US2441725A - Thermostatic switch - Google Patents

Description

May 18, 1948. N. J. sMlTH THERMOSTATIC SWITCH v Filed Aug. 15, 1944 2 Sheets-Sheet 1 nM z mb Q1 Q Qbv mmh. Sv v .N

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2 Sheets-Sheet 2 Uvafaaqys May 18, N 1 SMITH THERMOSTATIC SWITCH Filed Aug. 15, 1944 ONM. ik im. ...l wm Il .94.3 1i l mh Iaaveiaoa.- y Normas@ J'Smiu Patented May 18, 1948 UNITED STATES PATENT OFFICE THERMOSTATIC SWITCH Norman James Smith, Medileid, Mass.

Application August 15, 1944, Serial No. 549,518

12 Claims. l

or contact members within the case have a relatively low rate of thermal expansion relative to the case or shell. The longitudinal expansion or contraction of the case is effective to produce relative movement between the contacts in a direction transverse to the axis of the case. In one type, the switch may be designed to open an electrical circuit upon reaching 'a predetermined temperature. The contacts are held normally closed under a compressive force sufficient to prevent vibration which might result in defective operation. The relation between the longitudinal movement of the case and the lateral movement between the contacts constitutes the multiplication factor; for example, if the multiplication is 20 to l and the contacts are normally closed under a force of two pounds, a longitudinal force of 40 pounds is required to open the contacts.

It is desirable to have a relatively high multiplication factor in order that the switch will operate within a relatively small thermal difference and it would also be desirable to 4have a switch in which such multiplication factor may be adjusted.

In a common type of thermostatic switch of this general character, the contact members comprise a pair of bowed leaf springs. In such a switch, increasing the multiplication factor requires increasing the length of the case which may be commercially impracticable and such a switch cannot be designed for adjustment of the multiplication factor after it has been constructed.

Thermostatic switches of this general type are designed to undershoot" or overshoot; that is, provision is made for relative lost motion between the contact members and the case; for example,

' in one type of switch, the contacts are biased to open position. and held closed by compression in assembly, expansion of the case permits opening of the contacts and thereafter the case is free to expand or cvershoot without further separa- (Cl. 20G-137) Lwith the desired degree of pressure.

tion between the contacts. Similarly, in a switch of the type having contacts normally biased to closed position, the case is free to contract or undershoot after the contacts have been closed Such lost motion is usually provided between the contact carrying members and one end of the case in which they are mounted.

It is an object of the present invention to provide a novel type of thermostatic switch in which the contact carrying members are designed to operate on a new principle to permit variation in the multiplication factor without modifying the size or shape of the case and whereby the multiplication factor may be adjusted after the switch has been manufactured and before or after installation.

More specifically, it is the object of my invention to provide a thermostatic switch in which one contact carrying element is fixed to one end of the case and the other contact carrying element is mounted at the other end of the case, the two contact carrying elements being connected within the case by a transverse strut or post which is caused to deiiect or tilt to produce the relative lateral movement between the electrical contacts.

It is further an object of my invention, to provide for adjustments of the transverse post which connects the contact carrying elements to vary the transverse separation between elements.

It is another object of my invention to provide a thermostatic switch in which one of the contact carrying elements is constructed with relatively movable parts to permit the desired liberty of lost motion for an "undershoot or overshoot type of thermostat.

It is still further an object of my invention to provide for holding the contact carrying elements against rotation with respect to the case by means associated with the ends of the case rather than by a special configuration of the case itself.

It is another object of my invention to provide a novel type of adjustment for initial loading of the thermostatic switch by providing an adjusting screw having internal and external threads of differential pitch, the screw having relatively coarse external threads and relatively fine internal threads for setting a thermostat of the compression overshoot type having contacts normally biased to open, and the screw having relatively fine external threads and relatively coarse interlnal threads for setting a thermostat of the tension overshoot type having contacts normally biased to closed position.

Additional objects an advantages of my improvements will be more readily apparent from the following description lof preferred embodiments of my invention as disclosed in the attached drawings, in which;

Fig. 1 is a longitudinal sectional view taken on the plane indicated I-I in Fig. 4 through a thermostatic switch of the compression type in which the contacts are normally biased to open;

Fig. 2 is a. similar view showing a modified construction in which there is provision for adjustment of the multiplication factor;

Fig. 3 is a longitudinal sectional view taken on the plane indicated 3--3 in Figure 'l of a thermostatic switch of a type in which the contacts are normally biased to closed position, but are opened on assembly and tendto close on expansion of the case;

Fig. 4 is an end view of the thermostatic switch shown in Fig. 1 looking towards the right hand end and showing the adjusting screw;

Fig. 5 is a cross-sectional view taken on the plane 5-5 in Fig. 1;

Fig. 6 is a cross-sectional view on the plane indicated 6-8 in Fig. 2;

Fig. 'I is an end view of the thermostatic switch shown in Fig. 3 looking at the right hand thereof and illustrating the provision of means for locking the adjusting screw;

Fig. 8 is a cross-sectional view on the plane indicated B--8 in Fig. 3;

Fig. 9 is a fragmentary longitudinal section at one end of a switch of the tension undershoot type in which the contacts are normally biased to closed position;

Fig. 10 is a diagrammatic view showing the switch elements normally biased to open position and the forces which cause the contacts to close in assembly; and

Fig. 11 is a diagrammatic view showing the switch elements normally biased to closed position and the forces which act on the elements.

Referring to Fig. 1, there is shown a case or shell I0 having a bottom end I2 at the left hand end and a head I4 at the right hand end, the bottom end I2 has a screw threaded stud I8 mounting the turned up end I8 of the strut 20 which constitutes one contact carrying member. The part I8 is fixed against rotation with respect to the stud I6. Above the strut there is mounted a second strut or contact carrying member 22, the right hand end of which is received in the clevis 24 of the longitudinally ad.- justable pin 26 which is threaded into the adlusting pin or bolt 28. The pin or bolt 28 is threaded into the head I4. The pin 26 is held against rotation with respect to the case I0 by the provision of a vertical pin 30 which passes through the clevis 24 and through an opening in the end of the strut 22. The pin 30 is free to slide longitudinally but is held against rotation by being received in the clevis portion 32 of a stud 34 which is fixed to the head I4 by the rivet 36.

The external threads 38 of the adjusting pin 28 are shown coarser than the internal threads 40 which engage the pin 28. It will be understood, therefore, that turning of the pin 28 to cause it to thread into the head I4 and move in an inward direction will cause the pin 28 to thread into the socket of the pin 28 but that due to the differential in pitch between the external and internal threads, the pin 28 will also move inwardly with respect to the case I0.

The strut 22 has a pair of slots 42 and 44. A

4 bar or plate 48 is fastened to the strut 22 by the pins 48 and 80 which are received in the slots 42 and 44 respectively. 'I'he pin 80 also serves as a connection for the right hand end of the strut 20, the portion 82 of which is somewhat angularly disposed with respect to the strut 22. At the left hand end of the case, electrical contact members are mounted on the struts 20 and 22 as clearly shown in Fig. 5 in which a clip 84 carrying a contact 88 is fastened to the strut 28 but is separated therefrom by insulation 58 and a clip y8l) having a contact 82 is similarly fastened to the left hand free end of the strut 22 being separated therefrom by insulation 84. An electrical cable 88 is attached to theclip 54 and extends outwardly from the case through an opening 88 in the head I4. A cable Iii is similarly attached to the clip and extends through an opening I2 in the head I4. The' struts 28 and 22, as well as the bar 46 which forms a part of the upper strut 22 are composed of Invar or other ralloy having a relatively low rate of expansion. The switch shown in Fig. 1 is a compression type. During assembly, the strut or contact carrying member 22 is bent between the pin III and the pin 38 in such a manner that it is resiliently biased to open position of the contacts. Then by screwing in the adjusting pin 28, the contacts are caused to V close by compression. This will be more clearly understood by referring to Fig. 10 in which the strut 22, pin 50 and strut 20 are diagrammatically indicated. The arrows represent the compression forces oppositely applied to the struts 20 and 22 which tend to close the contacts. The screw 28 is adjusted so that the expansion of the case I0 will permit the contacts to open at the desired temperature. screwing the pin 28 inwardly with respect to the case will increase the set temperature. In Fig. 1 the switch-is close to the break point. Futher expansion of the case will cause the contacts 58 and 62 to open. After the contacts have opened, the case may further expand without further separation of the contacts since the strut 22 is free to move with respect to the pins 48 and 50. The slots 42 and 44 permit overshoot.

Fig. 9 shows a modified type of thermostatic switch very similar in construction to that of Fig. 1. It will be noted that the slot 44a in strut 22a is on the other side of the pin 80a and the fadjusting pin 28a has fine exterior threads 38a and coarse interior threads 40a. In this type, the strut 22a is bent during assembly to be resiliently biased to close the contacts. Screwing in the adjusting pin 28a will cause the pin 28a to thread into the pin 28a faster than the pin 28a moves into the case. Therefore, the struts 20a and 22a will be placed under tension as indicated by the arrows in Fig. 11 and the resilient pressure between the contacts will be relieved. Further expansion of the case will open the contacts. As the case contracts, the contacts close and after the set point has been passed the case may further contract without increasing the pressure or bending the strut 22a since the strut 22a is free to move with respect to the pin 50a.

Fig. 11 also illustrates the effective lever arm I1 is the arm between the pin 50a and the contact 82a. I2 is the vertical or lateral arm formed by the pin 50a. I may be made quite small with respect to I1 so the multiplication may be 20 to 1 or even as high as 40 to 1. Thus, the longitudinal expansion of the case may be highly multiplied to produce rapid separation of the contacts at the break point.

It may be very desirable to provide for adjustment of the multiplication factor to make the switch more or less sensitive or responsive to small changes in temperature. I have illustrated such. a switch in Fig. 2. The construction in most respects is quite similar to Fig. 1 and the same reference characters are used so far as possible. A U-sh-aped bar Il replaces the bar Il. The strut 22 is pinned to the upper leg of the bar Il by the pins 4I and Il while the strut 2l is pinned to the lower leg of the bar 8l by the pin l2. The free ends of the U-shaped bar 8l are adjustably spaced by a screw threaded pin Il. The upper end of the pin u is accessible for adjustment by vremoving a cover applied to an opening in the case. The strut 22 has the same slots 42 and Il to receive the pins 4l and il while in addition a slot Il is provided for the screw pin Il. The operation is the same as in Fig. 1. The switch may be made a tension undershoot type as well as the compression overshoot Aas shown. During assembly or after being installed. the multiplication factor may be adjusted by turning the screw Il.

It may be desirable to have the contacts open at normal temperature but arranged to close on an increase in temperature. Buch a switch is shown in Fig. 3. The construction is in many I' respects similar to Fig. 1. The pin or'stud i0 at the bottom end ofthe case mounts' a strut ll which has an extension t2 to which is attached a contact u, facing downwardly. The other strut l is similar to the strut I! in Fig. 1 and carries a contact Il facing upwardly to cooperate with contact Il. The strut is fastened to a bar IM by pins |02 and IM. The pin III serves to hold one end of the strut lll. It will be understood that the strut It is bent during assembly to resiliently press the contacts I4 and il together. but screwing in the adjusting pin II will place the struts under compression and open the contacts. Expansion of the case will allow the contacts to close as the compression or initial loading is relieved.

Fig. 3 also shows an arrangement which may be used for locking the adjusting pin 2l. A pair of plates I0! and ill are threaded on the pin 2l. The plates I" and Ill are squeezed together to tighten and bind on the screw 2l by tightening the screws H0 which pass freely through the outer plate i but thread into the inner plate I. The inner ends of the screws lll are received in the holes il! bored in the head il.

In all forms of the thermostatic switch one of the struts or contact carrying elements forms in effect an L-shaped lever since the pin to which the other strut or contact element is connected is rigid with the first strut in transmitting movement by tension or compression acting through the second strut. The spring rating is determined by the cross-section of the first strut between the lever arm pin and the supported end of the first strut and to a minor extent by the spring rating of the second strut. It should be noted that the contact clips are fastened on flat portions of the struts and do not effect the spring rating.

I claim:

1. In a thermostatic switch, a support. a pair of struts mounted on said support, cooperating electrical contacts on -said struts, said support having a thermal expansion rate greater than said struts, one of said struts having a relatively short transverse lever arm associated with said 6 Y v strut whereby said strut and said lever arm forms a rigid L-shaped lever strut, said L-shaped lever strut being resiliently mounted with respect to said support. and the 4other of said struts constituting a resilient leaf spring member fixed at one end to said support and at the other end to the short arm of said L-shaped lever strut whereby diiferential expansion of said support with respect to said struts will cause relative movement between said contacts. t'

2. In a thermostatic switch, a casing having a bottom end and a head, a leaf spring member fixed to the bottom end of said casing and ex tending towards the head thereof. a contact carried by said leaf spring member near the fixed end thereof. a lever member having a long arm and a relatively short transverse arm associated therewith. said leaf spring member being fixed to the short arm of said lever member. a contact carried by the long arm of said lever member and adapted to engage the first mentioned contact. said lever member having a rela'tlvely short resilient extension from the juncture between said lever arms for mounting said lever on the head of said casing, said casing having a thermal expansion rate greater than said leaf spring and said lever, said lever being initially biased by bending said resilient extension towards separation of said contacts, means for longitudinally adjusting the mounting of said lever with respect to said head to cause said contacts to close, whereby said contacts will open at a predetermined temperature as the casing expands. l 3. In a thermostatic switch. a casing having a bottom end and a head, a leaf spring member fixed to the bottom end of said casing and extending towards the head thereof, a contact carried by said leaf spring member near the fixed end thereof, a lever member having a long arm and a relatively short transverse arm associated therewith, said leaf spring member being fixed to the short arm of said lever member, a contact carried by the long arm oi' said lever member and adapted to engage the first mentioned contact, said lever member having a relatively short resilient extension from the juncture between said lever arms for mounting said lever on the head of said casing, said casing having a thermal expansion rate greater than said leaf spring and said lever,v said lever being initially ybiased to close said contact and means for longitudinally adjusting the mounting of said lever with respect to said head to adjust the pressure between said contacts whereby said .contacts will open at a predetermined temperature as the casing exbands.

4. In a thermostatic switch, a casing. a contact carrying member mounted in said casing and supported at one end thereof, a second contact carrying member mounted in said casing and supported at the opposite end thereof and means for producing relative lateral movement between the contacts oiv the contact carrying members by relative longitudinal movement between the supported ends of said members comprising a substantially rigid lateral lever arm associated with one of said members and a connection between the other member 'and said leverarm whereby oppositely directed f o r c e s acting through said members at spaced points on said lever arm caused by differential thermal expansion of said casing with respect to said members conttsrol the lateral movement between said contac 5. In a thermostatic switch, a tubular case having a bottom end and a head, a rst strut fixed to the bottom end of said case, a second strut mounted on the head oi' said case, a pair of contacts on said struts near the bottom end of said case, means xing said first strut to said second strut near the head of said case and forming in eiect a. relatively short lever arm between said struts. one of said struts having a slot to receive said means whereby oppositely directed forces acting at spaced points through said struts on said means will produce relative lateral movement between said contacts up to a predetermined point after which relative longitudinal movement between said struts may take place as said means is free to move with respect to said slot.

6. In a thermostatic switch, a tubular case having a bottom end and a head, a rst strut ilxed to the bottom end of said case, a second strut mounted on the head of said case, a pair of contacts on said struts near the bottom end of the case, means for fixing said rst strut to one side of said second strut near the head of said case whereby said means forms in eiect a relatively short lever arm between said struts and relative longitudinal movement between the ends of said case will produce oppositely directed forces acting on said lever arm to cause relative lateral movement between said contacts at a multiplied rate.

7'. In a thermostatic switch, a tubular case, a pair of contact carrying struts fixed respectively to the opposite ends of said case, 'means for causing relative longitudinal movement between the ends ci' said case to produce relative lateral movement between said contacts at a multiplied rate and means for adjusting the multiplied rate to vary the sensitivity of the thermostatic switch.

8. In a thermostatic switch, a tubular case, a pair of contact carrying struts fixed respectively to the opposite ends of said case, means for causing relative longitudinal movement between the ends of said case to produce relative lateral movement between said contacts at a multiplied rate and means accessible from the exterior o! the case for adjusting the multiplied rate to vary the sensitivity of the thermostatic switch.

9. In a thermostatic switch, a tubular case, a pair of contact carrying struts xed respectively to the opposite ends of said case, means for causing relative longitudinal movement between the ends of said case to produce relative lateral movement between said contacts at a multiplied rate and a screw threaded connection between said struts adjustable to change the multiplied rate to vary the sensitivity of the thermostatic switch.

10. In a thermostatic switch, a tubular casing having a bottom end and a head, a first strut fixed to the bottom end of said casing, an adjusting screw threaded into said head a second screw threaded into said adjusting screw, a second strut fixed to said second screw, a pair oi' cooperating contacts carried by said struts near the bottom end of said casing, means for fixing said first strut to said second strut and forming in eiect a short lever arm and means mounted on said head for holding said second screw against rotation while permitting relative longitudinal movement with respect to said head.

11. In a thermostatic switch, a casing, 9. pair of contact carrying elements mounted in said casing, means for connecting one of said elements to one end of said casing, means for connecting the other of said elements to the opposite end of said casing, a connection between said elements intermediate the ends of said casing and means at said connection for permitting limited longitudinal movement between said elements.

12. In a thermostatic switch, a casing, a pair of contact elements in said casing responsive to longitudinal expansion of .said casing by relative lateral movement between the contacts of said elements, one of said elements being composed of two parts. and means for fastening said parts together to permit limited longitudinal movement between the parts of said element.

NORMAN JAMES SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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