US2890306A

US2890306A - Electromagnetic circuit breaker

Info

Publication number: US2890306A
Application number: US667847A
Authority: US
Inventors: Albert B Rypinski; Walter S J Smith; Brunner Carl; Martin J Laverty
Original assignee: Murray Manufacturing Corp
Current assignee: Murray Manufacturing Corp
Priority date: 1957-06-25
Filing date: 1957-06-25
Publication date: 1959-06-09
Anticipated expiration: 1976-06-09

Description

June 9, 1959 A. B. RYPINSKI ET AL ELECTROMGNETIC CIRCUIT BREAKER Filed June 25, 1957 3 Sheets-Sheet 1 ATT RNEY June 9, 1959 A. B. RYPINSKI ET AL 2,890,306

ELETROMAGNETIC CIRCUIT BREAKER Filed June 25, 1957 3 Sheets-Sheet 2 ATTO EY June 9, 1959 2,890,306

A. B. RYPINSKI ET AL ELECTROMAGNETIC CIRCUIT BREAKER Filed June 25, 1957 s sheets-sheet s fui l I l i United States Patent O M ELECTROMAGNETIC CIRCUIT BREAKER Albert B. Rypnsk and Walter S. J. Smith, Brooklyn, and Carl Brunner, Floral Park, and Martin J. Laverty, West Hempstead, N.Y., assignors to Murray Manufacturing Corporation, Brooklyn, N.Y.

Application June 25, 1957, Serial No. 667,847

17 Claims. (Cl. 20D-87) Thisinvention relates to a device for opening and closing an electric circuit, and more particularly to a molded case magnetic circuit breaker, serving the dual function of a switch and a protective device.

`It is desirable to protect certain electrical equipment against continued overload and short-circuit, and it is also desirable to control the operation of the equipment with a manually operable switch. A circuit breaker may perform both these functions and, as a result of this versatility, has enjoyed wide and extensive use.

The protection provided by the circuit breaker is automatic, the breaker responding to a particular condition to open the circuit. Circuit breakers generally are classiiied by the condition which causes the automatic operation. For example, those which respond to excessive heat are known as thermal breakers, others which respond to heat-current are known as thermal-magnetic breakers, and those which respond solely to excessive current are called electromagnetic breakers. This invention relates to the electromagnetic circuit breaker.

The better known electromagnetic (hereinafter referred to as magnetic) circuit breakers, of the molded case type, operate on the toggle principle. As is well known, to operate a toggle device, force must be applied to a lever or handle until it reaches a predetermined position, whereupon the operated member snaps into the nal desired position. The linkage required for toggle action and the principle of operation are well known.

Toggle operated breakers suffer several disadvantages, the most significant being the cost of the toggle linkage and the peculiar arrangement required for such linkage.y

With the trend in the electrical industry towards minia turzation, simplification and consequent reduction in cost, considerable eiort and money has been expended in research for a smaller, more simple and less expensive magnetic circuit breaker.

Accordingly, it is a -tirst object of this invention to provide a novel magnetic circuit breaker which, in contradistinction to the toggle breaker, operates as a result of the application of a normal but continuous force until the breaker contacts are closed; in other words, the breaker operates by direct action.

It is a further object of our invention to provide a circuit breaker which is made of fewer parts, is less complicated and less expensive to manufacture, and is at least as efficient as the magnetic breakers now known.

`It is a feature of our invention to provide a breaker which is amenable to mass production techniques.

It is a further feature of our invention to provide a novel molded insulator case, comprising indentations and projections for mounting most of the breaker parts contained therein.

Theabove-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

y 2,890,306 Patented June 9, 1959 Fig. 1 is a side view of the magnetic circuit breaker casing;

Fig. 2 is an end view of the breaker showing a spring clip contact for connection to a bus-bar contact blade;

Fig. 3 is a view taken along lines 3--3 of Fig. 2;

Fig. 4 is a cross-sectional view of the breaker; and

Figs. 5-9 are views taken along the correspondingly numbered lines of Fig. 4.

Figs. 1 and 2 illustrate the simplicity of the casing which houses the magnetic circuit breaker. The casing comprises only two parts 1 and 2, which tit together inv interlocking engagement and are secured by three bolts 3 passing through the casing from one side and engage nuts 4 on the other side. In place of the bolts and nuts,

rivets may advantageously be used. One end of the.

casing (Fig. 2) is formed to leave an opening for passage to a spring clip 5 which is adapted to engage a blade-ofl a bus-bar (not shown) carrying the desired voltage. A

handle 6 is provided at the top of the casing emerging.-

from a raised portion 7 which tits into a complementary opening in a circuit breaker. mounting panel (not shown). Thus, for example, in a load center, a panel is provi-ded to accommodate a plurality of circuit breakers and only the raised. portions and handles are ap-` parent.

Referringnow to

Figs.y

3 and 4, the spring clip 5 constitutes the input to the breaker and a wire connector 8` nector 8. Thus, when the

contacts

11 and 12 are closed,y

a circuit is completed from input 5 to output 8.

The

ends

10 and 11 of the coil 9 may be brazed,v

welded or soldered to the clip 5 and contact spring mem.- ber 12 respectively, or if desired, the clip may be formed with a curved tongue 16, as shown, vwhich may bev crimped into the wire.

The contact spring 12 isa flat resilient member best seen in Figs. 5, 6) generally in the shape of a U..

The end which is brazed to the coil 9 maybe bent outwardly to serve as aplatform for the wire, or as shown,L

the end portion may beprovided. with a hole in which the Wire is brazed.

The spring 12 extends towards the base of the breaker` close to `an insulator wall 17 which serves the dual purpose of separating the spring 12 from the clip 5 and also limits the end-to-end movement of the spring when it is tensioned by closing the contacts. The spring 12 continues close to the base of the breaker, bearing against a slide assembly 18, tending to urge the slide assembly upwardly, and then between

insulator walls

19, 20. The wall 19 serves as a shield for the slide assembly against the hot gases produced by the arc which is de?.A veloped between the contacts as they break dueto an i overloaded circuit. Wall 20 limits the return movement t,

of the spring 12 when the contacts are opened.

If desired, a helper spring 21 may be utilized to urge'y the spring 12 into the open position. The spring 21 is maintained in position by one end engaging a V notchl in the wall 17 and held in tension between the base and the contact spring 12.

The leg of the spring 12 adjacent the wall 17 is restrained against longitudinal or lateral'displacement by ears 22 fitting into corresponding grooves formed in the casing. The portion ofthe spring directly below' the lower ear 22`is cut-awayjto provide clearance for the* end of the coil 9 which is connected to the spring cli 5.

'libe wire connector 8 comprises a loop-shaped body member 23, preferably made of a single piece of copper. The lower portion of the connector constitutes the extension which carries the contact 14. The end of the upper portion passes through a hole 24 formed in thc wall at the load end of the breaker. The hole 24 serves also as a wire 'insertion hole; the wire being secured to the connector by a tightening screw 25.

The connector is maintained in place by fitting snugly between insulator wall 26, extension 27 of wall 20 and a pair of rectangular projections 28 (Fig. 5) extending a small distance from the opposite sides of the casing into the space defined by the loop.

The spring clip 5 is mounted between the forward portion of wall 17 and the front end of the casing. The clip comprises a pair of blades 29 joined together at the top. An end portion of the clip, just below the top thereof, is struck-out to provide a U-shaped opening which slides over and embraces lip 30 extending from the wall 17. If desired, a second clip (not shown) may -be used to maintain the resiliency of the clip 5, the blades of the second clip extending at right-angles to, and embracing t-he blades ofthe clip 5.

Referring again to the electric circuit, the coil 9 constitutes the winding for a solenoid 31. The solenoid 31 comprises a hydraulically sealed, nonamagnetic tube 32 known as a magnetic time delay tube. As best seen in Fig. 8, the tube 32 contains a movable magnetic core 33, a spring 34, and is filled with a liquid of desired viscosity. A magnetic pole piece 35 in the form of a disk or rectangular plate is affixed to one end of the tube 32. The movable magnetic core 33 is normally urged to the end of the tube remote from the pole piece 35 by the spring 34. 'I'he length of the movable core 33 is approximately equal to the axial length of the coil and when the core is separated from the end pole piece, the reluctance of the magnetic circuit is maximum. During normal current flow through the coil 9, the developed by the magnetic flux is insufficient to overcome the spring bias and the damping effect of the liquid in the tube; thus, the movable core remains approximately in its normal po sition. As a result of a continuing overload, the increased current through the coil 9 produces a M.M.F. sufficient to cause the core 33 to move to the end pole piece 35, thereby closing the air gap and greatly increasing the flux.

The coil 9 is maintained in position between the end pole piece 35 and a shoulder member 36 made of magnetic material. The shoulder 36 is preferably in the form of a rectangular plate and is adapted to fit into slots 37 formed in the opposite sides of the casing as shown. Instead of the slots 37, other impressions in the casing may be utilized to support the shoulder member 36. If the pole piece 35 is in the form of a rectangular plate, it may be similarly mounted. However, if it is in the form of a disk, smaller in diameter than the width of the space between the sides of the `casing then an arcuate projection may -be provided on opposite sides of the casing abutting the sides of the pole piece to lodge it against movement.

The effect of the increased flux through the solenoid will now be described in conjunction with the control mechanism which may be operated manually to open and close the

breaker contacts

13, 14, or automatically to open the contacts upon an overload or short-circuit condition.

The control mechanism comprises a top member or armature 38 coupled to a side member or armature 39 which bears on the slide assembly 18. The armatures are made of a magnetic material and serve partially to complete a magnetic circuit for the solenoid.

The top armature 38, as best seen in Figs. 3, 4, and 5, is in the shape of a flatbar provided with a hole 40,

through which the upper part of tube 32 extends. One end of the bar rests on a support 41 extending from the casing. The other end is curved downwardly and has a portion `cut-out from the end thereof, leaving a pair of arms 42, 43. Close to the cut-out portion, a slot 44 is provided for receiving an end of the side armature 39.

The top armature is movable about the support 41, which acts as a fulerum, and its movement is guided by rails 45 (Fig. 5), molded as part of the casing, and fitting into notches 46.

The side armature 39, as best seen in Figs. 3, 4, and 7, is in the shape of a flat bar and is coupled to the top armature by its end or neck portion 47 fitting into the slot 44 of the top armature. At the upper end of the side armature, an ear 48 is struck-up and underlies the cut-out portion of the top armature.

The lower end of the side armature 39 is off-set, as shown, to extend `at right-angles to the bar for a short distance and then bent again to extend in the original direction. The right-angular portion bears on the slide assembly 18. The width of the bar is stepped to provide a portion 49 (Fig. 7) which is guided by projections 50 against sideward movement in the casing. The lower portion 51 of the side armature is narrowed further to fit freely in an opening 52 of a trip armature 53. The trip armature completes the magnetic circuit and its function will be described later in conjunction with the automatic opening of the contacts.

In the unoperated condition, that is, when the contacts are open, as shown in Fig. 3, the side armature is urged in the upright position by means of a slide assembly roller 54 bearing against the depending end of the Off-set portion, and also by a coil spring 55 acting on the underside of the ear 48, urging the neck portion 47 against the front face of the slot in the top armature.

As best seen in Figs. 3, 4, and 5, the coil spring 55 is mounted on a post 56, with its opposite ends positioned in a groove 57 formed in the wall 20. The intermediate portion 58 of the spring extends from the post (Fig. 5 and is shaped to lie between the arms 42, 43 of the top armature, bearing against the underside of the struck-up ear 48.

The circuit breaker is operated manually by moving the handle 6 towards the input end, to the position shown of oppositely disposed posts 59 fitting into corresponding holes provided in the handle. The handle is normally urged into the unoperated position by a spring 60, which is stretched when the handle is pivoted to the operated position. The bottom of the handle is angularly or cam shaped, with the corner of the angle acting directly on the top armature which serves as the cam follower. As seen in Fig. 3, side 61, of the cam surface, bears on the top armature while the handle is in the unoperated position.

When it is desired to close the contacts (i.e. operate the breaker), the handle is moved to the forward position as shown in Fig. 4. A V-shaped slot 62 in the top of the casing limits the travel of the handle 6. While the handle is being moved forwardly, the angle corner thereof causes the top armature to be pivoted downwardly about the support 41, thereby depressing the side armature 39. The side armature is moved down on the slide assembly 18 which exerts substantially only a radial force on the roller 54, whereby the roller has no tendency to turn. The direction of force applied by the side armature on the roller is critical and always passes through the center of the roller. This radial force is possible because only the linear component of motion of the top armature (cam follower) is translated to the side armature, thereby driving the side armature so that its direction'of force passes through the center of the roller.

Before the side armature is fully depressed, the curved arms 42, 43 of the top armature engage tbe intermediate portion of the spring 58 and relieve the tension from the asedaoe side armature; thus, the reason for the curved arms. The importance oli this and the importance of the position of the side armature relative to the roller will be apparent when the function of the trip armature is discussed.

When the handle 6 is moved fully forward, so thatV it is pivoted slightly beyond the equilibrium point of the angle corner the slide assembly 18 is depressed to its fullest extent, thereby correspondingly moving the contact spring 12 and closing the

contacts

13 and 14. The circuit breaker is now closed.

If it is desired to open the breaker, the handle is simply moved to the opposite position, the resiliency of the spring 12 is suficient to cause the slide assembly, side and top armatures to move as permitted by the handle. The

contacts

13, 14 open upon the slideleaving its extreme position.

The slide assembly 18 (Figs. 3, 4, 7 and 9-) comprises a journal housing which includes a bottomy 65, sides 66 (Figs. 7, 9) and a raised back portion 67 shielding the roller from the are developed between the opening contacts. The sides 66 are formed with outwardly extending ears 68 which together with the axle 63 guide the travel of the assembly; the axle being slidably mounted in a groove 69 and the ears 68 slidably embracing the walls on opposite sides of the groove.

One of the two important functions of the circuit breaker is to respond automatically to current exceeding a given magnitude for a period of time, immediately or upon short circuit, and open its contacts. This function is provided essentially by the solenoid andthe trip armature 53.

The trip armature is in the shape of an angle plate having one leg 70 positioned alongside the side armature and the other leg 7l below the pole piece 35 of the solenoid. As shown in Fig. 6, the leg 70 is provided with formed lugs 72 rockably mounted in depressions 73` in the casing. The depressions 73 are in the shape of a truncated triangle to permit the desired rocking motion.

The separation of leg 71 from the pole piece 35 is limited by a projection 74, extending from the casing.

The leg 71 is normally urged against the projection 74 by a coil spring 75 mounted on the post 56 (Figs. 5, 7) having one end 76 bearing against the leg 70, preferably in a groove provided therefor. The oppositeend of the spring is adjustably positioned by one of a plurality of molded pegs 77, whereby the tension of the spring-bearing on the trip armature may be adjusted.

Thus, upon the occurrence of a short-circuit or overload condition, the solenoid develops suliicient force to attract the trip armature which strikes the side armature imparting a force thereto tangentiall to thesurface of the roller, causing the roller to rotate and trip the side armature. Whereupon the slide assembly is immediately forced upwardly by the contact spring 12, thusopening the

contacts

13, 14. The position of the parts immediately being tripped is shown in Fig. 4 by dot and dash lines.

As the contacts 13, 14- are opening, as a result of overload, arcing occurs between the contacts. To suppress this arcing, a horseshoe shaped pieceof magnetic material 79 is mounted between the

walls

27 and 19, in a groove provided in the casing. The magnetic material is effectively a magnet during the presence of the arc and serves to cut-off the arc as it tends to pass betweenthe opposite ends thereof.

The hot gases produced by the are escape primarily between the

Walls

19 and 26 which is an open passage to the exterior of the breaker.

However, a small proportion of the hot gases and dust is projected towards the roller 54. As above mentioned, the raised back portion 67 of the journal housing protects the roller.

The advantages of our invention should now be apparent. The parts are of simple construction .and relatively few in number. Because the breaker operates on the direct action (as opposed to toggle action) principle, the arrangement of parts lends itself well to mass production techniques, and separate component parts such as pivot pins, posts, etc. have substantially been eliminated as a result of the novel molded casing.

While We have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. An electromagnetic circuit breaker, comprising a pair of complementary contacts, one of which is movable relative to the other, a time delay solenoid comprising a non-magnetic tubular member, a rst magnetic pol-e piece atiixed to one end thereof, a coil surrounding said tubular member and electrically connected to the movable contact, a magnetic core piece freely mounted within said tubular member in a fluid medium and movable towards said pole piece in response to flux of given magnitude passing `through said core, the ux increasing as the core piece approaches the end pole piece, a manually operable handle, a trippable mechanism linearly movable in direct response to the operation of said handle for opening and closing said contacts, and means for tripping said mechanism and thereby opening said contacts -in response to a given overload condition.

2. The breaker according to claim 1, and further comprising a contact spring carrying said movable contact and tensioned so that the contacts are normally separated, said mechanism comprising a slide assembly movable in a direction normal to said contact spring and bearing thereagainst, said slide assembly at one limit of its travel closing said contacts, a trippable member mounted on said slide assembly and directly responsive to the movement of said handle for moving said slide assembly, and a pivotally mounted trip armature under control of said solenoid, for tripping said trippable member, when attracted by said solenoid whereby the contact spring may move under its normal tension to open said contacts.

3. The breaker according to claim 1, and furthercomprising an input terminal and an output terminal, a ilat movable contact spring extending from `the input portion of the breaker to the output portion, one end of said solenoid coil being connected to said input terminal, the other end thereof being connected to one end of said contact spring, the other end of said Contact spring carrying said movable contact, the other of said complementary contacts being connected to said output terminal, said contact spring being tensioned so that the contacts are normally separated, and said `mechanism being operable on said contact spring .to open and close said contacts.

4; An electromagnetic circuit breaker comprising an input terminal and an output terminal, a solenoid coil, a at movable contact spring extending from the input portion of the breaker to the output portion, one end of said solenoid coil being connected to said input terminal, the other end Ithereof being connected to one end of said contact spring, the other end of said contact spring having a contact attached thereto, a fixed cooperating contact connected to said output terminal, said contact spring being tensioned so that the contact carried thereby is normally separated from said xed contact, a mechanism controlling the movement of said Contact spring comprising a manually operable handle having a cam operating surface, a cam follower, a trippable member movably mounted in a direction normal to said contact spring and actuated by said cam follower, and means responsive to the linear movement of said trippable member for moving said contact spring and closing said contacts.

5. The breaker according to claim 4, wherein said contact spring moving means comprises a roller rotably mounted on an axle, means for slidably mounting said axle in the direction of movement of said trippable member said trippable member comprising a bar-shaped member having one end off-set to engage said roller, and means for stably positioning said bar-shaped member.

6. The breaker according `to claim 5, and further comprising means for freely coupling said cam follower to said trippable member, said handle having an angularly shaped cam surface, a flat portion thereof bearing against Said cam follower when the handle is positioned so 'that the breaker is in the open position, and the angle corner of said surface actuating said cam follower during the movement of said handle to close said contacts, the linear component of motion of said cam follower being transferred Ito said trippable member, whereby substantially no turning force is applied to said roller.

7. An electromagnetic circuit breaker, comprising an insulator casing, separable `into two parts; an input end comprising a spring clip adapted to engage a blade contact, an output end comprising a wire connector, an interruptible electric circuit connecting said spring clip to said wire connector comprising a solenoid coil having one end connected to said spring clip, a movable spring contact connected to the other end of said solenoid coil, a button contact attached to an end of said spring contact, a complementary fixed button contact attached to said wire connector, said contact spring being tensioned so that said button contacts are normally separated, and when said contacts are closed, a circuit is completed from the input end to the output end; a mechanism for controlling manually and automatically the movement of said movable contact, comprising a solenoid body xedly mounted in said casing and having a magnetic pole piece attached to one end thereof, a top armature extending across the opposite end of said solenoid body and pivotally supported at one end thereof, a pivotally mounted handle having a cam surface for actuating said top armature and limiting the movement thereof in the upward direction, a side armature mounted transversely to said top ,j

armature, means coupling the other end of said top armaturc to one end of said side armature in linear motion transfer relationship therewith, the direction of movement of Said side armature being normal to said Contact spring, the opposite end of said side armature being off-set and having a portion extending perpendicular thereto, an assembly slidably mounted in said casing in a direction normal to said contact spring, said assembly comprising a roller, an axle therefor, a housing for said roller and axle having a base, means .normally positioning said side armature so that the off-set portion bears radially against said roller whereby there is no tendency for said roller to turn, said contact spring bearing against said base, a trip armature in the form of an angle plate, the trip armature being positioned so that one leg thereof is normally separated a given distance from said pole piece and `the other leg is positioned in close proximity to Said side armature, means rockably mounting said trip armature about a point intermediate said other leg so that when said one leg is attracted towards said pole piece, the base portion of said other leg strikes said side armature thereby `tripping it off said roller and permitting said Contact spring to move under its normal tension and open said contacts.

8. The breaker according to claim 7, and further comprising spring means mounted in said casing normally urging said one leg of said trip armature away from said pole piece, and a projection in said casing limiting the separation therebetween.

9. The breaker according `to claim 7, wherein said top armature comprises a pair of bent arms extending from one end thereof, an ear struck-up .from said side armature and located between said arms, a U-shaped spring, the base portion thereof bearing against the underside of said ear for positioning said side armature radially on 8 said roller, the arms being bent sufficiently so that when said side armature is fully depressed, the arms of said top armature engage the legs of said U-shaped spring relieving the tension on said side armature whereby the side armature is maintained in position by said top armature.

10. The breaker according to claim 7 wherein the cam surface of said handle is angularly shaped, a side portion thereof bearing against said top armature when said handle is in the open position, and the angle corner directly actuating said ltop armature during the movement of said handle to the closed position.

11. The breaker according to claim 7 and further comprising a horseshoe shaped magnetic member mounted in slots in the sides of said casing between said contacts when said contacts are open, whereby arcing caused by the opening of said contacts during the passage of current therethrough produces a magnetic eld across the ends of said horseshoe shaped member, which tends to suppress the arc.

12. An electromagnetic circuit breaker, comprising a pair of complementary contacts, one of which is movable relative to the other, a roller assembly slidable in the direction of movement of said movable contact and adapted to act thereon, a handle having a cam operating surface, a cam follower, a trippable mechanism coupled at one end to said cam follower and movably mounted transversely thereto, the opposite end of said trippable mechanism being positioned to bear radially on said roller assembly, whereby movement of said handle is transferred to said roller assembly for closing said contacts, means responsive to a given overload condition for moving said trippable mechanism olf said roller assembly thereby releasing said roller assembly, and means for urging said movable contact away from the other contact when released by said roller assembly.

13. The breaker according to claim 12, wherein said means for urging said movable contact consists of a pretensioned contact spring carrying said movable contact, the spring being mounted so that said movable contact is normally separated from the other contact.

14. The breaker according to claim l2, wherein said means for moving said trippable mechanism off said roller comprises a trip armature rockably mounted in close proximity to said trippable mechanism, and means for moving said trip armature so that in response to a given overload condition said trip armature imparts a tangential force .to said trippable mechanism relative to said roller assembly, thereby causing the mechanism to roll off said roller assembly.

l5. An electromagnetic circuit breaker comprising a pair of complementary contacts, one of which is movable relative to the other, a solenoid including a coil electrically connected to one of said contacts, a manually operable handle, a trippable mechanism movable in direct response to the operation of said handle for opening and closing said contacts, said trippable mechanism positioned to be attracted to said solenoid in response to an overload condition, a trip armature trippably coupled to said trippable mechanism and also positioned to be attracted to said solenoid in response to an overload current, whereby depending upon the nature of the overload, either said trippable mechanism or said trip armature may iirst be vattracted to said solenoid suiiciently to trip said trippable mechanism and open said contacts.

16. An electromagnetic circuit breaker comprising a pair of contacts, one being movable relative to the other, a manually operable handle having a cam-operated surface, a cam follower pivotally supported at one end thereof, a trippable member movably mounted transversely to said cam follower, means coupling the oppo site end of said cam follower to one end of said trippable member in linear motion transfer relationship; means mounted between the opposite end of said trippable member and said movable Contact and directly responsive to the linear movement of said trippable member for closing said contacts, comprising a roller assembly slidably mounted in the direction of movement of said trippable member, the *opposite end of said trippabie member comprising an offset portion bearing yagainst said roller, and means positioning said trippable member so that the force applied to said roller passes through the center thereof whereby there is no turning component on the roller.

17. The breaker according to claim 16 and further comprising means in close proximity to said tnippable member for imparting a tangential force thereto relative to said roller in response to a predetermined condition, causing said trippable member to roll oil said roller.

References Cited in the le of this patent UNITED STATES PATEBVL` S Epstein Dec. 10, 1957