US2658121A - Condition responsive electric switch mechanism - Google Patents

Description

Nov. 3, 1953 R. 1.. GRAY ET AL 2,658,121

CONDITION RESPONSIVE ELECTRIC SWITCH MECHANISM Filed July 30, 1951 2 Sheets-Sheet l T -El.

. \nven'tons:

Robert L. GraH, Charles 5. Grimshaw,

Their" A-t'torneg Nov. 3, 1953 R. L. GRAY ET AL 2,658,121

CONDITION RESPONSIVEZ ELECTRIC SWITCH MECHANISM Filed July 30, 1951 2 Sheets-Sheet 2 Inventors:

Robert 1... Gra g, Charles 5. Grimshaw,

T heir" Attovneg Patented Nov. 3, 1953 CONDITION RESPONSIVE ELECTRIC SWITCH MECHANISM Robert L. Gray, Schenectady, N. Y., and Charles S. Grimshaw, Erie, Pa., assignors to General Electric Company, a corporation of New York Application July 30, 1951, Serial No. 239,338

13 Claims. 1

This invention relates to electric switch mechanisms, and more particularly to such mechanisms which are utilized in the automatic regulation of a condition such as temperature or pressure.

The principal object of our invention is to provide a condition responsive switch mechanism which is inexpensive and easy to manufacture, and at the same time is reliable in operation and will provide a long life.

A feature of the invention is the provision of a snap action switch mechanism employing a non-overcenter spring.

Other objects and advantages of our invention will be in part obvious and are in part pointed out in the subsequent detailed description of a preferred embodiment of the invention.

In carrying out our invention in one form. we provide a switch mechanism which includes a fixed contact, a movable contact mounted on a cantilever spring, and a main operating lever arranged to move the cantilever spring member to open and close the contacts. The main operating lever is continually biased in the contacts closed direction by a non-overcenter spring. The operating lever is further biased in the opposite or contacts open direction by an additional spring which normally overpowers the non-overcenter spring and maintains and contacts open. Condition responsive means are provided for partially counteracting the second spring under certain circumstances to such an extent that the non-overcenter spring is able to close the contacts. When the counteracting action lessens sufficiently, the contacts are again opened.

For a clearer and more complete understanding of our invention, reference should be had to the accompanying drawing in which:

Fig. 1 is a side elevation View, partially in section, of a condition control device embodying our invention in a preferred form,

Fig. 2 is a left end View of the device of Fig. 1 ShOWing only a fragmentary part of the control knob,

Fig. 3 is a side view, partially in section, similar to Fig. 1 except omitting some parts and showing the contacts in the closed position,

Fig. 4: is an end view similar to Fig. 2 except partially in section,

5 is a sectional view along the lines 5-5 of Fig. 1,

Fig. 6 is a sectional view along the lines 6--6 of Fig. 1,

Fig. '7 is an end view along the lines 1-! of Fig. 1,

Fig. 8 is a top view partially in section of the device of Fig. 1, and

Fig. 9 is a partial side view in section of the device of Fig. 1 showing the details of the differential adjustment mechanism thereof.

In the drawing our invention is shown embodied in a refrigerator temperature control device which is designated generally by the numeral i8. As shown particularly in Figs. 1 and 3, this device includes a substantially U-shaped metal frame or support member I I having two substantially horizontal legs, the lower of which has an ofiset portion, and a vertical connecting or bight portion, which is to the left in these figures. The frame H has a vertical metal cross member I? rigidly connected between the two horizontal legs of member H, preferably by means of projections along the upper and lower edges of member I2 which engage corresponding openings in member ll, while an additional insulating support member 13 closes the free ends of member ll.

Referring to Figs. 2 and 4 in addition to Figs. 1 and 3, the device it) includes a face plate l4 rigidly secured to the vertical bight portion of support member l in a suitable manner such as by a pair of rivets I5. As illustrated, the face plate It has a pair of countersunk tapped openings It to provide for the attachment of device 10 to another apparatus or support. As seen best in Fig. 4, the device i0 also includes a pair of side plates I! of insulating material enclosing ,the rectangular opening formed by support member H and insulating support member [3. For ease of assembly, these insulating side members are preferably arranged to be held in place along their lower edges by upturned side portions IQ of a bottom plate 18, and along their side edges by right angle portions 15 of front plate Hi along one edge (see Fig. 8) and indentations 20 in insulating member [3 (one of which is shown in Fig. 3) along the other edge. As

"shown in Figs. 4 and 5, a U-shaped resilient member 2| fits down over the top horizontal leg of frame member ll; member 2| preferably has a plurality of indentations 2m (as shown in Fig. 6) forming projections 211) which snap into corresponding openings in side member ll, thus portions |.'-l form an auxiliary enclosure 11 beneath the lower horizontal member of frame H which houses a bellows 22. The bellows 22 may be connected by an external tube 87 to a source of fluid under pressure which actuates the bellows under conditions hereinafter discussed. The bellows 22 is rigidly secured to bottom plate l8 and when fluid pressure is applied to the inside of the bellows the upper surface 23 thereof expands upwardly. The vertical spacing in the bellows enclosure 'l! is preferably such that only a limited expansion of the bellows can take place before the upper surface of the bellows contacts the frame member I, in order to prevent damage to the bellows in the event of an abnormally high pressure. As an aid in firmly securing tube 81, it may be held by a split bracket 4|] forming a downward right angle extension of plate |8.

The upper surface 23 of the bellows has a rectangular indentation 24 located approximately centrally thereof in which is positioned a rectangular vertical operating rod 25. The operating rod has shoulder portions 25a (see Fig. a short distance above the lower horizontal leg of frame H, and a circular spring retaining member rests on these shoulders. A suitable rectangular indentation is provided in the lower surface of member 25 which engages the shoulders 25a and holds member 25 in place. Circular member 25 also has an elongated slot at right angles to the longitudinal axis of operating rod 25 and to the plane of Fig. 5, this slot being of such dimensions that the lower enlarged portion of operating rod 25 can be readily slipped through the slot in member 25 during assembly. Then when operating rod 25 is subsequently turned 90 to the position shown in Figs. 1, 3 and 5 and the indentation in the bottom surface of retainer 26 is engaged with the shoulder portions 25a, of the operating rod, the retaining ring is prevented from being dislodged.

The device l0 further includes a bracket 21 which is vertically slidable along cross member l2 of the frame. Bracket Zl includes a pair of vertically disposed right angle side members 28 which provide contact with cross member l2 near the top and near the bottom thereof along each side in order to provide a stable sliding bracket. Bracket 2'! also includes a centrally positioned horizontal right-angle portion 29 having an opening 30 therein through which rod 25 operates. A compression spring 3| which opposes the expansion of bellows 22 is positioned between horizontal portion 29 of the slidable bracket 21 and retaining ring 26.

Near the lower portion thereof, the bracket 21 is provided with a second horizontal projection 32 which extends (to the left in Figs. 1 and 3) through a suitable opening 33 in cross member l2. Extension 33 provides means for the vertical movement of bracket 21, and may have a set screw 34 positioned in a suitable tapped opening therein. The set screw 34 rides on a cam 35 (best seen in 2 and 4) which provides for a move ment of bracket 2'! upwardly and downwardly, with consequent changes in the force exerted by spring 3!. In Fig. 2, the cam is shown in the position which allows the greatest rise of bracket 21 thereby producing the minimum compression in spring 4|. In Fig. 4, the cam is shown rotated through almost a complete revolution counterclockwise from Fig. 2 to the position which produces the maximum downward movement of bracket 21 and consequently the greatest compressive force on spring 3|.

. cal cross member I2 of the frame.

The cam 35 may be integrally formed with a shaft 15 which projects through bight portion of support and face plate M. An operating knob may be mounted on the shaft and secured thereto in a suitable manner such as by a set screw 31, the knob thus providing means for rotating cam 35 to adjust the force of spring 3|. Cam 35 also includes an additional projection 38 in addition to the main cam surface, and the purpose of this projection and the operation of the cam are explained in detail hereinafter.

The device H) has a main operating lever 4| which is pivoted on a pair of lanced and formed arms, or bosses, 42 which project out from verti- Member I2 includes an opening 43 to provide for the extension of lever 4| therethrough, and arms 42 may conveniently be formed from metal which previously lay along the sides of this opening. Lever 4| has on opposite sides thereof a pair of downwardly turned portions 44 which rest on bosses 4-2, and each portion 44 is provided with an additional small projection 45 extending downwardly along the inner edge of boss 42 to aid in maintaining the alignment of lever 4|.

The lever 4| has a transversely disposed indentation 45 in the upper surface thereof in which is positioned an enlarged upper portion 41 of operating rod 25. Lever 4| also includes a slot at right angles with the indentation 45, which is similar to the slot previously described for retainer 26, to permit the passage of the lower enlarged portion of operating rod 25 through lever 4| during assembly. In the absence of pressure in bellows 22, the spring 3| exerts a downward biasing force which, acting through the enlarged upper portion 41 of the operating rod in indentation 46, tends to pull the right extremity (as seen in Figs. 1 and 3) of lever 4| downward.

Near its right end, lever 4| has secured thereto in a suitable manner such as by a pair of rivets 86 an extension 48. This extension is of insulating material and extends beyond the end of lever 4| to one side and below lever 4|, as shown best in Figs. 1, 3 and 8. A small projection 49 may be formed integrally with extension 48 on the lower surface thereof, and in the open contacts position shown in Fig. 1 this projection is in engagement with a larger horizontal projection 5|] on insulating support member l3. The engagement of projection 49 with projection 50 limits the downward motion of the right end of lever 4|.

The insulating member l3 may be assembled conveniently with metal frame member II by means of four rectangular projections 5| on member II which pass through four rectangular openings 52 in member I3 near the four corners thereof and are twisted as seen best in Figs. 1, 7 and 8, in order to engage suitable cam surfaces 18 on member l3 and hold the member l3 securely to member II. The upper and lower legs of member I I each have a pair of projections 79 against which member l3 abuts, to provide exact positioning of member l3.

The member I3 supports both the fixed and movable contact members of the device ID. As shown in Figs. 1, 6 and '7 particularly, the fixed contact member 53 is positioned on a right angle terminal member 54. One portion of member 54 is secured to a sunken outer surface portion 8| of member IS in a suitable manner such as by a pair of rivets 55, while the right angle portion of member 54 extends into the device enclosure through a suitable opening 8|] in mem ber l3 and supports fixed contact member 53 in horizontally disposed relation. Terminal member 54 may be provided with a screw 56 by which an electrical conductor such as conductor 51 may be secured thereto in order to provide an electrical connection to the fixed contact of the switching mechanism.

A movable contact member 58 arranged for movement into and out of engagement with fixed contact member 53 is secured to and is supported by a resilient member 59 which is positioned in transversely disposed operative relation with respect to extension 48 of lever 4| and is biased toward contact 53. As illustrated in Figs. 6 and 7 particularly, resilient member 59 has a reversely bent portion 60 overlying extension 48 to which the movable contact member 58 is secured. Resilient member 59 is secured to and supported by a second terminal member 6| which likewise extends through opening 8!! in member l3 and has a right-angle portion secured to the sunken outer surface 8| thereof in a suitable manner such as by a pair of rivets 62. A screw 63 is provided in terminal 62 for an electrical connection to a conductor such as 54.

For biasing lever 4| upwardly, i. e., toward the contacts closed position, there is provided a hairpin spring 65 which is best seen in Figs. 1, 3 and 9. Spring 65 has two indentations, one of which is engaged by a knife edge portion 6% on the right end of lever 4| while the other is engaged by a knife edge portion on a member 68. The latter member may conveniently be secured to the top leg of frame I! by a pair of rivets 69. The member 68 is preferably of resilient material and, as shown in Fig. 9, there may be provided a set screw 10 threaded through a suitable opening in insulating member |3 which provides for the adjustment of knife edge 51 to the left or right to decrease or increase the spacing between knife edges 66 and 61.

The parts of the switching mechanism, and particularly projections 49 and 50, are so portioned and arranged that knife edge 66 on lever 4| is always above a straight line connecting knife edge 61 and the pivot point of lever 4| where projections 44 rest on projections 42, i. e., spring 65 biases the right end of lever 4| continuously upward. Thus spring 65 always exerts a contact closing force on lever 4| tending to force the right end of lever 4| upward to cause projection 48 on the lever to engage the under side of projection 59 of resilient member 59 and force movable contact member 58 up against fixed contact member 53.

The force exerted by spring 65 on lever 4| may be looked upon as having two components, a horizontal component attempting to push knife edges 66 and 6! apart and a vertical component tending to force the right end of lever 4| upward. The latter component increases very rapidly as the right end of lever 4| moves upwardly, and this action aids very materially in causing our mechanism to move from the contacts open position to the contacts closed position with a snap action, as described in greater detail hereinbelow. Similarly, the very rapid decrease of this component of force aids in producing a snap action in the opposite direction of movement, i. e., from contacts closed to contacts open.

A cover plate 82, preferably of insulating material, may be provided for the sunken outer portion of insulating member l3 and secured to member l3 by means of a screw H which engages a suitable tapped opening 12 in the outer surface of member l3. This cover plate protects the terminals but preferably does not cover set screw ll] as it is generally desirable to leave this available for adjustment.

At the opposite, or left end, lever 4| is provided with an enlarged portion 13 having a'pair of slightly upturned edges 13a. In the extreme counterclockwise position of control knob 36, shown in Fig. 4, projection 38 on the cam engages portion 13 of lever 4| and holds it fixed so that the lever cannot be pivoted regardless of the actions of the bellows and the spring members;

As will be readily understood, this is the off position of the switching mechanism and when the knob is in this position the contacts 53, 58 cannot be closed. There is provided a projection 14 extending outwardly from the bight portion of support member into the path of projection 38 which limits the rotation of knob 36 and cam 35 slightly less than a complete revolution. In Fig. 4 the knob and cam are shown in the extreme counterclockwise position, while in Fig. 2, they are shown in the extreme clockwise position. In both extreme positions cam projection 38 is in engagement with projection 14.

The device illustrated and described herein embodying the switching mechanism of our invention may be advantageously used for automatically regulating the temperature of a house-v hold refrigerator, for example. In such a case, the tube 5! may contain a temperature responsive fluid. Upon an increase in the temperature. within the refrigerator to or beyond a predetermined value, the pressure in tube 87 and hence in bellows 22 is increased enough to overcome the downward biasing force of spring 3| sufficiently to allow the right end of lever 4| to be moved upwardly by spring 65. This results in the closing of contacts 53, 58 with a snap action,'

the snap action being due in large measure to the rapid increase in the upward component of the force exerted by spring 65, as discussed previously. Two other features also contribute to the snap action closing of the contacts, namely, the action of resilient member 59 which, as mentioned previously, is biased toward the fixed contact 53 and immediately moves the movable contact member 55 in this direction as soon as it is released, and the hammer blow provided by projection 48 on the under surface of projection 65 beneath movable contact member 58. When the temperature in the refrigerator is reduced sufficiently that the reduced pressure in bulb 22 is overcome by the spring 3|, the right-hand end of lever 4| again moves downwardly. This opens contacts 53, 58, with a snap action due to the rapid decrease in the vertical force com-'- open in the extreme counterclockwise position of cam and knob shown in Fig. i, by the action of cam projection 38 on lever 4|. At all other positions of the cam and knob, the contacts will close when the pressure in bellows 22 reaches a predetermined amount corresponding to a temperature selected by the knob position. These selected pressures and temperatures decrease as the knob and cam are rotated clockwise from the extreme counterclockwise position of Fig. 4.

position toward the contacts closed position, additional spring means biasing said lever continuously toward the contacts open position, said additional spring means normally overpowering said hairpin spring means, and condition responsive means for counteracting said additional spring means to such an extent that said hairpin spring means causes said contacts to close with a snap action.

4. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means for movement between two positons, a fixed contact member secured to said support means, a resilient member having as a portion thereof movable contact means arranged to engage said fixed contact member secured to said support mean in operative relation with said operating lever, said resilient member being biased to move the movable contact portion thereof in a predetermined direction and having means providing for the movement of the resilient member responsively to the movement of said lever, said contacts being engaged in one position of said operating lever and disengaged in the other position, spring means biasing said lever continuously toward a first of its two positions, additional spring means biasing said lever continuously toward the second of its two positions, said additional spring means normally overpowering said first spring means 9 and maintaining said lever in said second position, and condition responsive means for partially counteracting said additional spring means whereby said first spring means is enabled to move said lever from said second position to said first position.

5. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means for movement between two positions, a fixed contact member secured to said support means, a resilient member having movable contact means arranged to engage said fixed contact member as a portion thereof and secured to said support in operative relation with said operating lever, said resilient member being biased toward the contacts closed position and having means providing for the movement thereof responsively to the movement of said lever, said contacts being open in one position of said operating lever and closed in the other said position, spring means biasing said lever continuously toward said other position, said spring means providing an increasing component of force tending to close the contacts as the lever moves from said one position toward said other position, additional spring means biasing said lever continuously toward said one position, said additional spring mean normally over powering said first spring means and maintaining said contacts open, and condition responsive means for counteracting said additional spring means sufiiciently that said first spring means causes said lever to move from said one position to said other position and close said contacts with a snap action.

6. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means for movement between two positions, a fixed contact member secured to said support means, a resilient mem-' ber having movable contact means arranged to engage said fixed contact member as a portion thereof and secured to said support means in operative relation with said operating lever, said resilient member being biased to move the movcomponent of force tending able contact portion thereof in a predetermined direction and having means providing for the movement of the resilient member responsively to the movement of said lever, said contacts being engaged in one said position of the operating lever and disengaged in the other position, nonovercenter spring means biasing said lever continuously toward a first of its two positions, said non-overcenter spring means providing an increasing component of force on the lever aiding its movement as the lever moves from its second position toward the said first position, additional spring means biasing said lever continuously toward the said second of its two positions, said additional spring means normally overpowering said non-overcenter spring means and maintaining said lever in said second position, and condition responsive means for counteracting said additional spring means to such an extent that said non-overcenter spring means causes said lever to move from said second position to said first position.

'7. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, a flexible member secured to said support means in operative relation with said operating lever and having means for loosely receiving a portion of said lever, movable contact means forming a portion of said flexible member and arranged for engagement with said fixed contact member, said operating lever being movable between two positions in one of which it engages said flexible member and holds said contacts engaged and in the other of which it engages the flexible member and holds the contacts out of engagement, said lever being disposed to provide a hammer blow on said flexible member upon movement of the contacts to either the engaged or disengaged position, spring means biasing said lever continuously toward a first of its two positions, additional spring means biasing said lever continuously toward the second of its two positions, said additional spring means normally overpowering said first spring means, and condition responsive means for counteracting said additional spring means to such an extent that said first spring means causes said lever to move from said second to said first position.

8. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, a flexible member secured to said support in operative relation with said operating lever and having means for loosely receiving a portion of said lever, movable contact means forming a portion of said flexible member arranged for engagement with said fixed contact member, said operating lever being movable between two positions in one of which it engages said flexible member and holds said contact apart and in the other or" which it engages the flexible member and holds the contacts in engagement and said lever being disposed to provide a hammer blow on said flexible member upon movement to either of the two positions, spring means biasing said lever continuously toward the contacts closed position, said sprlng means providing a rapidly increasing to close the contacts as the lever moves from the contacts open position towards the contacts closed position, additional spring mean biasing said lever continuously toward the contacts open position, said strain 11 additional spring means normally overpowering said first spring means, and condition responsive means for counteracting said additional spring means to such an extent that said first spring means cause said contacts to close.

9. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, an elongated resilient member secured to said support in operative relation with said operating lever and having means for loosely receiving a portion of said lever, movable contact means mounted on said resilient member and arranged for engagement with said fixed contact member, said resilient member being biased to move said movable contact means in a predetermined direction with respect to said fixed contact member, said operating lever being movable between two positions in one of which it engages said resilient member and holds said contacts engaged and in the other of which it engages the resilient member and holds the contacts disengaged and said lever being disposed to provide a hammer blow on said resilient member upon movement to either the contacts engaged or contacts disengaged position, spring means biasing said lever continuously toward a first of its two positions, additional spring means biasing said lever continuously toward the second of its two positions, said additional spring means normally overpowering said first spring means and holding said lever in said second position, and condition responsive means for partially counteracting said. additional spring means whereby said first spring means is enabled to move said lever from said second position to said first position.

10. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, a flexible member secured to said support means in operative relation with said operating lever and having means for loosely receiving a portion of said lever, movable contact means forming a portion of said flexible member and arranged for engage ment with said fixed contact member, said operating lever being movable between two positions, in one of which it engages said flexible member and holds said contacts engaged and in the other of which it engages the flexible member and holds the contacts out of engagement, said lever being disposed to provide a hammer blow on said flexible member upon movement of the contacts to either the engaged or disengaged position, noneovercenter spring means biasing said lever continuously toward a first of its two positions, said non-overcenter spring means providing a rapidly increasing component of force on the lever aiding its movement as it moves from its. second position toward the said first position, additional spring means biasing said lever continuously toward the said second of itstwo, positions, said additional spring means normally overpowering the said non-overcenter spring means and holding said lever in said second position, and condition responsive means for counteracting said additional spring means to such an extent that non-overcenter spring means moves said lever from said second to said first position with a snap action.

11. An electric switch mechanism comp-rising,

support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, an elongated resilient member secured to said support in operative relation with said operating lever and having means for loosely receiving a portion of said lever, movable contact means mounted on said resilient member and arranged for engagement with said fixed contact member, said resilient member being biased to move said movable contact means in a predetermined direction with respect to said fixed contact member, said operating lever being movable between two positions in one of which it engages said resilient member and holds said contacts engaged and in the other of which it engages the resilient member and holds the contacts disengaged and said lever being disposed to provide a hammer blow on said resilient member upon movement thereof to either the contacts engaged or contacts disengaged position, non-overcenter hairpin spring means biasing said lever continuously toward a first of its two positions, additional adjustable spring means biasing said lever continuously toward the second of its two positions, said additional spring means normally overpowering said non-overcenter hairpin spring means and holding said lever in said second position, and condition responsive means for partially counteracting said additional spring means whereby said non-overcenter spring means is enabled to move said lever from, said second to said first position.

12. An electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, an elongated resilient member secured to said support in transversely disposed relation with said operating lever and having means for loosely receiving a portion of said lever, movable contact means mounted on said resilient member and arranged for engagement with said fixed contact member, said resilient member being biased to move said movable contact means in a predetermined direction with respect to said fixed contact member, said operating lever being movable between two positions in one of which it engages said resilient member and holds said contacts engaged and in the other of which it engages the resilient member and holds the contacts disengaged and said lever being disposed to provide a hammer blow on said resilient member upon movement thereof to either the contacts engaged or contacts disengaged position, non-overcenter hairpin spring means biasing said lever toward a first of its two positions, additional adjustable spring means biasing said lever toward the second of its two positions, saidadditional spring means including an operating rod operatively engaging said lever and positioned substantially normally thereto and a coil spring surrounding said operating rod, said additional spring means normally overpowering said non-overcenter hairpin spring means and holding said lever in said second position, and condition responsive means operatively connected with said operating rod for partially counteracting said additional spring means to such an extent that said non-overcenter spring means causes said lever to move from said second to said first position.

13. A condition responsive electric switch mechanism comprising, support means, an operating lever pivotally mounted on said support means, a fixed contact member secured to said support means, a cantilever spring member se cured to said support means in transversely disposed relation with said operating lever and having a reversely turned portion on the free end thereof for loosely receiving a portion of said operating lever, a movable contact member mounted on said reversely turned portion and arranged for engagement with said fixed contact member, said operating lever being movable between two positions, in one of which it engages said cantilever spring member and holds said contacts in engagement and in the other of which it engages said resilient member and holds the contacts apart, said lever being disposed to provide a hammer blow on said cantilever spring member upon movement to either the contacts closed or the contacts open position, non-overcenter hairpin spring means biasing said lever toward the contacts closed position, additional adjustable spring means biasing said lever toward the contacts open position, said additional spring means including an operating rod extending through an opening in said lever on the same side of the pivot point thereof as said contacts, said rod operatively engaging said lever and being positioned substantially normally thereto, a retaining ring secured to said rod, a slidable spring adjusting bracket, and a coil spring surrounding said operating rod and positioned between said retaining ring and said bracket, said additional spring means normally overpowering said non-overcenter hairpin spring means and maintaining said contacts open, a

bellows engaging an end of said operating rod for partially counteracting said additional spring means to such an extent that said non-overcenter hairpin spring means is enabled to move said lever from the contacts open to the contacts closed position, an adjusting cam engaging said bracket for adjusting the pressure at which said bellows counteracts said additional spring means sufiiciently to enclose said contacts, a projection on said cam for engaging said lever on the opposite side of said pivot to hold said contacts open regardless of the action of said bellows, and adjustment means for said hairpin spring to adjust the differential between the pressure at which said bellows causes said contacts to close and the pressure at which said additional spring means can overpower said hairpin spring means and said bellows to open said contacts again.

ROBERT L. GRAY. CHARLES S. GRIMSHAW.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,011,918 Cheney Dec. 19, 1911 2,458,518 Kohl Jan. 11, 1949 2,537,431 Stickel Jan. 9, 1951

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