GB2097896A - A valve in a paint spray gun - Google Patents

Abstract

A needle valve and retaining nut assembly for controlling the flow of air to the horns of a paint spray gun comprises an annular retaining nut 12 which threadedly engages the gun body and has an internally threaded portion of its internal bore, and a needle valve member 18A formed in plastics material with an integral collar 18B which, on the side of the collar opposed to the valve member, adjoins a tang member 18E. A control knob 20 in its normal range of travel threadedly engages the retaining nut and is formed at its front end with a sleeve portion 22 in which the tank member is a push fit. The arrangement is such that when the control knob 20 is turned in the valve opening direction to beyond its normal range of travel so that the thread becomes disengaged from the nut 12 the collar 18B retains the needle valve member 18A', and control knob 20 captive in the gun. Also provided in the gun body is a cup shroud threadedly engaged with the gun body and arranged at its lower end for attachment of a paint cup. Gas under pressure flows from the gun body through a transfer chamber and a control valve controls the flow of pressurised gas from the transfer chamber to the paint cup. When it is open, the control valve communicates with a pressure relief valve so that gas cannot be delivered to the paint cup beyond its safe working pressure. <IMAGE>

Description

SPECIFICATION Improvements in paint spray guns The present invention relates to spray guns for paint.

In a paint spray gun there is provided a spreader control valve which has the function of controlling the air supply to the horns of the air cap attached to the front of the gun. It is known to arrange this valve so that the front of the gun is not cluttered and the controls are grouped at the back. It is also known to make the valve as a detachable assembly so that it can be removed for maintenance or cleaning. In general, this part of the spray gun can be quite expensive to manufacture and the relevant sections are constrained from being accidentally unscrewed by use of circlips orE' clips. But for the less expensive spray guns, the needle valve is usually made from a plastics rod. The circlip must then be attached to the actuation knob which is extended through the body piece, as the unscrewing action of valve adjustment could break the plastic.Even lower cost valves comprise a plastic needle pressed into the end of a knob which screws directly into, and out of, the gun body and no restraint is fitted. It has been found that at operating pressures, such a valve can be inadvertently unscrewed out of thread engagement and be projected upwards of 20 feet with possible injury to the gun user or to a passerby. One object of the invention is to provide a detachable spreader control valve assembly which is inexpensive and safe to use.

In one aspect the invention provides a needle valve and retaining nut assembly for controlling the flow of air to the horns of a paint spray gun comprising: an annular retaining nut which threadedly engages the valve body and has an internally threaded portion of its internal bore; a needle valve member formed in plastics material with an integral collar which extends outward to prevent passage of the valve member through the retaining nut and on the side of the collar opposed to the valve member terminates in a tang member; and a control knob which in its normal range of travel threadedly engages the retaining nut and is formed at its front end with a sleeve portion in which the tang member is a push fit the arrangement being such that when the control valve is turned in the valve opening direction to beyond its normal range of travel so that the thread becomes disengaged the collar retains the needle valve member and control knob captive in the gun.

In another aspect the invention is concerned with a pressure relief safety valve for use on pressure feed guns fitted with a cup which can withstand only limited pressure. It is common to find that simple spray guns where the air supply is used to pressurise the paint, as well as to atomise and direct it, are limited in their maximum safe working pressure. Such spray guns are commonly supplied with small compressors of limited output and are themselvesl of a type known as a "bleeder" gun where the air fiows through the gun at all times. In use, the pressure reached rarely exceeds 25 PSI in the gun when connected to the compressor supplied.However, once the gun is sold, it is not possible for the manufacturer to control its use and even though the pressure limit is clearly marked on all guns of this type, a user could connect the gun to factory air line with potentially disastrous results. Also, it is not uncommon to supply such a gun with a compressing outfit which can develop over 100 PSI. A regulator is fitted to the compressor but its effectiveness cannot be guaranteed. We have designed a simple pressure relief valve which prevents over-pressuring of the paint cup and in a preferred aspect consists of a steel ball held in place by a leaf spring.

In a second aspect the invention provides a paint spray gun including a gun body, a cup shroud threadedly engaged with the gun body and arranged at its lower end for attachment of a paint cup, means defining a transfer chamber with said cup shroud in which gas under pressure can flow from the gun body and a control valve for controlling the flow of pressurised gas from the transfer chamber to the paint cup, the control valve when open communicating with a pressure relief valve so that gas cannot be delivered to the paint cup beyond its safe working pressure.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a side view of a spray gun showing also a shroud for a paint cup having a suction/pressure control valve; Figures 2a and 2b are diagrams of a spreader valve and associated bushing in the closed and open positions respectively; Figure 3 is a view of the front end of the spray gun in a vertical section; Figure 4 is a plan view of the gun; Figure 5 is a section of the suction/pressure control valve.

The captive spreader control valve In Figures 1, 2a and 2b a spreader valve 10 screws into an internally threaded annular nut 12 in a spray gun body 14 and is operable to control the air supply to the horns 16 of an air cap attached to the front of the gun. Thus when the spreader valve is closed the gun delivers a round spray but when it is open the gun delivers a fan spray. The spreader valve is, as seen in Figures 2a and 2b, a needle valve which fits into the body of the gun with a control knob at its rear end which is grouped at the back of the gun near the other controls.

The valve needle 1 8 is a one piece moulding in nylon, delrin or similiar material which is pressed into the end of a metal or plastics control knob 20 having a threaded portion 22 which engages the nut 12. The needle includes a shank 1 8A separated by a collar 1 8B from a piston section 1 8C which is gas-tightly sealed to a smooth front bore portion of the nut 12 by means of an '0' ring seal 18D. The piston section 1 8C is continued to define a relatively small diameter connecting tang 1 8E which locates in a sleeved front end of the knob 20. The collar 1 8B is of radial dimensions such that it will not pass through the nut 12.It will be noted from Fiqure 2b that the position of the collar 1 8B and the length of the thread 22 are such that the thread 22 disengages from the nut 12 before the collar 1 8B encounters it so that no separation force is exerted on the joint between the valve and the needle as the needle is unscrewed.

In use the threaded portion 22 is normally engaged with the nut 12 as shown in Figure 2a.

But if the valve is accidentally opened too far, air pressure forces the needle outwardly until the collar 1 8B encounters the inner end of the nut 12 after which further movement is prevented. The needle i 8 remains captive in the nut 12 and the '0' ring 1 8D continues to maintain a fluid-tight seal so that the 'blow-out' incident which was a danger to be guarded against in earlier designs of gun cannot take place.

The pressure relief safety valve In Figures 1,3 and 5, a pressure die-cast paint cup shroud 25 screws to the gun body 14 and includes an annular bushing piece 27 which locates a plastics paint delivery tube 29. A bushing 31 forms an air-tight seal between the lower end of the bushing piece 27 and the shroud 25 to define an annular transfer passage 33 which is in gas flow communication with the appropriate part of the gun body via a way 37. Gas flows between the transfer passage and the paint cup via a suction pressure control valve 39 more clearly seen in Figure 5. Thus depending on the position of a gate member 41 controlled by screw 42 a port 45 leading to the paint cup may be communicated either with a port 43 leading to the transfer passage 33 or with an air inlet port 47 for operation of the paint cup in pressure or in suction respectively. The port 43 also communicates via a passage with a discharge port 49 normally closed off by a ball 50 held in place by a spring 51 of strength such that the ball will rise to open the valve when exposed to a pressure in excess of a predetermined safe working limit, typically 50 PSI.

This pressure relief valve is free from the effects of paint overspray and lack of cleanliness because of its location in the gun and because of the freefloating nature of the leaf spring 51 which is formed integrally to a washer so that it may be held to the body of the shroud 25 by the valve 39 which is a screw fit therein. On overpressuring of the gun the air escaping from the valve will whistle and this will attract the user's attention to the pressure overload condition.

Claims (9)

1. A needle valve and retaining nut assembly for controlling the flow of air to the horns of a paint spray gun comprising: an annular retaining nut which threadedly engages the valve body and has an internally threaded portion of its internal bore; a needle valve member formed in plastics material which an integral collar which extends outward to prevent passage of the valve member through the retaining nut and on the side of the collar opposed to the valve member terminates in a tang member; and a control knob which in its normal range of travel threadedly engages the retaining nut and is formed at its front end with a sleeve portion in which the tang member is a push fit the arrangement being such that when the control valve is turned in the valve opening direction to beyond its normal range of travel so that the thread becomes disengaged the collar retains the needle valve member and control knob captive in the gun.

2. A needle valve and retaining nut assembly according to claim 1 , wherein the internal bore of the annular retaining nut is threaded only at its outer end but its inner end defines a smooth bore, and the needle valve member is formed with an integral piston member between the collar and the tang member which slides in the bore.

3. A needle valve and retaining nut assembly according to claim 2, wherein the piston member is recessed to receive an '0' ring arranged to maintain a gas-tight seal between the piston member and the retaining nut both during the normal range of travel of the needle valve member and at the position where outward travel of the needle valve member is restrained by the collar.

4. A captive needle valve and retaining nut assembly for controlling the flow of air to the horns of a paint spray gun substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

5. A paint spray gun including a gun body, a cup shroud threadedly engaged with the gun body and arranged at its lower end for attachment of a paint cup, means for defining a transfer chamber with said cup shroud in which gas under pressure can flow from the gun body and a control valve for controlling the flow of pressurised gas from the transfer chamber to the paint cup, the control valve when open communicating with a pressure relief valve so that gas cannot be delivered to the paint cup beyond its safe working pressure.

6. A paint spray gun according to claim 5, wherein the pressure relief valve comprises a port closed off by an external ball held in place by a leaf spring supported at one end.

7. A paint spray gun according to claim 6, wherein the control valve is threadedly engaged in the cup shroud and the leaf spring is defined by a tag portion of a washer which fits between the control valve and the shroud.

8. A paint spray gun according to claim 7, wherein the control valve is switchable between a pressure position and a suction position and includes a gas inlet passage for admission of air from the surroundings when in the suction position.

9. A paint spray gun including a pressure relief valve for the paint cup substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.