Patent Publication Number: US-6708786-B2

Title: Mounting attachment and bearing system for an industrial earth-boring cutter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This invention claims priority from U.S. provisional application serial No. 60/289,501, filed on May 8, 2001. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates generally to industrial earth-boring cutters and, more particularly, to the bearing system and attachments therefor for earth-boring cutters. 
     2. Background Art 
     Industrial earth-boring cutters, such as the type used in raise bore and shaft-drilling assemblies are well known in the art. An industrial earth-boring cutter  1 , as shown in FIG. 1, typically comprises a central journal assembly  2  on which a cutter body  3  is rotatably mounted. The cutter body  3  typically includes ribs, protuberances, or hard inserts  4  to break up and crush a formation  5  when the cutter body  3  is pressed against and rolled over the formation  5 . 
     The cutter  1  shown in FIG. 1 is a raised bore cutter. A ball bearing  10  and roller bearings  11  are disposed between the journal assembly  2  and the cutter body  3  to allow the cutter body  3  to rotate freely with respect to the journal assembly  2 . The ball bearing  10  is usually provided to carry axial load, and the one or more roller bearings  11  are typically provided to carry radial loads. In this configuration the roller bearings  11  are placed around the journal assembly  2  prior to sliding the journal assembly  2  into the cutter body  3 . Then the ball bearing  10  is put into place by inserting bearing balls through the ball hole  13  in the journal  2 . Once the bearing balls are in place, a ball plug  12  is inserted into the ball loading hole  13  and then a ball plug retainer  14  is inserted into the journal  2  to retain the ball plug  12  in place. 
     To prevent damage to the bearing balls of the ball bearing  10  and edges of the ball loading hole  13 , cutter designs known in the art have the ball hole  13  placed at 180 degrees from the load bearing zone of the journal assembly  2 . This placement is selected to prevent forcing the bearing balls against the rough edges of the ball loading hole  13  as they pass over the hole  13 . If the ball loading hole  13  were positioned in the load bearing zone, the bearing balls would forcibly impact the edges of the ball loading hole  13 , probably resulting in metal chips and debris being removed from the journal  2  so as to contaminate the lubricant and eventually destroy the bearings and seals. 
     Once assembled, the cutter  1  is typically attached to a rotatable headplate (not shown) by a support bracket  6  or similar structure. Typically the support bracket  6  includes a base attachable to the rotatable headplate (not shown) and legs  7  on each side of the base extending away from the base. Each leg  7  includes a yoke  8  at its distal end which is configured to receive and fixably couple to a support shaft  9  of the journal assembly  2  which extends axially outward at each end of the cutter  1 . 
     For many applications, industrial cutters are limited by the bearing capacity or bearing life. A major cause of bearing failure in industrial cutter systems is spalling of the non-rotating journal bearing surface. Spalling is the flaking off of material from a surface. Spalling of the non-rotating journal bearing surface is the result of a fatigue process caused by the rolling elements as they passed across the position the journal surface that carries the load. For example, as the rolling elements roll across the journal surface, the surface is repeatedly loaded and unloaded, which initiates subsurface cracks that ultimately cause spalling. When the journal surface spalls, hard steel debris contaminates the lubricant which causes rapid wear and damage to the rest of the operable bearing and seal components which eventually results in bearing failure. 
     Ideally, the load-bearing journal surface should be replaced with a new surface before it spalls so that the life of the bearing can be increased. This may be accomplished by rotating the journal during servicing of the cutter to place the previously unloaded journal surface in the load bearing position. One cutter design which allows for rotation of the journal by 180 degrees is shown in FIG.  2 . However, this design uses cylindrical roller thrust bearings instead of ball bearings. In this design, the ball bearing (shown at  10  in FIG. 1) is substituted by a plurality of small roller bearings  20  transversely disposed between the journal assembly  2  and the cutter body  3  along opposed upper and lower paths defined between a projection  21  extending from the journal surface and an internal recess  22  formed in the cutter body  3 . Because this design has no ball bearing, concerns regarding the placement of the ball loading hole ( 13  in FIG. 1) are eliminated. Therefore, it is possible to reverse the journal to expose a previously substantially unloaded surface as a replacement surface before significant spalling of the first load-bearing surface takes place. However, this cutter configuration requires very tight tolerances on four different axial bearing surfaces to maintain good control of axial loading and deflection. A closely toleranced cone bearing sleeve  23  is also necessary to assemble the thrust elements of the bearing. This sleeve  23  greatly restricts the outer bearing diameter, however, which limits radial roller bearing capacity. 
     In prior art cutter designs which use ball bearing retention, as previously explained, the ball loading hole is placed 180 degrees from the load zone. While this configuration ensures little or no load on the ball loading hole, this design does not allow for rotation of the journal. Therefore, the substantially unloaded surface of the journal bearing in these designs can not be later used during the cutter life. Further, if the journal were rotated, it would put the rough opening of the ball loading hole into a position of maximum radial loading, which would lead to premature bearing failure as described above. 
     It is desirable to have a simplified cutter which uses ball bearing retention and permits rotation of the journal so that a previously substantially unloaded surface may be subsequently used to carry load while maintaining the ball loading hole in a position outside of the load bearing zone so that the life of the bearing may be increased. 
     SUMMARY OF INVENTION 
     The invention is a rotary cutter mount for an earth-boring cutter. The mount includes a bearing journal adapted to be coupled to a cutter body. The bearing journal has a rotary cutter body rotationally coupled to an exterior bearing surface of the journal. A first mounting end of the bearing journal is shaped to enable rotationally fixed positioning in a corresponding yoke. The yoke is operatively coupled to the body of the earth-boring cutter. A ball race is formed in an exterior surface of the bearing journal, and a ball loading passage is formed in the bearing journal. The ball loading passage has an exit hole on the ball race. The exit hole is positioned so that a rotary orientation of the exit hole is disposed in a rotary orientation which is at a selected angular displacement from a direction of maximum radial loading on the bearing journal. A shape of the first mounting end of the journal and a shape of the corresponding yoke are adapted to enable mounting in a plurality of rotary orientations. Each of the selected rotary orientations is such that the exit hole is oriented other than in the direction of maximum radial loading. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 show examples of prior art industrial cutter structures. 
     FIG. 3 shows an exploded view of one embodiment of a cutter according to the invention. 
     FIG. 4 shows an exploded view of another embodiment of a cutter according to the invention. 
     FIGS. 5A and 5B shows one embodiment of a bearing journal according to the invention. 
     FIG. 5C shows one example of possible positions of an exit hole of a ball loading passage for various rotary orientations of a bearing journal according to the invention. 
     FIGS. 6A and 6B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 7A and 7B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 8A and 8B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 9A and 9B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 9C and 9D show examples of other embodiments of a mounting configuration according to the general concept shown in FIGS. 9A and 9B. 
     FIGS. 10A and 10B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 11A and 11B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 12A and 12B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 13A and 13B show another embodiment of a bearing journal and a corresponding mounting yoke. 
     FIGS. 14A and 14B show another embodiment of a bearing journal and a corresponding mounting yoke. 
    
    
     DETAILED DESCRIPTION 
     The invention provides a mounting system for an earth-boring cutter or other rotary systems having a journal bearing assembly subject to substantially one-sided loading. For example, this mounting system may be used for raised bore cutters, replaceable cutters on hole openers, underreamers, and reverse reamers used in trenchless utility boring. The invention may provide a substantial increase in bearing life for the rotary system. 
     An exploded view of one example of an earth-boring cutter  100  in accordance with the invention is shown in FIG.  3 . In this example, the cutter  100  comprises a generally cylindrical journal assembly  102 . The journal assembly  102  may be an integrally formed member or may comprise a plurality of members coupled together. The journal assembly  102  comprises a journal body  128  preferably having a plurality of recessed bearing rolling paths (not shown) defined thereon. 
     The cutter  100  further comprises a generally cylindrical cutter body  103  having a bore that extends axially therethrough for receiving the journal assembly  102  therein. The cutter body  103  may be tapered, as shown, and may include ribs, protrusions, or inserts which contact and cut through earth formations during drilling operations. The cutter body  103  further comprises an inner surface having a plurality of bearing rolling paths  131 ,  132 , and  133  defined thereon and corresponding to the rolling paths (not shown) on the outer surface of the journal body  128 . 
     A plurality of roller elements  129  are disposed between the cutter body  103  and the journal assembly  102 . The roller elements  129  are axially positioned to roll within the corresponding rolling paths ( 131 ,  132 ,  133 ) between the journal assembly  128  and the cutter body  103  to enable the relative rotation of the cutter body  103  with respect to the journal assembly  102 . In accordance with the invention, the rolling elements  129  include at least one set of ball bearings  112  and at least one other set of bearings, such as roller bearings  111  and  113 . The ball bearings  112  are provided primarily to carry axial load. The one or more other sets of bearings  111 ,  113  may be provided to carry radial or lateral loads. The one or more other sets of bearings  111 ,  113  may be cylindrical, crowned, logarithmic, or tapered roller bearings, or may be ball bearings. In this example, the other set of bearings  111 ,  113 , comprises a set of outer roller bearings  111  and a set of inner roller bearings  113 . For ball bearings primarily adapted for axial loading, a large ball race may be used to provide high thrust capacity and tight control of axial movement. Any type of race selected by one skilled in the art may be used for the ball bearings, for example an angular contact ball race design such as disclosed in U.S. Pat. No. 3,762,782 to Rumbarger. 
     The other components shown in FIG. 3 or the cutter  100  include a lubrication fitting  104 , an outer retaining ring  105 , an outer seal retainer  107 , O-rings  106 ,  116  and  119 , dowel pins  108  and  115 , an outer seal  119 , an inner seal  114 , an inner retaining ring  117 , an inner seal retainer  118 , a ball plug  123 , a ball plug retainer  124 , and a spring pin  125 . 
     A similar cutter is shown in exploded view in FIG.  4 . This cutter  100  includes an additional set of outer roller bearings at  111  for handling high radial loads. The type, number, and placement of the at least one other set of bearings in accordance with the invention may be determined by those skilled in the art and is not a limitation on the invention. 
     Referring to FIGS. 5A and 5B, in accordance with the invention, the mounting system for the journal on the cutter allows reorientation of the journal  152  such that the substantially unloaded portions of the journal bearing surface can be reoriented into the load-bearing position, such as when the cutters are serviced, without subjecting the ball loading hole  150  to maximum radial loading. This may result in a substantial increase in bearing life. To achieve reorientation of the journal for this type of roller retention earth-boring cutter configuration, the ball loading hole  150  must be located so that it is not subject to significant radial or lateral loading. This is achieved by positioning the exit of the ball loading hole  150  away from the load bearing zone, shown at  150 A in FIG.  5 A. In the embodiment shown in FIG. 5A the ball hole exit  150  on the journal  152  is located 90 degrees from the position of maximum radial load  150 A. This configuration enables the journal  152  to be rotated 180 degrees about the journal axis  154  during service of the cutter ( 100  in FIG.  3 ), while still orienting the ball loading hole  150  at a position which is about 90 degrees from the position of maximum radial load  150 A. 
     Thus, embodiments of the invention provide both apparatus and methods for reorienting the journal during the servicing of a cutter which may extend the life of the bearing. In some applications, the apparatus and method may effectively double the life of the bearing in comparison to prior art mounting systems. Embodiments of the invention may also be more cost effective and reliable than previous reversible systems. For example, using an integral ball race on the journal  152  and on the cutter body ( 103  in FIG. 3) reduces the design to a fewer number of bearing components, which may result in lower manufacturing costs. This may also lead to an improvement in reliability because the number of potential lubricant leak paths is reduced and tolerance stack-up is avoided in the axial direction. 
     Material which may be used for the roller elements may include any shock resistant tool steel, such as that known by the industrial designation S 2  and S 5 , or chrome alloy steel, such as known by the industrial designation 52A100. These materials are only listed here as examples of materials that may be used. Those skilled in the art will appreciate that any other suitable material may be used without departing from the spirit of the invention. 
     As shown in FIG. 5A, when the cutter is in use, only a portion of the journal  152  is subject to substantial load bearing, this portion being shown generally at  150 A. In accordance with the invention, after a first surface on the journal  152  is used, the journal  152  may be detached from its mounting and rotated about its axis  154 . After rotation, the journal  152  is then reattached such that the unworn surface is oriented toward the direction of maximum radial loading  150 A. 
     In accordance with the invention, the journal assembly is oriented such that the ball hole exit  150  is at an angle less than 180 degrees from the position on the journal  152  carrying maximum radial load  150 A. Preferably, the ball hole exit  150  is located between 45 degrees and 135 degrees away from position on the journal  152  carrying the maximum radial load  150 A. More preferably, the ball hole exit  150  may be located around 90 degrees away from position on the journal  152  carrying the maximum radial load  150 A. Locating the ball hole exit  150  respective of the maximum load-bearing position in this way allows for a rotatable or reversible journal system having the benefit of ball bearing retention, wherein the journal  152  can be rotated to expose a new area of journal surface to load bearing prior to significant spalling of the initially load-bearing surface. This may be done to postpone the effects of spalling and increasing the life of the bearing. An example of such orientation is shown in FIG.  5 B. 
     In another embodiment shown in FIG. 5C, the ball hole exit  150  may be located about 45 degrees away from the maximum radial load-bearing position  150 A. This positioning of the ball hole exit  150  allows for the journal to be rotated up to three different times in 90 degree increments, which may allow the cutter to be serviced as many as three times before it becomes necessary to replace the journal. 
     Those skilled in the art will appreciate that factors such as the load profile for the cutter, the design factors related thereto, and other factors such as the potential for load bearing on the edge of the ball hole, the rigidity of the mounting system, and the size of the ball hole should be considered when determining the selected angles at which the ball hole exit  150  is to be oriented during cutter operations. 
     To provide a rotatable journal for a cutter in accordance with the invention, a mounting system is required which allows for repositioning and securing in the journal in the selected orientations. In general, the mounting system comprises a contoured attachment mechanism disposed at each end of the cutter and rigidly coupled to the journal assembly, and a yoke having a complementary contour for receiving the contoured attachment mechanism and a means for rigidly coupling thereto. One embodiment of a journal mounting system in accordance with the invention is shown in FIGS. 6A and 6B. In this embodiment, the attachment mechanism comprises a generally octagonal cross-sectioned attachment shaft or pin  64  attachable to the end  153  of the journal assembly  152  in a rotationally fixed manner, such as by bolts, screws, or the like. The pin  64  may alternatively be or a shaft integrally formed with and extending from the journal assembly  152 . The external surface of the pin  64  is adapted to fit within corresponding surfaces of a yoke  60 . The pin  64  may be retained in the yoke  60  by a bolt or pin such as shown at  68 . 
     An embodiment shown in FIG. 6B, includes a threaded hole  65  in the pin  64 . A corresponding threaded hole  67  is provided in the yoke  60 , such that when the pin  64  is properly oriented in the yoke  60 , the holes  65 ,  67  of the pin  64  and the yoke  60 , respectively, align so that a bolt  69  may be passed therethrough to engage the holes  65 ,  67  and rigidly and removably couple the pin  64  to the yoke  60 . 
     As illustrated in FIGS. 6A and 6B, after the journal assembly  152  is used in an initial rotary orientation, the journal assembly  152  may then be detached from the yoke  60  by removing the bolt  69 , and rotated, as shown in FIG.  6 B. After rotating the journal assembly  152  such that a new bearing surface is oriented in the direction of maximum radial loading ( 150 A in FIG. 5A) the journal assembly  152  may then be reattached to the yoke  60  in the new rotary orientation. If desired, a second threaded hole (not shown) 90 degrees displaced from the hole  65  shown in FIG. 6B may be provided in the pin  64  to enable rotation of the journal assembly  152  in 90 degree increments. In other embodiments, attachment devices other than bolts may be used to attach the pin to the yoke without departing from the scope of the invention. 
     Another embodiment of a journal mounting device is shown in FIGS. 7A and 7B. In this embodiment, the mounting device comprises a generally cross-shaped attachment mechanism  74  forming or coupled to the end of the journal  152 , and a yoke  70  having a correspondingly cross-shaped cavity  70 A (FIG. 7B) for receiving the cross-shaped attachment mechanism  74  therein. At least one arm  71 , and preferably the opposing arm  71 A as well, of the cross-shaped attachment mechanism  74  is provided with a threaded hole  75  penetrating each arm  71 ,  71 A. Corresponding threaded holes  77  are provided in the corresponding shoulders of the yoke  70  as shown in FIG.  7 B. The journal assembly  152  is then attached to the yoke  70  by engaging bolts  79  in each of the holes  75  in the arms  71 ,  71 A of the cross-shaped attachment mechanism  74  and the corresponding holes  77  in the yoke  70 , as particularly shown in FIG.  7 B. 
     In accordance with the invention, after the journal assembly  152  is used in an initial rotary orientation, the journal assembly  152  can then be detached from the yoke  70  by removing the bolts  79 , and then rotated 180 degrees to allow substantially unloaded portions of the journal bearing surface to be reoriented into the maximum load-bearing position ( 150 A in FIG.  5 A). After rotation of the journal assembly  152 , the journal assembly  152  is then reattached in the same manner described above. In other embodiments, a second set of parallel axially aligned threaded holes (not shown) may be provided in the other two arms  71 B,  71 C of the cross-shaped attachment mechanism  74  to enable for rotation of the journal assembly in 90 degree increments. 
     Another embodiment of a mounting attachment is shown in FIGS. 8A and 8B. In this embodiment, the mounting attachment comprises an end of a generally cylindrical shaft  84  which extends at one end of the journal assembly  82 . The cylindrical shaft  84  is provided with a plurality of threaded holes  85  formed on the end face  84 A thereof. The mounting attachment further comprises a corresponding attachment yoke  80  having a slot or cutout  80 A formed therein which truncates in a shape adapted to receive the end  80 A of the cylindrical shaft  84  therein. The yoke  80  is also provided with a plurality of threaded holes  87  which extend through the wall thereof having the slot  80 A. When the cylindrical shaft  84  is in a selected rotary orientation in the yoke  80  the threaded holes  87  of the yoke  80  align with the threaded holes  85  in the end of the cylindrical shaft  84 . The shaft  84  can then be rigidly coupled to the yoke  80  by engaging a pin or bolt  89  in one or more, and preferably all of the aligned holes  85 ,  87 , as shown in FIG.  8 B. 
     After the journal assembly  152  is used in an initial rotary orientation, the journal assembly  82  can then be detached from the yoke  80  by removing the bolts  89 . The journal  152  can then be rotated by a selected angular amount to enable substantially unloaded portions of the journal bearing surface to be reoriented into the maximum radial load-bearing position ( 150 A in FIG.  5 A). Those skilled in the art will appreciate that this type of attachment configuration enables the journal assembly  152  to be configured to be rotated by any desired amount, such as 90 degrees or 180 degrees. The rotation angles available depend on the positions of the mating holes  85 ,  87 . The pattern shown in FIGS. 8A and 8B, which enables 90 degree incremental rotation is only one example of selected incremental rotation angles. After rotation of the journal assembly  152  to the next desired rotary orientation, the journal  152  is then reattached such that a different journal surface is subjected to the expected maximum radial load, as shown at  150 A in FIG.  5 A. 
     Another embodiment of a mounting attachment is shown in FIGS. 9A and 9B. In this embodiment, a contoured attachment mechanism  94  on the journal assembly  152  is configured to mate with a substantially triangular yoke  90 , wherein the contoured attachment mechanism  94  and the yoke  90  are coupled by bolts  99  passing through corresponding threaded holes  95 ,  97  of the contoured attachment mechanism  94  and the yoke  90 , as shown in detail in FIG.  9 B. This configuration enables reorientation of the journal assembly  152  in 180 degree increments to allow substantially unloaded portions of the journal bearing surface to be reoriented into the load-bearing position. The design shown in FIGS. 9A and 9B could be modified as shown in FIG. 9C to enable rotation of the journal assembly ( 152  in FIG. 9A) in 120 degree increments. Another embodiment shown in FIG. 9D is adapted to enable rotation of the journal assembly ( 152  in FIG. 9A) in 90 degree intervals. 
     Another embodiment of a mounting attachment is shown in FIGS. 10A and 10B. In this embodiment, a contoured attachment mechanism  204  on the journal assembly  152  is configured to mate with a yoke  200  having a substantially square yoke cavity configuration ( 200 A in FIG. 10B) which extends around the sides of the contoured attachment mechanism  204 . The contoured attachment mechanism  204  comprises a square-like cross section with beveled corners. The yoke  200  comprises upwardly extending legs which cradle the sides of the contoured attachment mechanism  204 . This embodiment of the yoke  200  has radial recessed corners. The radially recessed corners of the yoke  200  combined with the beveled corners of the contoured attachment mechanism  204  facilitate the insertion and removal of the contoured attachment mechanism  204  from the yoke  200 . In this embodiment, the contoured attachment mechanism  204  and yoke  200  each are provided with threaded holes  205 ,  207  which extend through opposing side surfaces. The contoured attachment mechanism  204  and yoke  200  may be coupled to each other by engaging a threaded member, such as a bolt  209  in the aligned holes  205 ,  207  as shown in FIG.  10 A. This configuration enables reorientation of the journal assembly  152  in 180 degree increments. In other embodiments, the coupling of FIGS. 10A and 10B may be modified by providing the other set of opposed sides of the contoured attachment mechanism  204  with holes to allow for a rotation of the journal assembly in 90 degree increments. 
     Another embodiment of a mounting attachment is shown in FIGS. 11A and 11B. In this embodiment, the contoured attachment mechanism comprises an elongated rectangular rib member  214  coupled to the end of the journal assembly  212  and having a hole  215  radially disposed therethrough. A corresponding yoke  210  comprises a slot configured to receive and retain the elongated rib member. The yoke  210  also comprises a hole  217  which corresponds in alignment with the threaded hole in the rib member when the rib member is inserted into the slot of the yoke. A member such as a bolt  219  may be used to couple the rib member and the yoke when aligned by threadably engaging in the holes when aligned. This configuration allows for reorientation of the journal assembly  212  by rotating it 180 degrees to allow substantially unloaded portions of the journal bearing surface to be reoriented into the load-bearing position. In other embodiments, this configuration may be modified to allow for a rotation of the journal assembly by a different amount. 
     Another embodiment of a mounting attachment is shown in FIGS. 12A and 12B. This attachment mechanism  224  is similar to that shown in FIGS. 10A and 10B. However in the embodiment of FIGS. 12A and 12B, the legs of the yoke  220  extend above the contoured attachment member  224  such that holes  227  in the upper portion of the legs of the yoke  220  align with a groove  225  formed along the top surface of the contoured attachment member  224 . A pin or bolt  229  which extends through the holes  227  in the yoke  220 , engage with the groove  225  in the contoured attachment mechanism  224  member thereby locking the contoured attachment mechanism  224  in place in the yoke  220 . 
     Another embodiment of a mounting attachment is shown in FIGS. 13A and 13B. In this embodiment, the contoured attachment mechanism  234  on the journal assembly  232  comprises a square-shape shaft having holes  235  provided therein. The yoke  235  comprises a generally rectangular shaped structure provided with a corresponding shaped cutout section configured to receive and couple with the square-shaped shaft extending from the journal assembly  232 . The contoured attachment mechanism  234  and yoke  230  are provided with corresponding threaded holes  235 ,  237  such that once the shaft is inserted into the cavity of the yoke  230 , a bolt  239  may be engaged therein to couple the contoured attachment member to the yoke. A wedge member  233  is also included in this configuration. The wedge member  233  is configured to be placed on top of the contoured attachment mechanism  234  when positioned in the cavity of the yoke  233 . The wedge  233  is provided with a threaded hole  231  extending down through the wedge from the upper surface. A corresponding threaded hole  237 A is provided in the yoke  230  to allow for threadably coupling the wedge to the yoke body to provide additional support for maintaining the shaft in place in the yoke. 
     Another embodiment of a mounting attachment is shown in FIGS. 14A and 14B. This attachment mechanism  244  is similar to that shown in FIGS. 6A and 6B. However, in the embodiment of FIGS. 14A and 14B the attachment mechanism  244  is a hexagon comprising sides each having substantially the same width. This attachment mechanism  244  couples to the yoke  240  similar to that for the mounting attachment shown in FIGS. 6A and 6B. However, this mounting attachment allows the journal assembly  242  to be rotated at 60 degree increments. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that numerous other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.