The invention relates to a fastening system for fastening a plate-like trim part, in particular to the underside of a support part, with a stud that has a threaded section and means for fastening it to the support part, and with a fastening element that has a stud receptacle for accommodating the stud and engagement means for engaging with the threaded section of the stud, wherein the engagement means are designed such that the stud can be inserted in the stud receptacle by means of a movement along its axis relative to the fastening element and can be secured in the stud receptacle in the inserted state.

A fastening system of the specified type is known from WO 2009/1 1 1620 A1 . Such fastening systems are used to attach plate-like trim parts to the underside of a vehicle floor and improve the aerodynamic characteristics of the vehicle by covering components such as exhaust pipes, fuel lines and brake lines. The fastening systems used for these purposes have to ensure stable and reliable attachment of the trim parts for reasons of safety. They should additionally permit easy and fast installation to keep the manufacturing costs low, and they should be removable for purposes of maintenance and repair.

In the fastening system known from WO 2009/1 1 1620 A1 , the stud is welded to the vehicle floor. In order to install the trim plate, the fastening element is first placed in a receiving hole in the trim plate and then is pressed, together with the trim plate, onto the stud in such a way that the stud penetrates the stud receptacle of the fastening element and is secured therein. In order to be able to remove the trim plate for maintenance or repair work, the fastening element can be unscrewed from the stud with the aid of a tool, wherein the fastening element rotates in the receiving hole of the trim plate. In order to avoid unintentional rotation of the fastening element relative to the trim plate, and thus the risk of unintentional loosening of the fastening element, a fastening element is known from EP 1 ,849,687 B1 that has a first retaining clip with a first flange and a second retaining clip with a second flange, wherein the two retaining clips can be connected to one another in such a way that an undercover intended to be fastened to the underside of a vehicle body can be clamped between the first and second flanges. The side of the first flange adjacent to the undercover is designed with a channel and the side of the undercover adjacent to the first flange is designed with a projection accommodated in the channel, by which means a rotational movement of the first retaining clip containing the stud receptacle relative to the undercover is limited. This design requires a projection to be formed on the undercover in addition to the receiving hole.

DE 199 44 685 A1 discloses a fastening arrangement for fastening a trim part to the underbody of a motor vehicle that is provided with downward projecting threaded studs, wherein the fastening element consists of a nut and a clip that can be inserted through a receiving hole of the trim part and can be connected to the nut through a snap-in connection in order to hold the nut on the trim part. The trim part with the fastening element held thereon is placed on a threaded stud and is held on the threaded stud by the clip. Then the nut can be screwed onto the threaded stud and tightened with the aid of a tool. The surface of the nut adjacent to the trim part in this design is provided with ribs that dig into the material of the trim part, thereby securing the fastening arrangement against loosening. This design is only suitable for trim parts that are relatively soft so that the ribs of the nut can dig in. On the other hand, the trim parts must be stiff enough to ensure secure tightening of the nut.

The object of the invention is to make available a fastening system of the initially mentioned type that is equally suitable for trim parts made of hard and soft materials. According to the invention the object is attained by a fastening system for fastening a plate-like trim part to a support part with a stud that has a threaded section and means for fastening it to the support part, and with a fastening element that has a stud receptacle for accommodating the stud and engagement means for engaging with the threaded section of the stud in order to secure the stud in the stud receptacle, and wherein the engagement means are designed such that the stud can be inserted in the stud receptacle by means of a movement along its axis relative to the fastening element, wherein the stud and the stud receptacle of the fastening element have cooperating and matched coupling regions, wherein each of the coupling regions is provided with a geometric shape that deviates from a circular shape concentric to the axis of the stud in such a manner that, once the stud has been joined to the fastening element, the interlocking coupling regions produce a resistance to rotation of the stud in the fastening element when a torque acts on the fastening element.

In the fastening system according to the invention, the stud and the stud receptacle of the fastening element have cooperating coupling regions, wherein each of the coupling regions is provided with a geometric shape that deviates from a circular shape concentric to the axis of the stud in such a manner that, once the stud has been joined to the fastening element, the interlocking coupling regions produce a resistance to rotation of the stud in the stud receptacle of the fastening element when a torque acts on the fastening element. Protection against unintentional loosening resulting from rotation of the fastening element thus is accomplished with the aid of the stud rather than with the aid of the trim part. As a result, the trim part can be configured in any desired way, and can be made of hard or soft material according to requirements. Moreover, the trim part need not be clamped tightly by the fastening system, but instead the fastening system is also suitable for movable attachment of a trim part. In order to be able to release the fastening system according to the invention for maintenance and repair work by rotating the fastening element using a tool, according to the invention at least one of the coupling regions, preferably the coupling region of the fastening element, can be designed to be elastically deformable or destructible by a torque overcoming the resistance of the coupling regions to rotation. The elastically deformable implementation has the advantage that the fastening system can be reused again for fastening after having been released, whereas when a coupling region has been destroyed, the destroyed part must be replaced by a new one. A defined elastic deformability of the coupling region of the fastening element can be achieved in a simple manner by an implementation of elastically resilient walls that delimit the coupling region of the fastening element.

The fastening element can also have an engagement region for engagement of a driving tool so that it can be rotated and unscrewed from the stud by using a driving tool while overcoming the resistance of the coupling regions.

According to another proposal of the invention, at least one of the coupling regions can have a guide means that effects an interlocking of the coupling regions when the stud is inserted in the stud receptacle of the fastening element. As a result of the guide means, the coupling regions of the stud and fastening element are rotated into the engagement position even in the case of unfavorable alignment, thereby simplifying achievement of the engagement position. Pre-alignment of the fastening element and stud prior to assembly can be dispensed with as a result.

According to the invention, the coupling regions of the stud can be located at one of the ends of the threaded section of the stud. Preferably, the coupling region is arranged on the end opposite the fastening end of the stud. At its fastening end, the stud can have a head designed for welding to the support part. However, its head can also be designed for an attachment of another type and be attached to the support part by riveting, clinching, or gluing, or by means of a nut, for example.

The coupling regions can have any desired shape, and can be designed to be rotationally symmetric or rotationally non-symmetric with respect to the stud's axis. Preferably, the coupling region of the stud has the shape of a polygonal body and the coupling region of the fastening element has the shape of a polygonal cavity into which it is possible to insert the polygonal body. However, the two coupling regions need not have the same shape, but absolutely can have different shapes. What is important is that the coupling regions can easily be inserted one into the other and that they can still transmit a certain torque subject to an upper limit.

An advantageous embodiment of the fastening element has, according to the invention, a tubular body with a cavity extending through it forming the stud receptacle and with a guide opening for the stud, wherein latching fingers that engage the stud thread are formed at a first section of the cavity adjacent to the guide opening, a wall with an opening forming the coupling region is formed in a second section of the cavity, and an engagement region for engagement of a driving tool is formed in a third section of the cavity. This design of the fastening element is suited to economical production from thermoplastic using the injection molding method

The wall of the fastening element forming the coupling region here can be designed to be sufficiently elastically deformable that rotation of the fastening element relative to the stud is possible with elastic deformation of the edges of the opening.

To increase the elastic deformability of the edges of the opening that form the coupling region, according to the invention slots can be provided at a distance from the opening in the wall containing the opening. As a result of these slots, portions of the wall sections adjacent to the edge of the opening can bend radially outward under the action of a torque loading the coupling regions to a sufficient degree that when a predetermined torque is reached, the resistance of the coupling regions is overcome and the fastening element can be rotated relative to the stud

For simple and economical installation of a trim part on the underside of a support part, it is additionally desirable to connect the fastening element to the trim part in advance. To this end, the fastening element according to the invention has an outward projecting flange for contact at a first side of the trim part and a plurality of elastic retaining fingers that jut outward from the fastening element and are designed to be pressed against the tubular body and pushed through a hole in the trim part in one direction and to engage a second side of the trim part in order to prevent removal of the fastening element from the trim part in the opposite direction. The fastening element designed in this manner can be connected to a trim part by a simple plug-in process, and can then be installed together with the trim part by being pushed onto a stud of a support part.

The invention is explained in detail below with reference to an exemplary embodiment that is shown in the drawings. They show:

Figure 1 a perspective view of a stud of a fastening system according to the invention,

Figure 2 a cross-section of a fastening element for a fastening system according to the invention,

Figure 3 a cross-section through a fastening arrangement using a fastening system according to the invention,

Figure 4 a view A of the fastening arrangement from Figure 3, and

Figure 5 a section of a variant embodiment of a fastening element according to the invention.

Figure 1 shows a stud 1 , with a cylindrical shank 2, that has a disk- shaped head 3 at one end, and a coupling region 4 at the other end. The head 3 has a greater diameter than the shank 2, and is intended for welding the stud 1 onto a metal component. The coupling region 4 has a coupling section 5 in the form of a square prism whose flat sides 6 and rounded edges 7 are parallel to the stud axis. Toward the end of the stud, a guide section 8 in the shape of a truncated cone adjoins the coupling section 5; the guide section serves to guide the stud 1 during insertion into a fastening element and its coupling region. The guide section 8 can also be designed such that it can bring about an alignment of the coupling section 5 relative to the coupling region of the fastening element in order to facilitate interlocking of the opposing coupling regions. Between the head 3 and the coupling region 4, the stud 1 is provided with a threaded section 9 that is composed of an external thread.

Figure 2 shows a fastening element 10 that can be connected to the stud 1 . The fastening element 10 has a one-piece, tubular body 1 1 , which is made of thermoplastic material in the injection molding process. Molded on one end of the body 1 1 is a dished flange 12 that is intended to rest against one side of a trim part. The flange 12 has elastic support elements 13 on its outer edge. The other end of the body 1 1 is provided with multiple elastic fingers 14, which serve to support the fastening element 10 on a support part, in particular on the underbody of a motor vehicle.

The body 1 1 has multiple retaining fingers 16 on its outside that project outward from the body 1 1 and are inclined toward the flange 12 at an angle of approximately 45° to the longitudinal axis of the body. The retaining fingers 16 are elastically resilient and can be bent toward the body 1 1 when the fastening element 10 is pressed into a receiving hole in a trim part with the fingers 14 foremost. At their free ends, the retaining fingers 16 have latching surfaces 17, which can rest against the side of the trim part facing away from the flange 12 in order to hold the fastening element 10 in place on the trim part. Adjacent to the retaining fingers 16 are supporting fingers 18 on which the retaining fingers 16 can be braced when relatively large forces act on the latching surfaces 17 of the retaining fingers 16. The tubular body 1 1 has a cavity 20 extending axially through the body 1 1 to receive a stud 1 . At the end facing the fingers 14, the cavity 20 is provided with a conical, inward-tapering guide opening 21 that serves to center and guide a stud 1 entering the cavity. Adjacent to the guide opening 21 is a stud receptacle 22 with two opposing latching fingers 23, 24. The latching fingers 23, 24 project inward into the cavity 20 and are provided at their free ends with ratchets 25 that are designed such that they can engage the threaded section 9 of the stud 1 . The latching fingers 23, 24 are elastically resilient and are inclined at an angle of approximately 45° to the longitudinal axis of the body 1 1 such that they approach the longitudinal axis as the distance from the guide opening 21 increases.

Approximately in its center, the cavity 20 has a wall 26, extending transversely to the longitudinal axis, with a central opening 27. The opening 27 forms a coupling region 28 that is designed to cooperate with the coupling region 4 of the stud 1 . For this purpose, the opening 27 has four wall surfaces 30 that are parallel to the longitudinal axis and that together form an opening with a square cross-section. The spacing between opposite wall surfaces 30 here is somewhat greater than the spacing of the surfaces 6 of the coupling region 4 of the stud 1 . The coupling region 4 can thus be inserted into the coupling region 28 without appreciable resistance.

So that a torque can be transmitted by the coupling regions 4, 28, the spacing of opposite wall surfaces 30 is less than the diameter of the coupling region 4 at the rounded corners 7. Consequently, the coupling region 4 placed in the coupling region 28 cannot rotate in the opening 27, but instead its rounded corners 7 come into contact with the wall surfaces 30. In this way, the interlocking coupling regions 4, 28 can resist a rotation of the fastening element 10 relative to the stud 1 . The magnitude of this resistance depends on the elastic or plastic deformability of the coupling regions. Hence, in the case of the fastening element 10, for example, because of the elasticity of the wall 26, the wall surfaces 30 can be spread apart by the rounded corners of the coupling section 5 of the stud 1 by means of a relatively large torque exceeding the torque specified for protection against unintentional loosening, so that the fastening element 10 can be rotated relative to the stud 1 . In this way, the fastening element 10 can be removed from the stud 1 .

In order to permit rotation for removing the fastening element 10, the part of the cavity 20 that extends from the wall 26 to the flange-side end of the body 1 1 is designed as an engagement region 32 for engaging a tool, for example a hex wrench.

Figure 3 shows an application of the fastening system according to the invention. The head 3 of the stud 1 is fastened to the underside of a support part 40 by welding. The fastening element 10, and with it a trim part 41 , is fastened to the stud 1 . During installation, the trim part 41 is first connected to the fastening element 10, so that the trim part 41 and the fastening element 10 can be fastened to the support part 40 by a single operation. The support part

41 has a receiving hole 42 into which the fastening element 10 is inserted, with the fingers 14 foremost, from the side facing away from the support part 40. In this process, the edge of the receiving hole 42 slides past the supporting fingers 18 toward the retaining fingers 16, by which means the retaining fingers 16 are pressed radially inwards toward the body 1 1 . Once the trim part 41 has slid past the retaining fingers 16, they spring back to their initial position shown in Figure 3, in which the ratchets 17 engage around the edge of the receiving hole 42, thereby holding the fastening element 10 in place in the receiving hole

42 of the trim part 41 .

In the next step, the trim part 41 with the fastening element 10 is brought up to the guide section 6 of the stud 1 and is aligned such that the guide section 8 enters the guide opening 21 of the fastening element 10. Next, the trim part 41 is pressed against the support part 40, in which process the fastening element 10, braced on the trim part 41 by the ratchets 17, is pushed onto the stud 1 . In this process, first the stud 1 enters the stud receptacle 22, with the latching fingers 23, 24 being spread apart and springing over the coupling region 4 and individual courses of the threaded section 9. The guide section 8 enters the opening 27 in order to center the opening 27 relative to the stud 1 and effect the entry of the coupling section 5 in the opening 27. The final assembled position is reached when the fingers 14 of the fastening element 10 rest against the support part 40 under preloading and the ratchets 25 of the latching fingers 23, 24 engage the threaded section 7. In this position, the coupling regions 4, 28 also engage one another, wherein they can absorb forces directed transversely to the stud axis and oppose rotation of the fastening element 10 relative to the stud 1 with a defined resistance. The trim part 41 is held securely between the flange 12 and the retaining fingers 16. Because of the support elements 13, the trim part 41 can be axially preloaded with a force by the fastening element in order to counteract vibrations of the trim part 41 .

The engagement of the square coupling section 5 in the square opening 27 in the wall 26 can be seen in the view in Figure 4. The drawing illustrates that rotation of the fastening element 10 is only possible if the walls of the opening 27 can be pushed radially outward. If the elastic deformability of the wall 26 is insufficient for this purpose, the square shape of the opening 27 is plastically deformed, and thereby destroyed, by forcible rotation of the fastening element. Thereafter, the fastening element is no longer suitable for reuse.

The required elastic deformability of the opening 27 of the coupling region 28 of the fastening element 10 can be achieved through the use of a very elastic material or through appropriate shaping. Thus, for example, as shown in Figure 5, slots 33 can be made in the wall 26 parallel to and at a distance from the wall surfaces 30, creating free spaces into which the wall sections 34 located between slots 33 and the wall surfaces 30 can be deformed. The exemplary embodiment described shows one possible cross- sectional shape for the design of the coupling regions. However, the invention is not limited to this. Instead, any other desired cross-sectional shapes that are suitable for transmitting a torque can be used for the coupling regions. The cross-sections can be symmetrical or asymmetrical with respect to the stud axis, and they can have a greater or lesser number of corners or projections. The cross-sectional shapes of the coupling regions of the stud and fastening element can also be different from one another, for example a five-sided coupling region can be combined with one that is six-sided.