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CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application, pursuant to 35 U.S.C. 111(b), claims the benefit of the earlier filing date of provisional application Serial No. 60/316,007 filed Aug. 30, 2001, and entitled “Improved Rollers for Coiled Tubing Injectors.” The present invention is also related to another provisional Patent Application Serial No. 60/304,681 filed Jul. 11, 2001 entitled “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for contacting and Supporting coiled tubing during its injection into and withdrawal from a well bore. More particularly, the invention relates to drive rollers having a central driven arcuate wheel and multiple idler tires mounted on either side of the central driven wheel. The idler tires have arcuate surfaces with the same diameter as the driven wheel surface and concentric with the arcuate surface of the central driven wheel, thereby supporting the tubing over a large arc without causing severe rubbing from differential motion between the wheel assembly and the tubing. 
     BACKGROUND OF THE INVENTION 
     Devices and methods for injecting coiled tubing into and retrieving it from wells are well known. Prior art coiled tubing injection systems include U.S. Pat. Nos. 6,142,406; 5,842,530; 5,839,514; 5,553,668; 5,309,990; 5,244,046; 5,234,053; 5,188,174; 5,094,340; 4,899,823; 4,673,035; 4,655,291; 4,585,061; and many other similar disclosures. In the prior art an injector at the wellhead is used to grip and control the injection and withdrawal of the tubing. 
     Conventional track injectors utilize gripper blocks mounted on two continuous parallel and opposed conveyor chains which are urged or pushed against the outer surface of the tubing. The interface forces between the gripper blocks and the tubing permit developing frictional forces which are used to transfer tangential loads from the conveyor chains to the tubing and vice versa. If insufficient interface force is applied to the tubing by the gripper blocks, slippage with attendant loss of control and wear occurs between the blocks and tubing. If excessive interface force is applied to the tubing by the gripper blocks, the tubing wall may be distorted and damaged or the injector may be damaged. A problem with such tracks results when the track is rotated into or out of engagement with the tubing from the sprockets at the ends of the track mounting assembly. This rotation can cause differential movement between the track and the tubing in the direction of the tubing axis so that rubbing occurs. As used in this description, the term “rubbing” represents any of the effects induced by metal pieces moving relative to each other when in contact, such as galling, abrasion, tearing, scrubbing, or skidding. Rubbing causes undesirable wear of both the tubing and the gripper blocks. 
     Historically, the approach used to increase the injection forces with conventional track injectors has been to lengthen the injector while maintaining a sufficiently safe interface force between the individual gripper blocks and the tubing. U.S. Pat. No. 5,842,530 for example shows provision of substantially more gripper blocks along the length of its injector. 
     Other injectors utilizing two continuous, parallel, and opposing track injectors having grooved shoes or blocks mounted thereon are known in the art. These opposing track units have facing portions where the multiplicity of gripping blocks run parallel for gripping the tubing therebetween and are typically positioned in line, directly adjacent and above the wellhead. 
     Another approach has been to utilize a large diameter driven wheel with an annularly grooved outer diameter to conform to and support the tubing. Relatively small-diameter hold-down idler rollers radially press the tubing against the wheel to provide extra interface force between the tubing and the wheel so that high tangential frictional forces can be imparted to the tubing by the wheel without maintaining large back tensions. These hold-down rollers have arcuate faces to match the tubing, but pronounced rubbing occurs between the tubing and the roller due to differential movement at different rotational diameters of the roller face. While the mechanism of wheel type injectors is simple, inexpensive, and reliable, wheel size can be a limitation, especially for larger tubing diameters. One such wheel type injector is disclosed in U.S. Pat. No. 5,839,514. 
     A more recent injector system known in the art is a linear injector which pulls on only one side of the tubing. For this type of device, coiled tubing is driven along a single linear section of an endless chain conveyor with an opposing linear array of small-diameter arcuate face hold-down idler rollers. These hold-down rollers are sized to conform to the tubing, but they as a result cause the previously mentioned differential rubbing motion between the tubing and the roller face. Such a linear or one-track injector eliminates the necessity of synchronizing the two opposed sides of a conventional track type injector and is less damaging to the surface of the coiled tubing, but it requires a much longer unit, which of necessity extends much higher and requires additional overhead clearance. Additionally, such an injector is more expensive because it requires a considerable number of gripper blocks and rollers and a longer support track. 
     Copending U.S. Provisional Patent Application “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender”, filed Jul. 11, 2001, utilizes a novel approach to imparting tangential injection forces to the tubing. That invention provides support over a larger portion of the tubing circumference by the driving means around the circumference of the tubing. By using a plurality of sets of opposed individually driven annularly grooved rollers which closely conform to the tubing and alternating the orientations of adjacent roller sets so that they are 90° apart about the through axis of the injector, excellent tubing support is provided. That invention is light weight, compact, easy to service and changeout for different tubing sizes, low cost, and efficient. However, the small-diameter arcuate rollers of this device exhibit the same undesirable rubbing action between the roller face and the tubing as the previously mentioned injectors. 
     A major problem with tubing injectors of all types is differential movement between the tubing and the portion of the injector mechanism which contacts the tubing. For instance, for opposed track type machines, when the drive chain carrying the gripper blocks has a link coming off of or entering onto the sprockets of the track drive, differential motion relative to the axis of the tubing occurs as a result of the difference in rotational radii for the surface of the support groove of the rotating gripper block. This differential motion results in an axial direction rubbing of the tubing surface which results in wear of both the contact block and the tubing. While this situation also exists for the large drive wheel of wheel type injectors, rubbing due to the small difference in circumferential speed at the different radii of the tubing support blocks is small enough to not be important for the wheel. This, however, is not the case for any small-diameter hold-down rollers used with either wheel-type injectors or linear injectors. Similarly, for the simple small-diameter arcuate drive rollers used in the copending U.S. Patent Application for “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender,” rubbing becomes more significant when the supporting arc of the drive roller is increased to provide more tubing support. 
     Elimination or minimization of the rubbing between the drive and hold-down rollers and the tubing in tubing injectors utilizing small diameter wheels is desirable for reducing both wheel and tubing wear. Minimizing such rubbing is particularly difficult when it is desirable to provide support for the tubing over a large arcuate surface in order to minimize tubing ovaling under the action of lateral loads. A significant need exists for improvements which will permit simultaneously minimizing the erosive action of rubbing and maximizing tubing support. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes a novel means and method for improving the injecting of coiled tubing into and from a well by reducing the rubbing action of arcuate wheels or rollers used to guide and react against the tubing. Several embodiments are multi-tired idler rollers applicable to most types of injectors. The third embodiment is a multi-tired driven roller which is also applicable to most types of injectors, but is particularly useful in conjunction with the linear mechanical injector described in copending U.S. Provisional Patent Application “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender” that is hereby incorporated by reference herein. 
     The embodiments of the present invention provide improved circumferential support to the tubing which they contact by supporting the tubing over a substantially longer arcuate surface than is used for other types of arcuate rollers. This increase of circumferential support thereby helps to minimize permanent ovaling of the tubing under the action of lateral forces. The deleterious differential rubbing in the tubing axial direction between the roller and the tubing which would normally result from using a longer wheel arc is minimized by segmenting the arc of the wheel into multiple independently rotating elements. By minimizing the difference between the maximum and minimum rotational diameters contacting the tubing of a wheel element, the differential movement of the contacting arcuate surfaces of the rotating element is minimized. This method and apparatus for providing additional support for the tubing by using roller assemblies consisting of multiple independent coaxial rollers having concentric arcuate contact surfaces of the same diameter markedly reduces the tubing and roller or wheel rubbing which normally occurs with grooved rollers having small diameters relative to the tubing. 
     When the rollers of the present invention are utilized to develop traction on the tubing in the linear mechanical injector described in copending U.S. Provisional patent application “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender,” the traction unit of that injector relies upon an array of multiple opposed pairs of annularly arcuately grooved driven roller assemblies which are urged into contact with the tubing. The pairs of roller assemblies are mounted in an alternating pattern 90° apart so that the tubing is well supported and urged into roundness. The arcuate surfaces of the driven roller assemblies are sized to closely conform to the nominal circular cross-section of the tubing. The driven roller assemblies consist of the drive rollers and coaxial mirror-image idler rollers on each side of the tubing contact portion of the drive roller. The arcuate surfaces of each of the drive rollers and their associated idler rollers have concentric arcs of the same diameter. 
     One aspect of the present invention is a roller assembly for supporting and applying transverse loads to tubing during its injection into and withdrawal from a wellbore. This roller assembly comprises: a central roller having a primary circumferential groove with a circularly arcuate cross-section; a first outer roller having a secondary circumferential groove with a circularly arcuate cross-section on an inner side of the external diameter of the first outer roller where the grooved surface is adjacent a first side of the central roller; and a second outer roller having a tertiary circumferential groove with a circularly arcuate cross-section on an inner side of the external diameter of the second outer roller where the grooved surface is adjacent a first side of the central roller; wherein the central roller and the first and second outer rollers are independently rotatable coaxial rollers, the primary, secondary and tertiary grooved surfaces having the same arc diameter and being mutually concentric to form a substantially continuous circularly arcuate tubing contact surface. The circularly arcuate tubing contact surface of the roller assembly comprises a primary groove surface that extends about 60° and a secondary and tertiary surfaces that extent on each side of the primary groove from about 30° to 40°. 
     Another aspect of the invention is an apparatus for supporting and applying transverse loads to coiled tubing during its injection into and withdrawal from a wellbore. The apparatus comprises: a central roller having a first and second mirror-image cylindrical outer rollers, the first and second outer rollers having respectively a secondary and tertiary circumferential groove with a circularly arcuate cross-section on a internal side of an outer diameter of said first and second outer rollers, one outer roller situated on each side of the central roller with the internal side facing the central roller and having a small clearance gap between the central roller and the outside rollers; wherein the central roller and the outer rollers are independently rotatable coaxial rollers, the primary, secondary and tertiary grooves having substantially equal arc diameters and being mutually concentric to form a substantially continuous circularly arcuate tubing contact surface; whereby when coiled tubing is placed in the circularly arcuate tubing contact surface, the movement of the coiled tubing will independently rotate the central and outer rollers. The apparatus may comprise a central non-rotating shaft passing through a through-bore in the central roller and the outer rollers, wherein the independently rotatable central roller and outer rollers are supported by the shaft and rotate about said shaft; or the apparatus may comprise a central rotating shaft passing through a through-bore in the central roller and the outer rollers, wherein the central roller is integral with the rotatable shaft and rotates with the rotatable shaft and the outer rollers are independently rotatable about the rotatable shaft. 
     Yet another aspect of the invention is a method for supporting and applying transverse and longitudinal loads to coiled tubing during its injection into and withdrawal from a wellbore. This method comprises the following steps: (a) feeding a coiled tubing through a functional path of a coiled tubing injector such that the coiled tubing is in contact with a plurality of roller assemblies. Each roller assembly having a central roller having a primary circumferential groove with a circularly arcuate cross-section, a first outer roller having an exposed secondary circumferential groove with a circularly arcuate cross-section on an inner side of said first outer roller where the secondary groove is adjacent a first side of the central roller; and a second outer roller having an exposed tertiary circumferential groove with a circularly arcuate cross-section on an inner side of the second outer roller where the tertiary groove is adjacent a second side of the central roller; wherein the central roller and the first and second outer rollers are independently rotatable coaxial rollers, the primary, secondary and tertiary grooves having the same arc diameter and being mutually concentric to form a substantially continuous circularly arcuate tubing contact surface; and (b) operating the coiled tubing injector to cause the roller assemblies to bear transversely on the coiled tubing so that tangential friction is developed between said rollers and the tubing, thereby permitting longitudinal driving forces to be transferred from the rollers to the tubing when the central rollers of the roller assemblies are rotationally driven; whereby when the coiled tubing moves by the circularly arcuate tubing contact surface the tangential friction between the rollers and the tubing cause the rollers to independently rotate. 
     The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed might be readily utilized as a basis for modifying or redesigning the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are believed to be characteristic of the invention, both as to its organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an oblique view of one embodiment of an idler roller with independently rotating center tire and two flanking outside tires mounted on a dead shaft; 
     FIG. 2 is a transverse sectional view along the roller shaft axis of the idler roller of FIG. 1; 
     FIG. 3 is an oblique view of another embodiment of an idler roller mounted on a live shaft and using the rollers of this invention; 
     FIG. 4 is a transverse sectional view along the roller shaft axis of the idler roller of FIG. 3; 
     FIG. 5 is an exploded oblique view of yet another embodiment consisting of a drive roller assembly which can be used in the drive modules of an injector; 
     FIG. 6 is an oblique view of a drive module for an injector which utilizes the drive roller assembly of FIG. 5; 
     FIG. 7 is a partial longitudinal sectional view of one of the injector drive modules of FIG. 6; 
     FIG. 8 is an oblique view of a linear injector using the drive roller assembly of FIG. 5; 
     FIG. 9 is an exploded oblique view showing two adjacent drive module pairs and their mounting in a segment of the injector body of FIG. 8; and 
     FIG. 10 is a partial, nonexploded transverse cross-sectional view of the segment of the housing of the injector corresponding to FIG.  9  and showing an opposed drive module pair installed in the housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and initially to FIG. 1, it is pointed out that like reference characters designate like or similar parts throughout the drawings. The Figures, or drawings, are not intended to be to scale. For example, purely for the sake of greater clarity in the drawings, wall thickness and spacing are not dimensioned as they actually exist in the assembled embodiment. 
     Referring to FIGS. 1 and 2, one embodiment of this invention is shown as a dead shaft idler roller. Dead shaft idler wheel assembly  1  mounts dead shaft roller assembly  7  on nonrotating dead shaft  3  which is in turn mounted in support frame  2 . Support frame  2  consists of an U-shaped base with a flat back which may be welded or bolted to a suitable. structural support and having symmetrical rectangular ends projecting normally from the flat back. Coaxial semicircular grooves which tightly fit to cylindrical dead shaft  3  are centrally located on the outer faces of the support frame  2  both parallel to and opposed to the flat back of support frame. Drilled and tapped holes symmetrically straddling the grooves in the outer faces of support frame  2  and perpendicular to the flat back mount threaded studs  5 . 
     Dead shaft  3  is a symmetrical round bar having a larger diameter central portion which serves as a bearing race and two outboard reduced diameter extensions which can be clamped into support frame  2 . At each outer end of the central bearing race portion of dead shaft  3  is an annular snap ring groove. Two identical shaft retainers  4  consist of rectangular bar stock with a central semicircular groove on one side perpendicular to the long axis of the shaft retainer. The semicircular grooves on shaft retainers  4  closely fit dead shaft  3 . Perpendicular to and equally offset from the axis of the semicircular groove of shaft retainer  4  are through bolt holes for the studs  5 . Dead shaft  3  can be clamped in support frame  2  by mounting shaft retainers  4  over studs  5  and using threaded nuts  6  mounted on studs  5  to force the shaft retainers against the shaft. 
     Dead shaft roller assembly  7  is rotationally mounted on dead shaft  3  and consists primarily of a center tire  8  and two flanking outside tires  14  with their supporting needle bearings  11 . Center tire  8  is an annular ring having an external symmetrical circularly arcuate face  9 , transverse ends, and a through bore with a counterbored bearing pocket  10  having a transverse end into which needle bearing  11  may be press-fitted. The generating diameter D of arcuate face  9  corresponds to that of the tubing which will be supported or contacted by center tire  8 , while the arc sector of the circularly arcuate face covers an angle θ of approximately 30° to either side of the middle of the arc. The minimum radius of rotation of the center tire  8 , denoted by R, occurs in the middle of the arc, while the maximum radius of rotation of the center tire is R+(D/2)×(1−Cos θ). 
     Outside tire  14  is an annular right cylindrical ring with an arcuate face  15  on one side of its outer diameter. The arcuate face  15  of outside tire  14  has its generating circle of the same size as that of arcuate face  9  of center tire  8  so that, when concentric with the arcuate face  9 , arcuate face  15  serves as a continuation of arcuate face  9  with only a small clearance gap. The arc for outside tire  14  will cover a sector of approximately 30° to 40°. The minimum rotational radius for outside tire  14  is R+(D/2)×(1−Cos α), while the maximum rotational radius is R+(D/2)×(1−Cos β). The central bore through outside tire  14  is sized to clear dead shaft  3 . The central bore is counterbored with a transverse bottom end from the side opposite the arcuate face  15  to form a bearing pocket  16 . A needle bearing  11  is press-fitted into bearing pocket  16 . One outside tire  14  is mounted on each side of center tire  8  on dead shaft  3  so that, with the exception of a small clearance gap between the outside tire  14  on each side and the center tire, a long continuous arcuate surface is created for contacting the tubing. One snap ring  17  per side is used in the snap ring grooves of dead shaft  3  to aid in retention of the center tire  8  and outer tires  14  on the dead shaft. The outline of the tubing  18  is indicated in FIG. 2 to show how the center tire  8  and outside tires  14  support the tubing. 
     Another embodiment of this invention is shown as a live shaft idler roller in FIGS. 3 and 4. Live shaft idler wheel assembly  20  mounts live shaft roller assembly  27  in a support frame composed of base plate  21  and two end plates  22 . Base plate  21  is rectangular and has a drilled and tapped hole normal to the large upper flat surface of the base plate symmetrically placed near each corner. Base plate  21  may be welded or otherwise attached to a suitable structural support. End plates  22  are rectangular and mounted with their larger flat faces perpendicular to the large upper flat face of base plate  21  and placed in mirror image positions about the middle of the base plate. End plates  22  are provided with vertical through bolt holes placed in their upper surfaces symmetrically about the midplane so that the through holes are coaxial with the tapped corner holes in the base plate  21 . Parallel to the base plate and on the vertical center plane of the base plate  21  and the end plates  22  is a through hole  23  with a flat-bottomed bearing pocket counterbore  24  on the inboard side of the end plates  22  when they are mounted on the base plate  21 . One stud  25  is threaded into each of the drilled and tapped corner holes of the base plate  21  and is passed through the corresponding coaxial vertical through bolt hole of end plate  22 . One nut  26  is used on each stud  25  to clamp end plate  22  to base plate  21 . 
     Live shaft roller assembly  27  consists of live shaft  28  having an integral center tire  29 , two mirror-image outer tires  34 , and the bearings and snap rings necessary to support the tires and shaft. Live shaft  28  is a symmetrical round bar having an enlarged integral center tire  29  having a circular arcuate face  30 , two adjacent larger diameter interior bearing journals  31 , and two reduced diameter outer bearing journals  32 . The outer ends of live shaft  28  fit loosely in the through bores  23  of end plates  22 . At each outer end of the interior bearing journals  31  of live shaft  28  is an annular snap ring groove  33 . As is the case for the first embodiment of this invention, the generating diameter D of arcuate face  30  corresponds to that of the tubing which will be supported or contacted by center tire  29 , while the arc sector of the circularly arcuate face covers an angle θ of approximately 30° to either side of the middle of the arc in order to provide tubing support. As before, the minimum radius of rotation of the center tire  29 , denoted by R, occurs in the middle of the arc, while the maximum radius of rotation of the center tire is R+(D/2)×(1−Cos θ). 
     Outer tire  34  is an annular right cylindrical ring with an arcuate face  35  on one side of its outer diameter. The arcuate face  35  of outer tire  34  has its generating circle of the same size as that of arcuate face  30  of center tire  29  of live shaft  28  so that, when concentric with the arcuate face  30 , arcuate face  35  serves as a continuation of arcuate face  30  with only a small clearance gap. The arc for outer tire  34  will cover a sector of approximately 30° to 40°. As with the previously described embodiment of this invention, the minimum rotational radius for outer tire  34  is R+(D/2)×(1−Cos α), while the maximum rotational radius is R+(D/2)×(1−Cos β). The central bore through outer tire  34  is sized to clear live shaft  28 . The central bore is counterbored with a transverse bottom end from the side opposite the arcuate face  35  to form a bearing pocket counterbore  36 . An inner needle bearing  38  is press-fitted into bearing pocket  36  and will run on an interior bearing journal  31 . One outer tire  34  is mounted on each side of center tire  29  on live shaft  28  so that, with the exception of a small clearance gap between the outer tire  34  on each side and the center tire  29 , a long continuous arcuate surface is created for contacting the tubing. An outer bearing  39  is pressed into the bearing pocket counterbore  24  of each of the end plates  22  to support live shaft  28 . One snap ring  40  per side is used in the snap ring grooves of live shaft  28  to aid in retention of the outer tires  34  adjacent to the center tire  29  on the live shaft  28 . The outline of the tubing  42  is indicated in FIG. 4 to show how the center tire  29  and outside tires  34  support the tubing. 
     Yet another embodiment of the invention is a driven roller which is shown in an exploded oblique view in FIG.  5 . Driven roller assembly  60  consists of drive roller  61 , two identical idler rollers  70 , and the needle bearings  73  and snap rings  74  required for assembly. Drive roller  61  has a central integral tire with a symmetrical circular arcuate drive face  62  and with a first outboard bearing journal  63  and second outboard bearing journal  64  of the same diameter at its opposed ends for support in drive module  80  by needle bearings  86 . Splined internal socket  65  is mounted on the outer end of first outboard bearing journal  63  for engagement with output shaft  101  of drive motor  100  so that the driven roller assembly  60  may be driven in either direction of rotation. An idler journal  66  of larger diameter than either of the outboard bearing journals  63  and  64  is located on each side adjacent to the central tire of drive roller  61 . Located at the outer ends of idler journals  66  and equispaced from the central tire of drive roller  61  are annular snap ring grooves  67 . As is the case for the other embodiments of this invention, the generating diameter D of arcuate drive face  62  of drive roller  61  corresponds to that of the tubing which will be supported or contacted by the drive face, while the arc sector of the circularly arcuate face covers an angle θ of approximately 30° to either side of the middle of the arc in order to provide tubing support. As before, the minimum radius of rotation of the center tire of drive roller  61 , denoted by R, occurs in the middle of the arc, while the maximum radius of rotation of the center tire is R+(D/2)×(1−Cos θ). 
     Idler roller  70  is an annular right cylindrical ring with an arcuate support face  71  on one side of its outer diameter. The arcuate face  71  of idler roller  70  has its generating circle of the same size as that of arcuate face  62  of the central tire of drive roller  61  so that, when concentric with the arcuate face  62 , arcuate face  71  serves as a continuation of arcuate face  62  with only a small clearance gap. The arc for idler roller  70  will cover a sector of approximately 30° to 40°. As is the case for the other embodiments of this invention, the minimum rotational radius for idler roller  70  is R+(D/2)×(1−Cos α), while the maximum rotational radius is R+(D/2)×(1−Cos β). The central bore through idler roller  70  is sized to clear the idler journals  66  of drive roller  61 . The central bore is counterbored with a transverse bottom end from the same side as that with the arcuate face  71  to form a bearing recess counterbore  72 . A needle bearing  73  is press-fitted into bearing pocket  72  and will run on an idler bearing journal  66 . One idler roller  70  is mounted on each side of the center tire of drive roller  61  so that, with the exception of a small clearance gap between the idler roller  70  on each side and the center tire, a long continuous arcuate surface is created for contacting and supporting the tubing. A snap ring  74  is inserted into each of the two snap ring grooves  67  in drive roller  61  to hold idler rollers  70  in position relative to drive roller  61  so that their arcuate support faces  71  are concentric with that of arcuate drive face  62 . 
     The driven roller assembly  60  shown in FIG. 5 is shown with an injector based on the injector shown in copending U.S. Provisional Patent Application “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender.” This coiled tubing injector is described in FIGS. 6-10. 
     Referring to FIGS. 6 and 7, a drive module  80  based on the driven roller assembly  60  of the third embodiment of this invention is shown. Multiple drive modules  80  are utilized in the coiled tubing injector of copending U.S. Provisional Patent Application filed Jul. 11, 2001, entitled “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender,” which is incorporated by reference herein. Drive module  80  consists of an approximately square cross-section drive module body  81 , a hydraulic drive motor  100 , and driven roller assembly  60 , along with associated hardware. Drive module body  81  is made from a square cross-section bar with radiused corners on the cross-section. Drive module body  81  has a through bore with two mirror-image counterbores having internal transverse shoulders  85 . Transverse square motor mount flange  82  is positioned at the first end of drive module body  81 . The flange face of motor mount flange  82  is configured to mount drive motor  100  and is appropriately drilled and tapped to receive the motor mounting screws  102 . In its middle, drive module body  81  has a transverse arcuate cross-section window  83  cut through two opposed sides and one of their adjoining sides to provide clearance for the tubing when the drive module  80  is positioned adjacent the tubing. Square outer flange  84  is mounted to the transverse second end of drive module body  81  by a comating pattern of drilled and tapped holes near the flange corners by outer flange screws  89 . The internal transverse shoulders  85  are located on each side of and adjacent to window  83  in the bore of drive module body  81 . 
     A needle bearing  86  is mounted in each of bearing retainers  87  and  92 , which are in turn inserted into drive module body  81  in order to support the driven roller assembly  60  by the first and second outboard bearing journals  63  and  64 , respectively, of drive roller  61 . First bearing retainer  87 , mounted on the drive motor side, is a short right circular cylinder having an on-center through hole larger than the journals of drive roller  61  and, on its side adjacent the driven roller assembly rollers, a counterbore with a transverse shoulder at its bottom. A needle bearing  86  is pressed into the counterbore of first bearing retainer  87  to bear against the transverse shoulder at the bottom of the counterbore. Right circular cylindrical first tubular spacer sleeve  88  is located between drive motor  100  and first bearing retainer  87  within the bore of drive module body  81  to maintain drive roller assembly  60  properly spaced from the motor and axially centered within drive module body  81 . The outer, motor end of first tubular spacer sleeve  88  is counterbored to receive the alignment boss of the case of drive motor  100  and thereby align motor  100  within the drive module body  81 . 
     Second bearing retainer  92 , mounted outboard of each bearing  86 , consists of a short cylinder with a small central through bore having two counterbores on its inner end. The smaller interior counterbore is sized to clear the second outboard bearing journal  64  of drive roller  61  of driven roller assembly  60 , while the larger counterbore is sized to mount needle bearing  86 . The needle bearing  86  mounted in second bearing retainer  92  supports second outboard bearing journal  64  of drive roller  61  of driven roller assembly  60 . Second bearing retainer  92  slip fits into the bore of drive module body  81  with its counterbores facing inwardly and the end of its larger counterbore abutting the end of bearing  86 . Right circular cylindrical second tubular spacer sleeve  93  is located between outer flange  84  and second bearing retainer  92  within the bore of drive module body  81  to maintain drive roller assembly  60  properly spaced from the motor and prevent axial play. 
     Drive motor  100  is a small reversible hydraulic motor of gear motor, piston motor, or gerotor construction and with a splined output shaft  101 . The output shaft end of the case of drive motor  100  has a short round mounting alignment boss which is mated with the counterbore of first spacer sleeve  88 . Drive motor  100  is mounted to motor mount flange  82  of drive module body  81  by motor mount screws  102 . Multiple hydraulic ports  103  handle the pressurized fluid supply for drive motor  100 , providing operator selectable pressure and return connections. Hydraulic case drain port  104  is also provided on the motor to handle internal leakage as shown in FIG.  9 . The position of the tubing is indicated by dashed circle  107  in FIG.  7 . 
     Referring to FIG. 8, a coiled tubing injector unit  120  for a coiled tubing rig based on the improved driven roller assembly of the third embodiment of this invention is shown in an oblique view. Except for the construction of the driven roller assemblies and their supports, the injector  120  has the same construction as that of the traction unit of copending U.S. Provisional Patent Application “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender”. The tubing is fed substantially coaxially through the injector unit  120  and into the well for performing well operations known to those skilled in the art. Typically the injector is connected sequentially to an adapter spool, blowout preventers, and the wellhead on its bottom end, while on its top end it may be connected to either a gooseneck or other apparatus. This other hardware is not shown here, but is well known by those skilled in the art of coiled tubing manipulation. 
     Injector unit  120  consists of traction drive body  121  and multiple drive modules  80  mounted therein. Lower transverse flange  122  at the bottom end and upper transverse flange  123  at the upper end are welded to traction drive body  121  to permit bolting the injector unit to the other hardware which is required. Traction drive body  121  consists primarily of a length of steel square structural tubing approximately 16×16 inches in cross section and having approximately a ⅝ inch wall. It may be seen in FIG. 8 that injector unit  120  has a repetitive array of multiple drive modules  80  extending from each of its four lateral sides. FIG. 9 shows an exploded oblique view of a portion of the injector unit  120  illustrating how traction drive body  121  holds two opposed pairs of drive modules  80 . The components shown in FIG. 9 are arrayed in a repetitive pattern up the length of injector unit  120 . Identical rectangular coaxial lower and upper drive module ports  126   a,b  and  127   a,b,  respectively, with rounded corners are transversely cut with mirror image symmetry about a longitudinal midplane of symmetry of traction drive body  121  to mount each opposed pair of drive modules  80 . Ports  126   a,b  and  127   a,b  are elongated slightly in the direction normal to the midplane plane of symmetry. In the same transverse plane containing ports  126   a,b  and  127   a,b  but normal to the aforementioned longitudinal midplane are two pairs of coaxial threaded squeeze cylinder mount holes  128  which are used to threadedly mount two pairs of opposed, inwardly looking hydraulic squeeze cylinders  129 . The holes  128  are symmetrical about the centerline of traction drive body  121 . 
     Short-stroke double-acting squeeze cylinders  129  each have a male thread on the rod end of their stub cylindrical bodies and are threaded into mount holes  128 . The squeeze cylinders may be seen more clearly in FIG.  10 . Each cylinder  129  has a piston rod  130  which has a flat outer end. Although it is not shown here, a cylinder bias spring may be mounted internally to squeeze cylinder  129  in order to bias rod  130  to extend. Such a bias spring is disclosed in copending U.S. Provisional Patent Application “Coiled Tubing Injector Utilizing Opposed Drive Modules and Having an Integral Bender”. Adjacent a first opposed pair of drive module ports  126   a,b  and  127   a,b  and its associated cylinder mount holes  129  is a similar arrangement of ports and cylinder mount holes which has its midplane of symmetry rotated 90° relative to the first set. These ports  126   a,b  and, similarly,  127   a,b  are configured to accept axial insertion and mounting therein of drive modules  80 . For clearance reasons, the drive modules  80  may be inserted from opposite directions into ports  126   a,b  and  127   a,b,  as is shown in FIGS. 8,  9 , and  10 . When drive modules  80  are being inserted into the mounting ports in traction drive body  121 , piston rods  130  of squeeze cylinders  129  are retracted. Cylinders  129  are positioned to urge drive modules  80  toward the centerline of traction drive body  121  so that the driven roller assemblies of the drive modules  80  can transversely contact any coiled tubing which is deployed through the injector unit  120 . Alternatively, one or both of the cylinders  129  can be replaced by one or more springs for urging the drive modules  80  toward the centerline of traction drive body  121  causing the drive modules to grip the tubing. 
     Operation of the Invention 
     The first embodiment of the invention can be used as an idler roller, a guide roller, or as a hold-down roller. In service, the dead shaft idler wheel assembly  1  is positioned so that the arcuate faces  9  and  15  of, respectively, the center tire  8  and the two outside tires  14  are in contact with the tubing  18  and exerting normal force on the tubing. As the coiled tubing injector moves the tubing by the roller, the tangential friction between the tires of the roller segments and the tubing cause the tires  8  and  14  to independently rotate on their bearings  11  about dead shaft  3 . All three tires  8  and  14  of the dead shaft roller assembly contribute to the lateral support of the tubing cross-section so that ovaling tendencies are minimized. 
     The operation of the first embodiment of the improved rollers of this invention, using a non-rotating dead shaft, benefits significantly from the separation of the arcuate surface of the dead shaft roller assembly  7  into independently rotating segments. The effective radius of curvature of the axis of any tubing  18  contacting the roller assembly  7  is substantially larger than the rotational radius of any portion of the roller arcuate faces  9  and  15 , so some rubbing due to differential movement in the axial direction will result. For practical purposes, the radius of curvature of the tubing may be assumed to be infinite; i.e., the tubing is straight. If the roller shown in FIG. 2 were integrated and not segmented, then the smallest rotational radius of the integrated roller would be R, while the largest would be R+(D/(D/2)×(1−Cos β), where β is the effective half-angle of the contact arcuate face. The effective radius of rotation, R E , the radius of the integrated roller which would have no relative slippage or rubbing with the tubing, would then lie somewhere between the smallest and largest rotational radius. Thus, the maximum amount of relative axial motion of the contacting integrated roller and the tubing would occur where the largest value of the difference between R E  and the rotational radius of the arc occurs. This difference and the attendant maximum rubbing action between the tubing and the roller could thus be quite large and objectionable. By segmenting the roller, as is done for dead shaft roller assembly  7 , effective radius of rotation R E1  for the arcuate face  9  of center tire  8  lies between R and R+(D/2)×(1−Cos θ). Likewise, for arcuate face  15  of outside tire  14 , effective radius of rotation R E2  lies between R+(D/2)×(1−Cos α) and R+(D/2)×(1−Cos β). The maximum amounts of rubbing are related to the maximum differences between R E1  and any point on the rotational radius of arcuate surface  9  of center tire  8  and R E2  and any point on the rotational radius of arcuate surface  15  of outside tire  14 . Therefore the rubbing for a segmented roller such as dead shaft roller assembly  7  is necessarily reduced relative to a non-segmented roller because arcuate face half-angles α and θ are both less than β and the possible range of rotational radius differences are accordingly reduced. 
     The application and operation of the second embodiment of this invention, the live shaft idler wheel assembly  20 , is very similar to that of the dead shaft idler wheel assembly  1 . The live shaft idler wheel assembly  20  embodiment of the invention can be used as an idler roller, a guide roller, or as a hold-down roller. In service, the live shaft idler wheel assembly is positioned so that the arcuate faces  30  and  35  of, respectively, the integral center tire  29  of live shaft  28  and the two outer tires  34  are in contact with the tubing  42  and exerting normal force on the tubing. As the coiled tubing injector moves the tubing by the roller, the tangential friction between the tires of the roller segments and the tubing cause the tires to independently rotate on their bearings  39  about live shaft  28 , while live shaft  28  rotates within its bearings  38 . All three arcuate faces  30  and  35  contribute to providing lateral support to the tubing  42  so that ovaling of the tubing is minimized. Since the same arcs and radii are used for each of the corresponding arcuate faces of the live shaft idler roller assembly  27  as for the dead shaft roller assembly  7  of the first embodiment and both embodiments have independent rotation of the roller segments, the rubbing tendencies of the described embodiments are identical. 
     The application and operation of the driven roller assembly  60  is very similar to those of the dead shaft idler wheel assembly I and the live shaft idler wheel assembly  20 . The driven roller assembly  60  embodiment of the invention can be used as a powered idler roller, a powered guide roller, a powered hold-down roller, or as a drive roller in a coiled tubing injector. In service, the driven roller assembly is positioned so that the arcuate faces  62  and  71  of, respectively, the integral center tire of drive roller  61  and the two idler rollers  70  are in contact with the tubing and exerting normal force on the tubing. Drive roller  61  is supported by needle bearings  86  of the drive module  80 . When torque is applied to drive roller  61  through splined internal socket  65  by motor  100  of drive module  80 , friction and the normal force between arcuate drive face  62  and the tubing permit drive roller to exert tangential drive forces on the tubing in the direction of the tubing axis. As the coiled tubing moves by the driven roller assembly, the tangential friction between the arcuate faces  71  of the roller segments and the tubing cause the idler rollers  70  to independently rotate on their bearings  73  about idler journals  66 . All three arcuate faces  62  and  71  contribute to providing lateral support to the tubing  107  so that ovaling of the tubing is minimized. Since the same arcs and radii are used for each of the corresponding arcuate faces of the driven roller assembly  60  of the drive module  80  as for the dead shaft roller assembly  7  of the other embodiment and both embodiments have independent rotation of the roller segments, the rubbing tendencies of the embodiments are identical. 
     The operation of a coiled tubing injector  120  using the improved rollers of the present invention is similar in many respects to that of conventional coiled tubing injectors in that it both inserts and withdraws coiled tubing from a well. However, certain critical differences exist between this device and both track-type and wheel-type injectors, as will be described below. 
     In order to feed tubing into the unit during initial loading, the hydraulic squeeze cylinders  129  of the injector unit  120  are pressurized to respectively permit moving their respective drive modules  80  away from the centerline path for the tubing. For squeeze cylinders  129 , the rods  130  are retracted so that the drive modules  80  can easily be displaced laterally within their lower  126   a,b  and upper  127   a,b  drive module ports to permit tubing passage. At this point, tubing can be fed through the injector unit  120  and thence into the blowout preventers and the well. 
     After the tubing is deployed through the units of the injector  120  and the tubing path has been inspected to ensure proper centralization, the squeeze cylinders  129  can be pressurized to extend their piston rods  130  to press on their respective drive modules  80 . This hydraulic inward biasing of the drive modules  80  results in the simultaneous and uniform gripping of the tubing between the opposed sets of driven roller assemblies  60 . The uniformity of squeeze by the driven roller assemblies  60  is ensured by manifolding all of the squeeze cylinders  129  together. 
     When driving the tubing in either direction, the appropriate ports  103  of the individual hydraulic drive motors  100  are selectably simultaneously pressurized to initiate their rotation and that of the attached driven roller assemblies  60  in the desired direction. The motors  100  are manifolded together, so only one control valve is required to actuate and control the injector unit  120 . For clarity, the interconnecting hydraulic tubing and the hydraulic system components are not shown, but these items are well known to those skilled in the art. Because squeeze cylinders  129  exert a substantial normal load on the tubing from the driven roller assemblies  60 , the frictional shear required between drive rollers  61  and the tubing in order to modify the axial force on the tubing can be developed. Since the tubing is well supported around its circumference by any opposed set of driven roller assemblies  60  and likewise is supported on a different axis rotated 90° apart by the adjacent sets of driven roller assemblies  60  on either side, ovalization of the tubing is minimized. At the same time, the segmentation of the driven roller assembly  60  results in much reduced tubing rubbing, so that the rollers and tubing both wear less. 
     Advantages of the Invention 
     Both the first embodiment dead shaft idler wheel assembly  1  and the second embodiment live shaft idler wheel assembly  20  can be used very satisfactorily as either guide rollers or as hold-down rollers for coiled tubing injection and handling systems. These particular applications of rollers are associated with large transverse loads being applied to the tubing, with an increasing tendency for tubing ovaling under the lateral load when the load is increased. Likewise, the tendency for rubbing is increased with increasing roller lateral load. A highly loaded non-segmented roller cannot have both a small radius of rotation R and a large half-angle of support β without causing very severe rubbing. However, the improved segmented rollers of this invention can simultaneously provide both large half-angles of support β for avoiding ovalization of the tubing and high transverse load capacities with a small radius of rotation R without severe rubbing. The ability to use smaller rollers than would otherwise be possible without this invention permits downsizing coiled tubing equipment based on these rollers, with associated weight and cost reductions. These same advantages pertain to the driven roller assembly  60 . 
     The injector of copending provisional Patent Application Serial No. 60/304,681 filed Jul. 11, 2001 offers several important advantages over conventional tubing injectors when it is used with the rollers of this invention. A very significant advantage is the relatively small size and weight of the injector. This feature is important for areas where significant weight limits are placed on vehicles. Another advantage is the modularity of the unit, which leads to fabrication savings, inventory minimization, and improved serviceability. Assembly and disassembly are both very simple for this construction, so the changing out of drive modules is easy and rapid. The use of multiple drive modules also adds a high level of redundancy to the system, thereby improving its reliability. 
     A further advantage is that load sharing of the drive modules is improved. For both conventional track-type injectors and wheel injectors, some slippage or tubing strain must occur because the strain in the tube builds in the direction of increasing tension, while for both track and wheel injectors, the strain in the track or wheel builds in the opposite direction. In the case of the injector of this invention, the individual roller contact patches on the tubing are relatively small and there is less influence of this effect. The alternation of tubing support directions by the drive rollers aids in avoiding ovaling of the tubing under side loads. This basically full support of the tubing is highly desirable for improving tubing life. 
     These and other advantages will be obvious to those skilled in the art. It may be understood readily that certain detail changes from the design herein are still within the scope of this invention. In particular, different types or combinations of bearings may be more suitable for supporting the tires, idlers, and rollers of the different embodiments. While the needle bearings shown offer good radial load capacity, they are limited in axial thrust capacity. Accordingly, bearings more suited to providing both radial load and thrust load capability may be substituted for the needle bearings. Alternately, separate thrust bearings may be used to bear on the outside transverse ends of the idlers to absorb the unbalanced axial thrust loads while the needle bearings are used to support the radial loads.

Summary:
A means and method for supporting and applying transverse loads to coiled tubing during its injection into and withdrawal from a well by using novel rollers having coaxial segmented arcuate faces. The arcuate faces have arcs with the same radius as that of the tubing to be supported and are mutually concentric, but are independently rotatable. The novel rollers may be both driven and undriven.