Patent Application
20090079534
Thermostatic switches, commonly referred to as thermal switches, are engineered for use in high reliability applications such as Space Science Satellites, Defense Satellites, Commercial Satellites, Manned Space Flight Programs and High-Value Terrestrial Applications. The operating and life specifications for thermal switches often require that the switches exhibit a high reliability while operating under extreme conditions such as within Space and Launch Vehicles. In addition, the thermal switches must often meet stringent temperature set point or threshold drift requirements over an operational life of typically twenty or more years.
The conventional thermal switches currently used for the above-identified applications may be bimetallic snap action type. A bimetallic disc is made of two dissimilar metals, where one metal has a low coefficient of thermal expansion and the other metal has a higher coefficient of thermal expansion. The bi-metal material is then punched into discs, formed, heat treated, and tested to meet desired temperature set point requirements.
The bimetallic disc deforms or actuates by changing from a convex state to a concave state at the desired temperature set point, which depends on the difference in thermal expansion coefficients of the two materials forming the bimetallic disc. Thus, the bimetallic disc alternates between a convex state and a concave state as the ambient temperature rises above or drops below the desired temperature set point.
At the set point temperature, the bimetallic disc moves either into or out of contact with a striker pin coupled to an armature, which may be a spring, such as a leaf spring. Depending on the design of the thermal switch, the deformation of the bimetallic disc causes the opening (e.g., open circuit) or closing (e.g., closed circuit) of a pair of electrical contacts or terminals. One example of a striker pin is described in U.S. Patent Publication No. 2004/0263311 (Thermal Switch Striker Pin) and is incorporated herein by reference in its entirety.
The components of the switch, such as the bimetallic disc, the striker pin, the armature, and portions of the terminals are located in a housing or case. The bimetallic disc is positioned between the striker pin and an internal surface of the case. Specifically, the amount of space or offset between the striker pin and the internal surface of the case is closely defined. By way of example, when the bimetallic disc is in the convex state it is in contact under force with the internal surface of the case due to its contact with the striker pin and when in the concave state it is in a free state under little or no force, yet remains in contact with the case.
Consequently, repeated actuation of the bimetallic disc has been known to cause an undesirable amount of wear to the disc, the striker pin, the case, or some combination of each. The amount of wear may become undesirable if it is sufficient to cause the set point temperature to “drift.” For example, the amount of wear may be undesirable if it causes a significant change in temperature in either the opening or the closing of the electrical circuit.
The present invention generally relates to a thermal switch of the bimetallic snap action type having a bimetallic disc. More specifically, the thermal switch includes a disc seat that cooperates with a spacer to retain the bimetallic disc. In addition, the disc seat may be plated with a substantially wear resistant substance to provide a smooth contact surface when in contact with the bimetallic disc.
In one aspect of the invention, a thermal switch includes a case having a substantially planar internal surface; a header assembly located in the housing, the header assembly having a striker pin coupled to an actuator spring; a spacer device concentrically positioned and closely received by the case; a bimetallic disc located in the case and deflectable between a first deflected state and a second deflected state based on whether a temperature of the disc is within a range of a desired set point temperature for the thermal switch, wherein in the first deflected state the bimetallic disc is in contact with the striker pin and in the second deflected state the bimetallic disc is out of contact with the striker pin; and a disc seat have a substantially planar body, wherein at least a portion of the body is plated with a wear resistant substance, the plated portion arranged in the case between the bimetallic disc and the substantially planar internal surface of the case such that the plated portion is in contact with the bimetallic disc when the bimetallic disc is in the second deflected state.
In another aspect of the invention, a disc seat for a thermal switch includes a substantially planar body having at least a portion of the body plated with a substantially wear resistant substance; and a flange coupled to the planar body and having a first shoulder surface and a second shoulder surface spaced apart in a stepped relationship from one another.
In yet another aspect of the invention, a method of actuating a thermal switch includes changing a temperature of a bimetallic disc such that the temperature of the bimetallic disc transitions through a desired temperature set point; and deflecting the bimetallic disc from a first deflected state to a second deflected state, wherein in the first deflected state the bimetallic disc is in contact under force with a disc seat and in the second deflected state the bimetallic disc is in a free state yet remains in contact with the disc seat, the disc seat having a substantially smooth surface plated with a wear resistant substance.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details or with various combinations of these details. In other instances, well-known structures and methods associated with thermal switches, armatures, electrical contacts or terminals, to include the operation thereof may not be shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.
The following description is generally directed to a thermal switch having a low abrasive and wear resistant disc seat for holding a bimetallic disc. The disc seat includes a disc body with a flange extending from a periphery of the disc body. A centrally-located through opening may be located in the disc body to prevent warping of the disc body during its manufacture and to help relieve residual stresses present in the disc seat. In addition, the disc seat may be made from brass where at least a first surface of the disc body is plated with TEFLON® Electroless Nickel, which may take the form of sub-micron particles of polytetrafluoroethylene with auto-catalytically applied nickel.
In one embodiment, a first hermetic glass seal 120 couples one terminal post 116 to the header 114, while a second hermetic glass seal 122 couples the other terminal post 118 to the header 114. An armature spring 124 is coupled to an end portion 126 of the terminal post 116. A stationary contact member 128 is coupled to an end portion 130 of the terminal post 118. A striker pin 132 is affixed to the armature spring 124 and is positioned in a spaced apart relationship from the bimetallic disc 104. In the illustrated embodiment, the bimetallic disc 104 is shown with a convex profile and out of contact with the striker pin 132, which in turn permits a closed circuit configuration where the armature spring 124 is in electrical contact with the stationary contact member 128.
As described above, the bimetallic disc 104 deforms from the convex profile to a concave profile when its temperature is above or below a desired set point temperature, again depending on the design of the thermal switch 100. In the illustrated embodiment, placing the thermal switch 100 in an open circuit configuration is accomplished when the bimetallic disc deforms from the convex profile to the concave profile (not shown). Upon reaching the concave profile, the bimetallic disc 104 contacts the striker pin 132, thus forcing the armature spring 124 to move out of contact with the stationary contact member 128.
The disc seat 110 is a low abrasive disc seat positioned within the case 102 and configured to reduce wear between the bimetallic disc 104 and the case 102. The disc seat 110 may help control a set-off distance 134 between the striker pin 132 and the bimetallic disc 104. Further, the disc seat 110 substantially eliminates much of the complex machining and other costs associated with manufacturing the case 102. In one current case design, the manufacturing of the case 102 requires costly complex dimensional control and a high quality finish where the bimetallic disc contacts the case. These advantages, as well as others, provide a less expensive thermal switch 100 with a lower temperature set point drift.
The temperature set point may be generally defined as the turn on and turn off points of the thermal switch 100. Thus, a drift in the temperature set point may be characterized as a change in the timing of when the thermal switch 100 either turns on or turns off. By way of example, the temperature set point for the thermal switch 100 may be specified to have a set point drift no greater than ±5° F. as measured in degrees Fahrenheit. A number of design and operational aspects may influence the temperature set point and cause an undesirable amount of set point drift over an operational life of the thermal switch 100. Some examples of such design and operational aspects are the bimetallic disc materials, the offset distance 134, the case stability or stiffness, the disc seat stiffness, the surface finish of the disc seat 110, relaxation or redistribution of residual stresses in the structural components of the thermal switch, and the effects of wear and/or abrasion. In recent testing of the disc seat 110 in a thermal switch, the temperature set point drift decreased by about 50% after 100,000 simulated operational cycles compared to the measured drift in a thermal switch without a disc seat 110.
The flange 142 may includes steps or shoulders 152. A first shoulder surface 154 cooperates with the spacer 108 (
In one embodiment, the disc seat 110 is made from brass that has been precision machined and at least the first surface 154 of the disc seat 110 includes TEFLON® Electroless Nickel, which may be applied by plating, coating, embedding, infusing, or some equivalent process. The plated surface 154 may include sub-micron particles of polytetrafluoroethylene (PTFE), such as TEFLON® made by Dupont, with auto-catalytically applied nickel. The resulting plated surface 154 is a dry-lubricated, low friction and low abrasive surface that is substantially hard and wear resistant. Additionally or alternatively, other comparable low abrasive materials may be used.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
