US3423712A

US3423712A - Thermal protective device having rapid response to sudden high overloads and delayed response to moderate overloads

Info

Publication number: US3423712A
Application number: US535217A
Authority: US
Inventors: John F Howard
Original assignee: General Electric Canada Co
Current assignee: General Electric Canada Co
Priority date: 1965-05-15
Filing date: 1966-03-17
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21
Also published as: DE1538398A1; DE1538398B2; DE1538398C3; GB1083650A

Description

Jan. 21, 1969 J. F. HOWARD 3,423,712

THERMAL PROTECTIVE DEVICE HAVING RAPID RESPONSE TO SUDDEN HIGH OVERLOADS AND DELAYED RESPONSE TO MODERATE OVERLOADS Filed larch 1'7, 1966 Sheet or 2 INVENTOR. JOHN F. HOWARD J- F- HOWARD THERMAL PROTECTIVE DEVICE HAVING RAPID RESPONS SUDDEN HIGH OVERLOADS AND DELAYED RESPONSE TO MODERATE OVERLOADS med March 17. 1966 Sheet INVENTOR. z/om/ [#awmw rrow/[Y Fl G. 3

United States Patent Office 3,423,712 Patented Jan. 21, 1969 7 Claims ABSTRACT OF THE DISCLOSURE An orverload relay for actuating an electrical switch mechanism which has a rapid response to sudden high overloads and a delayed response to moderate overloads. An electric heater energized by the current to which the relay is to be responsive is positioned on one side of a bimetallic strip which is supported at one end by a bimetallic support which extends on the other side of the bimetallic strip. Deflection of the bimetallic strip caused by heat received from the heater causes displacement of its second end and of a movable support connected thereto. Displacement of the movable support actuates the relay contacts. The bimetallic support which is more remote from the heater receives heat therefrom at a lower rate than the bimetallic strip, thus delaying its deflection. Deflection of the bimetallic support is eifective to increase the deflection of the bimetallic strip necessary to actuate the relay contacts, such that on moderate overloads actuation of the relay contacts is delayed.

This invention relates to overcurrent protective devices for electrical apparatus such as motors, and in particular to a bimetallic type of device which has rapid response to sudden high overloads and delayed response to moderate overloads.

Electromagnetic contactors for electric motors are usually equipped with overload actuated devices designed to protect the motor from damage due to excessive currents caused by overloads, defects, or stalling. These devices are generally referred to as overload relays. They usually employ a bimetallic element which deflects when heated by the motor current and trips a switching mechanism when the current becomes excessive. The bimetallic element is heated by all or a definite proportion of the current to the motor either directly by passing the current through the element, indirectly by passing the current through a heater located near the element, or by a combination of the two means. United States Patent No. 3,038,051, issued June 5, 1962, J. F. Howard, and assigned to the same assignee as this invention describes an overload relay employing a bimetallic element to actuate a switching mechanism in response to overcurrent conditions.

As is well known a bimetallic element must accumulate a certain amount of heat for it to deflect to the point where the switching mechanism is allowed to trip; consequently, time delays occur depending on factors such as the mass of the element, the mountings for the element, and the heat losses. Ideally, fast tripping is wanted for sudden overloads great enough to harm the motor, but much slower tripping is wanted for the lesser overloads which the motor is capable of withstanding for a limited time. The relays should remove the motor from the line very quickly if its rotor stalls or fails to start, and they should also allow the motor to carry a moderate overload for a short time and yet remove the motor from the line, if the overload persists. Relays fast enough to meet the first requirement tend to be too fast for the second and vice versa. As a result, the tendency in relay design has been to compromise between the two.

The trend in motor design is now to reduce the margin of overloading that the motor can safely stand. As a result, the time permitted for a stalled rotor has been reduced. However, during normal operation, the new motor is still capable of safely handling moderate overloads for short periods of time. This demands more stringent relay performance if the motor is to be adequately protected. The relays must, therefore, act very fast in removing the motor from the line if it stalls or fails to start, and yet they must not operate in response to moderate overloads of short duration. They must, however, remove the motor from the line if these moderate overloads last for any length of time in order to avoid overheating and possibly burn-out of the motor.

Therefore, an object of this invention is to provide a novel and improved relay capable of adequately protecting motors of the new rating.

A further object of this invention is to provide a thermally responsive device having a rapid response to sudden high overloads and a delayed response to moderate overloads.

In carrying out the invention in one form a thermally responsive device has a bimetallic strip loosely retained by its ends between a pair of spaced support members, one of which is a bimetallic support not movable by the deflection of the bimetallic strip and the other is readily movable by the deflection. A stop positioned on one side of the bimetallic strip intermediate its ends cooperates with the bimetallic support member to restrain deflection of the bimetallic strip upon being heated to movement or displacement of the end thereof supported by the movable support member. The movable support member may be an actuator of a switching mechanism. The bimetallic support member is also a strip of bimetal fixed at one end and adapted at the other end to loosely receive the end of the bimetallic strip. Heating of the bimetallic strip also heats the bimetallic support at a lower rate. Consequently, the bimetallic support will deflect and change the position of the fixed end of the bimetallic strip, whereby the trip ping characteristics of the device can be altered to meet the requirements specified above for the new motor ratings.

In order that the invention may be more readily understood, a preferred embodiment will now be described with reference to the accompanying drawings, in which:

FIGURE 1 is a perspective view of an overload protective device;

FIGURE 2 is an exploded view of the parts used in the device shown in FIGURE 1;

FIGURE 3 is a view of the device in elevation showing the parts of FIGURE 2 assembled in the housing;

FIGURE 4 is a simple diagram of the bimetallic strip and its bimetallic support; and

FIGURE 5 is a diagram similar to FIGURE 4 illustrating the bimetallic strip and its bimetallic support in deflected positions.

Referring now to IFII'GURES 1, 2, and 3, there is shown an overload actuated device of the type described in the aforementioned patent. This device comprises an insulating housing 1 and a side cover 2 cooperating with the housing to retain the parts in mounted condition. The device is adapted to be mounted by means of a screw threaded into a tapped hole 3 in the bottom of the housing. A reset lever 6 may be placed in one of two positions for either manual or automatic reclosing of a switching mechanism through the positioning of spring 7 either in slot 8 for automatic reclosing or in slot 9 for manual reclosing. The overload setting can be adjusted by rotating knob 5 through a partial turn stopped by a projection 5a on the housing which serves also as a pointer pointing to markings on the knob to show the particular setting. Terminals and 11 are provided for connecting the thermal unit in an electric circuit.

Referring particularly to FIGURE 3, the cavity in housing 1 is partitioned into two

separate compartments

13 and 14 by a wall 15 moulded integral with top wall 16 and bottom wall 17 and disposed generally parallel to

end Walls

18 and 19.

Terminals

10 and 11 are trapped in the moulded housing by the engagement of projections 20 with mating slots provided in top Wall 16 and cover 2. The top of compartment 13 is closed by a terminal strip 4 which supports an electric heater 12. The heater projects from the strip into compartment 13 near wall 15 and has its heating element connected to

terminals

10 and 11 by means of screws 21 which also secure the strip to the terminals. Hence the heater completes a circuit between the terminals.

A bimetallic strip 22 is positioned in compartment 13 nearly parallel to wall 15 between heater 12 and end wall 18, spaced from both, and is loosely retained in this position by means of

tongues

23 and 24 on the ends of the strip received into

mating apertures

25 and 26 respectively as a loose sliding fit, aperture 25 being in a bimetallic support 27 and aperture 26 in a movable support 28 (FIG- URE 2). The bimetallic strip is, in effect, hinged to the support for limited pivotal movement on an axi transverse to the bimetals. The low expansion side of the bimetallic strip faces the heater, and the spacing is great enough that the heater will not interfere with the deflection of the bimetal.

Bimetallic support 27 is a bimetallic strip bent transverse to the strip near both ends to provide end portions and 31 at an angle to a mid portion 29; both ends are bent over in the same direction from the mid portion with the high expansion side of the bimetal on the outside. End portion 30 is relatively short and contains aperture 25; the other end portion 31 is considerably longer and contains a relatively large central aperture 32. The support is positioned upright in compartment 13 with its low expansion side facing the high expansion side of bimetallic strip 22. Its end portion 31 is secured against the inner surface 34a of a boss or thickened portion 34 of end wall 18 by means of a tubular rivet 33 passing through aperture 32 in this portion and an aperture 44 in the boss; the rivet has its head bearing against the inner face of portion 31 and its other end turned over in the bottom of an enlargement of hole 44 countersunk from the outside of wall 18. Surface 34a may have a shoulder 34b thereon adjacent side wall 16 against which the edge of portion 31 rests to keep the support from turning. As shown in FIGURES 2 and 3 the bimetallic support has one end secured to an intermediate section of end wall 18 such that its relatively long mid portion projects outwardly from the wall and upwardly into compartment 13. Its other end 30 is located above aperture 26 in the movable support. The very end of portion 30 may be cut away as shown in FIGURE 2 to provide an end entrance to aperture 25 to facilitate assembly of the bimetallic strip to the support. The purpose and function of the bimetallic support will be described later.

Movable support 28 is spaced from the end portion 30 of support 27 a distance slightly less than the overall length of the bimetal and consists of a rigid strip, preferably insulating material, formed with projections 35 integral with one longitudinal edge of the strip and similar projections 36 integral with the other longitudinal edge. Projections 35 are adapted to fit loosely into groove 39 (FIGURE 3) in side wall 16, a groove which runs generally parallel to bottom wall 17, and projections 36 into a similar groove in cover 2 whereby support 28 is mounted for limited sliding movement lengthwise through slot 38 in partition 15. The length of support 28 is such that the end 37 thereof projects into compartment 14 while the other end containing aperture 26 remains in compartment 13. It is thus seen that the bimetallic strip spans the upper free end of support 27 and support 28 and is loosely retained therebetween for pivotal movement only along the line of contact between shoulders 40 and fixed support 27 and shoulders 41 and movable support 28. As shown in FIGURE 3, a small coil spring 79 may be used to lightly hold the non-movable end of bimetallic strip 22 against support 27. The ends of the spring can be hooked to the bimetallic strip and the support through a small hole in each at its center line. This arrangement tends to give the movable end of the bimetallic strip a little more accuracy than it otherwise might have.

A stop 42 on end wall 18 is disposed in compartment 13

intermediate supports

27 and 28 are adapted to engage the high expension side of the bimetallic strip such that it cooperates with support 27 to restrain movement of the bimetallic strip upon being heated to deflection of the end thereof supported on movable support 28. As clearly shown in FIGURE 3 support 27 restrains the upper end of the bimetal while movable support 28 allows the lower end of the bimetal to deflect to the right as indicated by arrow 43; the deflection of the lower end o the bimetal moves support 28 to the right. Preferably, stop 42 is made adjustable, and it may be a screw engaging an internal thread in the tubular rivet 33. The screw is driven far enough into the rivet that its point 45 engages the high expansion side of the bimetal on or near the longitudinal center line thereof. After assembly of the bimetallic strip to its

supports

27 and 28, screw 42 is driven into the rivet until its point 45 is positioned to provide the desired calibration of the bimetal; after which knob 5 is permanently fixed to its outer end. A coil spring 5b placed on the screw next to the knob is compressed between the knob and the bottom of the countersink in hole 44 to stiffen turning of the screw. Stops on knob 5 cooperate with projection 5a to limit turning of the screw to a fraction of a turn. This arrangement permits some adjustment of the bimetallic strip after calibration End 37 of movable support 28 actuates a switching mechanism located in compartment 14 when the support is moved towards end wall 19 by deflection of the bimetallic strip from its normal nondeflected position. The switching mechanism consists essentially of a frame 46 adapted to cause movement of contact arm 47 between contact make and contact break positions. Preferably, frame 46 is formed from a thin strip of resilient springlike material into a pair of spaced legs 48 integral with spaced

end portions

49 and 50; the legs and end portions define a window 51. End portion 49 has an inwardly projecting tip 52, and the other end portion 50 has a pair of notches 53 extending inward from opposite edges of the frame along an axis transverse to the longitudinal axis of the frame. There is also a small projection 55 raised from the flat surface of portion 50. A terminal 56 trapped in

recesses

57 and 58 in

walls

15 and 19 respectively at the lower end of compartment 14 has a pair o spaced legs 59, one of which is positioned along side wall 16 and the other along cover 2. The spacing between legs 59 is such as to allow each leg to fit freely into a corresponding notch 53 and thereby support frame 46 in an upright position for pivotal movement on the axis of the notches. The upper end portion 49 of the frame rests in recess 60 of which sides 61 and 62 serve as spaced stops to limit pivotal movement of the frame, and a coil spring 63 retained in recess 64 by means of projection 55 biases the frame for counter-clockwise rotation against stop 61. It is to be noted with respect to FIGURE 3 that spring 63 acts on frame 46 at a point below its pivotal axis While movable support 28 has its end 37 butting against the side of the frame above this axis. Movement of support 28 in the direction indicated by arrow 43 will cause the frame to pivot clockwise toward stop 62 against the bias of spring 63. It is therefore apparent that by biasing the frame counter-clockwise towards stop 61 spring 63 also biases bimetallic strip 22 for clockwise rotation about support '27 against stop 42. Hence when the bimetal deflects upon being heated, it does so against the bias of spring 63.

' Contact arm 47 is formed with a humped back 65 standing on a rounded base 66 which also supports a second upstanding arm 67. The contact arms are supported on terminal 56 for rocking movement on rounded base 66 on an axis adjacent and substantially parallel to the axis on which frame 46 pivots; a tongue 68 struck out from rounded base 66 fits freely into an aperture 69 in terminal 56 to prevent sliding movement of the arm assembly along terminal 56. A coil spring 70 compressed between the free end 71 of contact arm 47 and tip 52 on end portion 49 of frame 46 completes an over-center snap acting mechanism involving contact arm 47, frame 46 and coil spring 70.

In one position of the contact arms, i.e. that illustrated in FIGURE 3, a contact 74 on the free end of arm 47 makes with a fixed contact 72 and in the other position of the arms a contact 75 on the free end of arm 67 makes with a fixed contact 73, fixed

contacts

72 and 73 being clipped to end wall 19 of housing 1. When

contacts

74 and 72 touch, a circuit is completed between

terminals

56 and 72 through base 66 of the movable arm resting on terminal 56, and when

contacts

75 and 73 touch, a ,circuit is completed between

terminals

56 and 73 through the same route. A short, flexible lead may have one end soldered to the contact arms and the other end to terminal 56 to improve the electrical connection between the two. No lead is shown in the drawings.

In actual use of the overload device, heater 12 is connected into a power supply conductor by means of terminals and 11 such that the current flowing in the conductor passes through the heater to cause heating thereof. Under normal load conditions the heat produced by the heater is insuflicient to operate the device, but if the current flow rises above a predetermined value, sufllcient heat will then be produced to cause bimetallic strip 22 t0 deflect. As best illustrated in FIGURE 3 deflection of the bimetallic strip in the direction indicated by arrow 43 slides support 28 to the right whereby frame 46 rotates clockwise. As frame 46 rotates clockwise spring 70 is compressed and its axis brought towards alignment with the plane of the frame, i.e. the spring enters window 51 in the frame. When

contacts

72 and 74 are made as shown in FIGURE 3, spring 70 exerts a component of force tending to keep them closed, but continued rotation of the frame clockwise finally reverses this component of force thereby causing arms 47 and 67 to rotate counter-clockwise with over center snap action thus breaking

contacts

74 and 72 and

closing contacts

75 and 73. The construction of the switch mechanism is such that after actuation thereof due to deflection of the bimetallic strip frame 46 is free to rotate clockwise a considerable distance before striking stop 62 thus allowing for overtravel of the bimetal. After end portion 49 of frame 46 strikes stop 62 further overtravel of the bimetallic strip is possible because the frame preferably is formed from resilient material, such as thin spring steel strip, which will flex readily when urged to do so by movement of support 28. It is to be noted that the contact arms may take one of two st able positions, that is, one where

contacts

74 and 72 are closed and one where

contacts

75 and 73 are closed, the fixed

contacts

72 and 73 serving as stops to limit the travel of the arms. During each complete over center switching operation arms 47 and spring 70 pivot through window 51 in the frame. I

After a switching operation when the bimetallic striphas cooled so as to allow the frame to return to stop 61 due to the biasing action of spring 63 the contacts may be reset to their former FIGURE 3 position by manually depressing lever 6 which is slidably mounted in the casing and biased to its extended position by spring 7. In depressing lever 6 a cam surface 76 thereon engages hump 65 on arm 47 and forces the arm to rotate clockwise until it passes the over center point where spring 70 takes over to complete movement of the arm with snap action thus closing

contacts

74 and 72. This type of reset operation has been previously referred to as manual reset and it is possible when spring 7 is positioned in slot 9 (FIGURE 1). When spring 7 is positioned in slot 8, lever 6 is held in its depressed position by the spring; this has been previously referred to as automatic operation. During automatic operation cam 76 acts as a stop in the path of travel of hump 65; hence, when the bimetallic strip deflects upon being heated and moves frame 46 clockwise,

contacts

74 and 72 separate and remain open only as long as held that way by the bimetallic strip. As the bimetallic strip cools spring 63 drives the frame counter-clockwise to stop 61 over center with respect to spring 70 which in turn rotates the contact arms clockwise to reclose

contacts

74 and 72.

If the inside walls of the compartment enclosing the bimetallic strip are lined with a layer of heat reflecting material such as aluminum foil, some improvement in thermal characteristics are to be expected because heat losses will be reduced. Preferably, movable support 28 is made from an insulating material such as a white melamine plastic which does not conduct heat readily. The thermal insulation combined with bimetal supports of relatively small mass provides an arrangement where the bimetal looses very little of its heat. Insulating compartment 13 will also impede the transfer of heat into compartment 14 where the switching mechanism is located and heat is not wanted.

The main bimetallic element 22 can be compensated for ambient temperatures by making frame 46 from relatively thin, resilient bimetallic strip, and by placing the bimetallic frame in the device with its low expansion side facing in the direction of the main bimetal. As viewed in FIGURE 3, a rise in the ambient temperature causes the lower end of strip 22 to deflect to the right, and bimetallic frame 46 to bow to the right between its pivotal mounting on terminal 56 and stop 61; in other words, the frame bends in the direction actuator 28 is pushed by the main bimetal. The degree of compensation provided can be varied depending on the choice of the bimetals. Full particulars on the ambient compensation may be had from the applicants United States Patent No. 3,015,007 dated Dec. 26, 1961.

It will be seen from FIGURE 3 that bimetallic strip 22 is located close to heater 12, and that bimetallic support 27 is farther from the heater and on the other side of the strip. One side of the strip is exposed to radiation from the heater and the heat reflecting lining in the compartment, but the support is shielded from much of this radiation by the strip. However, heat will find its way to the support, as for example, through radiation and conduction from the strip, but at a much lower rate than to the strip. An increase in the current flowing in the heater will cause the strip to rapidly deflect, and a little later the support will also deflect. If the current is very high, as for example, due to a stalled rotor of a motor, deflection of the strip will be fast enough and great enough that the switch mechanism will trip almost immediately without causing any significant deflection of the bimetallic support, and thereby disconnectin the motor from its power source. If the increased current is due to a moderate overload on the motor the strip will, of course, deflect as before, but not as fast now because less current is flowing in the heater. This delay gives the bimetallic support time to deflect, and in so doing delays tripping of the switching mechanism longer than would be possible with a support unable to deflect. By using the proper combination of bimetals in the strip and support, fast tripping can be obtained on high overloads with much slower tripping on moderate overloads.

In many motor applications, moderate overloads occur frequently, but unless such overload conditions last long enough to be harmful to the motor, tripping is unwanted.

FIGURES 4 and 5 illustrate the operation of the novel arrangement of bimetals. In these figures, the low expansion side of strip 22 and support 27 face to the right.

Numerals

22 and 27 show the strip and support in nondeflected positions as they might be on a cool day before the motor is started. Suppose the motor is now energized but its rotor fails to start. Bimetallic strip 22 is heated very rapidly from heater 12 by the extremely high current flowing in the heater. In an extremely short time the strip deflects to the position indicated by dashed lines at 22a advancing its lower end 80 to the right to position 81. As the lower end of the strip advances, the movable support advances with it to the position 82 of its end 37. This is sufiicient movement of support 28 to the right to trip the switching mechanism and thereby disconnect the motor from its power source.

If support 27 remains undefiected in the position shown in FIGURE 4, a moderate overload on the motor for a little longer time will also cause strip 22 to deflect to position 2211. This may be a much shorter period of time than the motor can stand to be moderately overloaded; consequently, tripping of the switching mechanism occurs too soon. Assume now that the motor is moderately overloaded. The bimetallic strip deflects as before, but not so fast. After a short time before the strip has deflected to position 22a, support 27 also deflects in the way illustrated at 27a in FIGURE 5. The deflection of support 27 to position 27a causes the upper end portion 30 of the support to swing to the right from position 83 to position 84. Movement of end portion 30, the portion in which the upper end of the strip is loosely supported, to the right causes the strip to pivot clockwise on stop 42 to position 22b because movable support 28 is lightly biased for movement to the left by means of spring 63 (FIG- URE 3). Hence, by the time the strip assumes the curvature shown at 22a, it lower end has moved to position 85 rather than 81. Consequently, this amount of deflection causes far less movement of support 28 to the right; its end 37 has now been moved to position 89 rather than 82, a movement insuflicient to trip the switching mechanism. If, however, the overloading persists, the bimetallic strip will continue to deflect, and it will soon cause tripping of the switching mechanism. However, with a bimetallic support, it will take a little longer for tripping to occur. The movement of the strip and the supports occur simultaneously, and not individually as might be inferred in this paragraph. Individual movements were described only for the purpose of presenting a clear picture of the operation of the assembly. In FIGURES 4 and 5, 22a shows strip 22 in the same position and with the same curvature; in FIGURE 5, 22b shows the strip with the same curvature, but in a dilferent position due to the deflection of support 27. This curvature of the bimetallic strip is, of course greatly exaggerated.

FIGURE 4 illustrates the condition where a very high overload will cause almost immediate tripping, or where a moderate overload will cause tripping sooner than wanted. FIGURE .5 illustrates the condition where the bimetallic support also deflects, and in so doing decreases the movement of the lower end of the bimetallic strip. Hence, tripping will not now occur as soon on the moderate overload specified above, but a short time later if the overloading continues. This particular arrangement of the bimetallic members provides very fast tripping on heavy overloads, but delayed tripping on moderate overloads which the motor can tolerate for a short time and on which tripping is not wanted too soon. If, during a moderate overload, a heavy overload suddenly develops, the strip will absorb the additional heat much faster than the support and very fast tripping will, therefore, take place. It will be appreciated that the bimetallic members must be carefully selected as to dimensions, quality, rop

erties, etc., in order to obtain the optimum in tripping characteristics. However, this is considered to be within the skill of one familiar with the art.

Heretofore a bimetallic strip such as 22, for example, fast enough to trip the switching mechanism almost immediately that the motor stalls will also be fast enough to trip the mechanism too soon on moderate overloads. As pointed out earlier in connection with the new motor designs, extremely fast tripping is wanted for a stalled rotor but delayed tripping is wanted for moderate overloads. However, tripping must take place on moderate overloads after a brief delay, otherwise the motor may be harmed from overheating. Protective devices constructed according to this invention can be made to meet these conditions.

It is interesting to note from FIGURES 4 and 5 how the tripping action can be altered if bimetallic support 27 is formed with the high expansion side facing bimetallic strip 22. In this arrangement, the upper end portion 30 of the support will move to the left when the support deflects, and in so doing, the movement of the lower end of the main bimetal to the right will increase rather than decrease as described above. This will make the relay less sensitive to overloads when support 27 is undefiected and more sensitive when it is deflected. In other words, when the bimetal support is cold more current must flow in heater 12 to trip the switching mechanism than will be necessary for tripping it when the support is hot. This particular characteristic could be useful in relays used to protect motors started while heavily loaded, for example, loads such as fans, centrifuges, certain pumps, etc. This type of relay 'will accept the starting overcurrents because the bimetallic support is still cold, but in a relatively short time currents of similar magnitudes will cause tripping because the support is now hot and in a deflected position.

While particular embodiments of this invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects and, therefore, it is intended that the appended claims cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. A thermally responsive device comprising:

(a) a housing,

(b) a first bimetallic support having a first portion secured to a wall of said housing, and a second portion extending in a first direction and free to deflect, with the high expansion side of said first bimetallic support facing in the general direction of said wall,

(c) a bimetallic strip having a first end held from deflection by said second portion of said support and a second end free to deflect,

(d) a mechanism adapted to be operated by resultant displacement of said second end,

(e) an electric heater closely spaced from said himetallic strip, and more remotely spaced from said bimetallic support, heat from said electric heater being more quickly transferred to said strip and less quickly to said support to cause resultant displacement of said second end so as to operate said mechanism,

(f) a second support spaced from said second portion of said first bimetallic support on the low expansion side of said first bimetallic support for holding said second end of said bimetallic strip, said bimetallic strip extending in a direction substantially opposite said first direction, with the high expansion side of said bimetallic strip facing the low expansion side of said first bimetallic support, said second support operatively connecting said mechanism to said second end for operation thereby,

(g) a stop located intermediate said supports on the high expansion side of said bimetallic strip, and a means for biasing said bimetallic strip against said p,

(h) said first portion of said first bimetallic support being positioned against said wall and having an aperture aligned with an aperture extending through said wall, and

(i) a tubular rivet extending through said apertures of said wall and said first portion and having a head bearing against said first portion to secure said first portion to said wall, said stop comprising a screw threaded into said rivet for adjustment relative to said bimetallic strip.

2. A thermally responsive device comprising:

(a) a first bimetallic support having a first portion fixed and a second portion free to deflect when the temperature of said support changes,

(b) a bimetallic strip having a first end supported by said second portion of said support by a loose hingelike joint so as to provide limited pivoted movement of said bimetallic strip with respect to said first bimetallic support, a second end of said bimetallic strip free to deflect when the temperature of said strip changes, upon changes in the same direction of the temperatures of both said support and said strip, the deflection of said second portion of said support decreasing the resultant displacement of said second end of said strip upon deflection of said second end of said strip,

(c) a spring means holding said first end of said bimetallic strip in said loose hinge-like joint with said second portion of said support,

(d) a mechanism adapted to be operated by the resultant displacement of said second end, and

(e) means operatively connecting said mechanism to said second end for operation thereby.

3. The thermally responsive device of claim 2 wherein said support extends in a first direction and said bimetallic strip extends from said joint in a direction substantially opposite said first direction with the low expansion side of said first bimetallic support facing the high expansion side of said bimetallic strip, and a stop is located on the high expansion side of said bimetallic strip, against which said bimetallic strip deflects.

4. The thermally responsive device of claim 2 wherein said last-named means includes a second movable support for holding said second end of said bimetallic strip.

5. The thermally responsive device of claim 2 including an electric heater closely spaced from said bimetallic strip, and more remotely spaced from said first bimetallic support.

6. The thermally responsive device of claim 2 wherein said first bimetallic support extends in a first direction and said bimetallic strip extends from said joint in a direction substantially opposite said first direction with the low expansion side of said first bimetallic support facing the high expansion side of said bimetallic strip, and a stop is located on the high expansion side of said bimetallic strip, against which said bimetallic strip deflects.

7. A thermally responsive device comprising:

(a) a housing,

(b) a first bimetallic support having a first portion secured to a wall of said housing, and a second portion extending in a first direction and free to deflect, with the low expansion side of said first bimetallic support facing in the general direction of said wall,

(e) an electric heater closely spaced from said bimetallic strip, and more remotely spaced :from said bimetallic support, heat from said electric heater being more quickly transferred to said strip and less quickly to said support to cause resultant displacement of said second end so as to operate said mechanism,

(f) a second support spaced from said second portion of said first bimetallic support on the high expansion side of said first bimetallic support for holding said second end of said bimetallic strip, said bimetallic strip extending in a direction substantially opposite said first direction, with the high expansion side of said bimetallic strip facing the high expansion side of said first bimetallic support, said second support operatively connecting said mechanism to said second end for operation thereby, and

(g) a stop located intermediate said supports on the high expansion side of said bimetallic strip, and a means for biasing said bimetallic strip against said stop.

References Cited UNITED STATES PATENTS BERNARD A. GILHEANY, Primary Examiner.

R. COHRS, Assistant Examiner.

US. Cl. X.R.