Patent Publication Number: US-5530417-A

Title: Thermal cutoff with floating contact member

Description:
BACKGROUND OF THE INVENTION 
     This application relates to the art of electrical contact members and, more particularly, to electrical contact members used in electrical fuses. The invention is particularly applicable for use in thermal cutoffs and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be used in other environments. 
     A known type of thermal cutoff includes a generally mushroom-shaped floating contact member having an enlarged head and an elongated shank. The outer end of the enlarged head is flat and engages a sliding contact. Due to the large area of the outer end surface of the head, the unit engagement pressure between such surface and the sliding contact is relatively low. In some instances, the unit engagement pressure is so low that the electrical resistance is unacceptably high. It would be desirable to increase the unit pressure between a sliding contact and a floating contact member of the type described in order to maintain the resistance thereacross at acceptable levels. In thermal cutoffs of the type described, the elongated shank on the floating contact member has a sharp edge adjacent its outer end. During assembly of the thermal cutoff, the floating contact member may be cocked or tilted when a bushing is inserted for reception over the shank and the bushing may hang up on the sharp edge. It would be desirable to shape the terminal end portion of the shank to minimize the possibility of such hang-ups. 
     SUMMARY OF THE INVENTION 
     A thermal cutoff of the type described has a central depression of the outer surface of the enlarged head. This significantly reduces the surface area of the outer end of the enlarged head and increases the unit pressure between the remaining outer surface and a sliding contact engaged thereby. 
     In a preferred arrangement, the depression in the outer end of the enlarged head is substantially conical and occupies more than one-half of the outer end area of the enlarged head. The conical depression preferably slopes inwardly at an angle not greater than about 15°. Also, the depression preferably has a maximum depth not greater than about 0.010 inch in order to prevent undesirable weakening of the enlarged head. 
     The elongated shank is necked-in adjacent its terminal end opposite from the enlarged head. The necked-in portion is preferably smoothly curved or otherwise tapered for eliminating sharp edges in undesirable locations to minimize the possibility of hang-ups on a ceramic bushing that is assembled over the shank. 
     It is a principal object of the present invention to provide an improved floating contact member for use in thermal cutoffs. 
     It is an additional object of the invention to provide such a contact with an enlarged head having an outer end that is shaped for enhancing unit pressure between such end and a sliding contact. 
     It is another object of the invention to provide such a floating contact member with an elongated shank that is shaped adjacent its terminal end for minimizing the possibility of hang-ups with a ceramic bushing assembled over the shank. 
     It is an additional object of the invention to provide an improved thermal cutoff having an improved floating contact member incorporated therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional elevational view of a prior art thermal cutoff having a floating contact member therein; 
     FIG. 2 is a side elevational view of an improved floating contact member constructed in accordance with the present application; 
     FIG. 3 is a cross-sectional elevational view taken on line 3--3 of FIG. 2; 
     FIG. 4 is an end view of the enlarged head on the floating contact member of FIGS. 2 and 3; 
     FIG. 5 is a cross-sectional elevational view of a thermal cutoff having the improved floating contact member of the present application incorporated therein; 
     FIG. 6 is a partial cross-sectional elevational view showing an alternative shape for the enlarged head on the floating contact member; 
     FIG. 7 is a partial elevational view of the terminal end portion of a shank on a floating contact member; 
     FIG. 8 is a view similar to FIG. 6 and showing another embodiment; and 
     FIG. 9 is a view similar to FIGS. 6 and 7 and showing another embodiment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawing, wherein the showings are for purposes of illustrating certain preferred embodiments of the invention only and not for purposes of limiting same, FIG. 1 shows a prior art thermal cutoff including a tubular metal housing A having a related lead 10 attached to an end wall 12 thereof. A normally solid dielectric thermal pellet 14 is positioned within housing A adjacent end wall 12. Thermal pellet 14 may be of many different materials including caffeine or animal protein and liquifies at a predetermined temperature. 
     A compression spring 18 is compressed between discs 20, 22 that respectively engage thermal pellet 14 and a sliding metal electrical contact 24. Contact 24 has a plurality of circumferentially-spaced resilient fingers 26 resiliently engaging the interior of metal housing A. 
     A generally mushroom-shaped floating contact member B includes an enlarged head 30 and an elongated shank 32. Enlarged head 30 has a flat outer end surface 34 engaging slidable contact 24. The size of enlarged head 30 is minimized for reducing the area of end surface 34 to increase the unit pressure between such surface and sliding contact 24. Minimizing the size of enlarged head 30 requires the use of a tapered coil trip spring 38 that surrounds shank 32 and engages the rear surface of enlarged head 30. The other end of trip spring 38 engages end 40 on a ceramic bushing 42 received in the open end of housing A. Bushing 42 engages an internal shoulder 44 in housing A and the terminal end portion of housing A is crimped inwardly at 46 for securing bushing 42 within housing A. 
     An isolated lead 50 extends through a hole 52 in bushing 42 and has an enlarged contact head 54 thereon with a convex contact end surface 56 that cooperates with terminal end 58 of shank 32. A sealing compound such as epoxy 59 covers the outer end surface of bushing 42 and housing crimp 46, and surrounds a portion of isolated lead 50 to seal the central hole through the bushing. 
     The thermal cutoff is assembled by inserting components one at a time into housing A starting with thermal pellet 14. When floating contact member B and trip spring 38 are positioned within housing A, floating contact member 32 may be off center or may be tilted with shank 32 engaging the inner surface of housing A. Under such circumstances, it is difficult to then insert bushing 42 and its related isolated lead 50 into housing A because the end of bushing 42 will hang up on the end or edge of shank 32. 
     The internal components of the thermal cutoff are normally located as shown in FIG. 1. When the predetermined trip temperature is reached, thermal pellet 14 liquifies allowing compression spring 18 to expand toward housing end wall 12 while carrying disc 20 therewith. The biasing force of trip spring 38 then exceeds the biasing force of now expanded compression spring 18 so that floating contact member 32 and sliding contact 24 are also biased toward housing end wall 12 away from isolated contact 50. Separation of isolated lead contact surface 56 and shank contact surface 58 interrupts the electrical circuit between related and isolated leads 10, 50. 
     Referring now to FIGS. 2, 3 and 4, an improved generally mushroomed-shaped floating contact member C in accordance with the present application includes an enlarged head 60 and an elongated shank 62. Examples of dimensions will be given by example only and not by way of limitation simply to compare the previous floating contact member with the improved floating contact member. Elongated cylindrical shank 62 has a nominal diameter of about 0.060 inches which is the same as the diameter of the prior floating contact member. Circular enlarged head 60 has a nominal diameter of about 0.115 inches compared to a nominal diameter of about 0.105 inches for enlarged head 30 on the prior art floating contact member. Enlarged head 60 has a diameter that is about two times the diameter of shank 62. This means that it is within plus or minus 0.01 inch of two times the diameter of shank 62 (0.110-0.130). 
     A substantially centrally located conical depression 64 is formed in the outer end of enlarged head 60. Depression 64 preferably occupies at least one-half of the area of the outer end of enlarged head 60. This provides an outwardly facing head contact surface 66 that is annular or ring-shaped and located adjacent the outer periphery of enlarged head 60 as shown in FIG. 4. 
     Obviously, depression 64 may take other shapes. The depth of the depression is preferably minimized in order to maintain the strength of enlarged head 60. The depth of depression 64 shown in FIG. 3 is about 0.006 inches and is preferably not greater than about 0.010 inches. 
     The terminal end portion of shank 62 adjacent flat shank outer end 68 is necked-in as generally indicated at 70. The necked-in portion is preferably smoothly curved instead of being tapered along a straight line. The extent of necking-in may vary and in the arrangement shown the diameter of flat end 68 is 0.040 inches. 
     FIG. 5 shows the improved floating contact member of the present invention assembled within a thermal cutoff. The larger diameter of enlarged head 60 makes it possible to use a straight trip spring 78 instead of the tapered spring 38 of FIG. 1. Although it is still possible to use a tapered spring in the arrangement of FIG. 5, assembly is simplified by having a straight spring with a larger opening for receiving shank 62 on floating contact member 64. The reduced end surface area 66 on enlarged head 60 provides a substantially higher unit pressure between floating contact member C and sliding contact 24 to minimize resistance. 
     The necked-in terminal end portion 70 on shank 62 facilitates reception of shank 62 within bushing 42. Even if floating member C is off center or is tilted, the necked-in sloping outer surface of shank 62 provides self-centering action to facilitate positioning of bushing 42 within housing A. 
     FIGS. 6-9 show alternative embodiments. In FIG. 6, an enlarged head 60a has a substantially cylindrical depression 80 therein to leave a small annular projection 82 that provides the outer contact surface on the floating contact member. 
     FIG. 7 shows an alternative shank 62a having a rounded end portion 70a instead of a sharp corner as in the prior art arrangement of FIG. 1. 
     FIG. 8 shows shank 62b as having a tapered terminal end portion 70b intersecting outer terminal end 68. 
     FIG. 9 shows shank 62c having a tapered portion 70c intersecting a small diameter cylindrical portion 71 that extends to terminal end 68. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims.