Abstract:
A three-position, single-pole, double-throw, rotary ignition switch is disclosed that is water-tight and provides qualitative and quantifiable durability in the presence of high-current loads, even after long use. It withstands a continuous 20 A load, plus an additional occasional 20 A load, when in the “run” position. It withstands an additional 75 A inductive load when in the “start” position. Then even after 12,000 operational cycles, leakage current (with 28 VDC supply voltage) when the switch is in the “off” position, and between non-current-carrying terminals when the switch is in the “on” position, remains under 0.3 mA, and still allows leakage current not exceeding 10 mA at any time while each disconnected pair of terminals and between terminals and ground are exposed to 1,000±5 V rms  at a frequency of at least 60 Hz being increased 400 V/sec for one minute.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrical circuit maker in the form of an ignition switch for a vehicle. More specifically, the present invention relates to a vehicle ignition switch that provides improved durability characteristics for high-current applications. 
     BACKGROUND 
     Military units in the United States and other countries have used HMMWVs (High Mobility Multipurpose Wheeled Vehicles) for moderate- to heavy-duty transport activities for decades. Through those decades it has been found that the high currents passing through the ignition switch during operation of the vehicles (and their accessories) have weakened the insulators used in the ignition switches, resulting in short circuits and other current-leakage failures. In many embodiments, substantially all electrical power that is used in or on a vehicle passes through the ignition switch. There is, therefore, a need for an ignition switch that better withstands long-term use, repeated actuation, and high current flow without yielding to these failures. 
       FIG. 1  illustrates the external form factor of a single-pole double-throw rotary ignition switch  20  used in HMMWV light tactical vehicle (LTV, such as the Joint Light Tactical Vehicle (JLTV) currently being developed by the U.S. military) applications. This technology can also be applied to other vehicles and non-vehicular electrical systems without undue experimentation as will occur to those skilled in the art. 
     Assembly  22  extends from a main body  24  and provides the point of attachment through which torque is applied via a separate handle (ordnance part number 5381088) to change the state of the switch  20 . In this embodiment, stem assembly  22  includes screw  26 , washer  28 , nut  30 , and washer  32 . Torque is applied to switch  20  via a separate handle (not shown) to change the state of switch  20  between an “off” position, a “run” position, and a “start” position. Switch  20  is spring-biased to return automatically to the “run” position from the “start” position. 
     Body  24  has an opening in its end opposite stem assembly  22  that exposes terminals  34 ,  36 , and  38 . Terminals  34 ,  36 , and  38  are held within base  40 , which provides electrical isolation between the terminals  34 ,  36 , and  38 , and between each of them and rubber shell  42 . Rubber shell  42  provides additional insulation and facilitates water-tight connection with the terminals. 
     SUMMARY 
     Some embodiments of the present invention provide improved durability by using an insulator between ignition switch terminals that does not fatigue, cause carbon tracking, or abrade in the presence of normal frictional forces and with the effects of high-current use in the circuitry of a HMMWV, LTV, or other vehicle. Some of these embodiments provide open-circuit resistance across the insulator with dielectric strength sufficient to withstand 1000 VAC and yield leakage not exceeding 10 mA. These embodiments maintain a leakage current that does not exceed 0.2 mA at 28.0 VDC. Each of these tests applies after 12,000 cycles of operation. 
     Some embodiments use PLENCO 01581 as an insulating material, which in these embodiments yields tighter tolerances for dielectric strength, leakage current, and endurance testing based on verification methods described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an ignition switch for use in a HMMWV. 
         FIG. 2  is an exploded, partial-section elevation view of a housing assembly in an ignition switch according to one embodiment of the present invention. Sub-views  2 A,  2 B, and  2 C are elevation views of retaining washer  150 , plate  158 , and drive plate  168 , respectively. 
         FIG. 3  is an plan view of an insulating base component for use with the housing assembly shown in  FIG. 2 . 
         FIG. 4  is a section view of a base-shell assembly for use with the housing assembly shown in  FIG. 2 . 
         FIG. 5  is an exploded view of the overall assembly of the ignition switch embodiment of  FIGS. 2-4 , and  FIG. 5A  is a plan view of a contact assembly  229  used therein. 
         FIG. 6  is a partial section view of an assembled ignition switch according to the embodiment shown in  FIGS. 2-5 . 
         FIG. 7  is an exploded view of a terminal and contact carrier for use in the embodiment of  FIGS. 2-6 . 
     
    
    
     DESCRIPTION 
     For the purpose of promoting an understanding of the principles of the present invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the invention is thereby intended; any alterations and further modifications of the described or illustrated embodiments, and any further applications of the principles of the invention as illustrated therein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Assembly 
       FIGS. 2-6  illustrate an ignition switch according to one embodiment of the present invention. It is a 28 VDC, waterproof, rotary switch that complies with US Military Standard MIL-DTL-13623 (Type II, Class 1). It also conforms to U.S. Military drawing 12506826. It has three positions, or states, the positions being designated “off,” “run,” and “start.” The “start” position is a momentarily held position, and the switch is biased to return to the “run” position from the “start” position when no torque is externally applied to the switch mechanism. 
     Three terminals, in accordance with U.S. Army Standard A-A-52536, are provided and are designated “battery,” “run,” and “start.” When the switch is in the “off” position, no internal conductive path is provided between the terminals. When the switch is in the “on” position, current is supplied from the “battery” terminal to the “run” terminal. When the operator moves the switch to the “start” position, current is also supplied from the “battery” terminal to the “start” terminal. Current continues to be supplied to the “run” terminal as the switch moves from the “run” state to the “start” state and back. 
     The switch is rated to supply a continuous 28 VDC at 20 A to a resistive load on the “run” terminal. The switch is also rated to supply an additional 75 A in surge current to an inductive load on the “start” terminal, and a 20 A “lamp load” of light bulb(s) on either the “start” or “run” terminal (one at a time). When in the “off” position, leakage current through the switch does not exceed 0.2 mA. When in the “run” or “start” position, the voltage drop through electrically connected pairs of terminals does not exceed 75 mV. 
       FIG. 2  shows internal features of a housing assembly for use in an ignition switch according to the present embodiment. Generally, stem  140  extends through housing  124  to provide an external point of access by which torque is applied to the switch mechanism to change its state, such as when a key is turned to put the vehicle in an “on” state, or momentarily in a “start” state. Assembly  122  includes housing  124 , which includes recess  126 , bore  128 , and external threads  130 . In this embodiment, location nub  125  helps locate housing  124  in tooling during assembly. Housing  124  is preferably die cast from zinc according to UNS No. Z33520 in accordance with either SAE J468 or ASTM B 86, and is preferably coated in accordance with U.S. Military drawing MIL-P-53084. 
     Spring  132  is inserted in recess  126 , and ball  134  is placed on it. Spring  132  is preferably alloy coating music wire per ASTM A 228. Ball  134  is preferably made of SAE 52100 chrome steel, Grade 200, with a Rockwell C hardness between about 60 and about 67. Ball  134  and the area around it are then lubricated using Dow Corning® 55 O-Ring lubricant grease. 
     Washer  136  is placed around shaft  138  of stem  140 , which also includes bore  142 , groove  144 , hexagonal feature  146 , and head  148  as features. Stem  140  is preferably SAE 72 CDA  360  half-hard free-machining brass having Rockwell B hardness of at least 80 (and more preferably between about 85 and about 89). Stem  140  is plated per US Federal Specification QQ-P-416F, Type I, Class 3, or zinc plated per ASTM B633 FE/ZN 5 SC2 type I. 
     Retaining washer  150  is placed in groove  144  adjacent to washer  136 , and washer  152  is placed adjacent to retaining washer  150 . O-ring  154  is dipped into Dow Corning® 55 O-Ring lubricant grease and placed around shaft  138  adjacent to washer  152 , and the assembled stem is inserted through bore  128 . The assembled stem is staked into housing  124  to provide a water-tight seal. 
     As illustrated in  FIG. 2B , plate  158  includes as features hexagonal through-hole  160 , notch  162 , notch  164 , and wide notch  166 . The notches  162  and  164  partially receive ball  134  when the switch is in its “off” and “run” positions, respectively, in order to bias the switch toward retaining those positions. 
     As illustrated in  FIG. 2C , drive plate  168  includes curved tab  170  and tabs  172 , center through-hole  174 , and wide notch  176  as features. Positioning plate  158  and drive plate  168  are preferably made of steel with cadmium plating per Federal Specification QQ-P-416, class 3, type I, or zinc plating per ASTM B633 FE/ZN 5 SC2 type I, and the steel from which positioning plate  158  is made is preferably case hardened. Drive plate  168  is preferably added to housing assembly  122  so that through-hole  174  fits over head  148  of stem  140 , and is held in place there by retainer  178 . The assembly is staked in place using a suitable die and press. 
       FIG. 7  illustrates a contact carrier  230  and terminal  240  that are used in this illustrated embodiment. Contact carrier  230  includes a lower portion  238  and upper portion  232  made of a conductive substance such as brass. Transition portion  235  connects the two. Upper portion  232  supports contact  234 . Lower portion  238  defines hole  236  through which terminal  240  is inserted during assembly. Terminal  240  includes stem  242 , shoulder  244 , contact  248  and a silver brazed layer  246  that connects contact  248  to the rest of terminal  240 . Contact  248  is preferably made of 65 Ag 35 WC. During assembly of switch base  200 , there are two instances in which terminals  240  are placed through contact carriers  230 , and in each case shoulder  244  limits the axial movement of terminal  240  at the point where shoulder  244  meets lower portion  238  of contact carrier  230 . This arrangement puts a brazed cap of contact  248  in a substantially even level with contact  234 , and hole  236  in contact carrier  230  fits them into appropriate positions relative to other components of switch base  200  (see  FIG. 4 ). 
     Turning now to  FIG. 3 , base  180  has as features through-holes  182 ,  184 , and  186 , locating feature  188 , and contact carrier location depressions  190  and  192 . Terminal  198  is placed through through-hole  184  having ledge  202  to assist in positioning the terminal axially. As can be seen with additional reference to  FIGS. 4 and 7 , contact carriers  191  and  193  are placed into contact carrier location depressions  190  and  192 , thereby positioning contact caps  195  and  197  as shown. Terminals  194  and  196  are placed through through-holes  182  and  186  defined by the lower portions of contact carriers  191  and  193 . Each terminal  194 ,  196 , and  198  is then shaped mechanically to yield flanges  204 , which complete the positioning and hold terminals  194 ,  196 , and  198  substantially rigidly in place with respect to base  180 . Each contact on terminals  194 ,  196 , and  198  is preferably made of 65/35 silver/tungsten-carbide alloy. 
     Insulating shell  210  is placed over the extended points of terminals  194 ,  196 , and  198 , and retaining washers  212  are placed over each terminal to keep it in place. Base  180  and shell  210  are staked into place with retainers  212  in another die and press operation. Base  180  in the present embodiment is injection-molded from a polyester molding compound such as Plenco 01581 from the Plastics Engineering Company, Sheboygan, Wis. Plenco 01581 has a CTI (see below) of 600, which is within the preferred range (at least about 200) and more preferred range (at least about 500) of CTI values for insulating materials from which these components are made. Shell  210  is a rubber material per MIL-STD-417 of MIL-R-3065 that passes tests 2BC, 617, A14, C12, E034, F19 and Z1, and a dielectric test (see below) at 1000 VAC with no more than 50 mA present. 
     Turning now to  FIG. 5 , spring  214 , preferably four coils of either Zinc plating per ASTM B633 FE/ZN5 SC2 type I or cadmium-plated music wire from which the hydrogen embrittlement has been removed, is inserted into the interior of housing  124  adjacent to drive plate  168 . End  215  of spring  214  is turned in an axial direction and engages plates  158  and  168  as stem  140  is turned. Insulator  216  (molded polymer, such as Plenco 01581) includes recess  218 , which fits over head  148  of stem  140 . Springs  220 , each being 8.5 turns of inconel nicromate X750 wire with closed ends, fit into each of three recesses  222  in insulator  216 . Gasket  224  fits in annular groove  226  of housing  124 , and contact assembly  229  (see  FIG. 5A ) is placed into cavity  228  of insulator  216  into a “Y” area such that each end of contact  229  meets a spring  220 . With each of the three contacts  230  facing away from the springs  220 , base assembly  200  is fitted over the end into a particular groove in housing  124 . While placing base assembly  200  onto assembly  122  to complete the assembly, “Y” contact assembly  229  will be held into place using a suitable tool. The completed assembly is die-pressed into place. 
     Testing 
     Tests have been devised to evaluate the durability of systems that conform to the form factor illustrated in  FIG. 1 , and these have been applied to embodiments of the present invention. One set of tests is for current production quantities and is called the “control test.” This battery of tests includes an overload test, an endurance test, a dielectric strength test, a leakage current test, an insulation resistance test, and destructive disassembly and inspection. Each of these procedures will be discussed herein. Additional quantitative test data for certain materials is noted according to the Comparative Tracking Index (CTI) as defined by Underwriters Laboratories Inc. 
     In the overload test, the switch was energized by a 28.0+/−0.5 VDC source and the switch was exposed to a 75 A resistive load for each switch position through a minimum “on” time of 0.5 seconds with a 10.0+/−1.0 second “off” time repeated for 1000 cycles. Following the specified number of cycles, the load continues to be applied, and the internal voltage drop of the switch is measured between each pair of terminals that is connected in the relevant switch position. The “overload test voltage drop” is defined for purposes of this disclosure to be the greatest of these three measured voltage drops. In 100 tests, the illustrated embodiment has yielded an overload test voltage drop less than or equal to 75 mV in each test. 
     In the endurance test, the switch was energized by a 28.0+/−0.5 VDC source and was connected to the rated (lamp, resistive, and inductive) loads, and the switch was operated through 12,000 cycles. This test was run in accordance with the following sequence:
         Switching from the “off” position to the “run” position, and maintaining the “run” position for 13+/−1 seconds;   Switching to the “start” position and maintaining it for 10+/−1 seconds;   Allowing the switch to move back to the “run” position, and maintaining that position for 60+/−1 seconds; and   Switching to the “off” position for a maximum of 60 seconds of cooling time.   Each cycle shall last a total of 145+/−1 seconds.       

     During this phase of the test, there was no external evidence of malfunction in the illustrated embodiment. After the 12,000 cycles, the terminal-to-terminal voltage was measured for closed switch pairs, and the maximum voltage drop over all of those measurements is defined for the purpose of this disclosure as the “endurance test voltage drop.” The endurance test voltage drop in each test of the present embodiment was less than 75 mV. After the endurance test voltage drop test, the operating torque of the switch was not less than 30 ounce-inches. 
     The dielectric strength test is a variation on U.S. Military Hardware Standard MIL-STD-202G, Method 301, and was performed on a switch that had completed an overload test and a voltage drop test as described above. This test will also be performed after the endurance tests. In each switch position, an AC signal of 1000+/−5 V rms  at 60 Hz was applied between non-current-carrying parts. During each position change, the test was performed from terminal to housing and across each pair of terminals. In each position, the voltage between the terminals was checked at a frequency of at least 60 Hz, and the magnitude of the voltage was increased 400 V/s between terminals and between insulated terminal and ground for at least one minute on each application. Leakage current did not exceed 10 mA. Leakage current exceeding 10 mA would have constituted a failure. Switches were examined visually for evidence of damage such as burning, charring, loosening of components, smoking, or cracking, but no such evidence was observed. 
     The voltage source used for this test was rated to produce at least 0.5 KVA at 1000 VAC. In a series of tests, this voltage was applied between open-circuit contacts, between closed-circuit contacts, and to non-current-carrying parts. Leakage current in each of these situations was measured, and the maximum such current is defined as the “dielectric strength test leakage current.” In actual experiments, the dielectric strength test leakage current in the disclosed embodiment did not exceed 10 mA in any test. Further, there was no visible evidence of burning, charring, loosening of components, smoking, or cracking following this test of the disclosed embodiment. 
     In the leakage current test, the switch was placed in the “off” position, and a potential of 28 VDC was applied between each contact position. Then the switch was put in the “run” position, and the same test potential was applied between the “start” terminal and each of the “off” and “run” terminals. The maximum leakage current in this test is called the “measured leakage current” for purposes of this disclosure. In actual tests, the measured leakage current did not exceed 0.3 mA. 
     Other tests have been performed on this illustrated embodiment and may also be performed on other embodiments of this switch. For example, a shock test may be performed in accordance with MIL-STD-202G, Method 213B, Test Condition G. A vibration test may be performed in accordance with MIL-STD-202G, Method 201A. A corrosion test may include 240 hours of salt water spray exposure in accordance with MIL-STD-202G, Method 101E, Test Condition D. A fungus resistance test may be performed in accordance with ASTM G21 for a continuous 90-day period. An emersion/pressure test may be performed in accordance with drawing 12480561, as defined for a test of a type 1, class 2 device. A thermal shock emersion test may be performed in accordance with drawing 12480561, as defined for a test of a type 2, class 2 device, including a requirement that the switch remain operational before and after being exposed to thermal shock as specified in section 3.2.2 of that document, except that the test is performed for 10 cycles. A sand and dust test may be performed as outlined in MIL-STD-202G, method  110 A, including six hours at 68° F. to 86° F. (20-30° C.), followed by exposure to a temperature of 150° F. to 169° F. (6° C. to 76° C.) for an additional six hours minimum, with sand and dust velocity to the test chamber of between about 1,450 and about 1,950 feet per minute. After each of these tests, the switch according to the described embodiment showed no visible evidence of burning, charring, loosening of components, smoking, or cracking. 
     After the leakage current test, the disassembly and inspection of the switch was performed destructively and was primarily focused on water ingress and failure of internal insulator and base material. Neither charring, fungus, nor evidence of water ingress was visible. 
     When versions of an ignition switch that use prior technology were subjected to environments in situations no more severe than the testing described herein, many such switches caught fire, causing serious damage. The insulation material used in those switches was inadequate for further use in military vehicles. Failures would even occur that resulted in vehicles starting while the switches were in the “off” position, or vehicles continuing to run while in the “off” position. Switches were charring on the inside and becoming non-useable. 
     It has, therefore, been shown that the embodiment illustrated and described herein yields a more reliable ignition switch, and a correspondingly more reliable vehicle. Additional and alternate materials and assembly processes will occur to those skilled in the art in light of the present disclosure as a function of design priorities, including but not limited to cost, durability, environmental concerns, electrical conductivity, and the like. 
     All publications, prior applications, and other documents cited herein are hereby incorporated by reference in their entirety as if each had been individually incorporated by reference and fully set forth. While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.