GB2108319A - Electric switch - Google Patents

Abstract

A light duty electric switch has a contact-making member 7 in the form of a strip of resilient metal shaped into an elongated loop of which one side 14 engages a central contact 6 and includes a centrally disposed slight protuberance 15 in its outer surface. The opposite side includes a substantially central gap 13, the edges of which are defined by the ends of the strip and which receives a projecting portion 18 of an operating member 20 pivoted at 21 and causing the resilient loop forming the contact making member 10 to be held under compression across its width. In the illustrated position the member 7 bridges the central contact 6 and an outer contact 8. When the member 20 is rocked to its alternative position the member 10 first moves to the right, thus wiping both contacts. As soon as the protuberance 15 passes over the contact 6, the member 7 snaps over to its alternative, dotted line position in which it bridges the contact 6 and a second outer contact 9. <IMAGE>

Description

SPECIFICATION Electric switch This invention relates to light duty electric switches of the type including a contact-making member which is pivoted to rock about a central stationary member, which may form a contact, so as to engage either one of a pair of outer members, one or both of which may form contacts spaced equally from the central member If both the central member and one of the outer members are of insulating material so as not to form contacts, the contact-making member may operate to bridge a pair of contacts on the side opposite to the outer insulating member.

In order to operate a switch of this type, the contact-member has to be rocked from a position of stable equilibrium in engagement with one of the outer members, through a central position of unstable equilibrium and into a second position of stable equilibrium in engagement with the other output member. For this purpose, the operating member of the switch must be moved from a position in which it exerts resilient pressure on the contact-making member to one side of the central member to an alternative position in which it exerts similar to the other side of the central member, sliding along the surface of the contact-making member from one position to the other.This mode of operation requires an operating member comprising at least three parts, i.e. a pressure member for engagement with the contact-making member, a spring acting on the pressure member and a part which may be either pivoted or slidable which can be manually operated in order to transmit the necessary movement to the pressure member.

Although robust and positive in action, such a construction suffers from various disadvantages.

Quite apart from the cost aspect of the three-part operating member, the inevitable friction between the pressure member and the surface of the contact making member in moving from one position to the other has an adverse effect on the "feel" of the switch during operation. Since the contact-making member merely rocks from engagement with one outer member to engagement with the other outer member, the contact-making action itself is purely one of abutment and separation of the metallic surfaces without any element of mechanical "wiping"which, as is well known, is desirable in orderto effect self cleaning so as to offset the deterioration of the contact surfaces due to electrical burning and carbonisation.

An electric switch in accordance with the invention and of the type just described, comprises a contact making member in the form of a strip of resilient metal shaped into an elongated loop of which one side engages the central member and includes a centrally-disposed slight discontinuity in the angle of the strip having the effect of a slight protuberance in its outer surface and the opposite side includes a substantially central gap, the edges of which are defined by the ends of the strip and which receives a projecting portion of an operating member capable of movement in a direction generally parallel with the line of the contacts and spaced from that line by a distance such that the resilient loop forming the contact-making member is held under compression across its width.

The main effect of this resilient compression is to maintain contact between the contact-making member and the operating member without the need for any separate pressure member or spring forming part of the operating member and also to maintain the contact-making member in engagement with the central member without the need for any separate pivot on either component. Further results of the resilience of the loop can be described most simply in relation to a construction in which the central member and bothh the outer members take the form of electrical contacts. Thus the resilience also serves to hold the contact-making member in engagement with the respective outer contact in either of its stable operated positions.In such an operated position, the point of engagement between the contact-making member and the central contact is displaced from the mid-point on the contact-making member so that the thrust exerted on the contact making member by its reaction against the operating member serves to press the contact-making member into engagement with the respective outer contact.

When the operating member is moved in order to operate the switch and transfer the contact-making memberfrom engagement with one outercontactto the other, the first effect of movement of the operating member is to cause the contact-making member to slide bodily, thus producing an effective wiping action in relation to both the outer contact and the central contact. As the operating member eaches the mid-point of its travel and the reaction between the operating member and the contact making member passes through the central contact, the discontinuity on the outer surface of the loop engages the central contact and a sharply defined position of unstable equilibrium is obtained.

Shortly before this, the contact-making member moves away from the outer contact with which it has been in engagement and immediatelyafterthe mid-point of the movement, as the discontinuity on the surface loop slips over the central contact, the bodily movement of the contact-making member causes it to project further on the opposite side of the central contact so that the reaction with the operating member causes the contact-making member to rock into engagement with the other outer contact with a snap action which is accentuated by the passage of the discontinuity over the fulcrum formed by the central contact.The completion of the movement of the operating member then produces further wiping action with the central contact and a corresponding wiping action with the outer contact with which the contact-making member has just made engagement.

Preferably the discontinuity in the surface of the strip is formed by giving the strip a small permanent set to form a very shallow V-shape, but a similar result can be obtained for example by a punching operation.

As mentioned above, the part of the operating member which engages the gap in the contact-mak ing member needs to move in a direction generally parallel with the line of the contacts. In order to achieve this, the operating member as a whole may be capable of sliding movement in this direction or, alternatively, the operating member may be pivoted to turn through a predetermined angle so that the part engaging the contact-making member moves through a small arc of which the chord is parallel to the line of the contacts. Whatever the form of operating member, no separate plunger or spring is required and the overall construction is simplified accordingly.

An example of an electric switch in accordance with the invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is an elevation of the switch with a s de member removed to show the operating p2As; and Figure 2 is an elevation to an enlarged FC.?19 of a contact-making member forming part or the switch of Figure 1.

The switch shown in the drawings includes a housing having a base 1 and a cover 2 both made of insulating material, the cover being held in position by spigots 3. The base 1 carries three fixed contacts comprising a central contact 6 which forms a fulcrum for a contact-making member 7, and a pair of outer contacts 8 and 9. The three contacts lie in approximately a straight line although, as illustrated, the central contact 6 projects slightly above a straight line joining the outer contacts 8 and 9.

The contact-making member 7 is in the form of an elongated oval loop having generally parallel sides and rounded ends 10. This loop is formed from a parallel-sided strip of resilient metal which needs to be hard and to have good electrical conduction properties. Suitable metals for this purpose include brass, phosphor bronze, nickel silver and beryllium copper and the surface may with advantage be electro-plated with silver or other metal to reduce contact resistance. The upper side 12 of the loop includes a gap 13 defined by opposite ends of the metal strip and the lower side 14 includes a slight protuberance 15 caused by a discontinuity in the surface of the strip at its mid-point.

The contact-making member 7 is seen in more detail in Figure 2, the dotted line position corresponding to that of Figure 1 and the full lines showing the relaxed position prior to fitting in the switch. It will be seen that, when relaxed, the discontinuity at 15 is in the form of a very shallow V constituted by a set in the strip, but that when pressed inwardly against the resilience of the strip, the curvature of the strip on either of the discontinuity makes the latter slightly more pronounced.

The gap 13 in the upper side 12 receives a projecting portion 18 forming a reduced-thickness extension of a bar 19 which, in its turn, forms part of an operating member indicated generally as 20 and pivoted about a pin 21 fixed in the cover 2. The operating member 20 is thus in the form of a rocker of which the angular movement is restricted by abutment with shoulders 22 defined by the upper surface of the base 1. Owing to the action of the spring contact-making member 7, as will now be described, the operating member 20 has two stable positions in contact with either one of the shoulders 22 and, together with the contact-making member 7 moves from one stable position to the other with a snap-action.

In the position of the contact-making member 7 shown in full lines this member bridges the contacts 6 and 8 and is compressed against its own resilience in a direction across the width of the oval loop. This compression maintains a resilient contact pressure with the fixed contacts 6 and 8 and also holds the operating member 20 in the stable position illustrated. When the switch is operated by moving the operating member 20 to its alternative position, the projection 18 starts to move to the right along a small arc of which the chord is parallel with the general line of the contacts 6,8 and 9. Accordingiy, the member 7 first starts to move bodily to the right, thus having an effective wiping action on the contacts 6 and 8.When the movement of the operating member 20 reaches its mid-point, the member 7 reaches a symmetrical position of unstable equilibrium with the discontinuity 15 in engagement with the fulcrum defined by the contact 6 with the member 7 located symmetrically between the contacts 8 and 9. Shortly before this, the member 7 has left the contact 8 and immediately after the position of unstable equilibrium is passed, the discontinuity 15 moves to the right of the fulcrum and the reaction between the member 7 and the bar 19 causes the member 7 to snap into engagement with the outer contact 9. During the completion of its movement, i.e. until the operating member 20 rocks into contact with the left hand shoulder 22, the member 7 slides bodily to the right to carry out a wiping action on the contacts 6 and 9, until the stable position shown in dotted lines is reached.In this position, the reaction between the member 7 and the bar 19 maintains the contact pressure and hoids the operating member 20 in its alternative position.

It will be understood that the member 7 serves not only for contact-making purposes, but also to provide the necessary spring loading to the operating member 20. The provision of the gap 13 serves to key the member 7 to the operating member 20 while allowing the member 7 to rock in relation to the bar 19 through a significant angle, thus allowing the member to rock freely between the two alternative positions illustrated in the drawing. The rocking action itself takes place at or close to the centre of movement of the operating member so that the remainder of the movement in either direction results in the wiping action already described. The shaping of the rounded end 10 at each end of the oval loop facilitates the sliding movement onto and along the fixed contacts 8 and 9. Moreover, the operation of the member 7 just described transmits a snap-action "feel" to the operating member 20.

As alternatives to the construction illustrated in which the operating member 20 is in the form of a rocker, this may be in the form of a lever or slider, the only essential requirement being that the operating member should have a projecting portion equivalent to that shown as 18which moves along an arc or a straight line path which is generally parallel to the line of the fixed contacts. As already described, the metal strip forming the member 7 may be electro-plated, but for higher current ratings, the contact zones with the contacts 8 and 9 may have silver added by inlay or welding techniques.

The switch illustrated is a single-pole two-way switch, but as previously described, the operating principle illustrated is equally applicable to other types of switch. As a simple example, one of the contacts 8 or 9 may be replaced by an insulating stop so that a straightforward on - off action is obtained.

As another alternative, both the contact 6 and one of the outer contacts may be formed of an insulating material and a pair of contacts may be provided on the opposite side, the strip forming the member 7 being wide enough to bridge this pair of contacts in the "on" position.

Multi-pole operation can be obtained by the provision of a number of members 7 operated in parallel by a single operating member 20, the central, fulcrum-forming member being formed as a number of sections insulated from one another if necessary.

Claims (4)

1. A light duty electric switch of the type described and in which the contact-making member is in the form of a strip of resilient metal shaped into an elongated loop of which one side engages the central member and includes a centrally-disposed slight discontinuity in the angle of the strip having the effect of a slight protuberance in its outer surface and the opposite side includes a substantially central gap, the edges of which are defined by the ends of the strip and which receives a projecting portion of an operating member capable of movement in a direction generally parallel with the line of the contacts and spaced from that line a distance such that the resilient loop forming the contact-making member if held under compression across its width.

2. An electric switch according to claim 1 in which the discontinuity in the surface of the strip is constituted by a small permanent set forming a very shallow V-shape.

3. An electric switch according to claim 1 or claim 2 in which the operating member is pivoted to turn through a predetermined angle so that the part engaging the contact-making member moves through a small arc of which the chord is parallel to the line of the contacts.

4. A light duty electric switch substantially as described and as illustrated in the accompanying drawings.