GB2382465A

GB2382465A - Bimetallic switch

Info

Publication number: GB2382465A
Application number: GB0127370A
Authority: GB
Inventors: Robert Andrew O'neill; Kevin Edwin John
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2001-11-14
Filing date: 2001-11-14
Publication date: 2003-05-28
Anticipated expiration: 2021-11-14
Also published as: GB0127370D0; ATE330325T1; EP1446818B8; WO2003043043A1; DE60212435D1; EP1446818B1; CN1615533A; DE60212435T2; CN100440414C; GB2382465B; EP1446818A1

Abstract

A contact thermostatic switch has a dished, snap-acting bimetallic disc 5 cantilevered from one end of a diameter. The edge of the disc is loosely retained between a housing moulding 1 and a metal cap 6 secured to the moulding. At the other end of this diameter, the edge of the bimetallic disc engages a pip 4d on one end of a lever 4 which is pivotally-mounted in the housing at its other end. When the bimetallic disc 5 snaps to its oppositely dished configuration, it moves the pivotally-mounted lever 4 so as to determine the condition of a set of switch contacts by virtue of the end of the lever 4 which carries the pip 4d also having depending fingers 4c which abut a moving-contact carrying leaf spring 2b of the switch. The cantilevered mounting of the bimetallic disc and its interaction with the lever enables a smaller than conventional disc to be used, with consequent significant cost savings, without prejudicing the operation of the switch.

Description

IMPROVEMENTS RELATING TO THERMALLY-RESPONSIVE SWITCHES Field of the Invention: This invention concerns improvements relating to thermally- responsive switches, particularly bimetallic switches and more particularly, though not exclusively, so-called contactstats, namely contact thermostats designed to be juxtaposed with a body whose temperature is to be monitored ana incorporating a bimetallic device arranged to make good thermal contact with the body so as to be responsive to the temperature thereof, the condition of the bimetallic device determining the condition, open or closed, of a set of switch contacts.

Background of the Invention: Conventional contactstats tend all to have the same basic construction.

A typical example may be seen in US 4 754 252 (Elmwood Sensors).

Generally, a contactstat is provided with a housing which carries the terminals and contacts, with their leaf springs. There is a bimetal disc which is of slightly smaller diameter than the outside of the housing, and which has a push rod at its centre which transmits the movement at the centre to the contact leaf springs to determine the status of the contacts. Between the contacts and the bimetal is a spacer moulding, which guides the push rod and locates the bimetal disc against a metal cap. The spacer moulding also provides electrical isolation between the contacts and the normally earthed

bimetal end cap and reduces the possibility of flashover which can sometimes occur in confined spaces between arcing contacts and nearby earthed metal parts.

This approach is inefficient on two counts. Firstly, the spacer moulding represents a redundant part, whose functions could be performed by the housing and the push rod. Secondly the bimetal is used inefficiently, since its centre only moves a very small distance, while it generates far more force than is required to operate the contacts. This mismatch has led to a number of attempts to simplify the adjustment of the assembled contactstat to correct the relationship between the bimetal disc and the contacts to ensure that the snap region of the bimetal movement coincides with the opening movement of the contacts. Too far one way or the other will result in creep movement leading to contact wear and possible failure. Typical examples of attempts to resolve this are disclosed in US 4 669 182, US 4 825 186, GB 1 448 226, US 4 570 148, GB 2 098 003 and EP 0 182 487, which shows the extent of the problem.

The commonly used solution is to provide a push rod in a range of lengths, one being selected based on gauged measurements of the assembled switch.

Otter Controls have proposed a more efficient configuration of a bimetal disc in our Y1 product (GB 1 597 963) and more recently in GB 2 338 110 in which the bimetal disc is constrained at one end of a diameter, and the opposite end provides the movement and force to open the contacts. A fulcrum is provided at the centre of the disc to provide the reaction for this movement. This configuration provides twice the movement at half the force,

for a given disc, compared to the centre of the disc. The lower force is still adequate to operate the contacts, and the extra movement reduces the need to adjust the relative positions of the bimetal and contacts by half. This configuration has a problem in that the force is available at the edge of the disc, which makes the mounting of the contact springs require a part of the body which is of greater diameter than the diameter of the disk. This can be seen in the figures of the above mentioned Otter Controls patents, and leads to incompatibilities between a contactstat of this design and a"standard" contactstat. It needs more space.

Objects and Summary of the Invention: It is the principal object of the present invention to overcome or at least substantially reduce the abovementioned problems.

According to the present invention, in one of its aspects, there is provided a thermally-responsive switch comprising a dished, snap-acting, bimetallic disc retained in a housing such that movement of the bimetallic disc between its oppositely dished configurations causes one end of a diameter of the disc to move whilst the other end is substantially retained, and a member pivotally mounted in the housing and arranged at an end remote from its pivot to interact with a moving contact-carrying spring element of the switch, said pivotally mounted member co-operating with said disc in switch operations.

Other features of the invention are set forth in the subordinate claims and, together with the abovementioned aspect of the invention, will become

clear from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings.

Description of the Drawings: Figure 1 is an exploded perspective view of an embodiment of the invention; Figures 2A and 2B are cross-sectional views of the embodiment of Figure 1 with its terminals bent horizontally, Figure 2A showing the embodiment in contacts-closed condition and Figure 2B showing the contactsopen condition; and Figure 3 is a top plan view of the embodiment.

Detailed Description of the Embodiment: The drawings show an embodiment of the present invention which significantly reduces cost and complexity, whilst retaining interchangeability with"standard"contactstats. The design as shown is intended to be

compatible with"standard"half inch disc contactstats, but could be scaled to any required size. The customary 12. 5mm disc has been replaced in the embodiment with one of 10mm diameter and also has reduced thickness (1.25mm compared with 1. 5mm). This considerably reduces the cost of the disc-to 53%-which is significant since the bimetal is the single most costly component of the assembled switch. The smaller disc still has sufficient force and movement to operate the contacts. This reduction in diameter places the force conveniently within the body of the housing, allowing an almost

conventional layout for the contact springs. The force is applied directly beside the contacts so that none of the available movement of the bimetal is lost in flexure of the leaf spring.

The offset position of the force allows the use of a pivoted push rod, and the elimination of a spacer moulding. The push rod is guided in"V" grooves in the housing and needs no other alignment. Its profile is arranged to almost fill the area above the contacts, thus guarding the bimetal from possible flashover and, as will be described, providing a degree of air damping in its movement.

Referring to the drawings and particularly to Figure 1, the embodiment consists of just six components. A unitary moulded housing 1, which can be formed of synthetic plastics material or of ceramics material depending upon its temperature withstand requirements, provides accommodation for the other component parts of the switch. First and second switch terminals 2 and 3, the former a sub-assembly of a terminal proper 2a with a leaf spring 2b carrying a moving contact 2c of the switch, are received in a push-fit manner into appropriate accommodating recesses formed in the housing 1 and have barbed detents 7 which engage with the housing as shown in Figures 2A and 2B to ensure that they are retained in place. The second terminal 3 is turned over at its upper end and carries a fixed contact 3a of the switch.

A push rod or lever 4 is pivotally mounted in the upper part of the housing 1 and has knife-edge pivot parts 4a and 4b which locate in respective V -grooves 1 b (only one of which can be seen in Figure 1). The member 4 is

of generally rectangular shape and is arranged to make a relatively close fit (though not so close as to restrict its pivotal action) within the correspondingly shaped interior of the housing 1. At the end of the member 4 distant from its pivotal mounting, there are two spaced-apart depending fingers 4c which, in the assembled switch, extend on opposite sides of the fixed contact 3a of the switch and into engagement with the leaf spring 2b on opposite sides of the moving contact 2c. A formation 4d is upstanding from the top surface of the member 4 and, in the assembled switch, is engaged by the edge of the bimetallic disc 5.

The bimetallic disc 5, which is dished as is well known so as to be capable of switch-operating movement between oppositely dished configurations with a snap action, is received within the upper end of the housing I where it rests upon arcuate features 1 a. A metal cap 6 is clenched onto the housing 1 and holds the bimetallic disc 5 and, beneath the disc 5, the pivoted push rod or lever 4 in place, three pressed dimples 6a being provided in the upper surface of the cap 6 which form projections in the undersurface of the cap and assist in locating the bimetallic disc 5. A further dimple 6b is located to align with the centre of the bimetallic disc 5 and, on the underside of the disc, provides an abutment which acts as a fulcrum for switch-operating movement of the bimetallic disc 5. As will be appreciated, the disc 5 is substantially held at one point of its edge between the housing 1 and the cap 6 and pivots about this point so as to effect substantial movement of its

diametrically opposite edge point where it engages with the pip 4b on the upper surface of the member 4.

In operation of the thus described contactstat, when the bimetal disc 5 snaps through to its oppositely dished configuration, it forces the push rod 4 to rotate about the knife edges and opens the switch contacts. The close fit of the push rod within the housing also acts as an air damper to slightly damp the action of the bimetal, which will allow any arc to be extinguished while the contacts are close together, thus limiting the intensity of any arc, reducing its tendency to flash over to the cap and also enhancing the life of the contacts and reducing silver splash (the switch contacts aree formed of silver) which can compromise the integrity of the switch insulation over time.

This design of contactstat is believed to offer considerable cost advantages over previous designs. Others have tried to use a large flat push rod to transmit the force whilst being guided by the housing, see GB 1 267050 (Elmwood Sensors). The form shown in this patent must have significant clearance around the push rod because the disc can rotate out of alignment with the housing bore, which would otherwise cause jamming. Also such misalignment could lead to a change in the effective length of the pushrod as its point of action moves around on the spring and the bimetal as a result of the necessary clearance. The above described proposal overcomes this difficulty while retaining simplicity of construction.

Should it prove necessary to adjust the effective length of the push rod due to varying assembly dimensions, then the pip 4b may be made of different

heights on a range of push rods. However it is envisaged that the number of such variants will be considerably less than would be the case with a conventional construction.

Having described the invention in the foregoing by reference to a specific embodiment, it is to be well appreciated that the embodiment is exemplary only and that modifications and variations thereto will occur to those possessed of the appropriate skills without departure from the spirit and scope of the invention as set forth in the appended claims. For example, the fixed contact of the switch could be a silver contact coined into an opening in the turned-over part of the respective switch terminal or alternatively could be in the form of a plating provided on such turned-over terminal part, and likewise the moving contact could be a silver contact affixed to the end of the leaf spring or could be constituted by a plated portion of the leaf spring.

Claims

CLAIMS:

1. A thermally-responsive switch comprising a dished, snap-acting, bimetallic disc retained in a housing such that movement of the bimetallic disc between its oppositely dished configurations causes one end of a diameter of the disc to move whilst the other end is substantially retained, and a member pivotally mounted in the housing and arranged at an end remote from its pivot to interact with a moving contact-carrying spring element of the switch, said pivotally mounted member co-operating with said disc in switch operations.

2. A switch as claimed in claim 1 wherein said member pivotal ! y mounted in the housing is arranged substantially to fill the cross-section of an

internal chamber of the housing so that pivotal movement of said member is I subject to a degree of air damping.

3. A switch as claimed in claim 1 or 2 wherein said housing has a recess in which the bimetallic disc is received and a cap fitted on said housing serves to retain the bimetallic disc in said recess.

4. A switch as claimed in claim 3 wherein one or more formations are provided on the underside of said cap for assisting in the retention of the bimetallic disc.

5. A switch as claimed in claim 4 wherein said cap is formed of metal and said one or more formations comprise one or more dimples pressed into the metal of the cap.

6. A switch as claimed in claim 3 or 4 or 5 wherein said cap is arranged to provide an abutment for operating movement of the bimetallic disc.

7. A switch as claimed in claim 6 wherein said cap is formed of metal and said abutment comprises a dimple pressed into the metal of the cap.

8. A switch as claimed in any of the preceding claims wherein a formation on said pivotally mounted member engages an edge of the bimetallic disc.

9. A switch as claimed in any of the preceding claims wherein said spring element comprises a leaf spring carrying a moving contact of the switch at one end thereof and affixed at its other end to a first terminal of the switch, a second terminal of the switch carrying a fixed contact of the switch, said fixed and moving contacts co-operating in switching operations and said terminals being fixed in said housing.

10. A switch as claimed in claim 9 wherein said terminals are received in accommodating recesses formed in said housing in a push-fit manner with detents holding the terminals in place.

11. A switch as claimed in any of the preceding claims wherein said housing comprises a moulding of plastics or ceramics material.

12. A thermally-responsive switch substantially as herein described with reference to the accompanying drawings.

13. A thermally-responsive bimetallic switch comprising a cantilevered bimetallic element arranged at its free end to interact with a pivotally mounted member at a location remote from the pivot thereof, said pivotally mounted member being arranged to interact with a moving contact carrier of the switch.

14. A thermally-responsive bimetallic switch comprising a bimetallic element mounted so that one end thereof moves in response to thermal change, said one end interacting with a pivotally-mounted member arranged to determine the condition of a set of switch contacts.