Fastener with a Belleville head

Abstract

The present invention relates to headed push-in fasteners of the type produced by Penn Engineering & Manufacturing Corp. known as TackPins and TackScrews. More specifically it relates to a Tack Pin or a Tack Screw with a Belleville head that provides the functionality of a Bellville washer without needing a separate part. When installed, the compression of the Bellville shaped head will cause a permanent loading to be applied under the head of the fastener, essentially creating a clamping force to the members being attached.

Claims

1. A unitary fastener for joining two elements, a top and a bottom element face-to-face, comprising: a head including a downwardly divergent cone-shaped flange being the largest diameter of the fastener, said flange having an uninterrupted circular periphery adapted to resiliently deflect upwardly; and a shank extending immediately downward from said head, said shank including a displacer collar being the largest diameter of the shank for displacing material of the bottom element and means for engaging at least a bottom one of said two elements, said engaging means comprising an undercut adapted to receive the cold flow of metal from said bottom one of said two elements displaced by the collar.

2. A unitary fastener for joining two elements, a top and a bottom element face-to-face, comprising: a head including a downwardly divergent cone-shaped flange being the largest diameter of the fastener, said flange having an uninterrupted circular periphery adapted to resiliently deflect upwardly; and a shank extending immediately downward from said head, said shank including a displacer collar being the largest diameter of the shank for displacing material of the bottom element and means for engaging at least a bottom one of said two elements, said engaging means comprising helical threads adapted to receive the cold flow of metal from said bottom one of said two elements displaced by the collar.

3. An assembly of parts, comprising: a fastener according to claim 1 or claim 2; a top element in face-to-face abutment with a bottom element, said top element including an aperture through which the shank of said fastener passes; and the shank of said fastener engaged with a cavity in said bottom element to a depth where the periphery of the fastener head flange is in forcible contact with the top element and deflected sufficiently to apply a downward resiliently biasing residual clamp load between said elements, said shank adapted to receive the cold flow of metal from said bottom element.

4. The assembly of claim 3, said engaging means consisting of an undercut adapted to receive the cold flow of material from said bottom one of said two elements.

5. The assembly of claim 3 constructed and arranged so that the cold flow of metal is caused by a downward installation force applied to the head.

6. The assembly of claim 3 wherein the fastener is a tack pin.

7. The assembly of claim 3 wherein the fastener has a diameter of 1.0 mm.

8. The fastener of claim 1, said engaging means consisting of an undercut adapted to receive the cold flow of metal from said bottom one of said two elements.

9. The fastener of claim 1 constructed and arranged so that the cold flow of metal is caused by a downward installation force applied to the head.

10. The fastener of claim 1, wherein the fastener is a tack pin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 and 2 are cross-sectional views of prior art fasteners.

(2) FIG. 3 is a cross-sectional view of three embodiments of the invention.

(3) FIG. 4 is a cross-sectional view of an assembly utilizing the tack pin of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) Referring now to FIG. 3, three embodiments of the fastener of the invention are shown in side-by-side comparison with a Belleville washer. Any one of the embodiments is utilized to create an assembly in place of the fasteners of the prior art shown in FIG. 1 or 2 which join a top sheet to a bottom sheet. Each of the tack pin and tack screw embodiments has a head which includes a cone-shaped flange, a displacer collar 3 and an undercut or threads 5 which approximates the structure and performance of a Belleville washer shown at the far left.

(5) A Belleville washer creates force opposite to the direction of its compression. This figure depicts how a Belleville washer can be incorporated into the head of a tack pin, a tack screw, and a micro screw. The maximum force capability of a Belleville spring is given by equations that can be sourced from website Spring-I-Pedia at: http://springipedia.com/belleville-washers-formulas.asp.

(6) Referring now to FIG. 4, the tack pin 7 of FIG. 3 is shown in an assembly of a top element 8 affixed face-to-face with a bottom element 9 by the cold flow of metal into the undercut 5 of the tack pin 7. Upon installation, the periphery of the cone-shaped flange deflects resiliently upward as it forcefully contacts the top sheet. By the resilient nature of its material a continual biasing force is applied so that a residual clamp load is maintained.

(7) For the case of the tack pin with clamp load it is desirable to provide the minimum equivalent clamp force that can be provided by the yield of the metal in its undercut after installation. Incorporating the Bellville washer functionality into the head of at Tack Pin or screw effectively adds top sheet compression distance to the tack. This head design is advantageous where the top sheet changes in length to a greater degree than the tack pin or screw thus ensuring that a clamp load will be created. The deflection of the head in this instance is far greater than can be expected from the flat sheet alone. Similarly for the case of a micro screw, the design equation for a Belleville washer head, we set the force equation for the yield strength of the female threads engaged equal to the max force available for a Bellville washer.

(8) In the case of the screw, the fact that the screw is installed in tension creates the clamp load, and the necessity of the Bellville head to do this is not as important as it is for the tack pin or a tack screw. A Bellville head incorporated into the screw, however does provide the added benefits of: 1) Protection against the loss of preload of the screw 2) Over installation protection, such that a greater range for the tightening torque can be specified.

(9) From the above description of the invention it can readily be seen that the objects of the invention have been achieved. The incorporation of a Bellville washer type of head into the design of a tack pin provides the capability of supplying a clamp load to an assembly where tack pins are used, where previously only small clamp load was possible for a narrow range of conditions. Additionally the incorporation of this type of head to a tack pin adds tolerance to the installation pressing distance, that helps safeguard an over pressing, or over loading that can cause cosmetic damage. Finally for the tack pin, the elimination of a potential gap under the head, in the case of an under installed part, will give a more firm feel to the assembly. The added benefit of this type of head to a micro screw is that it provides protection against the loss of preload in the screwed joint, as well as installation tolerance on the required installation torque range. By combining the benefits of a separate washer into the structure of the fastener there are economies of manufacture and fewer problems with loose parts and the additional step of assembling the washer which may be impractical for miniature parts.

(10) Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.