MAGNETIC DOOR CATCHES

Abstract: A magnetic door catch (10, 20), for a door (100) and frame (301) assembly of an oven, has a first magnetic member (10) slidable in a housing (102), on the door (100), against resistance supplied by a leaf spring (204). The member (10) has opposed pole pieces (12, 13) to attract a striking plate (20) on the frame (301). The pole pieces (12, 13) are dimensioned so that there is substantially no variation in the magnetic force between the member (10) and the plate (20) with change in temperature of the oven. The plate (20) is resiliently mounted on rods (302,303) by helical-coiled springs (S, S.) Alternatively, the housing (102) may be resiliently mounted.

Description: MAGNETIC DOOR CATCHES This invention relates to magnetic door catches and it relates especially, although not exclusively, to a magnetic door catch for use in an oven. A magnetic door catch typically comprises two main component parts, a first part, which is mounted to the door itself, and includes a permanent magnet, and a second part mounted to the assembly against which the door fits. The second part usually comprises a ferromagnetic striking plate which is disposed to engage the first part when the door is closed and to which it is attracted, thereby effecting the latching action of the catch. Such catches tend to wear well, and moreover continue to function despite a displacement of the door from its intended mounting position - as often occurs when a heavy door is used. Whilst magnetic door catches of the above-described kind are particularly convenient when used in such applications as drawers or wardrobes, for example, they tend to be unreliable in relatively high temperature applications (up to 2000C, say) an oven door, for example. This is due to the strong temperature dependence of the permanent magnet, which, as the temperature rises, results in a decrease in the force which needs to be applied to unlatch the door. Thus, whilst the door may be adequately latched at room temperature, say, it is likely to become quite unsafe at relatively high operating temperatures (2000C, say). The si of the permanent magnet could4 perhaps, be increased so that the force necessary to open the door, at the elevated temperatures, is sufficiently high, but this would also tend to increase the force required to open the door at low temperature up to an intolerable and/or unworkable level. Moreover, where mechanical catches are used, for example, in ovens, problems in opening and closing the oven door properly tend to arise due to manufacturing tolerances, differential expansion and hinge wear. It is an object of the present invention to provide a door catch suitable for use both at ambient and at relatively high temperatures, and which may be used instead of a mechanical catch where the aforesaid problems are likely to arise. According to the invention there is provided a magnetic door catch for a door and frame assembly, the door catch comprising a first member formed of a body of magnetic material disposed between pole pieces and a second member which is magnetically attracted by the first member for latching therewith, the said pole pieces being dimensioned so that the force of attraction between the two members does not substantially vary with temperature, at least in the range 200C to 2100C, a first resilient means, in use, to urge the first member in a direction opposite to the said attractive force between the first and second members to ardegree insufficient to overcome the attractive force present when the door is closed. Further according to the present invention there is provided a door and frame assembly including one or more door catches as described in the immediately preceding paragraph. Still further according to the present invention there is provided an oven including a door and frame assembly as described in the immediately preceding paragraph. The first resilient means is preferably a leaf spring which in use is positioned behind the first member in a housing on the door or frame, the spring being arranged to bear against a rear wall of the housing to allow sliding movement of the first member in the housing. The first member may be attached to a door or frame by means of a cover plate which extends over the member but allows at least part of the pole pieces to be exposed to attract the second member. The cover plate may be fixed to a housing in which the first member is mounted. Preferably, a second resilient means is provided which, in use, biasses one of the-first and second members to permit the door to open from the frame before the first and second members are parted from one another. The second resilient means therefore, allows a more controlled latching and unlatching operation since it cuts down the initial force required to open the door, by allowing one of the first and second members to move initially against biassing and relative to its fixing on the door or frame. The second resilient means, preferably, comprises two helical-coiled compression springs mounted on opposed sides of one of the first and second members on rods lying parallel to the force of attraction between the first and second members, said one of the first and second members being movable with the rods as the springs are compressed. Two embodiments of a magnetic door catch in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figures la and 1b show isometric views of two main component parts of the first embodiment which are to be respectively attached to the door and frame assembly of an oven, Figure 2 shows an end view of one of the two main component parts fitted with a cover plate; Figure 3 shows a cut-away sectional side view of the first embodiment in which the two main component parts are respectively mounted on a door and frame assembly of an oven; Figure 4 shows a graph illustrating the variation with temperature of the force necessary to open the oven door shown in figure 3; ; Figure 5 shows a sectional side view of the second embodiment which is similar to Figure 3, and Figure 6 shows a spring clip. Referring firstly to Figure la, a first member in the form of main component part 10 of a magnetic door catch, which may be mounted to an oven door, comprises a body in the form of a flat plate 11 made of a permanent magnetic material, such as barium ferrite or strontium ferrite, for example, sandwiched between a pair of flat plates 12 and 13 made of a magnetically soft material such as soft iron or mild steel. Since the ferrite material is magnetised perpendicularly to its major surfaces the plates 12 and 13 are magnetically attracted thereto to form pole pieces having respective south and north poles at their edges 12' and 13'. The component part 10 is co-operable with a second member in the form of another main component part 20 comprising a flat, ferromagnetic striking plate shown at 20 in Figure 1b, which is attracted thereto so as to effect latching of the door. As shown in figure la, each plate 12, 13 conveniently has a rectangular notch 14, 15 cut into its outer edge (14a, 15a) so as to create respective upper and lower spaced projections. The outer edges 14a, 15a overlap the flat plate 11 to form a gap 16. As shown in Figure 2, a cover plate 30 is provided, which has four rectangular apertures through which the projections protrude. The component part 10 may then be mounted in a convenient manner into a door and the cover plate 30 fitted by suitable means such as screws, so as to lie substantially in the plane of the inwardly facing surface of the door. The attachment of the door catch to an oven door and frame assembly is shown in the sectional side view of Figure 3. Part of an oven door chassis is shown at 100 in the left hand portion of the figure and this has an aperture 101 which accommodates a housing 102 in which is slidably mounted the door catch component part 10. The housing 102 is screwed to the frame by screws 104 which pass through flanges 102 on the housing but alternatively the housing could be integral with the frame. The part 10 is shown in partly cut away form so that one of the plates forming a pole piece (shown at, 12, say, in Figure 1) is shown partially in side view at 201 whilst the other plate (13 say) forming the other polepiece is shown in sectional view at 202. The plates 12 and 13 and the interposed magnetic body 11 are urged towards the cover plate by first resilient means in the form of leaf spring 204 which is seated against a rear wall 103 of the housing 102. The body 11 is slidably in the housing 102. Other first resilient means, a rubber pad or a spring, for example, could alternatively be use9. The striking plate 20, against which the door 100 is latched, in the right hand portion of the drawing is mounted, in a manner to be described below, into a frame 301 of the oven. The graph shown in Figure 4 illustrates the variation with temperature of the force required to open the oven door shown in figure 3, that is the force required to overcome the attraction between the main component parts, 10, 11 of the catch. In initial experiments the plates 12, 13 which form the pole pieces of the arrangement had a thickness of 2.7mm and, as shown in curve (a), the force required to open the door was found to decrease from a maximum of 9kgm at room temperature, to only 0 about 3kgm at 200 C (a typically operating temperature of the oven). This variation was quite unacceptable for an operative device since although the door may latch reliably at room temperature, when the oven is turned off, the door is relatively easily opened at the higher temperatures. In accordance with this embodiment of the invention, when fitted to an oven door and frame assembly, and as illustrated in curve (b) of the graph, this undesirable variation in the opening force was substantially making by reducing the thickness of the plates, 12 and 13, 1.4mum; the opening force then remaining substantially 0 constant (at 5kg) over the range of temperature of 20 C to 0 210 C. It is also envisaged that the opening force would remain substantially constant for temperatures outside this range. It will be appreciated that while in the aforedescribed embodiment the thickness of the plates 12, 13 is 1.4 mm the present invention encompasses the provision of pole pieces having alternative dimensions in which the force of attraction between the first and second members does not substantially vary with temperature. Hitherto, as previously explained, in opening the door, a relatively large initial force is required to overcome the latching force of the catch, but once this latching force has been overcome and the door has started to open the required opening force falls rapidly to zero, resulting in an uncontrolled unlatching operation. To alleviate this problem in the aforedescribed embodiment the striking plate 11 is not fixedly mounted to the body of the oven but is mounted so as to be capable of executing a small movement relative thereto against the action of second resilient means which acts in a direction opposed to the movement of the door as it opens. This is achieved by mounting on opposed sides of the striking plate 11 two spaced rods 302, 303, which pass through holes in the frame 301 of the oven. Each rod 302, 303 has a nut 304, and a helical compression spring S is interposed between each nut and the frame 301 to effect a resilient biasing action. This arrangement provides for a controlled opening of the door since when the door is initially opened the striking plate remains in contact with the magnetic part of the catch and moves with the rods 302, 303 against the resistance of the compression springs S only, thereby overcoming the inertia of the door. Figure 5 shows an alternative arrangement in which the second resilient means S (which is similar to that shown in the figure) is arranged to bias the component part 10 rather than the striking plate 20. As the door 1CO is opened the component part 10 moves a small distance out of the door with its housing against the action of helical coiled compression springs (only one of which is shown in the figure) until the force is overcome and the striking plate 20 separates from the part 10. In addition a rubber pad 305 is positioned between the striking plate 20 and the door frame 301 to ciishion the plate on the frame and reduce sound effects. Alternatively, a leaf spring could be used instead of one pad. The pad 305 is 2 mm thick. The cover plate 30 may be, as in this embodiment, fixed to the housing by crimping the ends of the plate over flanges on the housing, and/or so that the housing is capable of moving with the cover plate relative to the door. Figure 6 shows a bifurcated sDr1n clIp 7 which may be used in conjunction with each rod and helical coiled spring S instead of nuts 30. The spring clip Z co-operates with an annular notch cut into the rod (which eliminates the need for the rod to be screw-threaded) to limit the movement of the helical coiled spring along the rod. Although, in the two embodiments described the first member is to be mounted to a door and the second to a frame it may be preferred to have an arrangement where the first member is mounted to the frame and second to the door. One o more door catches may he provided on the same door and frame assembly.

Claims: What we claim is: 1. A magnetic door catch (10, 20) for a door and frame assembly (100, 301), the door catch (10, 20) comprising a first member (10) formed of a body of magnetic material (11) disposed between pole pieces (12, 13) and a second member (20) which is magnetically attracted by the first member (10) for latching therewith, characterised by the said pole pieces (12, 13) being dimensioned so that the force of attraction between the two members (10, 20) does not substantially vary with temperature at least in the range 200C to 2100C, and a first resilient means (204), for example, a leaf spring, in use, to urge the first member (10) in a direction opposite to the said attractive force between the first and second members (10, 20) to a degree insufficient to overcome the attractive force present when the door (100) is closed. 2. A catch (10, 20) as claimed in Claim 1 further characterized by a cover plate (30) to attach the first member (10) to a door (100) or frame (301), so that the cover plate (30) extends over the first member (10) and allows at least part (12', 13') of the pole pieces (12, 13) to be exposed to attract the second member (20). 3. A catch (10, 20) as claimed in Claim 2 further characterized by a housing (102) fixed to the cover plate (30) and in which the first member (10! is mounted in the housing (102) and slidable relative thereto against said first resilient means (204). 4. A catch (10, 20) as claimed in any one of the preceding claims further characterized by a second resilient means (S, S) which, in use, biasses one of the first and second members (10, 20) to permit the door (100) to open from the frame (301) before the first and second members (10, 20) are parted from one another. 5. A catch (10, 20) as claimed in Claim 4 further characterized by the second resilient means (S, S) comprising two helical-coiled compression springs mounted on opposed sides of one of the first and second members (10, 20) on rods (302, 303) lying parallel to the force of attraction between the first and second members, said one of the first and second members (10, 20) being movable with the ross (302, 303) as the springs (S,S) are compressed. 6. A catch as claimed in Claim 4 or Claim 5 fu-tner characterized by the second resilient means (S, S) biassing the first member (10) and a third resilient means (305), for example, a rubber pad, is provided to cushion the second member (20) against sound effects of the door (100) closing on the frame (301). 7. A catch (10, 20) as claimed in any one of the preceding claims further characterized by the body of magnetic material (11) comprising a flat plate, the pole pieces (12, 13) comprising flat plates, each pole pIece (12, 135 being, for example, of thickness 1.4 mm and the second member (12) comprising a flat plate, the body of magnetic material (11) being of, for example a permanent magnetic material such as barlum ferrite or strontium ferrite and the pole pieces (12, 13) being of, for example, a magnetically soft material such as soft iron or mild steel. 8. A catch (10, 20) as claimed in Claim 7 further characterized by mutually opposed edges of the pole pieces (12, 13) overlapping the body magnetic material (11) to form a gap (16) and said edges are notched to provide four spaced projections (12', 13'). 9. A door and frame assembly (100, 301) including one or more door catches (10, 20) as claimed in any one of the preceding claims. 10. An oven including a door and frame assembly (100, 301) as claimed in Claim 9.

Inventors: ADAMS, FRANCIS JOHN, BLADES, ADRIAN PETER, PLACKETT, ANTHONY GEOFFREY
Applicants: KENWOOD MANUFACTURING COMPANY LIMITED
Application year: 1981
Main classification: E05C19
IPC codes: H01F7, F24C15, E05C19