US2404469A - Electromagnetic relay - Google Patents

Description

Juiy 23, 1945. 5 0013 2,404,469

ELEG'ZROMAGNETI G RELAY Filed June 12, 1944 FIG. I

INVENTOR. FREDRIO E. wooo ATTORNEY? Patented July 23, 1946 ELECTROMAGNETIC RELAY Fredric E. Wood, Oak Park, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application June 12, 1944, Serial No. 539,852

13 Claims.

The present invention relates in general to electromagnetic devices and, more particularly, to electromagnetic devices of the slow-to-release relay type.

An electromagnetic device of the slow-to-release relay type conventionally comprises a field structure including a core provided with a copper sleeve and a winding, a movable armature carried by the field structure, and one or more sets of switch springs operatively associated with th armature.

It is the object of the present invention to provide an improved electromagnetic relay of the type described which is light in weight, diminutive in size, economical in construction, reliable, long wearing and positive in operation and in which the release time is very stable and does not vary with any reasonable variation in voltage or switch spring pressure.

Another object of the invention is the provision of means for easily and readily changing the release time of the relay without requiring a major readjustment of the relay and in which after such release time is once set it will be maintained indefinitely.

The objects set forth above are in part realized by providing a fairly large copper sleeve, or slug, compared with the iron magnetic circuit in the field structure and so that the iron in the magnetic circuit is fully saturated by the counterelectromotive forc generated by the copper sleeve when the circuit through the winding is opened after the winding has been energized at approximately one third of the normal operating voltage. The copper sleeve, as is well known, due to this counterelectromotive force renders the relay armature slow-to-release in response to the interruption of current flow through the relay winding. Due to this construction, normal voltage variations or changes in the spring tension of the switch springs have very little, if any, effect on the release time of the armature. This relay is, therefore, virtually independent of normal voltage fluctuations and switch spring tension for controlling the release time of the armature. Also, due to this construction the relay has only one variable factor by means of which the release time of the armature may be varied and that is by variably adjusting the air gap between the relay core and the armature. Since this relay is of the diminutive type, being only one and one quarter inches in length, it is too small to have the usual standard residual screw in the armature, such as shown in my Patent 2,272,496,

issued February 10, 1942. y In addition the use of the usual residual screw would necessitate a hole in the armature thereby reducin the iron in the armature near the core and thereby reducing the efficiency of the relay. There is also another reason for not using a residual screw due to the fact that a residual screw is limited in size where it strikes the core and is poundeddown by use thereby losing its adjustment after a number of operations and necessitating frequent readjustment. In accordance with the invention the air gap between the armature and the core is adjusted by means of a fiat armature stop spring mounted in the switch spring pile-up and an adjustable stop nut. The stop nut is mounted on a threaded stud fastened to tho heelpiece of the field structure and the stud extends up through a hole in the armature stop spring. The adjustable stop nut may be screwed down against a washer, between the stop nut and armature stop spring, to position'the armature stop spring so as to limit the armature stroke thereby variably adjusting the air gap between the armature and the'core to provide the desired release time of the armature. The washer is partially rounded on one side so that the fulcrum point on the armature stop spring is notchanged when the adjusting nut is screwed down. The adjustable stop nut stays in the position set because of its fibre insert which frictionally maintains it in position. The flat armature stop spring is made from spring tempered phosphor bronze and the armature is chrome plated to present hard surfaces to prevent wear. The flat armature stop spring is slightly dished 'where the armature strikes it to provide the same striking surface to the armature regardless of the position to which the spring is adjusted. The adjustable stop nut may be easily turned by using pliers or a wrench to adjust the armature stop spring to the position which gives the desired release time ofthe armature. Due to the special construction of this adjusting nut, known commercially asan elastic stop nut, the adjustment is maintained indefinitely. If desired one or more relays may be released in sequence to produce longer predetermined timed periods.

The normal adjustment for certain release time requirements is such that the armature arm should strike the flat armature stop spring when the armature is approximately one or two thousandths of an inch from the core. At this point, the armature stop spring, due to its short fulcrum, greatly increases the spring tension tending to prevent further operation of the armature and it is this increasedspring tension, which is 'made variable by the adjusting nut, in combination with the air gap between the core and the armature which determines the release time of the armature.

It is also possible with this type of relay and adjustment to adjust the armature sto spring so that the armature will release when the voltage drops down to a predetermined value.

Other objects and features of the present invention will be apparent by reference to the following specification taken in connection with the accompanying drawing, in which:

Fig. 1 is a side view of an electromagnetic relay constructed and arranged in accordance with the present invention;

Fig. 2 is a right end View of the relay illustrated in Fig. 1;

Fig. 3 is an exploded perspective view of the electromagnetic relay illustrated in Fig. 1;

Fig. 4 is an enlarged sectional View showing 'the arrangement for pivotally supporting the relay armature;

Fig. 5 shows a partial cross section of the adjusting means for adjusting the time release of the armature.

Referring now to the drawing, the electromagnetic relay shown comprises a field structure including a longitudinally extending core I2 provided with a copper sleeve II and relay winding I3, a substantially L-shaped magnetic heelpiece I4, secured to the core I2 by means of screw III, and a movable magnetic armature I5 pivotally mounted upon the outer end of heelpiece I4 and cooperating with the core I2.

As best shbwn in Figs. 1 and 3, the armature I5 carries a pair of spaced-apart upwardly extending ears Iii provided with aligned holes therein and an operating arm II. A pivot pin 29 is inserted into the aligned holes and the outer ends of the pivot pin 20 are flanged over in order to rigidly secure the pivot pin 26 to the armature I 5. The operating arm I 1 extends along the side of the heelpiece and parallel thereto, and is provided at the end thereof with an inwardly extending operating projection Ila which overlaps the upper surface of the heelpiece I4 and normally rests upon a nonmagnetic rivet 22, secured to and extending upwardly from the upper surface of the heelpiece I4.

In order to provide a bearing surface for the pivotal armature, a nonmagnetic bearing plate I8, formed of bronze or the like, is carried upon the upper surface of the heelpiece I4 adjacent to the outer end thereof, and the pivot pin 29 is rotatably held against the upper surface of the bearing plate I8 in a transverse pivot pin receiving slot 2i formed in the bottom surface of the clamping plate I9. The arrangement for securing the armature I5 in pivotal relationship with the heelpiece I4 and the core I2 will be described in detail hereinafter.

To facilitate assembly of the electromagnetic relay, the switch spring set 25 is assembled independently of the relay structure itself, and,

after assembly, is secured to the heelpiece I4 together with the bearing plate I8 and the armature I5. With this arrangement different combinations of switch spring sets may be carried in stock by a relay manufacturer in order to meet the customers requirements for different numbers of springs as well as different combinations of make and break switch springs.

More particularly, the switch spring set 25 comprises a plurality of individual switch springs 26 and 21, an L-shaped back stop spring 28 for 4 spring 26 and a flat armature stop spring 29 arranged in superimposed relationship. The switch spring 21 is also an armature restoring spring and is provided with a bushing 4| which is held against the armature arm projection I'Ia by the tension in spring 21. Each of the springs comprise a base end and a free end and is provided with a cantilever mounting at its base end. Also, the springs are insulated from each other and from the clamping plate I9 at their base ends. The arrangement for assembling and insulating the base ends of the spring set 25 comprises a clamping plate I9, the plurality of insulators 23, tubular insulators 24, an end plate 39, the individual springs 26, 21, 28, the armature stop spring 29, and a retaining screw 3I. More particularly, the retaining screw 3I is inserted through the center tubular insulator 24 and threaded into the tapped hole 34 in the clamping plate I9 to securely clamp the springs and insulators between the clamping plate I9 and the end plate 39 in assembled relationship, until it is desired to fasten the spring set, by means of mounting screws 32, upon the heelpiece I4.

In order to assemble the spring set 25 upon the heelpiece I4, the bearing plate I8 is placed thereon with the three clearance holes therein in alignment with the three holes provided in the heelpiece I4. The armature I5 is then held in place with the pivot pin 29, in engagement with the pivot pin receiving slot 2| provided in the bottom surface of the clamping plate I9, as is clearly shown in the enlarged view Fig. l, and the holes in plate I9 placed in alignment with the holes provided in the heelpiece I4 and the bearing plate I8. Screws 32 are then inserted through the two outside holes in the end plate 39 and threaded into the tapped holes 33 in the heelpiece I4 to securely fasten the spring assembly 25, the pivot pin bearing plate IS, the armature I5, and the heelpiece I4 together into a unitary structure.

In order to permit adjustment of the air gap between the armature I5 and the front edge of the heelpiece I4, slotted holes are provided in the clamping plate I9 and the bearing plate l8, and the two outside tubular insulators 24 are provided with clearance holes therein which are slightly greater in diameter than the diameter of the mounting screws 32.

The switch springs 26 and 21 comprise a so called make spring combination in which the armature spring 2'! is adapted to be moved into engagement with the make spring 26. The general shape of these springs are somewhat similar to the springs shown in my previously mentioned patent except that these springs are clamped in the spring pile-up by a U-shaped portion of the springs. In order to facilitate ease of adjustment of the make spring 26, an L-shaped stop spring 28 is included in the spring pile-up, as indicated in the drawing, to hold the make spring 26 a predetermined distance from the armature spring 21,

The armature spring El is provided with a relatively long, flexible portion to facilitate ease of adjusting the pressure of this spring and bushing 41 against the operating projection Ila, whereby minimum perating power is required of the relay to overcome the tension of the armature spring 2'! in order to move it into engagement with the cooperating make contact 26. The armature spring 2'! is provided with an elongated slot 36 formed therein adjacent the free end thereof, which slot provides the armature spring with two spaced apart and independently flexible branches 31 and" 39 connected together at the free end thereof by curved'operating portion 39. The flexible portions 31 and 38 of the armature spring '21 each carry a metallic contact to cooperate with like metallic contacts on the Y- terially increasing the retarding effect on the armature and without materially changing the contact pressure between the circuit closing contacts. With this type of spring combination and a back stop spring, such as spring 28', the stop position of the armature l5 may be varied without the necessity of altering the tension or positions of the switch springs as is now commonly done on other types of relays using such a spring combination.

The armature stop spring 29 is included'in the spring pile-up in the position shown in the drawing and has a hole 42 in its bent portion approximately half way between its free end and the point where it is clamped in the spring pile-up. This hole is provided to enable the threaded stud 44 to pass therethrough for thepurpose of adjusting the free end portion of armature stop spring 29. A slightly dished portion 43 of armature stop spring 29 is dished as shown to provide a rounded'surface so that the projection arm Ila always strikes the same surface when the armature is operated regardless of the adjusted position of the armature stop spring. The threaded stud 44 is threaded into the heelpiece l4 and is held in place by the stud nut 45. The stud extends through the hole 42 in stop spring 29 and on top of spring 29 a washer 46 is placed over the stud 44. The washer 46 is rounded somewhat as shown so that when the elastic stop nut 41 is screwed down on the stud, the washer 45 does not change the fulcrum point. The elastic stop nut 41 is hexagonally shaped at its base and has the usual threads for cooperation with stud 44. In addition the nut 45 has an upper rounded extension in which a fibre is placed and held by friction. This fibre insert has a hole through which the stud 44 extends and which hole is just large enough to frictionally hold the nut on the stud in its set position. The nut 45 may be screwed down or up to adjust the dished portion 43 of armature stop spring 29 the desired distance before the projection arm Ila strikes it. The armature stop spring 29 may be adjusted to determine the point at which the projection arm l'la strikes the dished portion 43 thereby determining the air gap between the core I2 and armature |5 at this operating stage of the armature. At this point, the armature stop spring 29, due to its short fulcrum greatly increases the spring tension tending to prevent further operation of armature l5 toward the core l2.

In order to facilitate movement of the armature spring 21, a cup shaped metallic fixture 49 is welded to the lower surface of the operating portion 39, An insulating bushing 4| is arranged within the fixture 40 and frictionally secured thereto by spinning the side walls of fixture 40 into the bushing 4|. When the winding I3 is energized, the armature l5 is rotated about the pivot pin 20 in a counterclockwise direction as viewed in Fig, 1, thereby causing the armature to be moved from its normal position to its operated position. The pivot pin 20 rotates in the transverse pivot pin receiving slot 2| formed in the bottom side of clamping plate l9, the adjacent upper surface of the bearing plate |8 serving also as a bearing surface therefor. The operating projection Ila carried by arm I! of armature l5, acts upon the bushing 4| and armature spring 21 to flex spring 21 to engage the associated make spring contacts on switch spring 26.

The nonmagnetic rivet 22 underlying the projection Ila prevents the projection from sticking to the adjacent surface of heelpiec 4|, due to leakage of magnetic flux therebetween. In order to prevent objectionable play between the pivot pin 20 and the bearing surfaces, comprising the transverse pivot pin slot 2| and the bearing plate l8, when the entire relay is vibrated from an external source, the relatively long flexible armature switch spring 21 and its bushing 4| is tensioned against the upper surface of projection arm Ila between the point where the bottom surface of arm |'|a rests upon rivet 2'2 and the point where the arm joins the projection Ila. The rivet 22 acts as a fulcrum, so that pressure applied to the upper surface of the operating projection Ila at a point removed from the fulcrum tends to twist the armature l5, as is clearly shown in a slightly exaggerated form in Fig. 2. Consequently, if there is any play between the pivot pin 20 and the bearing surfaces indicated above, the

armature switch spring automatically takes up the play therebetween and thereby prevents vibration of the armature from an external source, either transverse to th axis of the pivot pin or parallel to the axis of the pivot pin.

In view of the fact that the armature switch spring 21 applies its pressure to the operating projection Ila at a point remote from the pivot pin 29 and also ata point remote from fulcrum 22, considerably less pressure is required to overcome any tendency of the armature IE to vibrate from an external source than would be required if the pressure was applied directly to the pivot pin itself, By actual test, it has been found that a very small percentage of the pressure required at the pivot pin to prevent vibration of the armature, is required when the pressure is applied to the operating projection |'|a at the remote end of the armature I5 in the manner shown.

Having described the construction and assembly of the relay, the utility and purpose of the invention will be set forth to enable the same to be more fully understood and appreciated.

It is well known that the provision of a copper sleeve, such as sleeve. H, or a copper slug materially'effects the release time of the relay armature by the generation of a counterelectromotive force when the circuit of the relay winding is opened. It is known that the size of the sleeve, or slug, is also a controlling factor in the release time of an armature. It is also common practice to provide residual screws in the armature to maintain an air gap between the relay core and the armature when a relay of the slow-to-release type is energized to prevent the armature sticking, eithen permanently or, too long due to the so called residual magnetism. Relays of this type have proven satisfactory for, use in telephone exchanges, but because of their 'size', weighta'nd other characteristics have not proven satisfactory for use in circuits used in airplanes. A small, or diminutive, type of relay was therefore developed for use in circuits or airplanes. Due to the small size of these relays, the use of a small residual screw for controlling the release time of the relay armature proved unsatisfactory because such small residual screws pounded out, thereby requiring constant readjustment as well as reducing the efficiency of the relay. It was necessary to develop a diminutive slow-to-release type relay which could be adjusted for different armature release time periods, within certain limits, and one in which the desired release time period, after once being set, would indefinitely maintain its adjustment, to meet the requirements for airplane use. In order to do this a small relay was designed which is virtually independent of normal voltage fluctuations and switch spring tension for controlling the release time of the armature, and one in which the only variable factor by means of which the release time of th armature may be varied, is the means for adjusting the air gap between the relay core and the armature. This relay is provided with a fairly large copper sleeve I I compared with the iron in the field structure comprising the core I 2, heelpiece I4 and armature I5. The amount of iron in the magnetic circuit of this field structure was carefully chosen so that the iron in the magnetic circuit is fully saturated by the counterelectromotive force generated by the copper sleeve II when the circuit through the winding I3 is opened after winding I3 has been energized at approximately one third the normal operating voltage. Due to this arrangement the release time of the armature is virtually independent of normal voltage fluctuations or changes in the switch spring tension of armature switch spring 21 and make switch spring 25.

In order to vary the release time of armature I5, an armature stop spring 29 is provided in the spring pile-up and an adjusting nut 41 is provided for adjusting the free end 43 of armature stop spring 29. When the winding I3 is energized the armature I5 is operated toward the core, the tension of armature spring 21 having very little, if any, retarding effect on operation of the armature I5, until such time as the projection arm I Ia of armature I5 strikes the dished portion 43 of armature stop spring 29. At this point, due to the short distance between 43 and the hole 42, located at the fulcrum point of armature stop spring 29, the armature projection arm I la encounters greatly increased spring tension tending to prevent further operation of armature I5 towards the core I2. Due to the adjustment of armature stop spring 29 and its tension, such spring may be in such a location as to prevent the armature I5 from actually coming in contact with the core I2 thereby leaving an air gap between armature I5 and core I2. As is well known, the greater the air gap the quicker the armature will release and since this air gap is made variable by adjusting the stop nut 41, the release time of armature I5 may be varied.

Depending upon the magnetic pull on the armature, the armature stop spring 29 may bend only slightly dependent upon the tension of spring 29 fromits fulcrum point to its free end 43 and this slight movement of the armature may permit actual engagement of the armature I5 with core I2. 29 exerts suflicient pressure to break the armature I5 away from the core I2 when the so called residual magnetism has died down to a prede- However, even in this case, the spring termined point to thereby initially start the restoring movement of armature I5. After the armature has once started in its restoring movement it will continue to its unoperated position.

It should also be noted that the washer 46 has a partially rounded surface next to the adjusting stop nut 41 so that regardless of the position of adjusting nut 41 the other surface of washer 46 does not change the fulcrum point on the armature stop spring 29.

This relay has proven very satisfactory and has met the requirements as to size, Weight and characteristics for use in airplane circuits and one which will maintain its release time characteristics indefinitely after once being set at the required time period. The release time may be easily changed by using pliers or a small wrench to turn adjusting stop nut 4! to in turn adjust the free end 43 of armature stop spring 29 to the position which gives the desired release time.

Having fully described the invention, what is considered new and desired to have protected by Letters Patent is specifically pointed out in the following claims.

What is claimed is:

1. In a relay comprising a relay core, an L- shaped heelpiece secured to said core at its L- shaped base portion and having an upright portion extending in a plane parallel with said core, an armature movably mounted on said upright portion of said heelpiece and having a portion extending over said core for completing the magnetic circuit of said relay, an arm integral with said core portion of said armature extending from said core portion parallel with said upright portion of said heelpiece towards said base portion of said heelpiece, a projection arm integral with and extending at right angles from said arm portion of said armature across the base portion of said heelpiece, a spring pile-up including an armature switch spring having a bushing tensioned against and engaging said projection arm at one point for maintaining said armature in its released position, an armature stop spring included in said spring pile-up having a free end extending over another point of said projection arm .and in the operating path of said projection arm, and means adjustably secured to said heelpiece for adjusting the distance between the free end of said armature stop spring and said projection arm for variably stopping said armature thereby variably adjusting the air gap between said core and said core portion of said armature when the relay is energized to change the release time of said armature.

2. In a relay comprising a relay core, a heelpiece secured to said core, an armature movably mounted on said heelpiece for cooperation with said core and for completing the magnetic circuit of said relay, a spring pile-up including a make switch spring and an armature switch spring, a back stop switch spring intermediate said make and armature switch springs for normally maintaining said make switch sprin out of engagement with said armature switch spring, a projection arm on said armature for operating said armature switch spring into engagement with said make switch spring to complete an electric circuit in response to the operation of said armature, an armature stop spring included in said spring pile-up having a free end located in the operating path of said projection arm, and means adjustably secured to said heelpiece for adjusting the distance between the free end of said armature stop spring and said projection arm for in said spring pile-up, an armature stop spring included in said spring pile-up and having its free end extending in the operating path of said projection arm, and an adjusting nut for adjusting the distance of th free end of said armature Stop spring from said projection arm to limit the operating stroke of said armature when the armature is operated.

4. In a relay comprising a magnetic structure including arelay core, a heelpiece securedto said core, an armature movably mounted on said heelpiece, and a spring pile-up secured to said heelpiece including switch springs for controlling an electric circuit, one of said switch springs having a free end operatively engaging said armature at one point for controlling said switch springs when the armature is operated, an armature stop spring included in said spring pile-up and having a free end extending over and in the operating path of said armature at another point of said armature, and an adjusting nutfor adjusting the distance between said free end of said armature stop spring and said armature to limit the operating stroke .of said armature when the armature is operated.

5. In a relay comprising a magnetic structure including a relay core, a heelpiece secured to said core, switch springs secured to said heelpiece, and an armature movably mounted n said heelpiece, one of said switch springs having a free end opeiatively engaging said armature at one point for controlling the operation of said switch springs when the armature is operated, an armature stop spring mounted on said heelpiece and having a free end extending over and in the operating path of said armature at another point of said armature, and an adjusting nut for adjusting the distance between said free end of said armature stop spring and said other point of said armature to limit the operating stroke of said armature when the armature is operated.

6. In a relay comprising a magnetic structure including a relay core, a heelpiece secured to said core, an armature movably mounted on said heelpiece, and a spring pile-up secured to said heelpiece including switch springs, a projection arm integral with said armature for controlling said switch springs when the armature is operated, an armature stop sprin included in said spring pile-up and having a free end extending ove and in the operating path of said projection arm, an adjusting nut having an insert for maintaining said nut in its last adjusted position, and said adjusting nut being manually adjustable for adjusting the distance of said free end of said armature stop spring from said projection arm to limit the operating stroke of said armature when the armature is operated in response to the energization of said relay.

'7. In a relay comprising a magnetic structure including a relay core, heelpiece secured to said core, an armature movably mounted on said heelpiece, and a spring pile-up secured to said heelpiece including switch springs for controlling an 10 electric circuit, a projection arm integral with saidarmature for controlling said switch springs when the armature is operated, an armature stop spring included in said spring pile-up and having a free end extending over and in the operating path of said projection arm, an adjusting nut having an insert for maintaining said nut in its last adjusted position, saidadjusting nut being manually adjustable-for adjusting-the distance of said free end of said armature stop spring. from saidprojection arm to limit the operating stroke of said armature-when the armature is operated in response to the energization of said relay, and a dished shaped. portion on said free end of said armature stop spring presentingthe same rou'nded striking surfaceto. said projection arm regardless of the adjusted. position ofsaid armature sto sprmg.

8. In a relaycomprising a magnetic structure including a relay core, a heelpiece and an armature movably mounted onsaid heelpiece, switch springs,.a projection armintegral withsaid armaturefor controlling said switch springs,.an armaturestopspring located in the operating path of saidprojectionarmior. limiting the operating strokeof saidiarmature towards saidcore,.means for operating. said armature, and means for adjusti'ngthe distance between said armature stop spring and said'projection arm for variably stoppingsaid armature before it reaches said core thereby variablyadjusting the air gap between saidarmature. andsaid core, when the armature is operated .to variably adjust the release time of saidarmature. a

9. Ina relay. comprising a. magnetic structure including, a relay core, a heelpieceand an armature.v movably' mounted onsaid heelpiece, an

armaturestop spring having .a base. portion secured to saidheelpiece andhaving a free end locatedinthe operating path of said armature, an adjusting nut located at a point intermediate the base portion and the free end of said armature stop spring for variably adjusting the free end of said armature stop spring variable distances away from said armature to limit the operating stroke of said armature towards said core to thereby vary the air gap between said core and said armature when the armature is operated, and a washer having a semi-rounded surface located between said adjusting nut and said armature stop spring for maintaining the same fulcrum point at said intermediate point on said armature stop spring regardless of the adjusted position of said armature stop spring and said adjusting nut.

10. In a relay including a relay core and a heelpiece secured thereto; a unitary assembly comprising a plurality of circuit controlling contact switch springs separated by insulators one of which is an armature restoring spring, a back stop spring for normally maintaining another of said switch springs from engagement with said armature restoring switch spring, an armature stop spring, and a clamping plate; an armature and a pivot pin therefor, means including said clamping plate for providing a bearing for said pivot pin, means for securing said unitary assemably to said heelpiece to rotatably secure said pivot pin in said bearing, an arm on said armature being engaged at one point by a bushing on the free end of said armature restoring switch spring, a free end of said armatur stop spring extending over another point of said arm, a threaded stud secured to said heelpiece, and an adjusting nut cooperating with said stud and manually operative to adjust the free end of said armature stop spring variable distances from said other point of said arm to thereby vary the air gap between the armature and said core when thearmature is operated towards said core in response to the energization of said relay.

11. In a relay including a relay core and a heelpiece secured thereto; a unitary assembly comprising a plurality of circuit controlling contact switch springs one of which is an armature restoring spring, an armature stop spring, and a clamping plate separated by insulators; an armature and a pivot pin therefor, mean including said clamping plate for providing a bearing for said pivot pin, means for securing said unitary assembly to said heelpiece to rotatably secure said pivot pin in said bearing, a bushing n the free end of said armature restoring switch spring engaging said armature for operating thereby, a free end of said armature stop spring located in the operating path of said armature, a threaded stud securedto said heelpiece, and an adjusting nut cooperating with said stud and manually operative to adjust the free end of said armature stop spring variable distances from said armature to thereby vary the air gap between the armature and said core when the armature is operated towards said core in response to the energization of said relay,

12. In a slow-to-release relay comprising a magnetic structure including a relay core, a heelpiece and an armature, a relay winding for operating said armature, and a copper sleeve around the relay core for retarding the release of said armature when the circuit through the relay winding is opened, the amount of iron in said magnetic circuit and the size of said copper sleeve being so proportioned that said magnetic circuit is fully saturated by the counter-electromotive force generated by said copper sleeve in response to the opening of the circuit through-said relay winding after said winding has been energized by approximately one third the normal operating voltage of said relay thereby rendering the release time of said armature unaffected by normal voltage fluctuations through said relay winding, and an adjusting nut adjustably secured to said heelpiece for adiustably limiting the operating stroke of said armature towards said relay core when the relay winding is energized in order to vary the air gap between said armature and said core to thereby vary the release time of said armature when said counter-electromotive force is generated by said sleeve in response to opening the circuit through said relay winding.

13. In a relay comprising a relay core, a heelpiece secured to said core, an armature movably mounted on said heelpiece for completing the magnetic circuit of said relay, a flexible armature switching spring tensioned against an operating arm of said armature, a back stop spring, a flexible make switching spring tensioned against said back stop spring and held out of engagement with said armature switching spring by said back stop spring, means for operating said armature to flex said armature switching, spring into engagement with said make switching spring and to flex said make switching spring a variable distance away from said back stop spring dependent upon the operating stroke of said armature, and an armature stop spring adjustable to various stop positions to stop said armature variable distances away from said relay core to change the release time of said armature without effecting the operate time of said armature and without appreciably changing the contact pressure between the armature switching spring and the make switching spring.

FREDRIC E. WOOD.

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