Locking screw for modular framework

A locking screw is disclosed having a shank and a frustoconical head with a cavity therein and a knurled annular end surface portion encircling the cavity. The locking screw is disposed within the arm of a joint connecting member which arm is inserted in a tubular structural member. The screw is turned out from the arm into engagement with the structural member holding the joint and structural member securely together.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to threaded fasteners and more particularly 
to screws having improved locking features for modular framework. 
2. Description of Related Art 
Many conventional frames, cabinets, tables, partitions and other similar 
items are made of modular components such as square tubular members which 
are interconnected by sliding them over projecting arms of a corner 
connector. Screws inserted into the arms are turned into engagement with 
the interior wall of the tubular members to secure the corner connector 
and tubular members together. Modular type assemblies of the 
above-described nature are widely used in the consumer industry for 
everything from patio furniture to garage cabinets because the components 
parts can be readily packaged and the end user can provide the assembly. 
Typical examples of conventional modular structures are illustrated in U.S. 
Pat. Nos. 3,858,989 and 3,854,831. In U.S. Pat. No. 3,858,989, a modular 
corner joint is disclosed which includes tubular members and a hub corner 
connector with protruding arms. Conical set screws are inserted into 
threaded holes which extend laterally through the arms. Once assembled the 
set screws are backed into a hole in the tubular member such that the 
screw engages the inner surface of the hole and ultimately locks the 
tubular member against the corner hub. In U.S. Pat. No. 3,854,831, a 
corner fitting has projecting arms. Before assembly bolts are inserted 
into threaded holes in the arms. The tubular members are slid over the 
arms, and the bolts are backed into the inner wall of the tubular members 
via an access hole in the wall of the tubular member. Each bolt has a head 
portion which extends into the access hole and pushes against the inner 
wall of the access hole, thereby urging the tubular member and the arm 
into clinching engagement. 
While the above described threaded fasteners have meet with a certain 
amount of success, the interconnected joint arrangements disclosed in 
these patents are still susceptible to loosening due to vibration or shock 
because the fastening screws do not provide sufficient surface resistance. 
Furthermore, in many applications modular joints are not used for 
fabricating cabinets, frames, tables or partitions because conventional 
modular structures are not sufficiently stiff or stable. Instead, welded 
structures are preferred which offer stability, but disadvantageously are 
very labor intensive to fabricate and therefore expensive. 
Therefore, it would be an advancement to the art to provide a simple, 
inexpensive means to fasten modular components together in stable 
relationship. 
SUMMARY OF INVENTION 
It is therefore an object of the present invention to provide fasteners for 
modular framework components that more readily resist loosening due to 
shock or vibration. 
It is another object of this invention to provide fasteners for modular 
framework which are simple and inexpensive to manufacture yet reliable. 
It is still another object of the present invention to provide fasteners 
for modular framework that can be utilized without the need for any 
special tools by lay persons without any special skills. 
A locking screw according to the present invention includes a threaded 
shank and a head portion at one end of the shank. The head portion has an 
outer frustoconical side surface and an essentially flat annular end 
surface portion, which encircles a recessed cavity. The flat annular 
surface portion has a roughened surface. A socket cavity extends through 
the bottom portion of the recessed cavity into the threaded shank, 
providing means to tighten the screw by means of a tool. In use, the 
locking screw is inserted into the arm of the a corner member, the arm 
being slid into a tubular member. The locking screw is backed into and 
tighten against the inner wall of the tubular member such that the flat 
annular end surface portion of the head engages the inner wall of the 
tube. The roughened surface grips the inner wall and readily resist 
disengagement due to vibration and shock. 
Other and further objects, advantages and characteristics features of the 
present invention will become readily apparent from the following detailed 
description of preferred embodiments of the invention when taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now with more particularity to the drawings, wherein like or 
similar parts are designated by the same numerals throughout the various 
figures, FIGS. 1-3 illustrate a locking screw 10 having a threaded shank 
12 and a head portion 14 disposed along a longitudinal axis 15. Head 
portion 14 has a generally frustoconical outer surface 16 which tapers 
outwardly from the shank 12 at a predetermined angle "X" to the 
longitudinal axis 15. The head portion 14 further has a central recessed 
cavity 20 extending inwardly from the end of the head 14, which cavity 
terminates into counterbore 28. The central cavity has a generally 
frustoconical inner surface 18 which tapers outwardly from counterbore 28 
at a second predetermined angle "Y" with respect to longitudinal axis 15. 
The outer and inner frustoconical surfaces 14 and 18 form wall 24 
therebetween, which wall terminates into annular surface portion 26, which 
encircles central cavity 20. The counterbore 28 is preferably hexagonal in 
cross-section and extends from the central cavity 20 into shank 12 along 
the longitudinal axis 15 providing a socket means for a tool such as an 
allen wrench or screw driver, for example. 
The predetermined taper angles Y, X for the inner and outer frustoconical 
surfaces 18, 14 are preferably selected such that the thicknesses of wall 
decreases from the bottom surface 22 to the annular surface portion 26. 
The predetermined taper angle for frustoconical outside surface 14 may 
preferably be in range from about 35.degree. to 45.degree. and more 
preferably in the range from about 38.degree. to 42.degree. with respect 
to longitudinal axis 15; while frustoconical inner surface 18 may form an 
angle of taper of 45.degree. to 55.degree. and more preferably 48.degree. 
to 52.degree. with respect to longitudinal axis 15. 
The annular surface portion 26 has a roughened texture to provide an 
improved gripping effect. Such roughened texture preferably takes the form 
of circumferential knurl 30. The circumferential knurling 30 comprises a 
plurality of alternating spaced crests 32 and groves 34. The crests and 
grooves may extend radially outward from the longitudinal axis. However, a 
preferred arrangement of crests 32 and grooves 34 for a right hand 
threaded screw is illustrated wherein the crests and grooves extend across 
the annular surface portion in a slanted clockwise direction. Alternately, 
for left hand threaded screws, the direction of knurling preferably would 
be the opposite, namely slanted counterclockwise (not shown). Slanting the 
crest and grooves across the annular surface portion increases the 
effective length of crests and grooves thereby providing for more griping 
power during use. 
An illustrated use of the locking screw is shown in FIG. 4, for example 
(reference may also be made to U.S. patent application Ser. No. 348,562 
which has been allowed and is incorporated herein by reference). The 
locking screw 10 is screwed into a threaded hole 38 of arm 40 of 
connecting member 42. The threaded hole 38 has a countersink 43 at one end 
thereof to receive the frustoconical head portion 14 of locking screw 10. 
Connecting member 42 may be a one piece monolithic element having a 
plurality of arms 40 for interconnecting one, two, three or more tubular 
structural members in a corner arrangement (in this example only one 
tubular structural member 46 is shown). The arms 40 may be of the same 
general cross-sectional configuration as the cross-sectional opening of 
structural member 46, but of a slightly small size so as to be slidably 
insertable into the end portion of the structural member. The arm 40 with 
locking screw 10 are inserted within hole 44 of tubular structural member 
46. The locking screw 10 is tighten by backing it out of the threaded hole 
38 and against the inside wall 48 of the tubular member 46 using a tool 
(not shown) such as an allen wrench via access hole 50. The annular 
surface portion 26 engages the inner wall 48 of the tubular structural 
member, the knurl crests of locking screw 10 penetrating and cutting into 
inner wall 48. The crests cut burrs from the inner wall which provide 
resistance to the screw from moving. The knurling therefore provides 
gripping power to add frictional resistance to inhibit undesirable 
loosening of the screw in shock or vibration environments. Additionally, 
the central cavity 20 allows a hole plug (not shown) to be inserted into 
access hole 50, thereby giving the exterior of the structural member 46 a 
smooth appearance. 
The above-described detailed description of preferred embodiments described 
the best mode contemplated by the inventor for carrying out the present 
invention at the time this application was filed and is offered by way of 
example and not by way of limitation. Accordingly, various modifications 
may be made to the above-described preferred embodiments without departing 
from the scope of the invention. For example, the head portion could be 
cylindrically shaped having a cylindrically shaped central cavity. 
Additionally, the roughened annular surface portion could have a plurality 
of rigids, protrusions or beads providing the frictional resistance 
desired to prevent loosening of the screw. Although metallic materials may 
be used to form the various above-described components, namely the 
structural members, corner members and locking screws, these components 
may be made of other materials, such as plastic. Accordingly, it should be 
understood that although the invention has been described and shown for 
various particularly embodiments, nevertheless various changes and 
modifications obvious to a person of ordinary skill in the art to which 
the invention pertains are deemed to lie within the spirit and scope of 
the invention as set forth in the following claims.