US2547610A - Thermostatic switch - Google Patents

Description

April 3, 1951 E. J. WENTWORTH THERMOSTATIC SWITCH 2 Sheets-Sheet 1 Filed May 10, 195-3 ob Q mm. aw 3 mm an MN Inveizivfl: 122222834 J. W

April 3, 1951 E. J. WENTWORTH 2 Sheets-Sheet 2 Filed May 10, 1950 8 m I\\ QM 8Q wfi. t I R! WT E \J J m. y Iv 11! .z r lll h b I I mm a mm P a Fm. mm

Patented Apr. 3, 1951 THERMOSTATIC SWITCH Elmer J. Wentworth, Framingham, Mass, assignor to Smith Control and Instrument Corporation, Needham, Mass., a corporation of Massachusetts Application May 10, 1950, Serial No. 161,106

5 Claims.

This invention relates to improvements in a thermostatic switch and more particularly to a thermostatic switch used for opening or closing an electrical circuit responsive to temperature changes and essentially of the type which constitutes a self-contained unit and may be described as the tubular type of a thermostatic switch.

A thermostatic switch of the character to which this invention relates essentially comprises a casing or container generally tubular in form and strut members within the case fixed to the opposite ends of the case. The strut members are made of material having a relatively low coefficient of expansion as compared with that of the case. The longitudinal expansion or contraction of the case produces relative movement between the strut members. The electrical circuit connections to the case lead to a pair of electrical contacts within the case. The relative movement between the strut members is effective through suitable operating mechanism to open or close the electrical circuit. Resilient means, such as leaf spring members, must also be used to normally bias the operating means either to close the circuit or to hold the circuit open. Suitable adjusting means is also desirable to preset the switch by loading the resilient means so that the electrical circuit will be either opened or closed as desired at the pre-determined temperature.

Thermostatic switches of this general type are disclosed in the patent granted to Norman J. Smith 2,441,725, issued May 18, 1948 and in the pending application of Norman J. Smith, Serial No. 120,631, filed October 11, 1949. In the Smith patent, No. 2,441,725, a resilient leaf-spring member forms a part of the strut. It is desirable to make the strut of Invar or other material which has a very low rate of thermo-expansion. On the other hand, a spring member which will retain over a long period of time a fixed degree of resiliency can not be made from Invar. The spring element should be made of Inconel or other high grade of spring steel.

It is an object of my invention to provide a thermostatic switch of the type above-described in which the strut members are made of Invar and the resilient member does not form an operating part of a strut and therefore can be made of Inconel.

It is a further object of my invention to provide a pair of strut members fixed to the opposite ends of the case and a lever member extending transversely to the struts which is moved by the relative movement between the free ends of the struts and a spring acting independently of the struts to provide a resilient load upon the lever member.

A still further object of my invention resides in the construction which permits ready assembly of the operating parts, which, it should be understood, are made of relatively small size. The strut member and the leaf-spring member have overlapping ends formed with openings in which may be inserted the T-shaped head of the lever member. The assembly of the lever member thereby serves to lock the ends of the struts and the leaf spring while permitting relative sliding movement between the ends of the struts and the spring member.

Further objects and advantages of my improvements will be more readily apparent from the following description of a preferred embodiment thereof as illustrated in the accompanying drawings: in which,

Figure 1 is a longitudinal section taken through the thermostatic switch and shown on an enlarged scale;

Figure 2 is an elevation showing the head end of the switch in Figure 1;

Figure 3 is a fragmentary horizontal section taken on the plane indicated 3-3 in Fig. 1;

Figure 4 is a horizontal section of the thermostatic switch;

Figure 5is an exploded perspective view showing the association of certain of the operating parts of the thermostatic switch; and

Figure 6 is a fragmentary vertical section showing a modified assembly of the struts and spring with respect to the lever.

Thermostatic switches of the type to which this invention relates may be constructed to be direct acting or reverse acting. In the direct acting type, which is the most common, the electrical contacts are normally biased to closed position and the expansion of the case will produce opening of the contacts. In the reverse acting type, the contacts are normally biased to open position and the expansion of the case will produce closure of the contacts. There is a further division between switches designed to undershoot or overshoot. That is, provision is made for relative lost motion between the strut members after the contacts are closed in the undershoot form, or provision is made for lost motion in the strut members after the contacts are opened in the overshoot form. The direct acting type of switch or the transverse acting type of switch may be made to undershoot or overshoot as desired. Thus there are four basic types of switches to suit special operating conditions. It will be further understood that in my improvements only simple changes in assembly are required to make a switch of the desired type. For example, a compression spring or a tension spring may be used and the lost motion may be made to be effective as desired.

My improvements are directed to a thermostatic switch of a well known type which is usually made of relatively small size and which essentially comprises an expansible case If! and operating parts within the case for opening or closing an electrical circuit. The case lllis preferably tubular in form and is .adapted to expand or contract responsive to temperature changes. The case H) has a head 2 at one end and a bottom closure M at the opposite end. Insulating mountings it are provided in the head for the electrical terminals [8. The inner ends 2!] of the terminals 18 are fitted with electrical contacts 22. It will, therefore, be understood that my switch does not require an insulated wiring within the case.

The pull strut 2 is made of Invar or other material'of relatively low co-efilcient of expansion. The strut M :is formed with an L-shaped end which is suitably welded and brazed to the bottom end M. A second strutmember 28 is provided; the end .of which overlaps the end of the strut 24. The head l2 of the case receives a sleeve 30 into which is threaded the adjusting bolt 32. The "bolt 32'is formed with an internal threaded bore-3 the threads of which are preferably finer than the exterior threads of the bolt 32. A threadedmember-tfi is received in the bore 34 and has an extension 38 which is suitably welded to the strut-member 28. The sleeve member 3!) is formed with slots 49 adapted to receive the fingers Q2 of strut 28. The strut E8 is therefore free .to slide with respect to the sleeve member 30 but is held against twisting. Turning the adjusting bolt 32 in one direction to move the bolt inward with respect to the sleeve 30 will at the same time draw the screw member 35 of strut member 28 into the bolt 38 at a slower rate, thereby adjusting the strut member 28 inwardly of the case with respect to the sleeve 30. Turning the adjusting bolt 32 in the opposite direction will similarly adjust the strutmember 28 by moving it outwardly with respect to the case.

The bow-shapedspring .member is is welded at one end it to the strut .24 near the end 25 of the strut. The opposite end 48 of the spring 4% is flattened and rests against the underside of the strut 2s. The spring-member i l may be made of high grade spring steeL-such as Inconel, since it does not need to have a low rate of expansion relative to the case.

Referring toFigure 5, it will be noted that the strut :28 has an opening 50; the strut 24 has an opening 52 and the spring 44 has an opening Each of these openings, 50, 52 and-54, is made of the same size and shape and it will be noted that in each-case the side edges of the openings or holes which extend'tr'ansversely of the parts are inwardly bowed. Figure 5 also shows the contact lever member '55. The lever 56 has the shoulders 58 which engage the bottom surface of the end '43 of the compression spring 34. The tongue portion 69 of the lever 55 is received in the three openings, 50, 52 and '56. The T-shaped head 62 of .thelever 55 looks on the top surface of the strut 28 as clearly shown in Fig. i. It will be understood that the lever 56 may be inserted at an angle through the holes, st, 52 and 54, and

4 then turned to lock the struts 28, 24 and the spring 44 together.

The space of each of the openings, 50, 52 and St between the bowed side edges is greater than the thickness of the neck portion of the lever 56 to permit a small degree of lost motion. The lever member 56 has a circular opening 64 to fit on the insulated bushing 66 which has a circular projection fitting into the opening 54. A similar insulated bushing 68 is held against the opposite side of the lever 56 and these parts are assembled on a pin 10 which has a riveted head 12. A washer M is interposed between the head 12 and the bushing Q8. The bushing 65 is engaged against the enlarged portion 76 of the pin 10. The pin member iii also serves as a mounting for a dumbbell-shaped common contact member 18 which is formed with two circular contact ends Elil adapted to engage the contacts 22. Thus, the contact member 18 forms a bridge between the contacts .22. Since the lever is free to rock in the openings,.5ll, 52 and 5 as it rests against the bowed sides thereof, the bridging .contact 18 will make a uniform pressure contact against each of the contacts 22.

The switch disclosed in Figure 1 is a direct acting undershoot type of switch. The leaf spring 34 is loaded under compression and the free end 48 of the spring i l is acting to move the lever 62 in a direction to hold the dumbbell bridge contact it against the contact buttons 22. The adjusting screw 32 may be set as indicated in Fig. 1 to oppose the thrust of the leaf spring, thus the struts 2d and 28 will be under an initial tension load. As the case It expands under a rise in temperature, the struts 24 and 28 will not expand at the same rate and the increased load on the struts will finally overcome the force of the leaf spring 24 and move the dumbbell contact l3 away from the contact buttons 22. It will be understood that the engagement of the strut 28 against the lever 55, as shown in Fig. 4, will act as a fulcrum point. The strut 24 will pull on the, lever at a point spaced from the fulcrum point and will oppose the action of the leaf spring.

The compression leaf spring it could be loaded under tension, and by suitable adjustment of the screw the struts could be placed. under a compressive load acting to oppose'the tension of the leaf spring. By this setting of the switch, contraction of the case would decrease the force of the struts tending to oppose the spring and allow the spring to open the contacts. This would be a switch of the direct acting overshoot type.

In order to make the switch of the reverse acting type, it is only necessary to assemble the strut 28 between the spring 46 and the strut 24 as shown in Fig. 6. In this case, the spring 24 is a tension spring tending to open the contacts and. the strut 2% is acting to oppose the opening of the contacts. The strut 28 will be acting as a fulcrum between the spring it and the strut 24. Expansion of the case will increase th load on the strut 24- and cause the lever 56 to rock in a direction to close the contacts. The reverse acting type of switch may also be made with a compression loaded spring with the struts also under a compressive load acting to oppose the spring.

I claim:

1. In a thermostatic switch, a tubular case adapted to expand or contract responsive to 'a change in temperature, a circuit closing lever within said case extending transversely thereof and adapted to rock toopen or close an electrical circuit, a short strut fixed with respect to one end of said case and adapted to form a fulcrum for said lever, a second longer strut fixed to the opposite end of said case and adapted to act upon said lever at a point spaced from the fulcrum of said short strut, said strut members having a coemcient of expansion less than that of said case and a leaf spring within said case acting upon said lever to oppose the force of said second strut, whereby said lever will be moved in one direction when the load on said second strut overcomes the force exerted by said leaf spring.

2. In a thermostatic switch, a tubular case adapted to expand or contract responsive to a change in temperature, a lever member mounted for rockable movement within said case and extending transversely thereof, means movable by said lever to open or close an electrical circuit leading into said case, a short strut fixed with respect to one end of said case and adapted to form a fulcrum for said lever, a second longer strut fixed with respect to the opposite end of said case and arranged to act upon said lever at a point spaced from the fulcrum of said short strut, said struts having cooperating openings to detachably receive said lever and a leaf spring acting upon said lever to oppose the force of said second strut, whereby said lever will be moved in one direction when the load on said second strut overcomes the force exerted by said leaf spring.

3. In a thermostatic switch, a case having a head closure at one end and a bottom closure at the opposite end, a pair of electrical circuit terminals mounted in the head end of the case, a pair of fixed electrical contacts supported by said terminals within the case adjacent the head end,

a pair of strut members supported by the opposite ends of said case, said strut members having overlapping relatively slidable ends within the case, the overlapping ends of said struts having openings adapted to receive a lever member extending transversely with respect to said struts,

a bridge contact member supported by said lever member adapted to engage said pair of contacts and resilient means separate from said struts acting upon said lever member, whereby expansion of the case will produce relative movement between said struts to tilt said lever and move said bridge contact member with respect to said fixed electrical contacts.

4. In a thermostatic switch, a tubular case having a head closure at one end, a bottom closure at the opposite end, a strut member fixed to the bottom member of said case, a second strut member adjustably mounted at the head end of said case for relative slidable movement with respect to first strut, said struts having overlapping ends, a lever member supported by the overlapping ends or" said struts and extending transversely with respect thereto, a pair of circuit terminals mounted in the head end of said case, contacts carried by said terminals within said case, a bridge contact member supported by said lever and adapted to engage said contacts to complete the electrical circuit and resilient means normally effective to move said lever in a direction to close the electrical circuit, whereby expansion or" the case will produce relative movement between said struts to tilt said lever and open said circuit.

5. In a thermostatic switch, a tubular case, a head closure at one end of said case and a bottom closure at the opposite end of said case, a first strut member fixed to the bottom end of said case, a second strut member supported from the head end of said case, said strut members having relatively overlapping ends, means for adjusting the position of the second strut member longitudinally of said case, a bow-shaped compression spring fixed at one end adjacent the bottom end of said case, said spring member having its free end slidably associated with respect to the inner ends of said struts, said struts and the free end of said compression spring having openings formed therein, a lever member having a T-shaped head adapted to be received by the openings in the ends of said struts and said spring, said lever forming a lock to hold the ends of said struts and said spring in slidable overlapping position and means to open and close an electrical circuit within said case responsive to expansion or contraction of said case producing relative movement between said struts and tilting of said lever.

ELMER J. WENTWORTH.

No references cited.

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