GB2083199A - An improved gas blow torch - Google Patents

Abstract

In a gas blow torch which is especially suitable for heating heat shrinkable plastics components used in the termination and/or jointing of electric cables and pipes, a simple and inexpensive needle valve comprises a valve seat 3 of tubular form having a circular bore 4 of uniform size throughout its length and an elongate needle 7 of circular cross-section which is axially positioned in the bore of the valve seat to define between the needle and the valves at an annular passage 5 of uniform cross-sectional area. The needle valve includes an axially adjustable shaft 6 which carries the needle 7 and which can be adjusted relative to the valve body 1 to adjust the length of the annular passage 5 and, hence, the amount of gas passing through the needle valve. <IMAGE>

Description

SPECIFICATION An improved gas blowtorch This invention relates to gas blow torches and is especially, but not exclusively, concerned with a gas blow torch which is suitable for heating heat shrinkable plastics components used in the termination and/or jointing of electric cables and pipes.

In a known form of gas blow torch, the flow of gas is controlled by means of a needle valve comprising a tapered needle which engages in a tapered valve seat of complementary shape; the angle of taper can be any angle within the range of a few degrees to 90 , depending on the function that the gas blow torch is to perform. Usually, the tapered needle is carried on a threaded shaft which engages in an internally threaded bore in the body of the valve. An increase in or reduction of the flow of gas in the torch is effected by appropriate axial movement of the tapered needle with respect to the valve seat by rotation of the threaded shaft. When the tapered needle is fully engaged in the valve seat, no flow of gas can take place.As the tapered needle is withdrawn axially from the valve seat by rotation of the threaded shaft, the cross-sectional area of the annular space bounded by the tapered needle and the valve seat gradually increases, thereby permitting a gradual increase in the quantity of gas flowing through the valve. It will be appreciated that the relative shapes of the tapered needle and the tapered valve seat are critical for efficient operation of the needle valve and, as a consequence, the cost of a needle valve constitutes an undesirable large proportion of the overall cost of a gas blow torch. Furthermore, such needle valves are relatively expensive to replace.

It is an object of the present invention to provide an improved gas blow torch having a novel form of needle valve which is simple and inexpensive.

According to the invention we provide, in a gas blow torch, a needle valve comprising a valve seat of tubular form having a bore of substantially uniform cross-sectional shape and size throughout the length of the valve seat, an elongate needle of substantially uniform cross-sectional shape and size throughout substantially the whole of its length, which needle is axially positioned in the bore of the valve seat to define between the needle and the valve seat a passage of substantially uniform cross-sectional area, and means for effecting relative axial movement between the needle and the valve seat to adjust the length of said passage.

Preferably, the bore of the valve seat and the elongate needle are each of substantially circular cross-section and, preferably also, the axes of the bore and the needle are substantially co-linear so that the passage bounded by the needle and the valve seat is of substantially uniform annular cross-section.

The means for effecting relative axial movement between the valve seat and the needle may take any convenient form but, preferably, the valve seat is fixed and means is provided for moving the needle axially with respect to the valve seat. The needle may be carried by an externally threaded shaft which screws into an internally threaded bore in the body of the valve, the position of the needle relative to the valve seat being adjusted by rotation of the shaft, or the shaft carrying the needle may be plain and caused to move axially with respect to the valve seat by a spring loaded lever or other means.

To facilitate assembly of the needle valve, preferably the extreme tip of the needle is tapered. Fluid-tight seals of elastomeric material or other suitable material will be positioned between the shaft and the valve body.

Because of the simple shape of the component parts of the improved needle valve, the valve is simple and inexpensive to manufacture. For example, the valve seat can be made by drilling a circular hole of small diameter in a block of metal. The needle can be a short length of wire, e.g. piano wire, having the same nominal diameter as that of the bit employed to drill the hole in the block of metal, the short length of wire being crimped, soldered or otherwise secured to another block of metal. No precision machining is thus necessary.

The cross-sectionai area of the passage between the needle and the valve seat restricts the flow of gas and remains substantially constant irrespective of the position of the needle relative to the valve seat. To vary gas flow, the needle is moved axially with respect to the valve seat, thus varying the length of the passage between the needle and valve seat. For a constant pressure difference across the valve, the resistance to gas flow is approximately proportional to the length of the passage, i.e. to the needle position relative to the valve seat. With the needle fully engaged, the gas flow is at a minimum but it is not stopped; with the needle fully withdrawn from the valve seat, the gas flow is at a maximum. It will be appreciated that a separate on/off valve is required on the gas blow torch to stop or start the flow of gas in the torch.

In a small gas blow torch, the length and cross-sectional area of the passage of the improved needle valve are designed to offer resistance considerably greater than that of the burner jet of the torch. Suitable values may be Range of Control 10:1 Needle Length 5 millimetres Needle Diameter 0.8 millimetres Minimum Resistance to Gas Flow 0.5 x that of the burnerjet Maximum Resistance to Gas Flow 5 x that of the burnerjet The invention also includes, for use in a gas blow torch, an improved needle valve as hereinbefore described.

The invention is further illustrated by a description, by way of example, of the preferred form of needle valve for use in a gas blow torch, with reference to the accompanying drawing which shows a diagrammatic cross-sectional view of the needle valve drawn on an enlarged scale.

Referring to the drawing, the needle valve has a valve body 1 having a main throughbore 2 extending throughout its length which is stepped at two longitudinally spaced positions along its length to form a valve seat 3 which has a bore 4 of circular cross-section. Positioned in the main throughbore 2 and axially movable with respect to the valve body lisa shaft 6 which, over a part (not shown) of its length, is in screw threaded engagement with an internally screw threaded part of the main throughbore 2 and which carries at its leading end an elongate needle 7 of circular cross-section which protrudes into the bore 4 of the valve seat 3 to form a passage 5 of annular cross-section.At least one fluid-tight seal 8 of elastomeric material is positioned between the shaft 7 and the valve body 1 and opening into the main throughbore 2 between the fluid-tight seal 8 and the bore 4 of the valve seat 3 is a gas inlet 9 which is connected, via an on/off valve (not shown), to a source of gas downstream of the bore 4 of the valve seat 3, the main throughbore 2 opens out to form a seating 10 in which can be secured a burnerjet 11.

To vary gas flow through the burner jet 1 lithe shaft 6 is moved axially with respect to the valve body 1 to cause the needle 7 to move axially with respect to the valve seat 3, thus varying the length of the passage 5 between the needle and the valve seat. With the needle 7 fully engaged, gas flow through the burner jet 11 is at a minimum and can be stopped by operation of the on/off valve; with the needle fully withdrawn from the valve seat 3, gas flow is at a maximum.

Claims (7)

1. A gas blow torch having a needle valve comprising a valve seat of tubular form having a bore of substantially uniform cross-sectional shape and size throughout the length of the valve seat, an elongate needle of substantially uniform cross-sectional shape and size throughout substantially the whole of its length, which needle is axially positioned in the bore of the valve seat to define between the needle and the valve seat a passage of substantially uniform cross-sectional area, and means for effecting relative axial movement between the needle and the valve seat to adjust the length of said passage.

2. A gas blow torch as claimed in Claim 1, wherein the bore of the valve seat and the elongate needle are each of substantially circular cross-section and are arranged with their axes substantially co-linear.

3. A gas blow torch as claimed in Claim 1 or 2, wherein the valve seat is fixed and means is provided for moving the needle axially with respect to the valve seat.

4. A gas blow torch as claimed in Claim 3, wherein the needle is carried by an externally screw threaded shaft which screws into an internally screw threaded bore in the body of the valve, the position of the needle relative to the valve seat being adjusted by rotation of the shaft.

5. A gas blow torch as claimed in Claim 3, wherein the needle is carried on a plain shaft which can be caused to move axially with respect to the valve seat by a spring-loaded lever.

6. A gas blow torch as claimed in any one of the preceding Claims, wherein the extreme tip of the needle is tapered.

7. A gas blow torch having a needle valve substantially as hereinbefore described with reference to and as shown in the accompanying drawing.