US2100660A

US2100660A - Electromagnetic tool of the percussion type

Info

Publication number: US2100660A
Application number: US47713A
Authority: US
Inventors: Greiner Max
Original assignee: Individual
Current assignee: Individual
Priority date: 1934-04-18
Filing date: 1935-10-31
Publication date: 1937-11-30
Anticipated expiration: 1954-11-30

Description

Nov. 30,'1937; M. GRElNER ELEGTROMAGNETIQ TooL 0F- THE PERCUSSION TYPE Filed Oct. 3l, 1935 2 Sheets-Sheet l 3 n M rhum m0 w A C. llmwlm. W3 Il /H M 3 2 M Wm 1./ w.. uw u. Il J f. 1 A /N/ /ll/ fr. u w 4| .Hh-WH n. 313 4 n :@:ll Q 2 H 1 *l .n n l .Il 3 m.. 0?/ M M M 7 /l f- A A l M. -Wm 7- v 4 1 4 lv, 2 :1jr f I/I 1 44 4 ..3 1 u.. h. 3. 5 w n Bannay. w u n w.. 2 2. Y 4

Patented Nov. 30, 1937 UNITED STATES PATENT OFFICE Max Greiner, Berlin-Charlottenburg, Germany Application october 31, 1935, serial No. 47,713 In Germany April 1s, 1934 19 Claims. 01. 172-7126) My invention relates to electro-magnetically operated reciprocatory tools of the percussion type, such as chisels or hammers, and, more particularly, to tools of this type operated by direct current'controlled by the reciprocatory mechanism of this tool.

An object of my invention is the provision of an electro-magnetically operated reciprocatory tool which is adapted for continuous service and will give a high specific power output in relation to its weight.

I attain this object by so dimensioning and combining the elements of the tool that the electric and magnetic losses and the consequent rise of temperature in continuous service is kept below satisfactory limits.

Further objects ci my invention are to provide improved means toi intermittently interrupt and close the electric circuits of the magnet coils of the tool and to provide improved spark-quenching condensers, whereby an unexcelled reliability of the interrupting contacts in continuous service is attained.

Further objects of my invention will appear from the description following hereinafter and the features of novelty will be pointed out in the claims.

My invention win be described nereinarier by way of its application toga reciprocatory tool of the percussion type, various modiiications of which are illustrated in the drawings.

Fig 1 is an axial sectionthrough my improved tool,

Fig. 2 is a section takenalong line 2-2 of Fig. 1,

Fig. 3 illustrates a side-view of the upper portion of Fig. 1, the casing being shown in section along line 3-3 of Fig. 2,

Figs. 4, 5, 6 and 7 represent various elements of the mechanism illustrated in Fig. 1, Fig. 4 being a section of the amature taken along line 4-4 of Fig. l,

Figs. 8 and 9 are partial axial sections through modiiied embodiments, the electrical controlling means and coils 'being omitted,

Fig. l is an elevation of a modified circuit interrupter for a tool constructed in accordance with the preceding figures,

Fig. 10a is a modication'of a detail of Fig. l0,

Fig, 11 is aplan-view of the mechanism shown in Fig. 10,

Fig. 12 is an elevation of a further modication of the circuitinterrupter,

Fig, 13 shows'a plan-view of Fig. 12.

In Fig. 1, I designates a mantle tube of soft steel which is closed at its upper end by a polepiece I3 having a peripheral flange overlying the end face of the tube I5 and is closed at its lower end by a pole-piece I2 each pole-piece having an axial boring. The parts I2 and I3 consist of soft steel too and are rmly united with the tube I5. For this purpose, they may be threaded and REiSSUED JAN 2 0 .1942

screwed into the tube I5. An annular member I4 preferably of a laminated structure comprising a plurality of sheet-steel rings of a thickness of about 11g ofan inch riveted together is loosely insertedl in the tube l5 intermediate the ends thereof.

As shown in\Fig. 6, the laminated annular member I4 is-preferably provided with a plurality of radial slots, the object of which will be explained later. The pole-pieces I2 and I3 which are preferably likewise provided with radial slots or recesses, as shown in Fig. 5, support a coaxially arranged tube I6 oi' smaller diameter which is iitted on cylindrical projections of the pole-pieces. This tube may consist of a suitable insulating material of high mechanical strength such as Bakelized paper or fabric, or it may consist of a diamagnetic metal, such as bronze.; In the latter case, the tube I6 is provided with two longitudinal slots separated from each other` and from the end faces of the tube by bridge-pieces ISG. These slots will reduce eddy-currents. 'Ihe tube I8 extends through and engages the inner periphery of the annular member I 4.

Within the tube I6, areciprocatory armature plunger I I is guided which also consists of soft ysteel and has an axial boring at its lower end,

such as a chisel, a riveting stock or the like.

The shank 24 is slidably guided in the axial boring of the member I2 and has a collar normally bearing against the lower end .face thereof. A helical spring 24' keeps the shankv 24 resiliently in its normal position and prevents the shank from being thrown out of the boring of the member I2. The spring 24 is screwed on threads provided on a lower cylindrical projection of the member I2 and its lower end of smaller diameter engages below the collar of the shank 24, l

The frame elements I2, I3, I4 and I5 and the armature plunger Il guided thereinr constitute two substantially closed magnetic fiux circuits of two coils. 25 and 26, the plunger II acting as a reciprocatory armature and the ring I4 being a1- ternately eiective in one or the other of the two 131.13 circuits. AThe function of the peculiar form of the inner projections of the pole-pieces I2 and I3 and of the associated ends of the plunger II will be explained later. The

coils

25, 26 which are alternately energized cause the plunger to be reciprocated. At the end of its downward stroke, the plunger strikes the tool shank 24. The upward movement of the plunger is limited by a rod I'I which is screwed or pressed into an upper cylindrical projection 64 of the plunger and pro- The cap I9 is preferablyl made of a high-grade steel, such as chromium-nickel steel, whereas the rod I1 consists preferably of a. diamagneticmaterial of high mechanical strength, such as diamagnetic nickel-steel, beryllium bronze or the like in order to improve the magnetic properties of the device.

The upper end ofthe rod I1 is reduced and threaded and provided with two annular nuts I8' and I8" which serve to slidably guide the upper end of the rod inthe cap I9 with little play and to operate the contact mechanism which will be described later. The spring-housing 2l of which a plan view is shown in Fig. 7, has two vertical borings through which the upper reduced ends of bolts 23 pass for attachment of the spring housing to a casting 44, as shown in Fig. 3, the housing being held in position on the bolts by nuts |23. The bolts- 23 are screwed into the ange of the member I3 and are provided with outer nuts 23' bearing on the top of the casting 44. This casting 44 has a tubular lower end engaging over the upper end of the tube I5 and `having an inner collar bearing upon themember I3. Hence, the bolts 23 and the nuts |23 and 23 will rmly unite the tube I5, the cover plate I3, the housing 2l and the casting 44. The upper end of the casting 44 is formed as a handle. To reduce its weight, it is preferably made of light metal.

The

magnet coils

25 and 26 are wound of insulated copper wire or aluminum wire and areimpregnated with varnish or another suitable compound and are baked in an oven, whereby the coils will be given the required rigidity which must be comparatively high because of the strong percussions acting on them in operation. Preferably, the coils are directly wound on the tube I6. If this tube is made of metal, ari-insulating layer 21' must be interposed, and discs 21 of insulating material are provided for supporting the coils on the interposed annular member I4 and on the pole-pieces I2 and I3. After the baking operation, the tube I6 and the coils, the annular member I4 and the insulating discs will constitute a rigid body which is preferably inserted in the tube I5 from below and is rmly clamped in position therein by themember I2 inserted into the lower end of tube I5.

Having now described the Aorganization of the mechanical and magnetical elements of the device, I shall now proceed to explain the circuit controlling means.

'I'he intermittent energization and de-energization of the coils is eilected by two pairs of contacts, one

pair

28, 28 being coordinated to coil 25 and one pair 29, 29' being coordinated to coil 26, compare Fig. 2. Preferably, each contact consists of a member of steel on which a thin plate of tungsten is welded. One of the contacts 28 which is not shown and the contact 29 are rigidly attached one above the other by welding, riveting or screwing in an angular bracket 30 of brass, bronze or steel which is rmly attached to the flange of member I3 by suitable bolts. The counter-contacts 28 and 29 are attached to springleaves 3I and 32 respectively, which consist of steel or bronze and are suitably clamped by bolts between a second bracket 36 (Fig. 2) and a counterplate 31, insulating plates being interposed between the spring-leaves 3 I, 32 and the

elements

36, 31. The bracket 36 is attached to the flange of member I3 by bolts.

In order to conduct the current to and from the contacts, two terminal members 38 of copper or brass are clamped in position below the counterplate 31, as shown in Figs. 2 and 3. The contact springs 3I' and 32 are so bent near their points of attachment to the bracket 36 that they tend to normally press the

counter contacts

28 or 29 firmly against the contacts 28 or 29', whereby the pairs of contacts will be given the tendency to resiliently stay in and return to closed position.

Cams 33 of bronze or steel are riveted to the springs 3I and 32 as shown in Fig. 3. These cams cooperate with a sliding member 34 of insulating material of high mechanical strength such as Bakelized fabric which is slidably guided on the vertical arm of bracket 30 and is attached to a lateral lug of a sleeve 35 slidably mounted on the rod I1. The sliding member 34 is so shaped that it may alternately ride on the two cams 33 and thereby open one or the other of the two pairs of contacts, whereas both contacts are temporarily closed when the sliding member 34 passes through its intermediate position.

The sleeve 35 which may -be bent of steel sheetmetal or may consist of a short piece of tube, may be alternately engaged by a shoulder 59 of the rod I1 and by the nut I8. Its length is so limited with regard to the distance of the shoulder from the nut that the sleeve 35 is positively carried along a short distance by the plunger II only, whenever the same approaches the ends of its strokes, and may then freely continue its movements under its inertia a little farther, whereby a rebound incident to the arrival of the sleeve 35 in its end positions will be avoided.

For putting the tool in and out of operation, I have provided a switch comprising two springleaves 42 carrying contacts 42 riveted thereto. The spring-leaves are bolted to the housing 2|, suitable insulating plates 43 being interposed. The end of the longer spring leaf carries an insulating piece 42" which enga-ges a lug of a nger piece 45 which is pivoted to the casting 44 and extends outwardly through a slot provided therein. Pressure exerted on the linger piece by the operators thumb will close the contacts 42, 42'.

Current is supplied to the tool from a suitable source by a ilexible cable 46 containing four conductors, two of which vserve to conduct the operating current, while two additional conductors of smaller cross-section connect the device with condensers 39 and 40 which are diagrammatically shown in Fig. 1 and will be described later. The cable 46 is attached to the casting 44 by a suitable clamping member 41.

In Fig. 1, I have diagrammatically shown the circuit connections of the coils, the contacts and the cable.

The positive terminal of a suitable source 4I of direct current which may be a generator or a battery, is connected by a lead 60 with the switch 42 from which the lead 60 extends to the terminals of the

coils

25 and 26. 'Ihe coils are so connected up as to energize their magnetic circuits in opposite directions, that is to say, the flux of coil 25 passes upwardly relative to the plunger Il, whereas the flux of coil 26 passes downwardly. Owing to thisv arrangement, any residual magnetism which otherwise might remain in the steel parts of the device after the reversal of the circuits, will be eliminated whereby the plunger Il is prevented from sticking magnetically in one of its end positions.

The ends of the coils are connected with the terminals 38 by leads 62, 63, as illustrated in Fig. 2, the coil 25 being connected to the contact spring 3l and the coil 26 to the contact spring 32 and thereby to 4the

contacts

28, and 29 respectively. The countercontacts are alternately connected through the bracket 30 with the structure of the device comprising the casting 44 and the tube l5, wherefrom a conductor 6l leads to the negative terminal of the source 4I of current. To reduce the sparking of the contacts, quenching condensers 39 or 40 respectively, are arranged in shunt to the pairs of contacts. The condenser 40 has a substantially larger capacity than the condenser 39 for a reason which will be explained later.

'I'he operation of the tool is as follows: When the sliding member 34 assumes the position shown in Figs. 1 and 3 and when the operator closes the contacts 42' by depressing the finger piece 45, the current flows by way of the pair of

contacts

29, 29 through the coil 26 only, whereas the coil 25 is not energized. Consequently, a flux is produced which will drive plunger Il upwardly with increasing velocity until the upper end of rod I 1 abuts against the end face of the cap I9. Shortly before the arrival of the plunger in its upper end position the sleeve 35 is engaged by the shoulder 59 and carried along upwardly together with the sliding member 34, whereby the sliding member 34 first permits the pair of

contacts

28, 28 to contact with each other and shortly afterwards opens the pair of

contacts

29, 29. For ayshort period both contacts are closed at the same time, whereby both coils will be energized simultaneously. Immediately after the contacts 29, 29' have been opened, the plunger H is resiliently arrested by the spring 20 which will reverse the motion and accelerate the plunger in downward direction.

Because of this fact, the plunger will immediately attain a considerable initial velocity on its downward effective stroke, this initial velocity corresponding to the energy mechanically imparted to the plunger by the spring 26. 'I'he energization of coil 25 whose circuit is closed shortly prior to the reversal of motion further increases the speed of the plunger Il on its effective stroke until the plunger strikes the shank 24 of the tool and transmits theretoall or nearly al1 of its momentum as the effective power output, the proportion of the momentum so transmitted depending on the'particular conditions of operation of the tool, for instance on the material worked upon. On the downward stroke of the plunger, the nut I8 will engage the sleeve 35 and carry the same along downwardly, where-` by the sliding member 34 will so control the contacts as to de-energize coil 25 and as to energize coil 26, thereby causing a reversal of motion, and the cycle of operation above described will be repeated.

stating that the buffer springis arranged to be struck by the armature.

The fact that the downward or striking stroke of the plunger takes place at a higher velocity than the upward or return stroke because of the initial speed imparted to the plunger by the buffer spring 20 and, therefore, takes place within a substantially shorter period of time, is of controlling importance for my invention, as will appear later. As the plunger itself positively controls the operation of the interrupter, the period of energization of the striking coil 25 is substantially shorter than that of the returning coil 26.

The broad principle of operation of the device and the combination of its elements were known prior to my invention except for certain features to be described later. I have been able for the first time to avoid excessive heating of thel device upon continued operation by so constructing, dimensioning and mutually arranging the elements of the device that the emciencyis considerably increased by a reduction of the losses, particularly the ohmic losses, in relation to the electric energy consumed. Furthermore, my invention results in a considerable increase of the effective energy of the percussion produced in relation to the weight of the device.

As a result of an extended investigation of the theory underlying the function of the device and of extended tests, I have found that the ohmic losses may be reduced to quite satisfactory limits by providing for a definite relationship between the mechanical motion of the armature plunger Il and the timed variable energization'of the magnet coils. More particularly, I have found that this relationship may be defined with satisfactory exactness by the ratio of the electric time constant of each magnet coil (computed for the position of the armature at the beginning of its -stroke produced by this coil) divided by the period of energization of the coil. This ratio will be termed the time constant ratio hereinafter. The period of energization depends on the number of reciprocations per time unit-due consideration to be given to the properties and timed operation of the interrupter-and is substantially shorter for the striking coil 25 than for the returning coil 26, as above explained. I have found that the percentage of the ohmic losses will become smaller the larger the time constant ratio is.

Therefore, according to my invention the electrical and mechanical elements of a reciprocatory tool of the percussion type are so dimensioned and organized that the time constant ratio of each coil will exceed the amount of .5 and will preferably vary between the limits of .7 and 1.0. As a rule, a further increase of this ratio beyond 1.0 would result in a design requiringexcessive cost of manufacture and would not materially improve the eiliciency.

My invention is based on the following phenomena: When the circuit of a coil of constant inductivity is closed, the current will gradually rise.to a maximum limitaccording to an exponential function of the time and will finally reach such limit determined by the voltage and the resistance. -The rate at which the current increases depends on the time constant of the coil. vIn any instant during the initial energization of the coil, the relative ohmic loss computed as a fraction of the iinal maximum input of` the coil is proportional to the square of the current computed as a fraction of the final maximum current. Hence, the specic ohmic loss at any instant computed as a percentage of the instantaneous input (which percentage determines the degree of efficiency) is directly proportional to the ratio of the instantaneous current to the nal maximumv current. Therefore, I endeavor to keep the instantaneous current as low as possible compared with the nal value it would reach upon a continued energization of the coil. The same theory applies to a coil having a. variable inductivity such as is the case with the coil of a reciprocatory tool because of the variation of its inductivity by the motion of the armature. If the motion of the armature is suiliciently rapid compared with the rate of the current increase, the current will never attain its theoretical final limit but will reach a certain maximum less than the iinal limit and will then drop because of the rapid increase in inductance consequent to the acceleration of the plunger until :dually the coil will be de-energized.

By properly controlling these conditions in accordance with the above rule regarding the magnitude of the time constant ratio, it has been possible to reduce the relative ohmic losses to a degree permitting of a continuous operation of an electric hammer of considerable power.

The above rule is a. result of extremely didicult and complicated theoretical investigations dealing with the timed development of the current and of the transformation of energy in the coils of tools of the reciprocatory armature type.

How the above-stated lower limits of the time 'constant ratio forboth magnet coils can be attained, will be evident for anyone skilled in the art, as the formulae for computing this ratio directly indicate the steps to be taken for increasing the ratio above the limit .5 or preferably above a limit between .7 and 1.0.

The attainment of this ratio will be considerably facilitated, however, by certain special measures which will be explained hereinafter.

The magnitude of the electric time constant of a. coil at the beginning of the stroke to be produced thereby is proportional to the product:

Square of the number of turns or windings multiplied with the permeance'of the magnetic circuit interlinked with the coil (the flux being computed for the initial position of the armature) and further multiplied with the electric conductance of the coil. The magnetic lpermeance and the electric conductance are proportional to the ratio of the cross-section to the length of the circuit. Therefore, the time constant ratio may be increased by the following measures:

(1) Increase of the number of turns and of the cross-section of the conductor. 'This measure, however, is apt to result in large dimensions with a consequent prohibitive cost of manufacture and weight of the tool and, therefore, is applicable with narrow limits only.

2) Increase of the permeance by a reduction of the length of the stroke and by increasing the diameter and by a suitable design of Vthe ends of the armature or plunger with a view to increasing the permeance of the air gap between the pole-pieces I2' and` I3 and .the opposed endfaceslvof the armature -II in the initial position of the latter. Preferably, the opposed faces are recess of the armature plunger II. 'I'his arrangement reduces the reluctance produced by the air gap.

As pointed out hereinabove, the period of energization of the striking coil 25 is substantially shorter than that of the returning coil 26. Because of this fact, I prefer under certain circumstances, particularly for tools of higher power, to dimension both coils and/or the permeance thereof diierently. Preferably, the permeance of the magnetic circuit interlinked with the returning coil is made larger than that of the striking coil. This will considerably aid in attaining a high time constant ratio of the returning coil.

In Fig. 9, I have illustrated a structure in which the initial permeance of the magnetic circuit of the returning coil is greater than that of the striking coil. The scale in which Fig. 9 is drawn is the same as that of Figs. 1 and 8, and

vthe same reference numerals are applied to corresponding parts. It will be noted, however, that the cooperating faces of the pole-piece 2I3 and the armature plunger 2II are conical, whereas the cooperating faces of the pole-piece 2I2 and the armature plunger 2II are plane. Owing to this arrangement, the permeance of the magnetic circuit of the upper or returning coil 26 is larger than that of the lower or striking coil 25. If desired, the conical profile of the elements 2I3 and 2| I may be replaced by a stepped profile as shown in Fig. 8.

(3) Increase of the frequency of oscillation of the armature by reducing the mass of the armature plunger without, however, reducing the cross-section thereof. The reduction of the mass need not impair the effective power of the tool, as the reduction in mass is compensated for by the incident increase of the velocity of the armature plunger.

The second object of my invention-increase of the specic percussive power-is attained by differently proportioning the magnet coils. This is an important feature of my invention. Preferably, the striking coil 25 is given a number of turns which is about half of that of the returning coil 26, and the cross-section of the wire of the striking coll 25 is about twice that of 'the wire of the returning coil. Owing to this arrangement, the ohmic resistance of the striking coil 25 is but a fraction of the resistance of the returning coil.` As the ohmic resistance of each coil circuit includes the resistance of the leads, the contacts etc., the ratio of the ohmic resistances of both circuits may be in the approximate order of from 1/2 to 1/6 depending on the particular requirements of the tool under consideration. I have found that in most instances, the ratio to be preferred is about 1A. A consequence of the dierent dimensions of the coils is that the striking coil will take up a substantially larger electric energy than the returning coil,

2,100,860 .whereby the difference between the periods of energization is compensated for. In spite of the substantially shorter period of energization, the striking coil may produce the same or even a larger percussive power per stroke than the returning coil, and the unavoidable production of heat by both coils in operation will be substantially alike. Therefore, the number of turns and the wire cross-sections of both coils may be so proportioned that with a given voltage the current taken up by both coils will reach the maximum limit which is permissible with regard to a rise of temperature in operation. It will be appreciated that the theoretical maximum of working power will be thus attained with a tool of given size.

As a part of the mechanical energy imparted to the plunger by the returning coil is dissipated when the motion of the plunger is reversed by the buffer spring 20 owing to imperfect elasticity thereof and to mechanical friction, I prefer to increase the energy imparted to the plunger by the striking coil in proportion to that imparted by the returning coil. To this end, I make the space accommodating the striking coil larger than that accommodating the returning coil, for

- instance in the manner indicated in Figs. 8 and 9, in which the annular member I 4 is arranged at a slightly smaller distance from the upper pole-piece than from the lower pole-piece. This has the further advantage of a reduction of the .rebound of the plunger in its upper position and of the consequent loss of mechanical energy owing to imperfect elasticity of the buffer spring 20 and of mechanical friction.

While each of the above-described novelfeatures of my invention, to wit a certain minimum limit of the time constant ratio and a difference between the dimensions of the coils, results in a substantial improvement per se, I prefer to combine both features and thus obtain a tool in which the mechanical energy transferred on the plunger in dependence of the electric energy taken up by the coils is so great and the mass of the armature plunger is so selected that the consequent frequency of oscillation will result in the desired magnitude of the time constant ratio.

While I prefer that each of the two coils should have a time constant ratio exceeding the aforestated limits, it is evident that a considerable improvement will result even if the striking coil only should have a time constant ratio of not less than .7,' particularly if the returning coil consumes a small share of the total electric energy only.

Further measures for reducing the losses are concerned with the elements conducting the magnetic flux. It is well known in the art that the losses owing to eddy-currents may be reduced by forming the entire flux circuit of a laminated structure. Because of the extraordinary stresses set up in a tool of the percussion type, I recommend, however, to make all those parts at least of compact steel which directly participate in the striking function. In the device shown in'Fig. l, these elements are the armature plunger II and the pole-pieces I2 and I3. These elements are subject to a variable magnetic flux even if the device is operated with direct current. While this flux will induce a useful counter-electromotive force in the coil energized at'` any time, it will at the Sametime produce undesired eddy-currents in the magnetic material.` If the magnetic elements would be formed by solid compact parts, these eddy-currents would become so strong as to materially reduce the eiiiciency. 'I'he production of eddycurrents would have the further disadvantage that the energy of the same will have to be supplied by the magnet coils, thus necessitating a larger current consumed and higher ohmic losses. Furthermore, the eddy-currents would partly countermagnetize the magnetic material, whereby the flux in the interior thereof would be considerably reduced, thus decreasing the permeance. In fact, the production of eddy-currents could practically jeopardize all of the above-explained advantages of my improvements.

In order to reduce the production of eddycurrents to an innocuous degree, I provide those elements conducting the flux, which promote the production of eddy-currents owing to a small circumference and a large cross-section, with suitably positioned slots subdividing the elements, these slots being so positioned that the mechanical strength of the elements with regard to the percussive stresses set up therein will not be materially affected. 'Ihis is the reason why in Fig. 1 the armature II and the pole-pieces I2 and I3 and the annular member I4 are provided with radial slots disposed parallel to the axis of the tool.

Fig. 4 illustrates a cross-section of the plunger II. It will be noted that the radial slots 66 extend to a depth of about 1/3 of the diameter and are substantially coextensive with the plunger in the axial direction thereof. Towards the ends of the plunger, however, the depth of the slots is slightly reduced with regard to the mechanical strength of the plunger. 'According to Fig. 2, eight such slots are provided. The larger the number of the slots is, the more eiective will they be. I'he radial slots 61 provided in the pole-pieces I2 and I3 extend clear through the pole-piece, as will appear from Fig. 5 showing a plan View of the pole-piece I2. 'I'he different numerous axially extending fingers formed by the slots 61 are united by a comparatively thin plate or flange integral with the sectors. Preferably, the radial slots 61 extend into the flange or plate to a certain depth, for instance, half of the thickness of the flange, as shown in Fig. 1, whereby the induction of eddy-currents therein is reduced.

In these flanges and in the annular member I4, the eddy-currents ow in a peripheral direction. For this reason, the annular member I4 is provided with radial slots B8 too, as shown in Fig. 6, to increase the electric resistance to eddy-currents. The slots may be provided in the outer periphery of the annular member Il only, as shown in the upper part of Fig. 6, or the slots may be alternately provided in the outer and in the inner periphery, as shown in thelower part of Fig. 6. Preferably, the annular member 'I4 is made of laminated material, the different sheetmetal plates being united by rivets.

The slots 61 and 68 in the pole-pieces I2 and I3 are preferably made sufficiently wide as to provide for ventilation of the interior of the tool for cooling purposes. Fresh air may be drawn into the tool from the outside atmosphere by the plunger through holes I5' provided in the mantle tube I5, as shown in Fig. 1.l The air flows through a gap left between the periphery of the coil and the inner surface of the mantle tube I5 and ows through theslots 61 and enters the interior of the tube I 6, thus cooling all of these elements. When the tool is Ato be used in an atmosphere filled 6 with dust, no holes |5 should be provided. Nevertheless, the intense circulation of the air agitated by the reciprocatory amature will promote the conveyance of the heat produced in the interior of the coils towards the outer surfaces, particularly to the mantle tube |5.

To reduce sparking and arcing on the

contacts

28, 28 and 29, 29 quenching condensers 39 and 40 are provided. The capacity of these condensers is of controlling importance for the development of the electric current upon interruption of the circuits and thus for the efficiency of the tool and, for this reason, should depend on the characters of the coils. This means that the capacity of the condenser cooidinated to the striking coil should be substantially larger, about twice, three times, or four times as large, than that of the condenser coordinated to the returning coil. As very high capacities amounting y material employed for the

contacts

28, 28 and 29,

29', that is to say, should lie below a limit from 25 to 30 volts. In this event, the risk is eliminated that an arc which might form on account of a temporary disturbance in the contact operation, would stayand would destroy the interrupter. If the tool were operated with a current of higher voltage, the formation of an arc would have destructive eiects. Furthermore, the use of a low voltage permits a further simplification of the interrupter, as the gap between the contacts need not belarger than 1/4 to 1 mm. Moreover, the insulation may be thin. As to the magnet coils, the low voltage has the advantage that on account of the smaller number of turns and of the greater cross-section of the conductors, the coils will represent a rugged structure which is not liable to be affected by the percussive stresses. The use of a .low voltage of operation is not limited to tools of small power. I have found that it oiers particular advantages if applied to large tools having a power of 1 kilowatt or more.

Although the above explanation of my invention will enable any electrical engineer to so design' the electric tool and to so dimension its elements that the benecial results of my invention will be obtained, suitable dimensions of the specific embodiment shown in Fig. 1 will be given hereinafter by way of example. Itis to be clearly understood, however, that my invention is in no way limited to these specic dimensions.

The diameter of the armature plunger may be 32 mm. and its stroke'35 mm. In this event, the striking coil 25 is preferably wound of wire havl ing a cross-section of 3.8 mm..2 and a diameter of 2.2 mm. The number of its turns is 180. The

wire of which the returning coil 28 is wound has a diameter of 1.3 mm. and a cross-section of 1.32

Imm?. The number of turns is 358. The operating voltage amounts to from 20 to 22 volts. With the interrupter as shown, the tool will have astroke' and the returning stroke require a dif ferent time, has the following result:

. The period of energization of the striking coil is about .015 second. The period of energization of the returning coil is approximately .036 second. The resistance of the entire circuit of the striking coil is .21 ohm. The magnetic conductivity of the ux at-the beginning of the downward stroke is 8.2 cm. The resulting time constant of the striking coil is .016 second.v The time constant ratio of the striking coil is 1.06. The resistance of the entire circuit of the returning coil amounts to .89 ohm. The permeance at the beginning of the upward stroke (with rod |1 consisting of magnetic material) amounts to 12.2 cm. and the resulting time constant of the returning coil computed for the beginning of the upward stroke is .022 second. The time constant ratioof the returning coil is .61. The electric energy consumption of both coils is about 420 watts.

The attainment of a high efficiency of the tool requires that the closure and the interruption of the circuit of the magnet coils be very accurate. While the accuracy of the interrupter illustrated in Figs. 1, 2 and 3 is quite satisfactory, I have devised improved interrupters of which two modications are shown in Figs. 10, 11, 12 and 13. These interrupters operate on the principle of the overcentering toggle. In Figs. 10 and 11, the elements corresponding to the embodiment shown in Fig. 1 are designated by the same reference numerals. I have modified the arrangement of the pairs of contacts |28, |28' and |29, |29 and of the spring-leaves |3| and |32 and of the terminals |38, while the function of these elements is the same as that of the corresponding elements in Fig. 1. It will be noted that the contacts |28' and |29 are mounted on a common spring-leaf |48 which may be bent upwardly or downwardly to interrupt and close the circuits alternately. The spring-leaves |3| and |32 mounted above and below the lspring-leaf |48 may be reinforced by auxiliary leaves |3|' and |32'. All of the springleaves and the terminals |38 are mounted on the ange of the pole-piece |3 and are bolted thereto by insulated bolts |50, suitable insulating pieces |49 being interposed. A short link |5| has onev end hinged to the free end of the middle springleaf |48 and the other end pivotally mounted in a rigid bracket |52 attached to the pole-piece. In

. the embodiment shown in Fig. 10, link |5| is formed by a helical spring which is so biased as to exert pressure on the free end of the spring |48. For the reduction of wear of the hinges or pivot points, small bushings |53 of bronze may be provided which are freely rotatable on pins |54 inserted in the free ends of the spring |48 and of the bracket |52 and are engaged by the ends of In the embodiment illustrated in Fig. 10a, the link 25| corresponding to the link |5| of Fig. 10, is rigid while the bracket 252 is made of springmaterial, for instance of a thin steel ribbon. The

link |5| or 25| and the spring-leaf |48 form an overcentering toggle tending under the pressure exerted by the link |5| or by the bracket 252 to assume the collapsed upper or lower position in which the spring |48 is deflected upwardly or downwardly, thus closing one or the other of the pairs of contacts. The alternate closure and interruption of the circuits of the magnet coils is directly effected by the armature. To this end,

the shoulder 59 and the nut I8 act on`the springleaf |48 in upward or downward direction through the intermediary of interposed helicalusprings |55 anc/i155. Owing to the provision of the springs, a percussive eiect on the spring-leaf |48 is avoided. The spring-leaves I3I` and |32 are provided with large openings aiording a free passage to the rod I1 and the helical springs |55 and |I55, whereas the opening provided in the spring-leaf |48 for the passage ofthe rod |1 is so small that the springs |55 and |55' may abut against the spring-leaf |48.

The modification illustrated in Figs. 12 and 13 is similar to that of Figs. 10 and ll diiering therefrom by the provision of a single comparatively thin spring-leaf 348 acting as a toggle. Both ends of this spring-leaf are firmly clamped between insulating elements 349 and 58 attached to the pole-piece I3 by bolts 35|) and 51 in such a way that the spacing of these clamping elements is slightly smaller than the free length of the interposedspring-leaf 348. Therefore, the spring-4 leaf is slightly biased and tends to assume an upper or a lower position in which it is bent. Two auxiliary spring-leaves 348" are riveted to the spring-leaf 348 and project into the path of shoulder 5S and of the elongated nut 3I8 on the rod I1, as will appear from Fig. 12. The three leaves 348 and 348" are provided with openings for the free passage of the rod I1. It is evident that the alternate engagement of one or the other 'of the spring-leaves 348" by the shoulder 53 or scribed with reference to Fig. 10, the various spring-leaves and the associated terminals 338 are rigidly mounted on the pole-piece I3 by means of interposed insulating plates 349 and of the.

insulated bolts 350.

If the right-hand end oi' the leaf 348 clamped between the insulating blocks 56 were cut off, the rod I1 would still actuate the contacts but, in this event, there would be' no snap action. From this consideration it will become apparent that the spring-leaf 348 may be regarded as composed of two sections, one constituting the contact carrier supported between the insulating blocks 349 for operation by the reciprocating rod I1 and the other section clamped between the insulating block 58 constituting overcentering means. Therefore, the overcentering means is integral with the contact carrying section of the springleaf in this embodiment.

This modification of the interrupter has the advantage of the absence of hinges or pivot points or of other elements subject to frictional wear. As the most delicate element of tools-of the type here under consideration is ordinarily the interrupter, the improvement thereof described hereinabove will greatly enhance the reliability of operation.

The described improvements resulting inl an increased eiliciency and specific percussive power are in no way limited to tools of the design illustrated in Fig. 1, but are broadly applicable to tools of any kind which are operated by direct current on the self-control principle.

Although I have described my invention with reference to certain species, I wish it to be clearly understood that numerous modifications may be made without departing from the spirit of my invention.

What I claim is:

1. Electromagnetic tool of the percussion type, comprising frame elements and a reciprocatory each associated with one of said circuits to induce a flux therein, a source of current, and an interrupter interposed between said coils and said source of current and adapted to be operated by said armature and to alternately close and interrupt the electric circuits of said coils, whereby said armature will be reciprocated by the alternate eect of the fluxes in said magnetic circuits, the interrupter, the number and cross-section of the turns of said coils and the mass, cross-section and stroke of the armature and of said frame elements being so dimensioned that the consequent electric energy consumption of. the coils and the consequent frequency of oscillation of the armature will result in a ratio of the electric time constant of each coil which, when computed for the position assumed by the armature before the stroke thereof produced by said coil, divided by the period of energization of said coil is not less than .5.

2. Electromagnetic tool of the percussion type, comprising frame elements and a reciprocatory armature guided therein constituting two substantially closed magnetic flux circuits, two coils each associated with one of said circuits to induce a flux therein, a. source oi. current, and an interruptor interposed between said coils and said source of current and adapted to be operated by said armature and to alternately close and interrupt the electric circuits of said coils, whereby said armature will be reciprocated by the alternate eiect of the iluxes in said magnetic circuits, the interrupter, the number and cross-section of the Aturns of said coils and the mass, cross-section and stroke of the'armature and of said frame elements being so Vdimermioned that the consequent electric energy consumption of the coils and the consequent frequency of oscillation of the armature will result in a ratio of the electric time constant of. at least one of the coils which, when computed for the position assumed by the armature before the stroke thereof produced by said coil, divided by the period of energization of said coil is not less than .7.

3. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein constituting two substantially closed magnetic flux circuits, resilient buier means anda tool-shank arranged on opposite sides of said ilux circuits to be struck by said armature, a returning coil associated with the ilux circuit adjacent to said buffer means, a striking coil having a smaller number of turns than said returning coil and being associated'with the flux circuit adjacent to said tool shank, a source of current, and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current, the resistance of the entire electric circuit of said returning coil being from three to six times as large as the resistance of the entire electric circuit of said striking coil.

-4. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein constituting two substantially closed magnetic ux circuits, a buier spring and va tool shank arranged on opposite sides of said ux circuits to be struck by said armature, a returning coil associated with the ilux circuit adjacent to said buffer spring, a striking coil having a smaller number of turns than said returning coil and being associated with theux circuit adjacent to said Vtool shank, a source of current and an interrupter interposed between said source of current and said coils and adapted to beoperated by said armature, the resistance of the entire electric circuit of said returning coil being from three to six times as large as the resistance of the entire electric circuit of said striking coil, the interrupter, the cross-section and stroke of said armature and of said frame velements and the number and cross-section of the turns of said coils being so dimensioned that the consequent electric energy consumption of the coils and the consequent frequency of oscillation of the armature will result in a ratio of the electric time constant of said striking coil which, when computed for the end position of the armature abutting against said buier spring, divided by the period of energization of said striking coil exceeds the amount of .7.

5. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein, two coils of different dimensions arranged to produce alternate iiuxes causing said armature to reciprocate, a source of current, and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current, the aforestated elements being so dimensioned that the consequent frequency of oscillation of said armature will result in a time constant ratio of each coil exceeding .5.

6. Electromagnetic tool of the percussion type comprising a mantle-tube of. magnetic material, pole-pieces inserted in the ends of said tube and having inward projections, an annular pole member inserted in said mantle tube between the ends thereof, an inner tube of diamagnetic material carried 'by said projections and extending through said pole member, an armature guided for reciprocation between said'projections within said inner tube, two coils mounted between said tubes on either side of said annular pole member, each adapted to induce a magnetic iiux through one half of said mantle-tube, through the pole-piece coordinated therewith, through said armature and through the annular pole member, a source of current, Vand an interrupter interposed between said source of current and said coils and adapted to`be operated by said armature and to alternately close and interrupt the electric circuits of said coils, whereby said armature will be reciprocated by the alternate eect of the uxes `produced by said coils, the' afore-stated elements being so dimensioned that the consequent electric energy consumption of the coils and the consequent frequency of oscillation of the armature will result in a time constant ratio of each of said coils exceeding the amount of .5.

. '7. Electromagnetic tool of the percussion type comprising a mantle-tube of magnetic material, pole-pieces inserted in the ends of said tube and having .inward projections, an annular polemember inserted in said mantle-tube between the ends thereof, an inner tube of diamagnetic material carried by said projections and extending through said annular pole member, an armature guided for reciprocation between said projections within said inner tube, a buffer spring and a tool shank arranged near the opposite ends of said mantle tube to be struck by said armature, a returning coil mounted between said tubes adjacent to said buier spring, a striking coil havinga smaller number of turns and a larger conductor cross section than said returning coil and being mounted between said tubes adjacent to said tool shank, a source of current and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current. the resistance of the entire electric circuit of said returning coil being from three to six times as large as the resistance of the entire electric circuit of said striking coil.

8. Electromagnetic tool of the percussion type 'comprising a mantle-tube of magnetic material,

pole-pieces inserted in the ends of said tube and having inward projections, an annular polemember insertedV in said mantle-tube between the ends thereof, an inner tube of diamagnetic material carried by said projections and extending through said annular pole member, an armature guided for reciprocation between said projections within said inner tube, a buffer spring and a tool shank arranged near the opposite ends of said mantle-tube to be struck by said armature, a returning' coil mounted between said tubes adjacent to said buier spring, a striking coil having a smaller number of turns than said returning coil and being mounted between of current,v and an interrupter adapted to be operated by said armature and to alternatelyy connect said coils to said source of current,'the resistance of the entire electric circuit of said returning coil,I being from three to six times as large as the resistance of the entire' electric circuit of said striking coil, the interrupter, the cross-section and stroke of said armature and the dimensions of said mantle-tube and said pole elements and the number and cross-section of the turns of said coils being so dimensioned that the consequent electric energy consumption of the coils and the consequent frequency of oscillation of the armature will result in a ratio of the electric time constant of said striking coil which, when computed for the end position of the armature abutting against said buier-spring, divided by the period of energization of said striking coil exceeds the amount of .7.

9. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein constituting two substantially closed magnetic ux circuits, resilient buffer means and a tool shank arranged on opposite sides of said ux circuits to be struck by said armature, a returning coilassociated with the ux circuit adjacent to said buer means, a striking coil having a smaller number of turns than said returning coil and being associated with the flux circuit adjacent to said tool shank, a source of current, an interrupter adapted to be voperated by said armature and to alternately connect said coils to said source of current, and two quenching condensers each coordinated to one of lsaid coils, the resistance of the entire electric circuit of said returning coil being from three to six times as large as the' resistance of the entire electric circuit of said striking coil, and the-capacity of the condenser coordinated to the striking coil being from two to four times as large as the capacity of the condenser coordinated to the returning coil.

10. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein constitutingtwo substantially closed magnetic ux circuits of different length, resilient buier means and a tool shank arranged on opposite sides of said ux circuits to be struck by ysaid armature, a returning coil associated with the ux circuit adjacent to said bufer means, a striking coll having a smaller number of turns than saidreturning coil and being associated with the ux circuit adjacent to said tool shank, a source of current, an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current, and two electrolytic condensers each coordinated to one of said coils and arranged in shunt with the contacts of said interrupter, the resistance of the entire electric circuit of said returning coil being from three to six times as large as the resistance of the entire electric circuit of said striking coil,'and the capacity of the electrolytic condenser coordinated to the striking coil being from two to four times as large as the capacity of the electrolytic condenser coordinated to the returning coil.

11. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature. guided therein, two coils arranged to produce` alternate fluxes causing said armature to reciprocate, a source of current, and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current, said interrupter consisting of two pairs of contacts, each pair comprising one contact mounted on said frame elements and one contact mounted on a spring-leaf common to both of said pairs and adapted to be deflected.

bysaid armature in alternate directions, and

overcentering means associated with said springleaf, the aforo-stated elements being so dimensioned that the consequent frequency of oscillation of said armature will result in a time constant ratio of each coil exceeding .5.

12. Electromagnetic tool of the percussion type comprising frame elements anda reciprocatory armature guided therein, two coils arranged to produce alternate fluxes causing said armature to reciprocate, a source of current, and1 an inter-v rupter adapted to be operated by said armature and to alternately connect said coils 'to said source of current, said interrupter including a contact-carrying spring-leaf, springs -interposed between said spring-leaf and opposed abutting faces of said armature, and overcentering means associated with said spring-leaf, the aforo-stated elements being so` dimensioned that the consequent frequency of oscillation of said armature will result in a time constantratio of each coil exceeding .5.

13. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein, two coils arranged to produce alternate fluxes causing saidarmature to reciprocate, a source of current, and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current, said interrupter including a contactcarrying spring-leaf, means for holding the ends of said leaf in fixed relationship to said frame elements `at a distance smallerl than the length of sa/.ii leaf, whereby the spring-leaf will bulge downwardly or upwardly, and resilient means Vcooperating with the armature and adapted, owing to the reciprocatory movement of thesame, to alternately cause said spring-leaf to jumpto one or the other of its bulged positions, the aforestated elements being so dimensioned that the consequent frequency of oscillation of saidarmature will result in a time constant .ratio of each4 coil exceeding .5.

14. Electromagnetic tool of the percussion type, A. comprising frame elements and a reciprocatoryfstantially closed magnetic ilux circuits, two coils each associated with one of said circuits to induce a flux therein, a source of direct current having a voltage below a limit of to 30 volts, and an interrupter interposed between said coils and said source of current and adapted to be operated by said armature and to. alternately close ,and interrupt the electric circuitsv of said coils,

whereby said armature will be reciprocated by the alternate effect of the fluxes in said magnetic circuits, the interrupter, the number andcrosssection of the turns of said coils and the mass, cross-section and stroke of the armature and of said frame elements being so dimensioned that the consequent electric energy consumption of the coils and the consequent frequency of oscillation of the armature will result in a ratio of the electric time constant of each coil which, when computed for the position assumed by the armature before the stroke thereof produced by said coil, divided by the period of energization of said coil is not less than .5.

15. Electromagnetic tool of the percussion type,

comprising frame elements and a reciprocatoryk armature guided therein constituting two substantially closed magnetic ux circuits, two coils each associated with lone of said' circuits to induce a flux therein, a source of direct current having a voltage below a limit of 25 to 30 volts, and an interrupter interposed between said coils and said source of current and adapted to beoperated by said armature and to alternately close and interrupt the electric circuits of said coils, whereby said armature will be reciprocated by the alternate effect of the fluxes in said magnetic circuits, the interrupter, the number and crosssection of the turns of said coils and the mass, cross-section and stroke of the armature and of said frame elements being so dimensioned that the consequent electric energy consumption of the c oils and the consequent frequency of oscillation of the armature will result in a ratio of the electric time constant of at least one of the coils which, when computed for ,the position assumed by the armature before the stroke thereof produced by said coil, divided by the period of energization of said coil is not less than .7.

16. Electromagnetic tool of the percussion: type comprising frame elements and a reciprocatory armature guided therein constituting two substantially closed magnetic flux circuits of different length, resilient buffer means and a tool Asource of current,the resistance of the entire .armature guided' therein, two coils 'of dilIerent dimensions arranged to' produce alternate fluxes causing said armature to reciprocate, a source 'of direct current having a voltage below-a limit of 25 to 30 volts, and an interrupter adapted to be operated by said armature and to alternately armature guided therein constituting two subconnect said coils to said source of current, the

afore-stated elements being so dimensioned that the consequent frequency of oscillation of said armature will result in a time constant ratio of each coil exceeding .5.

18. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein, two coils arranged to produce alternate iluXes causing said armature to reciprocate, a source of current, and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current, said interrupter consisting of two pairs of contacts, each pair comprising one contact mounted on said frame elements and one contact mounted on a spring-leaf common tov both of said pairs and adapted to be deflected by said armature in `alternate directions, mounting means for said spring-leaf, and overcentering means connected with said mounting means and integral with said spring-leaf, the aforestated elements being so dimensioned that the consequent frequency of oscillationof said armature will result in a time constant ratio of each coil exceeding .5.

19. Electromagnetic tool of the percussion type comprising frame elements and a reciprocatory armature guided therein constituting two substantially closed magnetic flux circuits, two coils each associated with one of said circuits to induce a i'lux therein, the rst coil having a larger number of turns and a smaller conductor crosssection than the second coil, a resilient buier means for said armature arranged adjacent to said rst coil, a tool shank adapted to be struck by said armature and arranged adjacent to said second coil, a source of current, and an interrupter adapted to be operated by said armature and to alternately connect said coils to said source of current.

MAX GREINER.