US3395375A - Snap-acting thermostatic control switch - Google Patents

Description

July 30,1968 J. A, msK ETAL 3,395,375

SNAP-ACTING THERMOSTATIC CONTROL SWITCH 5 Sheets-Sheet 1 Filed July 25, 1966 July 30, 1968 J. A. RISK ETAL 3,395,375

SNAP'ACTING THERMOSTATIC CONTROL SWITCH Filed July 25, 1966 3 Sheets-Sheet 2 FKSZ.

F f f Y w L KL 1 I03 July 30, 1968 J. A. RISK ETAL 3,395,375

SNAP'ACTING THERMOSTATIC CONTROL SWITCH 3 Sheets-Sheet 3 Filed July 25, 1966 L11 5 OP Ll- O Z 5 5 STOP POSITION FORCE OF I5 23 OF 9I DISPLACEMENT RESISTING FORCE P E OF 41 F I 6.7. I Y

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DISPLACEMENT I F C United States Patent 3,395,375 SNAP-ACTING THERMOSTATIC CONTROL SWITCH Jerry A. Risk, Versailles, and Raymond J. Ruckriegel, Fern Creek, Ky., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed July 25, 1966, Ser. No. 567,499 2 Claims. (Cl. 337-338) ABSTRACT OF THE DISCLOSURE A thermostatic switch in which a bimetal transmits force through a link to a snap acting blade having a variable spring rate adjusted to operate in the negative spring rate region. Bouncing of the blade is minimized by having it engage and lift a fixed contact which is resiliently biased into engagement with a rigid adjustabl-y mounted head away from the head. The setting of the head determines the temperature difierential of the switch.

The structure set forth herein is an improvement on that shown in our US. Patent No. 3,339,044.

Among the several objects of the invention may be noted the provision of a snap-acting thermostatic switch having adjustable operating temperatures for switching substantial electrical loads; the provision of a switch of the class described requiring minimal input force to instigate snap action; the provision of a switch of this class having the capability of close differential temperature operation which may be adjusted as to amount; the provision in such a switch of means for avoiding contact freezing and for preventing contact arcing be eliminating or at least minimizing contact closing bounce; and the provision of a switch of the class described which may be accurately built and calibrated in quantities at low cost. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the device and calibrating method therefor hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawings, in which one of various possible embodiments of the invention is illustrated,

FIG. 1 is a vertical section of a switch in closed-circuit position made according to the invention;

FIG. 2 is a plan view of FIG. 1 with certain parts broken away;

FIG. 3 is a horizontal section taken on line 3-3 of FIG. 1;

FIG. 4 is a horizontal section taken on line 44 of FIG. 1;

FIG. 5 is an enlarged detail view of portions of FIG. 1, showing parts in open-circuit position;

FIG. 6 is a schematic view of a spring blade, isolated from other parts, the fragmentary dotted lines illustrating moved positions;

FIG. 7 is a force-displacement diagram of the isolated spring blade of FIG. 6;

FIG. 8 is a schematic view of a thermostatic strip isolated from other parts, the dotted lines showing a moved position upon cooling;

FIG. 9 is a force-displacement diagram of the thermostatic strip shown in FIG. 8;

FIG. 10 is a schematic view of a combination of the spring plate of FIG. 6 and thermostatic strip of FIG. 8 shown in the combination, being isolated from other parts; and

FIGS. 11 and 12 are force-displacement diagrams for the combination illustrated in FIG. 10.

Corresponding reference characters indicate corre- Patented July 30, 1968 "ice sponding parts throughout the several views of the drawings.

The switch is described herein as being used, for example, in a circuit for controlling a motor-driven fan in the heating bonnet of a hot-air furnace supplying warm air to a separate heating space. Thus action of the fan is to be cut in by circuit closure upon temperature rise to drive warm air to the heating space and to be cut out upon cooling. The switch as illustrated is also applicable to the control of motor-driven compressors of refrigeration systems, and by a small change to be mentioned, is also applicable to systems requiring circuit opening upon temperature rise, as in the case of oven and like heating apparatus.

Referring to FIGS. 1-5 of the drawings, numeral 1 indicates an electrically insulating supporting body mounted on a metal supporting plate 3. Metal legs 5 extend through a metallic mounting plate 3 and are attached to the body 1 by clinching tabs 7. Lugs 2 hold the plate 3 in place. The legs 5 are connected at their lower ends by a cross member 9, forming a supporting structure for parts below plate 3. These are the parts that are to be inserted in the chamber under control by the device. The parts on the other side of the plate 3 will be outside. Struck up from the cross member 9 is a tab 11 in which is an opening 13 through which extends a springy composite thermostatic bimetal strip 15. Strip 15 is thin and its component layers are therefore not shown. For switchclosing upon temperature rise, its side of high thermal coefficient of expansion is lowermost and its side of low thermal coefficient of expansion uppermost. This arrangement is for switch closing upon temperature rise. For switch opening upon temperature rise, this arrangement would be inverted.

The bimetal strip 15 at its left end is provided with a dimple 17 resting upon the upper side of head 19 of an adjusting screw 21. The screw is threaded through collars 23 struck from one leg 5. The other end of the strip 15 is pivotally connectively interlocked as at 25 with the lower end of a connecting link 27. The strip 15 may be considered as a beam supported at 17 and 25 and downwardly loaded at fulcrum 13. This results in its part between 19 and 25 functioning as a simple beam, the tension in which may be adjusted by adjusting the support at 17 from screw 21. The pivotal interlock at 25 is conventional requiring no detailed description. A connected upper part 29 of link 27 is composed of insulating material such as melamine glass. This part 29 extends through an opening 31 in the metal plate 3.

Extending crosswise between the legs 5 is a support 33 for mounting a thermostatic high-limit safety switch 35. This switch may be of any known type such as a snap acting disc type, for example. The switch 35 has leads 37 extending through plate 3 from which they are insulated by insulating layer 39. The leads 37 extend through the body 1 to external terminals 40 on the body. The structure provides convenient means for mounting switch 35. However, switch 35, leads 37 and line terminals 40 may be omitted if it is desired to mount this switch elsewhere.

The body 1 on its underside provides space above plate 3 for containing snap-acting switch mechanism for normal temperature control. This mechanism comprises a spring switch blade 41 which has a fixed end 43 and a movable end 45. This blade is not snap-acting per se but is caused to become snap-acting by cooperative action in connection with the thermostatic strip 15, as will be made clear below. Blade 41 is slotted as shown at 46 (FIG. 3) to provide a central ribbon -47 flanked by sidewise ribbons 49, the latter being crimped as shown at 51. While not snap-acting per se, the blade is of the type that has a nonlinear action, i.e., a rising and falling resisting force in response to displacement, as illustrated in FIG. 7. It may therefore hereinafter be re ferred to as a spring blade formed to have nonlinear characteristics. The movable end 45 of the nonlinear blade 41 is provided with means 53 for pivot-ally interlocking it in a suitable slot in the upper end of the insulating part 29 of link 27. The pivotal interlock, which is of a usual type for the purpose, is indicated at 54. The movable end 45 of the blade 41 carries a movable switch contact 55.

At numeral 57 is shown a leaf spring, one end of which is dimpled as shown at 59. Its other end 61, along with the fixed end 43 of the blade 41, is attached to the base 1 by means of a rivet 53. The upper end of the rivet fastens a line terminal 65 on top of body 1. The spring 57 is thus mounted as a cantilever. By this arrangement the dimple 59 of spring 57 engages with the central part of the underside of the ribbon portion 47 of blade 41. The upper side of ribbon portion 47 is engaged centrally by the ball-shaped end 67 of an adjusting screw 69 threaded through the base 1 and having splined on its upper end a manually adjustable knob 71. Knob 71 has limited rotation effected by a stop 70 on body 1 engaged by opposite ends of an arcuate slot 74 formed on the underside of knob 71 and extending less than 360. Knob 71 may be suitably indexed, as desired. It may be pushed on the end of screw 69.

A triangular drag spring 72, sprung between a sunken part 76 of body 1 and the upper part of screw 69, serves to hold the screw 69 in any selected position into which it is adjusted by rotation. Thus the center of ribbon portion 47 of the blade 41 may be moved up and down by rotating the screws 69. The crimped sidewise ribbon portions 49 cause a nonlinear force-displacement action by the blade end 45 with the contact 55 thereon (see FIG. 7). A backstop is provided under the movable end 45 of the blade 41. This is constituted by an insulating head 75 supported on an adjustment screw 73 threaded through the plate 3. Head 75 acts as a stop.

Above the contact 55 on the movable end of the blade 41 is located a conductive spring 77 (see FIG. having a lower leg 79 and an upper leg 81 connected at a 180 turn. The upper leg 81 is formed with a sidewise extension 83 which is held by means of the lower head of a rivet 85 attached to and extending through the top of the body 1. The upper head of the rivet 85 fastens a terminal 87 on the top of the body 1. The lower leg 79 of spring 77 is formed with an end 89' which carries a movable terminal contact 91. The movable terminal contact 91 is always conductively connected with the line terminal through the spring 77.

On the underside of the spring end 89 is formed a dimple 93. This is resiliently engageable with the top of a lower head 95 formed on a reduced extension 97 of an adjusting screw 99 threaded through the base 1. Thus head 95 acts as a downward stop for the resiliently mounted movable terminal contact 91. The dimple 93 of spring leg 79 is capable of moving up from head 95. The upper end of the screw 99 has removably splined to it a manual adjusting knob 101 which may be suitably indexed. A triangular drag spring 103 sprung between the body 1 and the screw 99 maintains any given angular setting of the knob 101 and screw 99. By turning the knob 101, the elevation of the head 95 may be adjusted, thus changing the position of the movable line contact 91 when the dimple 93 rests on the head 95. It is to be noted that the movable switch contact 55 never engages head 95 but is engageable with the movable line contact 91 to lift it from head 95. The engageable and disengageable contacts 55 and 91 are not on a vertical line with the head 95 (see FIG. 3).

FIG. 6 schematically shows the blade 41 mounted in isolation for purposes of the following explanation, which relates to a tan control system such as above mentioned which requires circuit closure upon temperature rise.

FIG. 7 shows the spring-rate characteristics of the mounted isolated blade. The letter A in FIG. 6 corresponds to a free position of the blade 41. If the movable end of the blade 41 is forced down by some means, as indicated by dart M, the resisting force of the blade will at first increase. At a critical force P, position B will be reached on FIG. 6 and point B on FIG. 7. Position B corresponds to changed condition of the plate 41, which is to say that thereafter force to bring about further displacement will decrease (see point C in FIG. 7 and position C in FIG. 6). Letter C in FIG. 7 indicates the position corresponding to zero force. As will appear, the length of line BC is determined by the bottoming effect of member 75. It is important to note that thus far there has not been described any snap action elfected by the blade 41.

In FIG. 8 the bimetal strip 15 is shown as a beam but removed along with its supports from the remainder of the device. FIG. 9 shows certain force characteristics obtained by stressing by an upward force at its right-hand end. Force P can be obtained by bringing the right-hand end to the solid-line position, as shown by the dart. Hereinafter P is called the preload force. Any downward movement of the right hand end will cause the preload force P to decrease as shown in FIG. 9. In this event there is no evidence of snap travel.

For purposes of explanation, we will define a spring as having a positive spring rate if the resisting force increases as the free end is deflected away from its no-load position. Conversely, a spring has a negative spring rate if the resisting force decreases with added deflection of the free end away from its no-load position. FIG. 9 then shows the bimetal strip 15 as having a positive spring rate. FIG. 7 shows the spring blade 41 as having a positive spring rate from A to B and changing to a negative spring rate from B to C. It is important to notice that strip 15 has a positive spring rate as it is stressed which is less than the negative spring rate of blade 41. This is evidenced by the greater slope of line B-C on FIG. 7.

In FIG. 10, the spring blade 41 of FIG. 6 and the thermostatic strip 15 of FIG. 8 are combined and connected by the link 27, a part of the spring arm 77 with its terminal contact 91 being shown. It is then possible to adjust screw 69 and load the strip 15 by turning screw 21 to establish a preload force P on the end of blade 41 at which the force between the contacts 91 and will become balanced and equal to zero. Any cooling of the bimetal strip 15 and its downward movement will cause downward force increase and increase in the load P, but since the slope of the dotted line on FIG. 9 is less than that of the line B-C of FIG. 7, contacts 55 and 91 will then open. In this case the strip 15 drives the blade 41. The movable contact 55 will start from 0 velocity and accelerate toward the stop, thus achieving snap action for contact opening. When motion of the system is resisted by the stop 75, the bimetal will be pulling down- Ward with force Y and the blade 41 -will be pulling upward with smaller force Z (FIG. 11). The force against the stop will be Y minus Z.

Then if, as illustrated in FIG. 12, the bimetal strip 15 is heated, its force Y on the link 27 will decrease toward the value Z. The bimetal forces will ultimately become less than the upward forces of the blade. In this case the blade 41 will drive the strip in moving up from stop 75, thus to close contacts 55 and 91 so as to start the abovementioned fan. The temperature change does not affect the mechanical spring rate of the bimetal strip 15. As the end of the spring blade moves away from the stop 75, the force of blade 41 is always greater and is increasing faster then the force of the bimetal strip 15. It is for this reason that the movable contact 55 will be accelerated toward the line terminal contact 91 to drive the system with snap action back to its original closed-circuit position. Contact 91 acts as astop at one end of the travel of contact 55 and at member 75 as a stop at the other end of the travel, as indicated on FIGS. 11 and 12.

The tension in spring 77 and adjustment of screw 99 is such that when contacts 55 and 91 are open, spring 77 with contact 91 is pressed on head 95 of screw 99. The screw adjustment is such that upon closure of contact 55, spring 77 with contact 91 will be lifted from head 95. This has the effect of preventing any bouncing apart of contacts 55 and 91 during closure, since contacts 55 and 91 engage under load and remain engaged thereafter. It will be noted that the upward movement of spring 77 and line terminal contact 91 away from head 95 is unrestricted so that if as may be possible there is any contact vibration during closure, the two contacts 55 and 91 will vibrate together under load, without separation during such vibration. The spring rate of leg 79, which is positive, is such that after contact make and lifting of contact 91 from head 95, the contacts stop under a free condition of equilibrium without bottoming on any stop, thus avoiding any separating rebound, one from the other. If there is any vibration the contacts remain together throughout its occurrence. The contact movements after make also provide a wiping action, serving to shear any tendency for contacts 55 and 91 to weld together under heavy current such as are encountered in motor controls.

The screw 99 serves as means for changing the elevation of the line contact 91 when resting on the head 95, thus providing for changing the air gap between contacts 91 and 55 when open. This serves as a means for varying the temperature differential of the device, if desired. In any adjusted position the contact 91 is positioned against head 95 when contacts 55 and 91 are open and raised from the head when the contacts are closed. The screw 69 serves as an adjustment to stress the blade 41 in the contact-closed position so that when preloaded by downward pull from the end of thermostatic strip 15 the conditions are as shown in FIG. 1 with the contacts 55 and 91 ready to open. The screw 69 also serves as the operating temperature adjusting screw. Rotation of screw 69 will change the resisting force of the snap blade. This means that the bimetal must exert a different force to snap the system and a proportional change in operating temperature will occur.

If an adjustable differential temperature is required with adjustable temperature settings, the factory calibration may be as follows: Screw 99 is adjusted during assembly, knob 101 being pushed on at the setting for minimum temperature differential. Screw 69 is adjusted during assembly and its knob 71 is pushed on, being positioned at the maximum temperature setting. Screw 73 is positioned so that there is a wide air gap above stop 75. Screw 21 is started into its threads but not sufliciently to load the bimetal enough to open the contacts 55 and 91. The bimetal strip 15 is brought to the opening temperature required of the device at the maximum temperature setting. Screw 21 is then adjusted so that it loads the bimetal strip 15 just enough to open the contacts. This sets the opening temperature. The bimetal strip 15 is brought to the closing temperature required at the maximum temperature setting. Screw 73 is then adjusted, narrowing the air gap until the bimetal is loaded sufiiciently to cause the contact to close. This sets the closing temperature.

If a fixed differential is demanded with adjustable temperature, then calibration may be as follows: Screw 99 is positioned during assembly and is locked by conventional means such as an adhesive applied to its threads. Screw 69 is also adjusted during assembly and knob 71 is pushed on, being positioned at the maximum temperature setting. Screw 73 has been prepositioned so that there is a wide air gap. Screw 21 is then started in but doe-s not load the thermostat strip 15 enough to open the contacts. Strip 15 is brought to the opening temperaure required in the device at the maximum temperature setting. Screw 21 is then adjusted so that it loads the strip 15 just enough to open the contacts. This sets the opening temperature. The strip 15 is then brought to the closing temperature required in the device for maximum temperature setting. Screw 73 is then adjusted, narrowing the air gap until the bimetal is loaded sufficiently to cause the contacts to close. This sets the closing temperature.

For fixed-temperature, fixed-differential operation calibration may be as follows: Screw 99 is positioned during assembly and is locked by conventional means such as the adhesive above mentioned. Screw 69 is positioned during assembly and is likewise locked. Screw 73 has been positioned so that there is a wide air gap. Screw 21 is then started into its threads but not enough to load the strip 15 to open the contacts. The strip 15 is brought to the opening temperature required in the device. Screw 21 is then adjusted so that it loads strip 15 just enough to open the contacts. This sets the opening temperature. The bimetal is brought to the closing temperature required in the device. Screw 73 is then adjusted, narrowing the air gra-p until the bimetal is loaded sufficiently to cause the contacts to close. This sets the closing temperature.

It will be understood that all calibrations may be made without the switch 35 in circuit. As above stated the switch 35 with its leads 37 and terminals 40 may be omitted. When used, the switch 35 is wired into a suitable circuit for controlling any operation desired in addition to the operation controlled by the circuit through contacts 55 and 91. One such desired operation is to open the circuit under control between terminals 65 and 87. Thus switch 35 may function as a high-temperature-limiting switch.

Advantages are as follows:

The stopping of the downward springing movement of the contact 91 by head 95 at a definite point during each cycle results in a cycling action having more accurately repeatable characteristics. When the contacts are made, the system does not snap back to open-circuit condition by rebound upon closure. There is no positive stop determining the circuit-closed positions of engaged contacts 55 and 91. Thus there is no vibratory opening of the contacts. There is transverse wiping act-ion between contacts 55 and 91 as they close, thus serving to shear any incipient contact welding that might tend to take place. The electrical capacity of the switch is large because of the manner in which contact engagement and disengagement is accomplished. The bimetal has only to move slightly to cause the system to snap to switch-opening and closing positions. The provision in the assembly for inserting the high-limit switch 35, with its leads 37 and line terminals 40, is advantageous in that in one unit are conveniently assembled several control switches. The unit as shown in FIGS. 1 and 2 may be conveniently applied in heating bonnets or the like with the lower thermostatic components inside and the upper switch contact outside.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the device and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A snap-acting thermostatic control switch compris- (a) a supporting plate,

(b) a first structural assembly carried on one side of the plate for insertion into a chamber the temperature of which is to be controlled,

(0) a springy bimetallic temperature responsive strip in said assembly supported between its ends by a fixed assembly part, said strip having a positive spring rate and being coupled at one end thereof to a link extending through said plate,

(d)' an insulating housing forming part of -a second as- (e) a springy snap-acting switch blade in said housing and attached at one end to one of said terminals, its other end being movable, said blade having a range of spring rates including a negative rate which is greater in value than the value of the positive spring rate of said strip in the first assembly, said blade carrying a contact at its movable end, said movable end being articulated through said link in force opposition to said strip,

(f) calibrated first exteriorly accessible adjusting means carried on said housing and connected to the snapacting blade between its ends for varying stress in the blade, whereby force to bring about its snapaction may be varied to determine the temperature at which the blade will snap,

(g) second adjustment means forming part of said first assembly and connected with one end of the strip for stressing the strip to apply force through said link to the blade,

(h) a fixed head forming a first stop, resilient means conductively connected with the other of said terminals and having a free end engageable with said first stop -to fix a movable terminal contact carried by said free end when the terminal and blade contacts are separated in open position of the switch, said blade contact upon engaging the terminal contact in closed position of the switch moving said terminal contact and the free end of the blade from said first stop while maintaining resilient engagement with the terminal contact, thereby preventing circuit-opening bounce back of the blade contact from the terminal contact after these contacts initially close,

(i) third adjusting means carried by said housing for adjusting the position of said fixed head to adjust the contact pressure and determining the point at which said movable terminal contact is initially engaged by the blade contact to elfect circuit closure,

(j) a backstop for the movable end of the blade to limit switch-opening movement of the blade contact after it leaves the resiliently mounted terminal contact, fourth adjusting means for said backstop, the first adjusting means for the blade and the fourth adjusting means for the backstop being set so as to limit the spring rate of the blade within said negative range, the relative adjusted positions of said backstop and said head also determining the operating temperature differential of the switch.

2. A switch according to claim 1 including a thermostatic limit switch forming part of said first assembly,

conductive leads extending from said limit switch through said supporting plate, and additional line terminals for said leads carried upon said housing.

References Cited UNITED STATES PATENTS 3/1966 Bletz. Y 5/1958 Weber 200138 BERNARD A. GILHEANY, Primary Examiner. R. COHRS, Assistant Examiner.

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