GB2239353A - Microswitches - Google Patents

Abstract

A snap-action microswitch actuator comprises a first leaf member 28 and a second leaf member 29 pivotally connected thereto, knife edges (49, 50) (Fig 6A) formed on an outer portion of a cross-web (47) of the second member (29) engaging in complementary V grooves 37 (38) (Fig 5A) formed in the outer portion of a cross-web (34) of the first member 28, to form the pivot. A 'C' spring 48 formed integrally with (or separately from) second leaf member 29 is provided with a knife edge (51) (Fig 6B) at its free end which engages a complementary V groove (41) in a central limb (35) of first member 28 and acts to bias second member 29 towards or away from the first member 28 depending on the relative positions of said members 26, 29. A further embodiment is detailed (Figs 1-3). A switch unit incorporating the actuator is detailed (Figs 7A-G) wherein contacts (64, 65) may be thermostat contacts, and contacts (71, 72) may be pant of a thermal protection device operating if contacts (64, 65) do not open. Domestic appliance applications such as irons are exemplified. <IMAGE>

Description

MICROSWITCHES The present invention relates to microswitches and in particular to snap-acting microswitches and actuators therefor. By microswitch is meant a switch which is operated by a very small movement of an actuating member.

Snap acting microswitches are well known and are used in a wide variety of applications such as in the thermostatic control of domestic appliances such as irons. A typical snap-acting microswitch comprises a fixed contact and two generally rectangular apertured leaf members which overlie one another and are joined to each other, by welding for example, at corresponding ends. One leaf, which is fixed at its other end, has a central limb, extending into the aperture from that end, which engages one end of a 'C' spring. The other end of the 'C' spring engages or is integral with the other leaf member which carries a movable contact.The arrangement is essentially a spring over-centre snapacting geometrical mechanism, with the 'C' spring acting to bias the leaf member carrying the contact towards or away from the other leaf member to make or break with a fixed contact depending on the relative positions of the 'C' spring and the leaf members. In use, an actuating force for example generated by a bimetal temperature sensor is applied to the joined ends of the leaves to cause operation of the microswitch. Setting up the geometry to control the point at which the contact carrying leaf member snaps the contacts open or shut can be achieved by adjusting the position of the central limb.

Such prior art microswitches suffer from the disadvantage that contact pressure at the moment of contact break is low and tends towards zero as the parts line up geometrically which leads to electrical arcing and welding at the contacts. Furthermore the pull apart forces at the moment of contact break are low which means that micro-welds which develop during arcing during contact closing may not be swiftly broken. This leads to a phenomenom known as "spiking" in thermostatic controls utilising such microswitches wherein periodically a higher temperature than the nominal set operating temperature will be reached, as a greater actuation force, which is dependent on a higher temperature, must be applied to the microswitch to break these micro-contact welds. This is obviously undesirable.

It has been found that the problem of low contact pressure and pull apart forces is due largely to the geometric operation of the critical parts of the switch slowly coming into line and this is exacerbated by the inherent resilience of the leaf manner carrying the movable contact, which also counters the force produced by the 'C'spring.

According to a first aspect of the invention there is provided a snap-acting microswitch actuator comprising a first leaf member having three limbs extending from a connecting web, a second leaf member overlying the first leaf member and having two limbs extending from a connecting web, means for substantially freely pivotally interconnecting the outer limbs of the first leaf member and the limbs of the second leaf member at a location spaced from the connecting webs, and spring biasing means acting between the central limb of the first leaf member and the connecting web of the second leaf member so as to pivot the second leaf member either away from or towards the first member with a snap-action.

By providing a free pivotal connection between the leaf members, friction in the actuator is reduced, as is the effect of the resilience of the movable member.

Contact pressures and pull apart forces can therefore be improved.

Preferably the biasing means is a so-called 'C' spring i.e. an arcuate spring strip which extends between the end region of the central limb of the first leaf member and the connecting web of the second member.

The 'C' spring may be formed as a separate member or, preferably, integrally with the second leaf member as a central limb, to give a two piece actuator construction.

The 'C' spring is preferably freely pivotally mounted on the end region of the central limb of the first leaf member. The free end or ends of the 'C' spring is/are preferably preformed in such a manner as to minimise out of plane forces between the first and second leaf members.

The pivots between the first and second leaf member may for example be formed on the end regions of their respective outer limbs. In another construction the end regions of the outer limbs of one or both of the respective leaf member may be interconnected by a crossweb, and the pivot means may be formed on these respective cross-webs.

A particularly preferred method of pivotally interconnecting the leaf members is by providing the respective members with interengaging knife edges and generally V-shaped grooves. This form of pivot is well known in other contexts and may also be used between the spring biasing means and the centre limb of the first leaf member, and provides a particularly low-friction pivot. Knife edge in this context means edges which, with respect to the V shaped groove with which they are intended to engage, are suitably thin and profiled to locate positively and pivot freely in the grooves. The V shaped grooves may be formed in various ways as by machining, forming or bending.

In a preferred embodiment the spring biasing means is so arranged that the biasing means bias the knife edges into the V grooves.

The knife edges and V grooves may be formed on either leaf member. In one embodiment therefore, knife edges could be formed on the end regions of the first leaf member for engagement in V grooves formed on the end regions of the second leaf member, and the central limb of the first leaf member could be formed with a V groove to accept a knife edge formed on the 'C' spring biasing means.

Preferably, however, particularly when the 'C' spring is formed integrally with the second leaf member, the first leaf member is formed with V grooves only, and the knife edges are all formed on the second leaf member. This is a very significant feature greatly facilitating production, since it is difficult to produce V grooves on the second leaf member which, by virtue of its high spring properties is also generally very hard and cannot therefore be easily bent or worked.

The first leaf member although a spring member, need not be so resilient and therefore hard which means that the grooves thereon can be formed by bending.

The mode of operation of the preferred construction of actuator according to the invention is similar to that of known devices. It is essentially a geometric mechanism, with the 'C' spring acting to pivot the second leaf member towards or away from the first in dependence upon the relative positions of the pivot line of the 'C' spring relative to that between the first and second leaf members. When the pivot line of the 'C' spring goes over-centre in either direction with respect to the pivot line between the first and second leaf members, it will move the second leaf member in the opposite direction with a snap-action.

In use the first leaf member is fixed in a support at its connecting web end, and its outer limbs deflected at the other end by actuating means such as a bimetal.

This deflection, results in movement of the free end of the second leaf member by virtue of the pivotal connection between the two leaf members, . When that movement is sufficient to cause the 'C' spring to go over-centre, the other end of the second leaf member will be moved in a snap action either towards or away from the first member.

The set point of the device and hence the amount of deflection of the first and second leaf members necessary to cause this snap-action occurs may be varied by deflecting the central limb of the first leaf member towards or away from the second leaf member by means of a set screw or the like.

Of course it will be appreciated that the actuator would operate equally if the centre limb were moved by an actuating means and the outer limbs used to set the device.

The present invention also extends in a second aspect, to a microswitch comprising a snap-acting actuator in accordance with the first aspect of the invention and a set of electrical contacts, at least one of said contacts being movable by said actuator.

The second leaf member may be provided with an electrical contact at its movable end, and thus act as a switch contact. This is not essential however, and the movement of the second leaf member may instead operate a remote switch contact by means of an intermediate member or members. This is a particularly advantageous arrangement in circumstances where for example it is undesirable for the pivots or actuator to be current carrying, say in circumstances where they might have to carry high currents e.g. 5 to 10 Amps. The movable end of the second leaf member may thus operate remote contacts via push and/or pull rod means.

Preferably there should be a clearance or lost motion between the push and/or pull rod and the movable contact or the second leaf member to allow for contact wear. Thus for example a push rod could be mounted on the second leaf member, with a clearance to the contact or vice versa. It would be possible however to have the push rod in contact with both in which case these two interfaces may be so designed to wear at the same rate as the contacts so that contact wear does not affect the clearance and hence calibration of the switch.

Preferably however a push and/or pull rod clearance or lost motion is provided so that an impetus will be generated on operation of the leaf member to help break any micro-welds present.

Three embodiments of the present invention will now be described, by way of example only, with reference to the following drawings in which: Figure 1 shows, in section, an actuator in accordance with the invention; Figures 2A and 2B show plan and sectional views respectively of one component of the actuator of Figure 1; Figures 3A and 3B show plan and sectional views respectively of the other component of the actuator of Figure 1; Figure 4 shows a second actuator in accordance with the invention; Figures 5A and 5B show plan and sectional views respectively of one component of the actuator of Figure 4; Figures 6A and 6B show plan and sectional views of the other component of the actuator of Figure 4; and Figures 7A to 7C show a switch unit according to the present invention.

Referring firstly to Figures 1 to 3B, a first snapacting actuator 1 in accordance with the invention comprises first and second pivotally interconnected leaf members 2,3, between which extends a 'C' spring 24.

First leaf member 2 comprises three parallel limbs 4,5,6 which extend away from a connecting web 7 to form a generally W shaped configuration. The connecting web 7 is provided with apertures 8 which allow the member 2 to be mounted on a fixed support in use. The central limb 5 can be deflected with respect to the outer limbs 4,6.

The outer limbs 4,6 are formed with knife edges 9,10 at their free ends and are also bent up to form tabs 11,12 which, in use, engage actuating means (not shown) for deflecting the free ends. Flanges 16,17 are bent up along the outer edge of the outer limbs 4,6 to add rigidity to the leaf member 2.

The central limb 5 is bent up at its free end, and the upstanding portion 13 is formed with a V groove 14.

A tab 15 is released at the root of the centre limb 5 and acts as a backstop for the free end of the leaf member 3.

Leaf member 3 comprises two limbs 16,17 extending from a connecting web 18 in a generally U.shaped configuration. Connecting web 18 carries a switch contact 26. A flange 19 is bent down around the outer periphery of the leaf 3 to give it some rigidity. Tabs 20,21 which are bent down at the end of limbs 16,17 are formed with V grooves 22,23 for receiving the knife edges 9,10 formed on the outer limbs 4,6 of the first leaf member 2.

It will be seen that the biasing means i.e. the 'C' spring member 24 is formed integrally with the second leaf member 3 as a central limb. This is not essential and it could of course be formed as a completely separate member. In this embodiment the free end of the spring member is formed with a knife edge 25 for engagement with the V groove 14 formed in the central limb 5 of the first leaf member 2. As best shown in Figure 3B, the base of the 'C' spring 24 is coined across its width at 24a, i.e. it is reduced in thickness to provide a hinge line connection between the C spring and the member 3.

To assemble the actuator, the knife edges 9,10 are fitted into the grooves 22,23, and the spring member 24 bent to the configuration shown in Figure 1 so as to allow the knife edge 25 to engage in groove 14. It will be apparent that the spring member 24 maintains the grooves 22,23 in contact with the knife edges 9,10 as well as providing the force for the snap movement of the leaf member 3.

The operation of the actuator will now be described. The tabs 11,12 of the first leaf member are movable upwardly and downwardly as shown by the arrow 'M' in Figure 1 and in operation will be positively displaced in at least one direction by actuator means not shown. This could be a bimetal actuator or other thermally responsive member for example. As the free end of first leaf member 2 is displaced upwardly as shown in Figure 1, when the action of the 'C' spring on portion 13 goes over centre with respect to the pivot between the first and second leaf members 23, at 22,9;23,10, leaf member 3 will be pivoted away from the first leaf member 2 by the 'C' spring with a snap action. The contact 26 will then be snapped against a fixed contact 27 (shown in dotted lines in Figure 1).

When the free end of the first leaf member 2 moves back from its displaced position under its own resilience and/or under a displacing force, as soon as the 'C' spring 24 goes over centre, the second leaf member 2 and contact 26 will snap back against the back stop 15.

Calibration of the device can be achieved by providing means for deflecting the centre limb 5 up or down, thereby varying the amount of movement required at the tabs 11,12 required to trip the snap-action.

With reference now to Figure 4, a second actuator 27 in accordance with the invention comprises first and second pivotally interconnected leaf members 28,29. A 'C' spring 48 acts between the leaf members 28,29.

As can be seen from Figure 5A, the first leaf member 28 is generally rectangular in shape having two outer limbs 31,32, a connecting web 33 and a cross web 34. A central limb 35 extends from the connecting web 33 and together with the outer limbs 31,32 forms a generally W shaped configuration. As in the first embodiment apertures 36 are provided in the connecting web 33 for mounting one end of the leaf member 28 in use.

Tabs 37,38 which are in the form of V grooves 39,40 are bent up from the inner edge of the cross web 34 adjacent the ends of the outer limbs 31,32. The portion of the cross web 34 between the tabs 37,38 could be omitted. The end of the central limb 35 is also bent into a V groove 41. Tabs 42,43 are formed at the ends of the outer limbs 31,32 for engagement with external actuating means. A tab (not shown) may also be bent up at the root of the central limb 35 to act as a back stop.

With reference to Figures 6A and 6B, the second leaf member 29 is also generally rectangular in shape having outer limbs 44,45 connected at one end by a connecting web 46 and at the other end by a cross web 47. A 'C' spring member 48 is formed integrally with the leaf member 29 and extends as a central limb from connecting web 46.

Knife edges are formed on the inward facing edges 49,50 of the cross web 47 for pivotal engagement in the grooves 39,40 formed on the first leaf member 28, and also on the end 51 of the spring member 48. Thus in this embodiment all the grooves are formed on the first leaf member 28 and all the knife edges are formed on the second leaf member 29. This is a particularly advantageous arrangement since in practice it would be difficult to produce grooves in the second leaf member which is a high spring, high hardness material.

Both leaf members 28,29 are produced from cold rolled austentic stainless spring steel.

The leaf member 28 may typically be .25mm thick with a tensile strength of 1700-2050 Nmm-2 and a Rockwell Hardness of 520HV-580HV. Typically, after pressing a flat blank, the member may be bent to form the various pivot grooves and tabs in a single or multi-step process.

The leaf member 29 is stronger and harder than the first leaf member to provide a greater spring force for the 'C' spring 48. Typically it may be .15mm thick with a tensile strength of 2400 Nmm'2 and a Rockwell hardness of 625 HV, and it may be produced in a single pressing operation which also produces the required knife edges.

The spring member 48 may initially be formed in the plane of the leaf member as shown in Figure 6A and then bent to the configuration shown in Figure 6B. The free end portion of the 'C' spring 48 is so shaped as to minimise "vertical" out of line forces in the V groove 41.

The actuator of this embodiment is assembled in generally the same manner as that of the first embodiment, with the knife edges 49,50 first being located in V grooves 39,40 and the 'C' spring 48 then being flexed so as to engage the knife edge 51 in groove 41. A tab 52 may be provided on first leaf member 28 to facilitate correct positioning of the knife edges 49,50 in the grooves 39,40 during assembly.

The actuator of the second embodiment also functions in a generally similar manner to that of the first embodiment. As the free end of the first leaf member 28 is moved, the action of the 'C' spring 30 goes over centre with respect to the pivotal connection between the first and second leaf members 28, 29 snapping the free end of member 29 in the direction toward or away from the first leaf member 28. Again the amount of movement of the free end of the first member 28 required to snap the second member 29 may be varied by deflection of the central limb 35.

In this embodiment, the second leaf member does not carry a contact but is intended to operate a contact remotely. Figures 7A to 7C show one possible arrangement for operating a remote contact. In these figures, a switch unit is shown which incorporates an actuator substantially similar to the one shown in Figures 4 to 6B.

In Figures 7A to 7C, an actuator 53 is substantially as described with reference to Figures 4 to 6B. One end 54 of the actuator is mounted in a plastics housing 55, having live, earth and neutral terminals 66,67,68 respectively. Instead of fixing apertures 36, however, the first leaf member 56 is elongated at one end and is formed with a spring tag 57 which engages in an aperture in the housing 55. The second leaf member 58 does not carry an electrical contact, but instead carries a push-rod 59. The free end 60 of the first leaf member 56 is formed with a pair of fingers which engage either side of an insulating member 61 which is moved relative to the switch unit by an actuating means not shown. It will be noted that the tabs on the free end of the first leaf member point in the opposite direction to those in Figures 4 to 6.

In the position shown in Figure 7A, the actuator 53 is in a position of unstable equilibrium, and when the member 61 moves further in the upward direction the mechanism will go over centre and the second leaf member 58 will move with a snap action away from the first leaf member 56, and the push rod 59 will strike a leaf spring 63 which will break the contacts 64,65. This condition is shown in Figure 7B. It will be noted that the free end of the push rod 59 is not in contact with the leaf spring 63 while the contacts 64,65 are closed, in order to allow the push rod 59 to strike the leaf spring 63 with an impetus to break any microwelds and open the contacts 64,65. Opening the contacts breaks the live side power connection to lead 76 which is through terminal 66, link 75 and the contacts 64,65 and the second switch described below.

Also, as can be seen more clearly from Figure 7B, a set screw 69 is provided to deflect the central limb 70 of the first leaf member 56 to change the geometry of the device and thus the point at which the actuator will operate. When the member 61 moves back beyond its original position, the spring will go over centre in the opposite sense to snap the push rod 59 away from the leaf spring 63 to allow the contacts 64,65 to close once more.

The switch unit shown also comprises a second pair of switch contacts 71,72. Contact 71 is mounted on leaf spring 63 and contact 72 on leaf spring 73. Both leaf springs engage at their free ends with member 61. Leaf spring 63 is generally rectangular in shape with a tongue 74 released from its central region. The end of the tongue 74 carries the contact 64.

When the member 61 moves in a downward direction from the point shown in Figure 7A for example under the action of the leaf member 56,63 and 73, the contacts 71,72 will open as shown in Figure 7C. The live side supply to lead 76 is thus broken at this point.

Contacts 64,65 could therefore be thermostat contacts, for example, while contacts 71,72 could be part of a thermal protection device which operates if the contacts 64,65 do not open for some reason.

Of course while the push rod 57 has been shown mounted on the second leaf member 58, it could equally be mounted on leaf spring 63 with clearance from the second leaf member 58 to generate the same impetus effect on contact opening.

It will be apparent that many modifications may be made within the scope of the invention.

Claims (17)

CLAINS

1. A snap-acting microswitch actuator comprising a first leaf member having three limbs extending from a connecting web, a second leaf member overlying the first leaf member and having two limbs extending from a connecting web, means for substantially freely pivotally interconnecting the outer limbs of the first leaf member and the limbs of the second leaf member at a location spaced from the connecting webs, and spring biasing means acting between the central limb of the first leaf member and the connecting web of the second leaf member so as to pivot the second leaf member either away from or towards the first member with a snap-action.

2. An actuator as claimed in claim 1 wherein said biasing means is an arcuate spring strip which extends between the end region of the central limb of the first leaf member and the connecting web of the second member.

3. An actuator as claimed in claim 2 wherein said strip is formed integrally with the second leaf member as a central limb.

4. An actuator as claimed in claim 2 or 3 wherein said strip is freely pivotally mounted on the end region of the central limb of the first leaf member.

5. An actuator as claimed in any preceding claim wherein said first and second leaf members are pivotally interconnected at the end regions of their respective outer limbs.

6. An actuator as claimed in any of claims 1 to 4 wherein the outer limbs of one or both of said first and second leaf members are connected by a cross web, and pivot means are provided on said cross web or webs.

7. An actuator as claimed in any preceding claim wherein said means for pivotally interconnecting said first and second leaf members comprises interengaging V grooves and knife edges formed on the respective members.

8. An actuator as claimed in claim 7 wherein said biasing means biases said knife edges into said V grooves.

9. An actuator as claimed in claim 7 or 8 wherein said V grooves are formed on said first leaf member only and said knife edges are formed on said second leaf member only.

10. An actuator as claimed in any of claims 7 to 9 wherein said V grooves are formed by bending.

11. A microswitch comprising an actuator as claimed in any preceding claim and a set of electrical contacts, at least one of said set of contacts being movable by said actuator.

12. A microswitch as claimed in claim 11 wherein said second leaf member is provided with a switch contact at its movable end.

13. A microswitch as claimed in claim 11 wherein said second leaf member operates a remote removable contact via an intermediate member.

14. A microswitch as claimed in claim 13 wherein there is clearance or lost motion between said intermediate - member and said second leaf member or said movable contact.

15. An actuator substantially as hereinbefore described with reference to Figures 1 to 3B of the accompanying drawings.

16. An actuator substantially as hereinbefore described with reference to Figures 4 to 6B of the accompanying drawings.

17. A microswitch substantially as hereinbefore described with reference to Figures 7A to 7C of the accompanying drawings.