EP0022142A1

EP0022142A1 - Sealed electrical switch

Info

Publication number: EP0022142A1
Application number: EP79301321A
Authority: EP
Inventors: Keith Lewis
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-07-09
Filing date: 1979-07-09
Publication date: 1981-01-14

Abstract

An electrical switch comprising a hollow body (11) which slidably supports an operating plunger (12) for lengthwise sliding movement in the body. The operating plunger (12) is accessible from the exterior of the body, and housed within the body are fixed electrical contacts (13). A movable electrical contact (17) is also housed within the body and is movable relative to the fixed contacts (13) between a rest position and an operated position. A flexible sealing diaphragm (14) is secured at its periphery to the body (11) and extends between the plunger (12) and the contacts (13), (17). A first spring (18) acts between the plunger (12) and the body (11) to urge the plunger (12) to a rest position relative to the body (11). Movement of the plunger (12) under the action of the spring (18) is transmitted to the movable contact(17) by a flange (12c) of the plunger (12) to move the movable contact (17) to the rest position relative to the fixed contacts (13). said flange (12c) permitting movement of the plunger (12) against the action of said first spring (18) relative to the movable contact (17) so that the plunger (12) can be moved beyond the position corresponding to the operating position of the movable contact (17). A second spring (19) acts on the movable contact (17) to move the movable contact (17) from its rest position to its

Description

This invention relates to an electrical switch of the kind comprising a hollow body, an operating plunger accessible at the exterior of the body and supported in the body for lengthwise sliding movement relative to the body, fixed electrical contacts housed within the body, a movable electrical contact housed within the body and movable relative to the fixed contacts between a rest position and an operating position in response to movement of the plunger relative to the body, a lost motion arrangement whereby the plunger can move relative to the movable contact beyond a position corresponding to the operated position of the movable contact, and sealing means restricting access of liquid, for example, fuel, oil or water, to that region of the interior of the body containing the fixed and movable electrical contacts by way of the sliding interface of the body and the plunger.
Ve have preyiously proposed an electrical switch of the kind specified wherein a diaphragm seal extending between the slidable operating plunger and the contacts of the switch is trapped at its periphery by the body, and extends between the plunger and the movable contact so as to effect efficient sealing without susceptibility to moisture and foreign matter entering the contact region of the switch as a result of leakage around the plunger. However in order to provide for operation of the switch by movement of the plunger, together with the facility for movemenet of the plunger beyond the operated position of the movable contact it is necessary to provide a first spring acting between the diaphragm and the plunger, and a second spring acting between the movable contact and the body. Thus the first and second springs are in direct opposition to one another. The first spring must of course be stronger than the second spring in order that movement of the plunger can be transmitted by the first spring, and the diaphragm to the movable contact to move the movable contact against the action of the second spring relative to the fixed contacts. Thus in this switch the sealing is extremely efficient, but there arise problems in the choice of the first and second springs. If the first spring is considerably stronger than the second spring then its length must be accurately controlled in order to ensure that the movable contact occupies the desired rest position when the plunger is in its rest position. On the other hand, if the first spring is not greatly stronger than the second spring then some collapse of the first spring will occur during operative movement of the plunger and the amount of plunger travel necessary to operate the switch will be indeterminate.
It is an object of the present invention to provide a switch of the kind specified wherein the problems associated with choice of springs are minimised.
An electrical switch according to the invention comprises a hollow body, an operating plungeraccessible at the exterior of the body and supported in the body for lengthwise sliding movement relative to the body, fixed electrical contacts housed within the body, a movable electrical contact housed within the body and movable relative to the fixed contacts between a rest position and an operated position, a flexible sealing diaphragm secured at its periphery to the body, the diaphragm extending between the plunger and said fixed and movable contacts, a first spring acting between the plunger and the body to urge the plunger to a rest position relative to the body, means whereby movement of the plunger under the action of the first spring is transmitted to the movable contact to move the movable contact to said rest position relative to the fixed contacts said means permitting movement of the plunger against the action of said first spring relative to the movable contact so that the plunger can be moved beyond the position corresponding to the operating position of the movable contact, and a second spring acting on the movable contact to move the movable contact from its rest position to its operated position when the plunger is moved against the action of the first spring from its rest position.
Conveniently the movable contact, in its operated position engages the fixed contacts.
Desirably -said second spring acts between the body and the movable contact, and is weaker than the first spring.
Preferably an abutment on the plunger overlies a surface of the movable contact which is presented towards the operated position of the movable contact, a part of the diaphragm extending between said abutment and said surface, the movement of the plunger towards its rest position being transmitted to the movable contact by way of said abutment and said part of the diaphragm.
Conveniently the movable contact is an annulus and the diaphragm, the plunger, and the first spring extend through the annulus, said first spring acting against the body through the intermediary of the diaphragm.
In the accompanying drawings:-

Figure 1 is a diagrammatic sectional view of a prior art switch; and
Figure 2 is a sectional view of an electrical switch in accordance with one example of the present invention.

Referring to the drawings, in the known form of switch shown in Figure 1 a two-part hollow body 11 slidably supports an operating plunger 12. The plunger 12 is mounted for rectilinear sliding movement in the body 11 in the direction of its length, and is accessible from the exterior of the switch. Carried by a base part 11a of the switch and positioned within the hollow interior of the switch are first and second fixed electrical contacts 13 the electrical contacts 13 being integral with blade terminals projecting from the exterior of the base part 11a. A flexible sealing diaphragm 14 is trapped around its periphery between the two parts of the body 11, and divides the hollow interior of the body 11 into first and second chambers 15, 16 respectively. A movable electrical contact 17 in the form of a metal plate is secured to the diaphragm 14 in a position such that the plate 17 can be moved to engage simultaneously the contacts 13, thus completing an electrical circuit between the contacts 13. A first compression spring 18 acts between the plunger 12 and the diaphragm 14 and a second compression spring 19 acts between the movable contact 17 and the body part lla. The plunger 12 includes a shoulder 12a which engages an internal step of the body 11 to define a rest position of the plunger 12 relative to the body 11.
The switch is intended to operate as a normally open switch, and thus has a rest position wherein the movable contact 17 is spaced from the fixed contacts 13, and an operated position wherein the contact 17 engages the contacts 13. The spring 18 is stronger than the spring 19 so that movement of the plunger 12 inwardly of the body 11 is transmitted by the spring 18 to the diaphragm 14 to flex the diaphragm and move the movable contact 17 against the action of the spring 19 to engage the fixed contacts 13. Thereafter of course the movable contact and the diaphragm are held against further movement any further movement of the plunger 12 in the same direction is absorbed by collapse of the spring 18. Upon the release of plunger 12 the spring 18 restores to its original length, and the spring 19 moves the movable contact 17, the diaphragm 14 and the plunger 12 back to their rest positions.
The switch may be used in an environment where the plunger 12 is immersed in liquid for example water, oil, or petrol or diesel fuel. Since the plunger 12 is a sliding fit in the body 11 such liquid can find access to the chamber 15 by way of the sliding interface between the plunger 12 and the body. However it will be recognised that the diaphragm 14 constitutes a very efficient seal preventing access of liquid in the chamber 1 15 to the chamber 16. However, it will further be recognised that choice of the length and strength of the springs 18, 19 is critical. Clearly the spring 18 must be stronger than the spring 19 but in the event that the spring 18 is not greatly stronger than the spring 19 then the spring 18 will start to collapse before the movable contact 17 engages the fixed contacts 13. In an extreme situation this could result in the movable contact 17 never engaging the fixed contacts 13 but a more probable result is that the travel of the plunger 12 necessary to achieve contact closure is greater than the gap between the fixed contacts 13 and the movable contact 17 in the rest position of the contact 17. Since springs in a given batch of allegedly identical springs will necessarily differ in their characteristics, the movement of the plunger 12 necessary to achieve contact closure in a batch of apparently identical switches, will vary. Dn the other hand, if the spring 18 is chosen to be conaiderably greater in strength than the spring 19 then its length must be extremely accurately controlled in order for the movable contact 17 to be in the desired rest position relative to the fixed contacts 13, when the plunger 12 is in its rest position. Moreover, the strength of the spring 18 will be limited to some extent by the strength of the fixed and movable contacts of the switch, and the force available to operate the switch.
In the example of the present invention illustrated in Figure 2 the switch has a two-part body 11 comprising a base part 11a and a cover part 11b, the cover part 11b having an integral screw threaded hollow spigot 11c. The spigot 11c in use provides the mounting for the switch. A metal operating plunger 12 is slidably received in the hollow spigot 11c and extends into the hollow body 11. A pair of fixed electrical contacts 13 are carried by the base part 11a of the body 11 and are exposed within the body 11. A flexible rubber sealing diaphragm 14 is tightly trapped at its periphery 14a between the parts 11a, 11b of the body. The diaphragm 14 divides the interior of the hollow body 11 into first and second chambers 15, 16 the contacts 13 being within the chamber 16.
An annular copper contact plate 17 defines the movable contact of the switch, and has a diameter such that it can simultaneously engage both fixed contacts 13 to complete an electrical circuit therebetween. The diaphragm 14 is shaped to extend through the central aperture of the contact plate 17 and is arranged to grip the plate 17 on both faces thereof around the central aperture.
The plunger 12 extends through an electrically insulating rigid washer 20 and through the central aperture of the contact plate 17. The region of the diaphragm 14 passing through the central aperture of the plate 17 lies between the plunger 12 and the plate 17, and it is to be understood that the diaphragm 14 is complete, that is to say that it is not pierced by the plunger 12, the central region of the diaphragm 14 of the side of the plate 17 remote from the body part 11b defining a sac 14b extending towards the base part 11a of the body 11.
A first compression spring 18 acts between the plunger 12 and the base part 11a, the wall of the sac 14b being trapped between the spring 18 and the base part 11a. The spring 18 urges the plunger 12 in a direction away from the base part 11a. The plunger includes a radially outwardly extending flange 12c within the sac 14b, the flance 12c overlying the contact plate 17 but being spaced therefrom by the intervening thickness of the diaphragm 14. Thus during movement of the plunger 12 under the action of the spring 18,flange 12c causes the diaphragm 14 and the plate 17 to be moved with the plunger 12. A rest position of the plungar 12 and the movable contact plate 17 is defined by abutment of the washer 20 with the inner surface of the body part 11b. A second compression spring 19 acts between an internal shoulder of the body part 11b and the washer 20 to urge the washer 20, diaphragm 14, and contact plate 17 towards the base part 11a and thus to engage the movable contact plate 17 with the fixed contacts 13. However, the spring 19 is chosen to be weaker than the spring 18, and so the spring 18 prevents such movement of the contact 17.
Movement of the plunger 12 by an external force against the action of the spring 18 permits the washer 20, diaphragm 14 and movable contact 17 to follow the movement of the plunger 12 under the action of the spring 19, until the contact plate 17 engages the fixed contacts 13. Thereafter, further movement of the contact plate 17 diaphragm 14, and washer 20 is prevented, but the plunger 12 can of course be moved beyond this point against the action of the spring 18, the plunger 12 moving to accommodate additional movement of the component which effects movement of the plunger 12, but without applying any extra loading to the fixed contact 17, the diaphragm 14, and the washer 20.
It will be recognised that the arrangement of the springs 18, 19 shown in Figure 2 overcomes the problems of the spring arrangement of the construction shown in Figure 1. In Figure 2 construction, provided that the spring 18 is stronger than the spring 19 then no other parameters pf the springs are in any way critical, provided that the springs are of sufficient length and strength to fulfil their required functions, namely the spring 18 being adequate to return all of the parts to their rest position against the action of the spring 19, and the spring 19 being sufficiently strong to give the required contact pressure between the contact plate 17 and the fixed contacts 13.
In a modification of the construction shown in Figure 2 in order to remove the need for the spring 19 to be weaker than the spring 18, the plunger 12 is provided intermediate its ends with an upstanding shoulder against which the spring 19 acts. Thus in the modification the spring 19 does not act against the body, but acts only between the plunger 12 and the washer 20, and so its strength in relation to the spring 18 is irrelevant since both springs will be acting in the same direction on the plunger 12.

Claims

1. An electrical switch comprising a hollow body, an operating plunger accessible at the exterior of the body and supported in the body for lengthwise sliding movement relative to the body, fixed electrical contacts housed within the body, a movable electrical contact housed within the body and movable relative to the fixed contacts between a rest position and an operated position, a flexible sealing diaphragm secured at its periphery to the body, the diaphragm extending between the plunger and said fixed and movable contacts, a first spring acting between the plunger and the body to urge the plunger to a rest position relative to the body, means whereby movement of the plunger under the action of the first spring is transmitted to the movable contact to move the movable contact to said rest position relative to the fixed contacts said means permitting movement of the plunger against the action of said first spring relative to the movable contact so that the plunger can be moved beyond the position corresponding to the operating position of the movable contact, and a second spring acting on the movable contact to move the movable contact from its rest position to its operated position when the plunger is moved against the action of the first spring from its rest position.

2. A switch as claimed in claim 1 wherein the movable contact, in its operated position engages the fixed contacts.

3. A switch as claimed in claim 1 or claim 2 wherein said second spring acts between the body and the movable contact, and is weaker than the first spring.

4. A switch as claimed in anyone of claims 1 to 3, wherein an abutment on the plunger overlies a surface of the movable contact which is presented towards the operated position of the movable contact, a part of the diaphragm extending between said abutment and said surface, the movement of the plunger towards its rest position being transmitted to the movable contact by way of said abutment and said part of the diaphragm.

5. A switch as claimed in anyone of claims 1 to 4, wherein the movable contact is an annulus and the diaphragm, the plunger, and the first spring extend through the annulus, said first spring acting against the body through the intermediary of the diaphragm.

6. An electrical switch substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.