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: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to a tool for assembling threaded sucker rods of oil wells and other wells, and more specifically pertains to a switch on a rod tong that detects when the shoulder point of the rod&#39;s threaded connection has been reached. 
     2. Description of Related Art 
     Oil wells and many other types of wells often include a sucker rod pump for pumping oil or other fluid from deep within a well bore to the surface of the earth. A sucker rod pump is a reciprocating piston/cylinder type pump situated at the bottom of a long string of tubing that conveys the pumped fluid upward to the earth&#39;s surface. An oscillating drive at ground level is coupled to raise and lower the pump&#39;s piston by way of long string of sucker rods that may extend over 10,000 feet through the interior of the tubing. The string of sucker rods is comprised of individual solid rods of about 0.5 to 1.125 inches in diameter and about 25 to 30 feet long. Each sucker rod has an axial shoulder and male threads at each end that allow the rods to be tightly connected end-to-end by way of female threaded rod couplings (also referred to as boxes). The couplings also serve as a wear surface that protects the more expensive sucker rod from wear as the string of sucker rods may slide up and down along the interior of the tubing for millions of cycles over its lifetime. 
     Properly tightening each threaded joint of a string of sucker rods is critically important, as even a single improperly tightened joint can lead to a premature separation, fatigue cracking, or complete breakage of the string. This not only interrupts the ongoing operation of the well, but repairing a string of sucker rods is very expensive, due to its inaccessibility. Usually the entire string of sucker rods is removed from the well bore to repair a single joint. For a 10,000-foot string of 25-foot sucker rods, there are about 800 threaded joints. Thus, a reliable system is needed to properly tighten every single one. 
     Today, power rod tongs are possibly the most common tools for assembling and disassembling a string of sucker rods. Conventional tongs, such as those provided by BJ-Hughes Machinery of Houston, Tex., includes two sets of jaws: one set being driven to rotate relative to the other. To assemble a new joint, a sucker rod is first manually screwed hand-tight into each end of a coupling. The rod tong is positioned to engage one set of tong jaws with mating flats of one sucker rod, and the other set of jaws with mating flats of the other sucker rod. This places the coupling generally between, but spaced apart from, the two sets of jaws. Actuating the tong rotates one rod relative to the other, so that both rods screw tightly into the coupling generally at the same time. As the connection tightens, the tong eventually stalls at a torque or pressure preset by the operator. When the tong stalls, the operator assumes that the connection is properly torqued with the proper preload. 
     Some rod tongs may shut off automatically in response to a switch, as disclosed in U.S. Pat. No. 3,768,573. In this case, a hydraulically actuated switch cuts out the drive motor shortly after the hydraulic pressure exceeds a predetermined limit. The hydraulic pressure that actuates the switch is the same pressure that drives the hydraulic motor. This poses several possibilities for error, as the hydraulic pressure driving the motor is not a reliable indicator of the actual torque applied to the sucker rods. For example, at the beginning of the workday, the switch may be prematurely tripped by abnormally high pressure caused by cold, viscous hydraulic fluid or excessive friction in the drive mechanism of the tongs. Later in the day, as the hydraulic fluid warms up with use, switch actuation may be delayed, which may cause the sucker rods to be over tightened. 
     SUMMARY OF THE INVENTION 
     To improve the accuracy of tightening rod tongs, it is an object of some embodiments of the invention to consistently identify a shoulder point of a sucker rod connection, and do so substantially independently of the tong motor&#39;s hydraulic pressure. 
     A second object is to provide a rod tong with the ability to consistently identify a shoulder point of a sucker rod connection. 
     A third object is to provide a device for adjusting the point at which a rod tong identifies a shoulder point. 
     A fourth object is to provide a rod tong with a counter that accurately measures how far a first set of jaws rotates relative to a backup wrench. 
     A fifth object is to provide a rod tong with a pin connector that allows a backup wrench to pivot and slide relative to the tong&#39;s housing. 
     A sixth object of some embodiments of the invention is to enable rod tongs to identify a shoulder point of a sucker rod connection even if the motor driving the tongs is not a hydraulic motor. 
     A seventh object of some embodiments of the invention is to turn off a rod tong automatically and with repeatability by doing so substantially independently of the tong motor&#39;s hydraulic pressure. 
     These and other objects of the invention are provided by sucker rod tool that includes a switch that responds to movement of a backup wrench as the rod tool tightens a pair of sucker rods. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is front view two sucker rods about to be screwed into a coupling, with the coupling being shown in cross-section. 
     FIG. 2 is a side view of a sucker rod connection about to be tightened by a sucker rod tool according to one embodiment of the invention. 
     FIG. 3 is a top view of the FIG. 2, with one of the sucker rods shown in cross-section and portions of the tool cut away. 
     FIG. 4 is a side view of a sucker tool tightening a sucker rod connection. 
     FIG. 5 is a top view of the FIG. 2, with one of the sucker rods shown in cross-section and portions of the tool cut away. 
     FIG. 6 is a cross-sectional view taken along line  6 — 6  of FIG.  2 . 
     FIG. 7 is a cross-sectional view taken along line  7 — 7  of FIG.  4 . 
     FIG. 8 is a schematic diagram of a sucker rod tool showing control aspects of the tool. 
     FIG. 9 is similar to FIG. 6, but of another embodiment of the invention. 
     FIG. 10 is similar to FIG. 7, but of another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Two conventional sucker rods  10  and  12  about to be screwed into opposite ends of a threaded rod coupling  14  are shown in FIG.  1 . Upper sucker rod  10  and lower rod  12  each include a threaded pin  16  that screws into coupling  14 , a shoulder  18  adapted to tightly abut an axial face  20  of coupling  14 , and a drive head  22  that provides a set of flats  24  suitable to be engaged by a sucker rod tool used for tightening the sucker rods. 
     One example of a sucker rod tool is tongs  26  of FIGS. 1-7. Tongs  26  includes a rotational upper jaw  28  for engaging head  22  of upper rod  10  and a backup wrench  30  for engaging head  22  of lower rod  12 . In this example, upper jaw  28  includes two grippers  32  pivotally attached to a gear segment  34  (outer ring assembly) by way of pins  36 . Pins  36  allow grippers  32  to pivot in and out of engagement with head  22  of upper rod  10 , while gear segment  34  renders jaw  28  rotational relative to a tong housing  38 . FIGS. 2 and 3 show tongs  26  disengaged from the sucker rods, and FIGS. 4 and 5 show tongs  26  engaging the rods. 
     In FIGS. 3 and 5, portions of housing  38  are cutaway to more clearly illustrate a drive train  40  that couples a motor  42  (hydraulic, electric, pneumatic, etc.) to upper jaw  28 . Drive train  40  includes two drive gears  44  so that at least one of them remains in driving contact with gear segment  34  at all times, as gear segment  34  has a discontinuity or opening  46  for receiving and releasing rod  10 . A set of speed reducing gears  48  couples drive gears  44  to an output pinion gear  50  of motor  42 . Thus, motor  42  turning pinion  50  rotates gear segment  34  at a reduced speed to provide upper jaw  28  with sufficient torque to be able to tightly screw rods  10  and  12  into coupling  14 . To disassemble or unscrew at least one sucker rod  10  or  12  from coupling  14 , the rotational direction of motor  42  is simply reversed. Tongs  26  is quite similar in structure and function as a conventional set of tongs (e.g., tongs provided by BJ-Hughes, Inc. of Houston, Tex.), but with some important and novel modifications. 
     The modifications are primarily associated with backup wrench  30 . A head  52  of wrench  30  is adapted to engage flats  24  of lower sucker rod  12 . A pin  54  held between two lugs  56  and  58  of tong housing  38  pivotally couples a shank  60  of wrench  30  to housing  38 . This allows wrench  30  to pivot about a longitudinal centerline  62  of pin  54 , so head  52  of wrench  30  can move vertically to accommodate variations in the distance between the heads of upper and lower rods  10  and  12 . 
     To allow wrench  30  to move between a relaxed position of FIG. 6 and a torqued position of FIG. 7, sufficient clearance is provided between shank  30  and lugs  56  and  58  and between pin  54  and a hole  64  in shank  30 . Such clearance allows wrench  30  to slide axially in a direction parallel to centerline  62  and to rotate slightly about a vertical axis relative to housing  38 . Such movement serves as a trigger that enables a switch system  66  to determine accurately and repeatedly when tongs  26  applies a certain amount of torque to rods  110  and  12 . 
     This can be important when it is desirable to identify when a sucker rod connection reaches its shoulder point, i.e., when the connection is hand tight and further tightening begins stretching the rods&#39; threaded pins  16 . Once the shoulder point is reached, tongs  26  can rotate upper jaw  28  a predetermined amount to accurately achieve a proper preload or axial stretch within threaded pins  16 . 
     To accurately measure the extent to which rods  10  and  12  have been tightened beyond their shoulder point, tongs  26  are provided with a sensor  68  and a counter  71  (FIG. 8) that measure the rotation of jaws  28 , motor  42  and/or drive train  40 . Although sensor  68  can be any type of rotational sensor, in some embodiments, sensor  68  is a DZH series Hall effect sensor by Electro Corporation of Sarasota, Fla., which senses a magnetic disturbance created by each passing ferro-magnetic tooth of one of the gears of drive train  40 . A signal  70  from sensor  68  is fed back to counter  71 . Although counter  71  is schematically illustrated to represent any type of counter, in this example counter  71  is provided by a computer  72 , which encompasses any one of a variety of programmable or dedicated control circuits including, but not limited to, a microprocessor associated with appropriate memory and input/output boards; a microcomputer, computer, or PC; a PLC (programmable logic controller); and a myriad of hard-wired electrical circuits comprised of discrete electrical components and/or solid-state integrated circuits. After counting a predetermined number of pulses of signal  70 , computer  72  can record the results or generate an output signal  74  that stops motor  42 . Stopping motor  42  can be accomplished in different ways including, but not limited to, de-energizing an electric motor or stopping a hydraulic motor by shifting a directional valve  76  of a hydraulic circuit  78  to a neutral position, as shown in FIG.  8 . Valve  76  in its neutral position shunts any hydraulic fluid that a hydraulic pump system  80  would otherwise force in a forward or reverse direction through motor  42 . 
     However, to sense the shoulder point consistently (i.e., accurately and repeatedly), it is preferred to sense the movement or reaction of wrench  30  directly and independently of any hydraulic pressure associated with hydraulic circuit  78  (i.e., independent of any hydraulic pressure of any hydraulic fluid circulating through tongs  26 ). The term, “circulating” refers to a fluid that is in fluid communication with fluid that can travel along a path that eventually leads back to its starting point without having to backtrack. To this end, switch system  66  has a first portion  82  (e.g., a limit switch housing) that is relatively fixed relative to tong housing  38  and a second portion  84  (e.g., a spring-loaded plunger) that is adapted to engage and move with backup wrench  30 . Spring-loaded plunger  84  has one end  86  adapted to be engaged by shank  60  of wrench  30 . In some embodiments, plunger  84  comprises a bolt  88  disposed within a cavity  90  of lug  58 . With conventional tongs, cavity  90  is typically used to house a small hydraulic cylinder (similar to cylinder  92  of FIGS. 9 and 10) having a rod end that when pushed against by shank  60  of wrench  30  develops a pressure within the cylinder. A pressure gage on that cylinder would then indicate the force that wrench  30  exerts against the rod end of the cylinder. However, in the preferred embodiment of FIGS. 6 and 7 such a cylinder is replaced by a spring  94  compressed between a nut  96  on bolt  88  and a flange  98  at the base of cavity  90 . The location and structure of spring  94  is schematically illustrated to encompass any appropriately located structure that urges the bolt portion of switch system  66  toward wrench  30 . Examples of spring  94  include, but are not limited to, a compression spring, a tension spring, a pneumatic cylinder, a hydraulic cylinder, a resilient polymeric cylinder, one or more Belleville washers, etc. An electric limit switch  100  whose housing  82  is attached to tong housing  38  is actuated by an actuator arm  102  resting on a head  104  of bolt  88 . Switch  100  is schematically illustrated to encompass any device that can change states to create a signal  108 . 
     As tongs  26  applies a predetermined amount of torque to sucker rods  10  and  12 , backup wrench  30  shifts from its relaxed position of FIG. 6 to its torqued position of FIG.  7 . Upon moving to its torqued position, shank  60  of wrench  30  pushes against bolt end  86  and, with sufficient torque, overcomes a spring force  106 , as shown in FIG.  7 . This causes bolt head  104  to push actuator  102 , which changes limit switch  100  from a normal state of FIG. 6 to an actuated state of FIG. 7, thereby creating signal  108 . The switch&#39;s normal state could be open electrical contacts and its actuated state could be closed contacts, or vice versa. 
     With wrench  30  at or near its torqued position, spring force  106  urges wrench  30  back towards its relaxed position. An adjustment  110  adjusts the spring preload or amplitude of force  106  by adjusting the extent to which nut  96  is screwed onto bolt  88 . The further nut  96  is screwed onto bolt  88 , the greater is force  106 , unless, of course, wrench  30  forces spring  94  to bottom out. Increasing force  106  raises the required torque for tripping switch  100 . 
     In response to receiving signal  108 , computer  72  can respond in a monitoring or controlling manner. For monitoring the tightening of a sucker rod connection, tongs  26  tightens and stops the tightening process in a conventional manner (similar to other tongs and independent of computer  72 ). However, computer  72  can still determine whether the sucker rod connection has been properly tightened by counting the number of pulses of signal  70  that occur after signal  108  identifies the shoulder point. Computer  72  can record the results and/or provide the operator of the tongs with feedback, such as a green or red light to indicate respectively an acceptable or unacceptable connection. An acceptable connection would be when the number of pulses of signal  70  falls within a predetermined range. On the other hand, when controlling the tightening process, computer  72  would automatically stop tongs  26  after upper jaw  28  rotates a predetermined amount beyond the shoulder point. Signal  70  would indicate that the shoulder point has been reached, and computer  72  would measure the amount of rotation of upper jaw  28  by counting the pulses of signal  108  Computer  72  can stop tongs  26  by generating a signal  74  that shifts valve  76  to its neutral position, as shown in FIG.  8 . 
     In another embodiment of a rod tongs  112 , shown in FIGS. 9 and 10, a switch system  114  includes a hydraulic cylinder  92  coupled to a pressure switch  116  through a hydraulic line  118 . Upon tongs  112  tightening rods  10  and  12  to their shoulder point, backup wrench  30  shifts from its relaxed position of FIG. 9 to its torqued position of FIG.  10 . In the torqued position, shank  60  of wrench  30  pushes against cylinder  92  with a force  120  that builds hydraulic pressure inside cylinder  92 . Line  118  conveys the pressure to switch  116 . As the pressure within cylinder  92  reaches a predetermined limit indicative of the shoulder point being reached, switch  116  changes from its normal state of FIG. 9 to its actuated state of FIG.  10 . In the normal state, a set of normally closed contacts  120  of switch  116  are closed and a set of normally open contacts  122  are open. In the actuated state, contacts  120  open and contacts  122  close. Either set of contacts  120  or  122  can be used to generate signal  108 . The predetermined force indicative of the shoulder point is adjustable, as indicated by adjustment  124  of pressure switch  116 . In other words, pressure switch  116  has an adjustable pressure setting. However, since the hydraulic fluid in cylinder  92  and line  118  does not circulate, the actuation of switch system  114  is still independent of any hydraulic pressure of any hydraulic fluid that may happen to circulate through tongs  112 . 
     Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are well within the scope of the invention. For example, although tongs are preferably driven by a hydraulic motor through which hydraulic fluid circulates, tongs driven by electric or pneumatic motors are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the claims that follow.

Summary:
Tongs for assembling sucker rods of oil wells include a switch that senses movement of the tong&#39;s backup wrench relative to the tong&#39;s housing. When screwing two sucker rods into a threaded coupling, an upper jaw of the tongs rotates an upper sucker rod relative to a lower sucker rod, while the backup wrench holds the lower rod relatively stationary. When the tongs apply a torque that indicates that the threaded connection is beyond hand tight (i.e., at the shoulder point), a torque reaction force kicks the backup wrench to a position that trips the switch. A control responsive to the switch and a rotation sensor monitors and/or controls the threaded connection&#39;s circumferential displacement past the shoulder point. By sensing the position of the backup wrench directly, the switch can consistently identify the shoulder point independent of the tong&#39;s. hydraulic pressure, thus avoiding oil viscosity related errors.