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.

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.

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.