Abstract:
The invention relates to an assembly unit having an assembly part, which is penetrated by a bore and is to be fastened to a basic structure. The basic structure having a shaft, a head, and a fastener. The fastener having a fastening section on the end thereof facing away from the head, which is held captively and movably in the bore between a first axial position and a second axial position. The assembly unit having a spring, which encompasses the shaft and is formed from a wound spring wire and which holds the fastener captively in the bore. The spring wire is wound in a spiral shape such that the windings thereof do not overlap in the radial direction relative to the spring axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/EP2014/056872, filed Apr. 4, 2014, which claims the benefit of German Application No. 10 2013 005 751.5, filed Apr. 5, 2013. The entire contents of each of the foregoing patent applications are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an assembly unit. 
     2. Background and Relevant Art 
     In many branches of industry, components or assembly parts which have to be fastened or mounted on a basic structure are already provided with fastening means in a captive manner by the manufacturer such that such an assembly unit facilitates the subsequent assembly of the assembly part on a basic structure. The assembly parts or a fastening flange which is present thereon comprise at least one bore which penetrates them and which is penetrated by the shank of a fastening means, for example of a screw. The fastening means additionally comprises a head, which protrudes radially beyond the shank and overlaps the bore edge of the bore in a radial manner, and, on the end remote from the head of the fastening means, a fixing portion which is realized, for example, as a thread. For fixing the assembly part on the basic structure, the fixing portion engages in a fixing bore in the basic structure, for example a threaded bore. The fastening means is held in a captive manner in the bore so as to be movable between a first axial position and a second axial position. In the first axial position the shank protrudes from the assembly surface of the assembly part remote from the head at a maximum protrusion which comprises at least part of the fixing portion. In the second axial position the shank preferably does not protrude from the assembly surface or protrudes at a protrusion that is reduced compared to the first axial position. During the course of the assembly, the assembly part can consequently be positioned either up to a small spacing which corresponds to the reduced protrusion or, where there is no protrusion, can be positioned on the basic structure so as to form mutual surface contact. 
     DE 198 02 497 A1 makes known an assembly unit where the fastening means is held so as to be axially movable in the bore of the assembly part by means of a compression spring which is formed from helically wound spring wire and encompasses the shank of the fastening means. The compression spring is fixed with its one end, close to the head, on the shank of the fastening means and with its other end, remote from the head, on the assembly part. Disadvantageous in the case of the known assembly unit is, in particular, that the depth of penetration of the fastening means or of its fixing portion into a counter bore of the basic structure, in the case of a screw the depth of penetration thereof, that is necessary for sufficient fixing has an effect on the axial space required for the compression spring in the final assembly state in which the assembly part is clamped with the basic structure. The greater namely the depth of penetration of the fixing portion, the greater the number of spring wire windings at a given pitch of said spring wire windings. As the spring wire windings in the maximum shortened state of the compression spring abut against one another axially, the number of windings is added to a corresponding length. If, for example, the spring in its maximum shortened state is to be arranged fully inside the bore of the assembly part, an assembly part with a corresponding bore length or thickness is necessary. 
     Proceeding from here, it is the object of the invention to propose a correspondingly improved assembly unit. 
     BRIEF SUMMARY OF THE INVENTION 
     Said object is achieved with an assembly unit according to the preamble of claim  1  in that for captively mounting the fastening means on the assembly part a spring is provided, the spring wire of which is wound helically in such a manner that the windings thereof do not overlap in the radial direction with reference to the spring axis. As a result of said development, it is ensured that an increase in the number of windings of the spring on account of a greater displacement path of the fastening means or a greater depth of penetration into a counter bore of the basic structure does not affect the axial space required for the spring. As a result of the named type of windings, in its maximum shortened state, the spring takes on the form of a flat spiral spring, the dimension thereof in the axial direction depending purely on the axial thickness of the spring wire and not on the number of windings. When the number of windings is changed, just the outside diameter or the space required for the spring in the radial direction is changed. A spring of the type referred to can be used, consequently, in particular in the case of relatively thin assembly units which are penetrated by a bore with a smaller length, the fastening means thereof comprising a comparatively large length of penetration or screw-in and consequently protruding from the assembly surface of the assembly part with a correspondingly large protrusion in the first axial position. 
     The spring in the case of an assembly unit according to the invention can be a compression or tension spring, said spring comprising its maximum shortened state in each case in the first axial position of the fastening means. In this case, a compression spring assumes said state in the loaded state, that is with full spring compression, and a tension spring in the non-loaded state. 
     When the fastening means is positioned in its second axial position, the spring is situated in an elongated state in which it protrudes from the top surface which is located opposite the assembly surface of the assembly part. In said situation, a helical spring of the type known from DE 198 02 497 A1, in particular when the spring wire is thin, has low tipping stability or lateral stability. The result is that the fastening means can move into an unwanted oblique position with regard to the spring axis, which, among other things, makes introducing the fixing portion of the fastening means shank into a counter bore of the basic structure more difficult. An incorrect position of the fastening means of this type is countered in the case of a preferred development of the assembly unit as a result of the windings of the spring wire overlapping in the direction of the spring axis and, as a result of the overlapping, causing the spring to stabilize or stiffen laterally. In this case, there is a similar situation as in the case of tubes which are telescoped into one another. Quite particularly advantageous, in this case, is a development of the spring where the spring wire consists of a band with wide sides which extend in the direction of the center longitudinal axis. Generally speaking in this case, the dimension of the spring wire is greater in the axial direction than in the radial direction. 
     The fastening means, as mentioned above, is fixed on the assembly part by means of the spring so as to be axially displaceable between a first and a second axial position. The axial displacement path necessary in this case can be ensured just by the spring travel, i.e. the spring travel corresponds to the axial displacement path of the fastening means. If, however, as provided in the case of a preferred realization variant, the spring is fixed on the assembly part and/or on the fastening means so as to be axially movable about a displacement path, the displacement path is produced from the addition of the spring travel and the maximum displacement path of the spring. In the case of a predetermined axial displacement path of the fastening means, a shorter dimensioned and correspondingly more cost-efficient spring which is realized with a smaller axial installation height can then be used. 
     In order to reduce the axial installation height of the assembly unit further, the spring is fixed at least in part inside the bore. In this case, it is particularly advantageous when, in its maximum shortened state, the spring comprises a length which is smaller than the bore length, wherein the spring is arranged fully inside the bore. In this case, in the final assembly state in which the assembly part is clamped with the basic structure, the fastening means acts upon the assembly part by way of its head. The prestressing force of the fastening means is consequently introduced into the basic structure by means of the assembly part. 
     In the case of a preferred development, the length of the spring in its maximum shortened state or the axial spring wire thickness thereof is dimensioned such that in the final assembly state in which the assembly part is clamped with the basic structure, the head of the fastening means acts upon the maximum shortened spring in an axial manner, at least the predominant part of the prestressing force of the fastening means being directed into the basic structure in a main force-fitting manner by means of the spring. The spring consequently fulfills a dual function by, on the one hand, acting as a spacing element which transmits the prestressing force of the fastening means and, on the other hand, enabling the axial displacement of the fastening means relative to the assembly part. A spring acting as a spacing element is in particular expedient when the assembly part consists of a material, for instance an aluminum foam or a plastics material, which cannot be acted upon at least permanently with the prestressing force of the fastening element, e.g. because it tends to flow under load. As an assembly part consisting of such a material is simply clamped with the basic structure in a force shunt, it is exposed to a comparatively small load. So that the spring can be acted upon in the main force load and the assembly part can be acted upon in the force shunt, the length of the spring in its maximum shortened state must be matched to the length of the bore in the assembly part. The length of the maximum shortened spring is preferably slightly smaller than the bore length such that the assembly part is axially compressed to a certain extend in the bore region and the named force shunt can be realized. The assembly unit known from DE 198 02 497 A1 poses problems in this sense. By the spring windings being layered axially one above another in the maximum shortened state, which is also designated as a blocked state in the case of a helical spring, the tolerances thereof add up. The production of a helical spring with a predefined block length which is precisely matched to the bore length is consequently complex. 
     The spring can be fixed on the assembly part in different ways. Thus, it is conceivable for the spring end thereof remote from the head to be fixed in the bore of the assembly part by means of a press fit or also with axial mobility. In the event of assembly parts with a small thickness at least in the region of the bore it is, however, provided that the spring is fixed by way of its spring end, close to the head, on the top surface of the assembly part remote from the assembly surface. In the case of such a development, it is advantageous when fixing points, which are arranged on the top surface of the assembly part and hold the spring end remote from the head are at a radial spacing to the spring axis which is greater than the diameter of the fastening means head. In this way, it is ensured that the bottom surface of the head does not act upon the fixing points but on the side of the spring facing the head or on the spring wire windings which extend in a planar plane at least in the final assembly state. The named fixing points, in this case, are preferably tongues which are arranged on the top surface of the assembly part and clamp the outermost winding of the spring wire between them and the top surface of the assembly part. 
     In the case of the fixing or assembly of an assembly part on a basic structure, for example brought about by tolerances, a bore of the assembly part may comprise an axial offset in relation to a counter bore which interacts with it in the basic structure. Problem-free introduction of the fixing portion of the fastening means into the counter bore, however, is nevertheless possible when there is radial play between the spring and the assembly part and/or between the spring and the shank of the fastening means. 
     When the fastening means is a screw, i.e. when the fixing portion is realized as a thread, in the case of a preferred realization variant it is provided that the winding direction, in which the spring wire is wound from radially outside to radially inside, when viewed in a direction which extends parallel to the spring axis, away from the head of the fastening means and toward the end thereof remote from the head, corresponds to the direction of rotation of the thread. For example, in the case of a right-hand thread which has to be rotated clockwise so that it can engage with a counter thread, the spring wire is wound clockwise from outside to inside. The effect of the development in question is that where frictional locking is produced between the spring and the bottom surface of the head of the fastening means and/or the top surface of the assembly part when the fastening means is screwed into the counter bore of the basic structure, the result is a radial constriction of the spring wire windings and consequently an enlarging of the radial spacing between said spring wire windings. In contrast, where the winding direction is in the opposite direction with regard to the thread, there is the risk of the spring wire windings being radially widened and pressed against one another, which could obstruct the spring transferring into its maximum shortened state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below also with regard to further features and advantages by way of the description of exemplary embodiments and with reference to the accompanying drawings, in which: 
         FIG. 1  shows an assembly unit, including an assembly part, a fastening means and a spring which is developed as a compression spring, 
         FIG. 2  shows a perspective view of the spring from  FIG. 1 , 
         FIG. 3  shows individual representations of the individual parts of the assembly unit of  FIG. 1 , 
         FIG. 4  shows a representation which illustrates the assembling of a unit, which consists of fastening means and spring, and an assembly part, 
         FIG. 5  shows a pre-assembly state where the assembly unit is placed on a basic structure and the fastening means is situated in its second axial position, 
         FIG. 6  shows a representation corresponding to  FIG. 5 , in this case however a fastening means with a longer shank being used, 
         FIG. 7  shows a pre-assembly state where, differently to the images according to  FIG. 5  and  FIG. 6 , the assembly part is aligned in relation to the basic structure such that the center longitudinal axis of the fastening means is aligned with the center longitudinal axis of a counter bore which is present in the basic structure, 
         FIG. 8  shows a situation which, compared to the one shown in  FIG. 7 , differs by the fastening means already having been screwed a little further into the thread of the counter bore, 
         FIG. 9  shows the final assembly state of the assembly unit where the fastening means engages fully in the counter bore with its thread, 
         FIG. 10  shows a representation corresponding to  FIG. 1  of an exemplary embodiment where the spring is fixed on the assembly part so as not to be axially movable, 
         FIG. 11  shows a further exemplary embodiment with a spring realized as a tension spring, in a pre-assembly state in which the fastening means is situated in its first axial position, 
         FIG. 12  shows a representation that corresponds to  FIG. 6 , 
         FIG. 13  shows a representation that corresponds to  FIG. 7 , 
         FIG. 14  shows a representation that corresponds to  FIG. 9 , 
         FIG. 15  shows an exemplary embodiment with a tension spring which is fixed on the assembly part so as not to be movable, 
         FIG. 16  shows a sectional representation of the spring from  FIG. 15 , 
         FIG. 17  shows a further realization variant of an assembly unit in a pre-assembly state where the spring is fixed on the top surface of the assembly part, 
         FIG. 18  shows the assembly unit from  FIG. 17  in the final assembly state, 
         FIG. 19  shows a sectional representation corresponding to the line XIX-XIX in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
     The assembly units  1  shown in the illustrations include an assembly part  2 , at least one fastening means  3  and a spring  4 . The assembly part  2  is penetrated by at least one bore  5 . The fastening means  3  comprises a shank  6 , at the one end of which a head  7  is integrally formed. The diameter  8  thereof is dimensioned such that it overlaps the bore edge  9  of the bore  5  facing it in a radial manner. At least one end portion of the bore  5 , which extends up to the bore edge  9 , consequently comprises a diameter  10  which is smaller than the diameter  8  of the head  7 . The shank  6  carries a fixing portion  13  which interacts with a bore  15  that is present in the basic structure  14  for fixing the assembly part  2  on said basic structure. In the case of the exemplary embodiments shown in the illustrations, the fastening means is a screw, i.e. the fixing portion  13  is formed by a thread  11  which interacts with an internal thread  16  of the bore  15 . 
     The fastening means is held in the bore  5  of the assembly part  2  so as to be movable between a first axial position I and a second axial position II. The captive mounting in a first axial direction  18 , which extends away from the head  7  and toward the basic structure  14  or toward an assembly surface  17  of the assembly part  2  which abuts against said basic structure in the final assembly state, is ensured by the head  7 . In the first axial position I of the fastening means  3 , the head  7  abuts by way of the bottom surface  19  of its head against the top surface  20  of the assembly part  2  which is located opposite the assembly surface  17  or against the bore edge  9  or, where applicable, against the end  4   a , close to the head, of the maximum shortened spring  4 . In said situation, which corresponds to the final assembly state according to  FIG. 9 , the shank  6  protrudes from the assembly surface  17  with a maximum protrusion  23  ( FIG. 9 ). 
     The captive mounting of the fastening means  3  on the assembly part  2  in a second axial direction  24 , which extends from the end  4   b  of the shank  6 , remote from the head, and toward the head  7 , is ensured by means of the spring  4 . Said spring is a spring formed from a helically wound spring wire  29 , the windings  34  thereof encompassing the shank  6  of the fastening means  3  at a radial spacing. The spring wire windings  34  are wound such that they do not mutually overlap with reference to the spring axis  30  which extends along the center longitudinal axis  33  of the bore  5 . In this way, it is ensured that in the final assembly state or in the first axial position I, e.g. when the head  7  rests with the bottom surface  19  its head on the top surface  20  of the assembly part  2 , the spring  4  can assume a maximum shortened state with a minimum length L min  which corresponds to the axial length  35  of the spring wire  29 . The result is a correspondingly small axial installation height of the spring  4  and accordingly a small axial installation height of the assembly unit  1  fixed on the basic structure. The windings  34  of the spring  4  are additionally dimensioned in the radial direction such that at least some of them are overlapped radially by the head  7  of the fastening means  3 . 
     The spring  4  is connected to the assembly part  2  by way of its end  4   b , remote from the head, and to the shank  6  of the fastening means  3  by way of its end  4   a , close to the head. On account of the length of the spring  4  which is axially variable by a spring travel  27 , the fastening means  3  is displaceable into the second axial position II or is held in said position by the spring  4 . The displacement path  28  ( FIG. 9 ) or the axial spacing between the first and second axial position I, II corresponds at least to the spring travel  27 . The displacement path  28  is preferably chosen in each case in such a manner that in the second axial position II, the shank  6  of the fastening means  3  does not protrude from the assembly surface  17  of the assembly part  2  such that said assembly surface is able to be placed against a counter surface  22  of the basic structure  14  or moved into surface contact with said counter surface. 
     In the case of the exemplary embodiments shown in  FIGS. 1 to 10 , the spring  4  is fixed in the bore  5  of the assembly part  2  by way of its end  4   b , remote from the head. Its length L min  in the maximum shortened state is dimensioned such that it corresponds substantially to the bore length  40  ( FIG. 3 ) such that the spring  4  can be arranged substantially fully inside the bore  5 . Where assembly parts  2  are produced from solid material which withstands the prestressing force of the fastening means  3 , it is not necessary for the spring  4  to assume the function of a spacing element. The head  7  of the fastening means  3  is supported rather in the final assembly state on the bore edge  9  of the assembly part  2 , the prestressing force of the fastening means  3  being introduced into the basic structure  14  in a main force locking manner by means of the assembly part  2 . The length L min  of the spring  4  in the present case is expediently slightly smaller than the bore length  40 . 
     If assembly parts  2 , which consist of a less loadable material, are to be connected to a basic structure  14 , a spring  4  is provided, the minimum length L min  of which is dimensioned such that, in the final assembly state, the fastening means head  7  acts upon the spring  4  in an axial manner. In this case, the entire or at least a predominant part of the prestressing force of the fastening means  3  is directed into the basic structure  14  in a main force locking manner by means of the maximum shortened spring  4 . The minimum length L min  of the spring  4 , in this case, is expediently also slightly smaller than the bore length  40  such that in the final assembly state, on account of the influence of the prestressing force of the fastening means  3 , the head  7  thereof compresses the assembly part  2  axially to a certain extent in the region of the bore edge  9 . Part of the prestressing force of the fastening means, in this case, is utilized in the force shunt for pressing the assembly part  2  axially against the basic structure  14 . By far the greater part of the prestressing force, however, when as a result of the axial compression of the assembly part  2  the head bottom surface  19  approaches the spring  4  and finally acts upon the same in its bore edge region, is directed into the basic structure  14  in the main force load by means of the spring  4 . 
     The windings  34  of the spring wire  29  are dimensioned in the axial direction such that they also overlap in the second axial position II of the fastening means  3  in the direction of the spring axis  30 . The spring  4  is stabilized as a result in the lateral direction, that is in a direction which extends transversely with respect to the spring axis  30 , as a result of which a fastening means  3 , which is situated in the second axial position II, is centered in the basic structure  14  with reference to the center longitudinal axis  58  of the bore  15  during the course of the assembly. The introduction of a fastening means  3  into the bore  15  is facilitated as a result. The risk of the fastening means  3  tipping laterally (arrow  37  in  FIG. 1 ) is also at least reduced. The referred to stabilizing function of the spring  4  is then particularly marked when the spring wire  29  is formed from a band, the wide sides  36  of which extend in the direction of the spring axis  30 , and when the axial overlapping regions which are formed from such a spring wire  29  abut against one another, between which there is therefore not any or only a small radial spacing  70  ( FIG. 19 ). 
     As mentioned above, the displacement path  28  of the fastening means  3 , which said fastening means runs through between the axial positions I and II, corresponds at least to the spring travel  27  of the spring  4 . The displacement path  28  can be enlarged independently of the spring travel  27  when the spring  4  is fixed on the assembly part  2  and/or on the fastening means  3  so as to be axially movable. The maximum possible displacement path  28  is produced then from the sum of the spring travel  27 , the axial play  38  between the spring  4  and the assembly part  2  and the axial play  39  between the spring  4  and the shank  6  of the fastening means  3  ( FIGS. 1 and 3 ). The spring  4 , in this case, as is the case in the exemplary embodiments of  FIGS. 1-9 , can be fixed in the bore  5  so as to be axially movable in such a manner that it is displaceable beyond the assembly surface  17 , i.e. protrudes from said assembly surface with a protrusion  43  ( FIG. 1 ). 
     Along with a mounting of the spring  4  with axial play  38  in the bore  5 , a fixing where such an axial play is not provided can also be considered. Such a case is shown in  FIG. 10 . In this case, the spring end  4   b , remote from the head, or the spring wire winding  34 ′ which is located the furthest on the outside is fixed in an axially fixed manner, for example pressed into the bore  5  with a radial oversize and/or fixed by means of adhesion or welding. If axial play  38  of the spring  4  is provided, this is achieved by a positive-locking fit between the assembly part  2  and the spring  4 . To this end, an engaging-behind element  45 , which forms the named positive-locking fit with the spring end  4   b , remote from the head, protrudes radially inward from the wall of the bore  5 . A radially outwardly projecting counter element  46  is present on the spring end  4   b , remote from the head, or on the winding  34 ′ situated the furthest on the outside for this purpose. Said counter element extends so far in the radial direction that on its side facing the head  7 , it is engaged behind or radially overlapped by the engaging-behind element  45 , which is realized preferably as an annular projection. The counter element  46  is preferably formed from individual projections  47  ( FIG. 2 ) which are spaced apart in the circumferential direction of the spring  4 . The counter elements  46 , in the case of a band-like spring wire  29 , are tongues which are cut free from said band-like spring wire and bent radially outward. The counter elements  46 , in particular when they are realized as tongues, are flexible to a certain extent. When the spring  4  and the assembly part  2  are joined together, consequently the spring  4 , for example proceeding from the situation shown in  FIG. 4 , can be inserted into the bore  5  of the assembly part  2  as a result of being acted upon axially with a force F, the counter element  46  deforming elastically and passing the engaging-behind element  45 . 
     In addition to the above-mentioned positive-locking fit, the spring end  4   b , remote from the head, is fixed in the bore  5  with a second positive-locking fit in the first axial direction  18  which points away from the head  7 . Between the axial position of said positive-locking fit and the axial position of the positive-locking fit in the second axial direction  24  which points to the head, there is an axial spacing  48  which enables the axial mobility of the spring  4  or the axial play  38 . The axial spacing  48 , in this case, is greater than the axial length  50  of the engaging-behind element  45  ( FIG. 3 ). A second counter element  49  is present on the spring end  4   b , remote from the head, for realizing the second positive-locking fit. Said second counter element is preferably also formed by individual elements  47  which are spaced apart in the circumferential direction of the spring  4  and are tongues which are cut free from the spring end  4   b , remote from the head, or from the radially outermost spring wire winding  34 ′ and are bent radially outward. 
     The spring end  4   a , close to the head, is fixed on the shank  6  of the fastening means  3  with a positive-locking fit in the axial direction  18  which points away from the head  7 . The positive-locking fit is formed as a result of an engaging-behind element  53 , which is realized as an annular projection, protruding from the shank  6 . Said engaging-behind element engages behind a counter element  54 , which projects radially inward from the spring end  4   a , close to the head, or from the radially innermost spring wire winding  34 ″, on the side thereof remote from the head  7 . The counter element  54  is also realized in the form of individual projections  55  ( FIG. 2 ) which are spaced apart in the circumferential direction of the spring  4  and are cut free from the preferably band-shaped spring wire  29  and are bent radially outward. 
     In the case of the exemplary embodiments according to  FIGS. 1-10 , the spring  4  is a compression spring. Said compression spring comprises its maximum length L max  in the non-loaded state ( FIG. 3 ). The length  56  of the shank  6  is greater than the length L max  such that the fastening means  3 , at least when it abuts against the spring end  4   a , close to the head, by way of the bottom surface  19  of the head, protrudes with a protrusion  57  from the spring end  4   b , remote from the head, and where applicable from the assembly surface  17  of the assembly part  2 . When an assembly part  2  corresponding to  FIG. 1  is placed onto the counter surface  22  of a basic structure  14  during the assembly, the spring  4  and the fastening means  3  are displaced in the axial direction  24 . Such a displacement is possible on account of the axial play  38  between the spring  4  and the assembly part  2  as well as the axial play  39  between the spring  4  and the shank  6 . The axial play  39  is chosen such that in the pre-assembly position according to  FIG. 5  there is an axial spacing  61  between the spring end  4   a , close to the head, and the bottom surface of the head. When the assembly part  2  assumes on the basic structure  14  a position in which the center longitudinal axis  33  of the bore  5  of the assembly part  2  is in alignment with the center longitudinal axis  58  of the bore  15  that is present in the basic structure  14 , the shank  6  can be inserted into the bore  15  of the basic structure  14  without the spring  4  being axially compressed and without the application of force necessary for this by way of its end remote from the head, on which a radially narrowed search portion  59  is preferably present, and the thread  11  of the fixing portion  13  can be screwed a little way into the internal thread  16  of the bore. Only when the head  7  of the fastening means  3  acts upon the end  4   a  of the spring, close to the head, is said spring axially compressed or shortened as a result of the further screwing-in operation. If, as in the situation shown in  FIG. 7 , there is an offset between the center longitudinal axis  33  of the bore  5  of the assembly part  2  and the center longitudinal axis  58  of the basic structure, said offset can be compensated for by means of radial play  60  which is present between the counter element  54  or the individual projections  55  and the shank  6  and/or radial play  62  between the spring end  4   b , remote from the head, and the wall of the bore  5 . 
     The shank  6  of the fastening means  3  can be dimensioned axially such that the axial displaceability, which is produced from axial play  38  and  39  in addition to the spring travel  27 , is not fully utilized. Thus, in the case of the example shown in  FIG. 5 , the spring  4  is only displaced in the axial direction  24  relative to the assembly part  2  by a dimension corresponding to the protrusion  43  ( FIG. 1 ). In addition, the engaging-behind element  53  on the shank  6  is arranged at an axial spacing  61  to the counter element  54  which interacts with it for realizing a positive-locking fit. An exemplary embodiment where the shank  6  comprises a length  56  which requires full utilization of the axial play  38  and  39  in the pre-assembly situation, is shown in  FIG. 6 . 
     When, during the assembly operation, an assembly unit  2 , in a situation corresponding to  FIG. 1 , is placed onto the counter surface  22  of the basic structure, if the fastening means  3  tips relative to the center longitudinal axis  33  of the bore  5  of the assembly part  2  or is positioned incorrectly relative to the counter surface  22 , there is the risk of the fastening means  3  remaining in said incorrect position after it has been placed onto the counter surface  22 . Correct introduction of the shank  6  into the bore  15  of the basic structure  14  would then no longer be ensured. In order to avoid such an incorrect position of the fastening means  3 , the radially innermost winding  34 ″ of the spring  4  is developed such that the top edge  63  of the radially innermost winding  34 ″ facing the head  7  runs in a plane E 1  ( FIG. 1 ) which extends orthogonally with respect to the center longitudinal axis  33  of the bore  5  of the assembly part  2  when the spring is situated in its maximum extended state. In this way, it is ensured that the bottom surface  19  of the head, which rests on the innermost winding  34 ″, extends in the plane E 1  and consequently the fastening means  3  extends with its center longitudinal axis  31  at right angles with respect to the counter surface  22  of the basic structure or with respect to the assembly surface  17  of the assembly part  2 . 
     One possibility to ensure this is to develop the innermost winding  34 ″ of the spring  4  such that its top edge  63  in the maximum shortened state of the spring  4  extends in a plane E 2  which extends obliquely with respect to the center longitudinal axis  33  of the bore  5  ( FIG. 9 ). The inclination of the top edge  63  or of the plane E 2  is designed such that, in the maximum elongated state of the spring  4 , which it assumes in the axial position II of the fastening means or in a pre-assembly situation according to  FIG. 1 , it extends in the plane E 1  which is orthogonal with respect to the center longitudinal axis  33  of the bore  5 . It is also conceivable for the radially innermost winding  34 ″ of the spring  4  to be developed such that it or its top edge  63  always, that is independently of the respective spring deflection, extends in a plane E 1  which extends orthogonally with respect to the center longitudinal axis  33  of the bore  5 . 
     In the case of the exemplary embodiment shown in  FIGS. 11 to 14 , the spring  104  is a tension spring. Said tension spring is shortened to its minimum length L min  in its non-loaded state. The length L min , corresponds substantially to the bore length  40 . The minimum length L min , just as in the case of the above-described compression spring  4 , is determined by the axial length  35  of the band-like spring wire  29 . The spring  104  is developed substantially as the compression spring of the exemplary embodiments of  FIGS. 1 to 10 . In contrast to these, its maximum length L max  is dependent on the axial position of the engaging-behind element  53  in the axial position II of the fastening means  3 , in which said fastening means abuts against the counter surface  22  of the basic structure  14  by way of its shank  6  in the pre-assembly state ( FIG. 12 ). Whereas the compression spring  4  acts upon the counter surface  22  of the basic structure  14  and the bottom surface  19  of the head by way of its end  4   b , remote from the head, in the final assembly state, in the case of the tension spring  104 , there is no such axial force application. 
     When, during the assembly, an assembly unit  1 , which is situated in a situation according to  FIG. 11 , is placed onto the counter surface  22  of the basic structure  14 , the fastening means  3 , proceeding from the situation shown in  FIG. 11 , is displaced axially against the spring force of the spring  104 , the engaging-behind element  53  on the shank  6  of the fastening means  3  striking against the radially inwardly protruding counter element  54  and, as a result, elongating the spring and finally being held in its maximum elongated state in which it comprises the length L max . The fastening means  3 , in this case, is situated in its second axial position II. 
     The counter element  46  of the spring  104 , remote from the head, is in this case pressed against the side of the engaging-behind element  45  facing the basic structure  14  in the bore  5  of the assembly part  2 . As a result of the effect of the elongated spring  104 , the shank  6  of the fastening means  3  is pressed onto the counter surface  22  of the basic structure  14 . 
     The risk of the fastening means  3  being incorrectly positioned in the abovementioned manner also exists when a spring  104  is realized as a tension spring. There is no such incorrect positioning in the state of the assembly unit according to  FIG. 11  in which said assembly unit is supplied to a basic structure  14  insofar as the bottom surface  19  of the head is aligned orthogonally with respect to the center longitudinal axis  3  of the bore  5  in the first axial position I of the fastening means  3 . This is the case, for example, when the bottom surface  19  of the head of the fastening means head  7  abuts against the top surface  20  or against the bore edge  9  of the assembly part ( FIG. 11 ). Incorrect positioning of the fastening means  3  during the axial displacement of the fastening means  3  when placing the assembly unit  1  onto the basic structure can be avoided when the counter elements  54  on the radially innermost windings  34 ″ are arranged in a common plane E 1  which extends orthogonally with respect to the center longitudinal axis  33  of the bore  5 . Such a position of the counter elements  54 , which is independent of the respective operating position or lengthening of the spring  104 , can be achieved as a result of the radially innermost winding  34 ″ of the spring  4   a  being developed such that it maintains its position with reference to the spring axis  30  or the center longitudinal axis  33  of the bore  5  independently of the respective lengthening of the spring  104 . 
     When, proceeding from the situation shown in  FIG. 12 , the assembly unit  1  is aligned on the counter surface  22  of the basic structure  14  such that the center longitudinal axis  33  of the bore  5  is in alignment with center longitudinal axis  58  of the bore  15  of the basic structure, the shank  6 , with its search portion  59  in front as a result of the effect of the spring  104 , is inserted into the bore  15  until the end of the thread  11  of the shank, remote from the head, strikes against the internal thread  16  of the bore. The fixing portion  13  of the fastening means  3  can then be screwed fully into the bore  15  such that, in the final assembly state according to  FIG. 14 , the fastening means head  7  acts upon the bore edge  9  of the assembly part by way of the bottom surface of its head with the prestressing force of the fastening means  3 . In the case of the spring  104 , its minimum length L min  can also be dimensioned such that it serves or does not serve selectively as a spacing element. In the first-mentioned case, as depicted above, the main part of the prestressing force is introduced into the basic structure  14  by means of the maximum shortened spring  104  and acts upon the assembly part  2  in the region of the bore edge  9  in the force shunt with part of the prestressing force. 
       FIGS. 15, 16  show an exemplary embodiment of an assembly unit  1  where the spring  104   a  comprises only the counter element  46 , remote from the head, on its radially outermost spring wire winding  34 ′. For realizing a positive-locking fit in the second axial direction  24 , the bore  5  is widened at its end which opens out into the assembly surface  17  so as to form a radial shoulder  64 . The radial shoulder  64  forms the engaging-behind element  45  which interacts with the counter element  46  of the spring  104   a , it being engaged from behind by the counter element  46 . The second positive-locking fit, which also serves for captively mounting the spring  104   a  in the bore  5  and is produced in the case of the above-described exemplary embodiments by the interaction between the engaging-behind element  45  in the bore  5  and the counter element  49 , close to the head, is brought about by the engaging-behind element  53 , which is present on the shank  6  and is preferably realized as an annular projection, and the counter element  54 , which projects radially inward on the spring end  4   a , close to the head, or on the innermost winding  34 ″ of the spring wire and with the head  7  which overlaps the bore edge  9 . 
     The illustrations in  FIGS. 17 to 19  show an exemplary embodiment of an assembly unit  1  which includes a spring  204 , the end  4   b  of which, remote from the head, is fixed on the top surface  20  of an assembly part  2   a  which is realized as a sheet metal part. The spring  204  is also a helical spring, the spring wire  29  of which or the windings  34  formed from said spring wire not overlapping in the axial direction. The radially outermost winding  34 ′ of the spring wire  29 , which is realized, for example, with a round cross section, is fixed at fixing points  65  on the top surface  20  of the assembly part  2   a . The radial spacing  66  between the fixing points  65  and the spring axis  30  is greater than half the diameter  8  of the head  7  of the fastening means  3 . In the final assembly state, the head consequently does not act upon the fixing points  65  themselves by way of its bottom surface  19 , but upon the windings  34  of the spring  204  which are located radially inside the fixing points. 
     In the exemplary embodiment shown, the spring  204  is fixed on the assembly part  2   a  so as not to be axially movable. The fixing points  65  are formed in the form of tongues  67  which are cut free, for example, from the assembly part  2   a . The tongues  67  are bent beyond the top surface  20  of the assembly part  2  and extend radially inward, clamping the radially outermost winding  34 ′ between them and the top surface  20  of the assembly part. 
     There is also no axial play between the spring  204  and the shank  6  of the fastening means  3 . Rather, the innermost winding  34 ″ is clamped axially between the engaging-behind element  53 , which is present on the shank  6 , and the bottom surface  19  of the head. The radially innermost winding  34 ″ is wound such that the engaging-behind element  53 , which is preferably realized as an annular projection, engages behind the named winding at least over part of its circumference in the radial direction on its side remote from the head. The advantage of mounting the spring  204  on the shank  6  in an axially and radially fixed manner is that with this type of fixing, the fastening means can be held easier in an alignment which is parallel or coaxial to the center longitudinal axis  33  of the bore  5 . 
     The spring wire  29  of the spring  4 ,  104 ,  204  is wound such that its direction of winding  68  corresponds to the direction of rotation  69  of the thread  11  of the fastening means. For example, with a right-handed thread, in the case of a top view onto the thread, the thread flanks ascend to the right. In other words, a thread of this type has to be rotated in a clockwise direction of rotation when screwing into a counter thread. The direction of rotation is reversed with a left-handed thread. The winding direction  68  of the spring wire  29  is that direction in which the spring wire  29 —when viewed in a direction which runs parallel to the spring axis  30  and extends away from the head  7  of the fastening means  3  and toward the end thereof remote from the head—is wound from radially outside to radially inside. As can be seen in particular in  FIG. 19 , in the case of said development, when screwing a fastening means into a bore  15  of the basic structure  14  in the direction of rotation  69 , the windings  34  are entrained in the direction of rotation  68  as a result of friction with the bottom surface  19  of the head. In this case, the windings  34  are constricted so as to form or enlarge a radial spacing  70  which is present between them. In this way, when screwing a fastening means  3  into the bore  15  of a basic structure  14 , the axially overlapping windings  34  are prevented from pressing against one another so as to obstruct the shortening of the spring.