PCT/DE2012/200017 Description Title of Invention: Electrically conductive screw connection and spe cial bushing for a screw connection of said type Technical field 5 [0001] The present invention relates to an electrically conductive screw con nection between two parts which are screwed to one another by means of a screw having a screw head and a shank, wherein the screw runs through a bore in one part. The present invention also relates to a spe cial bushing for screw connections of this type. 10 Prior art [0002] Since modern motor vehicles are increasingly equipped with highly sensitive electronics, it is necessary for all electrically conductive parts, 15 i.e. substantially all metal parts of a motor vehicle, to be connected to earth so as to be electrically conductive with low resistance. [0003] In the past, additional large earth cables or other devices were pro vided for this purpose. This always led to the need to install additional parts, which involved additional costs and additional weight, wherein 20 these parts often did not fulfil their function satisfactorily, since frequently the contact was too poor, or deteriorated over time due to corrosion and electrochemical effects. For example, nowadays for a steering gear made of aluminium the automobile industry requires an earth contact with two cables each having a diameter of ten millimetres. 25 [0004] A direct contact between the individual components of a vehicle is no longer offered automatically nowadays, since for corrosion resistance the parts are generally already coated and/or varnished before the final in- -2 stallation, and because aluminium parts form an insulating passivation layer of aluminium oxide by themselves. [0005] Furthermore an electrically conductive screw connection for this pur pose is already known from German Utility Model 20 2010 007 769 Ul of 5 the Applicant, in which the low-resistance electrical contact between a part and the screw is produced by teeth on the workpiece-side surface of the screw head and/or of the nut or of a washer arranged between the nut and the one part, whereas the contact between the screw and the fur ther part is produced by raised portions on the shank of the screw, the 10 external diameter of which is slightly larger is than the internal diameter of the bore into which the screw is inserted, wherein these raised portions are arranged between the external thread and the screw head. [0006] This solution has proved reliable in principle, but it has been shown that when a screw of this type is replaced during servicing, contact prob 15 lems can occur due to manufacturing tolerances. Therefore an object of the present invention is to provide an electrically conductive screw con nection of this type which, even after being dismantled and assembled a number of times and even after replacement of the originally machined screw by a new one, is capable of producing an equally good, extremely 20 low-resistance contact between the screwed components. Moreover, an object of the present invention is to make available a low-resistance earth connection of this type which is structurally simpler to produce. Summary of the invention 25 [0007] According to the invention, this object is achieved by an electrically conductive screw connection in which a low-resistance electrical contact is produced between one of the parts and the screw by teeth on the workpiece-side surface of the screw head or of a washer arranged be 30 tween the screw head and the one part, whereas the contact between the screw and the other part is produced by a bushing which is formed so as to be radially elastic and which is arranged coaxially with respect to the -3 shank of the screw in a portion of the bore having an enlarged internal di ameter. [0008] Likewise the present object can be achieved by a special bushing into which a screw can be inserted and which can be inserted into in a corre 5 spondingly widened hole or into a correspondingly widened portion of a screw hole, and which is formed from a corrugated sheet metal strip, of which the corrugations are arranged parallel to the axis of rotation of the bushing. [0009] In this case it is particularly preferred if three corrugation peaks and 10 three corrugation troughs are arranged over a circumference of the bush ing. This is the simplest possible embodiment for the bushing, and en ables symmetrical and centred guiding of the screw in the bushing. [0010] Furthermore it is preferable that the sheet metal strip from which the bushing is formed is continuously closed. This increases the stability of 15 the bushing under radial stress. However, a bushing is also possible which is wound and of which the ends butt loosely in front of one another or in which the ends are connected by means of dovetailing or simple guiding. [0011] According to the invention, the corrugation peaks of the bushing can be 20 provided with stamped-out contact claws or with embossed studs or with a cross-knurl or transverse knurl in order to reduce the contact resis tance. [0012] For the same purpose, the surfaces of the bushing can be at least par tially corrugated or roughened. 25 [0013] Basically the bushing according to the invention can be inserted into an increased diameter of the bore on each end of the workpiece. [0014] However, it is particularly preferred that the portion of the bore having the enlarged internal diameter is arranged on the side of the one part fac ing the screw head, since then ejection of the bushing from the workpiece 30 during insertion of the screw is prevented. [0015] If the bushing is only to be introduced after the assembly of the parts, then it is preferred to provide the other part with a bore for the screw to -4 pass through, the internal diameter of the bore being greater than the ex ternal diameter of the bushing. In this case a solution may then be cho sen in which the electrical contact between two parts is produced exclu sively by the bushing, the bushing being inserted in the widened bores in 5 both parts, so that a corresponding special screw head or a correspond ing special nut or special washer can be completely omitted. By means of its elasticity, the bushing also compensates for slight offsets between the two workpieces. [0016] In the aforementioned solution it is also possible to install the bushing 10 directly under the screw head so as to be rotatable but non-detachable on the shank of the screw. In this way a special operation for installing the bushing is unnecessary and also the bushing does not have to be handled as a separate part. On the other hand, of course a specially manufactured screw is then necessary. 15 [0017] In this solution it is preferable that, for installation of the bushing above the external thread, the shank of the screw is provided with a radial bead, which runs around it and of which the external diameter is less is than the enveloping circle of the bushing. [0018] In order to improve the installation properties of a screw of this type, it 20 is also preferable if the circumferential bead is chamfered towards the ex ternal thread of the screw, so that with decreasing height the bead merg es into the external thread. Thus the screw centres itself in the enlarged hole portions. 25 Brief description of the drawings [0019] The present invention is explained in greater detail below with refer ence to the embodiment illustrated in the appended drawings. In the drawings: 30 [0020] Figure 1 shows a screw connection according to the invention with a special screw according to the invention in a sectional representation along the axis of rotation of the screw; -5 [0021] Figure 2 shows a special screw according to the invention from the front; [0022] Figure 3 shows a three-dimensional representation of a special bushing according to the invention; 5 [0023] Figure 4 shows a sectional representation through the special bushing according to the invention fitted onto a screw shank; [0024] Figure 5 shows the special bushing of the invention according to Figure 1 from the side; and [0025] Figure 6 shows a further embodiment of a special bushing according to 10 the invention. Best way of carrying out the invention [0026] Figure 1 shows a screw connection according to the invention in a sec 15 tional representation along the axis of rotation of the screw. [0027 In the embodiment illustrated here, a part of a steering gear housing made of aluminium as inner part 10 is surrounded by an axle support made of steel as outer part 12; both parts are connected by screwing by means of a screw 16, which extends through a bore 14 in the inner part 20 10 and two holes 11, 13 in the outer part 12, and a nut 28 having a washer 30. The hole 11 on the screw-head side and a part 15 of the bore 14 on the screw-head side have an enlarged diameter. [0028] In the present case the screw connection according to the invention is constructed so that it simultaneously produces a low-resistance electrical 25 contact between the components 10 and 12, even when they are painted, coated or covered with a passivation layer of aluminium oxide. [0029] In this case the screw 16 has, as is conventional, a screw head 18 hav ing a hexalobular external force application point 20 and a shank 22. As is conventional, the shank 22 of the screw 16 is provided with an external 30 thread 24. However, according to the invention, this external thread 24 does not extend as far as the screw head 18. Instead a circumferential bead 32 is arranged below the screw head 18 and serves to secure a -6 bushing 34 according to the invention below the head 18 of the screw 16. The more detailed construction of the bushing 34 is explained below with reference to Figures 3 to 5. [0030] The external diameter of the bead 32 is smaller than the enveloping 5 circle of the bushing 34 and the smallest diameter of the widened portion 15 of the bore 14 and of the hole 11 on the screw-head side. [0031] The diameter of the external enveloping circle of the bushing 34 may be chosen to be somewhat greater than the internal diameter of the wid ened portion 15 of the bore 14, so that as the bushing is inserted it is 10 slightly compressed radially and thereby a better electrical contact is pro duced. [0032] The hole 11 in the part 12 can either have a significantly greater inter nal diameter than the external diameter of the enveloping circle of the bushing 34, in which case the insertion of the screw 16 provided with the 15 bushing 34 is simplified, or a solution is also possible in which the hole 11 has just such a diameter as the widened part 15 of the bore 14. In this case, both the contact with the part 10 and also with the part 12 can be produced by the bushing 34. In such a case, if appropriate, further con tacting aids between the screw head 18 and the part 12 and between the 20 nut 28 and/or the washer 30 and the part 12 can be omitted. Such a solu tion is recommended in particular for cases in which the part 12 is sub stantially thicker than shown here. Furthermore when used for other pur poses and with a substantially thicker part 12, the bushing 34 can be used for producing a contact with the part 12, whereas the screw 16 for 25 example can be screwed into the part 10, so that no separate nut 28 is necessary, and the electrical contact between the screw 16 and the part 10 can be ensured by the screwing of this screw into the part, for exam ple by thread-forming or thread-cutting screws. [0033] Furthermore, with the illustrated combination of components 10 and 12 30 to be connected, it is possible to use a commercially available screw 16 when the bushing 34 is inserted before installation into the widened por tion 15 of the bore 14 of the part 10. Then the screw 16 only has to be -7 pushed through the bushing and the rest of the bore 14 in the part 10 af ter the part 12 is appropriately positioned. In this case widening of the hole 11 in the part 12 can also be omitted. [0034] In this type of installation, in which the bushing is previously inserted in 5 to the part 10, it is also not absolutely necessary to provide the widened portion 15 of the bore 14 on the screw-head side of the part 10. In such a case the widened portion 15 could also be provided on the nut side of the screw connection. [0035] When the external part 12 is painted or coated, for example by cata 10 phoretic painting (cataphoretic immersion painting - CIP), according to the invention a low-resistance contact must then be produced between this external part 12 and the screw 18. According to the invention, this can be effected by teeth 27 which are attached to the workpiece-side sur face of the screw head 18 and are illustrated in Figure 2. 15 [0036] In the present case the screw is screwed in by means of a lock nut 28 and a washer 30. In the embodiment illustrated here the washer 30 is al so provided with stampings 36 which have a rim produced by the stamp ing process and projecting in the direction of the workpiece 12, and thereby also create a good, low-resistance contact between the washer 20 and the external part 12 on this side. [0037] Alternatively the washer 30 could have been omitted and the nut 28 could also have been provided with appropriate teeth, and in this way an electrical contact can be produced between the nut 28 and the external part 12 in spite of painting or coating of this part 12. 25 [0038] Thus according to the invention, it is possible to use the teeth 27 below the screw head 18 for the electrical connection between the screw 16 and the external part 12, and as an alternative or in addition to this a washer 30 with appropriate teeth or raised portions can be provided, or this washer 30 can be omitted and the nut 28 can be provided with appropri 30 ate teeth and screwed directly on the external part 12.
-8 [0039] In most cases, however, it is sufficient according to the invention to produce the electrical connection between the screw and the external part 12 via the teeth 27 on the underside of the screw head 18. [0040] If the regions of the part 12 which are penetrated by the screw 16 are 5 thicker in other applications than shown here, the contact can also be produced exclusively via the bushing 34 as the bushing 34 extends through the appropriately widened hole 11 and the widened portion of the bushing 15 of the bore 14 and itself creates the contact to the two work pieces 10, 12. 10 [0041] Figure 2 shows a special screw 16 according to the invention as used for the screw connection according to Figure 1, viewed from the front, i.e. from the shank of the screw. [0042] In this case, in a deviation from the usual rules of technical drawing and for better understanding of the different diameter ratios, the bead 32 15 and the external thread 24 of the screw shank 22 are shown "as trans parent", so that the position and extent of the bushing 34 in relation to the other parts is clearly recognised. The outer edge of the bead 32 is illus trated by means of a part-circle on the left-hand side. On the right-hand half, the base circle diameter of the shank 22 and the external diameter 20 of the external thread 24 are shown, wherein the normal diameter of the shank 22 is shown by solid lines and the external diameter of the thread 24 is shown by dash-dot lines. [0043] The radially extending ribs 27, which are attached to the workpiece side surface of the screw head 18 and serve to improve the contact be 25 tween the screw head 18 and the surface of the workpiece 12, are clearly discernible here. The ribs 27 do not run as far as the external edge of the screw head 18, but merge there into a circumferential ring 38 which serves to prevent the penetration of moisture and thus corrosion phe nomena on the ribs 27. 30 [0044] The shape of the bushing 34 according to the invention is explained in greater detail in Figure 3.
-9 [0045] As can clearly be seen in Figure 3, the bushing 34 consists of a con tinuous cohesive sheet metal strip which is corrugated so that it has three corrugation peaks 40 and three corrugation troughs 42 over a circumfer ence. Thus, viewed in the passage direction of the screw, this produces 5 the shape of an equilateral triangle with very rounded tips and with slight ly concave side edges in the central region. For improved contact, the surface of the bushing 34 can be formed with a cross-knurl or transverse knurl or can be roughened, as represented here by the shading. This roughening does not have to be limited to the inside; the outside can also 10 be roughened. [0046] Figure 4 again shows the co-operation of the screw shank 22 and bushing 34 viewed in the direction coaxial to the screw shank 22, wherein the corrugation peaks 40 and corrugation troughs 42 are clearly discerni ble. 15 [0047] The corresponding external enveloping circle of the bushing 34 is indi cated by a dotted circular line. [0048] Figure 5 shows the bushing 34 from the side. It is clearly discernible here that the bushing 34 is provided in the region of the corrugation peaks 40 with small stamped-out claws 44. These serve to improve the 20 electrical contact with the inner face of the widened portion 15 of the bore 14. Alternatively for this purpose, studs can be can be embossed or a cross-knurl or transverse knurl or a roughening of the surface can be pro vided. [0049] Figure 6 shows a further bushing 34' according to the invention which 25 does not consist of a continuous sheet metal strip like the bushing 34 of Figure 3, but is wound from a sheet metal strip. In this way, the produc tion is simpler and more economical. [0050] The ends of the sheet metal strip forming the bushing 34' can either butt loosely in front of one another or, as shown here, they can be con 30 nected by a dovetail joint 46 or a simple guide. The embodiment by means of dovetailing or simple guiding has the advantage of greater sta bility.
-10 [0051] Moreover this bushing 34' also has three corrugation peaks 40 and three corrugation troughs 42 arranged alternately therewith, and is pro vided with stamped-out contact claws 44 in the region of the corrugation peaks 40. 5 [0052] Thus the present invention makes it possible for electrically conductive parts, preferably metal parts of a motor vehicle, to be already connected to earth so as to be electrically conductive with low resistance during in stallation of the connecting elements in order to comply with EMC re quirements. According to the invention, this object is achieved in that the 10 bushing 34 has contact on three flat strips with the screw 16 and the bushing 34 has, offset by 600 thereto, three further flat points of contact with the material in which the bushing is inserted. In contrast to the spe cial screw according to German Utility Model 20 2010 007 769 U1, the bushing 34 according to the invention can compensate by its special, cor 15 rugated form for tolerances of the hole in which it is inserted. At the same time the inward curvature in the direction of the screw leads to the screw 16 always being clamped even when the bushing 34 is radially com pressed from the outside. With other shapes the bushing would lift off from the screw due to the radial compression, and in this case according 20 to the invention the pressure on the screw is even increased. [0053] The bushing 34 itself can either be introduced into the element to be connected before the installation, and a quite standard screw (optionally comprising the special head 18 with the ribs 27) is used in the installation. Alternatively, as shown here, the bushing 34 can be pre-fixed on a spe 25 cial screw 16. For the insertion of the screw 16 and the bushing 34, the hole 11 in the workpiece 12 on which the head support is located must likewise be large enough for the bushing 34 to be passed through. Ideally for this purpose, the bushing 34 has a chamfer in order to avoid twisting. In this case the bead 32 on the special screw 16 is preferably provided 30 with a chamfer having a diameter increasing towards the head 18 of the screw 16 in order quite reliably to avoid twisting of the bushing 34. The -11 bead 32 must have a smaller diameter than the enveloping circle of the bushing 34 and the smallest diameter of the holes 11 and 15. [0054] The bushing 34 itself may be designed to produce contact in a wide va riety of ways. The contact can be made by small stamped-out claws 44 5 according to Figure 5; alternatively, embossed studs, similar to the sur face of a wood rasp, or a corrugated or roughened surface or a cross knurl or transverse knurl can serve for this purpose. Since for the use of the bushing 34 according to the invention only the widening 15 of the bore 14 in the one workpiece 10 and, in some circumstances (depending 10 upon the type of installation), an enlarged hole 11 in the other workpiece 12 have to be provided, this solution is also possible as a retrofit solution or a solution which can be implemented in the short term just before the start of series production. [0055] The special design of the bushing 34 according to the invention always 15 guarantees a secure seat for the screw 16, even when the bushing 34 is inserted into differently sized holes 15. Radial compression by the use in smaller holes does not lead to the screw 16 falling out but to an increas ingly secure seat. The bushing 34 according to the invention can prefera bly be used as a retrofit solution. Other solutions, such as for example 20 according to German Utility Model 20 2010 007 769, must always be tak en into consideration at an early stage in the design, since the parts must be co-ordinated accordingly. By contrast, the bushing 34 according to the invention can be introduced simply at a later stage, in this case for exam ple into a steering gear, by merely enlarging the through-hole there so 25 that the bushing 34 fits therein. Then at most a suitable special screw with corresponding head geometry or a corresponding nut or washer would have to be used in order to produce the electrical contact with the other part 12. All other parts can remain unchanged. [0056] The advantage of the present invention is that the earth connection be 30 tween different automobile parts can be ensured without additional earth cables even if the corresponding parts are already painted, coated or provided with a passivation layer before the screw connection. In addi- -12 tion, the present invention has the advantage relative to the previously customary earth cable that high-frequency interference is also very effec tively diverted. This is not necessarily offered with earth cables due to their unavoidable inductance. 5 [0057] On the other hand, according to the invention an appropriate contact between the screw 16 and the external part 12 is produced by the teeth 27 under the screw head 18 which cut through the paint layer on the ex ternal part 12. Alternatively or in addition, this can be effected by appro priate teeth or other projecting edges on a washer, wherein the washer 10 can be arranged both under the screw head and also under a nut. Fur thermore the nut can also be provided with corresponding teeth if no washers are used. [0058] The screw head, the washer or the nut can be provided with an exter nal circumferential ring in order to protect the screw connection, but in 15 particular the points of contact with the destroyed paint coating on the ex ternal part 12, from the penetration of water and thus from corrosion. [0059] In addition in the case of a connection by screw and nut on the work piece-side surface of the nut, a toothing or other projections can also be provided. When a connection having a nut and washer is used, the 20 washer may be appropriately provided with teeth or other projecting ele ments in order also to produce a good contact between the screw and the other part 12 also at the other end of the screw.