US2529652A - Limit switch - Google Patents

Description

Nov. 14, 1950 A. A. DICKE 2,529,652

LIMIT SWITCH Filed Nov. 16, 1948 2 Sheets-Sheet l INVEN TOR. ALLEN/4. D/CKE j mrm A T TOIQNEYS Patented Nov. 14, 1950 LIlVIIT SWITCH Allen A. Dicke, Montclair, N. J assignor to Crown Controls Company, Inc., New Bremen, Ohio, a

corporation of Ohio Application November 16, 1948, Serial No. 60,210

6 Claims.

This invention relates to an improved limit switch and particularly to thermostatically controlled electrical switches and more particularly to a duplex apparatus for automatically opening and closing different circuits alternately.

An object of this invention is to provide a switch of this type providing for rapid or snapaction. Another object is to provide a switch of this type comprising a flexible arm carried by an abutment and having a free end cooperating with a contact, the arm being formed between its ends with convolutio'ns to increase the resiliency thereof without increasing the distance from the abutment to the contact.

While the present invention is applicable to a wide range of uses, for illustrative purposes, but without intent to unduly limit the application or scope of the invention, it is herein shown and described as an automatic heating furnace control for either hold fire or high limit operation for which. it is especially desirable, but obviously is not limited thereto. The present instrument may be so mounted as to be responsive to temperature fluctuations of a furnace jacket bonnet or of a flue pipe or stack and serve to energize damper actuating means or fuel feeding mechanism as necessitated by heating requirements.

The present invention is a further development of an improvement on that disclosed in appli- 30 cation for Letters Patent of Wilhelm C. Muth,

Serial Number 683,054, filed July 12, 1946, now Patent No. 2,516,525, granted July 25, 1950. Said Muth application contemplates a limit switch of similar construction providing for snap-action contact operation by means of permanent magnets acting upon the contact arms. Due to the limited space available said prior construction utilized magnets which tend to pull the contact arms in the same direction as that toward which they are impelled by the thermo-responsive mechanism. This was found to be unsatisfactory under certain conditions and it is an object of the present invention .to overcome said difficulties and to provide a magnetically restrained snap-action objects in view as will more fully apear in the specification, the invention intended to be protected by Letters Patent consists of the features of construction, the parts and combinations thereof, and the mode of operation, or their equivalents, as hereinafter described or illustrated in the accompanying drawings.

In the accompanying drawings, wherein is shown the preferred, but obviously not necessarily the only form of embodiment of the invention:

Fig. 1 is a front view of an assembled thermostatically energized electric limit switch illustrating the present invention, the cover having been removed.

Fig. 2 is a vertical, longitudinal cross-sectional view taken on the line 22 of Fig. 1.

Fig. 3 is a fragmentary cross-sectional View taken on the line 33 of Fig. 1.

Fig. 4 is taken along line 44 of Fig. 1.

Fig. 5 is a fragmentary front view of part of the mechanism shown in Fig. l, but on an enlarged scale showing in dotted lines the position of the switch blade when in normal unactuated position and showing in full line the switch blade when partly actuated and just prior to its movement from one contact to the other.

Fig. 6 is a view similar to Fig. 5 with the parts shown in the positions which they occupy after the switch blade has been actuated, and Fig. 7 is a diagrammatic view of a system of electrical circuits under control of a limit switch of the type shown in Figs. 1 to 6.

Like parts are indicated by similar characters of reference throughout the several views.

In the drawings, I indicates a helically wound bimetal thermally responsive element through which extends a rock shaft 2, to the extremity of which one end of the bimetal element l is attached at 3.

The opposite endof the thermally responsive spring I is attached to a relatively fixed sleeve 4 through which the shaft 2 axially extends. This thermally responsive portion of the apparatus may be enclosed in a tubular. heat conductive limit switch in which a permanent magnet is so related to the contact blade as to temporarily restrain the contact blade against the action of the thermo-responsive actuator.

Another object is to provide such a switch in which the contacts are so arranged that the gap between contacts which are about to be closed is first gradually increased and is thereupon rapidly closed, thus increasing the velocity of 010- sure.

housing (not shown in drawings) closed at its extreme end and has at its opposite end a lock collar l surrounding the sleeve 4 and engageable with a mounting plate 8, which may be attached to a portion of a furnace jacket bonnet wall or a portion of a flue or stack pipe.

Mounted. on the forward end of the sleeve 4 is a housing 9 enclosing the contact members, their mounts and the actuating head, and having therein a bearing 2o for the shaft 2, mounted on With the above primary and other incidental Qt! abridge 9a. within the housing 9. Carried on the flanged face of the housing 9 are two contact mounting segments. One such mounting segment l i is normally fixed, but is adjustable within the range of segmental slots i2 through which engage clamp screws 94. The other mounting segment i5 is pivotally mounted concentrically with the housing 9 preferably on the rock shaft bearing for manual oscillatory adjustment thereabout. On each of the mounting segments H and It a bracket It is mounted for oscillatory adjustment about a pivot il within the range of an arcuate slot l8 through which a clamp screw l9 extends. Each of the brackets 16 carries a flexible spring contact arm or. blade. The spring arm or blade 28 of the stationary mounting segment l i is alternately engageable with each of a pair of relatively spaced contacts 2-! and 22. The corresponding adjustable mounting segment i5 carries a spring contact arm 23 alternately engageable with spaced contacts 24 and 25. The spring arms 20 and 23 are normally biased-toward each other by adjustment of the brackets it, whereby they tend to engage the innermost contacts 2i and 24 of the respective pairs thereof, under the inherent tension of the spring arms 26 and 23. Fixedly attached to the end of the rock shaft or stem 2, for unison motion intermediate the spring arms 28, and 23 is an oscillatory actuating member or cam 26. The cam 25 is rotated to and fro as the bimetal thermally responsive element is actuated in accordance with fluctuations of temperature. The cam 26 may engage one or the other of the spring arms, thereby overcoming the inherent bias of the engaged spring arm and breaking its contact with the inner or normally engaged'contact 2! or 24 causing it'to engage the other contact of the particular pair thereof.

In order to give to the spring arms actuated by the oscillatory cam a snap-action and definition to their movement, permanent magnets 2i and 24 are mounted on the corresponding mounting sectors adjacent to the contacts 2! and 2 2 respectively, which accelerates its movement and insures firm contact engagement.

A detachable cover cap'23'engageable withthe housing 9, encloses the spring arms, contacts and oscillatory sector. An arm 29' upon the adjustable mounting segment l5 extendsbeyond the periphery of the housing 9 and cover cap 28, to enable the segment and with it the spring arm 23'and coacting contacts 24 and 25, to be shifted circumferentially, thereby advancing or delaying the engagement of the cam 26 with the arm 23 and movement of the latter away from contact 24 into engagement with contact-25. When under increased temperature, the stress of the bimetal element increases, and, the shaft or stem 2 is rocked to move cam 26 out of its engagement with the spring arm 20 the latter disengages the contact 22 and, by itsinherent tension returns to engagement with the opposite contact 2|. By continued rotation the cam 26 engages the 0pposite spring arm 23 to break its contact with the contact 2d and flex the arm into engagement with the companion contact 25. The interval between the release of the spring arm 28 and engagement of the spring arm 23 is dependent upon the degree of rotative setting of the adjustable mounting segments II and i5. The adjusting arm 29 cooperates with a graduated scale upon a lip or segment 80 projecting from the housing 9 having, thereon a range of graduations from Mild through"lf lormal to Cold. The arrangement is such that a lower temperature is required to cause the cam 26 to engage the spring arm 23 when the mounting segment 55 is at one limit of its range of adjustment (Mild) than when it is oscillated to the opposite limit of its range of movement (Cold).

A connection panel 3| surmounts the housing 9 having thereon a series of circuit terminals, the conductors from which extend through the wall of the housing 9 to the various contacts.

Because of excessive range of temperature tending to actuate the cam 26 to an extreme position no positive stop is provided, but after having initially actuated the engaged spring arm, the cam 26 is capable of further idle operation in which the cam periphery merely wipes upon the spring arm under continuing influence of the thermostatic spring l without furtherv moving the engaged spring arm and therefore without straining or breakage of parts, and without causing the springs, either contact or thermostatic, to take a definite set.

In operation, the switching unit functions as a hold-fire or high-limit control, depending upon the condition of the fire as is reflected by the bonnet 0r stack temperatures. If thefire'is within normal limits, the cam 26 does not contact either spring arm 20 or 23. These arms therefore remain in their normal or biased positions, in engagement with the contacts 2i and H respectively. In this position, the switching unit performs merely a coupling function, serving as a terminal board through which the furnace control 32 is operatively connected with the-r0om thermostat 53. So long as the switching unit remains in its normal position, operation of the furnace control unit 32 is under the sole control of the room thermostat, and responds to a call for heat by the room thermostat increases combustion in the furnace, or by reducing combus tion in the furnace when the room' thermostat is satisfied.

Power for the furnace control 32 is supplied by the secondary of a transformer 34- through the wires 35 and 33. The room thermostat and the limit switch comprising the subject matter of the present invention are connected to the furnace control by means of the terminals 37, 38 and 39-. Terminal 33 is the off or stop terminal while the terminal 39 controls the on or start operation of the control 32. The two circuitsarecompleted through the common terminal.

As shown in Fig. 7 the system is in its off or stop position. Should the room thermostat 33 call for heat, the arm will move from the off or stop contact M3 to the on or start contact M. This movement of the room thermostat moves the furnace control'unit 32- to its on or start position, it being energized by'the following circuit; from the common terminal 31 conductor 52 leads to terminal lr' on the panel 35 of the switching unit. From there, through the internal wiring f the limit switch, the current is carried through conductor 44 to the spring arm 23 and thence through contact 24 and conductor 45 to the terminal it onthe panel 3i. From terminal 45 conductor 4i leads to the room thermostat 33. The circuit is completed from the thermostat through contact 4!, conductor 8 to the terminal isonthe panel-3l, and thence through conductor 5% to terminal 39 of the furnace control 32,

The furnace control remains in its on or start position until the room thermostat 33 is satisfied, at whichtime, the'thermostat moves from contact li to contact 53 thereb moving the furnace control to its off or stop position. The circuit for the off movement of the furnace.;.c ontrol is the same as for the on movement from thecommon terminal 31 of the furnace control to the thermostat 33. From the thermostat however, the circuit is completed through contact and conductor 5| to terminal 52 on the panel 3|. From terminal 52, the internal wiring of the switching unit carries the current through conductor 53 to the spring arm 20, thence through contact 2|, conductor 54, conductor 55 to terminal 56 on the panel 3|. Conductor 57 interconnects terminal 56 with the foff or stop terminal 38 on the furnace control thereby completing the circuit to move the furnace control to its oiff position.

Should an abnormal fire condition arise at any time, such condition is rectified by automatic operation of the limit switch of the present invention. For example, if during the on period of operation of ;the furnace control, as called for by the room thermostat 33 the flue or bonnet temperature should become dangerously high, the present switching unit responds to such abnormal condition by energizing the off or stop circuit f the furnace control. An increase in flue or bonnet temperature causes clockwise rotation of the cam 26 thereby moving the spring arm 23 away from contact 24 and into engagement with contact 25. Such movement of the arm 23 opens the circuit to the room thermostat thereby eliminating the overall control by the room thermostat, and substitutes a circuit to the off or stop terminal 38 of the furnace control. The new circuit is the same as before described from the common terminal 31 of the furnace control to the spring arm 23.

From the arm 23 however, the circuit is completed through contact 25, conductor 55, terminal 56 on the panel 3| and conductor 51 to the off or stop terminal 38 of the furnace control. The furnace is therefore shut off because of the increase in the flue or bonnet temperature above a predetermined safe maximum.

Another abnormal condition which is cared for and controlled by the present switching unit, is a decrease in flue or bonnet temperature when the room thermostat does not call for heat during extended periods. These periods usually occur in the early fall and late spring, when because of the ambient temperature, the thermostat remains satisfied for long periods of time. During these. periods it is quite possible for the fire to become quite low, or extinguished because of the lack of periodic firing operations. The low fire is reflected by a decrease in the stack or bonnet temperature, and when such temperature falls below a predetermined minimum, the cam 26 of the present switching unit b movement in a counter-clockwise direction, movesthe spring arm 23 from contact 2| into engagement with contact 22. Such movement ofthe arm 20 sets upa new circuit to energize the furnace control so as to move it to its on or start position. Such on operation of the furnace control is entirely independent of any demand by the room thermostat 33.

Referring again to Fig. '7 in which the system is shown in its off or stop position with the room thermostat fully satisfied as shown by its engagement with contact 40, upon movement of the arm 26 into engagement with contact 22, the on circuit is completed as follows. From the common terminal 31 of the furnace control the circuit to the thermostat 33 is the same as heretofore explained. The circuit to arm 20 is through contact 40, conductor 5|, terminal 52 on the panel 3| and conductor 53, The arm 20 being in en a ement with contact 22, conductor 58, terminal 49 on the panel 3|, and conductor 56. Thus the furnace control 32 is energized so as to move to the on or start position in-- dependently of the room thermostat 33.

Should the room thermostat 33 call for heat during the on operation of the furnace control under control of the arm 20, the thermostat moves from contact 40 to contact 4|, thereby establishing the normal on circuit which is completely independent of the on circuit controlled by the arm 2|]. Upon moving ofthethermostat from the contact 40 into engagement with the contact 4|, the special control circuit through the arm 20 is broken thereb relieving the switch 20-22 of all control functions, even though the arm 20 remains engaged with the on contact 22.

The contact surface of the cam or actuator 26 is concentric with its axis of rotation whereby it makes only point engagement with one or the other of the straight fiat circuit closing arms. The construction is such that after having initially engaged and moved the circuit closing arm, the actuator may continue torotate to greater or less degree, without varying the adjustment of the circuit closer arm.

Referring to Fig. 3, it will be noted that coritact 24 is in the form of a rivet passing through a ring, shaped permanent magnet 24' and through a hole in the L-shaped bracket 24 which bracket is mounted on the segment I 5 by means of a rivet 24a and preferably also" passing through a terminal clip 24b to which is connected the conductor 45. The contact blade 23 has riveted to it a washer 240 by means of a contact rivet 24d. The washer is made of soft iron or steel and acts as an armature in relation to the permanent magnet 24'.

Contact 25 is preferably in the form of a rivet carried on a bracket 25 which may be identical with the bracket 24" and is held in place b a rivet 25a to which is connected, underneath segment IS, the conductor 55. It will be noted that the brackets 24" and 25 are reversed in position. This is desirable so that the housing 9 may be made smaller in diameter as it is essential that the' rivet 25a be located within the housing 9.

The snap-action operation of the high-limit control switch is more clearly shown in Figs. 5 and 6. Referring to Fig. 5, the dotted lines indicate the position and form of contact arm 23 when not acted upon by cam 26. In full lines is shown the position and form of arm 23 when acted upon by cam 26 but prior to the movement of arm 23 to break contact 24. It will be noted that the attractive force of magnet 24 on its armature 240 is sufficient to cause considerable distortion or flexing of arm 23, considerable energy being stored therein. As soon as the distortion of arm 23 becomes great enough to overcome the power of magnet 24 contact 24 1s opened and the blade 23 moves rapidly to close contact 25. This is done by utilizing the energy stored in spring 23. By forming spring 23 with S shaped convolutions such as shown, the flexibility is substantially increased without increasing the distance from the point of support of spring 23 on bracket Hi to the contacts. Therefor it is possible to get the desired action in an instru ment of much smaller dimensions than if a straight arm 23 were used as in the Muth application referred to, It will also be noted that during the first action of cam 26 on arm 23 the gap of contact/ 25 is increased. This is most desirable in that the subsequent closing movement of contact 25 is therefore much more rapid. This action is made possible by locating the contacts 24-25 at different distances from the supported end of arm 23. Since during the initial stressing of arm 23 the contact 24 acts as a point of oscillation, the elements of contact 25 are separated, thus-offsetting any danger of having both contacts closed during that time.

The'operation of the low limit switch comprising arm 2i] and contact 2 l22 is the same as that just described with reference to the high limit switch except that automatic motion occurs as a result of contour clockwis movement of cam 26.

It will be noted that the constructions shown and described will serve admirably to accomplish the objects stated above. It is to be understood, however, that .the constructions disclosed above are intended merely as illustrative of the invention and not as limiting as various modifications therein may be made without departing from the invention as defined by a proper interpretation of the claims which follow.

I claim:

1. A limit switch for use in temperature control systems comprising, in combination, a thermo-responsive actuator, a mounting member, a supporting bracket on said mounting member, a flexible contact blade carried by said supporting bracket in a position where it may be engaged by said actuator, and at least one contact carried by said mounting member in a position where it is engageable by said contact blade, a magnet mounted adjacent said contact, and magnetic material associated with said contact arm so that the contact arm is attracted toward said contact, the contact arm being formed with convolutions between the point where it may be engaged by the actuator and the point where it is engageable with the contact, whereby the resiliency of the contact blade is increased so that energy may be stored therein by the actuator while the magnet is holding the contact blade in engagement with the contact, until the convoluted contact blade is strained sufficiently to overcome the pull of the magnet whereupon the contacts will be separated quickly by the energy stored in said convoluted contact blade.

2. A limit switch for use in temperature control systems comprising, in combination, a thermo-responsive actuator, a mounting member, a supporting bracket on said mounting member, a flexible con-tact blade. carried by saidsupporting bracket in a position where it may be engaged by said actuator, and at least one contact carried-by said mounting member. in aposition where it is engageable by said contact blade, a magnet mounted adjacent said contact, magnetic material associated with said contact arm so that the contact arm is attracted toward said contact, and a second contact on the other side of said contact arm and located further from said supporting bracket than said first contact, the contactarm being formed with convolutions between the point where it may be engaged by the actuator and the point where it is engageable with the first contact, whereby the resiliency of the contact blade is increasedso that energy may be stored therein by the actuator while the magnet is holding the contact blade in engagement with the contact, until the. convoluted contact blade is strained sufficiently to overcome the pull of the magnet whereupon the contacts will be separated quickly by the energy stored in said convoluted contact blade.

3. A limit switch for use in temperature control systems comprising, in combination, a thermo-responsive actuator, a mounting member, a supporting bracket on said mounting member, a flexible contact blade carried by said supporting bracket in a position where it may be engaged by said actuator, and at least one contact carried by said mounting member in a position where it is engagea-ble by said contact blade, a magnet mounted adjacent said contact, and magnetic material associated with said contact arm so that the contact arm is attracted toward said contact, the contact arm being formed as an S- shaped convolution between the point where it may be engaged by the actuator and the point Where it is engageable with the contact, whereby the resiliency of the contact blade is increased so that energy may be stored therein by the actuator while the magnet is holding the contact blade in engagement with the contact, until the convoluted contact blade is strained sufiiciently to overcome the pull of the magnet whereupon the contacts will be separated quickly by the energ stored in said convoluted contact blade.

4. The combination according to claim 1 wherein the contact blade is formed of a first section extending from the supporting bracket to a point beyond the point where it is engaged by the actuator, it being then bent back upon itself to form a second section extending to the vicinity of the supporting bracket and is then again bent back upon itself to form a third section extending beyond the first bend to and beyond the contact.

5. The combination according to claim 1 wherein the contact blade is formed of a first section extending from the supporting bracket to a point beyond the point where it is engaged by the actuator, it being then bent back upon itself to form a second section extending to the vicinity of the supporting bracket and is then again bent back upon itself to form a third section extending in a direction generally parallel to the first section and extending beyond the first bend to and beyond the contact.

6. The combination according to claim 2 wherein the contact blade is formed of a first section extending from the supporting bracket to a point beyond the point where it is engaged by the actuator, it being then bent back upon itself to form a second section extending to the vicinity of the supporting bracket and is then again bent back upon itself to form a third section extending beyond the first bend to and beyond both contacts.

ALLEN A. DICKE.

REFERENCES CITED The following references are of record in the

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