You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    This invention related to power tongs for making oil field connections and more particularly to power tongs for sucker rods and tubing connections. 
       BACKGROUND OF THE INVENTION 
       [0002]    Systems referred to as power tongs have been widely used for some time in oil field installations for making and breaking connections between end threaded products which are to be united into a string by couplings which join the products end to end. Such products include sucker rods which extend downhole within tubing or casing and provide drive power for pumping petroleum to the surface. Other strings are also made up using power tongs, and these include tubular products in the form of tubing and casing. 
         [0003]    As the technology has developed, the threaded connection between the elements in the string has had to become more precise and stronger because of increasing demands placed on the string. As the strings have increased in length consistent with wells drilled to greater depth, they have also encountered higher pressures, and ever higher loads and forces. More secure connections are thus needed to enable the downhole equipment to be utilized for longer periods of time, with higher reliability. 
         [0004]    Sucker rods have pin ends which are threaded without a taper, and reliance is placed on making a shoulder connection which is properly prestressed to withstand the forces that are to be encountered in cyclic pump operation over a long duration. Tubing and casing, on the other hand, utilize tapered threads, and are subject to both internal and external forces and combinations thereof. Also, the integrity of the connection between male and female threads is a consequence not only of the degree of engagement but of the dimensional tolerances that are permissible. 
         [0005]    An improvement in sucker rods is evidenced by U.S. Pat. No. 6,942,254 and application Ser. No. 09/960,391 of Kenneth J. Carstensen which both disclose a connection in which the end faces of the pin ends of the sucker rods engage each other either directly or via an intermediate torque disk. The connection is made up to a first operative point at which the pin ends are under initial compression and the coupling is then further tensioned to a further precise degree. This arrangement unites the component parts of the sucker rod connection in a manner such that they withstand the varying forces encountered during the action of a reciprocal or rotary pump, and resist the development of microcracks and consequent fatigue failures. 
         [0006]    The practical economic and throughput requirements at operating wells do not justify or permit the installation of expensive and complicated systems for instrumenting the measurement of torque or displacement values. It is much preferred to utilize a torque applicator, specifically a power tong system, to apply a precise amount of torsional force so that the connection is mechanically secure and repeatable. In this regard, the sucker rod configuration of the referenced Carstensen patents places a high premium on a capability for prestressing the sucker rod connection with a high degree of precision. Also, since the same power tong must also function in the break mode (disengagement) it should perform all the needed functions as they are required. 
       SUMMARY OF THE INVENTION 
       [0007]    A system for coupling the threaded ends of oil field connections to be made up into a string utilizes alternative sources for turning a rotary element engaged to the elements to be coupled together. A first motive source is a rotary drive for spinning the element to an initial engagement state, then a second longitudinally driven element with a variable but predetermined hydraulic pressure limit applies the desired final precise torsional force. The force applied by the longitudinally driven element can be precisely measured by a sensor, so that the torque applied can be raised to a present value within accurate limits. 
         [0008]    An improved power tong in accordance with the invention, more particularly, utilizes a combined dual function drive mechanism which is capable of operating the driven element, namely the sucker rod, tubing or casing in both a spinning mode and a precise torque application mode. As used for sucker rods, the wrench flat of the sucker rod is entered within a spinner mechanism and engaged by cam operated gripping mechanisms which are urged inwardly as a rotary drive is turned about the wrench flat. The rotary drive includes a hydraulic motor with internal step down gears turning a drive gear on a shaft adjacent the periphery of a large rotary cam gear with outer peripheral teeth. The drive gear is not coupled to the teeth on the ring drive directly but via idler gears on each side of it which engage the peripheral teeth. In an initial spinning mode, the motor turns the rotary ring drive which in turn drives the gripping mechanisms and the sucker rod. This continues until a shoulder on the sucker rod that is adjacent the wrench flat engages the end of the coupling sleeve in the sucker rod connection. Once this position is reached, the spinning is stopped, and a wholly different engagement mode is activated to complete precise torquing. A gear rack adjacent the idlers is shifted into engagement with the peripheral teeth of the idlers. Then a double acting hydraulic cylinder coupled to the gear rack moves it laterally until a selected and controlled limit is reached, by turning the ring drive and the engaged sucker rod until a precise rotational force level is established by an associated sensor. This prestresses the connection between the sucker rods, by virtue of the physical engagements of the sucker rods with the coupling sleeve, and provides superior realization of the benefits of the Carstensen sucker rod improvement referenced above. When a predetermined strain limit is reached, the drive cylinder is shut off and the gear rack is disengaged from the idler gears. The spinning action of the rotary ring drive is then reversed, and centrifugal force disengages the gripping heads from the wrench flat. The tongs can then be drawn away from the sucker rod via the passageway provided in the spinner section. Strain gage measurements show that the limit of torque that is applied to prestress the sucker rod connection is extremely accurate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A better understanding of the invention may be had by reference to the following description taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a cross-sectional plan view, of a power tong system in accordance with the invention; 
           [0011]      FIG. 2  is a side sectional view of the power tong system of  FIG. 1 ; 
           [0012]      FIG. 3  is an end sectional view of the mechanism of  FIGS. 1 and 2 ; 
           [0013]      FIG. 4  is a side sectional view of the mechanism taken from a different angle, showing further details of the system; 
           [0014]      FIG. 5  is a plan view of the tong housing with the cover and internal parts removed; 
           [0015]      FIG. 6  is a combined perspective and block diagram view of the power tong system; 
           [0016]      FIG. 7  is a fragmentary view of a backup mechanism used in gripping the sucker rod during make and break operations; 
           [0017]      FIG. 8  is a block diagram of the principal elements of a power tong for sucker rod connections in accordance with the invention, and 
           [0018]      FIG. 9  is an enlarged fragmentary view of a stress sensor mounted in a hydraulic shaft used in the lateral drive. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to  FIGS. 1-7 , a power tong in accordance with the invention is shown as it is configured for sucker rod operation. The component parts are disposed within a rugged tong housing  10  comprising a full height rear section  12  joined to a reduced height front section  14 . In the front section  14 , a wrench flat access slot or passageway  16  provides a pathway for receiving the wrench flat portion of a sucker rod at a position which will be called the wrench flat axis. This axis is usually vertical, and the entry pathway therefore is usually horizontal. A rotary cam gear  20  in the form of a robust ring drive disk having peripheral teeth  22  and an interior cam surface is concentric with the wrench flat axis and positioned in the front section  14 . Within the rotary cam gear  20 , the peripheral teeth  22  are positioned at mid-height between upper and lower rings  24 ,  26  which also form part of the rotary gear. The rotary cam gear  20  has a circumferential opening which provides a radial slot complementary to that in the front section  14  of the housing and is aligned with it when engaging and disengaging to wrench flats. The rotary gear  20  nonetheless rotates about the central axis because it is peripherally retained within a set of spaced apart rollers  27  disposed about the cam gear periphery, and each having upper and lower rollers  27   a  and  27   b  ( FIGS. 2 and 4 ) engaging the ring surfaces  24 ,  26  so as to hold the rotary cam gear in concentricity with the wrench flat axis as it is driven. 
         [0020]    An interior surface  28  ( FIG. 1 ) of the rotary gear  20  is configured to provide two opposing cam lobes  29 ,  29 ′ facing the wrench flat axis. The peripheral rollers  27  are mounted between flat upper and lower carrier plates  40 ,  42  and the opposite faces of the drive disk portion of the rotary cam gear  20  are held relative to the carrier plates  40 ,  42  by aluminum bronze friction segments  44  ( FIG. 4 ) which engage the rotary gear  20  to hold it in planar position transverse to the wrench flat axis. 
         [0021]    A pair of gripping heads  46   a  and  46   b  are disposed on opposite sides of the wrench flat axis and, in the position of the rotary gear  20  shown in  FIG. 2 , are also approximately perpendicular to the entry slot  16 . The gripping heads  46  each include inwardly facing teeth  47  (see  FIG. 5 ) directed toward the wrench flat axis and cam follower rollers  48  in contact with the cam surface  28 . When in contact with the interior surface of the rotary gear  20 , the cam follower rollers  48  and the gripper head assemblies are of sufficient size and strength to withstand the substantial forces involved in making a connection and applying torque. 
         [0022]    The drive in the rear section  12  operates in two modes. First, for spinning the sucker rod, it is coupled to a hydraulic motor  52  ( FIGS. 4 and 6 ) directly engaged to a shaft  54  that is vertical with respect to the principal plane of the rotary cam gear  20 . The shaft  54  is held between upper and lower bearings  56 ,  58  respectively and is coupled to a drive gear  60  ( FIG. 4 ) positioned adjacent but spaced from the peripheral teeth  22  on the rotary cam gear  20 . The hydraulic motor  52  may include internal step down gearing (not shown) to provide a desired combination of torque and rotational velocity. The shaft  54  mounted drive gear  60  engages separate adjacent idler gears  62 ,  63  (see  FIG. 1 ) which mesh with the peripheral teeth  22  on the rotary cam gear  20 . The idler gears  62 ,  63  rotate on short shafts (not shown) mounted in the top and bottom surfaces, respectively, of the rear section  12  of the housing. 
         [0023]    In the second mode of operation, the rotary cam gear  20  receives motive power from a gear rack  70  which is initially held at a space from the idler gears  62 ,  63 , as seen in  FIG. 1 . The gear rack  70  is attached to a double acting hydraulic cylinder  72  ( FIG. 3 ) which has a  10 ″ stroke and is operated by a hydraulic control  73  ( FIG. 6 ). The cylinder  72  is supported in a C frame  74  ( FIGS. 1-3 ) fixed to the housing  10  at its ends, which abut the sidewalls of the rear section  12 . Shafts  75 ,  75 ′ coaxial with the cylinder  72  and mounted in the housing wall provide axial sliding support for the cylinder  72  and associated gear rack  70 . A stress sensing transducer  152 , as described in more detail below in conjunction with  FIG. 9 , is disposed in one of the shafts for providing a precise measurement of the torque applied. 
         [0024]    The C frame  74  is movable in both directions, toward and away from the wrench flat axis, within a number of oval cam surfaces  76  (best seen in  FIG. 5 ) in the top and bottom walls of the C frame  74 , which receive cam followers  78  that are spaced apart in the transverse direction parallel to the cylinder  72 . These cam mechanisms allow a range of motion toward and away from the rotary drive  20 , so as to engage the gear rack  70  with the idler gears  62 ,  63 , and release it from same under hydraulic control. 
         [0025]    The drive motion for engaging and disengaging the gear rack  70  is provided by drive cylinders  80 ,  81  mounted in the rear section  12  against the back wall thereof, and positioned perpendicular to the gear rack axis. The drive cylinders  80 ,  81  engage a pair of drive brackets  83 ,  83 ′ ( FIGS. 1 and 2 ) which are coupled to the C frame  74 . After the initial spinning of the rotary gear  20  brings the shoulder on the sucker rod pin end into contact with the end of the coupling, the connection is ready for prestressing. To this end, the operator engages an actuator ( FIG. 6 ) which activates the control hydraulic controls  112 , which include the sensor  152  to shut off the gear rack drive when a selected stress limit has been reached. This sensor may be a strain gage, a piezoelectric transducer, or any other of the many devices for providing the needed degree of precision. 
         [0026]    Thus the power tongs in accordance with the invention utilize different modes of operation, so as to first engage the opposed gripping heads  47  ( FIGS. 1 and 5 ) against the wrench flat on the sucker rod by action of the double lobed cam surface  29 . Then the rotary gear  20  spins the sucker rod until the shoulder limit is reached and the drive stops, automatically or under operation control, as with the mechanisms shown in  FIG. 6 . Then the gear rack  70  is engaged to by first shifting in to engage the idler gears  62 ,  63  then driving to provide final increment of torque, but this time by lateral movement of the gear rack  70  so as to turn the sucker rod, until the predetermined limit is reached. The torque limit can be very precisely set, because it can be measured by modern strain gauge technology. The gear rack  70  can then be disengaged and the rotary drive reversed, this reversal causing the gripping heads to release by centrifugal force, so that the sucker rod can be removed and replaced with a new connection that is to be made up. 
         [0027]    A practical example of a system in accordance with the invention is shown in perspective view in  FIG. 6 , in fragmentary perspective view in  FIG. 7  and with the principal control elements being shown in the block diagram of  FIG. 8 . The sucker rod  100  extends vertically down through the front section  14  of the housing  10 , fitting within the passageway  16  that is provided for access. The sucker rod  100  includes a conventional wrench flat  102  ( FIG. 7 ) and an adjacent extending shoulder  104  with the pin end being threaded into a coupling  106  joined to a second sucker rod  108 . 
         [0028]    On the power tong assembly ( FIG. 6 ), control hydraulics  110 ,  112  are shown on opposite sides of the top of the tong housing  10 . Details of the valve and interconnections are not shown for simplicity and because numerous conventional hydraulic expedients are available. At one side of the housing is a control lever  114  coupled into the first control unit  110 , for activating the spinning mechanism. On the other side of the housing  10  are a pair of control levers  120 ,  122  engaged to the control hydraulics  112  on that side. These control levers control separate actuation of the spinning drive of the rotary cam gear  20 , and also allow separate lateral gear rack operation. Both of these actions are later reversed for disengagement functions. 
         [0029]    Handles  128  for manual operation of the tongs are disposed on each side of the housing to enable moving the power tongs, which are separately supported in conventional fashion, into operating position. The assembly, however, can alternatively be operated remotely in a robotic fashion, when assembling a string of sucker rods. In such an automatic operation, successive sucker rods are simply fed through the system, and automatically timed operations are undertaken in sequence, first spinning the sucker rod until shoulder engagement is encountered, then activating the gear rack to provide the selected level of prestress, and operating to disengage the tongs from the connection, so that the string can be advanced to the next connection point where the process is repeated. 
         [0030]    Details of the backup mechanism  120  are shown in the fragmentary perspective view of  FIG. 7 , in which it can be seen that gripper elements  121 ,  122  are spaced apart to receive opposite sides of a wrench flat  102  for a previously made connection, so that it can be held fixed as the upper sucker rod  100  is spun into position and then prestressed. 
         [0031]    The principal elements used in tightening a sucker rod connection to a first stop limit and then to a precise prestress limit are shown in block diagram form on  FIG. 8 , to which reference is now made. The operator controls  150  are exerted by the levers shown in  FIG. 6 , and start the spinner drive  5   b , which turns the drive ring  20  through the coupling gears  62 ,  63  until it reaches a physical stop in the improved Carstensen sucker rod configuration, as encountered. Then the spinner drive  50  is stopped, and the lateral drive  70  is first engaged to the coupling gears  62 ,  63 . Then the drive  70  is actuated by the operator, linearly moving in to engage the coupling gears  12 ,  13  and subsequently turn the drive ring  20  and the connection itself. 
         [0032]    The stress sensor  152  is coupled to a support shaft  74  or  75  for the lateral drive to signal that a chosen prestress limit has been reached. As seen in  FIG. 9 , powering of the drive rack  70  in either direction is caused by a concomitant increase in hydraulic pressure, which is sensed with high precision by the transducer  152 , so that the drive can stop automatically or under operator control. 
         [0033]    Various alternatives will suggest themselves to those skilled in the art, but it is to be understood that the invention encompasses all forms and variations in accordance with the appended claims.

Summary:
A power tong system for precisely making up a connection between two elongated elements, such as sucker rods, into an operative string for petroleum well installations. High precision is attainable to secure the full advantages of prestressing the coupling by combinatorial use of both a rotary drive to achieve a first contact position and a linear drive to secure a precise final torsioning. The mechanism for achieving this may employ a peripherally driven drive ring coupling gears engaged to the drive ring periphery, a rotatably driven drive and a linear gear rack which are both engageable to the coupling gear.