Abstract:
The invention relates to a device for adjusting and/or verifying the axial force in screwed connections. The inventive device contains a control device for limiting an axial force acting between two force application elements of the screwed connection. To this end, the invention provides that the control device will record signal values from a measuring element whose electrical resistance varies according to the acting axial force.

Description:
BACKGROUND OF THE INVENTION 
   The present invention concerns a device for adjusting and/or testing the axial tightening forces in screw joints. 
   The invention concerns above all mechanical screw joints which are used with different designs and in many ways in the field of mechanics. Basically, it can be applied to all known kinds of threads or screw joints. 
   Devices for adjusting the axial force in screw joints, including a check device for limiting an axial force operating between e.g. two force-applying elements of the screw joint, are already known. Thus so-called “torque wrenches” which allow mechanical overload control upon exceeding a given torque for a screw joint, are known. In this way, precise detection of the force between the force-applying elements (e.g. a bolt head on a bolt shank as the first force-applying element and a nut on a thread of the bolt shank as the second force-applying element) is not definitely possible. The torque which is applied to tighten a screw joint does cause a change in the axial force between the force-applying elements, as a rotational movement produces a change of distance between the force-applying elements and hence a change in axial force, due to the pitch of the thread of a bolt shank. However, the correlation between axial force and torque is under certain circumstances highly subject to errors, as the frictional forces of a force-applying element on a thread influence the torque critically. When the torques are identical, the tightening force of a screw joint can vary considerably, so that for bolts which slide easily there is a risk of overstretching, whereas in the case of stiff bolts the preselected torque is already attained when the parts to be connected are still loose. Apart from this imprecision, with the mechanical overload protection there is the drawback that only exceeding of the limit is possible, but not exact measurement of the axial force. 
   SUMMARY OF THE INVENTION 
   It is therefore the object of the present invention to propose a device for adjusting and/or testing the axial forces in screw joints, which can determine exactly the axial forces of a screw joint in a quick and cheap manner. 
   This object is achieved by a device for adjusting and testing the axial force in screw joints, wherein the device includes a check device for limiting an axial force operating between force-applying elements of the screw joint, wherein the check device has means for signal value pick-up from a measuring element whose electrical resistance is variable as a function of the operative axial force. 
   Due to the fact that the check device has means for signal value pick-up from a measuring element whose electrical resistance is variable as a function of the operative axial force, direct determination of the axial force is possible (without a diversion via torque measurement). The axial force can be applied by any force-applying elements (which can have any form and number), and it is essential that a resulting axial force is produced. A measurement of electrical resistance or a change of voltage occurring as a result in a measuring element which is mechanically loaded by the axial force, is used as the initial value. The measuring element can in this case be made of a material or coated with a material, as shown in DE 199 54 164 A1. This can be e.g. an amorphous, diamond-like carbon compound. To avoid repetition, reference is here made to all the materials which are listed in DE 199 54 164 A1. Naturally other piezoresistive materials are also possible for constructing the measuring element. 
   Advantageous developments of the present invention are provided in the dependent claims. 
   A particularly advantageous development provides that the device according to the invention has a component for fixing a rotatable force-applying element. The device can e.g. be designed as a spanner-like element which has, as the component for fixing a rotatable force-applying element, e.g. a nut, for encompassing a bolt. This nut can be in the form of known box spanner attachments, and also corresponding attachments for slotted-head or recessed-head bolts are possible. Naturally, any other forms are possible too, e.g. Allen key operations. The component for fixing the rotatable force-applying element (that is, e.g. a bolt head) can be rotatable relative to the remainder of the spanner (“ratchet” principle), but it can also be rigidly connected to the latter. 
   A further advantageous development provides that the fixing component includes means for signal pick-up. Thus e.g. a nut can have an electrical wire integrated or applied as a sheath, as well as contact pins which are electrically connected on the one hand to the measuring element and on the other hand to a measuring and control device belonging to the device according to the invention. Here, signal transmission can take place on the one hand by direct contact between electrically conductive surfaces. In addition to galvanic transmission, however, inductive or capacitive coupling of different signal value-transmitting wire sections is possible too (these variants are possible particularly when using the device in damp media or to reduce wear with relatively rotating components). 
   A further advantageous development provides that the means for signal value pick-up determine an axial force e.g. at only one location. For this purpose e.g. a contact pin is connected to a measuring element according to the invention (the voltage is then determined relative to an earth terminal which is provided via an electrical connection insulated from the contact pin, e.g. via electrically conductive bolt heads or nuts connected thereto. 
   It is, however, also possible to provide e.g. several contact pins for the simultaneous measurement of several signal values in order thus to obtain an even more precise averaged measuring signal (to compensate for temperature fluctuations or geometrical unevenness). 
   A development of the invention which is particularly suitable for practice provides that the component for fixing a rotatable force-applying element is designed for fixing a bolt head, a nut or the like. It is advantageous that here the means for signal value pick-up are likewise accommodated in the component for fixing the rotatable force-applying element (e.g. integral in a nut) and that these means for signal value pick-up directly contact, via e.g. contact pins, a washer to which an axial force is applied by the force-applying element (that is, bolt head, etc.). 
   A further advantageous development provides that a device for acoustic or optical indication of adjusted or existing axial force values is provided. In simple embodiments of the device according to the invention for detecting that values at the joint have been exceeded, a pilot lamp or signal sound device is necessary for this. For the indication of precise numerical values, an alphanumerical display is conceivable too. The tool according to the invention or the device according to the invention is, from experience, designed in such a way that on the basic tool, e.g. a ratchet, a cordless screwdriver or other screwing tools (including automatic screwdrivers), an electronic circuit with corresponding signal evaluation and indication as well as the necessary energy supply is provided. The energy can be provided by means of a battery or optically, capacitively or inductively coupled by telemetric devices using batteries or other energy storage means. The electrical supply of the measuring element can here be provided optionally by a d.c. or a.c. connection. 
   Basically, the device according to the invention can be used for all screw joints in the state of the art, that is, for screw joints with force-applying elements made of wood, metal or plastic. For fixing these screw joints, any elements from the state of the art can be used likewise, that is, recessed-head, slotted-head, hexagon, square or Allen-key bolts or the like. 
   Further advantageous developments of the present invention are provided in the other dependent claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with the aid of several figures. They show: 
       FIG. 1  part of the device for adjusting the axial force in a position placed on a screw joint, 
       FIGS. 2   a  and  2   b  details of a component for fixing rotatable force-applying elements, 
       FIG. 3  a detail of a contact pin for electrical contacting of a washer, 
       FIGS. 4   a  and  4   b  views of a device according to the invention for adjusting the axial force, 
       FIGS. 5   a  and  5   b  details of the electrical connection between a contact pin and an indicator according to the invention, and 
       FIG. 6  various possible ways of electrically connecting a component for fixing a rotatable force-applying element, to an indicator according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a screw joint with part of a device  1  for adjusting and/or testing the axial force in screw joints, the device including a check device for limiting an axial force operating between two force-applying elements of the screw joint. The screw joint is produced by a bolt shank  9  at the upper end of which is mounted a hexagon bolt head  7  and at the lower end of which a nut  8  is screwed onto a thread. Between the nut and the bolt head  7  are (listed from the bottom up) a component  11   a , a component  11   b , an ancillary washer  12 , an annular measuring element with piezoresistive coating, and a further ancillary washer. The axial force applied by turning the bolt head  7  therefore serves primarily for suitably adjustable connection of the components  11   a  and  11   b . For easier adjustment, the nut  8  is mounted non-rotatably in a manner no longer shown on the component  11   a.    
   The screw joint is shown only by way of example in  FIG. 1 . It may have any other form, e.g. a bolt with its thread can also be screwed direct into the component  11   a , and in this case the bolt head  7  forms a first force-applying element and the component  11   a  itself forms the second force-applying element corresponding to this, an axial force being built up between the force-applying elements. But any other screw joints are conceivable as well, with different embodiments of bolts, nuts and materials of these elements (wood, metal or plastic). Instead of a hexagon head, the bolt head  7  may of course be designed as a recessed-head, slotted-head, square or Allen-key bolt or the like. The means according to the invention for signal value pick-up of the check device which belongs to the device, are connected to a measuring element  2  in  FIG. 1 . This measuring element has the property that its electrical resistance is variable as a function of the operative axial force. Suitable for this can be an amorphous carbon layer with graphitic structures with sp 2  hybridisation in combination with diamond-like structures with sp 3  hybridisation. With respect to the possible multiplicity of further materials, comprehensive reference is made to German patent application 199 54 164 A1. However, other piezoresistive materials known to the expert are possible too. 
   The measuring element  2  is substantially designed as a hollow circular cylinder, the central opening being used for passage of the bolt shank  9 . The core of the hollow circular cylinder is made of a commercial steel (e.g. ST37). Naturally special steels, hardened steels, titanium, aluminium, etc. are also possible as materials, and also glass fibre-reinforced plastics. On the upper or lower side of this hollow circular cylinder are layers of material whose electrical resistance is variable as a function of the operative axial force (as described above). These layers do not, however, reach as far as the outermost circumference of the measuring element  2 , and the annular space remaining up to the edge of the measuring element  2  is a segmented or unsegmented annular region for electrical contacting. This region is suitable for connection to contact pins  14   a  or  14   b.    
   Via e.g. the contact pin  14   a , a change of voltage is measurable by varying the electrical resistance in the force-measuring layers  13 . The contact pin is connected in a manner which will be explained later to a control device of the device according to the invention. An earth terminal is also connected to this control device, and this electrical connection takes place via electrically conductive components between control device and measuring element  2  (that is, screwing attachment  15 , bolt head  7 , ancillary washers  12 , etc.). 
   Below, the construction of the screwing attachment  3  will be considered in more detail. Firstly it has a shape which is at least in one region complementary to the bolt head  7 , for turning the bolt head about the bolt shank axis. The screwing attachment  3 , which thus constitutes a component for fixing the force-applying element  7 , is made of an electrically conductive material, usually hardened special steel. The screwing attachment  3  can have a circular cylindrical shape and is coated on its outer peripheral surface with an insulator  16  (e.g. AL203 or an electrically non-conductive polymer). One or more spring-driven contact pins  14   a ,  14   b  are accommodated in an annular space which, seen radially outwardly, is defined by a further insulator layer in the form of a hollow circular cylinder. 
   It should be pointed out that the arrangement of contact pins  14   a ,  14   b  and washer  2  shown here is meant only as an example. Naturally it is also possible to provide a different form of a measuring element or to provide the washer in a protective case for protection against rotation. 
     FIGS. 2   a  and  2   b  show partial views of the environment of the contact pins  14   a ,  14   b . In  FIG. 2   a  it can be seen how an arrangement of a spring  5  and the contact pin  14   a , which is mounted between the cylindrical insulator layers  16  and  18 , is accommodated. The spring  5  is electrically conductive in order to make a connection between the measuring element  2  and the control device, not shown here. The contact pin  14   a  has at its projecting end a tip for better fixing to the washer and hence for ensuring a better measuring connection. Instead of and/or in addition to the spring, fixing can also take place with magnets  20  mounted at the tip (however, this requires a ferromagnetic measuring device). 
     FIG. 2   b  shows a top view of several contact pins, inter alia the contact pins  14   a  and  14   b . Basically, it is sufficient to provide a single contact pin  14   a . However, to improve the quality of measurement (equalisation of geometrical unevenness or temperature compensations) several contact pins may be used, which are electrically connected to each other or electrically insulated from each other (for this purpose, if occasion arises segmented regions electrically insulated from each other and each connected to a corresponding contact pin  14   a ,  14   b  are to be provided on the measuring element  2 ). 
     FIG. 3  shows a further embodiment of a contact pin  14   c  which differs from the above-mentioned embodiment only in that at its tip is provided a hook-like device as a crimping contact for better fixing/contacting of the measuring element  2 . 
     FIGS. 4   a  and  4   b  show views of a device  1  according to the invention for adjusting the axial force in screw joints 
     FIG. 4   a  shows a device  1  according to the invention in the form of a spanner. The latter has a handle  21  which is connected to a component  3  for fixing (a screwing attachment, see  FIG. 1 ). The handle  21  is non-rotatably connected to the screwing attachment  3 . Alternatively, it is also possible to provide the spanner as a “ratchet” in which a rotational connection which locks in one direction is provided. Depending on the type of connection, the electrical connection of screwing attachment  3  and handle  1  is also to be chosen. In  FIG. 4   a  it can be seen that the spring  5  is electroconductively attached to the handle  6  to the left of a contact, and this contact is connected by an electrical connection to an indicator  10  which is additionally connected by an earth terminal to the screw joint. Thus changes in electrical resistance in the measuring element  2  are measurable as a change of potential for the indicator  10 . 
   The indicator  10 , which belongs to the control device according to the invention, has an alphanumerical optical display for the indication of axial force values existing or adjusted in the screw joint. Here, as soon as the screwing attachment  3  is fitted on the bolt head  7  and the contact pins  14   a  (and others) are pressed onto the measuring element  2 , values for the axial force existing in the screw joint are provided. 
   Naturally, other indication options are possible too, for example, by a pilot lamp for providing an ordinary limit value, or acoustic indications. 
     FIGS. 5   a  and  5   b  show further details of the device according to the invention. In  FIG. 5   a  is again shown the screwing attachment  3  (as a box spanner). The insulating layers  16  and  18  enclose, in the manner described above, an annular space from which the contact pins  14   a  etc. are pushed out to the right by the relaxed spring  5  for making contact with a measuring element  2 . The spring  5  or an electrically conductive element arranged to the left of the spring connect to the electrical contact  6  for electrical connection to the indicator  10 . The contact  6  is constructed as a pin which is pushed by the spring  22  out of the handle  21  of the spanner, to improve electrical contact with the pin  14   a.    
     FIG. 5   b  shows a further arrangement of the annular space, in which the contact pin  14   a  is shown as an example. Here is shown a turret arrangement in which several contact pins are distributed over the circumference of the annular space. 
     FIG. 6  lastly shows a further type of connection between box spanner  3  (to which belong the means for signal value pick-up, that is, contact pins with insulators located round them). A component electrically connected to the contact pin  14   a  by the spring  5  can here too be contactlessly connected to a contact  6 ′ on the handle  21 . The contact  6 ′ is here too connected by a terminal wire to the indicator  10 . Between the contact  6 ′ and the electrically conductive element shown to the left of the spring  5  is provided a gap via which capacitive or inductive coupling can take place (see also the introduction to the specification). Alternatively, of course, galvanic coupling is always possible as well. 
   The device shown in the figures is a device  4  for adjusting and testing the axial force in screw joints, wherein the device includes a check device for limiting an axial force operating between force-applying elements of the screw joint, wherein the check device comprises means  4 ,  5  for signal value pick-up from a measuring element  2  whose electrical resistance is variable as a function of the operative axial force, wherein the device comprises a component  3  for fixing a rotatable force-applying element and the component  3  for fixing a rotatable force-applying element is designed to fix a bolt head  7 , a nut  8  or the like, and the means  4 ,  5  for signal value pick-up are likewise accommodated in the component  3  for fixing a rotatable force-applying element, for contacting a washer  2  arranged between the force-applying elements. 
   Hence devices which can be handled and with which, for example, in an industrial plant, the axial force of a bolt can be adjusted to the desired amount, are provided in particular for practical use. Here it is particularly advantageous that wholly commercially available bolts can he used, which do not have to be coated with additional force transducers etc. either. To determine the force there is used a washer  2  which, on account of its large circumference (the latter must inevitably be larger than the bolt shank), allows averaging of the data. This averaging is on the one hand to be established on the basis of the wide spatial extent of the washer and also on the basis of the plurality of several contact pins  14  which can come into contact electrically with the washer  2 . In this case it must also be taken into consideration that, on account of the rotation of the component  3 , spring mounting of the contact pins is sensible to compensate for unevenness of the washer during rotational movement. Also it is sensible to make the washer so rotationally symmetrical that the tips of the contact pins can slide over it and yet at the same time can transmit electrical currents.