US3038051A - Thermal device - Google Patents

Description

"June 5, 1962 Filed Nov. 25, 1959 J. F. HOWARD THERMAL DEVICE 2 SheetsSheet 1 INVENTOR. JOHN F. HOWARD A TTORNEY J.. F. HOWARD THERMAL DEVICE June 5, 1962 2 Sheets-Sheet 2 Filed NOV. 25, 1959 INVENTOR. JOHN F. HOWARD A TTORNEY United States Patent Oh ice 3,038,051 Patented June 5, 1962 3,038,051 THERMAL DEVICE John F. Howard, Peterborough, Ontario, Canada, assignor to Canadian General Electric Company, Limited, To-

ronto, Ontario, Canada, a company of Canada Filed Nov. 25, 1959, Ser. No. 855,289 Claims priority, application Canada Aug. 7, 1959 11 Claims. (Cl. 200-122) This invention relates to electric overload actuated devices and particularly to the thermally responsive element used in such devices.

An electric motor is very often connected to its power supply through an electromagnetic contactor provided with overload actuated devices designed to protect the motor against damage from excessive currents due to overload or stalled conditions. Such overload actuated devices ordinarily utilize a bimetallic strip disposed to actuate a set of contacts, and a heater element energized by the circuit to be controlled, the heater being placed near the bi-metal to cause heating and deflection thereof. In certain cases the bimetallic strip may be heated by passing at least a portion of the load current through it, or by such direct heating means in combination with a separate heater.

The means for supporting a bimetallic strip in an overload device must be such that the bimetal can be initially calibrated, and thereafter any given amount of deflection of the bimetal must consistently represent a predetermined current flow through the heater. Many applications of an overload device demand a bimetal supported for limited adjustment thereof to enable matching the device with a particular machine rating or to corn pensate for unusual load conditions to which the machine may be subjected, and throughout the entire range of adjustment the deflection of the bimetal must consistently represent certain predetermined load conditions. There are many known means for supporting a bimetallic strip, but seldom, if ever, does any one of such means provide the optimum in all the characteristics which may be required for a particular application. If, for example, a bimetallic strip is fixed to a support, the support is generally large enough to radiate a significant quantity of heat conducted from the bimetal. With certain supporting means it is difficult to calibrate the bimetal and maintain it properly calibrated, while other means do not readily lend themselves to adjustment of the bimetal. Another very important consideration in the manufacture of a device utilizing a bimetallic strip is cost; the device must be relatively inexpensive but, nevertheless, reliable.

Accordingly, it is an object of this invention to provide a novel means for supporting a bimetallic strip in a thermally responsive device, a means which is simple, inexpensive, and avoids relatively massive supports and close manufacturing tolerances.

It is another object of this invention to provide a means for supporting a bimetallic strip in a thermally responsive device whereby the strip can be readily calibrated and thereafter readily adjusted.

It is yet another object of this invention to provide a means for supporting a bimetallic strip in a thermally responsive device whereby very little heat can be conducted from the strip to the supports, and whereby substantially the full length of the strip can deflect.

It is a further object of this invention to provide a means for supporting a bimetallic strip in a thermally responsive device whereby the strip is not positively secured to a support thus avoiding the introduction of stresses in the bimetal due to welding, brazing, riveting, clamping, etc.

In accordance with a preferred embodiment of the invention a thermally responsive device is provided wherein a bimetallic strip is loosely retained by its ends between a pair of spaced support members, one of which is fixed and the other movable. A stop positioned on one side of the bimetal intermediate its ends cooperates with the fixed support member to restrain deflection of the bimetal upon being heated to movement of the end thereof supported by the movable support member. The movable support member may be a device actuator, often referred to as the tripbar.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawings in which:

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

FIGURE 2 is another perspective view of the device, the side cover being removed to expose the operating mechanism of the device;

FIGURE 3 is an exploded view of the parts used in the device of FIGURES 1 and 2; and

FIGURE 4 is a view of the device in elevation showing the parts assembled in the housing.

Referring now to FIGURES 1 and 2, the invention is shown as incorporated in an overload actuated device comprising 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 hold 3 in the bottom of the housing. External operating members are provided including knob 4 which cooperates with the moulded projection 5 on housing -1 as a stop, and reset lever 6 which may be positioned for manual or automatic operation through the positioning of spring 7 either in slot 8 for automatic operation or in slot 9 for manual reset operation. Terminals 10 and '11 are provided for connecting suitable conductors to resistance heater 12.

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 side 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 side wall '16 and cover 2. Heater 12 is secured in position inside compartment 13 near wall 15 but spaced therefrom by means of screws 21 threaded into apertures in terminals 10 and 11 whereby a circuit is completed through the heater between the terminals.

A bimetallic strip 22 disposed in compartment 13 generally parallel to wall 15 between heater 12 and end wall 18 is loosely retained in this position by means of tongues 23, 24 on the ends of the strip received into mating apertures 25, 26 respectively as a loose sliding fit, aperture 25 being in fixed support 27 and aperture 26 in movable support 28. Fixed support 27 is a sheet metal member having a flat body portion 29 formed with a bent-up portion 30 containing aperture 25 and an ear 31 bent in the opposite direction from an adjacent edge. The ear is apertured at 32 to receive screw 33 which secures the support to a boss 34 on the inside of wall 18. Movable support 28 is spaced from fixed 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 4) 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 supports 27 and 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.

A stop 42 on end wall 18 is disposed in compartment 13 intermediate supports 27 and 28 and adapted to en gage the high expansion side of the bimetal such that it cooperates with fixed support 27 to restrain movement of the bimetal upon being heated to deflection of the end thereof supported on movable support 28. As clear- 1y shown in FIGURE 4 fixed 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 of the bimetal moves support 28 to the right. Preferably stop 42 is made adjustable and may be a threaded shank screwed into a threaded aperture 44 in boss portion 34 of end wall 18 such that point 45 of the shank engages the high expansion side of the bimetal on or near the longitudinal center line thereof. After assembly of the bimetal to its supports 27 and 28, threaded shank 42 is driven into threaded aperture 44 until its point 45 is positioned to provide the desired calibration of the bimetal; after which knob 4 is permanently fixed to the outer end of the shank. Knob 4 cooperates with projection to limit turning movement of the shank to something less than one turn whereby the position of point 4 5 may be altered slightly to provide for limited adjustment of the bimetal. Indicia may be provided with which a pointer on the knob cooperates to indicate bimetal settings.

End 37 of movable support 28 actuates a switch mechanism located in compartment 14 when the support is moved towards end wall 19 by deflection of the bimetal from a normal undeflected position thereof. The switch 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 spring-like material into a pair of spaced legs 48 integral with spaced end portions 49 and 50, the legs and end portions defining 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, 58 in walls 15, 19 respectively at the lower end of compartment 14 has a pair of 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 whose sides 61, 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 counterclockwise rotation against stop 61. It is to be noted with respect to FIGURE 4 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 counterclockwise towards stop 61 spring 63 also biases bimetallic strip 22 for clockwise rotation about fixed 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 aperture 69 in terminal 56 to prevent sliding movement of the arm as s-embly' 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 action mechanism involving contact arm 47, frame 46 and coil spring 70. p

In one position of the contact arms, i.e., that illustrated in FIGURE 4, 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.

In actual use of the overload device, heater 12 is connected into a power supply conductor by means of terminals 10 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 heater 12 is insufficient to operate the device, but if the current flow rises above a predetermined value, suflicient heat will then be produced to cause bimetallic strip 22 to deflect. As best illustrated in FIGURE 4 deflection of the bimetal 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, 74 are made as shown in FIGURE 4, 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, 67 to rotate counterclockwise with over-center snap action thus breaking contacts 74, 72 and closing contacts 75, 73. The construction of the switch mechanism is such that after actuation thereof due to deflection of the bimetal, 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 bimetal 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 stable positions, that is, one where contacts 74, 72 are closed and one where contacts 75, 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 arm 47 and spring 70 pivot through window 51 in the frame.

After a switching operation where the contact arms have changed from the stable position shown in FIGURE 4 to a second stable position where contacts 74, 72 are open and 75, 73 closed, the contacts may be reset to their former FIGURE 4 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 force 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 7 4, 72 and returning the frame to stop 61? 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 to prevent overcenter switch action. Hence when the bimetal deflects upon being heated and moves frame 46 clockwise, contacts 74, 72 separate and remain open only as long as held that way by the bimetal. When the bimetal cools, contacts 74, 72 reclose automatically.

If the inside walls of the housing adjacent heater 12 are lined with a layer of heat reflecting material such as aluminum foil, some improvement in heat transfer from heater 12 to bimetal 22 is 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 can be expected to respond rapidly to overload conditions. Insulating compartment 13 around the heater will also impede the transfer of heat into compartment 14 where the switching mechanism is located.

It is Well known that a bimetallic strip tends to warp along its transverse axis as well as along its longitudinal axis when heated. Reformation of a strip along its transverse axis increases with increase of width of the strip. In order to reduce transverse deformation of the strip, the strip may be tapered inwardly from fixed support 27 to movable support 28 as shown in FIGURE 3. Hence the end of the strip which is free to deflect is of lesser width than the end restrained by support 27.

A tapered bimetallic strip such as that illustrated in FIGURE 3 is preferred because it performs well and can be cut from sheet stock with very little waste. Other suitable shapes, however, will occur readily to those skilled in the art. For example, a strip whose width is greatest in the region of stop 42 and tapers inwardly therefrom could be expected to perform well.

If desired, one or both of tongues 23, 24 could be offset laterally with respect to the longitudinal axis of the bimetallic strip, in which case apertures 25, 26 would also be offset in a similar way. When a tongue is offset, the strip will fit its support means in one position only, consequently improper assembly of the bimetal will become virtually impossible. This will facilitate manufacture.

The drawings show a heater 12 spaced from the bimetallic strip 22 and supported on terminals and 11. It is also well known to secure a heater to a bimetal in heat conducting relationship therewith or pass an electric current through a bimetal for the purpose of generating heat therein. It is to be understood that the bimetal described and claimed herein may be heated by any one of such means or by a combination thereof. The heating means employed will be determined by the nature of the load and the control function which the thermally responsive deviceis intended to perform.

Although the novel means for supporting a bimetallic strip has been described in combination with a particular switching mechanism, it can be readily adapted for use with other switching mechanisms such as, for example, that disclosed in a US. Patent to Ernst R. Wolff, No. 2,897,319, issued July 28, 1959.

Although a particular embodiment of the invention has been described, it is obvious that many modifications will occur to those skilled in the art.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A thermally responsive device comprising a pair of spaced support members, one of which is fixed and the other movable; a bimetallic strip bridging and engaging said members and supported thereon in a pivotal manner without being positively attached to either of said support members, said fixed support member restraining the associated bimetal end against substantial deflection; a stop disposed intermediate said support members against which said bimetallic strip deflects when heated, said stop cooperating with said fixed support member to re- 6 strain deflection of the bimetallic strip such that the end thereof supported on said movable support member moves and thereby moves the movable support member; and control means adapted to be actuated by the deflection of said bimetallic strip.

2. A thermally responsive device comprising a pair of spaced support members, one of which is fixed and the other movable; a bimetallic strip bridging and engaging said members and supported thereon in a pivotal manner without being positively attached to either of said support members, said fixed support member restraining the associated bimetal end against substantial deflection; a stop disposed intermediate said support members against which said bimetallic strip deflects when heated, said stop cooperating with said fixed support member to restrain deflection of the bimetallic strip such that the end thereof supported on said movable support member moves and thereby moves the movable support member; control means adapted to be actuated by said movable support member; and means for biasing said movable support member against movement of said bimetallic strip.

3. A thermally responsive device comprising an insulating casing; a fixed support in said casing; a switch actuator spaced from said support and slidably mounted in said casing; a bimetallic strip in engagement with-and loosely supported between said support and actuator, said support restraining the associated bimetal end against substantial deflection; a stop disposed intermediate said support and actuator against which said bimetallic strip deflects when heated, said stop cooperating with said support to restrain deflection of said bimetallic strip such that the end thereof supported on said actuator moves and thereby moves the actuator; contact means adapted to be actuated by said actuator; and means biasing said actuator against movement by said bimetallic strip.

4. A thermally responsive device comprising a pair of spaced support members, one of which is fixed and the other movable; a bimetallic strip bridging and engaging said members and supported thereon in a pivotal fashion, said fixed member restraining the associated bimetal end against substantial deflection; an adjustable stop member disposed adjacent an intermediate portion of said bimetallic strip against which said bimetallic strip deflects when heated, said stop cooperating with said fixed support member to restrain further deflection of the bimetallic strip such that the end thereof supported on said movable support member moves and thereby moves the movable support member; and control means adapted to be actuated by the deflection of said bimetallic strip.

5. An overload protective device comprising a fixed support member, a bimetallic strip having one end thereof engaging the fixed support member and supported thereby for limited pivotal movement without substantial deflection; an electric heater closely spaced from said himetallic strip along one side thereof; stop means disposed intermediate the ends of said bimetallic strip on the other side thereof; said support and stop means restraining deflection of said bimetallic strip whereby its other end will move toward said heater upon an increase in temperature of the bimetallic strip and switching means actuated by the movement of said other end of said bimetallic strip.

6. A thermally responsive device comprising a pair of spaced support members, one of which is fixed and the other movable; a bimetallic strip having a tongue formed on each end thereof; each support member having an aperture adapted to receive a tongue whereby the bimetallic strip is loosely retained between said support members; and a stop positioned intermediate the ends of said himetallic strip, said stop cooperating with said fixed support to limit the deflection of the bimetallic strip upon being heated to movement of the end thereof disposed in said movable support.

7. A thermally responsive device comprising an insulating housing; a fixed support in said housing and having an aperture; an actuator having an aperture and spaced from said support and slidaibly mounted in said housing; a bimetallic strip bridging said fixed support and actuator; a tongue on one end of said bimetallic strip adapted to be received into the aperture in said support as a freely slidable fit; a tongue on the other end of said strip adapted to be received into the aperture in said actuator as a freely slidable fit; sa-id tongues being the sole means for supporting the bimetallic strip on the fixed support and actuator; and a stop in said housing positioned intermediate the ends of said bimetallic strip, said stop cooperating with said fixed support to limit the deflection of the bimetallic strip upon being heated to movement of the end thereof disposed in said actuator.

8. An overload protective device comprising in combination an insulating housing divided into at least two separate compartments; a fixed support in one compartment and having an aperture; a switch actuator spaced from said support and mounted for sliding movement in said housing through a wall common to said two compartments; the end of said actuator in said one compartment having an aperture; a bimetallic strip disposed in said one compartment and bridging said fixed support and actuator; a tongue on one end of said bimetallic strip adapted to be received into the aperture in said support as a freely slidable fit; a tongue on the other end of said strip adapted to be received into the aperture in said actuator as a freely slidable fit; said tongues being the sole means for supporting the bimetallic strip on the fixed support and actuator; an adjustable stop projecting through an outer wall of said one compartment, said stop being positioned intermediate the ends of said strip and cooperating with said fixed support to limit the deflection of the strip upon being heated above a preselected temperature to movement of the end thereof disposed in said actuator; an electric heater in said one compartment spaced between said bimetallic strip and said common wall; and a switching device disposed in the other compartment, the movable contacts of said switching device being biased to a normal position and actuated to another position by means of said actuator moving in response to the deflection of said bimetallic strip.

9. An overload protective device comprising in combination an insulating housing divided into at least two separate compartments; a fixed support in one compartment having an aperture therein; a switch actuator spaced from said support and mounted for sliding movement in said housing through a wall common to said two compartments, said actuator having an aperture in the end of said actuator located in said one compartment; a flat bimetallic strip of uniform thickness disposed in said one compartment and bridging said fixed support and actuator, said strip tapering uniformly inward from said fixed support to said switch actuator; a pair of tongues, one on Cir each end of said bimetallic strip adapted to be received into the apertures in said support and said actuator respectively as a freely slidable fit, said tongues being the sole means for supporting the bimetallic strip on the fixed support and actuator; a threaded adjustable stop mem ber threaded through an outer wall of said one compartment, said stop being positioned intermediate the ends of said strip and cooperating with said fixed support to limit the deflection of the strip upon being heated above a preselected temperature to movement of the end thereof disposed in said actuator; an electric heater in said one compartment spaced between said bimetallic strip and said common wall; and a switching device disposed in the other compartment, the movable contacts of said switching device being biased to a normal position and actuated to another position by means of said actuator moving in response to the deflection of said bimetallic strip.

10. A thermally responsive device comprising a pair of spaced support members, one of which is fixed and the other movable, a bimetallic strip spanning and engaging said support members and loosely retained between them by the ends of the bimetallic strip, said fixed support member restraining the associated bimetal end against substantial deflection, and stop means positioned intermediate the ends of said strip, said stop means cooperating with said fixed support member to limit the deflection of the strip upon being heated to movement of the end thereof supported by said movable support.

11. A thermally responsive device comprising a pair of spaced support members, one of which is fixed and the other movable, a fiat bimetallic strip of uniform thickness spanning and engaging said support members and loosely retained between them by its ends, the edges of said strip tapering inward uniformly from the end atsaid fixed support, said fixed support member restraining the associated bimetal end against substantial deflection, and stop means positioned intermediate the ends of said strip, said stop means cooperating with said fixed support member to limit the deflection of the strip upon being heated above a predetermined temperature to movement of the end thereof supported by said movable support member.

References Cited in the file of this patent UNITED STATES PATENTS 2,308,091 McCullough Jan. 12, 1943 2,322,161 Schleicher June 15, 1943 2,336,408 Matthews Dec. 7, 1943 2,475,292 Osterheld July 5, 1949 2,897,319 Wolff July 28, 1959 2,932,707 Butler Apr. 12, 1960 FOREIGN PATENTS 707,569 Great Britain Apr. 21, 1954

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