Abstract:
A thermal protector for opening and closing an electric circuit, includes: a movable plate having a movable contact point mounted on a one end portion thereof and having a terminal mounted on an opposite end portion thereof; a bypass member joined to the movable plate at the end portions of the movable plate on which the movable contact point and the terminal are mounted; and a thermal responsive element for moving the movable contact point of the movable plate by a snap action thereof to open and close the electric circuit, wherein the bypass member is formed of a first metal material having a higher conductivity than a second metal material of which the movable plate is formed, and the movable plate and the bypass member are joined together and are subjected to heat treatment so that the first metal material of the bypass member is softened while the second metal material of the movable plate is precipitation-hardened.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The disclosure of Japanese Patent Application No. 2010-049443 filed on Mar. 5, 2010, including the specification, claims, drawings, and summary are incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a thermal protector, more particularly to a thermal protector suitable for a temperature switch which is activated at a high operating temperature such as over 200° C. and has a relatively large current rating. 
         [0003]    There has been known a thermal protector which uses a bimetal element as a thermal responsive element to open and close an electric circuit thereof using an inverting action of the element which occurs at a predetermined temperature. As an example of such a thermal protector, Japanese Patent Publication No. 2844026 discloses a thermal protector including a movable plate which has a movable contact point at a distal end thereof and constituted of a spring material; and a sheet-like thermal responsive element which performs an inverting action and is fixed to a terminal portion of the movable plate. Then, the movable contact point of the movable plate is pressed to a fixed contact point by the distal end of the thermal responsive element. 
         [0004]    Concerning a structure of a relay, Japanese Patent Application Publication No. 05-81983 discloses a system for connecting a contact point with a movable plate mounting portion through a flexible conductive wire in order to prevent the movable plate from being fused when a short-circuit current flows. Furthermore, Japanese Patent Application Publication No. 2004-133568 discloses a configuration of a power switch including a resistor and a contact point both connected in series with a bimetal, in which both ends of the bimetal and the resistor are short-circuited with another switch. As a result, when the bimetal was heated by a current and then inverted, the first switch is closed. 
       SUMMARY OF THE INVENTION 
       [0005]    A material used for a high-temperature thermal protector must be chosen according to a restriction based on a heat resistant temperature of the material itself and a temperature upper limit based on a related Safety Standard. Although as a material of the movable plate of the thermal protector, conventionally, copper alloy, which is representative of low-resistance materials, is usually used, the copper alloy undergoes an excessive deterioration in spring performance if it is placed under high temperatures. More specifically, a usage environment of the copper alloy is restricted to temperatures up to 230° C. and beryllium copper used for the springs cannot be used in such a high temperature range. 
         [0006]    Thus, in the range in which use of the copper alloy is restricted, steel (iron and its alloys) whose restriction temperature is 400° C. is generally used. Of the steels, as a spring material, particularly stainless steel is often used and there are a variety of stainless steel alloys. However, the stainless steels have a large specific resistance in common and when each of them is used for the movable plate of the thermal protector through which a large current flows, a large amount of Joule heat is generated when the current flows, so that an operating temperature of the thermal protector drops remarkably. 
         [0007]    Accordingly, in view of the above-described problems, an object of the present invention is to provide a thermal protector configured to be capable of preventing deterioration of the spring performance of the movable plate under high temperatures, maintaining a contact pressure at a contact point even under high temperatures, and keeping an internal resistance low to prevent the operating temperature from dropping. 
         [0008]    To achieve the above-described object, the present invention provides a thermal protector for opening and closing an electric circuit, including: a movable plate having a movable contact point mounted on one end portion thereof and having a terminal mounted on an opposite end portion thereof; a bypass member joined to the movable plate at the end portions of the movable plate on which the movable contact point and the terminal are mounted; and a thermal responsive element for moving the movable contact point of the movable plate by a snap action thereof to open and close the electric circuit, in which the bypass member is formed of a first metal material having a higher conductivity than a second metal material of which the movable plate is formed, and the movable plate and the bypass member are joined together and are subjected to heat treatment so that the first metal material of the bypass member is softened while the second metal material of the movable plate is precipitation-hardened. 
         [0009]    The second metal material of the movable plate may be stainless steel and the first metal material of the bypass member may be aluminum. The movable plate and the bypass member may have a substantially identical shape in top plan view between the end portions on which the movable contact point and the terminal are mounted. Preferably, the movable plate and the bypass member are not in contact with each other except at the end portions on which the movable contact point and the terminal are mounted. The thickness of the bypass member may be at most four times the thickness of the movable plate. The snap action of the thermal responsive element may be of single-operation type. 
         [0010]    According to the present invention, the movable plate and the bypass member are joined together at the specified end portions thereof and the assembly part of the movable plate and the bypass member is subjected to heat treatment to induce precipitation-hardening of the metal material of the movable plate. Consequently, hardness of the metal material of the movable plate is increased, thereby preventing deterioration of spring performance even when the thermal protector is heated to high temperatures. As a result, the contact pressure of the contact point can be maintained. Because the bypass member having a high conductivity is joined to the movable plate, the internal resistance of the thermal protector is decreased to suppress generation of Joule heat due to conducted current, thereby preventing the operating temperature of the thermal protector from dropping. By joining together the movable plate and the bypass member prior to the heat treatment for the precipitation hardening of the metal material of the movable plate, resistance welding can be executed easily because the metal material of the movable plate has no strong film and the metal material of the bypass member has excellent morphological stability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a plan view showing a thermal protector according to an embodiment of the present invention. 
           [0012]      FIG. 2A  is a plan view showing a bypass member of the thermal protector of  FIG. 1 . 
           [0013]      FIG. 2B  is a plan view showing a movable plate of the thermal protector of  FIG. 1 . 
           [0014]      FIG. 3  is a side sectional view of the thermal protector shown in  FIG. 1  indicating a state in which a movable contact point is in contact with a fixed contact point. 
           [0015]      FIG. 4  is a side sectional view of the thermal protector shown in  FIG. 1 , indicating a state in which the movable contact point is apart from the fixed contact point. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    Hereinafter, an embodiment of the thermal protector according to the present invention will be described with reference to the accompanying drawings. As shown in  FIGS. 1 to 4 , the thermal protector  1  of this embodiment includes mainly a base  10 , a bimetal element  20  which acts as a thermal responsive element, a movable plate  30 , and a bypass member  40 . 
         [0017]    The base  10  is made of an insulating sheet-like member. The bimetal element  20 , the movable plate  30  and the bypass member  40  are mounted on the surface of the base  10 , while a lead wire  12  is mounted on a back surface thereof. To connect the movable plate  30  and the bypass member  40  with the lead wire  12  electrically, the base  10  has a through hole  11  which passes through from a front surface to the back surface. 
         [0018]    The bimetal element  20  includes two metal plates, each having a different coefficient of thermal expansion, which are bonded together. This is a snap-acting type thermal responsive element whose curved direction is inverted instantaneously when the temperature rises up to a predetermined inversion temperature. Preferably, the inversion temperature of the bimetal element is 150° C. or more, and more preferably 200° C. or more. Preferably, the upper limit of the inversion temperature is 350° C. or less and more preferably, 300° C. or less. The bimetal element  20  has a circular outer shape and a central hole  22  is provided in a center thereof. 
         [0019]    The larger a difference between an operating temperature and a restoration temperature, the larger an output of the bimetal element  20  is. By setting the restoration temperature to a temperature which does not allow the bimetal to be restored under an ordinary environment, it is preferable to construct the bimetal element  20  into a single-operation device (SOD) type thermal protector  1  in which the operation thereof occurs once. In this case, preferably, the restoration temperature is, for example, −35° C. or less. The SOD allows an influence of metallic fatigue of a joint portion between the movable plate  30  and the bypass member  40  each made of a different metal material to be ignored. 
         [0020]    The movable plate  30  has a movable contact point  32  on a surface on the side of a base  10  at a distal end thereof. The movable plate  30  has a terminal portion  31  to be fixed to the base  10  at an end portion on the opposite side (hereinafter referred to as terminal side also). The terminal portion  31  has a through hole  33  for connecting to the lead wire  12  on a terminal side electrically. 
         [0021]    The movable plate  30  has a supporting shaft  34  extending vertically from the base  10  at a central portion thereof. To prevent a distal end of the supporting shaft  34  from disturbing the motion of the movable plate  30  due to a contact with a main body portion  30   a  of the movable plate  30 , a clearance hole  35  is provided in the central portion of the movable plate main body portion  30   a.  Further, the movable plate main body portion  30   a  has projecting portions  36 ,  37  projecting toward the base  10  such that they are adjacent to each of the terminal portion  31  and the movable contact point  32 . 
         [0022]    The bypass member  40  has a terminal portion  41  to be fixed to the base  10  and the movable plate  30  at an end portion of a side thereof. This terminal portion  41  has a through hole  43  for connecting to the lead wire  12  on the terminal side electrically. Additionally, the bypass member  40  has a movable portion  42  at an end portion on the opposite side with the movable contact point  32  of the movable plate  30  mounted on an opposing surface. The bypass member  40  has a main body portion  40   a  which is projected in an opposite direction to the movable plate  30  between the terminal portion  41  and the movable portion  42  in order to prevent the bypass member  40  from making contact with the movable plate  30 . 
         [0023]    Although, as shown in  FIGS. 1 and 2 , the main body portion  40   a  of the bypass member has an identical flat plane to the main body portion  30   a  of the movable plate  30 , the embodiment of the present invention is not restricted to this example. For example, the width of the main body portion  40   a  of the bypass member may be larger or smaller than the width of the movable plate main body portion  30   a.    
         [0024]    The movable plate  30  is formed of a precipitation hardening type metal material. The precipitation hardening mentioned here refers to hardening of metal material due to precipitation of fine crystal structure inside of the metal material by heat treatment. As the precipitation hardening type metal material, precipitation hardening type stainless steel is preferable. As the precipitation hardening type stainless steel, specifically, for example, SUS631 or SUS632 is preferable. Of those materials, particularly, SUS631 subjected to CH treatment for inducing precipitation hardening by heat treatment at 475° C±10° C. is preferable (JIS G 4313). In the meantime, when a spring material made of work-hardening type stainless alloy is subjected to heat treatment at 300° C. to 370° C., the spring performance is deteriorated. 
         [0025]    The thickness of the movable plate  30  is not restricted to any particular value as long as the spring performance of the movable plate  30  can be so maintained that the movable contact point  32  mounted at the distal end can contact or leave the fixed contact point  14 . For example, if stainless steel is used as a material, preferably, the thickness of the movable plate  30  is 0.1 mm to 0.2 mm, more preferably 0.15 mm, although it is related to the width of the movable plate and an output of the bimetal element. 
         [0026]    Although the material for the bypass member  40  is not restricted to any particular one as long as it is a metal material more flexible and having a higher conductivity than the movable plate  30 , it is preferred to be formed of aluminum or the like. As an aluminum, high-purity aluminum is preferred. The high-purity aluminum mentioned in this specification refers to aluminum or aluminum alloy whose content of aluminum is 99% or more. More specifically, it is preferred to use aluminum alloys each having an alloy number 1080, 1070, 1050, 1100, 1200 or 1N00 specified under JIS H 4000. 
         [0027]    The thickness of the bypass member  40  is not limited to any particular one as long as the bypass member  40  can maintain a flexibility not blocking the motion of the movable plate  30 . For example, when aluminum is used as a material, the thickness of the bypass member  40  is preferred to be 0.15 mm to 0.5 mm and more preferred to be 0.3 mm to 0.5 mm, although it is related to a width of the plate and an output of the bimetal. Thus, as a criterion, the thickness of the bypass member  40  is preferred to be four times or less the thickness of the movable plate  30 . In the meantime, the bypass member  40  may be formed by placing a plurality of thinner sheet materials than the movable plate  30  one on another. For example, it is permissible to stack three pieces of aluminum plates 0.1 mm in thickness and weld both ends of those sheets to produce the bypass member  40 . 
         [0028]    An assembly method of the thermal protector  1  will be described below. First, the movable plate  30  and the bypass member  40  are joined together. Because this joint requires connection stability under high temperatures, welding is preferable. Because for the movable plate  30 , a metal material prior to heat treatment for precipitation hardening without any strong film is used, resistance welding can be executed. For the bypass member  40  also, a metal material prior to heat treatment for precipitation hardening is used and therefore, a difference in hardness with respect to the movable plate  30  is so small that the resistance welding can be made. As for the welding, as shown in  FIG. 2 , the bypass member  40  is placed over the surface of the movable plate  30  and welded at three joining positions  39 . That is, on the terminal side, the terminal portion  31  of the movable plate  30  and the terminal portion  41  of the bypass member  40  are welded together at two of the joining positions  39 . On a distal end side, the back surface of the movable contact point  32  of the movable plate  30  and the movable portion  32  of the bypass member  40  are welded at one of the joining position  39 . In the meantime, the through hole  33  in the movable plate  30  is matched with the through hole  43  in the bypass member  40 . 
         [0029]    After welding, heat treatment for precipitation hardening is carried out on the metal material of the movable plate  30 . Upon CH treatment of stainless steel of SUS361 as such a kind of the heat treatment, this heat treatment is carried out for an hour at 475° C. ±10° C. according to JIS G 4313. When the precipitation hardening of the metal material of the movable plate  30  is completed, its hardness is increased so that the metal material is processed to a spring material having a large elasticity. On the other hand, in case in which the metal material of the bypass member  40  is, for example, aluminum, it is work-hardened by rolling. However, if the aluminum undergoes a heat treatment at high temperatures of 300° C. or higher, particularly 370° C. or higher in the hardened state induced by distortion generated inside by the rolling, recrystallization is progressed so that the distortion inside vanishes and the aluminum turns into an annealed state and becomes softened to yield flexibility. Such a softened bypass member has a poor morphological stability thereby making it difficult to perform resistance welding. Particularly when a high-purity aluminum is used, the aluminum turns into a full annealed state so that the hardness is decreased up to about 1/10 the initial period. In the meantime, the high-purity aluminum does not need to be age-hardened after the annealing process. 
         [0030]    After both the members are welded together in the above-described assembly process, heat treatment is carried out under the precipitation hardening conditions. As a result, a contact pressure of a contact point necessary functionally as a switch of the thermal protector  1  can be obtained with the movable plate  30  made of precipitation-hardened stainless steel and at the same time, an internal resistance can be greatly reduced by the bypass member  40  which connects the movable contact point  32  to the terminal portion  31  by bypassing the movable plate main body portion  30   a.  Further, the metal material of the bypass member  40  is softened because of its annealed state, thereby stopping the bypass member  40  from blocking the motion of the movable plate  30  and the inversion action of the bimetal element  20 . 
         [0031]    Next, the heat-treated assembly part of the movable plate  30  and the bypass member  40  is attached to the base  10  together with the bimetal element  20  and the lead wire  12 . More specifically, the bimetal element  20  is mounted such that the curve of the bimetal element  20  is projected with respect to the movable plate main body portion  30   a  and the supporting shaft  34  of the movable plate  30  is fit to the central hole  22  in the bimetal element  20  such that the supporting shaft  34  can move through freely. By inserting a terminal pin  13  into the through hole  43  in the bypass member  40 , the through hole  33  in the movable plate  30 , the through hole  11  in the terminal side of the base  10  and a through hole (not shown) in the lead wire  12  on the terminal side, the lead wire  12  on the terminal side is attached. Further, by inserting a fixed contact point pin  14  into the through hole  11  on the movable contact point side of the base and a through hole (not shown) in the lead wire  12  on the movable contact point side, the lead wire  12  on the movable contact point side is attached. 
         [0032]    In the thermal protector assembled as described above, as shown in  FIG. 3 , the movable contact point  32  provided on the distal end of the movable plate  30  makes contact with a head portion of the fixed contact point pin  14 . Because the spring elasticity of the movable plate  30  has been intensified by the heat treatment for the precipitation hardening, a contact pressure between the movable contact point of the distal end of the movable plate  30  and the fixed contact point  14  of the base  10  can be increased and at the same time, the contact pressure can be prevented from dropping even if the thermal protector  1  is used under high temperatures. 
         [0033]    An internal resistance of the thermal protector  1  is reduced largely by the bypass member  40 . For example, in case in which the bypass member  40  is of aluminum, the resistivity thereof is 2.5×10 −8 Ωm and in case in which the movable plate  30  is of stainless steel, the resistivity thereof is 100×10 −8 Ωm. That is, the bypass member  40  has a difference in resistance of 1/40 with respect to the movable plate  30 . Thus, most current flows through the bypass member  40  having a smaller specific resistance. That is, the internal resistance of the thermal protector  1  when the movable contact point  1  is closed is reduced and therefore, the entire resistance can be reduced to about several tens of percent or less although the value depends on the sectional area and the length of the bypass member  40  made of aluminum and conditions of the movable contact point and the terminal portion. Therefore, the thermal protector  1  can conduct a larger current than conventional ones. 
         [0034]    In this thermal protector  1 , as shown in  FIG. 4 , when the bimetal element  20  reaches a predetermined inversion temperature, the bimetal element  20  is inverted and curved so that it is projected against the base  10 . At this time, a central portion of the bimetal element  20  maintains contact with the base  10  and outside edges of the bimetal element  20  keep contact with the projecting portions  36 ,  37  of the movable plate  30 . Consequently, the movable plate  30  is pushed up by the bimetal element  20 , so that the movable contact point  32  is moved to leave the fixed contact point pin  14 . The movement of the movable plate  30  is performed by the elasticity of the movable plate  30 . Because the bypass member  40  has been softened by annealing as described above, the bypass member  40  never obstructs the movement of the movable plate  30  and the inverting action of the bimetal element  20 . 
         [0035]    Many other variations and modifications of the invention will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The above-described embodiments are, therefore, intended to be merely exemplary, and all such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.