Push in-Screw Out Fastening System

This invention relates to low cost fastening systems having fasteners that are easy and quick to install and simple to remove.

There are a myriad of fastening systems presently available to meet a great variety of different applications. Many of these systems use a molded tubular plastic fastener and receptacle in which the fastener is pushed or driven into the receptacle and is secured in the receptacle by friction. Removal is accomplished by pulling the fastener out of the receptacle. The fasteners are often referred to as "pine tree" or "Christmas tree" devices and include a circular head and a cylndrical shank having a plurality of thin, circular fins or ribs. The fins are deformed when the fastener is pushed into a receptacle that has an opening or orifice diameter that is less than the diameter of the ribs. The "memory" of the fastener material provides the frictional force that holds it securely in place in the receptacle. Such fasteners are often difficult to remove without damage, either to the fastener or to the components that are being held.

The fastener of the invention comprises, in its preferred embodiment, a molded plastic screw in which the screw thread is divided into a plurality of thread segments. The segmented construction imparts a degree of flexibility that enables the fastener to be pushed into a lesser diameter tubular receptacle without significant permanent distortion of the thread. The receptacle of the invention includes a single internal thread formed near its entry. The thread engages the thread segments when the fastener is turned into a counterclockwise direction and imparts a longitudinal removal force to the fastener. Thus with the invention, the fastener is driven or pushed into the receptacle by deflecting the resilient thread segments. When the thread segments pass through the one thread entry in the receptacle, they attempt to return to their undeflected positions and, in so doing, exert significant holding forces against the inner surface of the receptacle.

A principal object of the invention is to provide an improved low cost fastening system.

Another object of the invention is to provide a novel push in-screw out fastening system.

These and other objects and advantages of the invention will be apparent upon reading the following description in conjunction with the drawings, in which:

FIG. 1 is a side elevation of a screw fastener constructed in accordance with the invention;

FIG. 2 is a sectional view of the fastener of FIG. 1 taken along the line 2--2; and

FIG. 3 is a partial sectional view of an enclosure and panel incorporating the fastening system of the invention.

Referring to FIGS. 1 and 2, a threaded fastener 10 includes a circular head 12, having a screwdriver slot 14 therein, and a cylindrical shank 16. A spiral thread 18 is segmented into a plurality of thread segments 18a, 18b, 18c and 18d to form separated longitudinal slots 20, 22, 24 and 26 (best seen in FIG. 2). The end of shank 16 terminates in a generally cone shaped entry guide 30. Fastener 10 is preferably molded of plastic material with the slots 20-26 and thread segments 18a-18d being formed during the molding process. The thread preferably includes a taper on its lower side, such as indicated by reference numeral 28, to facilitate insertion of threaded fastener 10 into a receptacle. The segmented construction of the thread imparts a significant degree of flexibility to the threaded segments. It should be apparent to those skilled in the art that the number of slots and their size may be altered for different design applications, the criteria being that the fastener must be secured by the restoring force exerted by the deflected thread segments against the inner surface of the tubular receptacle, and that the thread segments must be strong enough to enable removal of the fastener by engaging a threaded portion in the receptacle entry.

In FIG. 3, a partially shown enclosure 32 has a tubular receptacle 34 formed therein that includes an inner surface 36. The entry of receptacle 34 has a thread 38 of the same pitch as thread 18 on fastener 10 formed therein. A partially shown closing panel 40 has a recess 42 with an aperture 44 at its bottom that the threaded shank of fastener 10 passes through. The fastener head 12 nests within recess 42 when the fastener is installed to secure panel 40 to enclosure 32. It will be apparent that fastener 10 may conveniently be captivated in recess 42 by making aperture 44 slightly smaller in diameter than the diameter of thread 18 and forcing fastener 10 through aperture 44 into position. In this connection, it should be noted that thread segments 18a-18d need not be pie-shaped but may have a generally cloverleaf shape, for example. The term thread diameter should therefore be understood to also mean the greatest distance or widest expanse across the thread segments.

As illustrated at 52 and 54, thread 18 is distorted when pushed into engagement with tubular receptacle 34 and the restoring force exerted by the thread segments against interior surface 36 of receptacle 34 secures fastener 10. A screwdriver blade 50 in slot 14 in head 12, when rotated in a counterclockwise direction as indicated by the arrow, turns threaded fastener 10 causing the segments of thread 18 to engage inner thread 38, formed in the entry of receptacle 34, to thereby permit fastener 10 to be unscrewed. The illustration in FIG. 3 has been idealized to assist in the explanation of the invention.

While the fastener of the fastening system of the invention may be fabricated from any suitable material, plastic is preferred because of its strength, low cost and ability to withstand distortion. Material such as polypropylene, polyethylene and nylon are quite suitable. The receptacle can be made of any type of polymeric material.

Those skilled in the art will readily envision many other arrangements for affixing a member, such as panel 40, to an enclosure, such as enclosure 32, with the fastening system of the invention. The described embodiment is merely illustrative of the preferred form of the invention. As mentioned, a greater or lesser number of thread segments and slots may be utilized and alignment of the thread segments is not required. Similarly, a full inner thead 38 is not a requirement, it being only necessary to have some thread engaging surface, preferably located at the entry of the receptacle. Thus it should be clear that numerous changes in the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit and scope. The invention is to be limited only as defined in the claims.