US2695968A - Commutator with constant tension spring - Google Patents

Description

Nov. 30, 1954 A. J. WELCH ETAL 2,695,968

COMMUTATOR WITH CONSTANT TENSION SPRING Filed Nov. 9, 1950 4 Shets-Sheet 1 F9 20 F9 I I I I 22 Z, 2 2,2,gig/223% i if fnvenfors CZT'Z /Qur'J Mel ck and J can J ecker' Nov. 30, 1954 A. J. WELCH ETAL 2,695,968

COMMUTATOR WITH CONSTANT TENSION SPRING Filed Nov. 9, 1950 4 Sheets-Sheet 2 fnverzj or- QTikZLT'JZZ/fiZCk and Jazz/can J ec/ ier NOV. 30, 1954 J w c ETAL 2,695,968

COMMUTATOR WITH CONSTANT TENSION SPRING Filed Nov. 9, 1950 4 Sheets-Sheet 3 r r [1 66 i: Hi F 1 '1: i

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fnz/erzibfsl QrZ /QUJ'J 062,671! and NOV. 1954 A. J. WELCH ETAL COMMUTATOR WITH CONSTANT TENSION SPRING 4 Sheets-Sheet 4 Filed Nov. 9, 1950 J7LUQILZLO715'I Gri /LL47" J ZUeZC/i and van J ec/ ier' United States Patent 0 COMMUTATlTOR'WITH-EONSTALNT TENSION SPRING This invention .is directed; to commutator ibrush :holders :andtpartsiherefor. More zspecifically, the {invention is directed to a commutator brush .holder -zassembly :wherein use is made of spring 'means shavingrsubstantially constant tension or :load characteristics :for athetactuation :oi the brush 'for iitstsfull effective length :andtlife.

Heretofore commutator or other electrical brushes have employed various means-zofunounting .wherein'ithere has'been exerted :on the.-brush,.-tension orgpressure'from an ordinary :helical, :spir'al, :leaf :or;motor .type spring. The eifectiveness of the brushes, and the wear thereofiris naturally lessened when the brushes. are-subjected =10 arc- .ing. This arcing hasrbeen-iinherenfly .:;present in :prior :brush ;c0nstructions :due sto :theztendenoy ,of &the "brush ;to zbounce or:moye away :from the commutator upon .hitting :an irregularity ;or rough :spot :thereon. 'Ashcre- ;tofore mounted and v with the .springs xheretofore :used, a rapidly revolving :commutator,may have a tendency ;to set up a bouncing tactionwwhich -:may:b.e likened to a -harmonic,. and which anay, in :fact have resulted :from :-the

harmonic :or rperiodicity of the brush spring.

.The main .object of the present invention :is to "pro vide 1a brushvholder employing :-a: .substantially constant load spring, .or whichahas a vsubstantially zero spring rate. The .usetof a substantially zerorate spring imalces unnecessary the usual: practice of :adding rmany. coils or-a great length ofpactivespring material ;as is necessary .and usually done iin' the casewof existing brush ,pressure springs of the. helicaLspiral, leaf .ortmotorespring types.

Another object'of .the:present.invention is to provide a brush holder-wherein theiinsulating componentsymay :be used to support a brush-withoutitheqaid of. the :usual roundor rectangular metal formed "101' drawn tubescmployed as. the intermediate easing, forvthe brush.

Another object of the, present invention is to provide -a brush holder in which the insulating-material can be .used to ,form the .maincasing sforpthewbrushexcept that one or more surfaces may-beecomprised'zof .;a metal insert whoseyfunetion :is ltO provide -:electrical paths :or act as aspr-ing support.

Another objectof :thejinventionis to provide a 1brush holder assembly .in whichithe spring .11sed-.to urge .the

brush toward the commutator is .in ithe imost intimate possible relationship with the brush and which does not employ the usual intervening :levers and devices-.to .convey the spring-pressure totthebrush. Thiszser-ves to reduce the .masses that; must react in order to, return .the brush to contact with :the commutator rafter-it has been disturbed during .theusual. course of commutation.

By constructing .the brushholder in accordance :W-ith the principles set forth, by the, present invention whereby the brush is :urged towards the-commutator with .a substantially constant pressure throughout its effective life,

the length of life of thetbrnsh.is.increasedieonsiderably.

,In rbrush holders constructed 135 ;.shown :herein, there .is ;-a.:minimum amount of 1- reactiue nnass which must :be .movedwhenever the ;brush is depressed awayfirom the commutator 'or moved towards the commutator. :Therefore, itis :possible, :when the brush is adeprcssedaway from the commutator, .;due to-its-engag ing an irregularity or "rough-spotthereomto return *thebrush into engagement with the commutator in the shortest possible time. A-further, advantage of a brush. holder of:the type disclosed {hereinis 'that 'there'is a grea-ter sensitivity of reaction when'the yspring'is :depressed and :this is due .to the fact that a amount of reactive mass ventional brush holder constructions.

2,695,968 Patented Nov. 30, 1954 '2 is moved when the .brush is displaced, as when it hits a rough spot on thecommutator.

When aconstant tension spring as disclosed herein is .used in the brush holder a lower optimum spring pressure is required to achieve satisfactory commutation,

and :this results from the .greatersensitivity of reaction and the shorter reaction .time when :the constant tension exerts a snubbing ,action when the brush is disturbed away from the commutator and thisresists .such factors which ,disturb the brush, minimizes the undesirable motion of the brush away from the commutator and aids in restoring it to intimate relationship with the commutator in the shortest possible time. It is a characteristic of 'thespringemployed in the present invention that small increments .of movement of the brush change the moment ,arm of .the reactive mass and because of this the maximumsor full load of the spring resists movement-of 'the brushaway from the commutator, while a somewhat lesser load urges the brush towards the commutator.

Anotherobject of'gthepresent invention is to provide a brushholder in which a spring in intimate relationship .with the brush can urge the .brush toward :the commutator and at the same time provide a component of pressure that stabilizes one side ofthe brush continuously .against-onesideof its .casing andthus prevent sidewise chatter of the brush within its containing cavity. This stabilizingefiectdoes not require the use of any auxiliary device and the degree of stability does not change through- .out the length and life of thebrush.

Due to the fact that intervening levers and devices for conveying spring pressure fromithe springto the brush are not necessary, it is possibleto provide 'a very compact arrangement of .partstand the reduction in the number of parts required in the brush holders disclosed "herein makesa brush .arrangement thatis more economical to manufacture and cheaper to maintain. The compactness of, design of the-brushholders also makes it possible to provide, greater circumferential spacing betweensuccessive brush holders and the possibility of flashovers is minimized considerably. The compactness of design permits smaller motor units and smaller and lighter mountings for the brushes. Because it is notnecessary to utilize intervening leversto convey the. ring pressure to the brush, aminimum amount. of radial space is required for housing the brush and this is particularly important in theconstruction of hand tools such as drills, the diameters of which .are generally determined/by. the space required to house-thebrush assembly. In applications where space is .not .a prime. consideration .a larger brush may be utilized than wouldbepossible withcon- This becomes important in applicationswherein the'brush'itseIf is required to last throughout the life. of the motor.

Anotherimportant'feature of thepresent invention is that the. springitselfdoes ,not contact the commutator when the brush has completely worn out and this eliminates a frequent sourceof damage to-thercommutator.

By the present inventionrthe spring can be divorced economically electrically from the electric current.

The intimacy of contact between the. spring and the brush, the small mass required .to react, the short reaction time, the snubbing action, and jthe absenceof fundamental frequencies in the arrangements disclosed by this inventionall ,act.to.retain greater intimacy of contact between thebrush andcommutatorwith subsequent less brush wearand better commutation.

.Other-objects, ,advantages and uses of'the invention will become apparent fromgtheffollowing detailed description when read in conjunction with the accompanying drawings wherein:

Fig. l is a fragmentary view showing in cross-section a commutator for a generator, electric motor or the like having the brush assembly mounted thereon;

Fig. 2 is a longitudinal cross-sectional view showing the detailed arrangement of a preferred form of commutator brush assembly, incorporating the present invention;

Fig. 3 is a cross-sectional view taken substantially on the line 33 of Fig. 2 looking in the direction of the arrows;

Fig. 4 is a view showing the shunt support or contact plate for the present construction;

Fig. 5 is a view showing the spring support plate;

Fig. 6 is a vertical cross-sectional view through a modified form of a commutator brush assembly, the view being on line 6-6 of Fig. 7 looking in the direction of the arrows;

Fig. 7 is a cross-sectional view taken on the line 7-7 of Fig. 6 looking in the direction of the arrows;

Fig. 8 is a plan view, partly broken away, of a commutator brush holder assembly mounting, incorporating still another modification of applicants invention wherein a split brush assembly is shown;

Fig. 9 is a vertical view partly in cross-section, taken on the line 9-9 of Fig. 8 looking in the direction of the arrows, the commutator being shown schematically in dotted lines;

Fig. 10 is a cross-sectional view taken on the line 1010 of the Fig. 9 looking in the direction of the arrows;

Fig. 11 is another cross-sectional view showing the mounting of the type disclosed in Fig. 9;

Fig. 12 is a plan view of another embodiment of the invention;

Fig. 13 is a side elevational view, shown partly in section, of the embodiment of the invention disclosed in Fig. 12;

Fig. 14 is a sectional view taken substantially along the line 1414 of Fig. 13;

Fig. 15 is a fragmentary side elevational view, showing the spring and insulator retaining latch in open position;

Fig. 16 is a front view of another embodiment of the invention;

Fig. 17 is a side view of the embodiment of the invention disclosed in Fig. 16;

Fig. 18 is a sectional view taken substantially along the line 18-18 of Fig. 17;

Fig. 19 is a detail view of the casing employed in the embodiment of the invention disclosed in Fig. 16; and

Fig. 20 is a detail view of the spring mounting plate employed in the embodiment disclosed in Fig. 16.

Referring now to the drawings, wherein like reference numerals designate like parts in the different figures, and referring first to the embodiment of the invention shown in Fig. 1, an electrical motor or electrical generator 2t) having a commutator 21 of standard construction and form is shown. The motor 20 has a pair of carbon brush holder assemblies 22 mounted in its housing and the as semblies 22 comprise one form of the invention disclosed herein. It will be noted that the carbon brush holder assembly 22 is of the cartridge type, which may be readily removed for servicing and replacement, and which may be mounted in the motor housing by any convenient means, such as, for example, a set screw (not shown) or the like.

The cartridge 22 may be made of any desired metal or, in its preferred form, it may be made of a plastic having electrical insulation properties. A garter spring 23 is seated in a peripheral groove 24 in each of the cartridges and constitutes the electrical connection to a lead wire 25.

The cartridge 22 is provided with a cavity 26 extending therethrough and has a pair of slots 27 and 28 in communication with the cavity. Mounted within the slot 27 is a spring support 29 which is provided with an elongated aperture 30. Mounted in the slot 28 is a contact plate or shunt support 31. The slots 27 and 28 do not extend completely through the cartridge 22, but rather have a configuration for seating shoulders 32 on the spring support 29 and shunt support 31. A cap 33 having an internal threaded portion for engaging an external threaded portion at the upper end of the cartridge 22 (as shown in'Fig; 2) retains the supports 29 and 31 rigid in the slots 27 and 28. A rectangular brush 34 which may be made of any suitable material is slidable longitudinally within the cavity 26 and engages the commutator 21. A shunt 35 is electrically connected to one end of the carbon brush 34 and is also connected to the shunt support 31 by means of a lug 36 on the shunt support which lug may be bent over as shown in Fig. 2 to afford an electrical connection between the shunt 35 and the shunt support 31.

The spring support 29 has a spring 37 secured thereto at 38. The spring 37 may be secured to the spring support 29 by means of a rivet, welding or any other convenient means. The spring 37 comprises any suitable type of resilient material, preferably spring steel, and is deformed such that each increment of its length has substantially the same radius of curvature as every other increment thereof. It is clear that when the spring 37 is allowed to relax it will form a plurality of relaxed convolutions, one of which is shown at 39. Each convolution 39 will be tightly engaged with the next inner and the next outer convolution due to the fact that the entire length of the spring 37 is deformed to a substantially constant radius of curvature. The outer convolution 39 is maintained in constant tangential and sliding engagement with a beveled surface 40 on the brush 34. Because of the beveled surface 40, the spring 37 continuously exerts a component of force urging the brush 34 to the right, thereby stabilizing the brush within the cavity 26.

It will be noted that the spring 37 is secured to the spring support 29 on the side thereof opposite the brush 34 and the elongated aperture 30 permits the spring to protrude through the support 29 whereby it can engage the brush 34. The spring 37, since each increment of its length is deformed with substantially a constant radius of curvature, is what may be termed a constant load spring or a constant tension spring. Because the spring 37 is a constant tension spring, it exerts a substantially constant force on the brush 34 at all times irrespective of the length of the brush 34 extending into the cavity 26. Because of this propensity of the spring 37 it lends itself particularly well to the present application as it is desirable to have the commutator brush 34 engage the commutator 21 with the same force no matter whether the brush is new or nearly worn out. When the brush extends into the cavity 26 to the depth shown in Fig. 2, a substantial portion of the spring 37 is unwound to almost a straightened condition. The portion of the spring 37 which is thus straightened is ineffective to exert any force on the brush 34 in a direction which would move the brush 34 towards the commutator 21. Likewise the convolution 39 which is relaxed also does not exert any force on the brush 34. It is therefore only that length of the spring 37 between the relaxed convolutions 39 and the straightened portion which urges the brush 34 downwardly. It will be seen that this length of spring 37 between the relaxed convolution or convolutions and the straightened portion is substantially the same no matter how much the brush 34 is worn. Since a substantially equal length of the spring 37 urges the brush 34 towards the commutator 21 at any one time, the force of engagement between the brush 34 and the commutator 21 is substantially constant.

As seen in Fig. 2, the brush 34 is engaged with the commutator 21 and is urged towards the commutator by the portion of the spring 37 between the relaxed convolutions and the substantially straight portion of the spring. This portion of the spring 37 acts on the brush 34 through a moment arm that extends from the center of the convolution 39 to this portion of the spring. It is apparent that this moment arm is considerably longer than the radius of the convolution 39. When a rough spot on the commutator 21 strikes the brush 34, the brush is displaced away from the commutator and due to the natural tendency of the spring to resist uncoiling the moment arm between the center of the relaxed convolution 39 and the portion of the spring between the convolution 39 and the straightened portion becomes somewhat shorter, with the straightened portion of the spring more nearly approaching a tangent to the relaxed convolution 39. Because of this shorter moment arm, when the brush 34 is displaced slightly from the commutator 21 a somewhat greater load, constituting substantially the full load of the spring, is applied to the brush 34 to resist displacement thereof. It is thus apparent that the spring 37 exerts asnubbing action such that the shorter moment arm of the spring causes substantially the full load thereof to resist movement of the brush away from the commutator while 'the slightly longermoment arm causes somewhat less than the full load of the spring to urge the brush towards the commutator when it is engaged therewith.

The peripheral groove 24 within which the garter spring 23 is positioned is deeper on one side of the cartridge 22 and actually communicates with the slot 28. The shunt support 31 has a bent over portion 41 which extends into the peripheral groove 24 thereby insuring electrical contact between the garter spring 23 and the shunt support 31.

The cap 33 has a knurled peripheral surface 42 for facilitating the manual placement of the cap on the cartridge 22. The cap 33 may also be provided with a transverse slot 43 for facilitating the placement of the cap on the cartridge by means of a screw driver.

Figs. 6 and 7 show a second embodiment of the present invention which differs from the Fig. 2 embodiment only slightly. In this embodiment of the invention the carbon brush 34 extends completely through a slot 50 in a casing 51 but still is designed 'to engage the commutator 21. The casing 51 may be bolted to the motor housing by means of bolts 52. The casing 51 is provided with cavities 53 which communicate'with the slot 50 and which are located on opposite sides of the slot 50. The spring 370 in this embodiment is made of the same material as the spring 37 disclosed in Fig. 2 but is deformed subs'tantially midway between its ends to form a U-shaped portion 54 for receiving the upper end of the brush 34 (as viewed in Fig. 6). The relaxed convolutions 39a of the spring 37a are housed within the -cavities 53 and tangentially and slidably engage walls 55 in the easing 51. The spring 37a is deformed in the same manner as the spring 37 so that no matter how far the brush 34 extends into the slot 50 a substantially constant force is applied thereto tending to hold it engaged with the commutator 21. y

The spring 37a shown in Fig. '6 tends to hold the brush 34 somewhat more rigidly than the spring 37 by reason of the fact that two reaction points for the spring rather than only one are provided. The spring 37a exerts a snubbing action just as the spring 37 of the first embodiment.

A third embodiment of the invention is disclosed in Figs. 8, 9, and '11 of the drawings. In this embodimentof the invention a pair of casings 60, which are substantially identical are provided. Each of the casings 60 is mounted on a shaft 61 and held rigidly thereon by means of bolts 62. The casings 60 are provided with suitable slots 63 for receiving a split brush having two sections 34a and 34b. A cavity 64 is also provided in the casing 60 for receiving a spring support 65. The spring support 65 may be held rigidly in the cavity 64 by any convenient means and, as shown in Fig. 8, is held against lateral displacement by the wedging surfaces 64a. A cover plate 66 is provided for each of the casings 60 and is held in place thereon by means of a bolt 67.

Each of the brush sections 34a and 34b has a beveled surface 40a and 40b respectively which enable the spring 37 to stabilize one of the sections against one side of the cavity formed in the casing 60 and the other section against the other side of the cavity. It will be noted from Figs. 9 and 11 that the beveled surfaces 40a and 40b together form an obtuse angle. The spring 37 is secured to the spring support 65 by any suitable means such as a rivet 68 and its relaxed convolutions 39 are seated within the groove formed by the beveled surfaces 400 and 40b in order to tangentially and slidably engage each of the brush sections 34a and 34b. The spring 37 in the embodiment of the invention shown in Figs. 9 and 11 functions in substantially the same manner as the spring 37 shown in the Fig. 2 embodiment and exerts a substantially constant force and a snubbing action on each of the brush sections 34a and 34b. The forces exerted on each of the brush sections 34a and 34b are also approximately equal so that each -of the sections wears at approximately the same rate.

A fourth embodiment of the invention is disclosed in Figs. 12-15. In this embodiment a casing 70 is provided with a slot 71 for receiving a brush 34d and with a slot 72, which communicates with the slot 71, for receiving a spring mounting insulator 73. The casing 70 is formed with a cylindrical aperture 74, for receiving a mounting shaft (not shown) and is slotted as at 75. A pair of arms 76 on either side of the slot 75 are slightly resilient and a bolt 77 threaded into suitable internal threaded apertures in the arms 76 may 'be used for rigidly connecting the brush holder to the mounting shaft seated within the cylindrical aperture 74.

The casing 70 is formedof any electrical conducting material, preferably brass, and a shunt '78 'co'nnected to the brush 34d, as shown in Fig. 12, is also electrically connected to the casing 70 by means of a bolt 79.

The casing 70 is provided with a pair of ears "80 and a shaft 81 extends through the cars 80 'to'pivotally mount a latch 82. The latch "82 is provided with an arm 83 that is designed to engage a beveled surface 84 formed on the casing 70. The latch 82 also has an arm 85 and an operating arm 86.

The portion of the latch '82 which connects the arm 83 with the remainder of the latch is narrower than a U-shaped notch 87 formed in the insulator 73. Consequently the insulator 73 is reciprocable with the narrow portion of the latch positioned within the notch 87. The insulator 73 is provided with a pair of outwardly extending projections 88 which engage the casing 70 to limit its downward movement in the slot 72. The insulator 73 has a substantially constant tension spring 37d, riveted thereto by means of a rivet 89. The spring 37a is the same type of a spring as the spring 37 'disclosed in Fig. 2 and a plurality of relaxed convolutions 39d tangentially engage a beveled surface 90 on the brush 34d. 7

When the brush 34d and insulator 73 are 'being assembled in the casing 70 the latch 82 is moved to the position shown in Fig. 15. The insulator 73 is pushed into the slot 72 with the convolutions 39d tangentially engaging the beveled surface 90 on the brush 34d until the base of the latch 87 is deep enough to permit the arm 85 to pass 'therethrough, as shown in Fig. 15, until the arm '83 engages the beveled surface 84 of the brush holder. The spring 37d urges the brush 34d againstthe usual commutator 21 and urges the insulator'73 upwardly, as viewed in Fig. 13. When-the latch 82 is in the position shown in Fig. 13 the spring 37 is effective to hold the insulator 73'upwardly against the latch 82 and to hold the brush 34d against the commutator '21. The insulator 73 engages the face of the arm 85 and thereby retains the latch 82 in the position shown in Fig. 13. When the latch 82 is retained in th'is'position-it prevents the insulator 73 from being forced upwardly out of the notch 72 due to the force of the spring 37d.

By this embodiment of the invention the constant tension spring 37d itself is effective to retain the movable parts of the brush holder in assembled relation and therefore a very simple and easily maintainable brush holder is provided. The spring 37d exerts a snubbing action on the brush 34d and the beveled surface 90 enables the spring 37d to stabilize the brush 34d in the slot 71. The brush holder disclosed in this embodiment of the invention is particularly applicable to large machines.

Another embodiment of the present invention is disclosed by Figs. l620 and this embodiment is particularly suitable for use with small electrical generators and the like. With particular reference to Fig. 16, a mounting plate for mounting a plurality of brush holder assemblies is provided. A casing 101 is formed as shown in detail in Fig. 19 and is provided with a pair of apertures for receiving rivet 102 that secures the casing to the mounting plate 100 and for receiving a bolt 103 that secures a shunt connector 104 to the casing 101. The casing 101 is formed so as to provide a cavity 105 for receiving a brush 34s. A shunt 106 connects the brush 342 with the shunt connector 104.

A spring mounting plate 107 has a constant tension or constant load spring 37e secured thereto by means of a rivet 108 and when the mounting plate is assembled with the casing 101 the spring 37e tangentially engages the brush 342 to continuously urge it into engagement with the commutator 21. The casing 101 is cut out to provide a pair of outwardly protruding lugs 109 and the spring mounting plate 107 is provided with a pair of outwardly protruding lugs 110. The portion of the mounting plate 107 which is cut out to provide the lugs 110 provides suitable notches 111 for receiving the lugs 109 on the casing 101.

In assembling the brush holder disclosed in Figs. 16-20 the brush 34a is inserted within the cavity 105. The spring mounting plate, having the spring 37:: secured thereto is then placed adjacent the casing 101 with the spring 37e in engagement with the brush 342. As viewed in Fig. 16 the mounting plate 107 is moved downwardly until the portions 112 thereof slip past the lugs 109. The mounting plate then snaps into engagement with the casing 101 and the spring 37s urges the mounting plate 107 upwardly whereby the lugs 109 are positioned within the notches 111 and grasp the portions 112 of the mounting plate 107 to retain it in latched engagement with the casing 101. In this embodiment of the invention the spring 372 is effective to retain the casing and spring mounting plate engaged and to simultaneously maintain the brush 342 in engagement with the commutator 21, and further the spring exerts a snubbing action on the brush as in all the other embodiments.

By the present invention there is provided a brush holder for electric motors or generators which is very eflicient in its operation. Because of the fact that the spring at all times exerts a substantially constant force on the brushes the normal harmonic bouncing of the brushes due to the irregularities in the surface of the commutators is substantially eliminated. Also because of the constant force applied by the spring throughout the life of the brushes, their rate of Wear is uniform.

It is contemplated that numerous changes and modifications may be made in the present invention without departing from the spirit or scope thereof.

We claim:

1. In an article of manufacture, a cartridge having an opening therethrough, a contact plate secured within said opening, a support secured within said opening, a commutator brush slidable within said opening, a shunt electrically connecting said brush and said contact plate, a spring fastened at one end to said support and engaged with said brush for biasing said brush towards the other end of said opening, said cartridge having a peripheral groove, a portion of said contact plate protruding into said groove, and a garter spring in said groove engaging the protruding portion of said contact plate.

2. In an article of manufacture, a cartridge having an opening therethrough, a contact plate secured Within said opening, a support secured Within said opening, a commutator brush slidable within said opening,

a shunt elec- I trically connecting said brush and said contact plate, a spring engaged with said brush and fastened to said support for biasing said brush towards one end of said opening, said cartridge having a peripheral groove, a portion of said contact plate protruding into said groove, and a garter spring in said groove electrically engaging the protruding portion of said contact plate.

3. In an article of manufacture, a casing having an opening therethrough, removable means for closing one end of said opening, a contact plate secured Within said opening, a commutator brush slidable Within said opening, a shunt electrically connecting said brush and said contact plate, a spiral ribbon spring mounted within said opening for continuously urging said brush towards the other end of said opening with a substantially constant force irrespective of the position of said brush within said opening, said casing having a peripheral groove, a portion of said contact plate protruding into said groove, and a garter spring in said groove electrlically engaging the protruding portion of said contact p ate.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 122,288 Smith Dec. 26, 1871 422,265 Van De Poele Feb. 25, 1890 510,892 Coifman Dec. 19, 1893 585,872 Lundell July 6, 1897 697,432 Bassett Apr. 15, 1902 728,273 Mix May 19, 1903 1,488,575 Wagner Apr. 1, 1924 1,495,203 Roberg May 27, 1924 1,964,280 Witchger June 26, 1934 1,977,546 Fornelius Oct. 16, 1934 2,270,122 Toelle Jan. 13, 1942 2,273,644 Hope Feb. 17, 1942 2,280,735 Weissinger Apr. 21, 1942 2,647,743 Cook Aug. 4, 1953 FOREIGN PATENTS Number Country Date 440,030 Great Britain Dec. 16, 1935 560,231 Germany Sept. 29, 1932 603,159 Germany Sept. 25, 1934 658,776 Germany Apr. 11, 1938 OTHER REFERENCES Product Engineering, July 1949, pages 136 to 140.

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