Abstract:
An electric alarm siren of low power has a relatively high sound intensity and a good timbre due to a runner with arch-shaped blades. 
     The electric alarm siren includes a central part which is divided by a partition wall into a driving part with a built-in motor being enclosed in a compact whole by a cover, and into an acoustical part comprising the runner and a front part.

Description:
The technical problem that the invention should solve is the construction of a siren having a good timbre and relatively high sound intensity and the siren and electromotor being relatively small sized, compact and reliable. 
     An electrical alarm siren of low power is disclosed in the Yogoslav Pat. Appln. No. P 816/80, which corresponds to U.S. Pat. No. 4,393,374. The siren consists of a supporting part onto which a driving electromotor is fixed with a cover over the motor. The front sound generating part is made in the form of a funnel-shaped horn with air outlet orifices along the circumference. A runner is fastened to the motor shaft and is placed in the front part at the height of the orifices and has radial blades provided with air cutters at their ends. The siren has a good sound timbre, but does not reach a maximum efficiency, i.e. maximum loudness. 
     Another siren of this type is disclosed in the Patent FR 2 371 034. The siren has no housing, the runner is provided with a ring along the outer edge of the air cutters and rotates in the front part with orifices along the circumference. The runner has radial blades, the front part is not formed as a funnel-shaped horn. The sound efficiency is still lower than the siren according to the Yogoslav Pat. Appln. P 816/80, which corresponds to U.S. Pat. No. 4,393,374. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is the aim of the invention to provide an electric siren of low power with relatively high intensity and good timbre of sound. The low power electric alarm siren in accordance with the invention is made of aluminium casting in such a way that the runner has arch-shaped blades, which substantially affects the intensity and colour of the sound. The runner rotates in the supporting central part. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail with the aid of the drawings, which show: 
     FIG. 1 illustrates a side view of the runner 1; 
     FIG. 2 illustrates a front view of the runner 1; 
     FIG. 3 illustrates a front view of the siren in accordance with the invention; 
     FIG. 4 illustrates a side view of the siren in accordance with the invention; 
     FIG. 5 illustrates a front view of the central part 3; 
     FIG. 6 illustrates a side view of the central part 3; 
     FIG. 7 illustrates a side view of the siren in accordance with the invention with a partial sectional view. 
    
    
     DETAILED DESCRIPTION 
     The electric alarm siren in accordance with the invention includes the following parts: a runner 1 with arch-shaped blades 1c, a front part 2 having a funnel-shaped nozzle, a central part 3 with fixing accessories for an electromotor 5, a front part 2 axially extending outboard from the central part, and a cover 4 with slots 3c, a cover 4 and an electromotor 5. As shown in FIG. 7, front part 2 extends axially outboard from central part 3 which means that part 3 is closer to the longitudinal centerline (as viewed in FIG. 7 along the axial extent of the siren) as compared with part 2. Therefore, &#34;outboard&#34; refers to items farther from the longitudinal centerline or center plane as compared to another referenced item. Contrariwise, &#34;inboard&#34; means the item is closer to the center plane as compared to another reference item. Central part 3 includes a first portion which has circumferential slots 3c. Axially outboard from the first portion is front part 2. As shown in FIG. 7, front part 2 has a funnel-shaped nozzle open to the axial front end of the siren. 
     The runner 1 includes a circular plate 1a whose axially inboard back is flat, whereas in the centre of the front, at the radially innermost portion of the axially outboard front thereof, there is a convexity 1d with a bore for the shaft 5a of the electromotor 5. As can be seen by comparing FIGS. 1 and 7, the axially inboard face of runner 1 (the face on the right-hand side of the runner in FIG. 7) is flat. In contrast, the axial outboard face of runner 1 is not flat but has varying dimensions, i.e., compare in FIG. 1, rib 1b, convexity 1d and blades 1c. The convexity 1d extends as a continuity radially into several low ribs 1b that continue into higher blades 1c. Both the ribs 1b and blades 1c are arch-shaped away from the radius in such a way that the blades 1c tangentially approach the circumference of the runner 1, where they are formed as air cutters 1g. The outer arc 1e of the blade 1 c is shorter than the inner arc 1f, the outer arc 1e being approximately as long as the air cutter 1g. There are spaces between the air cutters 1g being as long as the air cutters 1g themselves. 
     The runner 1 according to FIG. 2 is suitable only for a clockwise rotation. 
     The inwardly bent front part 2 of the siren forms a funnel-shaped nozzle and has a circular opening 2a in the middle. The inner circumference of the front part is intended to be fixed to the central part 3, viz. to the ring 3j. 
     The central part 3 is cylindrically shaped member having at its first half air slots 3c in the shape of rectangles being uniformly distributed in pairs arranged along the circumference. 
     The number of pairs of slots 3c equals the number of the air cutters of the runner 1. The first half ends with an axially outboard ring 3j onto which the front part 2 is fixed. In the second half of the central part 3, two legs 3a are fixed to the siren. The second half also ends with a narrow ring 3b of somewhat smaller diameter than the central part 3 itself; there are three right-angled appendages 3d with screw holes on it. At one leg 3a there is an opening 3f for a connecting cable. 
     In the middle of the central part 3 there is a partition wall 3e with a bearing part 3g intended to receive the shaft 5a of the electromotor 5; the wall is smooth and separates the acoustical portion of the siren from the driving portion. There are two bores 3h in the partition wall 3e for screw fastening of the motor 5. On the partition wall 3e at the bores 3h, four brackets 3k support the core of the electromotor 5. 
     The cover 4 encloses the electromotor 5 thereby providing a protection and compactness of the siren. Under the cover 4 there is beside the electromotor 5 a space provided for a relay. There are also three screw holes 4a on the cover 4. 
     The siren is assembled by sticking the shaft 5a of the electromotor into the bearing part 3g and by fixing the electromotor 5 with screws through the bores 3h in the partition wall 3e of the central part 3. 
     The cover 4 is fastened to the central part 3 by means of screws through the screw holes 4a and appendages 3d. A power supply cable extends through opening 3f. 
     The runner 1 is stuck on the shaft 5a, thereupon the front part 2 is firmly pressed upon the border of 3b.