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BACKGROUND OF INVENTION 
     The present invention relates to power tongs typically used in the oil and gas industry to make up and break apart threaded joints on pipe, casing, and similar tubular members. 
     Power tongs have been used for many years and are generally employed in the oil and gas industry to grip and rotate tubular members, such as tubular. The tubular members are gripped with high compressive forces while applying a high degree of torque to break apart or tighten threaded tubular member connections. In most cases, power tong designs employ a cam mechanism for converting a portion of the torque into a gripping (compressive) force normal to the tubular member. This conversion is often accomplished using a power-driven ring gear having an interior cam surface. As the ring gear rotates, cam follower (roller) on a jaw member rides upon the cam surface, causing the follower (and thus the jaw member) to move into contact with the tubular member. U.S. Pat. No. 4,404,876 discloses such an arrangement. 
     Most current power tong designs include a ring gear cam with an open slot or throat (“open-throat power tongs”), through which the tubular member passes to position the jaw members around the tubular member. However, some tong designs employ a ring gear cam that has no open throat and is thus a solid circular member. This solid circular ring gear design is generally referred to as a closed-head power tong. 
     When the drilling environment allows the use of a derrick or similar overhead structure from which a power tong may be suspended, an openthroat power tong is often a more efficient tool. Open-throat power tongs easily disengage and/or retract from the tubular member when the operator desires to raise, lower, or otherwise manipulate the tubular member. 
     On the other hand, a closed-head power tong is more difficult to retract from the tubular member because it has a closed throat and must pass over the end of a tubular member. However, there are common drilling environments where there is no structure from which an open-throat power tong may be suspended and insufficient workspace to engage and retract open-throat power tongs. In such environments, a closed-head power tong may be the only practical alternative. Closed-head power tongs are highly useful during operations where snubbing units are employed. Typically, closed-head power tongs are positioned over the drill string with the individual tubular members forming the drill string extending through a center aperture in the closed-head power tong. A tubular member is moved vertically through the center aperture until the threaded joints for connecting adjacent tubular members are in position to be made up (screwed together) or broken out (unscrewed). 
     To increase efficiency, as many successive tubular member connections as possible are made-up or broken out without interruption, i.e. having to move the center aperture of the tong out of alignment with the drill string. However, the drill string may include a down hole tool or other device that has a diameter greater than the diameter of the center aperture of the closed-head power tong. In these situations, the prior art closed-head power tongs typically require that the drill string be broken and any tubular positioned in the center aperture at that point be removed therefrom. The closed-head power tongs are then removed from alignment with the drill string, and the oversized tool is removed from the drill string or re-positioned vertically along the drill string above or below the power tong such that it is not required to pass through the power tong. The closed-head power tong can then be re-aligned with the drill string. To re-establish the connection of the drill string through the center aperture, the unconnected tubular joint must be positioned above or below the center aperture to reconnect to a continuous length of drill string. 
     What is need in the art is a closed-head power tong design which allows the center aperture to be readily increased in diameter without the necessity of removing the drill string from the center aperture. The closed-head power tong design should provide for center aperture enlargement with a minimum lost time and with such simplicity that unskilled workers could perform the task. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     With the aforementioned considerations in mind, it is therefore an object of this invention to provide a power tong assembly used in connection with a conventional snubbing unit. 
     It is a further object of the present invention to provide a closed head power tong that is adapted to increase the gripping aperture therein to allow passage of objects having a diameter larger than the normal gripping aperture. 
     It is a further object of the present invention to provide a closed head power tong that may be quickly and efficiently operated without the necessity of removing the power tong from alignment with the drill string. 
     The invention herein comprises a closed-head power tong having a power tong body. The tong body has a ring gear positioned within the body, and the ring gear comprises at least one cam surface. The tong body also has a cage plate assembly comprising at least two mating inserts and a jaw aperture formed in at least one of the mating inserts. 
     The invention may also comprise a power tong body having a ring gear positioned within the body wherein the ring gear has at least one cam surface. A cage plate assembly is removably positioned at least partially within the body and configured to have a gripping aperture therein. The gripping aperture has first dimension and the cage plate assembly is adapted to increase the gripping aperture to a second dimension sufficiently sized to allow passage of an object having a dimension larger than the first dimension. 
     A method of making up or breaking one or more section of tubular members using the closed head power tong invention is also disclosed herein. The method comprises the steps of (a) removing the cage plate assembly from the power tong body, thereby increasing the diameter of the gripping aperture to allow passage of a tool; and (b) passing the tool through the power tong body a sufficient distance so that the cage plate assembly may be repositioned within the power tong body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a side view of the invention employed in a conventional snubbing unit. 
     FIG. 2 a  illustrates a side view of the invention. 
     FIG. 2 b  illustrates an upper perspective view of the invention with the top plate removed. 
     FIG. 2 c  illustrates a cross-sectional view of the spring assembly used to mount the power tong to the leg assembly. 
     FIG. 2 d  illustrates a side view of the link assembly that mounts the power tong to the leg assembly. 
     FIG. 3 illustrates an upper perspective view of the mating inserts inserted within the upper cage plate shown without the jaw assemblies positioned in the jaw apertures. The lower cage plate is not shown. 
     FIG. 4 illustrates a lower perspective view of the mating inserts inserted within the upper cage plate shown without the jaw assemblies positioned in the jaw apertures. The lower cage plate is not shown. 
     FIGS. 5 a  and  5   b  illustrate an embodiment of the mating inserts shown attached (FIG. 5 a ) and detached (FIG. 5 b ). 
     FIGS. 5 c  and  5   d  illustrate another embodiment of the mating inserts shown attached (FIG. 5 c ) and detached (FIG. 5 d ). 
     FIGS. 6 a  and  6   b  illustrate embodiments of the ring gear with three camming surfaces engaging the jaw assemblies. 
     FIG. 7 illustrates an upper view of the power tong with the upper tong plate and upper cage plate removed. 
     FIG. 8 illustrates an upper view of the power tong with the cage plate assembly removed. 
     FIGS. 9 a  and  9   b  illustrate embodiments of the back-up used in combination with the present invention. 
     FIG. 10 illustrates a cross-section view of the interrelation of the upper and lower cage plates, the ring gear, and the mating inserts taken along the A—A axis shown in FIG.  3 . 
     FIG. 11 illustrates an exploded view of the invention shown with the mating inserts removed from within the power tong. 
     FIG. 12 illustrates a partial bottom view of the power tong showing the brake band&#39;s engagement with the brake ring. 
    
    
     DETAILED DESCRIPTION 
     Illustrations of construction, design, and methods of operation of the invention are set forth below with specific references to the Figures. However, it is not the intention of the inventor that the scope of his invention be limited to these embodiments. 
     FIG. 1 illustrates the application of the power tong-back-up combination  100  used in connection with a snubbing unit  101  positioned over an existing well. Snubbing unit  101  is equipped with slip assemblies  160 ,  170  that are used in conjunction to raise and lower tubular members  110 ,  120  as is known in the art. Snubbing unit  101  may also be equipped with a hydraulic rotary table  180  that rotates equipment positioned thereon. 
     When lengths of tubular members are joined (“made-up”) or disconnected (“broken”), tubular members  110 ,  120  are passed through the respective gripping apertures  205 ,  901  (shown in FIG. 2 b ) in power tong  200  and back-up  300 . When joint  130  (as seen in FIG. 1) is suitably positioned between power tong  200  and back-up  300 , power tong  200  engages and grips first tubular member  110  and a back-up  300  engages and grips second tubular member  120 . Power tong  200  rotates first tubular member  110  and back-up  300  grips and holds stationary second tubular member  120 . Pipe joint  130  is made-up or broken, depending upon the direction of torque applied to first tubular member  110  by power tong  200 . 
     Referring to FIG. 2 b , closed-head power tongs  200  have a power tong body composed of lower plate  704  and an upper plate  802 , both having a center aperture  801  passing therethrough as shown in FIGS. 7 and 8. FIG. 8 illustrates upper plate  802  and motors  103  operatively positioned thereon. A collar  803  having leg apertures  803   a  is positioned at each corner of upper plate  802  to assist in the mounting and operation of power tong  200  on leg assembly  102  as described below and shown in FIG. 2 b . FIG. 7 illustrates how motors  103  rotate respective gears  701 , providing rotational energy to ring gear  600  through interaction of teeth  702  and teeth  601 . 
     Referring back to FIG. 1, top plate  104  is positioned atop leg assembly  102  and fixes the position of legs  102   a  relative to each other. Top plate  104  has a passageway  107  extending therethrough substantially aligned with gripping apertures  205 ,  901 . Optionally, a tapered pipe inlet  105  is positioned atop top plate  104  and passageway  107  to guide tubular members  110 ,  120  into closed-head power tong  200  when tubular member  110 ,  120  are inserted into a well. As shown in FIGS. 2 a  and  4 , a pipe inlet  402  is attached to mating inserts  301 ,  302  to guide tubular member  110 ,  120  into center aperture  801  in power tong  200  when tubular members  110 ,  120  are raised from a well. 
     FIG. 9 a  illustrates back-up  300 , which comprises a body having a lower section  910 , a cover  920  (shown in FIG. 9 b ) and two or more, preferably three, hydraulically powered jaw assemblies  904  positioned therein. As shown in FIG. 9 a , jaw assemblies  904  have jaw carriers  903  with jaws  902  facing back-up power tong gripping aperture  901  through which tubular members pass. Jaw assemblies  904  are very similar to the jaw assemblies found in U.S. Pat. No. 4,649,777 to Buck, which is incorporated herein by reference. Back-up  300  mounts onto leg assembly  102  via apertures  905  that correspond to each leg  102  substantially as shown in FIG. 2 b.    
     Referring to FIG. 1, closed-head power tong  200  and closed-head back-up  300  are positioned on snubbing unit  101  using leg assembly  102  that allows relative vertical displacement of the power tong unit  100 . As seen in FIG. 2 b , power tong  200  locks onto legs  102   a  using spring assembly  106  (shown in FIGS. 2 c  and  2   d ) or any other suitable member known in the art. 
     Viewing FIG. 2 c , each spring assembly  106  comprises a spring tube  230  positionable over one leg  102   a  Spring tube  230  has a first end  270  comprising a collar  111  and a second end  249 . Spring tube  230  is fixedly positioned relative to leg  102   a  using a nut  232 , washer  232   a , and bolt  231 . Bolt  231  inserts coaxially through bolt passageway  233  in spring tube  230 , through holes  240  in leg  102   a , through the other side of bolt passageway  233 , through washer  232   a , and into nut  232 . As also shown in FIG. 2 d , the lower edge  251  of end  270  (collar  111 ) provides a shoulder against which first end  238  of spring cap  235  rests. 
     The second end  249  of spring tube  230  has threads  247  that mate with corresponding threads  248  on spring retainer  246 . The upper edge  256  of spring retainer  246  provides a shoulder upon which the first end  243  of spring cap  242  may rest. Spring  250  is positioned between shoulder  237  of spring cap  235  and shoulder  245  of spring cap  242 . Upper power tong plate  802  is positioned above and may rest upon spring plate lip  236  so that the weight of power tongs  200  may be supported in part by each of the four spring assemblies  106  and so that spring  250  biases spring cap  235  and upper plate  802  away from end  249 . 
     As power tong  200  grips and rotates first tubular member  110  and back-up  300  grips and holds stationary second tubular member  120 , first tubular member  110  is either forced toward or away from second tubular member  120  by action of the corresponding threads at joint  130  on tubular members  110 ,  120 . The construction and design of spring assemblies  106  allow power tong  200  to move vertically to accommodate the vertical motion of the tubular members  110 ,  120 . 
     When a joint  130  is made-up, first tubular member  110  moves toward second tubular member  120 . Accordingly, power tongs  200  move downward. Upper tong plate  802 , already in contact with lip  236 , forces spring cap  235  to compress spring  250 . Note that in an “at rest” position, upper tong plate  802  rests upon lip  236  due to gravity. When the make up is complete and the jaw dies  610  release tubular member  110 , the potential energy of spring  250  forces upper plate  802  (and hence power tong  200 ) back to its normal position. 
     When a joint  130  is broken, first tubular member  110  moves away from second tubular member  120 . Accordingly, power tongs  200  move upward. 
     Lower tong plate  704  moves upward and abuts lip  244 , causing spring cap  242  to compress spring  250 . When the joint  130  is broken and jaw dies  610  release tubular member  110 , the potential energy in spring  250  forces lower plate  704  (and hence power tong  200 ) back to its normal position. 
     Referring to FIG. 2 d , links  106 A movably attach power tong  200  to two of legs  102   a  via collar  111 , spring tube  230  and spring assembly  106 . Each of these legs  102   a  is configured with one or more holes  240  that allow bolt  231  to insert through holes  233  in collar  111  and through holes  240  in leg  102   a  Bolt  231  fixedly positions collar III relative to leg  102   a.    
     Each link  106 A comprises a first arm  112  that connects to collar  111  and a second arm  116  that connects to upper tong plate  802  (see FIG. 2 b ). Each arm  112 ,  116  connects to a third arm  113  which allows arm  112  to move vertically relative to arm  116 , allowing power tong  200  to move vertically relative to legs  102   a  as previously discussed. A locking pin  117  (shown in FIG. 2 b ) may be inserted through either pivot hole  114 ,  115  to prevent vertical displacement of power tong  200  by preventing third arm  113  from pivoting. Links  106 A also help prevent the small amount of rotational movement that can occur by power tong  200  relative to legs  102   a  It is generally desirable to engage locking pin  117  when power tong  200  is being transported or handled to prevent unexpected movement between power tong  200  and back-up  300  from injuring workers or damaging equipment. 
     Referring to FIG. 10, cage plate assembly  204  generally comprises annular upper and lower cage plates  203 ,  202 . FIG. 3 is a perspective view showing upper cage plate  203  but having lower cage plate  202  removed. While the following description refers primarily to FIG. 10, cross-reference to FIG. 3 will aid in understanding the subject matter discussed. Rollers  710  sit within power tong  200  and mount on shafts  711 , which are held in place by nuts  712 . Rollers  710  support ring gear  600  by supporting ring gear teeth  601  therewithin. Ring gear  600  is positioned between lower and upper cage plates  202 ,  203  using cam followers  330  mounted from respective plates  202 ,  203 , with nuts  331  positioned in corresponding apertures  340 ,  341 . 
     Cage plates  202 ,  203  are appropriately spaced using spacer tube  321 , as seen in FIG. 10, so that ring gear  600  and plates  202 ,  203  may freely rotate relative to one another. Cage plates  202 ,  203  are fixedly positioned relative to each other using a bolt  308  inserted through aperture  320  in upper cage plate  203 , through spacer tube  321  and into bolt hole  324  in lower cage plate  202 . 
     Viewing FIGS. 6 a  and  6   b , ring gear  600  also has one or more cam surfaces  607  that face jaw assemblies  609 . The rotation of ring gear  600  about the jaw assemblies  609  causes engagement and retraction of the jaw assemblies  609  (and jaw dies  610  thereon) with the tubular member as rollers  609 A roll upon cam surfaces  607 . Causing jaws  610  to ride upon cam surfaces  607  requires relative rotation between ring gear  600  and cage plates  202 ,  203  attached to mating inserts  301 ,  302 , which in turn carry jaw assemblies  609 . For example, see U.S. Pat. No. 4,404,876 to Eckel or U.S. Pat. No. 5,291,808 to Buck, which are incorporated by reference herein. A preferred jaw assembly  609  used with the present invention is a low friction jaw assembly such as that disclosed in U.S. Pat. No. 5,819,605 to Bangert, et al. 
     To allow initial relative rotation between ring gear  600  and cage plates  202 ,  203 , a brake band  1125  (see FIG. 12) typically applies a limited frictional force to cage plate  202  and allows ring gear  600  to rotate relative to the cage plates  202 ,  203  until jaws  610  engage the tubular member. Cage plates  202 ,  203  and ring gear  600  then rotate in unison, thereby applying torque to the tubular member. 
     Jaw assemblies  609  are shown retracted in FIG. 6 a  and engaged in FIG. 6 b  with mating inserts  301 ,  302  not shown. The lines denoted as  620 ,  621  show the minimum and maximum retraction and extension points of jaw dies  610 . 
     Viewing FIG. 12, to initially hold cages plates  202 ,  203  stationary while ring gear  600  rotates sufficiently to close jaw dies  610 , a brake band  1125  acts upon lower cage plate  202 . In the embodiment shown, brake band  1125  actually contacts brake ring  1205 , which acts as an extension of cage plate  202  as best seen in FIG.  10 . Brake band  1125  applies an initial frictional force to lower cage plate  202  through brake ring  1205 , holding cage plates  202 ,  203  stationary and allowing ring gear  600  to move relative to cage plates  202 ,  203 . Generally, it is desirable to make brake band  1125  adjustable such that it may be tightened or loosened in order to vary the amount of frictional force applied to cage plate  202 . 
     After relative rotation begins, jaw assemblies  609  mount cam surfaces  607  via jaw rollers  609 A and close on the tubular. After closing on the tubular, ring gear  600  continues to transfer torque to jaw assemblies  609  and therefore to cage plates  202 ,  203  eventually causing cage plate  202  to overcome the resisting frictional force of brake band  1125 . Because cam surfaces  607  translate torque into radial force, a higher torque needed to overcome the resistance of brake band  1125  results in a higher initial radial force being placed on the tubular prior to cage plates  202 ,  203  beginning to rotate. Therefore, the frictional resistance of the brake band  1125  is adjusted to regulate the initial radial load or initial “bite” with which jaw assemblies  609  grip the tubular. 
     The embodiment of brake band  1125  as shown in FIG. 12 is similar to the brake band disclosed in U.S. patent application Ser. No. 08/897,185, which is incorporated herein by reference. Brake band  1125  is biased against brake ring  1205  using a spring  1134 . Spring  1134  pivotally connects to brake arm  1111 . Spring  1134  also pivotally connects to power tong lower plate  704  using spring retainer  1138  and pivot pin  1139 . Brake band  1125  constantly maintains frictional resistance between brake band  1125  and brake ring  1205  during the entire operating sequence of power tong  200 . 
     As best seen in FIG. 11, positionable within the annulus of cage plate  203  (hence within center aperture  801 ) are at least two mating inserts  301 ,  302 , preferably only two as shown in FIG. 5 d . Viewing FIG. 5 d , mating inserts  301 ,  302  also have an upper lip  314  that rests upon the upper surface  318  of plate  203  (as shown in FIGS.  3  and  10 ). Mating insert sidewalls  317  extend downward from lip  314  and into center aperture  801  as seen in FIG.  11 . 
     As shown in FIG. 5 d , one or more apertures  316  are positioned within the upper surface  318  of inserts  301 ,  302  and extend through lip  314 . Apertures  316  allow upper cage plate  203  and lower cage plate  202  to be fixedly positioned relative to each other via bolts  308  and spacer tube  321  as seen in FIG.  10 . 
     Referring to FIG. 10, one or more bolts  308 , having a threaded lower end  324  extend through inserts  301 ,  302  (via bore  320  in upper cage plate  203 ) through spacer tube  321  and into bolt bore  324  in lower cage plate  202 . Bolt  308  inserts completely within aperture  316  so that inserts  301 ,  302  may be removed from center aperture  801  (as seen in FIG. 11) without removing bolt  308 . Because aperture  316  is configured only slightly larger than the head of bolt  308 , the head of bolt  308  is configured with an internal engagement surface (shown in FIG. 10) to allow a tool, such as an Allen wrench, to engage and remove or install bolt  308 . Alternatively, aperture  316  may be sized sufficiently so that a conventional wrench may engage the perimeter of the head of bolt  308 . 
     One or more connecting members  309 , such as bolts and associated washers (shown in FIGS.  3  and  10 ), connect inserts  301 ,  302  to upper cage plate  203  via bolt hole  326  so that upper and lower cage plates  203 ,  202  and inserts  301 ,  302  may rotate concurrently. 
     FIG. 11 illustrates a unit having three jaw assemblies  609 . Viewing FIGS. 4,  5   a  and  5   b , sidewall(s)  317  are configured to have jaw apertures  307  extending therethrough to allow jaw assemblies  609  (also shown in FIGS. 6 a  and  6   b ) to be positioned therein with the jaw dies  610  facing gripping aperture  205 . Viewing FIG. 11, each jaw  610  is biased in an retracted position using jaw springs  305  connected between pins  311  (best seen in FIG. 3) and the upper stems  630  on jaw assemblies  609 . Stems  630  slide within notches  306  formed in upper wall  319 . Notches  306  aid in biasing jaw assemblies  609  in a retracted position. 
     As shown in FIG. 4, mating inserts  301 ,  302  have recesses  312  formed in sidewalls  317 . Spacer tubes  321  (not shown in FIG. 4, see FIG. 10) fit into recesses  312  so that cage plates  202 ,  203  may be operatively connected by bolt  308 . Each jaw aperture  307  has a corresponding recess  401  formed in the lower portion  408  of cage plate  203  to allow jaw assembly  609  to be lifted from within cage plate  203  as seen in FIG.  11 . 
     Pipe inlet  402  is attached to the lower end  315  of mating inserts  301 ,  302  and may itself comprise mating sections  410 ,  411  that form inlet  402  when mating inserts  301 ,  302  are mated. Sections  410 ,  411  attach to lower end  315  using any suitable means, preferably using one or more bolts  406 . Inlet  402  has a tapered sidewall  403  that converges from lower end  404  to upper end  405  to assist tubular members entering gripping aperture  205 . The converging sidewall  403  is also shown in FIG.  10 . 
     Viewing FIG. 10, insert sidewalls  317  have a pin hole  322  configured therein having an opening  327  in the outer surface  342  of sidewall  317  facing spacer tube  321 . Bolt  406  has a hole therein that allows pin  323  to slide within hole  322  and through the hole in bolt  406 . When inserts  301 ,  302  are positioned within center aperture  801 , spacer tube  321  prevents pin  323  from disengaging bolt  406 , preventing inlet  402  from detaching from inserts  301 ,  302 . 
     FIGS. 5 a  and  5   b  illustrate one embodiment of mating inserts  301 ,  302  used in cage plate assembly  204 . FIG. 5 a  illustrates the mating inserts  301 ,  302  removed from cage plate assembly  204  and removed from within center aperture  801  in power tong  802 . Viewing FIG. 5 b , insert  302  comprises at least one male mating member  501 , more preferably male mating members  501 ,  502  positioned at each mating side  505  of insert  301 . Male mating members  501 ,  502  engage and abut female mating members  504  formed in the corresponding mating inserts  301 ,  302  as recesses. When mated, apertures  313  in inserts  301 ,  302  are substantially aligned with lower apertures  503  positioned in the lower male mating members  502  so that a pin  304  (see FIG. 3) or other suitable member, may be positioned therein to position mating inserts  301 ,  302  relative to each other. 
     FIGS. 5 c  and  5   d  illustrate an alternate embodiment of the mating inserts  301 ,  302 . FIG. 5 c  illustrates mating inserts  301 ,  302  removed from cage plate assembly  204  and removed from within center aperture  801  in power tong  802 . As shown, inserts  301 ,  302  are simply sectioned and have only mating sides  505   a  with no mating members to hold inserts  301 ,  302  together. 
     FIG. 5 c  illustrates gripping aperture  205  having its smallest diameter, referred to as a first dimension and marked as first dimension α in FIG. 5 c . FIGS. 5 d  and  11  illustrate how sections  301 ,  302  can be removed from within center aperture  801  and detached to allow the passage of a downhole tool  150 , having a dimension larger than first dimension α but smaller than the dimension β of center aperture  801  as best seen in FIG.  8 . As used herein “dimension,” when used to refer to the size of an aperture shall mean the distance necessary for an object, such as downhole tool  150 , to pass therethrough. When the downhole tool  150  has passed through power tong  200 , sections  301 ,  302  reattach to each other using pin  304  and repositioned within center aperture  801 . 
     As shown in FIG. 7, a ring gear  600  is positioned within power tong body on rollers  710  (see also FIG.  10 ). Ring gear  600  comprises outer gear teeth  601  that mate with the gear teeth  702  on gear  701 , rotated by motors  103  as shown in FIG.  7 . Viewing FIG. 6 a , ring gear  600  further comprises cam follower slot  605  so that followers  330  (see FIG. 10) may be positioned therein. Slot  605  has one or more slits  603 ,  604  configured therein which receive a pin  602 , commonly known as a reversing pin in the art. FIG. 3 illustrates how reversing pin  602  engages one of two apertures  602   a  formed in cage plate  203 . 
     When jaw assemblies  610  retract, jaw rollers  609 A roll along cam surface  607  until rollers  609 A reach a recess  608 . At this point, pin  602 , which is positioned in either slot  603 ,  604 , stops the relative rotation between ring gear  600  and cage plates  202 ,  203  so that jaw rollers  609 A do not roll further upon an adjacent cam surface  607  and re-grip the tubular. Pin  602  limits the travel of rollers  609 A along cam surface  607  when tubular is gripped, thereby preventing rollers from traveling to an adjacent neutral surface and preventing jaws  610  from crushing the tubular. 
     While not shown in the Figures, one modification of the present invention could include the positioning of a conventional load cell thereon. The load cell could be used to measure the torque imparted to power tong  200  relative to back-up  300  during operation of the system. For example, a load cell is first affixed to power tong  200 . A vertical pressure plate is then affixed to back-up  300  such that the pressure plate are adjacent to the load cell. When power tong  200  applies torque to a tubular, there is a tendency for power tong  200  to rotate relative to back-up  300 . This tendency to rotate causes the vertical pressure plate to load the load cell. In this manner, the torque imparted to the tubular by power tong  200  can be measured. Those skilled in the art will recognize that this is but one way to measure torque. Load cells could be mounted in numerous ways upon power tong  200  to achieve the same effect. Moreover, the measurement techniques are not limited to load cells, hydraulic or otherwise. Many alternate devices for measuring loads could be employed to determine the torque imparted on the tubular. 
     As used herein, “vertical” shall mean substantially along the y-axis or plane created by the y-axis shown in the Figures, while “horizontal” shall mean substantially along the x-axis or plane created by the x-axis shown in the Figures where the axes are shown. 
     Finally, while many parts of the present invention have been described in terms of specific embodiments, it is anticipated that still further alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.

Summary:
Closed head power tongs used to make up and break threaded pipe joints are disclosed herein. The closed head power tongs have removable mating inserts that allow the gripping aperture of the power tongs to be increased so that objects larger than the gripping diameter of the tongs can be raised or lowered through the center aperture of the power tong without removing the tongs from alignment with the drill string.