FRAME ASSEMBLY OF A PHOTOELECTRIC SMOKE SENSOR

Abstract: A frame assembly wherein a frame (11) is large enough to cover the whole shield plate (10). and the frame (11) is engaged with and supported by a base plate (2) via the shield plate (10), to make it possible to simply and quickly assemble a photoelectric smoke sensor in which the frame (11) having a light-emitting unit (14) and a light-receiving unit (15) is se­ cured to the base plate (2) via the shield plate (10).

Description: The present invention relates to a light-scattering or light-reducing type photocell-type smoke sensor, and more particularly to a frame assembly for a photocell-type smoke sensor wherein a frame provided with a light emitting part and a light receiving part is secured to a base through a shield plate. There exist two sorts of photocell-type smoke sensors, i.e. a light-scattering-type in which smoke entering a dark box scatters a light beam from a light emitting part so that light reaches the light receiving part, and the other is a light-reducing type in which smoke entering the dark box shields the light beam from the light emitting part so that the light beam cannot reach the light receiving part. In both types it is known to provide a frame having a light emitting part and a light receiving part, and a shield plate to shield from outside noise, the frame being screwed to the base of the detector with the shield plate which forms the upper surface of the dark box lying therebetween. Thus, since in this conventional photocell-type smoke sensor the frame and the shield plate are secured to the base by means of screws, assembly is troublesome and it takes a long time, reducing productivity. Further, the shield plate used as an upper surface of the dark box is made of a metallic plate taking account of its function, and since it is larger than the frame there exists a portion which is not covered by the frame and could reflect the light beam. Therefore, in order to prevent such reflection a dark paint for absorbing light must be applied to the shield plate, so that the time for the assembly process is further increased. It is also known from other prior art smoke sensors to connect parts by interlocking cams (FR-A- 2 454 661) or snap-in elements (JP-U-136 981). However, such connections are restricted to secure two directly adjacent parts only without an intermediate plate. In view of the above circumstances it is a principal object of the present invention to provide a frame assembly for an initially mentioned photocell-type smoke sensor which allows a simple and rapid assembly in order to improve productivity. In accordance with the present invention this object is attained in that a frame assembly is provided in which the frame is dimensioned to wholly cover the shield plate, and a pair of elastically deformable engaging members are formed on the base or on the frame, each member having a hook portion to engage in a corresponding hole in the frame or in the base respectively to firmly secure the frame, the base and the shield plate together. In the course of assembly of a photocell-type smoke sensor in accordance with the present invention the shield plate is put on the base. When in the next step the frame is put on the shield plate the engaging members pass through the corresponding holes and the hook portions formed on the engaging members preferably engage stepped portions formed around the peripheries of the holes so that the frame is securely fixed to the base with the shield plate lying therebetween, and at the same time the whole of the shield plate is covered by the frame. Fig. 1 is a vertical sectional view of one embodiment of the present invention; Fig. 2 is a plan view of the base shown in Fig. 1; Fig. 3 is a plan view of the shield plate shown in Fig. 1; and Fig. 4 is a plan view of the frame shown in Fig. 1. Fig. 1 indicates a photocell-type smoke sensor 1 of the light-scattering type. A base 2 of the sensor has an integral shield case 3 and printed circuit board 4. As shown in Figs. 1 and 2, base 2 is provided with receiving portions 5 for receiving the terminals of a light emitting element and a light receiving element, a pair of holes 7 for engaging and holding elastic hook portions of a dark box 6, the holes 7 being oppositely disposed diametrically, and a pair of elastic engaging members 8 similarly oppositely disposed diametrically. These engaging members 8 are elastically deformable and have hook portions 9 formed at their ends each projecting radially inwards. A frame 11 is secured to the base 2 with a shield plate 10 lying therebetween. The shield plate 10 is, as shown in Fig. 3, made of a circular metallic plate having a diameter d, and formed with a pair of holes 12 for introducing the engaging members 8 formed on the base 2, and receiving holes 13 for inserting the projecting portions of a light emitting part 14 and a light receiving part 15 provided on the frame 11, respectively. The frame 11 is, as shown in Figs. 1 and 4, of a circular disk configuration having the same diameter d as the shield plate 10 to wholly cover it, and is provided at its lower surface with the light emitting part 14 and the light receiving part 15, a pair of holes 16 being adapted to receive the engaging members 8 formed on the base 2 and to pass them therethrough. The optical axes of the light emitting part 14 and the light receiving part 15 are inclined and intersect at a predetermined angle. On the radially inward edges of the holes 16 for the engaging members 8 are formed stepped portions 17 to engage the corresponding hook portions 9 of the engaging members 8. As shown in Fig. 1 the black box 6 comprises a protecting cover 18, an insect screen 19 and a labyrinth 20, and its inner space is in communication with the outer atmosphere. Further, in Fig. 1 the reference numeral 21 indicates a monitoring space, 22 a lens mounted in front of the light emitting element 14, and 23 a ceiling to which the smoke sensor 1 is mounted. Next, the assembly of the sensor will be explained. First the shield plate 10 is put on the base 2 and the engaging members 8 of the base 2 are introduced into the holes 12 formed in the shield plate 10. Thereafter the frame 11 is placed on the shield plate 10 by passing the engaging members 8 formed on the base 2 through the holes 16 formed in the frame 11. The hook portions 9 of the engaging members 8 of the base 2 are therefore elastically urged radially outwards by the inner peripheries of the holes 16 for the engaging members 8, and upon passing through the holes 16 the hook portions 9 project over the surface of the frame 11 and abut the stepped portions 17 owing to their elasticity, thus firmly pressing and securing the frame 11 to the base 2. At the same time the shield plate 10 is interposed between the frame 11 and the base 2, and the whole surface of the shield plate 10 is covered by the frame 11. It will be appreciated that the present invention is not limited to the embodiment described above and illustrated. For example, the elastically deformable hook portions 9 may be provided on the frame 11 whereas the holes 16 with stepped portions 17 may be formed in the base 2. With the described constitution of the sensor the frame can be secured to the base with the shield plate lying therebetween by merely putting the shield plate on the frame and urging them against the base. No screwing is required for the assembly. Therefore, contrary to the conventional smoke sensor of this kind the assembly can be done simply and rapidly, remarkably improving the productivity. Further, since the frame entirely covers the surface of the shield plate the plate need not be darkly coloured to absorb light. The elimination of the painting operation also increases the productivity.

Claims: A frame assembly for a photocell-type smoke sensor wherein a frame (11) provided with a light emitting part (14) and a light receiving part (15) is secured to a base (2) through a shield plate (10), characterized in that the frame (11) has a dimension (d) to wholly cover the shield plate (10) and a pair of elastically deformable engaging members (8) are formed on the base (2) or on the frame (11), each member having a hook portion (9) to engage in a corresponding hole (16) in the frame (11) or in the base (2) respectively to firmly secure the frame (11) , the base (2) and the shield plate (10) together. A frame assembly as claimed in claim 1 wherein stepped portions (17) are formed on the peripheries of the holes (16) to engage the hook portions (9). A frame assembly as claimed in claim 1 wherein the shield plate (10) is formed with holes (12) for passing of the engaging members (8) with the hook portions (9) therethrough.

Inventors: OHWASI ETUICHI NOHMI BOSAI KOG, OHWASI, ETUICHI NOHMI BOSAI KOGYO KABUSHIKI KAISHA
Applicants: NOHMI BOSAI LTD, NOHMI BOSAI LTD.
Application year: 1986
Main classification: G08B17
IPC codes: G08B17, G01N21, G01D11