GB2135519A - Electrical safety switches - Google Patents

Abstract

A safety switch of the kind used to cut off the supply of electricity when a machine guard is lifted, comprises metal stops (96, 98), into engagement with which the contacts of a contact member (60) are moved, on movement of the contact member into a circuit- opening position, the stops affording heat sinks into which heat from the contacts will flow. The switch may comprise an actuator mechanism comprising a first part (26) and a second part (28) mounted on the first part for limited movement against the action of a spring (27). The second part may comprise a fast-action switching means (60, 62, 69) and the first part may carry a directly-actuated switching means (50, 52). In the operation of the switch in a circuit-opening operation, the fast action switch means is actuated first, followed by the directly-actuated switch means, and in the operation of the switch in a circuit-closing operation, the directly-actuated switching means is actuated first, followed by the fast- action switch means. <IMAGE>

Description

SPECIFICATION Electric switches This invention is concerned with improvements relating to electric switches, particularly of the kind (hereinafter referred to as being of the kind specified) comprising a pair of fixed contacts, and a contact member, movable towards and away from the fixed contacts between a closed position in which a conductive bridge is provided between said fixed contacts and an open position, and particularly of the kind in which the contact member is mounted for speedy movement, such as by the use of an overcentre device, between said positions.

Such switches frequently comprise a further pair of fixed contacts, so positioned that when the contact member is in its closed position in relation to said one pair of contacts, it is in its open position in relation to said further pair of contacts, and vice-versa. Such a switch may be used with one pair of contacts in a power circuit, and the other pair in a signalling circuit in which a lamp is illuminated when the power circuit is open.

However it may be desired to utilise a switch of the kind specified without such a further pair of contacts. For example in the specification of our application filed contemporaneously herewith there is described a safety switch comprising (a) a primary switching means comprising a pair of contacts and a directly-operated contact member, (b) a secondary switching means comprising a pair of contacts and a snap-action contact member, (c) an actuator mechanism operative in a circuitopening mode initially to move the contact member of the secondary switching means and subsequently the contact member of the primary switching mean, the two pair of contacts being electrically connected in series with one another.

Thus opening of the circuit by the primary switching means may be wholly in accordance with safety regulations, whilst prior to such operating the circuit is opened by said secondary switching means with minimum arcing. In such circumstances, utilising a further set of contacts for operation by the secondary set of contacts is not appropriate.

In such a switch desirably the snap-action contact member is moved into its open position into engagement with a stop, conveniently provided by the housing of the switch.

When switches of the kind set out in the last preceding paragraphs but one and two are tested, the contact member is moved back and forth repeatedly to open and close the power circuit, during which process the contact member gets hot. Where the contact member is moved between two pairs of contacts, no problem is encountered. However where the switch comprises only one pair of contacts associated with said contact member, and wherein when the contact member is moved away from said pair of contacts, the contact member is moved into engagement with a stop, the temperature attained by the contact member during testing tends to melt the plastics material from which the stop is provided.

According to this invention there is provided a switch of the kind specified, in which the contact member is associated with a single pair of contacts, wherein when the contact member is moved to its open position, the contacts are moved into engagement with stops affording heat sinks.

Thus preferably the stops are of metal, and are preferably provided by metal inserts provided in the switch at the positions in which the further pair of contacts would be provided were the switch to be of the kind set out the last preceding paragraph but four.

There will now be given a detailed description to be read with reference to the accompanying drawings, of a switch which is a preferred embodiment of this invention and which has been selected for the purposes of illustrating the invention by way of example. In the accompanying drawings: FIGURE lisa plan view of the preferred embodiment, the terminals thereof having been removed for clarity; FIGURE 2 is a longitudinal sectional view of the line 2-2 of Figure 1; FIGURE 3 is a longitudinal sectional view taken on the line 3-3 of Figure 4; and FIGURE 4 is a front elevation of a contact assembly of the switch.

The switch which is the preferred embodiment of this invention is a safety switch, and is designed specifically to comply with British Standards Institution Reguiation BS 4794 Part 2, Section 2.20, entitled "Control Switches".

The switch comprises a generally rectangular housing 6 moulded of plastics material and defining an interior cavity 7 within which an actuator member 8 is mounted for a longitudinal sliding movement, the actuator member being urged by a spring 9 into a first position, as shown in the drawings, in which it abuts against an interior wall 10 of the housing with a boss 12 protruding therefrom. Screwed into the end of the boss is an adjustment element in the form of a screw 14.

Extending transversely of the housing are two pairs of webs 1 6a, 1 6b and 1 8a, 1 8b, said webs guiding the actuator member 8 for sliding movement, and dividing the cavity 7 into three pairs of outer chambers 20a, 20b, 22a, 22b and 24a, 24b respectively (see Figure 1).

The actuator member 8 comprises a primary slide member 26 and a secondary slide member 28, constructed and arranged so as to be capable of being relatively easily assembled together in a manner in which the member 28 is capable of longitudinal sliding movement, said secondary slide member 28 being urged by a spring 27 into a first position relative to the primary slide member 26.

Mounted in the chambers 20a and 20b, in channels 32 thereof, are terminals 34, 36 respectively, carrying at their lower ends contacts 35, 37 respectively. Mounted in the chambers 22a and 22b, in channels 32 thereof, are terminals 38 and 40 respectively, carrying at their lower ends contacts 39 and 41 respectively. Mounted in the chambers 24a and 24b, in channels 32 thereof, are terminals 42, 44 respectively, carrying at-their lower ends contacts 43 and 45 respectively.

The switch which is the preferred embodiment of this invention also comprises a first contact assembly 50 carried by the primary slide member 26, comprising a conductive bar 52 which carries, at its opposite ends, contacts 54 and 56. The bar 52 extends through an aperture 58 in the slide member 26, and is urged by a spring 59 towards a first position in which the contacts 54 and 56 are in engagement with the contacts 35 and 37.

Carried by the secondary slide member 28 is a secondary contact assembly 60, comprising a conductive bar 62 which carries at its opposite ends contacts 64 and 66. The bar 62 is provided with a central aperture 68, through which a longitudinal part of the secondary slide member 28 extends, and an overcentre spring 69 extending between said longitudinal part and the bar 62 (see Figure 4).

Carried by the primary slide member 26 is a third contact assembly 70, comprising a conductive bar 72 which carries at its opposite ends-contacts 74and 76. The bar 72 extends through an aperture 78 in the primary slide member 26 and is urged by a spring 79 towards a first position in which the contacts 74 and 76 are in engagement with the contacts 43 and 45.

In use of the switch which is the preferred embddiment of the.invention, the supply of electricity which isto be controlled is connected to terminals 34 and 38 by means of screw clamps 34a and 38a thereof respectively, and the terminals 36 and 40 are bridged, and a circuit including a source of electricity and a signalling member (which may be a light bulb) is connected across the terminals 42 and 44.

- The housing 6 is connected in relation to a switch actuator, comprising a member 80 which moves, on operation of the actuator, in the direction A (Figure 1) into engagement with the screw 14 to move the actuator member 8 in said direction. During initial movement of the actuator member 8 from its first position, the primary and secondary slide members 26 and 28 move together producing, movement of the overcentre spring 69 relative to the bar 62 of the second contact assembly, and when said spring 69 is moved beyond an overcentre position, rapid movement of the secondary contact assembly 62 in the opposite direction is produced, causing the contacts 64, 66 to move out of engagement with the contacts 39 and 41 respectively, opening the circuit between the terminals 34 and 38.

On continued movement of the actuator member 8, the tongue 29 of the secondary slide member 28 engages an end wall of the housing 6, to prevent further movement of the slide member in the direction A, and hence prevent further movement of the second contact assembly 60.

The aperture 58 of the primary slide member 26 is sufficiently elongate in the longitudinal direction, that when the first contact assembly is in its first condition, as is shown in Figure 1, said first contact åssémbiy extends generally centrolly - through the aperture in the longitudinal direction.

Thus during the initial movement of the actuator member 8 as hereinabove described, the first contact assernbly is retained in its first position by the spring 59. However, after the actuator member 8 has been moved a sufficient distance to ensure under normal operating conditions actuation of the overcentre spring 59, an end wall 57 of the aperture 58 engages the bar 52 of the first contact assembly and moves the first contact assembly from its first position, separating the contacts 54 and 56 from the contacts 35 and 37.

Thus in the event of failure of the second contact assembly to open the circuit between the terminals 34 and 38, the first contact assembly 50, under direct actuation from the member 8, will be effected to open the circuit.

On final movement of the third actuator member 8 by the member 80, the contacts 74 and 76 of the third contact assembly 70 are moved into engagement with the contacts 43 and 45 respectively, closing the signal circuit and operating the signalling device.

If for any reason it is desired to dispense with the second contact assembly, the bridging between the terminals 36 and 40 may be removed, and the supply of electricity to be controlled may be connected directly across the terminals 34 and 36.

On retractive movement of the member 80, the third contact assembly 70 first opens the signalling circuit, and the first contact assembly 50 then establishes electrical connection between the terminals 34 and 36. However, since the terminals 38 and 40 are opened, this is established in the absence of flow of electric current. On final retractive movement of the member 80, the second contact assembly 60 is operated to close the circuit between the contacts 38 and 40, thus establishing a closed circuit between the contacts 34 and 38.

When the contact assembly 60 is in its closed position, the contacts 64 and 66 respectively engage the fixed contacts 39 and 41, the contact assembly providing a conductive bridge between said fixed contacts. However, when the contact assembly is in its open position, the contacts 64 and 66 would in a conventional arrangement engage stops afforded by the plastics body of the housing. Whilst under normal use of the switch this would present no significant problem, when switches of the kind set out above are tested, the contact member is moved back and forth repeatedly to open and close the power circuit, during which process the contact member gets hot.Thus, when such a switch is tested, the contact member is moved into engagement with the plastics material, and the temperature attained by the contact member during such testing tends to melt the plastics material from which the stop is provided.

Thus, the preferred embodiment comprises metal insert members 96 and 98 which are of inverted JTshape, being mounted on the webs 1 6a 4-nd 16b, with the iimbs of the members being located in. slots provided in the-web.

In this manner, when the switch is subjected to testing, during which the contact assembly 60 is moved repeatedly from its closed to its open position, the contacts 64 and 66 are moved into engagement with the members 96 and 98, which afford heat sinks, reducing the temperature attained by the contact assembly and hence the tendency for the contact assembly to damage the plastics body moulding.

Additionally, should the overcentre spring 69 break during use, there will be no possibility of it providing any undesired conductive path.

It will be appreciated that whilst the invention has been described in relation to a safety switch comprising a contact assembly operated by an overcentre spring, other forms of snap-action devices may be used to provide for fast movement of the second contact assembly.

Whilst in the preferred embodiment the terminals 36 and 40 are bridged, if desired the bridge may be removed, and the device utilised as universal 6-terminal switch, in which a pair of normally closed, slow-operated contacts are wired to terminals 34 and 36, a pair of quick make and break contacts are wired to electrically separate terminals 38 and 40 and'also to 42 and 44, and a pair of slow-operated normally open contacts may be wired to terminals 42 and 44.

Claims (3)

1. Q switch of the kind specified, in which the contact member is associated with a single pair of contacts, wherein when the contact member is moved to its open position, the contacts are roved into engagement with stops affording heat sinks.

2. A switch according to Claim 1 wherein the stops are of metal, and are provided by metal inserts. provided in the switch at the positions at which a further pair of contacts would otherwise be prqvided.

3. A switch according to Claim 1 comprising: (a) primary switching means comprising a pair of contacts and a directly-operated contact member; (b) a secondary switching means comprising a pair of contacts and a snap-action contact member; (c) an actuator mechanism operative in a circuit-opening modeinitially to move the contact member of the secondary switching means and subsequently the contacting member of the primary switching means, the stops being provided for engagement by the snapraction contact member.