Abstract:
Machines and methods capable of evaluating torque-tension and prevailing torque performance of prevailing-torque threaded fasteners. Such a machine may include one or more of the following features: a coordinated drive unit for achieving relative linear and rotational movement of a nut and bolt of a nut-bolt assembly; a sensor unit with multiple reaction torque sensors in series, wherein a lower-range torque sensor is protected from being overstressed while torque is transferred therethrough to a higher-range torque sensor; an optical temperature measurement unit oriented to detect a temperature at a thread interface of a nut and bolt through an opening in a device adapted to rotate the nut; and a bolt recess measurement unit that determines a bolt-through condition of a nut-bolt assembly by detecting a bolt recess of the nut-bolt assembly that has been assembled such that a prevailing torque feature thereof has been encountered.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/096,022, filed Dec. 23, 2014, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to machines and methods capable of evaluating threaded fasteners. The invention particularly relates to machines and methods of performing torque-tension evaluations of prevailing-torque threaded fastener assemblies that comprise a locking feature, resulting in the fastener assembly exhibiting a prevailing torque during assembly. 
     The fastener industry has utilized various inspection procedures for qualifying prevailing-torque threaded fasteners. These procedures typically specify certain parameters of particular interest, including but not limited to drive torque, thread torque, prevailing torque, tension, etc. As used herein: “tension” will refer to the tensile loading of a threaded bolt caused by driving a threaded nut onto the bolt such that the bearing surfaces of the nut and bolt are drawn toward each other and the nut-bolt assembly generates a clamping load therebetween; “drive torque” will refer to the torsional resistance to assembling a threaded nut onto a threaded bolt, measured as the torque required to drive the nut onto the bolt and including the torque required to develop tension in the bolt as the bearing surfaces of the nut and bolt are drawn toward each other during assembly; “thread torque” will refer to the torsional resistance to assembling a threaded nut onto a threaded bolt, measured as the torque required to prevent the bolt head from rotating and including the torque required to develop tension in the bolt as the bearing surfaces of the nut and bolt are drawn toward each other during assembly; “final assembly torque” will refer to drive and/or thread torque; “prevailing torque” will refer to the torsional resistance to assembling a threaded nut onto a threaded bolt, measured as the torque required to prevent the bolt head from rotating while there is no tension in the bolt as the bearing surfaces of the nut and bolt are drawn toward each other during assembly or away from each other during disassembly; and “bolt-through” will refer to the condition during assembly of a threaded nut onto a threaded bolt at which the end of the bolt opposite its head and bearing surface is flush with the exit end of the nut (i.e., the end opposite the bearing surface of the nut). 
     Standard inspection procedures for qualification of some prevailing-torque threaded fasteners often impose various requirements on the above and other parameters. Some of these requirements and problems that may be encountered are briefly summarized below. 
     Standard inspection procedures sometime require the measurement of prevailing torque (measured before the nut-bolt assembly develops any clamping tension) and final assembly torque (measured when the nut-bolt assembly develops the final clamping tension). These procedures generally require stringent percent-of-point accuracy for both the prevailing torque measurements and the final assembly torque measurements. The ratio of the magnitude of prevailing and final assembly torque measurements can be 1:10 for a given nut-bolt assembly. It is also usually desirable that equipment used to perform torque measurements are capable of testing a range of fastener sizes and material strength grades, which may further necessitate a 1:10 ratio of torque measurements between the torque levels required for nut-bolt assemblies of the smallest size and/or lowest strengths relative to the torque levels required for nut-bolt assemblies of the largest size and/or highest strengths. It is difficult with a single sensor to economically and accurately measure torque over a range that, based on the foregoing, may encompass a torque ratio of 1:100. 
     Standard inspection procedures usually require that one member of a nut-bolt assembly is constrained from rotating with a constraining tool, while the other member is rotationally driven by a socket or other suitable drive tool. Due to the helical nature and geometry of screw threads, the relative rotation between the driven and constrained members of a nut-bolt assembly will cause relative linear motion between the members of the assembly as the members axially translate relative to each other. Unless the relative linear position of the drive tool and constrained member is accurately coordinated with their relative rotational position, there will be dragging (friction) forces and/or torques introduced into the nut-bolt assembly that can introduce errors into the test measurements. These forces can occur if the driven member is able to slide within or otherwise shift relative to the drive tool, the constrained member is able to slide within or otherwise shift relative to the constraining tool, or the drive tool rubs the bearing surface of the constrained member. In addition, if the relative linear position of the drive tool and constrained member is not coordinated with their relative rotational position, it may be necessary for the constraining tool or drive tool to be deep enough to accommodate the change in the relative linear position between the members of the nut-bolt assembly. 
     Standard inspection procedures may also require the measurement of temperature at the thread interface of a nut-bolt assembly to ensure that the temperature remains within a prescribed allowance from room temperature. One way to monitor this temperature is to continuously observe the thread temperature where the bolt thread exits the nut with the use of an optical measurement instrument. However, because the linear distance between the bearing surfaces of the bolt and nut changes as the nut-bolt assembly is driven rotationally to assemble or disassemble, a “moving target” is presented to the optical measurement instrument unless steps are taken to continuously adjust the aim of the instrument or to fix the targeted bolt thread where it exits the nut. Another obstacle is that the nut is usually constrained or driven by a socket, which often completely envelopes the nut including the targeted bolt thread where it exits the nut, obscuring the bolt thread from optical temperature measurement. 
     Standard inspection procedures also sometimes require that torque measurements are summarized and recorded at defined intervals during a test. Some of those intervals are defined relative to the bolt-through position where the end of the bolt opposite its head is flush with the exit end of the nut (the surface opposite its bearing surface), often taken as when the bolt starts to protrude from the exit end the nut. Observing a bolt-through event in real-time is problematic because the tool or adapter that constrains or drives the nut often completely obscures the event if simple measurement or detection methods are used. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides machines and methods capable of evaluating the torque-tension and prevailing torque performance of prevailing-torque threaded fasteners, and particularly nut-bolt assemblies. 
     According to certain aspects of the invention, a machine for evaluating a prevailing-torque threaded nut-bolt assembly may include one or more of the following features: a coordinated drive unit for achieving relative linear and rotational movement of a nut and bolt of the nut-bolt assembly; a sensor unit comprising multiple reaction torque sensors in series, wherein a lower-range torque sensor is protected from being overstressed while torque is transferred therethrough to a higher-range torque sensor; means for rotating a nut, wherein at least one opening is defined therein and an optical temperature measurement unit is oriented to detect a temperature at a thread interface of the nut and bolt through the opening; and/or a bolt recess measurement unit that determines a bolt-through condition of the nut-bolt assembly by detecting a bolt recess of the nut-bolt assembly that has been assembled such that a prevailing torque feature thereof has been encountered. 
     Other aspects and advantages of this invention will be appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a machine capable of evaluating a prevailing-torque threaded fastener in accordance with nonlimiting embodiments of the invention. 
         FIG. 2  schematically represents a unit of the machine of  FIG. 1  that provides coordinated relative linear and rotational movement of two members of a threaded nut-bolt assembly. 
         FIG. 3  represents a partial sectional view of a reaction torque sensor unit of the machine of  FIG. 1   
         FIGS. 4A and 4B  represent, respectively, perspective and side views of a portion of the reaction torque sensor unit of  FIG. 3 , and  FIG. 4C  represents a cross-sectional view taken along section line  4 C- 4 C of  FIG. 4B . 
         FIGS. 5A and 5B  represent, respectively, perspective and cross-sectional views of an optical temperature measurement unit of the machine of  FIG. 1 . 
         FIG. 6  represents a cross-sectional view a bolt recess measurement unit of the machine of  FIG. 1 . 
         FIG. 7  shows a summary graph representative of torque data obtained from tests performed on three samples with the machine of  FIG. 1 . 
         FIG. 8  shows a detail graph representative of torque data that may be obtained during a test performed on a sample with the machine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a machine adapted to measure torque and tension characteristics of a prevailing-torque threaded nut-bolt assembly, and  FIGS. 2 through 6  depict certain components of the machine  10 . The machine  10  is preferably compatible with automatic torque-tension and prevailing torque qualification test processes of types by which fastener lots are evaluated. The nonlimiting embodiment of the machine  10  shown in  FIG. 1  is particularly configured as a torque-tension test machine suitable for automatic testing of the prevailing torque and torque-tension characteristics of a nut-bolt assembly in which the nut is a locknut, and therefore has a locking feature (element) that results in the nut exhibiting a prevailing torque during assembly and disassembly with a bolt. 
     As seen in  FIG. 1 , the machine  10  comprises an enclosure  12  situated on a counter top  14  of a cabinet  16 , and certain components of the machine  10  enclosed within the enclosure  12 . The machine  10  can be seen as further including a monitor  18 , computer mouse  19 , keyboard  20 , joystick  22 , and emergency stop button  24  as user interface devices with which a user can control the machine  10  and its operation. The joystick  22  can be used to manually control the operation of the machine  10  while the enclosure  12  is open, and in particular the manual (jog) operation of rotary and linear actuators  26  and  28  that are part of a coordinated drive unit  30  ( FIG. 2 ) of the machine  10 . Opening the enclosure  12  preferably inhibits automatic operation of the drive unit  30 , and activating the emergency stop button  24  preferably inhibits all operation of the drive unit  30 . Certain components of the machine  10  may be enclosed within the cabinet  16 , including control, processing and data storage equipment that preferably form part of a computing system associated with the machine  10 . The computing system may use a standard personal computer and use a Windows7 operating system and custom programs for controlling the machine  10  and processing and storing data. Additional components of the machine  10  that may be enclosed within the cabinet  16  include the aforementioned rotary and linear actuators  26  and  28 , a drive torque sensor, rotational (angular) position sensor  46 , translational (linear) position sensor  48 , and controller  50 , discussed in further detail below. The computing system may communicate via an intranet, for example, to automatically retrieve testing schedules and requirements data, automatically record and store test data, etc. The machine  10  and its computing system are preferably, though not necessarily, equipped to perform tests during which the following measurements are recorded continuously or at predetermined update periods: angular position of one or more components of a nut-bolt assembly, bolt-through condition, total torque, thread torque, clamp load, and temperature.  FIGS. 7 and 8  show graphs that are representative of data that may be obtained with the machine  10  of  FIG. 1 . 
       FIG. 2  represents the coordinated drive unit  30  of the machine  10  that provides coordinated relative linear and rotational movement of two members (a nut  32  and a bolt  34 ) of a threaded nut-bolt assembly. The drive unit  30  includes the linear actuator  28  employed as an axial drive to position (raise/lower) a constraining tool  36  that is coupled to the head  34 A of the bolt  34  and adapted to prevent rotation of the bolt  34  about the thread axis of the nut-bolt assembly. The drive unit  30  further includes the rotary actuator  26  employed as a torsional drive to rotate the nut  32 . The rate at which the constraining tool  36  is raised or lowered by the linear actuator  28  is assigned by the computing system according to the rotational speed and direction of the rotary actuator  26  and the pitch and handedness (left or right) of the threads of the nut-bolt assembly. The result of this coordinated translation of the bolt  34  and rotation of the nut  32  fixes the apparent axial location of the nut  32 , while not directly constraining it to that axial location. Though the machine  10  is shown in  FIG. 1  as having the axis of the nut-bolt assembly oriented in the vertical direction, it is within the scope of the invention that the axis of the nut-bolt assembly could be oriented in any direction. 
     In the nonlimiting embodiment of the drive unit  30  shown in  FIG. 2 , a member  38  is constrained from translating in all axes, and is rotationally driven by the rotary actuator  26  about the thread axis of the nut-bolt assembly to install or remove the nut  32  from the bolt  34 . The configurations of the member  38  and the shaft coupling the member  38  to the rotary actuator  26  in  FIG. 2  are for illustrative purposes only, and it is foreseeable that measures could be taken to reduce the mass of each to reduce their moments of inertia. A constraining member  40  is rigidly attached with circumferentially-spaced posts  42  to the member  38 , such that there can be no relative rotation between the members  38  and  40 . The constraining member  40  accepts the nut  32  such that there can be no relative rotation between the member  40  and the nut  32 . The nut  32  is not required to be axially constrained by the member  40 . In combination, the actuator  26 , members  38  and  40 , and posts  42  serve as means adapted to cause rotational movement of the nut  32  about its axis of rotation.  FIG. 2  further shows a member  44  constrained from rotating in all axes, and linearly driven by the linear actuator  28  in parallel with the thread axis of the nut-bolt assembly. The bolt  34  is fixed relative to the member  44  by the constraining tool  36  (which in combination are adapted to serve as means for constraining rotational movement of the bolt  34 ), and therefore moves linearly with member  44  (which in combination with the actuator  28  is adapted to serve as means for causing linear movement of the bolt  34 ). 
     The operations of the rotary and linear actuators  26  and  28  are shown in  FIG. 2  as being controlled with the controller  50  using feedback from the rotational and translational position sensors  46  and  48 , which in combination are adapted to serve as means for coordinating the linear movement of the bolt  34  with the rotational movement of the nut  32  so that linear movement of the nut  32  does not occur. The computing system and the controller  50  in particular can coordinate the respective rotational and translational motions of the rotary actuator  26  and linear actuator  28  according to the equation
 
 x=θ*P  
 
where x is the linear distance that the members  34 ,  36  and  44  move, θ is the angular distance (e.g., revolutions) that the members  38 ,  40  and  32  are rotated, and P is the pitch of the threads in units of length/distance per thread or full revolution (angle). For example, if the pitch P is 0.05 inches and the angle θ is 5 revolutions, the translation distance x is 5*0.05=0.25 inches. If the coordination of the rotational and translational motions of the nut  32  and bolt  34  is continuous, the nut  32  appears fixed along the axis of the threads as the rotary actuator  26  acts to install and remove the nut  32  from the bolt  34 .
 
       FIG. 3  represents a torque sensor unit  52  of the machine  10 , and  FIGS. 4A, 4B, and 4C  represent several views of a portion of the sensor unit  52 . The torque sensor unit  52  addresses the difficulty of economically and accurately measuring torque over a wide range, for example, encompassing a torque ratio of 1:100, by utilizing multiple reaction torque sensors in series. One reaction torque sensor  54  shown in  FIG. 3  is preferably capable of accurately measuring torque in a first measurement range. At least one other reaction torque sensor  56  shown in  FIG. 3  is preferably capable of accurately measuring torque in a higher measurement range. The lower and higher ranges are selected to overlap such that the accuracy of the measurements at the minimum of the measurement range for the higher-range torque sensor  56  is sufficient at the maximum of the measurement range for the lower-range torque sensor  54 . Such a capability is provided by protecting the lower-range torque sensor  54  from being overstressed while the torque being sensed is still otherwise transferred through the sensor unit  52  to the higher-range torque sensor  56 . 
     The sensor unit  52  represented in  FIGS. 3 and 4  preferably allows measurement of torque from zero torque up to a defined saturation torque level of the lower-range torque sensor  54 . Ideally this saturation torque level is between the upper extent of the specified measurement range of the lower-range torque sensor  54  and the specified overload level of the lower-range torque sensor  54 . 
     In the nonlimiting embodiment of the sensor unit  52  shown in  FIGS. 3 and 4 , the members  38  and  40  are rigid and fixed to each other through the posts  42 , and transmit prevailing torque to the nut  32 . The bolt  34  is assembled (threaded) into the nut  32 . During assembly, the nut  32  transmits prevailing torque to the bolt  34 . The bolt  34  is rotationally constrained to the constraining tool  36 . The constraining tool  36  is rigidly fixed to a member  58 , which is rigidly fixed to the lower-range torque sensor  54  at a sensor input plane  60  ( FIG. 4C ). Input torque is transferred through the bolt head  34 A, the constraining tool  36 , and member  58  to the lower-range torque sensor  54  via the input plane  60  of the sensor  54 . Torque is transferred through the lower-range torque sensor  54  through a sensor output plane  62  to a rigid member  64 , and then through a rotationally flexible member  66  to a rigid member  68 , to which the rigid member  64  is further connected with a bearing  69  to preserve the axial alignment of the unit  52  under torsional load. The rigid member  68  is rigidly connected to a rigid member  70  through rigid members  72  and  74 . The rigid member  70  serves as the output of that portion of the sensor unit  52  that is represented in  FIGS. 4A, 4B and 4C , and is shown coupled with the higher-range torque sensor  56  in  FIG. 3 . The thread torque sensed at the head  34 A of the bolt  34  is transferred to the lower-range torque sensor  54  through the bolt head  34 A, the constraining tool  36 , and member  58 . Under this torsional load, the flexible member  66  allows the lower-range torque sensor  54  and member  64  to rotationally flex relative to the member  68 , the extent of which is limited by two gaps  76 , each between a portion  58 A of the member  58  nested or disposed between portions  74 A ( FIG. 4B ) of the member  74 . As torque increases from zero but while still lower than the saturation torque level of the lower-range torque sensor  54 , the torsional load is entirely transferred through the bolt head  34 A, constraining tool  36 , and the member  58  to the lower-range torque sensor  54 , and then through the members  64 ,  66 ,  68 ,  72 ,  74 , and  70  to the higher range torque sensor  56 . If torque continues to increase to a level at or above the saturation torque level of the lower-range torque sensor  54 , the member  58  sufficiently rotates relative to the rigidly-interconnected members  68 ,  72  and  74  so that one of the gaps  76  between the members  58  and  74  (depending on the direction of rotation) is closed and the portion  58 A of the member  58  comes in contact with one of the portions  74 A of the member  74 , with the result that torsional loads up to the saturation torque level are transferred through the bolt head  34 A, constraining tool  36 , member  58 , the lower-range torque sensor  54 , and then members  64 ,  66 ,  68 ,  72 ,  74 , and  70  to the higher range torque sensor  56 , whereas torsional levels above the saturation torque level are solely transferred to the higher range torque sensor  56  through the bolt head  34 A, constraining tool  36 , and members  58 ,  74 , and  70 , bypassing the lower range torque sensor  54  and members  64 ,  66 ,  68 , and  72 . 
     In view of the above, torque up to the saturation level of the lower-range torque sensor  54  is transferred through the lower-range torque sensor  54  to the higher range torque sensor of the sensor unit  52  prior to the gap  76  closing, and torque in excess of the saturation level is transferred through the sensor unit  52  to the higher range torque sensor in parallel to but bypassing the lower-range torque sensor  54  after the gap  76  closes. In this manner, though the deflection and measurement of the lower-range torque sensor  54  are mechanically limited, the sensor unit  52  can transfer torque in excess of the saturation limit of the lower-range torque sensor  54 , during which time the lower-range torque sensor  54  only experiences and reports the saturation torque value. Logic in the computing system used to perform the data analysis can recognize the limitation of the lower-range torque sensor  54  and use the higher-range torque sensor  56  reading above the measurement range of the lower-range torque sensor  54 . 
     In view of the above, a benefit of the sensor unit  52  is that torque levels below the saturation torque of the lower-range torque sensor  54  can be accurately measured with the lower-range torque sensor  54 , and torque levels above the saturation torque are transferred around the lower-range torque sensor  54  to prevent overstress and damage to the lower-range torque sensor  54 . Furthermore, two torque sensors in series (one lower-range and one higher-range) can be used to accurately test a large range of fastener sizes and materials, where traditionally several sensors would otherwise be required and used one at a time. This sensors-in-series arrangement eliminates the requirement to acquire and maintain (including periodic calibration verification) numerous sensors, each having a unique range, and the need to swap the sensors in and out of the test equipment whenever a test would otherwise be out of the installed sensors accurate range. 
       FIGS. 5A and 5B  represents an optical temperature measurement unit of the machine  10 . The temperature measurement unit is used to monitor heat generated at the nut-bolt thread interface due to a significant portion of the work imparted on the nut-bolt assembly being converted to heat, causing a temperature rise during assembly and disassembly. The temperature measurement unit exploits aspects of the coordinated drive unit  30  shown in  FIG. 2 , which ensures that the nut  32  does not axially translate during its installation on the bolt  34  as a result of the position of the bolt  34  being controlled and coordinated with the thread pitch and rotation of the nut  32 . As a result, temperature can be measured at a stationary point where the threads of the bolt  34  exit the nut  32  because the nut  32  can remain fixed relative to an optical temperature measurement instrument  80  mounted in proximity to the nut-bolt assembly under test. The instrument  80  can be aimed and, if properly fixtured, will remain on-target for all subsequent assembly and disassembly cycles of a given inspection procedure. Because different platings and coatings commonly used on fasteners will have different emissivity properties, the instrument  80  is preferably capable of being calibrated for the particular finish of the bolt threads. 
     Another obstacle to obtaining accurate temperature measurements and avoided with the coordinated drive unit  30  is achieved with the constraining member  40 . In the nonlimiting embodiment represented in the drawings, the constraining member  40  is shown as a disk that is oriented perpendicular to the axis of rotation of the nut  32 , is not thicker than the height of the nut  32 , and is axially positioned relative to the nut  32  such that the constraining member  40  does not obscure the optical temperature measurement instrument  80  from the target at the nut-bolt interface, represented in  FIG. 5B  as a surface of the bolt  34  at or in close proximity to the exit end of the nut  32 . In addition, the constraining member  40  is driven by the member  38 , which does not continuously obscure the measurement instrument  80  from the target. In the nonlimiting embodiment shown in  FIGS. 2, 3, 5A and 5B , two or more posts  42  can be circumferentially-spaced about the axis of rotation to transfer the drive torque to the constraining member  40  and nut  32 . The posts  42  define relatively large openings or windows  43  therebetween. Though optical observation of the target is momentarily obscured each time one of the posts  42  passes between the measurement instrument  80  and its target, this momentary obstruction is predictable and/or detectable, and the observed temperature during the passage of a post  42  can be filtered accordingly. The momentary obstructions generally cause a cooler temperature observation than the target=s temperature. The observed measurements can be filtered by selecting local temperature maxima, which would not include the cooler measurements of the posts  42 . 
     The constraining member  40  has a hole through its center that is coaxial with the axis of rotation of the nut  32  and is sized and shaped such that it will drive the nut  32  with sufficient torque to accomplish the goals of a test procedure without relative rotation between the constraining member  40  and nut  32 . The hole in the constraining member  40  has a size and shape that is suitable for coupling with the peripheral shape of the nut  32 , for example, a rectangular or hexagonal shape for use with a hexagonal-shaped nut  32 . 
     Torque tests of prevailing-torque fasteners often use the bolt-through condition as a reference point to define intervals at which subsequent torque measurements are summarized and recorded. A “bolt-through” condition occurs during the assembly of a nut and bolt when the end of the bolt opposite its head is flush with the exit end of the nut (the surface opposite its bearing surface). The bolt-through condition is often taken as occurring when the bolt starts to protrude from the exit end the nut. 
     A common characteristic of prevailing-torque fasteners is that they are usually required to be assemblable by hand for a minimum amount prior to encountering a prevailing torque feature (element) of the fastener assembly. Disassembling and reassembling a nut-bolt assembly to this encounter point is sufficiently repeatable, such that if the nut and bolt are assembled to the encounter point and the distance is measured between the exit end of the nut and the end of the bolt opposite its head (referred to herein as the bolt recess or negative protrusion), the nut and bolt can be disassembled and then reassembled and reliably returned to the encounter point at which the bolt recess was measured. Using this characteristic and the knowledge of the thread pitch (distance per revolution/thread) of a nut-bolt assembly, by monitoring the angular position (rotations) of the nut  32  (for example, with the aforementioned rotational position sensor  46  ( FIG. 2 ) and performing a pre-test measurement of the bolt recess at the start of a test, the instant of the bolt-through condition can be predicted accurately and in real-time without needing to be measured or detected directly. 
     For this purpose,  FIG. 6  represents a bolt recess measurement unit  82  that employs a linear transducer (linear potentiometer)  83  to measure the distance between the end of the bolt  34  opposite its head  34 A and the exit end of the nut  32 . For a nut-bolt assembly in which the nut  32  is a prevailing-torque locknut and the bolt  34  is a standard test bolt, the two components can be assembled together (usually by hand) to the point where the prevailing-torque feature of the nut  32  is first encountered by the threads of the bolt  34 . The linear transducer  83  of  FIG. 6  is represented as a spring-loaded linear distance transducer having a body  84  and rod  86 . A probe  88  is coupled to the rod  86  and protrudes through a collar  90  and through a reference plane  92  defined by a surface of the collar  90 . The nut-bolt assembly  32 - 34  is positioned on the collar  90  so that the end of the probe  88  contacts the end of the bolt  34  and the spring load of the transducer  83  is overcome such that the exit end of the nut  32  contacts the surface of the collar  90  that defines the reference plane  92 . The length of the probe  88  between the reference surface  92  and the end of the bolt  34  corresponds to the bolt recess of the nut-bolt assembly. The transducer  83  can be calibrated such that its output value is zero when the end of the probe  88  is even with the reference surface  92 , such that when the transducer  83  is assembled with a nut-bolt assembly having a known recess, the measurement obtained from the transducer  83  corresponds to the known recess. As represented in  FIG. 1 , the bolt recess measurement unit  82  can be mounted in the counter top  14  and within the enclosure  12  of the machine  10  to facilitate performing a bolt recess measurement prior to a torque tension test. As shown in  FIG. 1 , only the probe  88  and surrounding collar  90  need be exposed to perform a bolt recess measurement, with the remainder of the unit  82  located within the cabinet  16 . The assembly of the nut  32  and the bolt  34  can be disassembled after the measurement of bolt recess and then reassembled into the constraining tool  36  and member  40  of the torque tension test apparatus for the torque tension test. Reassembly of nut  32  and bolt  34  to the same position from which bolt recess was measured is generally repeatable to the accuracy required of the test. 
     The bolt recess measured by the measurement unit  82  can be divided by the thread pitch (distance per revolution/thread) of the nut-bolt assembly to determine the number of revolutions required for the nut  32  to travel the axial distance from the measured bolt recess position to the bolt-through condition. The test is performed, recognizing that the determined number of revolutions to the bolt-through condition can also be used to establish intervals at which torque measurements can be subsequently obtained, summarized and reported during testing of a nut-bolt assembly. Determination of the bolt-through condition can be performed automatically with appropriate test execution software based on the thread pitch of the nut-bolt assembly and measured bolt recess serving as input parameters of the test. 
     While the invention has been described in terms of specific nonlimiting embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the machine  10  and its subsystems could differ in appearance and construction from the embodiments shown in the Figures, the functions of each component of the machine  10  and subsystems could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, and various processing and materials could be substituted for those noted. Accordingly, it should be understood that the invention is not limited to the specific nonlimiting embodiments illustrated in the Figures. It should also be understood that the phraseology and terminology employed above are for the purpose of disclosing the illustrated embodiments, and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims.