Technical field - This invention relates to interlocking devices for doors giving access to pressure vessels, electrical switch enclosures, machines, and other hazardous areas.

Background art - In using electromechanical switches for safety interlocking purposes, it is known to be preferable to use switches with contacts which can be mechanicall • forced into the open or off position, and such switches are preferably arranged so that door opening is accompanied by a forced movement and not by the action of a return spring in the off direction. Similar considerations apply to pneumati hydraulic and process valves, where it is also preferable to have a positive mechanical action to the off position. The mechanical forcing to the off position is known as positive mode operation, and is designed to overcome failures due to sticking, or in electrical switches, contact welding.

In most cases, forcing into the off position is achieved by a rotary motion, usually associated with a cam, (for most switches, and. for many position-sensing valves), but sometim associated directly with the rotary operation of a valve. The rotary motion may be derived from a machine guard hinge on which a cam is mounted, but this type of system has limit sensitivity and cannot be applied to removable guards. Methods are known for interlocking using rotary mechanisms driven by an exchange key (which is moved from switch to gua bolt) ; by a captive key, captive in the guard, which engages axially with the rotary switch or valve mechanism; or by a tongue device which engages with the rotary mechanism tangentially. This invention, relates to a development of th last mentioned device. It is well known in the domestic goods industry to have interlocks combined with various forms of latch mechanism, these latches helping to seal doors; vaguely similar systems exist in the-- utoclave (pressure vessel) industry. In neither case is the valve or switching mechanism positively operated, and no such devices exist

in fields where this mode of operation is required, though they would be very useful. This invention relates to this type of development.

Drawings - The invention will now be described by way of example by reference to the accompanying drawings, in which: Figure 1 shows an arrangement of the switch contacts, the rotary cam which operates them, and the arrangement of the tongue device which operates the rotary cam.

Figures 2 and 4 show arrangements of switch and lever operated bolt in ^" which the bolt acts as the operating tongue device.

Figure ^' 3 shows an adaptor for the arrangement in figure 2, enabling it to be applied to sliding doors.

One embodiment of the invention (figures 1, 2) comprises switch contacts 1 which are pressed towards the closed position by a spring 2. The contacts may be directly driven as shown, or of a type where snap action is followed by a direct break, or of snap acting type alone. For the switch may be substituted any valve, pneumatic or hydraulic, which is operated by axial movement of a plunger, or by other linear movement. The switch or valve is operated by a cam 3. The device may instead of linearly driving switch contacts or valve ports by means of cam 3, equally well drive a rotary control element directly. The cam is driven round directly, or indirectly via any known lost motion and overcentring mechanism, .by the tangential engagement of a tongue 4. A lip 14 on the tongue engages with a mating surface 15 of, the rotary device or cam to provide a positive drive in the "off" direction. Corresponding pairs of surfaces 16, 17 drive in the "on" direction.

In known devices, the tongue is fixed to the door to engage the switch and associated parts as the door is closed. In embodiments of the invention, it is arranged as shown to act as a fastening device for the door 5, operable to affect the switch only when the door is closed. Operation is by sliding, or as shown, by manual operation of a rotary handle

6 through a rack 7 and pinion 8, or other known mechanisms for door handles. It may be arranged with a spring 9 to have a slam action, and a ramp 10 permitting the door to be slammed without the bolt being retracted by the handle. The handle may incorporate lost motion as in a conventional household door handle.

In an alternative embodiment, as shown in figure 4, device may be made more secure by using a known arrangement of a magnet 11 in the door operating a spring 12 returned contact shielding device 13.

Increased security may be generated by using two tongues and two switch assemblies.

The tongue device may itself be designed to have a surface suitable for latching, or may be associated with a separate bolt driven simultaneously with it, or interlocked mechanically with it and operating separately.

Frequently it is needed to hold a door shut during the period of existence of a hazard, it not being possible to rely on the immediate disappearance of the hazard if the switch controlling it is switched off. Time delays or hazard sensors may be used therefore to operate a solenoid bolt, a- thermally operated bolt, or pneumatic or hydraulic or other bolts (depending on the types of control media available) to either (a) retain the door or (b) retain the latch mechanism or bolt. An arrangement of the latter type is shown inset in figure 2, wherein a retainer 18 is operated by a solenoid 19.

In the embodiment of figure 4, the parts are numbered to correspond with similar parts in figure 2. Locking means 21 m y be provided to prevent operation of the handle.

A solenoid bolt 34 is arranged to detain the latch bolt, in the embodiment of figure 2, being a miniature solenoid inserting a bar 35 in the path of the latch bolt.

For sliding doors, a ramp device 20 is used in lieu of the ramp 10 on the embodiment of figure 2.

It will be appreciated that the device of figure 2 may be mounted either inside or outside the enclosure, and the latch operating mechanism likewise, if appropriate to the

purpose of the enclosure.

Application - The invention may be applied to any domestic appliance or item of industrial plant or ^" machinery where an interlock and a door latching system are desired or required, and can be combined in a single unit.