GB2076600A

GB2076600A - Electrostatic Spray Gun

Info

Publication number: GB2076600A
Application number: GB8114887A
Authority: GB
Original assignee: Graco Inc
Current assignee: Graco Inc
Priority date: 1980-05-19
Filing date: 1981-05-15
Publication date: 1981-12-02
Also published as: FR2482480A1; DE3119441A1; JPS5710365A; CA1163796A

Abstract

An electrostatic spray gun comprises: a body (46, 48), a high voltage cable (30) extending into the body and an electrically insulating sleeve (44) disposed in a passage in the body, the inside of the sleeve receiving a resistor (32) and an end of said high voltage cable. The sleeve has an external annular groove (58) adjacent the end of the sleeve enclosing the resistor, and an internal annular groove (60) adjacent the end of the sleeve enclosing the end of the high voltage cable. An O-ring (55, 57) is fitted in each of the annular grooves, one O-ring being in air sealing relation between the sleeve and said passage and the other O-ring in air sealing relation between said sleeve and said high voltage cable. <IMAGE>

Description

SPECIFICATION Electrostatic Spray Gun This invention relates to electrostatic spray guns.

Electrostatic spray guns utilised in the painting industry create an ionizing zone adjacent a paint spray tip, the ionizing zone being created by electrostatic high voltages delivered by means of high voltage cable attached to the spray gun and electrical conductors embedded in a spray gun body.

It has been customary to deliver high voltage to an electrostatic spray gun by a heavy cable having a conductor surrounded by suitable dielectric plastics material having good electrical insulation properties, and enclosing the plastics material with a metallic grounding sheath. The cable is typically connected to a metal portion of a spray gun handie and good electrical contact is made between a metallic grounding sheath of the cable and the handle itself to ensure that the handle will be held at ground potential. The central portion of the cable, consisting of the plastics material and the conductor is typically extended a relatively considerable distance into the spray gun body, and the conductor is brought into electrical contact with a resistor having a relatively large resistance value located in the spray gun body.The other end of the resistor is typically electrically connected by means of various contacting mechanisms to an ionizing needle which projects forward of the spray gun. A typical electrostatic spray gun of this type is disclosed in U.S. Patent Specification No. 4 1 82 490.

The portion of the conductor extending into the spray gun body is usually quite long, on the order of 1 5-30 cm (6-12 inches), so as to space the exposed conductor as far as possible from the metal portion of the handle to prevent electrical arcing. As a further defence against arcing it is common to enclose snugly the cable inside a plastics sleeve insulator which is also inserted into the spray gun body. This reduces the air gap surrounding the cable and further lessens the tendency for electrical arcing. As a further precaution against arcing it is common to pack the inside of the sleeve with a high dielectric grease material to attempt to fill all air voids in the space between the sleeve and the conductor it encloses. Such an approach is disclosed in U.S.

Patent Specification No. 3 81 5 820 and U.S.

Patent Specification No. 3 459 374. It has therefore been well recognised that potentially serious electrical arcing problems can occur inside the body of an electrostatic spray gun between any exposed conductor and the grounded metal handle of the spray gun.

Since the construction of any high voltage electrostatic spray gun necessarily involves the utilization of components having inherent capacitance, there is the further problem of protecting against voltage discharge between capacitive elements and grounded elements, or between capacitive elements and any conductive path within the spray gun at a different voltage potential. Any air gap within the spray gun body represents a potential discharge path, and it is therefore desirable to reduce as far as is practicable all such air gaps. However, since such spray guns necessarily are constructed from components which are replaceable, and replacement must necessarily require some dimensional tolerances between components and the spray gun body, it is impossible to completely eliminate all air gaps within the spray gun and its internal components.Various approaches, some of which have been described above, have been utilized in an attempt to solve these problems.

The present invention seeks to provide a simple solution to many of the problems described above, whilst permitting replaceable components to be utilized inside a spray gun body without requiring critical dimensional tolerances to ensure tight fits between the respective components, and without requiring the use of messy greases and other materials to attempt to pack the voids within the spray gun body.

According to one aspect of the present invention there is provided an electrostatic spray gun comprising: a body: a high voltage cable extending into the body and having an electrical contact between a conductor of the cable and an electrical component; an insulating sleeve within the body and concentrically positioned around said cable and said electrical component, said sleeve having an internal annular groove adjacent one end and an external annular groove adjacent the other end; an O-ring seated in each of said annular grooves, said O-rings being sized for air sealing fit between said sleeve and the respective concentric adjacent component.

According to another aspect of the present invention there is provided an electrostatic spray gun comprising: a body: a high voltage cable extending into the body; an electrically insulating sleeve disposed in a passage in said body, the inside of said sleeve receiving a resistor and an end of said high voltage cable; said sleeve having an external annular groove adjacent the end of the sleeve enclosing said resistor, and an internal annular groove adjacent the end of the sleeve enclosing said end of the high voltage cable; an O-ring fitted in each of said annular grooves, one O-ring being in air sealing relation between said sleeve and said passage, and said other O-ring in air sealing relation between said sleeve and said high voltage cable.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 shows a side cross-sectional view of an electrostatic spray gun according to the present invention; and Figure 2 shows an expanded partial crosssectional view of a portion of Figure 1.

Referring first to Figure 1, there is shown an electrostatic spray gun 10 according to the present invention of the type generally known as an "automatic" spray gun. Spray guns may be generally classified as either "automatic" or "manual". Automatic spray guns are intended for affixing to a support mechanism adjacent a conveyor line and are automatically actuated by remote control for applying spray in sequence with the arrival of articles along the conveyor line.

Manual spray guns typically have a handle for grasping by an individual, and a built-in trigger mechanism for actuating the spray gun under manual operation. The spray gun 10, being an automatic spray gun, is actuated by means of air pressure applied from a remote source (not shown) through an inlet 31 to actuate an air cylinder 28 and thereby to retract a valve rod 26 against the action of a spring 21 located in a chamber 16. The retraction of the valve rod 26 causes a paint valve 20 to lift from its seat, and thereby to open a passage 22 to the flow of paint via an inlet 12 and a passage 14. Paint is thus emitted from the spray gun through a nozzle 24.

A high voltage cable 30 is attached to spray gun 10 by means of a suitable cable connector 33 in a metal body portion 46 of the spray gun. The cable 30 has a braided metal layer (not shown) which serves as a grounding sheath, and which is clamped between the body- portion 46 and the connector 33 in the conventional manner. The cable 30 has an inner dielectric insulator 42 which encloses a high voltage conductor 43 and extends into the body portion 46. A body portion 48, preferably made from insulating plastics material such as, for example, nylon, is threadably clamped against the body portion 46 by means of a locknut 50. A passage in the body portion 48 is in alignment with a similar passage in the body portion 46 when the locknut 50 is attached.A plastics tube or sleeve 44 extends through the aligned passages in body portions 46, 48 and encloses the insulator 42 and a resistor 32 of relatively high resistance value. The resistor 32 has conductive end contacts 52, 54. The conductor 43 electrically contacts the contact 52 of the resistor and a conductive member 34 electrically contact the contact 54 of the resistor.

A threaded conductor segment 36 contacts the conductive member and a needle 40, which passes through the nozzle 24 to make the electrical contact.

The sleeve 44 has an external annular groove 58 (Figure 2) adjacent one end thereof, and an internal annular groove 60 adjacent the other end thereof. The annular grooves 58, 60 accept respective O-rings 55, 57 of a resilient material such as rubber. For example, the O-ring 55 is a rubber O-ring sized to fit snugly in the groove 58, and the O-ring 57 is a rubber O-ring sized to fit within the groove 60. Since the O-ring 55 fits over the outside of the sleeve 44 and the O-ring 57 fits along the inside of the sleeve 44, the two O-rings are not of identical sizes. For example, in a preferred embodiment, the inside diameter of the passage drilled through the body portions 46, 48 is 1.42 cm (0.56 inches). The sleeve 44 has an outside diameter of 1.37 cm (0.54 inches) and in inside diameter of 0.79 cm (0.31 inches).The grooves 58, 60 are machined to a depth of about 0.010 cm (0.004 inches) and a width of about 0.23 cm (0.09 inches). The O-ring 55 has a nominal inside diameter of 1.1 cm (0.4375 inch) and an outside diameter of 1.43 cm (0.5625 inch); while the O-ring 57 has a nominal inside diameter of 0.64 cm (0.25 inch) and an outside diameter of 0.95 cm (0.375 inch). The foregoing selection of sizes provides optimum compression of the O-rings when placed in position, and ensures an adequate air seal between the respective sides of the O-rings.

In operation, when high voltage of the order of 50-100 kilovolts is applied to the cable 30, a corresponding high voltage potential is found at contacts 52, 54. The high voltage potential at the contact 54 is effectively isolated from potential discharge along the air gap between the sleeve 44 and the body portion 48 by means of the 0rings 55. The high voltage potential at the contact 52 is effectively insulated from a potential electric discharge through the air space along the inside of the sleeve 44 and the insulator 42 by means of the O-ring 57. In both cases, the potential electrostatic discharge could otherwise occur between the high voltage point and metal body portion 46, which is nominally at ground potential. However in both cases the discharge path is eliminated because of the effective air seal provided by the respective O-rings.

Claims

1. An electrostatic spray gun comprising: a body: a high voltage cable extending into the body and having an electrical contact between a conductor of the cable and an electrical component; an insulating sleeve within the body and concentrically positioned around said cable and said electrical component, said sleeve having an internal annular groove adjacent one end and an external annular groove adjacent the other end; an O-ring seated in each of said annular grooves, said O-rings being sized for air sealing fit between said sleeve and the respective concentric adjacent component.

2. An electrostatic spray gun as claimed in claim 1 in which said external annular groove is disposed adjacent said electrical component.

3. An electrostatic spray gun comprising: a body; a high voltage cable extending into the body: an electrically insulating sleeve disposed in a passage in said body, the inside of said sleeve receiving a resistor and an end of said high voltage cable, said sleeve having an external annular groove adjacent the end of the sleeve enclosing said resistor, and an internal annular groove adjacent the end of the sleeve enclosing said end of the high voltage cable; and an O-ring fitted in each of said annular grooves, one O-ring being in air sealing relation between said sleeve and said passage, and said other O-ring in air sealing relation between said sleeve and said high voltage cable.

4. An electrostatic spray gun as claimed in any preceding claim in which said O-rings are made of a resilient and compressible material.

5. An electrostatic spray gun substantially as herein described with reference to and as shown in the accompanying drawings.

6. In an electrostatic spray gun of the type having a high voltage cable connected to the gun and having an internal electrical contact point between the cable conductor and another electrical component; the improvement comprising: an insulating sleeve concentrically positioned around said internal cable and said other electrical component, said sleeve having an internal annular groove proximate one end and an external annular groove proximate the other end; an O-ring seated in each of said respective annular grooves, said O-rings sized for air sealing fit between said sleeve and the respective concentric adjacent component.

7. An electrostatic spray gun having a high voltage cable attached to the spray gun and having internal electrical connections to couple said cable to a forward electrode, comprising: (a) a passage in said spray gun between said high voltage cable attachment and said electrode; (b) an electrically insulating sleeve in said passage, the inside of said sleeve adapted to accept a resistance and an end of said high voltage cable; said sleeve having an external annular groove proximate the sleeve end enclosing said resistance, and an internal annular groove proximate the sleeve end enclosing said high voltage cable; (c) an O-ring fitted in each of said annular grooves, said one O-ring in air sealing relation between said sleeve and said passage, and said other O-ring in air sealing relation between said sleeve and said high voltage cable.