PILOT TUBE ASSEMBLY AND METHOD FOR GAS APPLIANCE RANGES

BACKGROUND OF THE INVENTION

This application claims the benefit of provisional application number 60/691,895,

filed June 17, 2005.

1. Field of the Invention

The present invention relates to a pilot tube assembly for gas appliances, including

gas ranges, gas furnaces, gas grills and other gas fired appliances wherein the pilot line

ignites the main gas flame.

2. Description of the Prior Art

Pilot tubes are used in gas appliance ranges and stoves. The pilot tube is an assembly of a gas inlet tube of carbon steel fuel line, having one end attached to a special

fitting called a pilot tip. This attachment needs to be free from gas leakage even when the

pilot tube is subjected to repeated cycles of varying temperature. An assembly comprised of

a pilot tip and a gas inlet tube is commonly referred to as a pilot tube assembly. The pilot

tube assembly provides a continuous flame that is always available for the lighting of a

range burner.

There are over ten major stove and range manufacturers in the US and across the

world, all of which require a pilot tube assembly for safe operation. Stove and range

manufacturers have essentially adapted one of three commonly accepted methods for

manufacture of pilot tube assemblies. Those methods commonly used to manufacture pilot tube assemblies include: a) compression fitting assembly; b) brazing assembly; and c)

deformation assembly. Each of these three prior art assembly methods and their

disadvantages are discussed.

The prior art compression fitting assembly of pilot tubes uses compression fittings.

A threaded pilot tip is connected to a brass compression sleeve having a brass compression

nut carried by the gas inlet tube, as illustrated at 10 in the photographs shown in Fig. 1. The

Fig. 1 photographs show the pilot tube assembly both in the opened-out configuration and

the assembled configuration. The pilot tip is threaded at one end 11. It is connected to the

gas inlet tube using industrial brass compression sleeve 12 and nut 13. The assembled pilot

tube assembly is shown on the photograph to the right. This assembly operation is manual

and time consuming and the machined components, including the threaded pilot tube, are

very expensive. The connection of the gas inlet tube to the pilot tip compresses the

compression sleeve 12 by the nut 13, with the result that misalignment therebetween results

in gas leakage. The gas tube has to be thrown away since it is deformed by the compression

fitting, which is permanently deformed over the gas inlet tube.

The prior art brazing assembly of the pilot tube assembly is illustrated at 20 in the

photographs of Fig. 2. A machined fitting slid over the gas inlet tube is positioned at the end of the pilot tip and the parts are brazed together using a braze filler. The braze filler fills the

gap between the gas inlet tube and the machined fitting as well as the gap between the

machined fitting and the end of the pilot tip. The brazed joint is shown at 21. During

brazing, movement of the parts or non-uniform braze fill in either of the two joint locations,

may result in gas leakage. The machined fitting and the brazing operation of pilot tube

assembly components are inherently expensive, whether the operation is carried out

manually or in an automated fashion.

The prior art deformation assembly of the pilot tube assembly is illustrated at 30 in

the photographs of Fig. 3. A loose fitting pilot tip fitting 31 is placed on the end of a pilot

tip. Using a chisel-type devise, the pilot tip is deformed onto the gas inlet tube. The left

photograph shows the loose pilot tip fitting 31 slid over the gas inlet tube 32. The right

photograph shows the deformed pilot tip creating a pilot tube assembly. The deformation

location of the pilot tip is shown at 33. This deformation process tends to be unreliable.

Manufacturers using this assembly method report that a high percentage of fittings become

loose and fall off, causing concern in the gas range industry.

US Patent No. 4,854,855 to Rajewski discloses a flare igniter assembly. The flare

igniter assembly comprises an elongated tubing assembly including a first air supply

conduit and a fuel gas supply line enclosed inside the first air supply conduit. The fuel gas

supply line is provided with a member having a venturi-type orifice through which fuel gas

and intake air pass. The orifice effects the initial mixing of fuel gas and air. The fuel gas

supply line further includes a combustion zone and means for transferring a fuel gas-air

mixture downstream from the orifice to the combustion zone. The combustion zone includes an inspirating baffle member firmly positioned within the fuel gas supply line. The

terminal portion of the fuel gas supply line extends into the second air supply conduit,

within which an ignited flame is burning. The second air supply conduit is adjacent to a

porthole in the flare stack, near the upper end thereof, to ignite combustible waste gases

passing through the flare stack. The disclosed flare igniter assembly is a waste gas igniter.

The construction method of the igniter involves the use of a venturi in the gas line to

aspirate air from the second air tube, which surrounds the gas tube. The air is not aspirated

from the ambient and there is no pilot tip in this device.

US Patent# 6,814,570 to Zink, et al. discloses a venturi mixer and combustion

assembly. The combustion assembly includes a venturi mixer and a combustion tip, which is attached to a discharge end section of the venturi mixer. The discharge end section of the

venturi mixer is provided with a divergent interior wall. Because of the reduction in

pressure drop at the venturi exit, the device is able to induce more combustion air. Thus, the

premix gas burner tip or pilot tip / burner is capable of operating over a wider air to fuel

ratio range. Further, the flame propagation speed increases as a result of the induction of

more combustion air with increased flame stability. The gas inlet tube has an orifice and the

venturi mixer is placed in front of the gas inlet tube. It is not bonded to the gas inlet tube in

any manner. The gap between the gas exit tube with the orifice and the venturi mixer is

critical and is not controlled.

US Patent No. 5,716,003 to Streetman discloses a jet pilot tip. The jet pilot tip is adapted for use in an in-line connection of a fuel supply. The jet pilot tip includes a

body with a threaded bore, which is capped during use, and an open surface with a side

conduit member connected to a fuel supply line. The cap is removed and a pin is inserted to

clean the nozzle of the jet pilot tip. Gas is injected out of the jet pilot tip into the ambient.

The pilot tube does not mix gas and air to create a readily combustible fuel air mixture.

There remains a need in the art for a pilot tube assembly that functions reliably in gas appliances. Also needed is a manufacturing process for producing a low cost pilot tube

assembly in an automated assembly operation. Gas leaks in the pilot tubes are undesirable

and result in safety concerns. Since the pilot tube is always turned on and the main gas

flame impinges on the pilot tube, its construction must repeatedly withstand severe thermal

cycles without failure.

SUMMARY OF THE INVENTION

The present invention discloses a pilot tube assembly that is economical to

manufacture and provides an extremely reliable joint between the pilot tip and the gas inlet

tube. The pilot tube assembly does not require compression fittings, brazing operations, or

operations that deform the pilot tip, which make conventional pilot tube assemblies less

reliable to manufacture and shorten their service live.

The pilot tip has a central aperture to deliver gas and later apertures to aspirate

ambient air. The gas stream and air stream are mixed and delivered to the tip of the pilot

tube assemble to create a continuous stable pilot flame. The pilot tip is provided with a taper

in a central bore that is intended to join with the gas inlet tube. The diameter of the tapered

bore at the gas inlet tube entry point is slightly larger than that of the gas inlet tube's outer

diameter. The bore entry point may be as large as 1.005 to 1.05 times the outer diameter of

the gas inlet tube. The assembly machine forces the pilot tip onto the gas inlet tube and the

two pieces swage-lock together. This intimate connection between the pilot tip and the gas

tube inlet has several key embodiments. For this swaging process to occur, the taper angle

has to be selected within a narrow range, in order to be operable within a certain pressure

range applied by the assembly machine. The gas inlet tube deforms to match the taper that is

present in the pilot tip. A smaller taper angle allows this swaging operation to occur at a lower pressure, provides large contact area between the pilot tip interior surface and the gas

inlet outer surface. The gas inlet tube also travels a larger distance into the pilot tip. If the angle is larger, the reverse holds true. When the angle exceeds a certain value, it is

practically impossible to force the gas inlet tube into the tapered bore of the pilot tip since

plastic deformation of the gas inlet tube outer surface is incapable of accommodating the

deformation required. Experiments with various taper bore geometry of the pilot tip have

resulted in the selection of a rather narrow range of taper angles that result in the reliable

swaging operation. A taper angle of 1 to 2 degrees from the centerline, preferably 1.25 to

1.75 degrees, and more preferably 1.5 degree, provides optimum swage lock bonding.

The selection of the angle of taper affects the swaged part in several key aspects.

First, it determines the distance that the gas inlet tube penetrates into the pilot tip. The

assembly machine has to be designed to provide the force over this displacement distance

for reliable swaging. Secondly, the angle also determines the contact area between the gas

inlet tube and the pilot tip. Since the main gas flame impinges on the pilot tip, its

temperature rises causing the pilot tip to expand. If the gas tube is not in intimate contact

with the pilot tip, it has a lower temperature than the pilot tip, causing the loosening of the

joint between the pilot tip and the gas inlet line. Intimate contact between the two

components results in smooth heat flow, keeping their thermal expansions similarly

matched, thereby maintaining intimate contact even when the assembly is thermally cycled.

The pilot tip may be cast and machined to required dimensions and this operation is

conducted at a minimal cost since no threading is needed. The assembly machine uses a

standard gas inlet tube and swages the two components together reliably by the application

of selected pressure and distance of movement using an air cylinder ram. The assembly

process allows for semi-automatic assembly and greatly reduces labor costs.

Significant advantages are realized by practice of the present invention. Key features

of the pilot tube and method therefor include, in combination, the components set forth below:

1) a pilot tip has apertures for aspirating air attached to a gas inlet line providing a

gas air mixture that permits reliable combustion to produce a continuous flame capable of

igniting a main gas flame in a reliable manner, while sustaining multiple thermal duty

cycles;

2) the pilot tip is provided with a tapered inlet end to receive the gas inlet tube;

3) the taper is in die range of 1 to 2 degrees from the centerline, preferably 1.25 to

1.75 degrees, and more preferably 1.5 degrees;

4) the gas inlet tube is forced into the tapered bore of the pilot tip by the application of pressure from an air cylinder ram in an assembly machine, thereby swaging, deforming

and mechanically locking the gas inlet tube into the pilot tip by deformation of the outer surface of the gas flow tube and thereby producing the pilot tube assembly; and

5) the intimate contact between the gas flow tube's outer surface and the pilot tip's inner surface transmits heat from the main gas flame, maintaining approximately equal thermal expansion of the pilot tip and gas inlet tube, thereby permitting multiple thermal

cycles of the pilot tube assembly; whereby the pilot tube assembly is produced in an assembly line operation with extremely high reliability at reduced manufacturing costs, producing a gas tight seal resistant to repeated thermal cycling.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description of the preferred embodiments of the invention and the accompanying drawing, in which:

FIG. 1 is a photograph depicting a prior art compression fitting assembly of a pilot tube;

FIG. 2 is a photograph showing a prior art brazing assembly of a pilot tube;

FIG. 3 is a photograph illustrating a prior art deformation assembly of a pilot tube;

FIG. 4 is an illustration of a pilot tip according to the present invention showing the taper provided to receive the gas inlet tube;

FIG. 5 is a photograph depicting a pilot tip having a taper designed to receive the gas inlet tube; and

FIG. 6 is a photograph showing details of an assembly machine, which inserts a bent gas inlet tube into the pilot tip using a pair of jaw grips.

DETAILED DESCRIPTION OF THE INVENTION

Pilot tubes are used in gas appliance ranges, stoves and industrial furnaces. The pilot tube is an assembly comprising a gas inlet tube, typically from a carbon steel fuel line, which is attached on one end to a special fitting called a pilot tip. The pilot tip has several openings on the side to aspirate ambient air. A high velocity incoming gas stream is mixed with aspirated air, providing a ready to combust fuel air mixture. The well-mixed combustion mixture is delivered at the end or tip of the pilot tip, producing a continuous flame. Once the main flame is ignited, it is in close proximity with the pilot tip and is subject to heating. When the main flame extinguishes, the pilot tip returns to its original temperature. The pilot tube assembly is subjected to large temperature variations. Moreover, the attachment between the pilot tip and the gas inlet tube must be maintained as a gas leak- tight and mechanically secure device. Any gas leakage, or in the worst case, separation of the tip from the assembly, results in the release of raw gas into the ambient, which is undesirable and can cause safety concerns.

These problems, inherent to conventional pilot tube assemblies, have been overcome by the pilot tube assembly and method of the present invention. It has been found particularly advantageous in this respect, to provide an assembly wherein a pilot tip has a

taped bore which receives a standard gas inlet tube, which may be straight or bent. The gas

inlet tube is forced and swaged into the pilot tip by an assembly machine, producing

mechanical interlocking thereof. The assembly machine is provided with a pair of grips,

which hold and insert a bent or straight gas inlet tube into a pilot tip.

The pressure applied causes a swaging action, and depends greatly on the angle of

taper provided in the pilot tip. In general, the larger the angle, the higher is the pressure

needed to create the swaging action and the contact area between the pilot tip and the gas

inlet tube. If the angle is too large, the expansion of the pilot tip may result in its breakage.

We have found that there is a very narrow range of angles within which the swaging effect

occurs, creating a reliable bond between the pilot tip and the gas inlet tube. Advantageously,

the joint thus created is immune to thermal cycles to which the pilot tube assembly is

subjected throughout its service life. The angle of the pilot tip is 1 to 2 degrees, preferably

1.25 to 1.75 degrees and more preferably 1.5 degrees. The tolerance of this angle is

typically +/- 0.25 degrees. The gas inlet tube is typically of standard dimensions and is cylindrical. No

preparation is needed to prepare this tube for insertion, other than removal of burrs and cutting irregularities. Generally, the dimension of the pilot tip at the insertion point is

slightly larger (about 1.005 to 1.05 times larger) than the outer diameter of the gas inlet

tube. This permits an easy insertion of the tube. Upon application of pressure using the

assembly machine, the outer surface of the gas inlet tube deforms to match the taper and

creates a swaged, mechanical bond. The surface area of contact between the pilot tip inner

surface and the outer surface of the gas inlet tube is sufficiently large at these selected

angles that heat is readily conducted away from the pilot tip to the gas inlet tube. Therefore,

the temperature of the pilot tip and the gas inlet tube are approximately the same. The tip

and tube thermally expand in a similar manner, and the joint is less likely to be

compromised. This is the reason why the joint can sustain repeated thermal cycling without

damaging the components of the pilot tube assembly.

The assembly machine inserts the pilot tip and brings forth the gas inlet tube using a

pair of jaw grips. An air cylinder applies pressure to the pilot tip and forces the gas inlet

tube into the pilot tip, thereby creating the pilot tube assembly.

Figure 4 is an engineering drawing illustrating the pilot tip with the taper provided to

receive the gas inlet tube. The taper is shown to have the most preferable angle of 1.5

degrees +/- 0.25 degrees. This angle is between the inner surface of the taper in the pilot

and its centerline. Gas flow occurs through the central aperture in the pilot tip, causing air to be aspirated from the lateral apertures.

In Figure 5 there is shown a photograph of the pilot tip with the taper provided to

receive the gas inlet line. The angle between the sidewalls of the taper is 3 degrees, as

shown.

Figure 6 shows a photograph of the assembly machine 60 during operation. The

pilot tip 61 is brought into the machine with the tapered side facing the right side of the

photograph. The gas inlet tube 62, which is bent, is held by grips 63. The air cylinder ram

64 applies pressure to insert the gas inlet tube 62 into the taper of the pilot tip 61, swaging

the pilot tube assembly.

A number of tests are carried out to evaluate the leak tightness of the pilot tube

assembly and its structural integrity, especially after repeated thermal cycling.

Leak test - A pilot tube assembly was produced and the holes for air inlet and gas

outlet were plugged. The gas inlet tube was connected to air pressure. The assembly was

immersed in water to look for any bubbles. The goal of this test is to exceed 30 psi,

approximating maximum values encountered in a gas line. The pilot tube assembly

sustained 100 psi without any bubbles while immersed in water as shown in Table 1 below.

Table 1

Pull Test - The pilot tube assembly was inserted into a universal testing machine

which is capable of applying tensile loading. The load at which the inserted gas inlet tube

separates from the pilot tip was measured. The goal was to exceed 25 pounds of pull force.

The actual separation value was 118 pounds of tensile load as shown in Table 2 below.

Table 2

Heat Cycle Test - The pilot tube assembly was heated in an oven set at a

temperature of 700 degrees F. The assembly was taken out of the furnace and cooled to

ambient temperature and the pressure leak was tested at 25 psi. The cycle was repeated 100

times. The goal was to pass the pressure leak test for 100 cycles. The results are shown in

Table 3 below.

Table 3

Yield of present invention as compared to prior art methods - Table 4, shown

below, documents the yield comparison between various manufacturing methods heretofore

disclosed and utilized for the pilot tube assembly and the pilot tip taper assembly of the

present invention.

Table 4

Having thus described the invention in rather full detail, it will be understood that

such detail need not be strictly adhered to, but that additional changes and modifications

may suggest themselves to one skilled in the art, all falling within the scope of the invention

as defined by the subjoined claims.