Electrical switch

Description

[0001] This invention relates to an electrical switch. Many forms of electrical switch are known, in which a resilient movable contact arm is urged into engagement with a fixed contact by means of an operating member in order to close the switch.

[0002] In some such switches such as that disclosed in EP-A2-0004420 the operating member does not move solely longitudinally of the movable contact arm, but moves transversely thereof, and thus the movable contact arm does not perform only the up and down movement relative to the fixed contact required for making and breaking the connection, but is also moved transversely of its axis.

[0003] This -transverse movement of the movable contact arm results in the surface of the contact portion of the arm being rubbed across the surface of the co-operating contact portion of the fixed contact, such rubbing resulting in fretting of the contact surfaces. This fretting in time results in so-called fretting corrosion of the contact surfaces, thus resulting in high, across-contact electrical resistance and possibly even non- electrical contact when the switch is closed.

[0004] According to this invention an electrical switch in which a resilient movable contact arm is urged into engagement with a fixed contact by means of an operating member in order to close the switch, the operating member moving transversely of the contact arm to effect such closure, and the contact portion of the fixed contact being convex as seen by the contact arm, is characterised in that the contact portion of the contact arm is concave as seen by the fixed contact, and has a larger radius of curvature then the co-operating contact portion of the fixed contact.

[0005] In a switch according to this invention the surface of the contact portion of the contact arm rolls across the surface of the contact portion of the fixed contact on operation of the switch, and thus there is little if any fretting of the surfaces, and thus little is any fretting corrosion occurs. Further, in view of this rolling action each part of the surface of the contact portion of the contact arm and of the surface of the contact portion of the fixed contact, is engaged by the other surface for only a small part of the time taken for operation of the switch, and thus any possibility of fretting of any part of either surface is kept to a minimum. In fact, the highest possibility of fretting will occur at the beginning of a closing and at the end of an opening operation of the switch, while in the actual contact position of the switch the operating member will be positioned over the fixed contact and axially aligned with the surfaces of the contact portions of the contact arm and fixed contact, in which position there is the minimum possibility of transverse movement of the surfaces relative to each other and thus of fretting.

[0006] The invention will now be described by way of example with reference to the drawings in which:-

Figure 1 is an exploded perspective view of a rotary electrical switch according to the invention;

Figure 2 is a perspective view of the underside of the operating member of the switch of Figure 1;

Figure 3 is a cross-sectional view through the switch of Figure 1;

Figure 4 is a diagram illustrating operation of known switches; and

Figure 5 is a diagram illustrating operation of switches according to this invention.

[0007] The switch shown in Figures 1, 2 and 3 is similar to that described and claimed in EP-A2-0004420, and comprises a circular base 1 moulded from electrically insulating plastics material, and carrying an input contact 2 and four fixed output contacts 3. The contacts 2 and 3 are press-fitted in the base 1, and each has an outwardly projecting pin portion for receipt in a hole in a substrate (not shown) thereby to connect the contact 2 or 3 to a conductor on the substrate which may be a printed circuit board. The inner ends of the output contacts 3 constitute contact portions the surfaces of which are convex, and project above the adjacent surface of the base 1, while the inner end of the input contact 2 has a flat head on which is welded a substantially planar bridging contact 5 having four resilient contact arms 6 associated with the four fixed output contacts 3 respectively.

[0008] The output contacts 3 are located at positions spaced at about 90° around the centre of the base 1 and at mutually different distances, that is radii, from the centre of the base 1, and the input contact 2 is offset from the centre of the base 1 such that it is substantially equispaced from each of the output contacts 3. Thus, the effective lengths of the four contact arms 6 of the bridging contact 5 are also equal.

[0009] The base 1 is formed with an upstanding peripheral flange 7 within which is received a circular operating member 8 which completes the switch. The flange 7 has an inwardly directed segmented lip 9 which engages over a segmented annular shoulder 10 on the operating member 8 to mount the operating member 8 on the base 1.

[0010] The inner surface of the operating member 8 is formed with a plurality of arcuate cam projections 11 having mutually different radii measured from the centre of the operating member 8, that is from the axis of rotation thereof, which axis passes through the centre of the base 1.

[0011] There is at least one cam projection 11 having a radius of curvature equal to the distance of each of the output contacts 3 from the centre of the base 1. Each cam projection has end surfaces 12 which slope from the surface of the operating member 8 up to a level middle surface 13 which is parallel to the surface of the operating member 8.

[0012] The operating member 8 is formed with an outwardly directed bar 14 which can be gripped by a user's fingers to rotate the operating member 8 relative to the base 1.

[0013] The flange 7 on the base 1 is divided into a plurality of circumferentially spaced segments 15 by slots 16. Two diametrically opposed segments 15 are narrower than the others and the lip portions 9' thereon are enlarged and engage in slots 17 in the shoulder 10 of the operating member 8 to provide an indexing action on rotation of the operating member 8 relative to the base 1, the slots 17 and lip portions 9' thus together defining a number of stable rotary positions of the operating member 8 relative to the base 1.

[0014] Each contact arm 6 of the bridging contact 5 comprises a single inner limb 18 extending from the input contact 2 and carrying at its outer end a three-legged arrangement comprising three outwardly extending substantially equal length legs 19 lying in a common plane. The free end of the centre leg 19 constitutes a contact portion for engagement with the associated output contact 3 and is concave in transverse cross-section as seen by the output contact 3 and has a larger radius of curvature than the co-operating contact portion of associated output contact 3. The two outer legs 19 are joined by an arcuate linking member 20 which extends towards the operating member 8 for engagement by the cam projection for projections 11 associated therewith.

[0015] The arrangement is such that when the linking member 20 of a contact arm 6 is not engaged with an associated cam projection 11 on the operating member 8, the centre leg 19 of that contact arm 6 is out of engagement with the associated output contact 3, as shown to the left in Figure 3, but when the operating member 8 is rotated relative to the base 1 to a position in which the linking member 20 is engaged by an associated projection 11, then the centre leg 19 is held in engagement with the associated output contact 3, as shown to the right in Figure 3, whereby the bridging contact 5 provides a connection between the input contact 2 and that output contact 3.

[0016] The cam projections 11 on the operating member 8 are arranged such that for each of the stable rotational positions of the operating member 8 relative to the base 1 a unique combination of connections between the input contact 2 and the output contacts 3 is established by the bridging contact 5, these connections enabling a coded decimal output to be obtained from the output contacts 3 from a voltage applied to the input contact 2.

[0017] The three-legged form of each of the contact arms 6 of the bridging contact 5 is advantageous in that it gives a two stage closing of the centre leg 19 on to the associated output contact 3, this ensuring ease of operation of the switch with a final high contact force between the centre leg 19 and the output contact 3. When the sloping surface 12 of a cam projection 11 first engages the linking member 20 of a contact arm 6, the whole contact arm 6 including the inner limb 18 is deflected about the connection of the contact arm 6 to the input contact 2 until the surface of the contact portion of the centre leg 19 of the contact arm 6 engages the associated output contact 3. Thereafter only the two outer legs 19 are deflected relative to the centre leg 19 thereby increasing the contact force between the contact portion of the centre leg 19 and the output contact 3 until the linking member 20 is engaged with the planar surface 13 of the cam projection 11. Further, since all three legs 19 are of substantially equal length whereby the linking member 20 engages the cam projection 11 substantially over the associated output contact 3 a high contact force is obtained for a relatively low applied force, and thus the switch is easy to operate.

[0018] In known switches, as shown in Figure 4, the surface of the contact portion of the centre leg 19 of each resilient contact arm 6 is planar, and the co-operating surface at the contact portion free end of the associated fixed output contact 3 is also planar. In such known switches as the operating member 8 of the switch is moved to close a particular contact arm 6 on to its associated fixed output contact 3, as indicated by the arrow in Figure 4, the leading sloping surface 12 of the cam projection 11 which is to effect the closure engages the linking member 20 of the contact arm 6, and moves the contact arm 6 towards the fixed contact 3, and the contact portion of the contact arm 6 engages the fixed contact 3. However, since the cam projection 11 is not moving solely longitudinally of the contact arm 6 but is moving transversely thereof, the contact arm 6 is not moved only downwards on to the fixed contact 3, but is moved across the fixed contact 3, and thus the surfaces of the contact portions of the contact arm 6 and fixed contact 3 are rubbed together this leading to fretting and corrosion as discussed above. Similarly such rubbing occurs when the operating member 8 is moved further to open the connection between the contact arm 6 and the fixed contact 3, as shown to the right in Figure 4. As is shown in Figure 4 such rubbing affects the whole surface of the contact portions of the contact arm 6 and fixed contact 3.

[0019] Referring now to Figure 5, as described above, in the switch of this invention the surface of the contact portion of the fixed output contact 3 is convex as seen by the contact arm 6, and the co-operating surface of the contact portion (centre leg 19) of the contact arm 6 is concave as seen by the fixed contact 3, the contact portion of the contact arm 6 having a larger radius of curvature than the co-operating contact portion of the fixed contact 3.

[0020] With such an arrangement, on movement of the operating member to close a particular contact arm 6 on to its associated fixed output contact 3 the same transverse movement of the contact arm 6 relative to the fixed contact 3 as described with reference to Figure 4 occurs, but in view of the shape of the co-operating contact portions, the contact portion of the contact arm 6 in effect rolls across the contact portion of the fixed contact 3 with the position of contact between the two portions moving across the surfaces thereof as shown from left to right in Figure 5. Thus, there is little if any rubbing of the surfaces of the contact portions over each other, and fretting and corrosion are kept to a minimum. Further, the parts of the surfaces of the contact portions, which are in contact when the stable closed position is established, are not in contact during the periods of maximum transverse movement of the contact arm 6 relative to the fixed contact 3, that is when the linking member 20 is in engagement with the sloping surfaces 12 of the cam projection 11, and thus fretting of these parts of the surfaces of the contact portions is kept to an absolute minimum.

[0021] Although the possibility of fretting has been discussed above with reference to Figures 4 and 5 in relation to a rotary switch of the type shown in Figures 1 to 3, it will be appreciated that a switch in accordance with this invention can be of any type in which there is the possibility of transverse movement of the resilient contact are relative to the associated fixed contact with consequent possibility of fretting of the surfaces of the contact portions of the contact arm and fixed contact.

Claims

1. An electrical switch in which a resilient movable contact arm (6) is urged into engagement with a fixed contact (3) by means of an operating member (8) in order to close the switch, the operating member (8) moving transversely of the contact arm (6) to effect such closure, and the contact portion of the fixed contact (3) being convex as seen by the contact arm (6), characterised in that the contact portion (19) of the contact arm (6) is concave as seen by the fixed contact (3), and has a larger radius of curvature than the co-operating contact portion of the fixed contact (3).

Ansprüche

1. Elektrischer Schalter, bei dem ein federnd nachgiebiger, bewegbarer Kontaktarm (6) mittels eines Betätigungselements (8) zum Schließen des Schalters in Berührung mit einem feststehenden Kontakt (3) gedrängt wird, wobei sich das Betätigungselement (8) zum Bewirken eines derartigen Schließens in Querrichtung des Kontaktarms (6) bewegt und der Kontaktbereich des feststehenden Kontakts (3), gesehen vom Kontaktarm (6) aus, konvex ist, dadurch gekennzeichnet, daß der Kontaktbereich (19) des Kontaktarms (6), gesehen vom feststehenden Kontakt (3) aus, konkav ist und einen größeren Krümmungsradius besitzt als der mit ihm zusammenwirkende Kontaktbereich des feststehenden Kontakts (3).

Revendications

1. Commutateur électrique dans lequel une lame mobile flexible (6) de contact est sollicitée de façon à porter contre un contact fixe (3) au moyen d'un élément (8) de manoeuvre afin de fermer le commutateur, l'élément (8) de manoeuvre se déplaçant transversalement à la lame (6) de contact afin d'effectuer cette fermeture, et la partie de contact du contact fixe (3) apparaissant convexe à la lame (6) de contact, caractérisé en ce que la partie (19) de contact de la lame (6) de contact apparaît concave au contact fixe (3) et présente un rayon de courbure supérieur à celui de la partie coopérante de contact du contact fixe (3).

Drawing