US2255666A - Pressure switch - Google Patents

Description

sept- 99 9461- A. l.. JUb'soN 2,255,666

PRESSURESWIT'CH Filed July 2, 1938 2 Sheets-Sl'xeerI l Elm l lol 9 Eff/k@ el EL . l 2, 2

\\ 22' n ze 66 i 2' Albemtlmarudson Gttorneg Sept 9, 1941 A. L. JUDsoN 2,255,666

PRESSURE SWITCH zFiled July .2, 195s (Ittorneg Patented Sept. 9, 1941 PRESSURE SWITCH Albert L. Judson, Portland, .0reg., assgnor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application July 2, 1938, Serial No. 217,244

(Cl. 20G-83) 11 Claims.

This invention relates generally to the art of automatic switching and is more particularly concerned with a safety feature to be applied to a pressure or temperature operated switch.

It is now common practice to operate a mercury or open contact type of switch by means of a bellows, which may be connected with either a source of pressure or a bulb containing a thermostatic fluid, and to oppose operation of the bellows by means ofv a main adjusting spring. A second spring is provided with these instruments for the purpose of producing a diiferential of operation, which is accomplished by iurnishing a stop for said spring so that movement of said bellows is opposed during only part of its travel. The bellows will therefore operate the switch in one direction after it has picked up the diiierential spring against the action of both springs, and in the other direction under the influence of only the main adjusting spring after the differential spring has engaged its-stop.

As used in the field of refrigeration control the main bellows just described is connected with the suction line leading from the evaporator to the compressor to maintain avsubstantially constant low pressure in the evaporator. In this case the bellows will operate to open the switch on a predetermined low pressure or temperature to stop the compressor motor and close the switch on a predetermined higher pressure or temperature to start it again.

'I'hese instruments are frequently provided with a high pressure cut-out in the form of a bellows adapted to open the switch through a strain release connection upon the occurrence of a predetermined high pressure in the high pressure line leading from the compressor to the condenser. This is a safety feature to prevent the'pressure from attaining a dangerously high value.

Itis an object of thevpresent invention to provide an instrument of the above type with a means responsive to an overload condition of the compressor motor for opening the circuit thereto independently of the two pressure controls. A

A further object is to provide an overload device which is' responsive to the heat produced by an excessive amount of current iiowing in a load circuit and which opens the switch in series with theA load, requiring manual operation to reset the mechanism and close the switch.

A further object is to provide an overload cut-out for an instrument of the described type and also a manualmeans for positively holding the switch in an open position.

A still further object is to provide an overload cut-out for opening a switch which has a manual lever which is operated in one direction to reset the device while holding the switch in open position, and in the other direction to positively hold the switch in open position when it is desired to shut down the system for any reason.

These and other objects will become apparent as the following specification is read in the light of the accompanying drawings in which:

Figure 1 is a front view of an instrument embodying my invention shown partly in section taken on the line I-I of Figure 3 looking in the direction of the arrows;

Figure 2 is a section taken on the line 2-2 of Figure 1 looking in the direction of the arrows;

Figure 3 is a section taken on the line 3-3 of Figure 1 looking in the direction of the arrows;

Figure 4 is a front elevation of the overload cut-out mechanism showing the heating element;

Figure 5 is a section taken on the line 5-5 `of Figure 4 looking in the direction of the arrows;

Figure 6 is a side elevation of the overload cut-out showing the manual reset mechanism and looking toward the right as seen in Figure 4, and

Figure 7 is a rear view of the overhead cut-out mechanism.

The mechanism which comprises this invention is mounted in a casing II and includes an angle arm I2 which is fastened to a depressed portion I3 at the back of casing II by means of screws Irl.` The angle arm I2 has a forwardly extending portion I5 'which is provided on the lower edge with a pair of knife edges I6. These.

knife edges are adapted to seat in a pair of depressions II in a main operating lever I8 which vis provided at one end with a bearing member I9 adapted to cooperate with an operating pin 20. The pin 20 is carried by a bellows (not shown) which is mounted within a casing 2I connected outside of the casing I I and at the bottom thereof by means of screws 22. A connection 23 is provided at the bottom of casing 2| for the purpose of connecting it to a source of pressure or to a thermally responsive bulb by means of capillary tubing (not shown); The lever I8 has a hole 24 near the center thereof for accommodating a carriage 25 which is provided with a pair of knife edges 2S adapted to seat in dwells 2'I in 38, the other end of which is rigidly connected to a nut 3| having an extension arm 32 the extreme end of which is inserted through a slot in the side of the casing II. Depending loosely through a hole in the top of the casing II and threaded through the nut 3| is an adjusting screw 33 which upon rotation causes vertical movement of the nut 3| due to the fact that it is held against rotation by means or the extension arm 32. The -movement of nut 3| varies the tension of the spring 30 to provide an adjustment of the pressure at which the bellows will move thev lever I8. Sutably connected to the side of the casing II and near the bottom thereof is a bracket 34 provided with two stops 35 and 36 between which the end 31 of the lever I8 is adapted to move. The purpose of these stopsis to limit the `travel of lever I8 aboutl the knife edges I6 under the inuence of the bellows and spring 30. Suitably connected to the lever I 8 as by means of rivets 29 is an upstanding arm 38. Pivotally mounted on arm 38 as shown at 39 is a lever 48 which is provided with an arcuate slot 4I which cooperates with a headed pin 42 connected to the arm 38. A spring 43 is connected at its lower end to thel end 31 of lever I8 and at its upper end with a projection 44 on the lever 48 and operates to normally maintain the lever 48 in the relation shown in Figure 1 with respect to the arm 38 so that the pin 42 is engaged with the extreme right end of the slot 4I. Mounted on the rear of the casing I I immediately above the lever 48 is a pin 45 on which is mounted a U-shaped switch carrier 46 having two legs 41 and 48. The leg 48 carries a switch clip 49 in which is mounted a mercury switch 58. 'Ihe leg 41 is 'bifurcated at its lower end and the two pieces thus formed straddl-e operating bar which is mounted on the lever 48 just above the slot 4I. Rotation of the lever 48 will therefore cause rotation of the switch carrier 46 about the pin 45 and will actuate the switch 58 to open and closed positions. Pivotally mounted on one side of the casing II adjacent the bracket 34 as shown at 52 is a lever 53 having an arm 54. A second lever 55 is adjustably pivoted to the lever 54 as shown at 56 by means of a pin and slot connection indicated at 51. The tension spring'58 is connected at its lower end to the hole 59 in lever 55 and at its upper end is rigidly attached to nut 60 which has an extension arm 6Iv the extreme outer end of which passes through a slot 62 in the side of casing II. 4A screw 63 projects loosely through a hole in the top of casing II and is threaded through the nut 68 so that rotation of the screw 63 will cause vertical movement of the nut 68, due to the fact that the extension arm 6I will prevent its rotation, and thereby adjust the tension of the spring 58. The arm 54 on lever 53 is provided with a projection 65 which is adapted to engage the stop 36 at which time an upturned portion 6'6 on the arm 54 will be located just beneath the end 31 of lever I 8 and about midway of its travel between the stops 35 and 36.

llt will be seen from the `structure described above that the main adjusting spring 38 constantly opposes upward movement of the pin 28 under the iniiuence of the bellows and tends to hold the bearing I9 against the pin 29. Adjustment of the tension of this spring by the screw 33 will therefore adjust the pressure at the lever I8. Looped through a hole 28 in the carriage 25 is one end of a main adjusting spring which the bellows will start to rotate the lev-er I8 in a counter-clockwise direction. Assuming that the lever is engaging the stop 35 an increase in pressure within the casing 2| will cause upward movement of pin 28 which will cause the lever I8 to rotate in a counter-clockwise direction aboutthe knife edges I6 and the end 31 of the lever I 8 will leave the stop 35 at a pres- .vsure which is determined by the tension on the spring 38. `As it.continues its rotation it will eventually strike the upturned endA 66 of the arm 54 and any continued movement of the 1ever I8 will then move the lower end of the spring 58. This means that the pressure at which the end 31 will strike the stop 36 is determined also by the tension of the spring 58 in addition to that ofv spring 38. Moving in the other direction tension ofboth springs will determine the pressure at which the end4 31 leaves the stop 36 and after the projection 65 on the arm 54 strikes the stop 36 the further movement of the end 31 until it engages the stop 35 will be determined only by the tension of the spring 38.

It will be noted that there is a slight amount of lost motion between the pin 5I and the legs formed by the bifurcated end 41 of the switch carrier 46. The mercury switch 58 is so positioned on the carrier 46 that the switch will be closed after the end 31 of the lever I8 has engaged the arm 66 and picked up the spring 58 and upon movement of the lever in the opposite direction the switch will not open until after the end 31 of lever I8 has separated from the arm 66 with the result that the pressure at which the switch will be closed will be determined by the combined force exerted by the spring 58 and 38 and the pressure at which it will be opened will 'be determined only by force exerted by the spring 38. In other words, the spring 38 provides for the main adjustment of the switch and the spring 58 provides for an adjustment of the di'erential of operation of the switch. The ends of the extension arms 32 and 6| as stated before extend through slots in the side of the casing II and the position of these ends is an indication of the force exerted by their respective springs due to the fact that they indicate the position of the nuts carrying the upper end of theseA springs. .In order to take advantage of this fact a scale plate 68, a glass protector 69 and a frame 18 have been provided about these slots for the purpose of indicating the positions of the arms 32 and 6I so that the main adjustment of the switch and the adjustable dii-erential of the switch may be easily read.

An arm 1| is loosely suspended from a pivot 12'near the upper end of the casing and is provided with a pointer 13 at the bottom thereol' which cooperates with a mark 14 on the back of the casing II for the purpose of indicating when the instrument is in a proper vertical position so that the mercury switch used will function correctly. i

On the side of casing II and opposite the adjusting spring is attached a second bellows casing 15 ln which is located a bellows (not shown) which carries an operating pin 16. The casing 15 is provided with a connection 11 which may cooperate with a capillary tube to connect the bellows with either a source of pressure or a thermal bulb as may be desired. Pivotally mounted on pin 18 within the casing is a lever 19 having a at portion 88 adapted to cooperate with the pin 16. Lever 48 has at its upper end an extension 8| which normally lies adjacent to an arm 82 on the lever 19. In the event of a high temperature or pressure, whichever the bellows within the casing 15 is adapted to respond to, the pin 16 will be forced to the left as viewed in Figure 1 which will cause a clockwise rotation of the lever 19 and a consequent clockwise rotation of the lever 40 about it pivot 39. The lever I3 and arm 38 will remain stationary under this action and the spring 43 will be elongated and the pin 42 will slide in the slot 4| so that the pin 5| will tilt the switch carrier 46 to open the switch 50 independently of the lever |8 and the bellows within the casing 2|. The purpose of this is to provide a safety device for breaking the circuit independently of the main control. After the condition to which the b'ellows 15 responds has returned to a safe value the pin 16 will move tothe right and the spring 43 will cause the lever 40 to assume its original position with respect to the arm 38 and the control of the switch 50 will be placed again on the bellows within the casing 2 One of the main objects of this invention is to provide an instrument oi this character with an overload cut-out for independently opening the' switch 50. Such a cut-out is supported by means of a post 85 which is connected through the angle arm I2 to the back of casing by means of screw 86. The post 85 carries at its opposite end by a suitable connection a plate 81 which is additionally connected to the casing by means of bracket 88 and screw 89 which passes through the hole 90 in the plate 81. The

plate 81 is provided with a pair of ears 9| one n on each side* of the plate as shown in Figure 7 and between which is mounted a spindle 92. Freely rotatably mounted on the spindle 92 by means of a pair of arms 93a, shown in Figure '7, is a lever 93 which adjustably carries a headed screw 94 which is heldin its adjusted position by means of a locknut 95. Also rotatably mounted on the spindle 92 is a second lever 96 which is normally biased in a counterclockwise direction as viewed in Figure 5 by means of the spring 91 one end of which engages the plate 81 and after being wrapped around the spindle 92 the other end engages an upstanding arm 98 on the lever 96. An arm 99 of the lever 96 (see Figure 3) has adjustably mounted thereon by means of a pivot and a slot and eccentric screw connection 0|, a second arm |02 having extension |03 which lies immediately over an arm |04 which extends from the lever 40. Mounted on the plate 81 by means of screws |05 is a block of insulation |06 through the center of which is a hole |01. Mounted in a cut-away portion about the hole |01 and adjacent the plate 81 is a bimetallic snap acting disc |08 which is retained in position by means o f an insulating disc |09 which is held in engagement with the plate 81 by means of the block of insulation |06.y 'I'heplate 81 is provided with a pair of holes WL| I0 ini which are inserted a pair of projections on the insulating disc |09 for the purpose of properly centering it with relation to the plate 81. A screw ||2 is threaded through the center of the thermostatic disc and project-s through a hole ||3 and the insulating disc |09 and through hole 4 in the plate 81 and carries at its outer end a bearing member which is adapted to engage the headed screw 94 carried by the lever 93. It

lwill be noted that bearing member ||5 has been omitted from Figure 5 in order to show up other parts of the mechanism more clearly. A pair of Cil metallic conducting plates and |2| which are attached to the insulating block. |06 by means of screws |22 and |23 support an annular insulating member |24 on which is mounted an electric heater |25 one end of which is electrically connected to the plate |2| the other end being electrically connected to the plate |20. Conductor |26 connects the plate |20 with one contact of the mercury switch and conductor |21 connects the other contact of the mercury switch with screw |29. 'I'he conducting plate |50 electrically connects screw |29 to-terminal |5| and conducting plate |52 electrically connects screw |23 to terminal |53. The load circuit will be connected in series with the terminals |5| and |53 so that the current will pass from the terminal |53 through plates |52 and |2|, heater |25, plate |20, screw |22, conductor |26 through the mercury switch 50; conductor |21, screw |29', and plate |50 to the other terminal |5I. 'Ihe plate 81 is provided with a projection |30 on which is pivotally mounted an arm 3|. A handle |32 is provided on one end of the arm so that it may be manually operated. The other end of the arm is provided with an outside cam surface |33 comprising a high point |34, a low point |35, a second high point |36, and a shoulder |31. The arm 98 on the lever 96 is adapted to cooperate with the cam surface and the spring 91 acts to maintain the arm 98 in constant engagement with the surface |33. The arm |3| is provided with a slot which has an inside cam surface 4| having a low point |42 and a high point |43. An extension |44 on the lever 93 extends within this slot and is adapted to cooperate with this internal cam surface |4|. The arm 98 has a projection |45 which is adapted, at certain times to be disclosed later, to cooperate with the arm |44. A spring |46 is wrapped around the pin |41 on which the arm |3| is pivoted and the two ends engage opposite sides of a projection |48 on the arm |3| and also on opposite sides of an extension |49 on the spindle 92. The function of this spring is to bias the manually operable arm |3| to a mid or neutral position.

In considering `the operation of this overload cut-out we will assume the parts to be in the position shown in Figure 6 at which time the thermostatic disc will be in its cold position which is shown in Figure 5. The lever 96 will be in the position where the extension |02 will be raised so that the lever 40 will be in the relationship shown in Figure 1 with respect to the arm 38 on the lever I8 at which time the mercury switch will be under the control of the bellows within the casing 2|. Assuming now that there is an overload in the circuitv to which the heating element 25 is connected the said heating element will then become overheated causing the thermostat 08 to snap abruptly to the right from the position shown in Figure 5. This will cause the bearing member ||5 to engage the headed screw 94 rotating the lever 93 in a clockwise direction as shown in Figure 5 at which time the projection |44 on the lever 93 will engage the projection |45 on the lever 96 rotating that in a clockwise direction as viewed in Figure 5 and depressing the extension |03 shown in Figure 3 which will cause lever 40 to rotate in a clockwise and in this case it will also be independent of the condition of the bellows within the `casing 15. The construction of the thermostaticdisc |08 is such that cooling of the heating element |25 will not cause it to snap back to its original position, hence the parts will remain in the position just described wherein the switch 50 is in open position until the device is reset manually. In order to reset the device the handle |32 is raised which causes the arm |3| to lower at which time the high point |43vof the inside cam surface |4| will engage the extension |44 on the lever 93 causing it to rotate in a counter-clockwise direction' as viewed in Figure 5 at which time the headed screw 94 will press against the bearing member on the screw ||4 thus snapping the thermostat |08 back to its original position. At the same time it will be noted that f the low part |35 of the outside cam surface due to the action of spring 91 and the switch 50 will be returned to closed position. The reason for maintaining the switch 50 in open position while the device is being manually reset is so that it will be impossible to manually hold the switch closed while an overload may exist in the circuit. In other words it will be impossible for the switch 50 to be in closed position while an overload Y exists'except for the time necessary to heat the thermostat |08.

If it is desired to place the switch in an off position and leave it there for an indefinite period of time the handle |32 'is depressed from the position shown in Figure 6 until the extension |38 on the lever 96 engages the high part |36 of the outside cam and engages the shoulder |31. This rotates the lever 96 in a clockwise 'direction as viewed in Figure 5 causing opening of the switch due tothe coaction of the extension |03 with arm |04 as'described above and upon release of the handle |32 the shoulder |31 will prevent the spring |46 from returning the arm |3| to the -neutral position. In this manner the switch may be kept in open position as long as desired and when it is again desired to place the system in operation the handle 32 may be manually raised which will cause the extension |38 to again engage the low portion |35 of the outside Vcam surface which permits closing of the switch 50 and places the control of this switch back on the bellows within the casing 2|.

It will therefore be seen that I'have added to the low pressure and high pressure operated switch which is now in common use, an overload cut-out which is` simple in operation, durable in construction, and which may be easily positioned in the instrument casing. It will be noted that if this overload cut-outv feature is notdesired it is limiting. For the limits of my invention reference should be had to the appended claims.

I claim as my invention:

1.' A device of the character described comprising in combination, an operating member, a iirst condition responsive means for moving said operating member back and forth, an actuating member normally mova'lie with said operating member, a switch mounted for actuation by said actuating member, a support, a lever mounted on said support, a one way acting-bimetallic thermostat mounted on said support'for rotating said lever, connections between said lever and said actuating member whereby said thermostat is capable of moving said actuating member independently of said operating member, manual means for-actuating said thermostat in one direction, and connections between said manual means and said lever whereby said manual means is capable of moving said lever and hence said actuating member independently of said condition responsive means and said thermostat.

2. In a device of the class described, in combination, an operating member, a bellows for moving said member back and forth, an actuating member pivotally mounted on said operating member and normally biased to move therewith,

necessary only to remove the screws 66 and 89 and Y disconnect the conductors |26 and |21 at which time the overload mechanism along with the positive manual shut-oli may belifted out of the casing as a unit.

Modicationsof this invention may occur to those who are skilled in the art and' it is therefore to be understood that the 'embodiment disclosed is intended to'be illustrative rather than 75 a pivotally mounted mercury switch, means on said actuating member for rocking said mercury switch about its pivot, a support, a one way acting bimetallic thermostat mounted on said support, an electric heating element mounted adjacent said thermostat, a lever pivotally mounted on said support, connections between said lever and actuating member whereby the thermostat upon overheating by said electric heating element will actuate said lever to move the actuating member relatively to said operating member to open said switch, and manual means for actuating said thermostat in one direction.

3. In a device of the class described, in combination, an operating member, a bellows for moving said member back and forth, an actuating member pivotally mounted on said operating member and normally biased to move therewith, a pivotally mounted mercury switch, means on said actuating member for rocking said mercury l thermostat mounted adjacent said heating element and adapted to snap over center upon being overheated by said heating element but being incapable of returning to its original position by itself, a lever pivoted on said support, and overlying a portion of saidactuating member, and connections between said 4thermostat and said lever whereby said thermostat in snapping due to overheating causes said lever to move said actuating means to open said switch independently of said operating member.

4. In a device of the class described, in combination, an operating member, a bellows for moving said member back and forth, an actuating member pivotally mounted on said operating member and normally biased to move therewith, a pivotally mounted mercury switch, means on said actuating memberfor rocking said mercury switch about its pivot, a support, an electric heating element mounted on said support, a bimetallic thermostat mounted adjacent said A'heating element and adapted to snap over center-upon being overheated by said heating element butfbeing incapable of returning to itsoriginal position', by itself,a lever pivoted on said support, and overlying between said thermostat and said lever whereby said thermostat in snapping due to overheating causes said lever to move said actuating means to open said switch independently of said operating member, a manually operated arm having a neutral position, means on said arm for snapping said thermostat to its original position from its overheated position as said arm is moved in one direction from neutral position, and means on said arm for maintaining said switch in open position while said thermostat is being snapped.

5. In a devce of the class described, in combination, an operating member, a bellows for moving said member back and forth, an actuating member pivotally mounted on said operating member and normally biased to move therewith, a pivotally mounted mercury switch, means on saidactuating member for rocking said mercury switch aboutits pivot, a support, an electric heating element mounted on said support, a bimetallic thermostat mounted adjacent said heating element and adapted to snap over center upon being overheated by said heating element but being incapable of returning to its original position by itself, a lever pivoted on said support, and overlying a portion of said actuatingl member, connections between said thermostat and said lever whereby said thermostat in snapping due to overheating causes said lever to move said actuating means to open said switch independently of said operating member, a manually operated arm having a neutral position, means on said arm for snapping said thermostat to its original position from its overheated position as said arm is moved in one direction from neutral position, means on said arm for maintaining said switch in open position while said thermostat is being snapped, and means on said arm for moving said lever to open said switch independently of said bellows and thermostat as said arm is moved in the other direction from neutral position.

6. A device of the character described comprising in combination, a support, a rst lever pivoted on said support, a second lever pivoted on said support, means biasing said second lever into engagement with said first leverwhereby they normally move in unison, a switch mounted for actuation by said second lever, a heating element, a snap action thermostat engaging said rst lever and snapping'from a iirst position to a second position in response to an excess of heat from said heating element for actuating said second lever to open said switch, and manual means for engaging said second lever to open said switch independently of said heat responsive means.

7. A device Vof the character describedl comprising in combination, a support, a rst lever pivoted on said support, a second lever pivoted on said support, means biasing said second lever into engagement with said first lever whereby they normally move in unison, a. switch mounted for actuation by said second lever, a-heating element, thermal means engaging said iirst lever and snapping from a rst position to a second position in response to an excess of heat from said heating element for actuating said second lever to open said switch, said means being incapable of returning to said nrst position by itself, and manual means for exerting a force on said thermal means to return it to said first position while holding said second lever relatively stationary and said switch open.

8. A device of the character described comprising in combination, a support, a i'lrst lever pivoted on said support, a second lever `pivoted on said support, means biasing said second lever into engagement with said rst lever whereby7 they normally move in unison, a switch mounted for actuating by said second lever, a. heating element, means engaging said rst lever and snapping from a first position to a second position in response to an excess of heat from said heating element for causing said second lever lto open said switch, said means being incapable of returning to said rst position by itself, a manually operable arm having a neutral position, connections whereby movement of said arm in one direction from its neutral position will return said hea-t responsive means to its rst position while holding said second lever relatively stationary and said switch open, and connections whereby movement of said arm in the other direction from its neutral position will move said second lever to open said switch independently of said heat responsive means.

9. A device of the character described cornprising in combination, a support, a, nrst lever pivoted on said support, a second lever pivoted on said support, means biasing said second lever into engagement with said first lever whereby they normally move in unison, a switch mounted for actuation by said second lever, a heating element, a snap acting bimetallic thermostat mounted adjacent said heating element and engaging said first lever, said thermostat snapping from a rst position to a second position to cause said second lever to open said switch in response to an excess of heat from said heating element, a projection on said second lever, a manually operable arm, and a cam connected to said arm,

movement of said arm in one direction causing said cam to rotate said second lever independently of said rst lever and open said switch.

l0. A device of the character described cornprising in combination, a support, a first lever pivoted on said support, a second lever pivoted on said support, means biasing said second lever into engagement with said first lever whereby they normally move in unison, a switch mounted for actuation by said second lever, a heating element, a snap acting birnetallic thermostat mounted adjacent said heating element and engaging said iirst lever, said thermostat snapping from a first position to a second position to cause said second lever to open said switch in response to an excess of heat from said heating element, said thermostat being incapable of snapping back to its iirst position under its own power on cooling, a projection on said rst lever, a projection on said second lever, a manually operable arm pivoted for rotation, and a cam on said arm having iirst and second cam surfaces, rotation of said arm causing said rst cam surface to engage the projection on said first lever and move it in a direction to return said thermostat to its flrst position, said second cam surface engaging the projection on said second lever to hold it relatively stationary while said rst lever is being moved, thereby maintaining said switch in open position` during the operation.

11. A device of the character described comprising in combination, a support, a first lever pivoted on said support, a second lever pivoted on said support, means biasing said second lever into engagement with said rst lever whereby they normally move in unison, a switch mounted for actuation by said second lever, a heating element, a snap acting bimetallic thermostat mounted adjacent said heating element and engaging said rst lever, said thermostat snapping from a lirst position to a second position to 4cause said second lever to open said switch in response to an excess of heat from said heating element, said thermostat being incapable of snapping back to its rst position under its own power on cooling, a projection on said rst lever, a projection on said second lever, a manually operable arm pivoted for rotation, means to bias said arm to a. neutral position, a cam on said arm having rst and second cam surfaces, rotation of said arm in one direction from neutral causing said rst cam surface to engage the projection on said first lever and move it in a direction to return said thermostat to its first position, said second cam surface engaging the projection on said second lever to hold it relatively stationary While said first lever is being moved, thereby maintaining said switch in open position during the operation, said biasing means returning said arm to neutral position upon release thereof, a third cam surface on said cam, rotation of said arm in the other direction from neutral causing said third c am surface .to engage the projection on said second lever and move it to open said switch independently of said first lever and thermostat,

and means preventing said biasing means from returning said arm to neutral position upon release thereof.

' ALBERT L. J UDSON.

CERTIFICATE OE CORRECTION. Patent No. 2,255,666. september 9, 1911;.

ALBERT L. JUDsON.

It is hereby certified that error appears in the printed specification ofthe above numbered patent requiring correotionas follows: Page l, second column, line 5l, for the word "overhead"v read overloadg page 5, second column, line LL, claim 8, for "actuating" read actuation; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 11th day of November, A. D. 1911.1.

Henry Van Arsdale, (Seal) I Acting Commissioner of Patents.

CERTIFICATE OF CORRECTON. Patent No. 2, 255, 666. September 9, 19LL1.

ALBERT L. JUDSON.

It is hereby certified that error appears in the printed specificati on of the above numbered patent requiring correction as follows: Page l, seci ond column, line 5l, for the Word "overhead" read overload--g page 5, second column, line claim 8, for actuating" read --actuation; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 11th day of November, A. D. 19M..

Henry Ven Arsdale,

(Seal) I Acting, Commis sioner of Patents.

Images