US2949909A - Electromagnetic hammer - Google Patents

Description

Aug. 23, 1960 P. A. MACCHIONI ET AL ELECTROMAGNETIC HAMMER 5 Sheets -Sheet 1 Filed May 19, 1958 g- 1950 P. A. MACCHIONI ETAL ,9 9,909

ELECTROMAGNETIC HAMMER Filed May 19, 1958 5 Sheets-Sheet 2 lllllllllllllll l fllllllllllwillllll mmm lllllllll llllllllll 1 Aug, 23,1960

Filed May 19, 1958 P. A. MACCHIONI ETAL ELECTROMAGNETIC HAMMER Fig. 7

5 Sheets-Sheet 3 1960 P. A. MACCHIONI ETAL 2,949,909

ELECTROMAGNETIC HAMMER Filed May 19, 1958 5 Sheets-Sheet 4 l 1950 P. A. MACCHIONI ETAL 2,949,909

ELECTROMAGNETIC HAMMER Filed Kay 19, 1958 5 Sheets-Sheet 5 United States ELECTROMAGNETIC HAMMER Pietro Aurelio Macchioni and Luigi Macchioni, both of Piazza Statute, Turin, Italy This invention relates to an electromagnetic hammer of the type in which the striker adapted to act on the end or spigot of .a tool supported by the hammer casing, is securedly fixed to the movable portion of the armature of an electromagnet, the stationary portion of which together with the energizing winding are mounted in the hammer casing, resilient means, preferably helical metal springs being provided to oppose the attraction on the movable portion of the electromagnet armature, thereby determining the width of the gap or gaps where the electromaguet comprises in a well-known manner a three-branch armature, two branches embracing from the outside the coil, and a central branch being surrounded by the coil. In this known hammer construction both the striker and tool spigot are guided on their rectilinear reciprocating motion in suitable guides or bushes, so that it was essential to provide for periodic adequate lubrication of the sliding parts of the hammer by superior lubricants, more particularly lubricants resisting high temperatures (up to 140 C.) on account of the heavy work and stresses to which the said sliding parts are subjected.

The necessity of providing for periodic adequate lubrication of the hammer was per se a serious drawback and did not fully avoid the still more serious inconvenience deriving from the fact that both the striker and tool spigot slide in suitable guides provided in the hammer frame. Now, it is known that however perfect the sliding surface in the direction of the striker axis may be, friction cannot be fully avoided, since a quick reciprocating motion is involved apart from the braking effect due to magnetic leakage and ensuing magnetization of the movable parts. A further cause of friction at this region resides in a small torque set up periodically at the compression stage of theopposing spring acting on the striker. A further friction set up during operation of the hammer is generated between the tool spigot and its guide fast with the hammer frame or tapped collar in which the spigot is inserted. Experiments carried out on a number of unsatisfactorily lubricated hammers disclosed that the said friction leads to a loss in efliciency up to 50%. This can be explained by considering that, assuming one gram friction only on each striker stroke which performs 3000 up to 6000 strokes/minute, most of the power employed for performing .a certain work is consumed merely for overcoming the said friction. It was found that an insufficient lubrication may lead to a reduction in stroke of the striker such that the striker no longer strikes the tool. This annuls the hammer efliciency and gives rise to objectionable wear of the sliding surfaces. Further drawbacks reducing efliciency of known electromagnetic hammers are attributable to the unsatisfactory structure of the magnetic circuit of the electromagnet or, better said, the electric motor driving the hammer. In fact, eleotromagnets employed for driving electromagnetic hammers ,or'similar devices have their gaps extending perpendicular ;to the energizing coil axis,-which reduces atent attraction between the movable and stationary portions of the armature as flux is set up, or at an oblique direction with respect to the coil axis and direction of flow, or even of curvilinear shape. Though the latter arrangement is more advantageous at the initial attraction stage, it is no longer so at the end stage, on account of the fact that one component only extending in the direction of the coil axis or flux of the attraction force between the movable and stationary armature portions can be exploited.

This invention provides an improved electromagnetic hammer fully eliminating the above drawbacks which can be operated without any lubrication, the said hammer practically fully avoiding the friction sources mentioned above, so that the hammer can be operated without any lubrication, and maintain a high effectiveness and increased emciency owing to the absence of passive resistances, this resulting in considerable economy in operating costs.

A further object of this invention is to provide an electromagnetic hammer, the magnetic motor of which is equipped with a magnetic circuit of the improved structure adapted to minimize any loss in eificiency from the electric hammer part and aiford an optimum synchronism between the displacements of the movable portion of the electromagnetic armature and variations in magnetic flow.

A further purpose of this invention is to provide an electromagnetic hammer which is simple in construction, of low weight and at the same time sufliciently tough for withstanding heavy operating conditions imposed thereon.

A further purpose of this invention is to provide an electromagnetic hammer which can be operated with a satisfactory efficiency both on short and long strokes of the strikers at .frequencies of 3000 up to 6000 strokes/ minute, provided that the current frequency is 50 periods, and further employed for uses for which electromagnetic vibrators only were suitable.

According to this invention, the above purposes are attained by suspending the striker, movable portion of the electromagnet securedly fixed thereto within the hammer frame by means of eight springs, out of which four rear springs or opposing springs are adapted to space the movable portion of the armature from the stationary portion thereof and determine the width of the gap, the four or synchronizing springs being adapted to support the movable portion and follow the striker on its on and return stroke avoiding the latter from being released at any moment of its stroke.

A further object of this invention is to arrange the magnetic circuit of the electromagnet in such a manner that the outer gaps at least comprise a rectilinear portion extending perpendicular to the coil axis followed by a curvilinear portion, the gaps provided in the outer armature branches being arranged at the region situated between the striker and adjacent end of the energizing coil, the inner gap being spaced from the other coil end by an extent which is about /3 the overall coil length.

The above and further characteristic features and advantages will be clearly understood from the appended description referring to the accompanying drawings which show by way of a non-limiting example some embodiments of the invention.

On the drawings:

Figure 1 is .a longitudinal section of one embodiment of the improvedhammer,

Figure 2 is a part sectional elevational view of the hammer shown in Figure 1 looking over the arrow F,

Figure 3 is a view from below of the hammer shown in Figure 1,

Figure 4 is a sectional view on line IV-IV of Figure 1,

Figure 5 is a side view of a constructional detail of thehammer,

' Figure 6 is a plan view of the detail shown in Figure 5, v Figure 7 is a longitudinal sectional view of a further hammer embodiment,

Figures 8 and 9 are side views of two tools adapted to be fitted to the hammer shown in Figure 7,

Figure 10 is a lateral part sectional elevational view of a further embodiment of the improved hammer,

Figure 11 is a plan view of the tool connected with the hammer shown in Figure 10.

Figure 12 shows a constructional detail of a modification of the hammer shown in Figure '10,

Figures 13 to 15 show diagrams of the operation of the improved hammer,

Figure 16 is a side view of a further constructional detail of a modification of the hammer,

Figures 17 and 18 are part longitudinal sectional views of two further modifications of the hammer.

As shown in Figures 1 and 2 of the drawings, the hammer casing comprises a rear portion made up of two symmetrical members 1, 2, respectively, interconnected by a pair of nuts and bolts 3 and 4.

The portions 1, 2, are each formed with a rear extension shaped to partly form the handle 5 which supports a suitably positioned press button 6 acting through the spring 6a and a plunger 6b on the switch 7 secured to a body 8 of insulating material arranged at the bottom of the inside of the casing.

A connection 9 is situated at the level of the insulating body 8 on the remote side of the switch 7, for attachment of a current supply cable 10.

An energizing coil 11 is arranged within the casing portion formed by the members 1 and 2 coaxially with the longitudinal hammer axis. The coil 11 rests on the remote side of the handle 5 against the walls 12 transversely extending beyond the members 1, 2 and carrying on top perforated flanges which are connected by pairs by means of joints 13 to nuts and bolts visible in Figures 2 and 4.

The coil 11 rests on the remote side of the walls 12 on the stationary portion of the armature of the electromagnet, comprising a bundle of sheets and having an inner branch or countercore 14 and two outer branches 13 extending between the outer coil periphery and inner walls of the members 1 and 2. The movable armature, likewise comprising a bundle of sheets insulated from one another, has an inner branch 15 or movable core of the electromagnet and its outer branches 16 made of one piece with the core 15. The bundle of sheets forming the movable armature 16, 15 is arranged on the remote side of the counter-core 14 in a recess in the rear portion of the striker 17. The gaps between the counter-core 14 and movable core 15 and outer branches 13, 16, respectively of the stationary and movable armature are arranged and shaped to afford a maximum efficiency in operation of the hammer at any time. As shown in Figure 1 each gap comprises a straight portion extending transversely of the coil axis and a curvilinear portion. The experiments carried out disclosed that the best efficiency is obtained when the ratio of length taken across the coil axis of the rectilinear to the curvilinear gap portion is about 1:4. The air gaps are arranged in a novel manner with respect to the coil 11. The air gap 34 between the core 15 and counter-core 14 is spaced from the coil and bearing on the stationary armature by about one third of the axial coil length. The result is that the movable core 15 of the movable portion is encircled by a high number of turns of the coil 11. The outer gaps 35 are fully outside the coil, i.e. beneath its end resting on the walls 12 and outside its outer peripheral outline. This arrangement and configuration of the gaps afford the following advantages:

(1) Any interference between the dispersed flux portions, which unavoidably arises at the gaps, is avoided;

(2) A considerable eifective component is aiforded at any stage of the hammer operation.

For most of the flux is transferred at the initial stage of the magnetic attraction to the curvilinear gap portion, constantly yielding an effective attraction component tending to overcome the action of the opposing springs 23. At the end stage of attraction most of the flux is transferred to the rectilinear gap portions and, its attractive force being directed in a truly axial direction with high efficiency, thereby affording full synchronism between the variations in flux and movements of the movable armature and striker, and minimizing the unavoidable delay constantly arising between the sinusoidal variation in fiow and movement of the movable armature and striker.

All the above advantages favourably affect operation of the hammers both with long striker strokes (8 to 12 mm. at a speed of 3000 strokes/minute) and short strokes (from 5 to 6 mm. at a speed of 6000 strokes/minute).

A convenient modification of the outer gaps 34 is shown in Figure 16. This embodiment afforded the best results in practice. The striker structure is more clearly visible in Figures 5 and 6, showing the cross recess 18 receiving the bundle of sheets 15, 16 and four ribs in X-shaped arrangement, each ending by a cylindrical enlargement 2.0. The cylindrical enlargements are each formed with a through bore and two slightly conical seats on opposite sides 21, 22, respectively. The ends of four opposing springs 23 bear and are centred in the four seats 21, the seats 22 receiving the ends of four synchronizing springs 24. The purpose of these springs will be explained in detail hereafter. The four springs 23 bear by their other ends on four screwthreaded washers 25 fitted in seats 26 formed in the members 1, 2 and screwed on one end of the tie-rods 27 extending with a clearance through the bores in the enlargements 20 on the striker reaching beyond the said bores. The ends of the tie-rods 27 extending from the seats 26 have screwed thereon lock nuts 28. The ends of the four synchronizing springs 24 remote from the ends bearing and centred in the seats 22 are received by similar seats formed in the washers 29 screwed on a screw-threaded portion of the tie-rods 27. Lock nuts 30 are provided for locking the washers 29. The end of the rods 27 remote from the bolts 28 has screwed thereon a further bolt 31 securing the lower portion of the hammer body or cap 32 which closes from the outside the striker 17 and synchronizing springs.

The X-shape of the striker improves cooling of the coil by setting up an efiicient air draught flowing to the inside of the coil and issuing through a plurality of holes in the casing on the near side of the handle 5. The cap 32 has a seat 33 formed in its central forward portion, said seat receiving the bush or tapped collar 134 for the tool secured to the cap 32 by means of bolts 135. The rear portion or spigot 360: on the tool 36 is seated within the tapped collar 134. The spigot 36a is guided in the collar 134 through the interposition of a special linear ball bearing comprising a plurality of ball rows 37 mounted in a tubular cage 38. Grooves 39 are cut in the outer periphery of the spigot 36a and guide the balls 37 therein. Similar guides can be provided in the inner periphery of the tapped collar 134. The linear ball bearing shown in Figure 1 is adopted where the hammers are used for demolition purposes, when they operate under particularly heavy conditions and high frictions arise between the tool spigot and tapped collar as the operator presses the apparatus by his full weight to drive the tool into the material or release it therefrom. The adoption of linear ball bearings aifords a reduction of any sliding frictions as may arise between the spigot and threaded collar. This was confirmed by actual practice. The linear ball a bearing can be made otherwise than shown in Figure 1 and may comprise balls arranged in the supporting cage along a helical path. The latter embodiment is more particularly suitable when the tool should be capable of rotating during operation of the hammer about its own axis. 40 denotes annular flanges on the spigot 36a adapted to cooperate with the cross pin 41 for preventing accidental slipping of the tool oft its location in the collar 134 and preventing access of foreign bodies to the linear ball bearing.

The end of the spigot 36a extends beyond an axial opening in the cap 32 into the hammer to contact with the striker 17 as the opposing springs 23 return the striker towards the tool on ceasing of the flux. Figure 1 shows the striking stage of the tool. The clearance between the flange 40 and inner rib 134a on the collar 134 is visible. The opposing or rear springs 23 are adapted to push the striker forwardly and determine the gap width in the armature of the electromagnet.

The resilient properties of the springs 23 are selected so that the compression load required for fully compressing said springs is subjected at the end of the striker stroke due to the action of the magnetic attraction during energizing of the coil somewhat greater than the force of said magnetic attraction. The resilient properties of the four forward synchronizing springs 24- are such that these springs act on the striker by a total load lower than the load of the springs 23. A further purpose of the set of the above described eight springs is to follow the striker on its on and re turn stroke, preventing it from becoming released at any moment during the hammer operation. The above described structure fully avoids frictions and passive resistances on movement of the striker during operation of the hammer.

The hammer modification shown in Figure 7 differs from the embodiment described with reference to Figures 1 to 6 in that the central gap 34 is fully rectilinear. This arrangement is of course better suited with short stroke hammers (stroke length about 8 to mm. and 6000 striker strokes per minute).

The above hammer embodiment diflfers from the last described one in the structure of the tapped collar for the tool. The collar comprises two bushes 42, 43 receiving springs 44, 45 interposed between the opposite collar ends and collars 40a provided on the tool spigot 36.

The spring 44 is larger in cross section than spring 45 so as to be less resilient and avoids an untrained operator excessively pressing the tool thereby tending to reduce the striker stroke and unfavourably affecting the etficiency of the apparatus. The smaller spring 45 is more resilient but is more strongly compressed in order to hold the tool spigot constantly backward in the striker direction and ready to be impinged upon by the latter. On impingement the spring 45 which is more resilient easily yields allowing forward movement of the tool towards the work piece.

It will be seen from Figure 7 that the tool 36 is actually a tool holder into which a chisel 46 is inserted by means of a wedge connection.

The chisel could be replaced by other tools, such as for instance a fork 48 for vibrating reinforcing iron on casting concrete into moulds, as shown in Figure 8, or a ram 47 for vibrating moulds shown in Figure 9. In the modified hammer embodiment shown in Figure 10, the cap 32 is deprived of a tool-holding collar and is merely formed with an axial bore for penetration of the spigot of the tool 50 which is directly connected with the striker 17.

In the illustrated embodiments the spigot portion extending from the cup 32 has connected therewith a tubular member 51 having a tip 51a and provided with a plurality of lateral wing pairs 52, 53, 54 inclined to the axis of the tubular member 51.

The last described hammer embodiment together with a peculiar tool serves for vibrating concrete where considerable mass has to be shaken. The provision of the wings 52, 53, 54 allows transmission of vibrations directly to the mass without any loss in power. The vibrations set the sand granules in motion at a frequency which is approximately the frequency resonant with the hammer. The sharp end 51a of the tube 51 facilitates penetration thereof into the concrete mass. In the modification shown in Figure 12 the hammer is of a construction similar as shown in Figure 10. In this case also the tooi is directly connected with the striker. However, it comprises a fiat vibrating plate mounted for oscillation about a pivot 152 journalled in a fork shaped projection 153 on the cap 49.

The arm 154- securedly fixed to the plate which forms a bell-crank lever with the former, is articulated to the end of the spigot 155 secured to the striker.

Figures 13 and 14 show the variations in flux and feed current for short stroke hammers (about 5 to 8 mm.) and long stroke hammers (8 to 12 mm.) at frequencies of 6000, 3000 blows, respectively, per minute.

In the diagram shown in Figure 13, the hatched portions of the half waves correspond with the attraction periods of the striker, the portions 60 between the attraction periods corresponding with the rope ling periods of the movable armature connected with the striker, at the end of which the striker abuts the end of the tool spigot.

Figure 15 shows a characteristic diagram for the improved hammer of the behaviour in time of the striker displacements in connection with concrete vibration. This diagram shows the effectiveness of the blows upon the tool spigot at the end of the striker repelling stroke.

In the modified embodiments shown in Figures 17 and 18 the striker 1'7 is held between two springs 81, 82, respectively. The spring 81 is a helical spring and is interposed between an annular projection of the striker 17 and the bottom of the forward casing 32. secured by means of nuts 83 and bolts 84 to the hammer body. The spring 32 is a conical spring and is interposed between the other side of projection 80 of the striker 17 and an annular seat 85 hearing on the stationary electromagnet portion 13. This structure which employs a conical spring 82 having its larger diameter end facing the electromagnet, aifords the following important advantages:

(1) The conicity of the spring affords a small size structure at the hammer end supporting the tool as well as shortening of the overall hammer length, for a portion of the movable core 15, 16 can be arranged within the outline of the spring 82. In this case the structure of the lateral gaps can be conveniently selected of the type shown in Figure 16.

(2) Should the spring 32 break, the hammer is automatically arrested, for the conicity of the spring causes the portions 14 and 15 of the electromagnet armature to readily abut each other.

The embodiment shown in Figure 18 is advantageous where a hammer with a longer stroke of the striker 17 is desired without increasing the gaps. This purpose is attained by making the central electromagnet core 15 similarly to Figure 17 as well as the counter-core 14 of a movable type and causing the latter to bear on a reaction spring 86. In order to reduce the lateral gaps the core 15 and counter-core are guided by rectilinear bearings 88 quite similar to the bearing shown in Figure 1 adapted to guide the tool tail, Since during operation of the hammer the core 15 and counter-core 14 abut each other, they are provided at their facing ends with rims in the form of rings 87, 89 of highly resilient and tough antimagnetic material in order to reduce the core wear.

What we claim is:

1. An electromagnetic vibrator comprising a portable casing having a handle secured at one end and an opening provided in its other end, a coil coaxial with said openmg inside said casing arranged near the end of the latter opposite said opening, a magnetic core for said coil extending through-the latter, a complementary stationary armature embracing said coil from the outside, at least a part of said core facing said opening in the casing being movable in the axial direction of said coil, a striker inside the casing arranged between said coil and said opening, said striker being fast with said movable part of the core, a first and a second set of four springs symmetrically arranged around the common axis of said coil and said opening, the springs of both sets bearing each by its one end on a seat provided in the striker body, the other end of each spring of said first set hearing by its other end on a seat provided inside said casing near said coil and the other end of each spring of said second set bearing by its other end on a seat provided inside said casing near said opening, and means arranged at the end of said casing in which said opening is provided for supporting and guiding the end of a tool to be acted upon by the striker.

2. An electromagnetic vibrator comprising a portable casing having a handle secured at one end and an opening provi ed in its other end, a coil coaxial wtih said opening inside said casing arranged near the end of the latter opposite said opening, a magnetic core for said coil extending through the latter, a complementary stationary armature embracing said coil from the outside, at least a part of said core facing said opening in the casing being movable in the axial direction of said coil, a striker inside the casing arranged between said coil and said opening, said striker being fast with said movable part of the core, a first and a second. helical spring wound around said striker, a collar on the central portion of said striker, a pair of annular seats at opposite sides of said collar each for abutting one end of said springs, a seat for abutting the other end of said first spring situated inside said casing near said coil and a seat for abutting the other end of said second spring situated inside said casing near said opening, said first spring being arranged between said collar and said seat near said coil being of conical shape and having its end of larger diameter abutting on said seat lying near said coil, a linear bearing comprising a bush fast with the casing and surrounding a portion of the striker and several ball rows extending parallel with the bush axis and held by a common cage, said balls of each row being guided in grooves provided inside said bush and on the outer periphery of said portion of the striker, and means arranged at the end of said casing in which said opening is provided for supporting and guiding the end of a tool to be acted upon by the striker.

3. An electromagnetic vibrator as set forth in claim 2, having in front of said movable part of the core fast with the striker a counter-core movable in the axial direction of the core, a linear roll bearing for supporting and guiding the end of said counter-core opposite said part of the core fast with the striker and a spring interposed between said end of the counter core supported by said linear bearing being further provided for guiding the counter-core and for permitting the latter to yield under impacts it may receive from the movable core part fast with the striker during operation of die vibrator.

4. An electromagnetic vibrator comprising a portable casing having a handle secured at one end and an opening provided in its other end, a coil coaxial with said opening inside said casing arranged near the end of the latter opposite said opening, a magnetic core for said coil extending through the latter, a complementary stationary armature embracing said coil from the outside, at least a part of said core facing said opening in the casing being mavoble in the axial direction of said coil, a striker inside the casing arranged between said 8 coil and said opening, said striker being fast with said movable part of the core, a first and a second set of four springs symmetrically arranged around the cornmon axis of said coil and said opening, the springs of both sets bearing each by its one end on a seat provided in the striker body, the other end of each spring of said first set bearing by its other end on a seat provided inside said casing near said coil and the other end of each spring of said second set bearing by its other end on a seat provided inside said casing near said opening; a tool holder collar secured to the part of the casing in which said opening is provided, said collar being coaxial with said opening, a linear bearing comprising a ball row within said collar, a tool having its one end introduced in said casing through said collar and said opening, said bearing supporting and guiding the said end of a tool which is introduced through said opening in said casing for receiving impacts from the striker, and a cage for holding said balls aligned in rows parallel to the axis of said collar, the inner wall of said collar and the outer Wall of said tool end having provided therein axial grooves forming rolling tnacks for said balls.

5. An electromagnetic vibrator comprising a portable casing having a handle secured at one end and an opening provided in its other end, a coil coaxial with said opening inside said casing arranged near the end of the latter opposite said opening, a magnetic core for said coil extending through the latter, a complementary stationary armature embracing said coil from the outside, at least a part of said core facing said opening in the casing being movable in the axial direction of said coil, a striker inside the casing arranged between said coil and said opening, said striker being fast with said movable part of the core, a first and a second set of four springs symmetrically arranged around the common axis of said coil and said opening, the springs of both sets bearing each by its one end on a seat provided in the striker body, the other end of each spring of said first set bearing by its other end on a seat provided inside said casing near said coil and the other end of each spring of said second set healing by its other end on a seat pro vided inside said casing near said opening, a tool holder collar secured to the part of the casing in which said opening is provided and coaxial to the latter, annular bent in flanges at the ends of the collar, two springs inside said collar each bearing on one of said flanges, a tool having its one end introduced inside said casing through said collar and said opening for receiving impacts from said striker, tWo spaced flanges at the portion of said tool end arranged inside said collar for abutting the other ends of said springs bearing on said annular flanges, said spaced flanges having an outer diameter smaller than the inner diameter of said collar.

6. An electromagnetic vibrator comprising a portable casing having a handle secured at one end and an opening provided in its other end, a coil coaxial with said opening inside said casing arranged near the end of the latter opposite said opening, a magnetic core for said coil extending through the latter, a complementary stationary armature embracing said coil from the outside, at least a part of said core facing said opening in the casing being movable in the axial direction of said coil, a striker inside the casing arranged between said coil and said opening, said striker being fast with said movable part of the core, a first and a second set of four springs symmetrically arranged around the common axis of said coil and said opening, the springs of both sets bearing each by its one end on a seat provided in the striker body, the other end of each spring of said first set bearing by its other end on a seat provided inside said casing near said coil and the other end of each spring of said second set hearing by its other end on a seat provided inside said casing near said opening, a tool having the form of a tubular rod having its one end 9 introduced in said casing through said opening, means for securing said end of the tool to said striker, a plurality of lateral wing pairs inclined to the axis of said tubular rod secured to the part of the latter projecting outwardly of said casing.

7. An electromagnetic vibrator comprising a portable casing having a handle secured at one end and an opening providing in its other end, a coil coaxial with said opening inside said casing arranged near the end of the latter opposite said opening, a magnetic core for said coil extending through the latter, a complementary stationary armature embracing said coil from the outside, at least a part of said core facing said opening in the casing being movable in the axial direction of said coil, a striker inside the casing arranged between said coil and said opening, said striker being fast with said movable part of the core, a first and a second set of four springs symmetrically arranged around the common axis of said coil and said opening, the springs of both sets bearing each by its one end on a seat provided in the striker body, the other end of each spring of said first set hearing by its other end on a seat provided inside said casing near said coil and the other end of each spring of said second set bearing by its other end on a seat provided inside said casing near said opening, a rod secured to the striker and projecting outwardly from said casing through said opening, a bell crank lever having an arm hinged to the end of said rod projecting from said casing, a pin for rockingly supporting said bell-crank lever fast with the casing end at which said end is provided, said pin having its axis perpendicular to the axis of said opening and being laterally offset with respect to the latter, a blade secured to the other arm of said bell-crank lever, said other arm extending normally parallel with the axis of said opening.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,622 Weyandt July 31, 1945 2,399,977 Bardos May 7, 1946 2,436,992 Ernst Mar. 2, 1948 2,439,230 Weyandt et a1. Apr. 6, 1948 2,641,720 Stewart June 9, 1953 FOREIGN PATENTS 614,242 Great Britain Dec. 13, 1948 741,536 Great Britain Dec. 7, 1955

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