Thread System

Abstract

A threaded member capable of being driven into low density materials has a sharp thread with its leading angle substantially larger than its trailing angle.

Description

BACKGROUND OF THE INVENTION

This invention relates to threaded members and more particularly to screw threaded fasteners for use in low density materials.

The expression "low density materials" is intended to include materials having a density below about 40 pounds per cubic foot and penetratable by a threaded member. Widespread use of some low density materials as a substitute for natural wood has been hindered by the inability of conventional wood screws to penetrate and hold in such materials. For example, conventional wood screws cause low density particle board materials to crumble between the screw threads and thus prevent the screw from penetrating the material properly. Some low density particle board materials such as Novaply permit conventional wood screws to be driven thereinto, but such screws do not hold well enough in that they loosen or are pulled out under too low a load to make their use practical. When an attempt is made to drive conventional wood screws into laminated materials such as natural wood veneer bonded to a thicker core of polyurethane foam of 20 pounds per cubic foot density, the screws lift the veneer from the core. The result has been that relatively expensive fasteners, or fasteners that are difficult to use, have had to be used instead of screws. Also, when an item is made from materials having vastly different densities, several different kinds of fasteners must be stocked by its manufacturer.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a threaded fastener capable of being screwed into and capable of holding well in low density materials.

Another object is to provide a threaded fastener usable in materials having a wide range of densities.

Another object is to provide a threaded fastener that can be driven without requiring a pre-drilled pilot hole.

Another object is to provide a threaded fastener that can be made in a single pass by rolling a blank between dies.

Another object is to provide a threaded fastener that does not crumble low density particle board between its threads.

Another object is to provide an easily manufactured, relatively inexpensive fastener that is rugged and durable.

Still other objects and advantages of the invention will be revealed in the specification and claims, and the scope of the invention will be set forth in the claims.

Briefly stated, in accordance with one aspect of the invention a screw threaded member has sharp-edged, asymmetrical threads of increased surface area and spacing that penetrate and hold the member in materials having a wide range of densities.

DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a side view of a threaded member in accord with this invention.

FIG. 2 is an enlarged side view of a portion of the thread from the member shown in FIG. 1.

FIG. 3 is an enlarged side view of a portion of the member shown in FIG. 1.

FIG. 4 is a plan view taken along the line 4--4 in FIG. 3.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows how this invention would be applied to a wood screw 1 having a conventional round head 2 with a slot 3 facilitating driving of same, it being understood that this thread system can be used to anchor essentially any type of threaded member, such as a bearing rod having a threaded end. Thread portion 4 is a single, continuous, sharp-edged, asymmetrical screw thread spiraling down the root portion 5 of fastener 1 and terminating at sharp pointed tip 6.

As shown in FIG. 2, the thread leading angle is 30.degree. and the trailing angle is 15.degree.. This results in an included angle at sharp edge 7 of 45.degree. that permits thread portion 4 to bite into and hold well in materials having densities in the range of about 15 to about 60 pounds per cubic foot. The threads do not crumble or damage low density materials, and it is not necessary under most circumstances to pre-drill a pilot hole for member 1 before driving. These thread angles are critical, and the advantages of this invention will not be attained if there is any significant deviation therefrom.

FIG. 3 shows the relationship between the depth T of thread portion 4 and the diameter R of root 5. Thread depth T must always be at least 30 percent of the root diameter R and preferably is between 30 and 40 percent. Thus, the thread system of this invention provides substantially more thread depth than the prior art. This results in a greater thread bearing surface area which reduces crumbling and other damage to low density materials while member 1 is being driven, as well as increasing its holding power thereafter. One factor in achieving this desirable result is that relatively less material is displaced when screw 1 is driven.

FIG. 4 shows the relationship between thread surface area and root area for this thread system. This thread system has substantially greater thread surface area than do comparable prior art systems, and this greatly increases the ability of this system to hold in low density materials. This greater thread surface area results from the above relationship between thread depth T and root diameter R, and from the specific values specified for the leading and trailing angles.

In FIG. 1 the distance between threads is designated D. For threaded members in accord with this invention to perform properly, it is necessary that the thread spacing D be at least double the thread depth T, and preferably 2.2 to 3 times the thread depth. This results in fewer threads per inch, and contributes to substantial elimination of the crumbling that occurs when prior art threads are driven into low density particle board.

Tests have been run to demonstrate the greater holding power of screws in accord with this invention. Size 10 gauge screws in accord herewith driven one-half inch into 18-20 pound per cubic foot urethane foam required a pull-out force averaging 215 pounds, while conventional type A screws of the same size resisted an average of only 158 pounds before being pulled out of the same material. This is an improvement of about 36 percent. The same test showed these screws to be about 20 percent more resistant to pull-out from particle board materials. The same test in other materials revealed that screws in accord herewith always resisted a larger average pull-out force than did conventional types of wood screws. This shows that the holding power of fasteners with threads in accord with this invention in low density materials is substantially greater than conventional wood screws of the same size. The tests also revealed increased holding power in materials with densities up to about 60 pounds per cubic foot. Furthermore, conventional screws showed a much greater deviation in pull-out resistance values than did the screws of the instant inventions.

It has thus been shown that screw threaded members in accord with this invention are easily driven into low density materials without damaging such materials and also have substantially increased holding power when compared to conventional structures. Threaded members as disclosed herein are also readily usable in high density materials, and this means that industrial users can reduce their fastener inventories. Also, these threaded members are especially suited for use in mass production operations where they are driven by power rotated tools (as contrasted with hand rotated tools). The threaded members of this invention can be economically manufactured in a single pass between dies in a conventional rolling operation because the metal flows easily and fills out the sharp crested thread when the leading angle is 30.degree. and the trailing angle is 15.degree..

While the present invention has been described with reference to a particular embodiment, it is not intended to illustrate or describe herein all the equivalent forms and ramifications thereof. Also, the words used are words of description rather than limitations, and various changes may be made without departing from the spirit or scope of the invention disclosed herein. It is intended that the appended claims cover all such changes as fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A screw threaded member for use with materials having a density in the range from about 15 to about 60 pounds per cubic foot comprising a root portion of predetermined diameter, a single continuous external sharp-edged thread portion having a leading angle of 30.degree. and a trailing angle of 15.degree., the depth of said thread portion being from 30 to 40 percent of the diameter of said root portion, and the spacing between threads being from 2.2 to 3 times as great as said thread depth.