ELECTRICAL SWITCH

Abstract: In an electric switch the contact portion of a fixed contact (3) is convex while the co-operating contact portion of a mov­ able contact arm (5) is concave and of larger radius of curva­ ture whereby lateral operating forces on the contact arm (5) cause the contact portions to roll rather than slide over each other in order to keep fretting to a minimum.

Description: Electrical Switch. 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. In some such switches the operating member does not move solely longitudinally of the movable contact arm, but moves transversely thereof, for example as in many rotary switches, 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. 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. 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, is characterised in that the contact portion of the contact arm is concave as seen by the fixed contact, and the contact portion of the fixed contact is convex as seen by the contact arm, the contact portion of the contact arm having a larger radius of curvature than the co-operating contact portion of the fixed contact. 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 if 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. This 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 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. The switch shown in Figures 1, 2 and 3 is similar to that described and claimed in European Patent Publication No. 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. The output contacts 3 are located at positions 0 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 equi-spaced 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. 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. 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. 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. 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. 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. 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 or projections 11 associated -therewith. The arrangement is such that when the linking member 20 of a contact arm 6 is not engaged with an associated cam projection ll 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. 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. 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 tç 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. 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. 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. 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. 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 arm 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: Claims: 1. 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, characterised in that the contact portion (19) of the contact arm (6) is concave as seen by the fixed contact (3), and the contact portion of the fixed contact (3) is convex as seen by the contact arm (6), the contact portion (19) of the contact arm (6) having a larger radius of curvature than the co-operating contact portion of the fixed contact (3). 2. A switch as claimed in Claim 1, characterised by a circular base (1) of electrically insulating material carrying an input contact (2) and a plurality of fixed output contacts (3), the input contact (2) having mounted thereon a bridging contact (5) having a plurality of resilient contact arms (6) respectively associated with the -fixed output contacts (3), and further comprising a circular operating member (8) mounted on the base for rotary movement relative thereto, the inner surface of the operating member (8) being formed with a plurality of cam projections (11) adapted and arranged such that on rotation of the operating member (8) the cam projections (11) engage the contact arms (6) of the bridging contact (5) and urge the contact arms (6) into engagement with the associated fixed output contacts (3), the operating member (8) having a plurality of stable rotary positions relative to the base (1), in each of which positions a particular combination of the contact arms (6) of the bridging contact (5) is closed on to the associated fixed output contacts (3). 3. A switch as claimed in Claim 2, characterised in that the base (1) is formed with an upstanding peripheral flange (7) having an inwardly directed lip (9) which engage over an annular shoulder (10) on the operating member (8) to mount the operating member (8) on the base (1). 4. A switch as claimed in Claim 3, characterised in that the flange (7) on the base (1) is divided into a plurality of circumferentially spaced segments (15) whereby the inwardly directed lip (9) is segmented, at least one of the lip portions (9') being adapted and arranged to engage in slots (17) in the shoulder (10) on the operating member (8) thereby to provide an indexing action on rotation of the operating member (8) relative to the base (1) and define the stable rotary positions of the operating member (8) relative to the base (1).

Inventors: GILISSEN, HERMANUS PETRUS J., SOES, LUCAS, TEN BERGE, EDUARDUS FRANCISCUS A.
Applicants: AMP INCORPORATED
Application year: 1981
Main classification: H01H19
IPC codes: H01H1, H01H19