bearing ring  74  by first spacer  72 . Second bearing ring  74  is separated from first sleeve  30  by retainer  78 . Third bearing ring  80  sits above spacer  84 . Third bearing ring  80  separates the upper end of eccentric sleeve  32  from upper mandrel  20  when rotary steerable tool  10  is assembled. 
     FIG. 7C  is an exploded view of eccentric sleeve  32 , deflection pad  36 , and alignment sleeve  112  associated with the present invention. Eccentric sleeve  32  is hollow with recess  100  in thick wall  34 . Eccentric sleeve  32  has thin wall  98  opposite thick wall  34 . Recess  100  receives pistons  40 , piston springs  104 , washers  108 , retaining rings  110 , and deflection pad  36  when rotary steerable tool  10  is assembled. Retaining bolts  38  attach deflection pad  36  to pistons  40 . Piston springs  104  exert pressure against the shoulder of pistons  40  to retain deflection device  36  within recess  100  when eccentric sleeve  32  is un-pressurized. 
   Alignment sleeve  112  has sloped surface  156  on one end and bearing ring  114  beneath its opposing end. Sloped surface  156  terminates in a point and has a generally elliptical shape when viewed at elevation from its side. Alignment sleeve  112  is attached to the inside of eccentric sleeve  32  by any convenient method, such as welding. Alternatively, alignment sleeve  112  and eccentric sleeve  32  can be machined as a single piece. 
     FIG. 7D  is an exploded view of lower mandrel  50  and lower external bearing assembly  45  associated with the present invention. Lower mandrel  50  is hollow with male threads  52  on the exterior of one end. The end opposite male threads  52  receives male threads  142  of upper mandrel  20  (see  FIG. 7A ) when rotary steerable tool  10  is assembled. Third collar  46  has male threads  146  on one end that attach to female threads  148  in one end of sleeve  48  when rotary steerable tool  10  is assembled. Third bearing ring  130  fits below collar  46  and is separated from fourth bearing ring  136  by spacer  134 . Fourth bearing ring  136  is separated from second sleeve  48  by retainer  140 . 
   With respect to the above description, it is to be realized that the optimum dimensional relationship for the parts of the invention, to include variations in size, materials, shape, form, manner of operation, assembly, and use are deemed readily apparent and obvious to one of ordinary skill in the art. The present invention encompasses all equivalent relationship to those illustrated in the drawings and described in the specification. The novel spirit of the present invention is still embodied by reordering or deleting some of the steps contained in this disclosure. The spirit of the invention is not meant to be limited in any way except by proper construction of the following claims.

Summary:
The invention is an improved rotary steerable tool. The improved rotary steerable tool comprises a control tube that slides vertical within a mandrel in response to changes in drilling fluid pressure, thereby opening and closing a channel between the mandrel and a piston chamber in a rotationally isolated sleeve. With the channel open, a piston in the piston chamber is exposed to the drilling fluid. When the drilling pressure is sufficient to cause the piston to move, the piston forces a deflection pad outward. After the deflection pad engages a borehole wall, any additional increases in pressure force the opposing side of sleeve toward the opposite wall, thereby tilting the direction of any attached drill bit. An optional guide lug and alignment sleeve orient the deflection pad with respect to other components.