U.S. patent number 3,592,100 [Application Number 04/727,939] was granted by the patent office on 1971-07-13 for binding screw having electrically insulated head.
This patent grant is currently assigned to Harvey Hubbell, Incorporated. Invention is credited to Czeslaw Mackiewicz, James E. Meehan.
United States Patent |
3,592,100 |
Mackiewicz , et al. |
July 13, 1971 |
BINDING SCREW HAVING ELECTRICALLY INSULATED HEAD
Abstract
A binding screw having a threaded shank and an enlarged head
coated with a plastic insulating material. The head includes a
usual slot which is cut through the insulating material exposing
the base metal.
Inventors: |
Mackiewicz; Czeslaw (Trumbull,
CT), Meehan; James E. (Bridgeport, CT) |
Assignee: |
Harvey Hubbell, Incorporated
(Bridgeport, CT)
|
Family
ID: |
24924729 |
Appl.
No.: |
04/727,939 |
Filed: |
May 9, 1968 |
Current U.S.
Class: |
411/403; 174/159;
411/914; 174/54; 411/903; 427/185; 411/908; 425/270; 470/9;
470/11 |
Current CPC
Class: |
H01R
4/70 (20130101); F16B 33/004 (20130101); Y10S
411/903 (20130101); Y10S 411/908 (20130101); Y10S
411/914 (20130101); H01R 4/30 (20130101) |
Current International
Class: |
H01R
4/70 (20060101); F16B 33/00 (20060101); H01R
4/30 (20060101); H01R 4/28 (20060101); F16b
023/00 (); F16b 035/00 () |
Field of
Search: |
;85/1C,10.1,1JP,9,1,54,37,45 ;151/38,7 ;10/10,27P,27,5,3
;117/21,113,116P,128.4,127,132C,230--33,4 ;174/166,193,54,58 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
MATERIALS IN DESIGN ENGINEERING, February, 1960, pp. 91--95, copy
available in 117--21..
|
Primary Examiner: Britts; Ramon S.
Claims
What we claim is:
1. A binding screw comprising: a threaded metallic shank; an
enlarged metallic head integrally formed with said shank at one end
thereof, said head having a top and sides; a relatively thin layer
of plastic electrical insulation material secured to and
surrounding the top and sides of said head; and a transverse slot
extending through said layer of plastic insulation material into
said head, thereby exposing the base metal of said metallic head
throughout the sidewalls of said slot.
2. The binding screw defined in claim 1 wherein said layer of
plastic insulation material is bonded to said head.
3. A method of forming a binding screw comprising the steps of:
forming a head screw blank; heating the blank; immersing the heated
head in a mass of plastic insulation material for a predetermined
time thereby bonding the insulation material to the head and
building up an insulation layer of desired thickness; removing the
coated blank from the mass of insulation material; threading the
shank of the screw blank; and slotting the head of the screw blank
through the insulation layer and into the metal head.
4. The method of forming a binding screw as defined in claim 3
wherein said mass of plastic insulation is a fluidized bed of nylon
particles formed by continuously agitating the nylon particles with
an airblast passing therethrough.
5. The method of forming a binding screw as defined in claim 3
wherein said mass of plastic insulation is a bath of liquid plastic
material.
Description
BACKGROUND OF THE INVENTION
This invention relates to a binding screw and a method of making
the same and, more particularly, to such a screw having a layer of
plastic insulation material deposited upon its head. This invention
further relates to the use of insulated head binding screws with
grouped switches to prevent arcing between adjacent screwheads.
It is common to group switches in an outlet box for conveniently
providing a group of AC switches at a single location. However,
when high-voltage circuits, for example, 20 a. 277/480 v. AC, are
controlled by the switches a potentially hazardous condition exists
in the outlet box. The usual laterally outwardly extending binding
screws of each of the closely mounted switches present exposed live
metal parts across which arcing may occur when a 700 volt potential
exists between adjacent binding screws. To prevent this condition,
the National Electrical Code provides in part that: "Snap switches
shall not be grouped or ganged in outlet boxes unless they can be
so arranged that the voltage between exposed live metal parts of
adjacent switches does not exceed 300 volts."
One solution to the problem of grouping AC switches in high voltage
circuits has been to eliminate the binding screws altogether and to
back wire current-carrying leads to the switch bodies. The leads
may be captured by suitable internal spring clips in a known
manner. However, these clips often lose their resilience and become
unreliable. Therefore, it would be desirable to retain the positive
mechanical securing action of a binding screw of either a
back-wired switch or a binding-head type while ensuring that no
arcing will occur between adjacent switches when closely mounted in
an outlet box.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of this invention to provide a
binding screw with an electrically insulated head and a method of
forming the same.
Another object of this invention is to provide a binding screw
having a layer of plastic insulating material deposited upon its
head and having the base metal material exposed in the slot.
A further object is to provide a plurality of grouped switches with
the binding head screw described in the above paragraphs.
To accomplish the primary object, in one form, there is provided a
binding screw having a threaded shank with an enlarged electrically
insulated slotted head at one end. The head includes a layer of
plastic insulation material surrounding the metal head of the screw
and the slot extends through the layer of plastic insulation
material and into the metal head to expose the base metal of the
head.
BRIEF DESCRIPTION OF THE DRAWING
Other objects and further details of that which is believed to be
novel and the present invention will be clear from the following
description and claims taken with the accompanying drawing
wherein:
FIG. 1 is a front elevational view of an outlet box with the cover
removed housing a series of grouped AC switches having binding head
screws constructed in accordance with this invention;
FIG. 2 is a perspective view of a screw blank with a coated
head;
FIG. 3 is a perspective view of the screw blank of FIG. 2 in which
a shank has been threaded;
FIG. 4 is a perspective view similar to that of FIGS. 2 and 3 in
which the head has been slotted;
FIG. 5 is a side elevational view partly in section showing a
binding head screw constructed in accordance with the invention;
and
FIG. 6 is a schematic side elevational view of a holding rack of
screw blanks positioned above a trough of coating material.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With particular reference to the drawing there is illustrated in
FIG. 1 a plurality of AC switches 10 mounted in an outlet box 12
upon mounting lugs 14. Each switch includes a cover 16 made of a
plastic insulating material, a toggle 18, and a plurality of
laterally outwardly extending binding screws 20. A mounting strap
22 carrying securing screws 24 retains each switch in position in
the outlet box upon the mounting lugs. The lateral position of each
binding screw 20 is determined by the size lead captured thereby
when the lead is secured by the head. Therefore, it should be noted
that the heads of the binding screws of adjacent switches may be
extremely close together and in fact may be in direct contact. In
order to satisfy the above-quoted requirement of the National
Electrical Code the binding screws 20 have been formed to eliminate
the conventional exposed live metal parts of these screws.
The novel binding screw 20 of this invention is clearly illustrated
in FIGS. 4 and 5. This screw includes a metallic threaded shank 26
having an enlarged metallic head 28 at one end. A layer of
synthetic plastic insulating material 30, such as nylon, is bonded
to the top and sides of head 28 and a slot 32 is formed through the
plastic layer 30 into the head 28 (see FIG. 5). The dielectric
coating material has been selected to be hard enough to allow it to
be machined in the slotting operation without burring while also
being resilient enough to prevent chipping and cracking. The design
of this novel binding screw 20 relies on a dielectric coating
thickness of at least 0.010 inch to provide adequate insulation. It
has been determined that when binding screws constructed in
accordance with this invention are positioned with their heads
touching one another (i.e. having a total insulation thickness of
0.02 inch) they are able to withstand approximately 3,000 volts
therebetween without breakdown of the dielectric. Therefore, it
should be apparent that their use in 277/480 v. AC circuits is well
within acceptable limits. It should also be pointed out that
although these binding screws 20 have been developed to solve a
particular problem--viz grouped AC switches--they will find usage
in a great number of applications where exposed live binding screws
are present.
Although the screwheads 28 may be coated after slotting, this has
been found to be disadvantageous for several reasons. First, the
screwdriver will often cut through the insulation in the slot and
tear out sections of insulation in the slot and on the top surface
of the head. Second, the coated slot generally does not have flat
walls to transmit applied torque from a screwdriver which has a
tendency to slip out of the slot. Third, the resilient insulating
material in the slot presents a plastic "cushion" between the screw
and the screwdriver which results in an uncertain feeling when
tightening the screw. Fourth, the slot does not coat uniformly
giving a poor appearance to the finished article. Finally, there is
no visual means of inspecting for adequate coat thickness. The
novel binding screws 20 illustrated in FIGS. 4 and 5 overcome the
above disadvantages with the base metal material exposed in the
slot thus eliminating the false sense of security which might be
associated with a totally coated head.
The binding screws 20 of this invention are formed by heading a
usual screw blank 34, loading a plurality of blanks 34 in a holding
rack 36 (note FIG. 6), heating the screw blanks and immersing the
heads of the blanks into a trough 28 of suitable insulating
material 40 which may be in the form of an air-agitated fluidized
bed of nylon powder or a liquefied plastic. Holding rack 36 is
moved downwardly to allow only the head of the blank to be immersed
in the insulating material. The immersed heated screwheads melt the
nylon particles or react with the liquefied plastic so that a layer
of plastic insulation is firmly adhered to and is built up upon the
screw blanks. The thickness of the layer is controlled by the
temperature of the blanks as well as the length of immersion time.
The coated screw blank 42 (note FIG. 2) is removed from the holding
rack 36 and may be transferred to a threading apparatus where a
thread 26 is rolled upon its shank in a known manner. It then may
be slotted as at 22 to form the finished article 20 (note FIGS. 4
and 5).
It is believed that the many advantages of this invention will now
be apparent to those skilled in the art. As set forth in the
objects above there is provided an inexpensive binding screw which
may be used in high-voltage circuit AC switches, particularly those
grouped in a common outlet box to prevent arcing between binding
head screws of adjacent switches. Of course, the binding screw will
find numerous applications in other environments.
It should be understood that the present disclosure has been made
only by way of example and that numerous changes in details of
construction and the combination and arrangement of parts may be
resorted to without departing from the true spirit and scope of the
invention as hereinafter claimed.
* * * * *