Patent Application
20090315394
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.
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.
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 housing 24. Rubber shell 42 provides additional insulation and facilitates water-tight connection with the terminals.
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.
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.
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.
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
As illustrated in
Turning now to
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
Testing
Tests have been devised to evaluate the durability of systems that conform to the form factor illustrated in
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:
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 Vrms 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 110A, 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.
