Patent Publication Number: US-2022221358-A1

Title: Screw load testing device and method for performing a load test on a screw

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to International Application Serial No. PCT/EP2020/062235, which is hereby incorporated herein in its entirety by this reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a screw load testing device wherein such screw load testing device is particularly suitable for testing strength values of a screw. Furthermore, the invention relates to the use of a screw load testing device according to the invention and to a method for performing a load test on a screw using a screw load testing device according to the invention. 
     BACKGROUND OF THE INVENTION 
     Screws are well-known. According to VDINDE 2645 Part 2:2014-09, screws enable establishing a detachable threaded connection that joins two or more parts so that they behave like one piece under all existing operating forces. During the production of screws, random samples of screw batches are tested for compliance with specified strength values. This so-called load testing is carried out by using a screw load testing device. 
     Such a screw load testing device having the features of the preamble of claim  1  is known from WO2008/151799A1, which corresponds to US Patent Application Publication No. 2010-0162829, which is hereby incorporated herein in its entirety by this reference for all purposes. This known screw load testing device used for load testing of conventional screws comprises a housing having a plurality of housing members wherein a screw head axially abuts against an end face of a first housing member and a threaded nut cooperating with a threaded portion of the screw contacts an end face of a second housing member. When a tightening torque is applied to the screw, the two housing members are clamped against one another by the screw. A measuring device may be used to measure the amount of clamping as an axial force and/or a torque. 
     Such a screw load testing device is useful for testing conventional screws either in a non-destructive test or in a test that uses a load resulting in destruction of the screw. Generally, destruction of the screw during a destructive load test particularly occurs in the threaded portion of the screw so that after destruction of the screw it is relatively easy to separate the two housing members from each other or to remove screw fragments from the screw load testing device. 
     However, such a screw load testing device known from the prior art is of only limited use in a destructive load test of a breakaway screw. A breakaway screw has two screw heads arranged one after the other along its longitudinal axis. A rear screw head breaks off when a predetermined tightening torque is applied. Breakaway screws are used in many technical fields as a safety element or an anti-theft device. The reason is that once the rear screw head has broken off, specialized tools must be employed for removing the remaining screw body from the screw connection. Breakaway screws vary widely especially with respect to the shape and size of the rear screw head. 
     Thus, in a destructive load test of a breakaway screw the two housing members of the screw load testing device are clamped against one another by the axial force and the space for applying the specialized tool is structurally blocked by the housing members. Therefore, it is impossible to reach the appropriate tool engagement surfaces with the specialized tool and to remove the screw fragments from the screw load testing device. 
     In practice, it has been tried to solve this problem by providing the breakaway screw to be tested prior to the load test with additional tool engagement surfaces in those areas of the breakaway screw that are still accessible on the screw body from the outside after the screw head, i.e. the original tool engagement surface, broke off. On the one hand, this has the disadvantage that attaching or forming such additional tool engagement surfaces that merely serve for loosening the breakaway screw after the destructive load test requires additional work. Furthermore, it is considered to be a particular disadvantage that mechanical processing of the breakaway screw for forming the additional tool engagement surfaces is typically associated with thermal stressing of the material of the screw body which may lead to a change in the microstructure of the material and thus to altered strength properties of the screw body. This may falsify the measurement results during the load test of a breakaway screw. 
     EXEMPLARY OBJECT AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a screw load testing device allowing for easy removal of the screw from the screw load testing device after the end of a load test or after destruction of the screw. 
     This object has been achieved by the features described more fully below. 
     The invention relates to a screw load testing device comprising a housing configured to arrange a screw and a mating element that interacts with a threaded portion of the screw and is in particular formed as a threaded nut, said housing comprising a first housing member configured to interact at least indirectly with a portion that is in monolithic connection to the screw, said housing further comprising a second housing member configured to interact with the mating element, wherein the two housing members are arranged movable with respect to one another at least in the direction of a longitudinal axis, and wherein the screw load testing device further comprises a measuring device for detecting an axial force F acting on the screw and/or for detecting a torque M acting on the screw, wherein a clamping device is provided for moving the two housing members from a first position to at least a second position, and wherein in the first position a first distance in the direction of the longitudinal axis between two contact surfaces configured to cooperate with the screw and the mating element is smaller than a second distance of the two contact surfaces in the second position. 
     The idea underlying the present invention is that after a (destructive) load test was performed on the screw it is essential for removing the screw fragments to reduce an axial force applied on the two housing members cooperating with the screw for the load test so that the still intact screw body can be released from its mating element (nut) arranged in the housing, ideally by hand and without providing specific tool engagement surfaces. 
     In view of the above, the teaching of the invention provides for constructing the screw load testing device in such a way that the two housing members cooperating with the screw may be moved by means of a clamping device from a first position into at least a second position, and in such a way that in the first position a first distance in the direction of the longitudinal axis between two contact surfaces configured to cooperate with the screw and the mating element is smaller than a second distance of the two contact surfaces in the second position. 
     Advantageous further embodiments of the screw load testing device according to the invention are provided in the dependent claims. 
     In a particularly preferred embodiment of the invention at present, resetting means eliminates the need for the user to manually move the two housing members of the housing after destruction or the end of the load test of the screw from their second position in which they are arranged at a distance from one another into their first position in which they are arranged at a smaller distance from one another. The resetting means are configured to move the two housing members of the housing in the direction of the first position. Thus, when the clamping device is deactivated, then the housing members are automatically reset in the direction of their first axial position. 
     There are several options with regard to the specific construction of the resetting means. A mechanical structure is preferred where the resetting means are formed as at least one spring element. A particular advantage of this construction is that it will achieve resetting of the housing members in the desired manner without needing additional actuation or auxiliary structures. The spring element is in particular a compression spring. A specific constructional realization provides for the at least one spring element to be arranged in the area of the clamping device between two members of the clamping device configured to be movable relative to one another in the direction of the longitudinal axis. It is further particularly preferred to provide not only one spring element or one compression spring but rather a plurality of spring elements or compression springs. In this case, it is particularly preferred to provide these arranged at uniform angular intervals on a pitch circle diameter around the longitudinal axis of the housing members. Furthermore, it should be noted that instead of mechanical resetting means or spring elements it is also conceivable to form the resetting means as pneumatic or hydraulic resetting means by using compressed air or a hydraulic medium, for example. 
     With regard to the clamping device for generating a preloading force that acts on the two housing members in the direction of the second position it is particularly preferred to provide a clamping device in the form of a hydraulic clamping device. A hydraulic clamping device comprises a piston and a cylinder. The piston is pressurized by hydraulic medium and drives the piston up through the cylinder during which the piston is guided in the cylinder. A particular advantage of using a hydraulic clamping device is that relatively high clamping forces acting in the axial direction may be absorbed and that, furthermore, the piston will not exhibit any perturbing stroke variations because the hydraulic medium is incompressible. 
     In a further advantageous embodiment, pressurization by the hydraulic medium is achieved by means of an electric pump or a pneumatic system. 
     In order to be able, on the one hand, to use one and the same housing or one and the same housing members for (breakaway) screws having different thread diameters and, on the other hand, to ensure that an axial force acting on the (breakaway) screw and/or a torque acting on the (breakaway) screw after a tightening torque was applied only acts in the area of a contact surface of the mating element cooperating with the screw and of the screw head, respectively, and may be detected by a measuring device in a precise manner, another structural embodiment of the housing provides for the housing to be configured for receiving the screw between the two contact surfaces forming a radial gap therebetween. 
     In a further advantageous embodiment of the screw load testing device, in order to be able to use the screw load testing device for a variety of screw sizes without need for elaborate modifications or long setup times, the screw load testing device is provided with a size change member that preferably has a plurality of through-holes with various diameters and is provided as a mating element for the portion in monolithic connection to the (breakaway) screw, and the mating element may be positioned in a rotationally fixed manner in axial contact to the first housing member of the housing. 
     Furthermore, a plurality of options also exist with respect to the arrangement and construction of the measuring device. A particularly advantageous embodiment with respect to construction provides for arranging the measuring device between the second housing member and a further member wherein the second housing member and the further member are arranged to be movable relative to one another in the direction of the longitudinal axis and about the longitudinal axis, and wherein the measuring device is preferably arranged in direct contact with the second housing member and the further member. 
     The manufacture of the individual housing members of the housing is particularly easy and economical when they are formed as turned members. In this respect, a further embodiment of the invention includes at least the first and second housing members in the shape of sleeves or rings. 
     The invention further comprises the use of a screw load testing device according to the invention as described above for performing load tests on breakaway screws. Such a use is particularly advantageous since due to the construction of the screw load testing device according to the invention it is very easy to remove the fragments of a breakaway screw present on the housing after a destructive load test of the screw. 
     Furthermore, the present invention also encompasses a method for performing a load test on a screw, in particular a breakaway screw, by means of a device according to the invention as described above. The method according to the invention is characterized at least by the following steps: 
     Moving the first and second housing members from the first to the second position while increasing the axial distance between two contact surfaces for the screw by activating the clamping device;
 
Inserting the screw and the mating element that cooperates with the screw;
 
Applying a tightening torque to the screw;
 
Deactivation of the clamping device after the end of the load test;
 
Removing the screw.
 
     In a further embodiment of the method, resetting means are provided for moving the two housing members from the second position towards the first position after deactivation of the clamping device. 
     In a further advantageous embodiment of the method, an axial gap for removing the screw is created between the first contact surface and a portion of the screw cooperating with the first contact surface. A particular advantage of such a method is that the axial force previously acting on the screw is completely eliminated so that the screw may be released from the mating element simply by manually turning or screwing the screw. 
     Further advantages, features and details of the invention will be apparent from the following description of preferred exemplary embodiments thereof as well as from the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a longitudinal section through a screw testing device according to the invention after a destructive test of a screw before it is removed from the screw testing device, and 
         FIG. 2 through 5  show each in a perspective longitudinal section the screw testing device according to  FIG. 1  during different phases of screw testing. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     Throughout the Figures, identical elements or elements having the same function are designated by the same reference numerals. 
     The screw load testing device  10  as shown in the Figures is for non-destructive or destructive load testing of screws  1 . In particular, the screw load testing device  10  is suitable for performing load tests on breakaway screws  2 . The breakaway screw  2  is shown in its undestroyed state in  FIG. 3  and is characterized by a cylindrical portion  3  having a threaded portion  4  which is only visible in  FIG. 1  on at least one end face portion thereof. The threaded portion  4  is configured to cooperate with a mating element  5  formed, for example, as a threaded nut  6 . At the other axial end portion of the breakaway screw  2  is defined a (short) cylindrical portion  7  followed by a shear-off portion  8  in the shape of an hourglass that is in monolithic connection to a head portion  9  of the breakaway screw  2 . The head portion  9  comprises tool engagement surfaces for applying a tightening torque to the breakaway screw  2  in a manner known per se. In particular, the tool engagement surfaces are formed as planar surfaces so that they are able to cooperate with a wrench or a similar torque applying device, for example. 
     In practice, such a breakaway screw  2  shown in  FIG. 3  for example is used for connecting members which must be ensured to be no longer detachable or separable from each other in an easy manner after the members are mounted or connected to one another. For this purpose, a high tightening torque is applied to the breakaway screw  2  via the head portion  9  thereof during mounting of the breakaway screw  2  which leads to its destruction in the region of the shear-off portion  8  or to shearing off of the head portion  9  in the region of the smallest cross-section of the shear-off portion  8 . There are no tool engagement surfaces on the remaining cylindrical portion  7  or the shear-off portion  8  so that it is usually impossible to remove such a breakaway screw  2  after mounting without using further auxiliary means or a lot of effort. 
     The screw load testing device  10  is used to test random samples of batches of screws for compliance with strength values during the manufacture of screws. 
     The screw load testing device  10  comprises a housing  12  formed from a plurality of members. Housing  12  comprises a first plate-shaped or annular housing member  14  that defines on one opposite surface thereof, a longitudinal groove  20 . As shown in  FIG. 1  for example, the longitudinal groove  20  is configured to receive therein, a size change member  16 . The size change member  16  is axially interposed between the annular housing member  14  and the cylindrical portion  7  of the screw  2  and defines a first contact surface  19  that is configured and disposed to axially abut against the end face  17  of the cylindrical portion  7  of the breakaway screw  2  on the side of the breakaway screw  2  that faces away from the shear-off portion  8 . 
     In the embodiment shown, the size change member  16  has the shape of a rib and comprises through-holes  18  with different diameters so that the size change member  16  may be used for different sizes or diameters of the breakaway screw  2 . For this purpose, the cylindrical portion  3  of the breakaway screw  2  passes through a through-hole  18  with a small radial clearance. Furthermore, the first housing member  14  comprises a longitudinal groove  20  that is adapted to the width of the size change member  16 , wherein the size change member  16  axially abuts against the bottom of the groove  20 , thus forming a rotationally fixed connection between the first housing member  14  and the size change member  16  about a longitudinal axis  22  of the housing  12 . 
     Axially, i.e. in a plane perpendicular to the longitudinal axis  22 , the first housing member  14  rests against a clamping device  24 . The clamping device  24  is preferably a hydraulic clamping device. The clamping device  24  comprises a first member  26  having a through-bore  27  extending concentrically with the longitudinal axis  22 . The diameter of the through-bore  27  is adapted to the diameter of the cylindrical portion  3  of the breakaway screw  2  so that at least a small radial gap is always formed between the cylindrical portion  3  and the through-bore  27 . The first member  26  of the clamping device  24  is radially enclosed by a second member  28  of the clamping device  24 . The second member  28  has the shape of a sleeve and comprises a hydraulic connection portion  30  and a vent connection element  36 . A hydraulic element  34  is disposed in the hydraulic connection portion  30 , the hydraulic element  34  being connected to a source of hydraulic fluid, not shown, and to a valve device. The vent connection element  36  serves to vent the clamping device  24 . An annular pressure chamber  38  is arranged between the two members  26 ,  28  of the clamping device  24 . 
     The two members  26 ,  28  of the clamping device  24  may be moved in the direction of the longitudinal axis  22  from a first position in which the two members  26 ,  28  rest against each other with axially opposite abutment surfaces  39 ,  40  ( FIG. 5 ) in the direction of the arrow  41  ( FIG. 1 ) to a second position that is axially spaced-apart in the direction along the longitudinal axis  22  from the first position. 
     Annular sealing elements  42 ,  43  such as elastic O-rings are provided between the two members  26 ,  28  under radial pressure from the clamping device  24  for sealing the pressure chamber  38  from the environment. Preferably, the first member  26  comprises corresponding annular grooves for receiving the annular sealing elements  42 ,  43 . This presently preferred embodiment is particularly space-saving. Alternatively, the second member  28  may comprise the corresponding annular grooves for receiving the annular sealing elements. 
     A plurality of blind holes  44  are formed, preferably at uniform angular intervals from each other, in the first member  26  of the clamping device  24  on a pitch circle diameter around the longitudinal axis  22  on an end face facing away from the first housing member  14 . A spring element  45  in the form of a compression spring is arranged in each of the blind holes  44 . The spring elements  45  serve as resetting means  46  for applying a resetting force to the two members  26 ,  28  of the clamping device  24  in the first position described above. For this purpose, the spring elements  45  are axially supported on an annular support plate  48  that is axially clamped to the first member  26  by means of screws  49 . The support plate  48  also comprises a through-bore  51  that receives the cylindrical portion  3  of the breakaway screw  2  forming a radial gap therebetween. 
     With its second member  28 , the clamping device  24  axially protrudes on the side facing away from the first housing member  14  into a recess  52  of a further housing member  54  which is part of a measuring device  70 . The further housing member  54  comprises a bore portion  56  extending coaxially with the longitudinal axis  22  in which an annular washer  58  is arranged forming a radial gap  59 . The annular washer  58  radially receives a portion of a sleeve-shaped insert  60  acting as a second housing member  62 . The annular washer  58  and the second housing member  62  are connected to one another in a rotationally fixed manner, for example, by means of a plurality of dowel pins  63  ( FIG. 2 ). The insert  60  or the second housing member  62  forms the surface for axial contact of the threaded nut  6  against an end face  64  of the second housing member  62  acting as the second contact surface  61 . Furthermore, the annular washer  58  is connected in a non-rotatable manner to a further washer  65 , for example by forming the annular washer  58  and the further washer  65  monolithically. 
     The measuring device  70  mentioned above is accommodated between two surfaces  66 ,  67  arranged at a distance apart from one another in the direction of the longitudinal axis  22  and extending annularly around the longitudinal axis  22  on the further washer  65  and the further housing member  54 . By way of example only, the measuring device  70 , which is for example configured in a manner known per se, further comprises a plurality of strain gauges (not shown) or similar measuring elements which serve to detect an axial force F and/or a torque M acting on the breakaway screw  2  or the threaded nut  6  in the direction of the longitudinal axis  22 . By an electrical plug connection  72 , the measuring device  70  is connected, for example, to an evaluation unit and/or to a display unit, not shown, configured to evaluate, display and/or store the axial force and/or the torque detected by the measuring device  70 . 
     The washer  65  is clamped in the axial direction via the measuring device  70  against the further housing member  54  by means of a torque measuring body  74 . 
     In the following, the functioning of the screw load testing device  10  described above in a load test of a breakaway screw  2  will be explained with reference to the sequence of Figures shown in  FIGS. 2 to 5  as follows:  FIG. 2  shows the situation with the breakaway screw  2  not yet mounted on the screw load testing device  10 . The pressure chamber  38  is filled via the hydraulic connection portion  30  with pressure medium being under a pre-pressure of, for example, 30 bars, and a return flow of the pressure medium from the pressure chamber  38  is prevented by a respective position of a valve in the hydraulic fluid supply. The hydraulic pressure prevailing in the pressure chamber  38  causes the clamping device  24  and the two members  26 ,  28  to be moved against the spring forces of the spring elements  45  from their first position to their second position. The second axial position is characterized by a distance a 2  between the two contact surfaces  19  and  61  for the breakaway screw  2  in the direction of the longitudinal axis  22 . The axial movement is thereby limited by a contact between the support plate  48  and the extended second member  28  of the clamping device  24  wherein optionally the second axial position is monitored by a proximity sensor (not shown). 
       FIG. 3  shows the breakaway screw  2  mounted in the screw load testing device  10  by screwing the threaded portion  4  into the threaded nut  6 . At the same time, it can be seen that the cylindrical portion  7  is in axial contact with the size change member  16  with its end face  17  on the side facing the housing  12 . As shown in  FIG. 3 , a tightening torque is now applied to the breakaway screw  2  via the tool engagement surfaces of the head portion  9  of the breakaway screw  2  until it is destroyed in the area of the shear-off portion  8 . This situation of the head portion  9  having been severed from the cylindrical portion  7  is shown in  FIG. 4 . Increasing the tightening torque of the screw  1  also increases the hydraulic pressure of the hydraulic medium in the pressure chamber  38  of the clamping device  24 . 
     An axial force F acting on the breakaway screw  2  after application of the tightening torque and/or a torque M acting on the breakaway screw  2  after application of the tightening torque is thereby detected by means of the measuring device  70 . In this respect, those skilled in the art may measure only the axial force F or only the torque M or may measure both the axial force F and the torque M. 
     After the end of the test it is necessary to remove the fragments of the breakaway screw  2  that are now present under the axial load in the screw load testing device  10 . For this purpose, first the hydraulic medium is allowed to flow back from the pressure chamber  38 , in particular by opening the valve (not shown). Subsequently, the resetting means  46 , i.e., the spring forces of the spring elements  45 , cause the two members  26 ,  28  of the clamping device  24  to move from the second axial position back into their original first position. The resulting axial distance a 1  between the two contact surfaces  19  and  61  is less than the second axial distance a 2 . In addition, as shown in  FIG. 5  an axial gap  76  of, for example, 5 mm, is formed between the size change member  16  and the cylindrical portion  7  in the vicinity of the end face  17  of the breakaway screw  2 . Thereby, it is particularly easy to unscrew the breakaway screw  2  via the cylindrical portion  7  or the shear-off portion  8  by hand from the mating element  5  or the threaded nut  6  and to remove the breakaway screw  2  from the housing  12  in the axial direction. 
     The screw load testing device  10  described above may be altered or modified in a variety of ways without departing from the spirit of the invention. In particular, it is noted that the screw load testing device  10 , although it is described only in connection with a load test of breakaway screws  2 , may of course also be used for load tests performed on conventional screws  1 . 
     LIST OF REFERENCE NUMERALS 
     
         
           1  screw 
           2  breakaway screw 
           3  cylindrical portion 
           4  threaded portion 
           5  mating element 
           6  threaded nut 
           7  cylindrical portion 
           8  shear-off portion 
           9  head portion 
           10  screw load testing device 
           12  housing 
           14  first housing member 
           16  size change member 
           17  end face 
           18  through-hole 
           19  first contact surface 
           20  longitudinal groove 
           22  longitudinal axis 
           24  clamping device 
           26  first member of clamping device 
           27  through-bore 
           28  second member of clamping device 
           30  hydraulic connection portion 
           34  hydraulic element 
           36  vent connection element 
           38  pressure chamber 
           39  abutment surface 
           40  abutment surface 
           41  arrow 
           42  sealing element 
           43  sealing element 
           44  blind hole 
           45  spring element 
           46  resetting means 
           48  support plate 
           49  screw 
           51  through-bore 
           52  recess 
           54  further housing member 
           56  bore portion 
           58  annular washer 
           59  radial gap 
           60  sleeve-shaped insert 
           61  second contact surface 
           62  second housing member 
           63  dowel pin 
           64  end face 
           65  further washer 
           66  surface 
           67  surface 
           70  measuring device 
           72  plug connection 
           74  torque measuring body 
           76  axial gap 
         a 1  first distance 
         a 2  second distance