THREADED FASTENER WITH RESILIENT LINKING MEANS

Abstract: A threaded fastener comprises a head component 12 for the application of torque to the fastener, an externally threaded shank portion 30, resilient linking means 20 for interconnecting the head component 12 and the shank portion 30 a certain axial distance apart and for resiliently biasing against axial displacement of the shank portion from the head component a distance other than such certain axial distance, and torque transmitting means 50 for transmitting torque from the head component 12 to the shank portion 30. The torque transmitting means 50 is able to transmit sufficient torque from the head component 12 to the shank portion 30 to cause axial displacement of the shank portion 30 in an aperture in a work piece 45 against the resilient biasing of the resilient linking means, whereby the resilient means is elongated and put in axial tension.

Description: The present invention is directed to a threaded fastener with resilient linking means and, more particularly, to a threaded fastener adapted to maintain a plurality of fastened work pieces together under compression. It generally is desirable when fastening together work pieces, for example when fastening together multiple sheets or panels or fastening a panel to a frame member, to insure that the work pieces remain tightly fastened together under some degree of compressive force. Thus, .for example, in the context of motor vehicle assembly, coplanar panels may be fastened together at an area of overlap or a body panel may be fastened to a structural frame rail. In either case, it is desirable to maintain a tight assemblage during extended usage. Certain factors, however, are known to work against maintaining a tight connection between work pieces held together in assembly by a threaded fastener. A connection may loosen, for example, if the threaded fastener holding the work pieces together slips, that is, rotates out of the fastener aperture in the work pieces. In addition, particularly where work piece(s) and/or fastener is made of plastic, such as fibre reinforced plastic body panels for a motor vehicle, there may be erosion of the material at the interface between the fastener and the work piece or at the interface between one work piece and another. In addition, thermal expansion and contraction of the fastener and/or the work pieces under the influence of changing ambient temperature can cause an otherwise tight connection to loosen. DE-A-2,751,067 discloses a metal screw having an extension piece of synthetic plastic material which is threaded as a continuation of the thread of the metal screw and which has an elastic bendable part of its centre. The plastic extension piece is moulded around a projection from the threaded shak of the metal screw, the perfection being such as to retain the plastic extension piece against both rotation and axial displacement relative to the projection. It is an object of the present invention to provide a threaded fastener which can maintain a tight connection between work pieces -- a fastener which can accommodate a certain amount of fastener slippage or dimensional change, particularly axial foreshortening of an assembly, while maintaining a tight connection between the pieces. This and other objects and advantages of the invention will be made apparent by the following disclosure of the invention and discussion of preferred embodiments. According to the present invention, there is provided a threaded fastener for fastening work pieces together under compression, said fastener comprising a head component for the application of torque to the fastener, said head component comprising a torque receiving end portion and coaxial and unitary therewith an upper shaft portion having a diameter less than that of said end portion, an externally threaded lower shank portion formed of plastic and having a smaller diameter than said end portion and the same diameter of the upper shank portion, said lower shank portion being coaxial with said upper shank portion, resilient linking means for resiliently interconnecting said upper shank portion and said lower shank portion a certain axial distance apart when said threaded fastener is in a free state, characterised in that said resilient linking means is arranged to provide bias against axial displacement of said lower shank portion from said upper shank portion a distance other than said certain axial distance, said resilient linking means comprising a block of elastomeric material of which at least that portion spanning said certain axial distance between said upper shank portion and said lower shank portion is cylindrical, coaxial with said upper shank portion and said lower shank portion, and of diameter no larger thank that of said upper shank portion and said lower shank portion and, torque transmitting means for transmitting torque from said head component to said lower shank portion, said torque transmitting means comprising a rigid shaft extending through said cylindrical portion of said resilient linking means coaxially therewith from a first T-shaped end of said rigid shaft housed in an axially extending slot in said upper shank portion to form a first non-rotational engagement therewith to a second T-shaped end housed in an axially extending slot in said lower shank portion to form a second non-rotational engagement therewith, said torque transmitting means being able to transmit sufficient torque from said head component to said lower shank portion to axially displace said lower shank portion in a work piece against the resilient biasing of said resilient linking means to elongate said resilient linking means and put it in axial tension, and being fractureable at a yield torque which is less than said threaded fastener's, failure torque. Threaded fasteners according to the invention as disclosed above provide several significant advantages which those skilled in the art will recognise to comprise a significant advance in the art. A novel and critical feature of the new fastener of the invention is the provision of a resilient linking means which resiliently biases the fastener to a certain axial dimension, most significantly biasing it against axial elongation. During installation of the fastener, a second critical and novel feature of the invention, the aforesaid torque transmitting means, acts to transmit torque from the head component of the fastener to the threaded shank portion. In this way the threaded shank portion is rotationally driven into a work piece. After travel of the head component of the fastener stops when it seats in or against an exterior surface of a work piece, continued transmittal of torque to the threaded shank portion causes it to be screwed further into the fastener aperture overcoming the bias of the resilient linking means. The linking means is thereby axially elongated and, being then in tension, puts the fastened work pieces in compression by pulling the threaded shank toward the head component of the fastener. As discussed further below, it will be apparent to those skilled in the art that the engagement of the torque transmitting means with the threaded shank portion at one end and with the head component at the other end preferably are such as to allow certain axial travel even while not permitting free relative rotational displacement, so that the threaded shank portion is free to travel into the work piece, elongating the resilient linking means. Thereafter, the elongated linking means can accommodate a change in axial dimension of the assembly, such as thermal contraction of the attached work pieces or by erosion of material at the fastener/work piece interface. The elongated linking means accommodates such change in axial dimension by contracting back toward its original axial dimension, i.e., its free state axial dimension. In this way the fastener of the invention maintains a tight connection in the assembly notwithstanding such change in the axial dimension of the assembly. A second advantage of the invention flows as a consequence of the advantage described immediately above. Specifically, the head component and the upper externally threaded shank portion of the threaded fastener can be formed of plastic or other material notwithstanding the thermal expansion/contraction of plastic materials or the lower dimensional stability in general of suCh materials in comparison to metal fasteners. Thus, a greater variety of materials, including plastic, metal, ceramics, etc., will be found suitable for the fastener of the invention than would be suitable for fasteners according to prior known designs. This is particularly advantageous where a plastic component is to be fastened. A fastener of the type disclosed herein can be formed of plastic for such application, thereby avoiding a metal-to-plastic interface which under the influence of vibration or component shifting can produce squeaking or like noises and which can result in accelerated erosion of the plastic at such interface. Of course, numerous fastener features well known to the skilled of the art can be employed together with the above described critical features of the invention. Thus, for example, thread lock features such as that known commercially as Nylock can be provided on the threaded shank portion. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a threaded fastener in accordance with the present invention, Figure 2 is a section view of the fastener of Figure 1 taken through 2-2 of Figure 1 and shown in an assembly fastening together two work pieces, and Figure 3 is a section view of the fastener Figure 1 taken through line 3-3 of Figure 1. In view of the present disclosure those skilled in the fastener arts will readily recognise the usefulness of the present invention in numerous applications in which an assembly is formed, for example, by attachment of a first component to a second component. Thus, for example, the threaded fastener of the invention can be used to fasten together coplanar plates or panels, to fasten a panel to a frame member, such as in the assembly of a motor vehicle body, etc. The threaded fastener of the invention is particularly useful for attaching together two or more coplanar sheets or panels having different coefficients of expansion. The axial resiliency of the fastener facilitates lateral shifting of fastened components over each other without buckling even when one or both are plastic. In such applications it is particularly preferred that the fastener components having an interface with the fastened components comprise plastic material, rather than metal or the like, to reduce or eliminate erosion of the panel at such interface. Also, the resilient linking means according to certain embodiments of the invention provides a substantial measure of lateral flexibility as well as axially elastic resiliency. This can be advantageous in allowing a fastened assembly to absorb lateral impact and/or to allow relative lateral shifting of one component relative another in the assembly due to thermal expansion/compression or the like. It will be apparent to the skilled of the art in view of the present disclosure that the engagement of the resilient linking means with the lower shank portion and with head component can be a freely rotating engagement. Referring now to Figures 1-4, a fastener 10 is seen to comprise a head component 12 adapted by virtue of its hex-head perimeter end portion 14 and/or hex-socket 16 for the application of torque to the fastener. Head component 12 further comprises upper shank portion 18 which can be seen to be of smaller diameter than end portion 14. Shank portion 18 is, according to this preferred embodiment, unitary with the end portion and extends axially therefrom to resilient linking means 20. In the preferred embodiment shown the upper shank portion 18 is threaded. Optionally, however, this portion of the fastener may be unthreaded so as to slide freely within an aperture in a work piece. Resilient linking means 20 interconnects the head component 12 to lower shank portion 30 a fixed axial distance apart when the fastener is in its free state. Because the linking means 20 is resilient, it provides bias against axial displacement of the lower shank portion from the head component a distance either more or less than such fixed distance. Linking means 20 can be seen to be coaxial with the head component and with the lower shank portion, such that axial force is transmitted substantially uniformly from the head component to the lower shank portion. In the embodiment shown the portion of the elastomeric block spanning the distance between the two shank portions is cylindrical. The resilient linking means 20 is smaller in diameter than the end portion 14 of the head component and is smaller in diameter than the two shank portions. Optionally, however, the resilient means may be of diameter equal to that of the shank portions. The resilient linking means comprises a block of elastomeric material. Numerous suitable materials are commercially available and will be apparent to the skilled of the art in view of the present disclosure. Exemplary such materials include natural and synthetic rubbers. Particularly preferred materials include elastomeric polyurethanes. Alternative suitable resilient linking means include a spring element, such as a coil spring, and the like. Lower shank portion 30 is seen to be externally threaded. Particularly where upper shank portion 18 is to make threaded engagement with a fastener aperture, lower shank portion 30 will be of the same diameter as the upper shank portion 18, so as to pass through that portion of the aperture with which upper shank portion 18 will form a threaded engagement. Referring specifically now to Figure 2, fastener 10 is shown in assembly with a first work piece 40 and a second work piece 45. Lower shank portion 30 forms a threaded engagement with an aperture in work piece 45 while upper shank portion 18 forms a threaded engagement work piece 40. Shoulder surface 15 of end portion 14 bears against exterior surface 41 of work piece 40. Referring now particularly to Figures 2-4, an additional critical element will be described in connection with the made of operation of the invention. Specifically, the fastener is seen to comprise a torque transmitting means 50. In the preferred embodiment illustrated, torque transmitting means 50 comprises a rigid shaft 52 extending coaxially with the resilient linking means 20. To provide good distribution of torque to the lower shank portion, the torque transmitting means should be coaxial with the head component and with the lower shank portion. Thus, in the illustrated embodiment it extends axially through the centre of the resilient linking means, through axial aperture 22 thereof, from a first non-rotational engagement with the head component to a second non-rotational engagement with the lower shank portion. Such non-rotational engagements comprise a T-shaped end 54 and 55, respectively. The T-shaped end 54 is housed or received in an axially extending slot 56 in the head component, specifically, in the lower shank portion of the head component. The lower T-shaped end 55 of the torque transmitting means is housed in axially extending slot 57 in the lower shank portion. The axially extending slot in the head component and in the lower shank portion of the fastener can be formed by any of various techniques known to the skilled of the art. These components of the fastener can be moulded of plastic or other suitable material in two longitudinal halves that are assembled with the other components (the torque transmitting means and the resilient linking means) and then welded or adhesively bonded together. Alternatively, the resilient means and torque transmitting means can be insert moulded into the fastener according to methods and techniques well known to the skilled of the art. Also, so called lost core moulding techniques can be employed. Alternative suitable techniques and materials for fabrication of the fastener will be readily apparent to the skilled of the art in view of the present disclosure. The engagement of the torque transmitting means with the head component and with the lower shank portion of the fastener, as described above, allows axial displacement of the torque transmitting means relative those other two components of the fastener while preventing rotation of the torque transmitting means relative either of them. There preferably is a degree of free axial movement in engagement of the torque transmitting means with the head component and/or with the lower shank portion so that elongation (and in some embodiments both elongation and compression) of the resilient linking means can occur by axial displacement of the lower shank portion away from (or toward) the head component. Relative rotational movement, however, is prevented so that the torque transmitting means can effectively transmit torque from the head component to the lower shank portion. In this way the lower shank portion can be rotated into a fastener aperture against the resilient biasing force of the resilient linking means. Axial displacement of the lower shank portion into a fastener aperture will elongate the resilient linking means and put it in tension, whereby it can accommodate axial foreshortening of the fastened assembly in the manner described above. Alternative means for accommodating axial elongation and compression of the resilient linking means will be apparent to the skilled of the art in view of the present disclosure. Certain embodiments of the invention, for example, employ a torque transmitting means with ends immovably embedded in the head component and lower shank component and having a telescoping shaft with means, such as, flutes or a non-circular cross section, to prevent free rotation of one shaft portion relative another. According to preferred embodiments of the invention, the torque transmitting means is rotationally yieldable at a torque, referred to herein as the "yield torque" of the torque transmitting means, which is less than the threaded fastener's failure torque. By rotationally yieldable is meant that the torque transmitting means at its yield torque ceases to transmit significant torque to the lower shank portion of the fastener. As the resilient means is elongated and put in tension, the torque needed to further drive the lower shank portion into a fastener aperture increases. At a certain torque, the yield torque of the torque transmitting means, it will be unable to further drive the lower shank portion. Where the yield torque is less than the threaded fastener's failure torque, this can provide significant protection against failure of the fastener due to over tightening. By failure of the fastener is means that the threads are stripped or permanently deformed sufficiently to significantly reduce the strength of the connection formed by the fastener or that the resilient linking means is extended beyond its elastic deformation point, e.g. to the point of rupture, or that the end portion of the head component is sheared from the fastener, or any like damage which would significantly interferes with the performance of the fastener. The torque transmitting means may become rotationally yieldable by being fracturable at the yield torque. Thus, for example, it will be in the skill of the art to design a torque transmitting means which will fracture, shatter, snap etc. at or about a preselected torque. With reference to the embodiment shown in the drawings, shaft 52 or the entire torque transmitting means 50 can be fabricated of tool steel or like brittle material such that it will be fracturable at or about a predictable torque. Other suitable materials for the torque transmitting means include, for example, other metals, glass, ceramic and the like.

Claims: A threaded fastener for fastening work pieces together under compression, said fastener comprising a head component (12) for the application of torque to the fastener, said head component comprising a torque receiving end portion (14) and coaxial and unitary therewith an upper shaft portion (18) having a diameter less than that of said end portion (14), an externally threaded lower shank portion (30) formed of plastic and having a smaller diameter than said end portion (14) and the same diameter of the upper shank portion (18), said lower shank portion (30) being coaxial with said upper shank portion (18), resilient linking means (20) for resiliently interconnecting said upper shank portion (18) and said lower shank portion (30) a certain axial distance apart when said threaded fastener is in a free state, characterised in that said resilient linking means is arranged to provide bias against axial displacement of said lower shank portion (30) from said upper shank portion (18) a distance other than said certain axial distance, said resilient linking means (20) comprising a block of elastomeric material of which at least that portion spanning said certain axial distance between said upper shank portion (18) and said lower shank portion (30) is cylindrical, coaxial with said upper shank portion and said lower shank portion, and of diameter no larger than that of said upper shank portion (18) and said lower shank portion (30) and, torque transmitting means (50) for transmitting torque from said head component (12) to said lower shank portion (30), said torque transmitting means comprising a rigid shaft (52) extending through said cylindrical portion of said resilient linking means (20) coaxially therewith from a first T-shaped end (54) of said rigid shaft (52) housed in an axially extending slot (56) in said upper shank portion (18) to form a first non-rotational engagement therewith to a second T-shaped end (55) housed in an axially extending slot (57) in said lower shank portion (30) to form a second non-rotational engagement therewith, said torque transmitting means (50) being able to transmit sufficient torque from said head component (12) to said lower shank portion (30) to axially displace said lower shank portion (30) in a work piece against the resilient biasing of said resilient linking means (20) to elongate said resilient linking means (20) and put it in axial tension, and being fractureable at a yield torque which is less than said threaded fastener's, failure torque. A threaded fastener according to claim 1, wherein said upper shank portion is threaded.

Inventors: GOURD, JAMES TEAGUE
Applicants: FORD FRANCE S. A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1989
Main classification: F16B35
IPC codes: F16B5, F16B33, F16B35