Abstract:
The invention relates to a threaded bolt provided with measurement planes for use in ultrasonic length measurement. After the threaded bolt has been fixed in a connection, such measurement is performed to determine the length change of the threaded bolt, in order to find out the tension within the threaded bolt in that way.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 09/955,522, filed on Aug. 18, 2001, now U.S. Pat. No. 6,712,570, which was a continuation of PCT/NL00/00087, filed on Feb. 11, 2000, which claimed priority to Netherlands application no. 1011591, filed Mar. 18, 1999. 

   TECHNICAL FIELD OF THE INVENTION 
   The invention relates to a threaded bolt provided with measurement planes for use in ultrasonic length measurement. More specifically, after the threaded bolt has been used to form a connection, a measurement is taken to determine the length change of the threaded bolt, and therefore determine the tension within the threaded bolt. 
   BACKGROUND OF THE INVENTION 
   It is known to provide a threaded bolt with measurement planes. Typically, the measurement planes are formed either at a recess or an elevation with respect to the actual end planes of both the head end and the insertion end. The measurement planes are often formed by means of cold deformation and finishing. One such threaded bolt is described in European patent application 0.459.365 which issued May 28, 1991, the contents of which are incorporated herein by reference. 
   Another threaded bolt is known from U.S. Pat. No. 4,846,001 which issued Jul. 11, 1989, in which the threaded bolt is shown as having a head which has been provided with a recess including a measurement plane. The measurement plane has been made sufficiently smooth/level by finishing, in order to be used in ultrasonic measuring. The recess is used for accommodating a piezo-electric sensor. In another embodiment the recess is absent and the sensor is placed on top of the end plane at the head. 
   Frequently, existing threaded bolts exhibit several radial planes at one or both ends, situated at various axial distances measured along the center line of the threaded bolt, which can adversely affect the ability to achieve accurate measurement results. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to reduce the number of radial planes exhibited by the threaded bolt, thereby improving the accuracy with which ultrasonic length measurements on the bolt can be performed. 
   One embodiment of the invention provides a threaded bolt having a proximal head end and a distal insertion end, wherein the proximal head end and the distal insertion end have been provided with a single, exposed radial measurement plane for use in ultrasonic length measurement in order to determine the tension in the threaded bolt after it has been used in a connection. The measurement plane at the distal insertion end is formed by the flat bottom of a recess at the distal insertion end, wherein the recess is bounded by a circumferential wall. The circumferential wall transitions into the flank of the distal insertion end via a buckle or curve. Therefore, multiple radial planes at the insertion end other than the measurement plane are prevented. Therefore, the ultrasonic length measurement is related only to the measurement plane at that location and double signals are prevented. The measurement plane is protected from pollution and damage by the recess. 
   Another embodiment of the invention provides a threaded bolt having a proximal head end and a distal insertion end, wherein the proximal head end and the distal insertion end have been provided with a single, exposed radial measurement plane for use in ultrasonic length measurement in order to determine the tension in the threaded bolt after it has been used in a connection. The measurement plane at the distal insertion end is formed by the flat bottom of a recess at the distal insertion end, wherein the recess is bounded by a circumferential wall and the outermost distal end of the threaded bolt is formed by a circular line. 
   In preferred embodiments, the transition from the circumferential wall of the recess into the flank of the distal insertion end may run according to a convex course, but preferably has a sharp buckle shape, thereby ruling out the occurrence of murmur while taking measurements as much as possible. 
   Preferably the circumferential wall of the recess forms a conical surface which is oblique with respect to the bolt center line, the angle of which is maximally 75° with respect to the bolt center line, preferably also more than 45°. 
   Preferably the flank in the distal end portion, which is contiguous to the circumferential wall of the recess, forms a flank conical surface oblique with respect to the bolt center line, the angle of which is maximally 45° with respect to the bolt center line. Preferably the flank conical surface at the proximal side changes into the cylindrical plane via a flank conical surface which is less oblique with respect to the bolt center line, for instance at 25–35°, which cylindrical plane may or may not be contiguously provided with the thread. Alternatively, the distal end of the bolt provided with the aforementioned recess is truncated at the flank. 
   Preferably the recess has been formed by means of an upsetting treatment of the insertion end. As a result of the upsetting treatment, the distal end of the bolt can be provided with its final form in one step. Alternatively, apart from the upsetting treatment for forming the recess, the oblique flank can be obtained by means of a machining treatment. 
   It is a further object of the invention to provide a threaded bolt which can easily be manufactured and is able to provide reliable measurement results. 
   To that end, a preferred embodiment of the invention provides a threaded bolt having a proximal head end and a distal insertion end, the proximal head end and the distal insertion end being provided with measurement planes for use in ultrasonic length measurement in order to determine the tension in the threaded bolt after it has been used in a connection. The measurement plane at the insertion end forms the complete end plane and has been arranged on the insertion end without a final processing operation. 
   In preferred embodiments, the measurement plane has been arranged at the distal insertion end by means of an upsetting treatment. The upsetting treatment can advantageously be performed during a heading treatment performed in a bolt-machine, also known as “bolt maker”. In such a bolt machine, the upsetting of the head takes place in one operation. During the upsetting treatment, the measurement planes can be realised at both ends. Therefore, the exact axial distance between both measurement planes can be determined. 
   Alternatively, the upsetting operation for making the measurement planes may take place during the subsequent shank reduction. 
   By way of a further alternative, instead of by means of the upsetting treatment, the measurement plane at the insertion end can be made in the same bolt machine, but in a following station, where the point is made. This then takes place by means of a machining treatment. 
   The invention furthermore provides a method for manufacturing a threaded bolt according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be elucidated on the basis of the examples shown in the attached drawings, in which: 
       FIG. 1  shows a threaded bolt according to the invention, having a recessed measurement plane at the distal end; 
       FIG. 1A  shows a detail of the distal end of the threaded bolt of  FIG. 1 ; 
       FIGS. 2A and 2B  show two exemplary embodiments of another threaded bolt according to the invention. 
       FIG. 3A  shows a view of a bolt machine for manufacturing a threaded bolt according to the invention; 
       FIG. 3B  shows consecutive stations within the bolt machine of  FIG. 3A ; and 
       FIG. 4  shows a possible embodiment of the point making machine accommodated in the bolt machine according to  FIG. 3B . 
   

   DETAILED DESCRIPTION 
   The threaded bolt  1  shown in the  FIGS. 1 and 1A  comprises a shank  2  having a widened head  3  with flange  4 , in which a thread  5  has been arranged on the shank  2 . The distal end  6  of the shank  2 , and thus the bolt  1 , is shown enlarged in  FIG. 1A . As shown in  FIG. 1A , a recess  7  has been made in the distal end  6 , the recess being axially bounded by a flat bottom  8  which extends perpendicular to the center line X of the bolt  1 . The recess is radially bounded by an inclined circumferential plane  9  running concentrically about the center line X, the plane  9  is at angle a of 45°–75° with respect to the center line X. Via a sharp, line-shaped transition  10 , which forms a buckle and which in end view forms a circular line, the distal end  6  changes radially to the outside into an inclined circumferential plane  11  which is also concentric with respect to the center line X. Inclined circumferential plane  11  is at an angle β with respect to the center line X, β maximally being 45°. It can be seen that the inclined circumferential plane  11 , via a buckle, changes into a comparable inclined circumferential plane  12 . However, inclined circumferential plane  12  is at an angle γ with respect to the center line X, which is different from the angle β and which here is 25 to 35°. 
   At the head  3  a flat plane  14  has also been provided on the end plane  13 , which just like plane  8  is perpendicular to the center line X is suitable to serve as a measurement plane in ultrasonic measurements. The plane parallelism here amounts to a least 0.05 mm. 
   The recess  7  and the measurement plane  8  can be formed in a simple manner by upsetting the distal end  6  of the bolt (preferably made of steel) in a pointing machine. In the same upsetting treatment the circumferential planes  11  and  12  can be made. Alternatively, said planes  11  and  12  can be made by means of a machining treatment, as is discussed hereinafter in relation to  FIG. 4 . 
   During measuring, the sensor will be placed against the measurement plane  14 , possibly with the help of a very thin layer of contact adhesive. With the help of ultrasonic measurement, the distance S, measured in a direction parallel to the center line X between the planes  8  and  14  (against which the sensor has been placed), can be exactly measured and determined. With that distance, the tension in the bolt can be determined. The accuracy of the ultrasonic measurement is improved as a result of only one radially oriented plane  8  being present in the distal end  6 , therefore preventing multiple measurement signals. 
   By way of example, with a diameter of the circular line  10  of approximately 12 mm, the diameter of the plane  8  may be 4 mm. 
   Just like the measurement planes of the threaded bolt to be discussed below and shown in the  FIGS. 2A and 2B , the measurement planes  8  and  14  may be formed without final processing operations, meaning that for plane  8  the upsetting treatment can be the last treatment of the plane. 
   In  FIG. 2A , comparable bolt parts as compared to the bolt in  FIGS. 1 and 1A  are shown with comparable reference numbers, increased by  100 . Thus, the bolt  101  is provided with a distal measurement plane  108  and with a proximal measurement plane  114  which forms the bottom of a recess  115  in the proximal end plane  113  of the bolt  101 . A sensor may be placed in the recess.  115 . In this case, the distance S between the measurement planes  108  and  114  is exactly known: beforehand. The recess  115  and the measurement plane  114  have been formed during flat pre-upsetting (also see  FIG. 3B ), without final processing treatments (such as grinding) of the measurement plane. The measurement plane  108  has been formed here during upsetting, also without final processing treatments of the plane. 
   In  FIG. 2B  an alternative threaded bolt  201  has been shown, in which the same parts have the same reference numbers again, increased by 100 over those in  FIG. 2A . In this case the measurement planes  208  and  214  have been formed at both the proximal end and at the distal end of the bolt  201 . Just like the measure plane  108 , the measurement plane  208  extends over approximately the entire diameter of the distal end, or insertion end. 
   In  FIG. 3A , a bolt machine  320  is shown with which the bolts according to the  FIGS. 1 ,  2 A,  2 B, and alternative embodiments can be made. Metal wire  321  is supplied from a roll  322 , passes through the supply end  323  of the bolt machine  320  and is processed into threaded bolts. The threaded bolts are provided with measurement planes according to the invention and are discharged at the discharge end  324  where they are collected in a tray  325 . 
   The bolt machine  320  includes a thread cutter for cutting a workpiece for the bolt (see  FIG. 3B ), a station  402  for pre-upsetting (after which the workpiece  401   a  is obtained), a station  403  for heading (after which a workpiece  401   b  is obtained), a station  404  for reducing the shank (after which a workpiece  401   c  is obtained), and a station  405  for trimming (after which a workpiece  401   d  is obtained). The workpieces  401   d  are subsequently subjected to a point-making treatment in a pointing machine. accommodated in the same bolt machine  320 , and after that to a thread rolling treatment, also in the same bolt machine  320 . 
   As stated, the measurement planes in the bolts according to the invention can be arranged by cold deformation during heading, in station  403 , either by integrally forming a recess in the head or not. During this upsetting of the head use is made of an especially formed stamp in station  403 . For making the measurement plane at the distal or insertion end a push-out pin  426  or  427  especially formed for that purpose can be used for deforming the distal end in the wanted manner by upsetting in station  402  or  403 . 
   Thus, without particular further provisions, a threaded bolt can be formed having measurement planes which are suitable for ultrasonic length measurements. 
   The pointing machine  530  of  FIG. 4 , accommodated in the bolt machine  320 , can alternatively be used for making the measurement plane at the distal end which is formed by the end plane of the distal end. As can be seen, two cutter plates  531  and  532  have been arranged which are hingeable and can be adjusted in angular position. The cutter plate  531  is positioned inclined and the cutter plate  532  horizontally to cut the measurement plane. Here again, the exact distance between both the measurement/end planes is known. Both cutter plates are rotated about the insertion end, whereas the bolt is held against rotation.