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Description  |
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BACKGROUND OF DISCLOSURE
This disclosure is directed to machine or wood screws and is more
particularly directed to screws often termed security screws, which are
purposely made difficult to remove by unauthorized persons not having the
required instrument needed for removal thereof.
Security screws now generally available consist of the following types:
one-way screws, drilled spanner, slotted spanner, pinned socket screw, and
the Holt screw. The screws are shown, respectively, in FIGS. 1A, B to 5A,
B and are indicated as prior art.
The one-way screw, FIGS. 1A and 1B, consist of a standard slotted screw
which has the head milled away so that it can only be driven one way. It
is a necessarily expensive screw because the head has to be slotted and
milled into shape. The head configuration is obtained in three steps: (1)
cold heading, (2) slotting, and (3) milling. While the one-way screw is
effective, it is expensive and unsightly and very difficult to remove for
servicing. It is installed with a regular screwdriver.
The drilled spanner is shown in FIGS. 2A and 2B and the slotted spanner is
shown in FIGS. 3A and 3B. They are both, by nature, expensive as the head
configurations require either drilling or milling after cold heading. Both
screws are installed and removed by using a special spanner wrench.
Unauthorized persons can easily remove them by rotating them with a pin
punch and hammer.
The hex socket security screw, FIGS. 4A and 4B, is a standard socket head
screw which has a pin installed in the center of the socket cavity. It can
only be installed and removed by using a hex key wrench that has a hole
drilled in the end to slip over the pin. A counterfeit wrench can be made
only by annealing a standard wrench, drilling the hole, and rehardening
the wrench. The screw can be removed by unauthorized persons by breaking
the pin off using a small punch and then using a regular hex key wrench.
These screws are very expensive to manufacture. The head cavity is first
broached to its hex shape. A hole is drilled to accommodate.
The Holt screw, FIGS. 5A and 5B, is apparently relatively new to the
market. It is installed and removed by using a special socket wrench which
has the exact configuration for each screw head size. The screw is driven
by the three near vertical slots in the periphery of the binding type
screw head.
The screw can easily be removed by unauthorized persons by using a hammer
and pin punch to rotate the head. Its cost is moderate compared to the
other security screws, but it requires a cold heading operation and a
subsequent slot-forming operation to manufacture.
The screw head form described in this disclosure has none of the
disadvantages of any of the above screws. It is the most secure because it
has no protrusions or cavities in which one might use a pin punch or other
tool. Its profile is very low and tapered to thwart the use of pliers. It
can only be readily removed by the use of the socket wrench made for its
particular size.
Additionally, the preferred embodiments are inexpensive to manufacture
since only one cold heading operation is required for their manufacture.
No secondary drilling or milling operations are needed.
BRIEF DESCRIPTION OF THE DISCLOSURE
The security screw of this invention has a low profile head and a threaded
portion coupled thereto and is characterized in that the peripheral
surface of the head embodies an odd number of sides arranged symmetrically
with respect to the center of the head and inclined to the axis of the
screw to prevent removal of the screw by means of the use of pliers. In
one form there are three sides and in another five sides. Optionally, the
head in either peripheral configuration may be provided with a diametrical
slot and the crown slashed at opposite sides and at opposite ends of the
slot to provide raised, tapered shoulder portions for rotating the head in
one direction to insert the screw, but to prevent its rotation in a
direction to remove the screw.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are top and side views of a prior art slotted security
screw;
FIGS. 2A and 2B are top and side views of a prior art drilled spanner
security screw;
FIGS. 3A and 3B are top and side views of a prior art slotted spanner
security screw;
FIGS. 4A and 4B are top and side views of a prior art hex security screw;
FIGS. 5A and 5B are top and side views of a prior art Holt security screw;
FIG. 6 is a top plan view of one form of security screw wherein the head is
of rounded-off triangular configuration;
FIG. 7 is a side elevational view of the screw of FIG. 6;
FIG. 7A is a sectional view taken along line 7A--7A of FIG. 6;
FIG. 8 is an isometric of the security screw shown in FIGS. 6, 7 and 7A;
FIG. 9 is a view illustrating the preferred manner for developing the
rounded-off triangular configuration of the head;
FIG. 10 is a top plan view of an alternate embodiment of the disclosure to
permit the security screw to be rotated in one direction by a screwdriver;
FIG. 11 is a front elevational view of the screw of FIG. 10;
FIG. 12 is a side elevational view of the screw of FIG. 10;
FIG. 13 is a sectional view taken along line 13--13 in FIG. 10;
FIG. 14 is an isometric view of the screw of FIGS. 10-13;
FIG. 15 is an isometric view of a security screw of rounded-off pentagonal
configuration;
FIG. 16 is a plan view illustrating the preferred manner of developing the
rounded-off pentagonal configuration of the head shown in FIG. 15; and
FIG. 17 is an elevation of the head showing the configuration of the crown.
DETAILED DESCRIPTION OF THE DISCLOSURE
Referring to FIGS. 6 to 9, the screw comprises a head 20 and a conventional
threaded portion 30. The head has a curved crown or cap 21, a peripheral
skirt 22, and a flat annular bottom side 23. In accordance with one
preferred form, the skirt comprises three major inclined surfaces 22-1,
22-3 and 22-5 arranged symmetrically with respect to the center of the
head which impart to the head the triangular configuration shown in these
figures and three minor inclined surfaces 22-2, 22-4 and 22-6. The said
surfaces terminate, respectively, in the top skirt line 24 and the bottom
skirt line 25 and are defined by six arcs.
The arcs defining the bottom skirt line 25 are preferably determined as
follows with reference to FIG. 9; assuming the full diameter to be 2R, lay
off lines 0A, 0B, and 0C. Each line is equal to R. Locate points D, E and
F which are 1/2 R from the origin 0. At points D, E and F, describe three
circles with radius 1/2 R. From point D, describe arc JK such that the arc
becomes tangent to the circles about E and F. From point E, describe arc
LG such that the arc becomes tangent to the circles about F and D. From
point F, describe arc HI such that the arc becomes tangent to the circles
about F and D. The line H,I,J,K, L and G now describes the bottom skirt
line or bottom periphery of the head.
The head cross section is shown in FIG. 7A. Its profile is most preferably
about 1/2 lower than the ASA standards for binding head screws in order to
prevent removal by pliers. In addition, to prevent gripping of the head by
pliers, the skirt surfaces 22-1 to 22-6 are tapered so that plier jaws
will slip off as the screw is attempted to be firmly grasped to turn it.
It has been found that the skirt surfaces 22-1, 22-3, and 22-5 (see FIG.
7) should be set at an angle (angle of inclination) between about
40.degree. to 70.degree. and most preferably 50.degree. to 60.degree. with
respect to a line which is perpendicular to the flat bottom 23. The most
preferred angle being about 56.degree. for best results. It has also been
found that the skirt surfaces 22-2, 22-4, and 22-6 oppositely disposed to
surfaces 22-1, 22-3, and 22-5 should be at a smaller angle (angle of
inclination) of preferably about 3.degree. to 10.degree. and most
preferably 5.degree. with respect to a line perpendicular to the bottom 23
so that the plier jaws are further prevented from obtaining a good grip on
the screwhead.
While the above angular relationships are most preferred, it should be
understood that the angle would be quite acceptable if it was about
0.degree. to 15.degree.. Most preferably, the surfaces 22-1, 22-3 and 22-5
are inclined at an angle which is at least four times that of surfaces
22-2, 22-4 and 22-6.
The security feature of this invention is most advantageous on the screws
in the #2 to #12 machine screw range in order to prevent their removal by
the use of cold chisels. A table of head sizes most suitable for this
invention for #2 to #12 machine screws appear below. For thread size,
consult ASA table for binding head screws #B18. 6-1947 in Machinery's Hand
Book. Representative overall head heights and skirt heights are given in
the table below for #2 to #12 machine screws of the diameters as shown:
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MACHINE HEAD
SCREW Nominal OVERALL SKIRT
(Nominal Diameter HEAD HEIGHT HEIGHT
Size) 2 R (inches)
Y (inches) X (inches)
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2 .186 .035 .017
3 .200 .038 .019
4 .220 .041 .020
5 .250 .047 .023
6 .280 .052 .026
8 .340 .064 .032
10 .386 .072 .036
12 .438 .082 .041
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The length of the threaded portion 30 is preferably 1/4" or less, e.g.,
1/8", 3/16", etc., although larger lengths may advantageously use this
invention.
FIGS. 15 to 17 illustrates the now most preferred form of screw comprising
a head 20 and a conventionally threaded portion 30. The head comprises a
curved crown or cap 32, a peripheral skirt 34 and a flat annular bottom
side 36. The skirt comprises five major inclined surfaces 34-1, 34-2,
34-3, 34-4 and 34-5 which are arranged symmetrically with respect to the
center of the head and which impart the pentagonal configuration to the
head. The said surfaces terminate, respectively, in the top skirt line 38
and in the bottom skirt line 40, said skirt lines being defined by 10
arcs.
The arcs defining the bottom skirt line 40 are determined as follows:
Referring to FIG. 16, lay off the lines AB, BC, CD, DE and EA within a
circle having its center at 0 such that the five lines form a pentagon.
Now lay off the lines AM, BN, CP, DK and EL such that these lines pass
through the center of the circle 0 and the points A, B, C, D and E,
respectively. Describe five circles at the points F, G, H, I and J whose
radii are one-half the length of the segment AQ (or BR or CS or DT or EU)
and describe five arcs with centers at the points Q, R, S, T and U such
that the arcs terminate at the point of tangency with the five smaller
circles. The bottom skirt line 40 peripherally of the head is now defined
by the five large arcs 34-1 to 34-5 and the five arc segments of the five
small circles.
The head cross section, FIG. 17, like that of the screw shown in FIGS. 6 to
9, is preferably about one-third lower than the ASA standards for binding
head screws in order to prevent removal by pliers. In addition, to prevent
gripping of the head by pliers, the skirt surfaces 34-1 to 34-5 are
tapered at an angle in the order of 40.degree. to 70.degree. and most
preferably 50.degree. to 60.degree. with respect to a line which is
perpendicular to the flat bottom 40. The most preferred angle is about
56.degree. for best results. The inclination at the points of tangency of
the small circles with the large circle which are, respectively, the
surfaces 34-6, 34-7, 34-8, 34-9 and 34-10 would be at a smaller angle of
inclination of preferably about 3.degree. to 10.degree. and most
preferably 5.degree. with respect to a line perpendicular to the bottom
40.
While the above angular relationships are most preferred, it should be
understood that the angle would be quite acceptable if it was about
0.degree. to 15.degree. and the angle being at least 4 times that of the
angle.
Reference should now be had to FIGS. 10 to 14 which show a modified form of
the screw. In these Figures, the skirt 22, the bottom 23 and the threaded
portion are the same as described with reference to FIGS. 6 to 9 and
therefore like numbers are used. However, the top of the head 20 is in
these figures provided with two shoulders 26 with curved camming portions
26-1, facings 26-2 and outer curved portions 26-3.
The arced triangular-shaped facings 26-2 are provided to permit a
screwdriver to be inserted to drive the screw in one direction to insert
same. The curved camming portions 26-1 are sloped as shown to prevent
turning of the screw by a screwdriver in a reverse direction to remove it.
In order to remove the security screw of this invention, a special socket
having the shape of the head formed, e.g., by molding or casting
techniques well-known in the art is used.
Thus, this invention provides a new and improved security screw uniquely
suitable for retaining covers of business machines, such as punches used
in providing betting tickets, securely in place. The screws of this
invention, because of their shape, are extremely difficult to remove
except by those authorized individuals having the special tool needed to
remove the screws.
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