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BACKGROUND OF THE INVENTION 
     The invention generally relates to rotary drilling heads for the oil industry and more particularly to a rotary drilling head that includes a diamond enhanced bearing assembly which can be retrieved through the rotary table of the drill rig and which increases rotational drilling speeds and lengthens service intervals. 
     Referring initially to FIG. 1, there is shown a conventional rig  10  for rotating a drill bit  12  on the end of a drill string  14  for drilling a well bore  16 . The drilling rig  10  includes a rotary table  18  located on the floor  20  of rig  10  for transmitting torque to the drill string  14 . The drill string  14  extends through a blowout preventer (“BOP”) stack located beneath the rig floor  20  and includes a kelly  22  at its upper end and a plurality of drill pipes  24  including a plurality of drill collars  26  connected at it lower end to the drill bit  12 . The drill string  14  transmits rotational and axial movements to the drill bit  12  for drilling the well bore  16 . 
     Referring now additionally to FIGS. 2 and 3, there is shown a typical kelly  22  having threaded rotary shouldered connections  28  at its top and bottom and a center section  30  with a polygonal outer cross section. The rotary table  18  includes a clearance hole, typically 17.5″ or 22.5″ in diameter, for housing a drive bushing that corresponds to the polygonal geometry  30  of kelly  22  for applying torque to kelly  22 . Kelly  22  in turn transmits torque to the drill string  14  and bit  12  at the bottom of well bore  16 . 
     Drilling fluids, often referred to as drilling mud, are pumped downward through the flowbore of the drill string  14  under high pressure, through drill bit  12  and then returns upwardly via the annulus  44  formed between well bore  16  and drill string  14  to remove the cuttings to the surface. The returning mixture of drilling fluids and cuttings is diverted beneath the rig floor  20  to a mud reservoir by means of a device commonly referred to in the industry as a rotary drilling head assembly  46 . 
     A rotary drilling head assembly  46  is typically mounted below the floor  20  of the drilling rig  10  on the top of the BOP stack to redirect the drilling fluid returning from the well bore  16  and to allow rotation and deployment of the drill string  14  through the rotary table  18 . During normal drilling operations, the blowout preventers are maintained in the “open” position, leaving only the rotary drilling head to divert the returning pressurized drilling fluids away from the rig  10 . 
     FIG. 2 illustrates a typical prior art rotary drilling head assembly  46  having an outer stationary housing or bowl  48  and an inner drive ring  50  with a bearing assembly  52  disposed in between allowing drive ring  50  to rotate within bowl  48 . Outer bowl  48  includes a flange  54  for mounting the assembly  46  to the BOP stack and a flow diverter port or outlet  56  having a flange  58  for the attachment of a pipe extending to the mud reservoir. Assembly  46  further includes an inner stripper assembly  60  slidably received within drive ring  50  and connected to the upper end of drive ring  50  by a retaining clamp  62  allowing stripper assembly  60  to rotate with inner drive ring  50 . Stripper assembly  60  includes an outer housing  66  bonded by a rubber insert  68  to inner drive bushing  32 . The lower end of outer housing  66  is bolted to a flange  64  which is bonded onto stripper rubber  42 . A primary non-rotary seal  70  and a secondary non-rotary seal  72  serve to statically seal the outside of stripper assembly  60  from bearing assembly  52  and rig floor  20 . Bearing assembly  52  includes an upper set of roller bearings  74  and a lower set of roller bearings  76 . Upper and lower roller bearings  74 ,  76 , respectively, are separated axially by a bearing spacer  78 . An external pressurized oil system lubricates the bearings  74 ,  76  through hydraulic quick connects  80 , and is maintained by rotary lubrication bearing seal members  82  above and below the bearing assembly  52 . Bearing seal members  82  are stationary even while there is full 360° rotation of stripper assembly  60  and drive ring  50  within outer bowl  48 . Since the clamp assembly clamps the rotating side of the bearing assembly, the clamp assembly must also rotate. 
     The rotary drilling head assembly  46  counteracts forces due to the upward pressure from the returning drilling fluids, the radial wobble of the drill string  14 , and the downward engagement forces of drill string  14 . The bearing assembly  52  of a conventional drilling head assembly  46  includes tapered roller bearings to enable rotation of the drive ring  50  with respect to the outer bowl  48  and to overcome these various forces. Previous designs utilize two horizontally opposed tapered roller bearings  74 ,  76  spaced apart axially to handle the loads encountered during drilling operations, as shown in FIG.  2 . Because the design of tapered roller bearings allows them to counteract loads in both the thrust and radial directions, the lower set of bearings  76  encounters the upward annular fluid forces and radial wobble forces simultaneously, while the upper set of bearings  74  encounters the downward drill string and radial wobble forces. This arrangement allows radial and axial forces to be countered regardless of the direction that they may be acting upon rotary drilling head  46 . 
     During operation, individual sections of drill pipe  24  are connected to the upper end of drill string  14  with their upper end attached to the lower end of kelly  22 . The new section of drill pipe  24  is then lowered through the stripper assembly  60 . As the rotary table  18  rotates, rotary table  18  rotates kelly  22  and thus kelly bushing  34  disposed within drive bushing  32  and around kelly  22 . As shown in FIG. 3, drive bushing  34  includes an inside cutout geometry  36 , an outside geometry  38 , and a split cut  40 . Inside geometry  36  corresponds to polygonal section  30  of kelly  22 , and outside geometry  38  corresponds to a drive bushing seat  32  of a stripper assembly  60  hereinafter described. Split cut  40  facilitates the assembly and disassembly of drive bushing  34  about kelly  22 . Drive bushing  34  is slidably engaged both about polygonal section  30  of kelly  22  and within the corresponding geometry of drive bushing seat  32 . Kelly bushing  34  thereby allows kelly  22  to pass through the rotary drilling head  46  while also transmitting torque from the rotary table  18  to the drill string  14  and stripper assembly  60  of the drilling head  46  simultaneously. 
     Stripper rubber  42  seals with drill string  14  as the drill string  14  moves axially through stripper assembly  60 . Kelly  22 , drill pipes  24 , and threaded pipe connections  28  therebetween may be of many different sizes and shapes and yet must pass through stripper rubber  42 . Therefore, the stripper rubber  42  of rotary drilling head assembly  46  must be flexible to sealingly engage and accommodate the various sizes of the components of drill string  14 . Rubber stripper  42  also diverts the drilling mud through side port outlet  56  of drilling head  46  in maintaining the sealing engagement with drill string  14 . 
     From time to time the stripper assembly  60  must be removed to replace the stripper rubber  42 . This requires disconnecting the retaining clamp  62  to release outer housing  66  of stripper assembly  60 . When the outer housing  66  is larger than the opening through the rotary table  18 , the stripper assembly  60  must be removed from beneath the rig floor  20  which is expensive. 
     Further, when service intervals dictate, the bearing assembly  52  must be replaced. This requires that the drilling head assembly  46  be dismantled and the bearing assembly  52  lifted out of outer bowl  48 . This is done by removing bearing retaining screws  84  that secure bearing assembly  52  to outer barrel  48 . Once removed, bearing assembly  52  can be inspected, replaced or repaired if no longer functioning properly. To prevent disrupting operations with time consuming disassembly procedures, the clearance diameters of rotary table  18  and any other equipment between it and rotary drilling head  46  must be larger than the maximum diameter of bearing assembly  52 . If smaller rig floor equipment is used, then rotary drilling head assembly  46  must be removed from beneath the rig floor  20  for disassembly. 
     One major limitation of prior art rotary drilling head designs is that the roller bearing assemblies require a large radial clearance. Thus, prior art drilling head designs either require a large hole in the rotary table  18  or must be removed from beneath the rig floor  20  for dismantling. It is desirable to produce a rotary drilling head assembly  46  that has a small radial clearance that will allow the stripper assembly  60  and bearing assembly  52  to be removed through the opening in the rotary table  18 . 
     During drilling operations, the seals that maintain the lubrication oil on the drilling head bearing packages may fail prematurely. In the event that a lubrication seal is lost, the roller bearings are destroyed and must be immediately replaced. When seal failure occurs, the entire drilling operation must be stopped so that the rotary head bearing assembly  52  can be replaced. To replace the roller bearings, the whole rotating bead must be removed from the well casing. To prevent costly outages and repair regimens, a more durable bearing design that can function following a lubrication seal loss is desirable to minimize down time. 
     Diamond bearings are disclosed in U.S. Pat. No. 4,410,054 for use in downhole mud motors. The present invention overcomes the deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     The rotary drilling head assembly of the present invention includes a housing having a bore for receiving a drive member. The drive member has an outer diameter of less than  17{fraction (1/2+L )} inches so as to pass through the    17{fraction (1/2+L )} inch opening in a rotary table. A bearing assembly is disposed between the housing and drive member allowing the drive member to rotate within the housing and includes an outer stationary portion and an inner rotating portion maintained in place by upper and lower threaded retaining rings. Retaining clamps attach the outer stationary portion to the housing and the rotating portion to the drive member. Rotary seal assemblies isolate the bearing assembly and its lubrication system from the drilling fluid to prevent premature wear and failure of the bearing.    
     The bearing assembly includes a plurality of opposing disc-like members that have flat bearing surfaces meeting on a substantially planar surface of contact. The disc-like members are preferably made of a polycrystalline diamond material. The highly wear resistant polycrystalline diamond bearing resists drill string and axial loads. 
     The bearing assembly includes at least two long-lasting diamond bearings to carry axial thrust loads. Each bearing includes annular bearing plates each supporting a plurality of friction bearing members having bearing faces of highly wear resistant polycrystalline diamond to carry the thrust load. 
     The diamond enhanced bearing of the present invention is more compact than equivalent roller bearing assemblies of the prior art rotary head assemblies. By reducing the space required in the radial dimension, the diamond enhanced rotary drive assembly fits through the opening in, a 17.5″ rotary table which was not possible with the over 20″ diameter roller bearing design of the prior art. Some current roller bearings are small enough to be retrieved through a 17.5″ rotary table but their load carrying capacity is limited by their diminished radial envelope. 
     Additionally, the diamond enhanced bearing package of the improved rotary drilling head assembly is designed to be symmetrical. In the event that one side of the bearing wears faster than the other, the bearing may be removed and reversed to allow the drilling head to continue in service. Rotary seals are positioned below and above the diamond enhanced bearings of the present invention encapsulating a lubricant fluid that provides lubrication to the bearing members. The use of diamond bearings, however, makes it possible for the bearings to be safely cooled and lubricated by the drilling fluid in the event of a lubrication seal failure. 
     By incorporating the inherent fail-safe properties of diamond enhanced bearings into the rotary drilling head assembly of the present invention, considerable advances in drilling head life can be achieved. By utilizing a more durable rotary drilling head with a higher maximum rotational speed, production costs can be reduced by both reducing the number of expensive bearing replacement operations and being able to drill at a faster rate than before. 
     Other objects and advantages of the present invention will become apparent from the following description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of a preferred embodiment of the invention, reference will now be made to the accompanying drawings wherein: 
     FIG. 1 is a schematic of a conventional drilling system for a well; 
     FIG. 2 is a sectional view of a prior art rotary drilling head assembly with roller cone bearings; 
     FIG. 3 top view of a kelly bushing for use with the prior art drilling head assembly of FIG. 2; 
     FIG. 4 is a sectional view of a rotary drilling head assembly constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 5 is a top view of a diamond bearing showing an overlap geometry in accordance with the preferred embodiment of the present invention; 
     FIG. 6 is an enlarged cross sectional view of one side of the rotary drilling head assembly of FIG. 4; 
     FIG. 7 is an enlarged cross-sectional view of the bearing assembly of the preferred embodiment of the present invention shown in FIG. 4; and 
     FIG. 8 is an enlarged cross-sectional view of an alternative embodiment of the bearing assembly shown in FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to now FIGS. 4-8, the rotary drilling head assembly  100  of the preferred embodiment includes an outer stationary housing or bowl  102  and an inner drive ring  104  with a bearing assembly  106  disposed between the drive ring  104  and bowl  102 . Assembly  100  further includes a stripper assembly  110  slidably received within drive ring  104  and mounted to the top of drive ring  104  by fastener members such as bolts  108 . Stripper assembly  110  includes a drive bushing  112  having a stripper rubber  114  bonded at  116  to its lower end. Seals  118  are provided to seal between drive bushing  112  and drive ring  104 . 
     Outer bowl  102  includes an inlet mounting flange  122  for connection to the BOP stack and an outlet port  124  with a flange  126  for connection to a pipe extending to the mud reservoir. A bushing sleeve  128  is disposed within the upper cylindrical bore  130  in bowl  102 . The outer diameter of bushing sleeve  128  is less than the diameter of the opening in the rotary table and typically is 17.5 inches or less so as to allow sleeve  128  to pass through the opening in the rotary table. 
     Drive ring  104  and bushing sleeve  128  form an envelope for housing bearing assembly  106 . A retaining ring  132  is threaded onto the lower end of drive ring  104  to position and retain the bearing assembly  106  within the drive ring  104 . Bushing sleeve  128  rests on an upwardly facing shoulder  136  on bowl  102  extending inwardly into bore  130  and also includes a inwardly extending flange  138  forming an upwardly facing shoulder  140 . Sleeve  128  is maintained in position by a stationary retaining clamp assembly  142  which engages an outwardly extending flange  144  on the upper end of bowl  102  and bears against the upper terminal end of sleeve  128  forcing sleeve  128  against upwardly facing shoulder  136 . 
     Bushing sleeve  128  also includes upper and lower hydraulic ports  146 ,  148 , respectively, communicating with hydraulic ports  150 ,  152 , respectively, for providing lubricating and cooling fluids to bearing assembly  106 . Seals  154  are provided to seal around ports  146 ,  148 . Upper and lower seal assemblies  156 ,  158  are disposed above and below bearing assembly  106 . Each seal assembly  156 ,  158  includes a seal housing  160  having a passageway  162  communicating with either hydraulic port  146  or  148  and the inner surface of seal housing  160  between a pair of seal grooves housing seal members  164 ,  166 . A check valve  95  is disposed in passageway  162 . Upper and lower bushings  168 ,  170  are disposed between seal assemblies  156 ,  158  and drive ring  104  with a seal member  101  sealing therebetween. 
     Referring particularly to FIG. 7, bearing assembly  106  includes a housing  172  having a plurality of upwardly facing apertures  174  and a plurality of downwardly facing apertures  176  for housing disc-shaped bearing members  178  such as members  180 ,  182 , respectively. Housing  172  also includes a inwardly facing being race  184  for housing a plurality of radial load carrying roller bearings  186 , such as needle bearings, equally spaced about the outer diameter of bearing race  184  with their axis extending parallel to the central axis  188  of rotary drilling head assembly  100 . Assembly  106  also includes an outer spacer bushing  190  which bears against roller bearings  186 . Bearing assembly  106  further includes an upper bearing ring  192  and a lower bearing ring  194 , each having a plurality of apertures  196 ,  198 , respectively, for also housing disc-shaped bearing members  178  such as members  200 ,  202 , respectively. 
     As shown on the left hand side of FIG.  4  and in closer detail in FIG. 8, roller bearings  186  may be replaced by a journal bearing with hard surface facing  204  on the outer radial surface of housing  172 . Hardened surface  204  bears against the inner diameter of bushing  190 . Hardened surface  204  can also be manufactured of diamond material. Alternatively, instead of disposing the radial bearings  186  between the upper and lower assemblies  206 ,  208 , radial bearings may be disposed outboard of the upper and lower assemblies  206 ,  208  with radial bearings above the upper bearing assembly  206  and radial bearings below the lower bearing assembly  208  to increase stability and eliminate any pivoting about radial bearings  186 . 
     Bearing members  178  are generally in the shape of cylindrical studs that are secured in their respective mounting apertures by conventional methods and are able to withstand large compressive loads and vibrations. The material of the bearing members  178  is a hard material such as tungsten carbide and is capable of bonding well with the polycrystalline diamond compound that is secured thereon. The diamond substrate is applied to the exposed circular faces and about the periphery of the cylindrical bearing members  178  for the purpose of reducing frictional wear on the members, is extremely wear and heat resistant once applied, and offers performance that well exceeds that of roller bearings. The diamond coated surfaces of each bearing member  178  in their respective mounting ring collectively act as a single hardened bearing surface. The bearing members are preferably cylindrical studs having flat faces with initially flat disc-shaped diamond wafers supported thereon. There is preferably one more of the diamond bearing wafers on one of the annular bearing plates than on the other and it is preferred to have all diamond wafers be of the same size and diameter. Wafers currently manufactured by Megadiamond Industries that are 13 mm in size are acceptable for this application. 
     The diamond bearing of the present invention utilizes a thrust surface that is only ½″ wide radially. Roller bearings of the same load capacity would require  1{fraction (1/2+L )}″ to  2″ of radial width. Diamond bearings may save as much as 1″ of radial space (at least 2″ on the diameter) over the roller bearings of the prior art. To accomplish this space savings with roller bearings, the size of the roller bearings may have to be reduced which would reduce their load carrying capacity. Even though smaller, the diamond bearings exceed the load carrying capacity of the roller bearings by 5 to 10 times. 
     Diamond bearings are able to run at higher temperatures than other types of bearings under similar loads. The diamond wafers of the present invention do not begin to deteriorate until they reach 1300° F. The diamond bearings allow the rotary drilling head assembly to operate at speeds much higher than previously allowable with roller bearings. Whereas diamond drilling head assemblies utilizing diamond bearings can operate at speeds up to 200 RPM and up to 600 RPM in special situations, current rotary drilling head assemblies typically allow the drill string to only rotate at 100 RPM or less. Since the bearing seal components only operate effectively below 400° F., a chiller system may be required to keep the lubrication system cool. 
     Upon assembly, upper bearing ring  192  with apertures  196  housing members  200  is disposed opposite upwardly facing apertures  174  with members  180  on housing  172  to form an upper diamond bearing assembly  206 . Similarly, lower bearing ring  194  with apertures  198  housing members  202  is disposed opposite downwardly facing apertures  176  with members  182  on housing  172  to form a lower diamond bearing assembly  208 . Housing  172  thus provides two arrangements of bearing members  178  that face in opposite directions from each other in the axial direction. This arrangement allows housing  172  to act both as bottom ring for bearing members  180  for upper bearing assembly  206  and as top ring for bearing members  182  for lower bearing assembly  208 . 
     Referring particularly to FIG. 5, upper and lower bearing assemblies  206 ,  208  thus form upper and lower polycrystalline diamond enhanced thrust bearing surfaces. Each of the two complimentary surfaces are horizontally opposed and include rings of disc-shaped bearing members  178  that are equally spaced into a circular pattern within their mounting plates. To ensure simplicity of design, all bearing members  178  are of a standard size and diameter. Since the bearing members  178  are all of the same diameter, the number of bearing members  178  in each of the two bearing rings within a bearing assembly,  206  or  208 , differs in number by one. This difference is necessary to ensure that at any position that the bearing assembly may encounter, no more that one pair of bearing members  178  may line up perfectly with one another such that all other engaging bearing members  178  overlap. Alternatively, bearing members  178  may have different diameters for each bearing ring and yet still accomplish the same result. 
     In the assembly of the bearing assembly  106  in the envelope formed by drive ring  104  and bushing sleeve  128 , an upper retainer ring  214  is threaded onto the upper end of bushing sleeve  128  thereby compressing upper and lower seal assemblies  156 ,  158  and upper and lower bearing rings  192 ,  194  with bushing  190  therebetween against shoulder  140 . Likewise lower retainer ring,  132  is threaded onto the lower end of drive ring  104  compressing the bushings  168 ,  170 , housing  172 , and spacer rings  210 ,  212  together against a downwardly facing shoulder  216  on drive ring  104 . 
     Housing  172  is thus attached to drive ring  104  and thereby rotates with drive ring  104 . The upper and lower bearing rings  192 ,  194  are attached to the outer geometry of bearing assembly  106  and thus to outer bowl  102 . Thus, the bearing rings  192 ,  194  are stationary and do not rotate. 
     In operation, as drill string  218  is rotated by the rotary table  18  (shown in FIG.  1 ), drive ring  104  rotates within outer bowl  102 . As the drill string  218  passes downwardly through the stripper assembly  110 , downward drill string forces  220  are placed on bearing assembly  106 . This causes the bearing members  182 ,  202  to engage to absorb these downward axial forces on the drilling head assembly  100 . The downhole pressure on the returning drilling fluids also places upward annular forces  222  on bearing assembly  106 . This causes the bearing members  180 ,  200  to engage to absorb these upward axial forces on the drilling head assembly  100 . Only one bearing assembly  206 ,  208  engages at any one time. When upper bearing assembly  206  engages, lower bearing assembly  208  separates and disengages and when lower bearing assembly  208  engages, upper bearing assembly  206  separates and disengages. The radial forces on the drilling head assembly  100  caused by the rotation of the drill string  218  are absorbed by the roller bearings  186 . Bearing housing  172  and diamond bearing members  180 ,  182  also rotate within their respective diamond enhanced bearing assemblies  206 ,  208 , serving to counteract axial forces  220 ,  222  experienced by rotary drilling head assembly  100 . 
     The preferred embodiment of the present invention incorporates a bearing assembly with two bearing systems that each contain sets of horizontally opposed bearing members that meet with each other in a planar geometric fashion. Such an embodiment is highly effective in countering axial thrust loads but does not offer any resistance to radial drill string loads. Since loads in the radial direction are much lower than those in the axial direction, the secondary radial bearing system incorporated into the present invention is in the form of roller bearings  186 . 
     An external pressurized cooling and oiling system  225  communicates through hydraulic quick connects  224  in fluid communication with seal members  164 ,  166  above and below bearing assembly  106  to cool and lubricate the bearings. System  225  may include an oil chiller. Seal members  164 ,  166  are constructed to remain in place while allowing full 360° rotation of the mating drive ring  104  with maintenance of seal integrity. 
     Because forces from the annular return of drilling fluids can be greater than downward drill string forces  220  by 10 times or more, upper bearing assembly  206  wears at a faster rate than lower system  208 . To compensate for any uneven wear between bearing assemblies  208 ,  206 , bearing assembly  106  is constructed symmetrically so that it may be removed from rotary head assembly  100 , reversed, and reinstalled so that the lesser worn bearing system opposes smaller downward drill string loads  220 . By utilizing the less worn bearing system of the assembly in place of the heavily worn bearing system, use of drilling head assembly  100  can be continued on a temporary basis until a replacement bearing assembly  106  can be located and installed. 
     Alternatively, a cost saving embodiment for the bearing assembly design includes; a diamond bearing assembly to offset the larger annular return forces and a roller bearing assembly to offset the smaller drill string forces. Such an alternative reduces cost but would not be reversible. Where vertical height restrictions are critical, a still further alternative utilizes only one bearing assembly which offsets both upward and downward forces. Such an alternative reduces the vertical height of the drilling head assembly but requires that the stripper assembly  110  float up and down. 
     When it is time for bearing replacement, stationary retaining clamp  142  is removed and drive ring  104  and bearing assembly  106  are removed through the opening in rotary table  18 . The current art design presented is capable being retrieved through a 17.5″ clearance rotary table. The prior art presented requires a 22.5″ rotary table clearance. 
     In the alternative to the preferred embodiment, the diamond bearing members  178  of each opposing bearing ring meet each other at a contact surface with a conical profile. Two horizontally opposed conical diamond bearing systems would suffice to oppose both radial and thrust forces encountered in drill string operations. However, to do so, each of the wafers would have to have a concave surface which is properly oriented. The concave surfaces could be created by arranging the initially flat diamond bearing members in a conical arrangement and rotating them under load until they are “broken in” and obtain the conical profiles. 
     Although the invention is described with particular reference to a rotary drilling head assembly used in well drilling operations, it will be recognized that certain features thereof may be used or adopted for use in other applications. Inventions relating to oilfield well drilling have applications in other industries that also require earth drilling including, but not limited to, the drilling of water wells, underground electrical conduits, fluid pipelines, or geo-thermal energy systems. While the preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. For example, the relative dimensions of various parts, the materials from which the components are made and other parameters can be varied. 
     The embodiments described herein are exemplary only, and are not limiting. Many variations and modifications of the invention and the principles disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.

Summary:
The rotary drilling head assembly of the present invention includes a housing having a bore for receiving a drive member. The drive member has an outer diameter of less than 17 ½ inches to pass through a 17½ inch opening in a rotary table. A bearing assembly is disposed between the housing and drive member allowing the drive member to rotate within the housing and includes an outer stationary portion and an inner rotating portion. Retaining clamps attach the outer stationary portion to the housing and the rotating portion to the drive member. Rotary seal assemblies isolate the bearing assembly and its lubrication system from the drilling fluid and prevent premature wear and failure of the bearing. The bearing assembly includes a plurality of opposing disc-like members that have flat bearing surfaces meeting on a substantially planar surface of contact. The disc-like members are preferably made of a polycrystalline diamond material.