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Title:
TEMPERATURE OR PRESSURE SENSING ELEMENT
Document Type and Number:
WIPO Patent Application WO/1983/002157
Kind Code:
A1
Abstract:
A temperature or pressure sensing element of the kind having an expansible capsule or bellows (1) connected a capillary tube (2) by way of an hermetic seal is fabricated by sealing the capillary tube (2), which may be of aluminium, stainless steel, brass or copper, to the bellows or capsule (1) with a synthetic resin bonded joint (14). The resin joint also serves to connect the capillary tube (2) to a rigid support plate (9).

Inventors:
Nelson
Charles
Frederick, Sleeman
Alan
Digory
Application Number:
PCT/GB1982/000345
Publication Date:
June 23, 1983
Filing Date:
December 07, 1982
Export Citation:
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Assignee:
RANCO CONTROLS LIMITED NELSON
Charles
Frederick, Sleeman
Alan
Digory
International Classes:
G01L7/12; G01K5/32; G01K5/42; G01L7/10; G12B1/04; H01H35/32; H01H37/38; (IPC1-7): G01K5/32; H01H37/36; G12B1/04
Foreign References:
DE2320141A11974-01-17
FR2480938A31981-10-23
GB2057686A1981-04-01
DE2414813A11975-10-09
US3654578A1972-04-04
US2698766A1955-01-04
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Claims:
CLAIMS
1. A temperature or pressure sensing element comprising an expansible capsule or bellows device (1) connected to a capillary tube (2) in which the capsule or bellows devic (1) is attached to a rigid support plate (9) havingan aperture through which the capillary tube (2) passes, the capillary tube (2) being secured to a surrounding part of the support plate (9) and being hermetically sealed to the capsule or bellows device (1) , characterised in that the capillary tube (2) is bonded to the support plate (9) and sealed to the capsule or bellows device (1) by a synthetic resinbonded joint (14).
2. A sensing element according to Claim 1, in which the capillary tube (2) is fabricated in aluminium, stainless steel, brass or copper.
3. A sensing element according to Claim 1.or Claim 2, in which the capillary tube (2) is bonded to the capsule or bellows device (1) by an *.epoxy resin joint (14).
4. A sensing element according to Claim 1, Claim 2 or Claim 3, characterised in that the resiπboπded joint (14) is reinforced by a metal sleeve (15) which surrounds the end of the capillary tube (2) adjoining the capsule or bellows device (1) and which is embedded at least partly in the resin of the joint (14) .
5. A* sensing element according to Claim 1, characterised in that the capsule or bellows device has a rigid base member (6) having a tubular portion (8) which projects through the aperture in the support plate (9) and which is swaged to attach the base member (6) to the support plate (9JV the said tubular portion (8) being embedded in the resin joint (14) .
6. A sensing element according to any one of Claims 1 to 5 , in which the end of the capillary tube remote from the capsule or bellows device is closed by the resin bonding of a ferrule or cap to the said end .
7. A method of making a temperature or pressure sensing element comprising an expansible capsule or bellows device ( 1 ) connected to a capillary tube (2) , c har ac t er i sed in th a t one end of the capillary tube (2) is connected to a support plate (9.) of the capsule or bellows device (1) by the formation of a resinbonded hermetically sealed joint (14) and the other end (23) of» the capillary tube (2) is closed by a forging operation to form a hermeticall sealed system.
8. A method according to Claim 7, in which the cold forging operation is preceded by a cold welding or crimpin step to close the said other end of the capillary tube.
9. A method according to Claim 7, in which the cold forging operation is carried out. while the said other end of the capillary tube (2) is in communication with a supply of charging fluid.under pressure (18).
10. A method according to Claim 7, Claim 8 or Claim 9, in which the coldforging is effected by an impact tool (22) acting axially on the said .othe end of the capillary tube.
11. A method according to Claim 7, in which the capillary tube (2) is fabricated in aluminium, copper or other ductile metal.
Description:
Temperature or Pressure Sensing Elemen This invention relates to the manufacture of temperature or-.pressure sensing elements, i particular sensing elements of the type comprising an expansible capsule or bellows device connected to a capillary .tube which is closed at its end remote from the capsule or bellows device to form a hermetically sealed system.

Fluid filled hermetically sealed temperature sensing elements of the type referred to above are commonly employed in thermostatic controls and the like, the fluid filling of the sealed system comprising a gas, vapour, or liquid/vapour in equilibrium, according to the intended practical application and range of temperatures for which the element is to be used.

The walls of the expansible capsule or bellows device are commonly made of spring steel and the capillary tube is most commonly made of copper. The capsule or bellows device is usually supported by a mild steel support plate in the region where the capsule or bellows device adjoins the capillary tube. The joint between the capillary tube and.the capsule or bellows device has to be mechanically strong and hermetically sealed- This joint is, in the known manufacturing technique, formed by soldering. Thus in the most usual arrangement the expansible capsule or bellows device is connected to a rigid support plate having an aperture through which the capillary tube passes, this aperture being sealed by the solder joint, which also forms a mechanical bond between the support plate and the adjoining end of the capillary tube. After soldering the joint between the capillary tube and the capsule or bellows device the assembly has to be wire-brushed to remove solder flux residues and the assembly is then heated in vacuo to effect complete dehydration before the introduction of the gas or liquid filling.

The high cost of copper renders the use of

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alternative materials for the capillary tube attractive. The use of aluminium capillary tube in place of copper capillary tube has certain advantages, not least of which is a significantly lower.cost. A problem arises however, in the provision of a -satisfactory sealed joint between the capillary tube and the capsule or bellows device to which it is connected, given that a conventional soldered joint cannot be used to effect a sealed bond between aluminium and steel surfaces. It has been found that a satisfactory sealed joint can be made using a resin bonding material. Accordingly the present invention provides, in one aspect, a temper- -ature or pressure sensing element comprising an expansible capsule or bellows device connected to a capillary tube in which the capsule or bellows device is attached to a rigid support plate having an aperture through which the capillary tube passes, the capillary tube being secured to a surrounding part of the support plate and being hermetically sealed to the capsule or bellows device, characterised in that the capillary tube is bonded to the support plate and sealed to the capsule or bellows device by a synthetic resin-bonded joint.

By using a resin-bonded joint it is possible to form an extremely strong connection between the capillary tube and the capsule or bellows device, using a fabrication method which is significantly cheaper than the soldering technique used when copper capillary tube is employed. The capillary tube may be fabricated in, for example, aluminium, stainless steel or brass. Where aluminium is employed the capillary tube is preferably anodised, at least on its external surface. Various resins may be employed to form the resin bonded joint. An epoxy resin has proved satisfactory, preferably a single part epoxy resin which is heat-cured. The heat-curing of the resin joint assists the dehydration of the assembly. A further incidental advantage of employing resin as the bonding material is that no separate wire brushing step is necessary to remove flux residues.

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The resin bonded joint is preferably reinforced by a metal sleeve which surrounds the end of the capillary tube adjoining the capsule or bellows device and which is embedded at least partly in the resin of the joint. The capsule or bellows device may have a rigid base member having a tubular portion which projects through the - aperture in the support plate and which is swaged to attach the base member to the support plate , the said tubular portion being embedded in the resin joint. The resin forming the j oint enters the tubular portion surrounding the end of the capillary tube , and forms an effective seal, between the capillary tube and the rigid base member , as well as sealing the goint between the capillary tube and the rigid support plate . When the sensing element according to the invention is used for sensing temperature the capillary tube is closed at its end remote from the capsule or bellows device to form a hermetically sealed system. The present invention in another aspect provides a method of making a temperature sensing element in which one end of a capillary tube is connected to a support member of a diaphragm or bellows device by the formation of a resin-bonded hermetically sealed joint and the. other end of the capillary tube is closed by a forging operation. The forging operation , which is preferably, but not necessarily, cold forging, may be preceded by a welding or crimping step to close the said other end of the capillary tube . Alternatively, the forging operation may be carried out while the said other end of the capillary tube is in communication with a supply of charging fluid under pressure. Where the capillary is sealed by cold forging this is preferably effected by an impact tool acting axially on the said other end of the capillary tube . The invention will be further understood by reference to the following drawings , given by way of example i which:

Figure 1 is a diagrammatic sectional view pf a temperature sensing- element fabricated by a known process according- to the prior art;

Figure 2 is a diagrammatic sectional view, corresponding to Figure 1, illustrating a -temperature sensing element according to one embodiment of the present invention;

Figures 3 and 4 are diagrammatic sectional views, similar to Figure 2, illustrating alternative embodiments of temperature sensing elements according to the invention;

Figure 5 illustrates diagrammatically the sequence of operations in one method of sealing the end of the capillary tube in a temperature sensing element according to the invention, and Figure 6 illustrates diagrammatically an alternative method of sealing the end of a capillary tube in a temperature sensing element according to the invention. Throughout the drawings the same reference numerals are used to indicate the same or corresponding component parts.

Referring irst to Figure 1, a typical temperature sensing element made by a Jnown prior art abrication process is illustrated. A flexible-walled expansible capsule or bellows device 1 is connected to a capillary tube 2 which is closed at its end remote from the bellows device 1 , forming a hermetically sealed system containining a gas, vapour or liquid/vapour filling. The bellows device 1 comprises two stainless steel diaphragms 3, 4 which are welded together peripherally. A rigid mild steel boss 5 is projection welded to a central part of the diaphragm 3 and acts as a force transmitting member for applying an actuating force to a thermostat or other mechanism (not shown) in response to temperature variations in the sealed system. A rigid base member 6 of mild steel is projection welded to a central part of the other diaphragm 4, which has a central hole 7 aligned with and slightly smaller in diameter

than a central hole in the base member 6. The base member 6 is formed with an upstanding tubular portion

8 which projects through an aperture in a mild steel support plate 9. The boss 5 and base member 6 of the bellows device are electroplated with nickel sulphamate to resist corrosion and assist welding. The mild steel support plate 9 is copper plated..

The bellows device 1 is secured to the support plate 9 by swaging of the tubular portion 8, the diameter of the latter being slightly greater than that of the capillary tube 2. The capillary tube passes through the tubular portion 8 of the base member 6 and abuts the portion of the diaphragm 4 surrounding the hole 7. Before being fitted to the bellows device 1 the capillary tube 2 is formed with a bend so that the end of the capillary tube which is joined to the bellows device 1 is perpendicular to the plane of the bellows device 1 and its support plate 9. The capillary tube 2 is anchored to the support plate 9 by means of deformable lugs 10 formed integrally with the support plate 9. ' With the capillary tube 2 correctly positioned sealing is then effected by the injection of solder 11 into a shallow annular well 12 in the support plate 9 surrounding the capillary tube 2. The solder 11 forms a mechanical joint between the support plate 9 and the capillary tube 2, and also enters the space between the capillary tube 2 and the tubular portion 8 of the base member 6, forming a hermetic seal. After cooling of the soldered joint the joint itself is wire brushed to remove flux residues and the entire assembly is then dehydrated by heating in a chamber in vacuo. This is followed by charging of the interior of the capillary tube 2 and the bellows device 1 with the appropriate quantity of gas or liquid, following which a cold weld is made in the end of the capillary tube 2 ' remote from the bellows device 1 to seal the

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interior of. the capillary tube. The sealed end of the •capillary tube.is then swaged, crimped and fused to form a sealed tip 13. The completed and sealed temperature sensing element may be electroplated with tin prior to inspection. The present invention makes use of an alternative material for the capillary tube 2, in this example

•r aluminium capillary tube. The aluminium tube 2 is . anodised on its external surface. The assembly of the temperature sensing element is similar to'that illustrated in Figure 1, except that the bonding of the capillary tube 2 to the support plate 9 is effected by means of a synthetic resin bonding material 14, in this case a single part epoxy resin. A suitable resin for this purpose is ESP 109 resin manu actured byPermabond Adhesives Ltd. The resin 14 forms a joint similar to the soldered joint 11 ofFigure 1 , hermetically sealing the capillary tube both to the- support plate 9 and to the base member 6 of the bellows device 1. Curing of the resin is effected by heating, for example to 180 C for 20-30 minutes. The heat-curing of the resin also assists dehydration of the bellows device and capillary tube 2, avoiding the need for this as a separate step.

Reinforcement of the resin bonded joint is afforded by a metal sleeve 15 which surrounds the end of the capillary tube 2 adjoining the bellows support plate 9, the end of the sleeve 15 adjacent the bellows device 1 being flared outwardly and located in the well 12, so that, upon formation of the resin joint 14, it is embedded in the resin. By providing a pre-anodised aluminium capillary tube 2 the need for a separate finishing coating of electro- -plated tin can be avoided, particularly if the bellows support plate 9 is electroplated, for example, with nickel sulphamate, prior to assembly. Figure 3 illustrates a variant of the embodiment shown

in Figure 2, in which a weld 16 is formed between the aluminium capillary tube 2 and the crimped end of the tubular, portion 8 of the base plate 6. This weld 16, which may alternatively comprise a soldered joint, is carried out before the formation of the resin bonded joint 14, and forms a preliminary seal.

Figure 4 shows a further variant of the temperature sensing element illustrated in Figure 2 in which the reinforcing sleeve 15 is omitted, the joint 14 being formed with resin only. The end of the capillary tube 2 adjoining the bellows device 1 may be strengthened by the provision of a preformed bulge portion 17, as shown diagrammatically.

The filling of the capillary tube 2 of the temperature sensing element, and its subsequent closure, can be performed in a single cold-forging operation when aluminium is used as the material for the capillary tube. One sequence of operations for closure of the capillary tube 2 is illustrated in Figures 5(a) -(d). The capillary tube 2 and its associated bellows device 1 is charged with fluid under pressure, in this example, refrigerant gas, from a charging manifold or reservoir 18 (Figure 5(a)) with which the open end of the capillary tube 2 communicates through a seal 19. After filling to the required pressure the capillary tube 2 is crimped externally of he charging manifold 18 at 20 (Figure 5(b)) . The crimping operation forms a cold-weld and is followed by parting of the capillary tube 2 at the site of the crimp 20. The closed end of the capillary tube 2 is then finally sealed by a cold-forging operation in which the end of the capillary tube 2 is inserted in a - die 21 and deformed by a punch 22 which acts axially on the closed end of the capillary tube 2, forming an enlarged sealed end 23 (Figure 5(d)). The resulting sealed end 23 would not necessarily have an enlarged diameter as shown. An alternative method of closing and sealing the capillary tube 2 is illustrated in Figure 6, in which the cold-forging of the end of the capillary tube to form the enlarged end 23 is effected in a single

operation. The end of the open capillary tube 2 is " inserted in the charging manifold 18 through the seal 19, as \ iHustrated ' in Figure 5 (a) and is held firmly in a clamp or jaws. A punch 22 moves axially with respect to the end of the capiliary tube 2 and passes- through the charging manifold 18, through a further seal 24. The punch 22 cold-forms the enlarged end 23 of the capillary tube 22 in the manifold 18, immediately after the charging operation, forming a finished end 23 similar to that illustrated in Figure 5(d).

The cold-forged sealed end 23 of the capillary tube 2 is stronger and less vulnerable to damage than the sealed capillary tube ends formed by the traditional method. Moreover, the joint between the capillary tube and the bellows device is stronger and more easily made than the traditional soldered joint.

Other capillary tube materials which may be bonded using the method of the present invention are stainless steel, copper, brass and copper plated mild steel. Aluminium is," however, preferred because of its relative cheapness and its greater flexibility, rendering the sensing element easier to install.

The capillary tube 2 in the temperature sensing element of the present invention is preferably pre- anodised, as stated previously. Alternative surface treatments may, however, be employed, including a pre-coating with paint or plastic materials including epoxy, epoxy polyester of ethyl vinyl.=acetate applied by dipping, spraying, electrophoretic or powder coating methods. The coating material may be applied either as a precoat to the capillary or may be applied as a coating after the assembly of the sensing element. Similarly, other environmentally compatible plating finishe may be applied to the other component parts of the sensing element according to the intended field of application.

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When using copper as the capillary tube material the finished end of the tube may be coated with solder in a solder-dipping step to seal any micropores in the surface of the capillary tube. When using the resin-bonding technique of the present invention to attach:-.one end of a capillary tube to a capsule or bellows device, the other end of the capillary tube need not necessarily be closed by a cold- forging operation as described. In soire cases it may be preferred to seal the end of the capillary tube by the bonding of a -ferrule or cap to the tube, using a resin bonding agent such as, for example, acrylic resin or the same resin as used to bond the capsule or bellows device to. the capillary tube. Although the invention has beendesc : ribed with particular reference to a temperature sensing element, it will be appreciated that it is also applicable to pressure sensing elements, in which case the capillary tube connected to the capsule or bellows device would be open-ended.