US2236680A - Electric limit switch - Google Patents

Description

April 1, 1941. v, FRY j 2,236,680

ELEcTfuc LIMIT. SWITCH Filed April 7, 193s n Patented A pr. l, 1,941

ELECTRIC LIMIT SWITCH Virgil Homer Fry, Freeport, Ill., assigner to Micro Switch Corporation, Freeport, lll., a corporation of Illinois.

Application April 7, 1938, Serial No. 200,658

` s claims.- (erano-47) The present invention relates generally to electric switches and more particularly to limit switches for use on power machinery and machine tools.

Objects of the present invention include the provision of a rugged, compact. low-cost limit switch of the precision type, suitable for use on power machinery and power machine tools, the provision of a precision type of limit switch operable in response to a small motion of its actuating member whle permitting a relatively large overtravel of the actuating member beyond the switch operating point thereof, the provision of a precision type of limit switch that is proof against damage or loss of .precision from ordinary accidental damage or from mechanical blows or the like imparted to its actuating member, and the provision of an improved electric switch construction. 'I'hese and other objects of the invention will become apparent from the following description of a specic embodiment thereof, which embodiment serves by way of example to illustrate the manner in which the invention may be carried out, Without thereby limiting the invention to the specific details of the example. In the drawing: l Fig. 1 is an elevational view, partly in section, of a limit switch construction embodying my present invention;

' Fig. 2 vis a sectional dview taken along the line 2-2 of Fig. 1 and viewed in the direction of the` arrows;

Fig. 3 is a sectional view taken along the line 3--3 of Fig. 1; and,

Fig. 4 is an elevational view taken lfrom a direction opposite that of Fig. 1. In the drawing, a steel switch housing i0 includes a case i2 and a cover I4. The case i2 in turn includes a mounting plate i6 and a frame i8. Mounted in the frame I8 Ais an actuating plunger assembly 20 for actuating a rocker arm or lever 26 also mounted in the frame I8. The rocker arm 26 in turn actuates a precision type sensitive electric snap switch or contact mechanism 28.

The frame I8 is formed from steel strip welded into a. continuous loop and is provided withl formed ears such as 30 and 32, to which the cover it and mounting plate i6 may be fastened. The

sembly 20 is reciprocable in a turned brass bushing 34 which is riveted into the steel :frame I6,

and which extends down into the interior ofthe case at one end thereof. The plunger 20 consists of a pin 24 with a head 44'at its lower end, and a cap 22 pinned onto the shouldered upper end of pin 24. The pin 24 ts the small bore at the bottom of the bushing 34 and the cap 42 itself iits the larger bore at the upper end of the bushing. AIncluded within the bushing 34 are a feltpackingl washer 36, a metal washer 38, and a Y spring 40. The headed pin 24 is inserted in the bushing 34 before the bushing is riveted into the steel frame I8. After the bushing 34 has been riveted into place solder is run along the riveted joint to seal it against seepage` of oil. Next the vwashers 36 and 38and spring 40 are inserted in the bushing over pin 24 and the cap 22 is pressed onto the upper end of pin 24 and seated against its shoulder. While a clamp holds the cap 22 rmly in place, a hole is drilled transversely through the cap 22 and pin 24 and a locking pin 42 is driven into the hole.

The spring 4D holds the actuating plunger assembly 2li normally extended from the switch housing as shown in Fig. 1 with the head 44 bearing against the lower end of bushing 34. The clearance space between the head 44 and the bottom of frame i8 is greater than the distance that cap 22 extends above the frame so that cap 22 may disappear into the bushing 34. Insmuch as the switch is intended to be used on power machinery, and on power machine tools, where it will be exposed to oil, grit and chips, a clearance of several thcusandths o f an inch is left between the'cap 2 2 and the interior surface of the bushing 3 so that grit and small metal chips will not likely wedge between the cap 24 and the bushing 34 to prevent free operation. The felt-packing washer 36 provides a seal to prevent the entrance of oil into the switch casing through this plunger and bushing assembly. The lowerefid of bushing 34 isV recessed at 46 to permit a lost motion for the end of rocker arm 26 between the recessed face of the bushing and the head 44 of the pin 24'when the plunger assemblyl 20 is in its normal position as shown.

As shown in Fig. 1, the switch 28 is mounted at an angle in the case i2 to leave above it a tapered space into which the connecting leads may be brought thrigh the opening lili), and to leave below it another tapered space just large enough vfor the rocker arm or lever 26.

The rocker arm 26 is formed of steel and case ihardened. It is channel shaped and supported on a shaft 48 vcarried by the pair of ears 32 which extend along theentire channel-shaped lower side of the frame i8. A pair oggrease soaked felt washers 49 space the rocker armv24 between the ears 82 of the channel-shaped frame I8 and provide a supply of lubricant for the rocker arm bearings. A spring 58 urges the rocker armv 26 to rotate in a clockwise direction as seen in Fig. 1 so that its tongue 52 bears normally against the head 44 ofthe actuating plunger 28. When the actuating plunger 28 is depressed against the force of spring 40, the spring 5I) rotates rocker arm 28 to follow the movement of actuating plunger 2D to operate the switch 28.

When the bushing 84 and its actuating plunger assembly, and the rocker arm 26 and its spring.

58 have been mounted in place in the frame I8, the mounting plate I6 and its gasket I'l are put in place and fastened to the frame I8 by four screws 56, shown in Fig. 4. The mounting plate I6 includes a pair of mounting lugs 58 whichfextend beyond the margins of the frame I8 and which are provided with dowel holes 60 and clearance screw holes 62 for mounting the switch on a machine with which it is to be used. The mounting plate I6 is moved under the heads of. the screws 56 to bring the top of the cap 22 into the correct position with respect to the dowel holes 6U, as by bringing the dimension A shown in Fig. 1 to a predetermined value. When the mounting plate i6 has been properly located on the frame I8 with respect to the normal or rest position of cap 22, a pair of holes are drilled and dowel pins 64 are inserted to insure that the setting will not thereafter be disturbed. Solder is then run over the dowels 64 and screws 66 t0 hold them securely and to provide an oil tight seal.

The switch 28 is a precision snap switch of the type shown and described in U. S. patent to Phillip K. McGall, 1,960,028, May 22, 1934. It includes a molded insulating housing 66 in which are mounted a forked, spring supporting membei-.68, a system oftoggle leaf springs 'I4 carrying moving contact. 16, and a pair of stationary contacts 'I8 and 88. A plunger 82 reciprocable in the insulating cover 84 of the switch housing is adapted to be pushed upwards, as seen in Fig. 1, against the cantilever tension spring 'I8 to deiiect it past the pivotecl support of bowed compression spring 12 at the extreme right end `of the spring support member 6B, to induce a snap motion of contact 16. The contact 16 snaps from its normal position where it engages stationary contact 88 as shown in Fig. 1, to its operated position where it engages the other stationary contact 18. When the plunger 82 is released the stressed cantilever spring 'I8 recovers its original position and snaps the moving contact 'I6 back to its normal position against stationary contact 88. The snap switch 28 is provided with holes in its insulating housing 66 to t over mountingdowels 83 and 64 carried by the mounting plate I6.

At the time that the mounting plate I6` is assembled onto the frame I8, it is provided only with dowel pin 83. After the mounting plate I6 has been pinned onto the frame I8 with the dowels 64, switch 28 is f itted onto dowel 88. The partly assembled device is mounted in a jig having pins for locating the dowel holes 68, and having also a micrometer head for operating the actuating plunger 28. Switch 28 is adjusted to such a position that it snaps its contact 16 when plunger 20 is` depressed 'a predetermined position, as for example a position a few 'thousandths of an inch below its normal position in which its head 44 stops against the lower end 75 of brassbushng 24. The switch 28 is gripped with a clamp, and with the hole in the insulating 83 and shouldered dowel pin 84 is riveted into theA mounting plate I6. l

Connecting leads 86, 81 and 88 are soldered to the terminal lugsf98, 92 and 94 of the switch 2l, a sheet of insulating fibrous material 98 is folded over the terminals of switch 28, the leads are kthreaded through the opening |80 in the frame Il and the switch 28 is slipped into place on the two dowel pins 88 and 84. The leads 86, 81 and 88 are sealed with wax IIII in the aperture IDU of the frame I8. and cover I4 and its gasket I5 are put into place to complete the oil tight closing of the switch mechanism.

It is to be noted that when the actuating plunger 28 is depressed to operate the switch, the

plunger 28 itself does not directly actuate the electric switch mechanism 28. Rather the head 44 of the pin 24 releases the tongue 52 of rocirerv arm 26 and permits spring 58 to rotate the rocker arm. 26 slightly to operate the switch 28. Ac-

cordingly a rapidly mpving cam or the like, actuating the plunger 28 with a hammer-like blow, I cannot deliver that blow to the comparatively fragile and delicately adjusted switch mechanism 28. Similarly an abrupt release of plunger 28 which permits the spring 48 to, strike the head 44 of plunger pin 24 against the tongue 52 of rocker arm 26 cannot deliver that blow to the switchv mechanism 28 because the effectof that blow and of the action of the spring 46 `is to move the tongue 54 of rocker arm 26 away from the plunger 82 of the switch 28. Accordingly, no matter how the plunger 20 may be operated, abrupt blows and excessive strains cannot\be imparted to the delicately adjusted precision switch mechanism 28.

The movement differential or movement required of plunger 82 of the snap switch mechanism 28 to snap contact 16 back and forth between stationary contacts 18 and 8U is, in certain commercial constructions, of the order of .001

inch or even .0001 inch. Accordingly only a very slghtmotion is required of rocker arm 28. In fact its motion is restricted by the stop 4 8 on bushing 34 and by the snap switch mechanism 28 itself to limit any hammer blow action that the inertia of the rocker 26 itself might impose on the switch 28.

Since the clearance provided between head 44 of plunger pin 24 and the lower wall of frame I8 ls greater than the distance that the cap 22 of the plunger 20 extends above the suriace of the switch housing, the head 44 cannot be made to jam against the frame I8 unless the top of the cap 22 is driven below the upper outer surface of the frame I8. A workman or mechanic adjusting a cam or lever to operate the limit `switch on a piece of machinery will ordinarily take care to prevent such a cam or lever from striking against the frame I8 itself, but may not necessarily take the addition and separate precaution of making certain that the plunges/28 be not driven beyond the clearance space provided for it within the switch housing.

In many installations of switches of this type the dimension A in Fig. 1 is unimportant for the normal position of plunger 20, but is important only in connection with that position of plunger 28 at which the switch 28 operates. In this connection it is to be observed that the switch housing maybe deformed, as by a blow against the upper portion of the frame Il, to displace the bushing 3l from its original position Without` which the switch 2lil operates. The dimension A (shown in Fig. 1 for the operation oi' switch 28 depends not upon .the position of bushing 3l but rather upon' the length of plunger 2li, and upon the positions of shaft 48 and switch 2l relative to the mounting plate I6. mounted directly on the mounting plate Ii and the pin 48 is mounted in the frame I8 on the side away from the projecting cap 22 of actuating plunger 20. 'I'here is very little likelihood that the lower portion of frame I8 (as seen in Fig. l) will ever be damaged by accidental -blows and the like from the cams, levers, etc. intended for the operation of actuating plunger 20. The clearance space IIS between the bushing and head 4l, for

the tongue 52 of rocker 26, insures that plunger 20 will stop against the bushing 34 and not against tongue 52 and it permits a greater tolerance in the position of bushing 3|. Consequently this clearance 46 enables the limit switch better to survive any slight deformations of the upper portion (as seen in Fig. 1) of the frame I8.

The precision snap switch 28, has a contact gap of the order of .010 inch and uses light contact The switch 2l isv tion of the class described, a rectangular shallow housing, an actuating plunger extending from said housing for external actuation, a lever adapted to be operated by said plunger, said actuating 'plunger and lever extending along two adjacent sides of said rectangular housing, but spaced out from the corner included between said two sides to leave clearance space for movement of said plunger, said lever lying at an angle to the sides of said rectangular housing, a unitary rectangular switch mounted at an angle in the space between the other two sides of said rectangular housing and said plunger andv lever, said switch being so oriented in said case asto lie at an angle to the sides of said rectangular housing along side of and approximately parallel to the direction of said lever and so as to leave a tapered Y space to one-side of said switch for the entrance pressures. Consequently it is particularly susceptible to failure if oil seeps into it. Oil between the engaging contact surfaces becomes carbonized by the arc incident to breaking a circuit and then prevents the contact pieces from making good electrical contact with each other. It is partly for this reason that all joints are gasketed and soldered to make the limit switch oil tight. While the wires 8B, 8l and 88 are preferable sealed in the opening Ill with pitch as shown in the drawing, under certain conditions a conduit may extenid from this opening to protect the wires.

Since in such a case, the conduit itself may be so congured, or may have its opening so located as to prevent entrance of oil therethrough, the pitch seal may be omitted.

The present invention provides an extremely simple construction for a. precision type of limit switch. Its simplicity permits it to be manufacv tured at a low cost and permits also a compact ruggedconstruction, theadjustments of which are not easilydisturbed after the switch is once completely assembled.

One commercial form of switch has overall dimensions of less than three and three-quarter inches by two inches by one inch, requires less than .001 inch motion of its actuatinlplunger 20 between the on and oi operations of snap switch 28 and yet is capable of controlling directly two one-half horsepower A. C. motor circuits.

'I'he limit switch is oil and water tight and is proof against failure, or even misadjustment, from any ordinary type of accidental damage. Since the present construction positively `avoids the imposition of any excessive operating strains upon the delicately adjusted snap switch 28 itself, the limit switch will continue to operate and maintain its precision of operation over long periods of time.

It will be apparent to those skilled in the art .that the present invention is capable o! numerous modifications and variations and that the embodiment herein shown and described serves simply by way of example. Accordingly the invention is to be limited only in accordance with Iclaim:

of the connecting lead wires into said housing for connection to said switch, said switch being adapted to be operated by said leve'r, said parts lying compactly in said housing and substantially lling it except for saidtapered space required for the connecting leads.

2. In combination in a limit switch construc- `tion of the class described, an integral switch member including spring means for defining a normal position of said switch member, an intermediate member, second spring means o perable to move the intermediate member to oppose the action of said rst spring means to operate said -switch member, primary actuating means, third spring means operable to move the primary actuating means for opposing the spring of said intermediate member, a stop for opposing the action of the third spring means on said pri mary actuating meansto deiine a normal position of said primary actuating means, means for limiting the motion of :said intermediate member, and means providinga lost motion connecy therefrom, and including spring means tending to maintain said switch mechanism in the normal position thereof, a. rocker adapted to engage said plunger to operate said switch, and a spring urging said rocker against said plunger to operate said switch against the force of its own spring, a primary actuating plunger, a spring and stop for dening a normal position of said primary actuating plunger, saidprimary actuating 1. In combination ln a nina switch construe:

plunger being :adapted in its normal position to engage said rocker and hold said rocker against the force of its own spring to permit said switch to move to its normal position, said primary actuating plunger having a lost-motion connection with said rocker so' constructed and arranged that as said plungermoves out of itsnormal po-` sition it permits said rocker, under the force of its own spring to i'ollovc' said actuating plunger but does not permit saidactuating plunger to drive said rocker in the direction in which said rocker is urged by its own spring.

4. The combination of claim 3 wherein there is Included a stop for said rocker to limit its motion in the direction in which it is driven by said primary actuating plunger, but wherein said stop is spaced to permit a-slight motion of said rocker beyond the extreme position to which it maybe driven by said primary actuating plunger, wherein said primary actuating plunger is operable to drive said rocker only against the force of said rocker spring, and wherein said rocker is operable to drive the switch phmger only in the direction in which said rocker is urged by its own spring. n 5. In combination in a limit switch construction of the class described, an enclosing case, a

bushing supported in a wall of said case, a plung' er reciprocable in said bushing, said bushing including a stop for vlimiting the extension ot said plunger from said bushing, a spring for holding said plunger normally extended, a. lever in said case, said plunger being adapted to bear against said lever when said plunger is in its normal position. a spring urging said lever to follow sai plungerin the direction in which said plunger moves out of its normal position, contact mecha-- nism including spring means for giving it a nor-

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