US2756355A - Radio shielded magneto - Google Patents

Description

July 24, 1956 R. E. LEAR ET AL RADIO SHIELDED MAGNET Filed Nov. 18, 1954 INVENTOR5 R. E. LEAR BY B.A. GLASSMACH ER United States Patent RADIO SHIELDED MAGNETO Robert E. Lear and Bernard A. Glassmacher, Columbus, Ohio, assignors to Surface Combustion Corporation, Toledo, Ohio, a corporation of Ohio Application November 18, 1954, Serial No. 469,675

8 Claims. (Cl. 310153) This invention relates to a radio shielded magneto, and more particularly, to such a magneto having substantially reduced size and weight as compared with previously known radio shielded magnetos.

In many aircraft installations, for example, the use of magnetos is advantageous. As a specific example, magnetos are used in ground heaters to generate an E. M. F. for providing an igniting spark. Such ground heaters are frequently used at military air bases, as well as at commercial air fields. The use of previously known magnetos in such heaters has been impossible because of radio interference unless the entire magneto was enclosed in a suitable shield which blocked the air transmission of energy from the magneto. The presence of such a shield around a magneto greatly increases the difiiculty of servicing, and adds substantial bulk and weight to the unit. Particularly in the case of portable ground heaters, which must sometimes be carried by air, for example to advance bases, the extra bulk and weight of the shielding are also objectionable.

The instant invention is based upon the discovery of a radio shielded magneto having virtually the same bulk and weight of a corresponding unshielded magneto, and one where there is no problem of servicing because connections are made in the usual manner directly to the magneto, rather than through a surrounding sheath or shield.

It is, therefore, an object of the invention to provide an improved radio shielded magneto.

Other objects and advantages of the invention will be apparent from the description which follows, reference being had to the accompanying drawings, in which- Fig. l is a plan view of the stator of a magneto according to the invention, showing various electrical parts attached thereto;

Fig. 2 is a vertical sectional view generally along the line 2-2 in Fig. 1, showing the assembled magneto; and

Fig. 3 is a view in vertical elevation of the exterior of the stator of the magneto.

Referring now more particularly to the drawingsythe specific radio shielded magneto according to the invention shown in Figs. 1 through 3 comprises a flywheel or rotor 11, and a stator 12 carrying various electrical apparatus conventional in magnetos, as well as part of the shielding mechanism according to the invention.

The main essential electrical apparatus carried by the stator comprises a pair of coils 13 wound on separate arms of a core 14 attached to the stator by machine screws 15, a grounded contact 16, a second contact 17 urged toward the contact 16 by a spring 18 electrically connected thereto, and supported on an insulating base 19 pivotable around a stud 20. A wire 21 connects one of the coils 13 to the spring 18; a wire 22 connects a condenser 23 to the spring 18; and a shielded wire 24 connects an operating switch member (not illustrated) to the spring 18. The Wires 21, 22 and 24 are retained in position by a machine screw 25. An E. M. F. is generated in the coils 13 from which it is available through wires 26, for ex- 2,756,355 Patented July 24, 1956 ample for providing a current to energize igniter plugs, by virtue of rotation of the rotor 11 which carries permanent magnets 27. The portions of the magneto of Figs. 1 through 3 thus far discussed, and the operation, are conventional, and constitute no part of the instant invention.

The radio shielding of the magneto of Figs. 1 through 3 comprises, in the specific embodiment of the invention shown, a knitted Monel gasket 30 retained by a circular angle member 31 mounted on the stator 12 by machine screws 32, in electric and mechanical contact with an outer surface 33 of the stator. As will be seen by reference to Fig. 2, the knitted Monel gasket 30, when the magneto is in operation, makes rubbing mechanical and electrical contact with the rotor 11. It has been found that merely the provision of a knitted Monel gasket 30 positioned and supported as shown in Fig. 2, in mechanical and electrical contact with the stator and in rubbing mechanical and electrical contact with the rotor, accomplishes substantial radio interference reduction in the frequency range from 0.15 megacycle to megacycles. The knitted Monel gasket has a service life in excess of 250 hours of continuous operation, and is, therefore, within 1954 specifications set by the United States Air Corps. It has also been found that other gasket materials can be substituted for the knitted Monel gasket 30, with similar effect upon radio interference suppression. For example, a solid copper gasket has been substituted for the Monel gasket 30 and found satisfactory from the standpoint of radio interference suppression in the range of frequencies from 0.15 megacycle to 150 megacycles. The copper gasket, however, is commercially impractical because it lacks abrasion resistance, and has, therefore, a short useful life.

In a preferred embodiment of the invention, as will be seen by reference to Fig. 2, at least the inner surface 34 of the rotor 11 has a comparatively high electrical conductivity, for example by virtue of a cadmium plating 35 provided at least on such surface. In the case of a magneto having a stamped steel rotor, although the provision of the Monel gasket 30, as described, effected substantial suppression of radio interference, the interference level of the magneto provided with the Monel gasket was still above 1954 U. S. Air Corps specifications. However, the provision of cadmium plating 35 on the surface 34 of the rotor reduced the level of radio interference to a point within such specifications over the entire range indicated. In addition to reducing radio interference, cadmium plating as described protects a ferrous metal rotor against corrosion in service conditions, and thus avoids the necessity for one of the usual protective coatings, which have been found to increase radio interference.

Although the invention is not limited to the following theoretical explanation, it is believed that a radio shielded magneto according to the invention is operative because electrostatic charges generated during operation of the magneto are interrupted by the conductive cadmium plate 35, conducted along the surface thereof, through the Monel gasket to the stator 12, and ground ed through the metal portion of the apparatus carrying the magneto, for example a gasoline engine, to which the stator is attached. In this connection, it has been observed that a commercially available magneto having a stamped steel rotor and provided with a knitted Monel gasket, as described, emitted more radio interference signals than permitted by 1954 U. S. Air Corps specifications in the frequency range previously mentioned when the stamped steel rotor supplied by the manufacturer was used, and also when this rotor was plated with copper by ordinary commercial plating techniques. When, however, a like magneto was used for test purposes, except that the rotor was plated with cadmium by ordinary commercial techniques, the emission of radio interference signals was reduced to a point within the said specifications. The electrical conductivity of cadmium is approximately one-fourth that of copper, but it is known that the deposition of a continuous copper plate is considerably more diflicult than is the deposition of a continuous cadmium plate. It is also known that electrical energy, in the frequency range involved, is conducted as a surface phenomenon, and will develop, within practical limits, whatever voltage is necessary to cause its conduction. it is believed, therefore, that the superiority of cadmium plating, over copper plating or an unplated magneto rotor, is attributable to the increased continuity, as regards electrical conductivity, achieved by cadmium plating of at least the interior surface of the rotor. It has also been demonstrated that cadmium plating on only the interior of the rotor is equally as effective as cadmium plating on both the interior and exterior thereof. It is preferred that at least the interior surface of the rotor have a resistivity not greater than 20 microhms per centimeter cube. It will be apparent that, for radio shielding according to the invention to be effective, the rotor, the stator, and the gasket 30 or its equivalent must form an enclosure for the various electric components of the magneto.

The ideal radio shielded magneto according to the invention is one wherein a continuous, high electrical conductivity path is provided from the interior surface of the rotor, and through the stator to ground. As a practical matter, however, it is preferred to sacrifice the high conductivity of copper plating, for example, on the interior of the rotor, because of the greater ease with which continuity is achieved by cadmium plating, for example. It is also preferred to sacrifice the high electrical conductivity of a copper gasket making connection between the rotor and the stator in favor of the greater abrasion resistance of Monel, for example.

It will be apparent that various changes and modifications can be made from the specific details disclosed and discussed without departing from the spirit Of the attached claims.

What we claim is:

1. In a radio shielded device for generating electricity, comprising an armature carried by a stator, at least one magnet carried by a flywheel, and wherein the stator and the flywheel are relatively movable and form an enclosure for the armature and the magnet except for an annular space separating the stator and the flywheel,

the improvement which includes a resilient, knitted,

Monel gasket interposed in and substantially filling the annular space, attached to and in electric contact with the stator and in rubbing electric contact with the flywheel, and a substantially continuous layer of cadmium plate constituting at least the interior surface of the flywheel.

2. In a radio shielded device for generating electricity, comprising an armature carried by a stator, at least one magnet carried by a flywheel and wherein the stator and the flywheel are relatively movable and form an enclosure for the armature and the magnet except for an annular space separating the stator and the flywheel, the improvement which includes a resilient, knitted, wearresistant metal gasket interposed in and substantially filling the annular space, attached to and in electric contact with the stator and in rubbing electric contact with the flywheel, and a substantially continuous layer of a metal having an electric resistivity not greater than 20 microhms per centimeter cube constituting at least theinterior surface of the flywheel.

3. In a radio shielded device for generating electricity, comprising an armature carried by a stator, at least one magnet carried by a flywheel, and wherein the stator and the flywheel are relatively movable and form an enclosure for the armature and the magnet except for an annular space separating the stator and the flywheel, the improvement which includes a resilient knitted metal gasket interposed in and substantially filling the annular space, attached to and in electric contact with the stator and in rubbing electric contact with the flywheel.

4. In a radio shielded device for generating electricity, comprising an armature carried by a stator, at least one magnet carried by a flywheel, and wherein the stator and the flywheel are relatively movable and form an enclosure for the armature and the magnet except for an annular space separating the stator and the flywheel, the improvement which includes a resilient, knitted, wearresistant metal gasket interposed in and substantially filling the annular space, attached to and in electric contact with the stator and in rubbing electric contact with the flywheel.

5. In a radio shielded device for generating electricity, comprising an armature, first plate means for supporting the armature, means for creating a magnetic field, second plate means for supporting said last-named means, and wherein the first plate means and the second plate means are relatively movable and form an enclosure for the armature and the means for creating a magnetic field except for an annular space separating said plate means, the improvement which includes a resilient metal gasket interposed in and substantially filling the annular space, and in electrical contact with both of the plate means.

6. In a radio shielded device for generating electricity, comprising an armature, first plate means for supporting the armature, means for creating a magnetic field, second plate means for supporting said last-named means, and wherein the first plate means and the second plate means are relatively movable and form an enclosure for the armature and the means for creating a magnetic field except for an annular space separating said plate means. the improvement which includes a resilient knitted metal gasket interposed in and substantially filling the annular space, attached to, and in electric contact with one of the plates and in rubbing electric contact with the other of the plates.

7. In a radio shielded device for generating electricity, comprising an armature, first plate means for supporting the armature, means for creating a magnetic field, second plate means for supporting said last-named means, and wherein the first plate means and the second plate means are relatively movable and form an enclosure for the armature and the means for creating a magnetic field except for an annular space separating said plate means, the improvement which includes a substantially continuous layer of a metal having an electric resistivity not greater than 20 microhms per centimeter cube constituting at least the interior surface of the movable plate, and means electrically interconnecting the two plate means.

8. In a radio shielded device for generating electricity, comprising an armature, first plate means for supporting the armature, means for creating a magnetic field, second plate means for supporting said last-named means, and wherein the first plate means and the second plate means are relatively movable and form an enclosure for the armature and the means for creating a magnetic field except for an annular space separating said plate means, the improvement which includes, a resilient, knitted, wear resistant metal gasket interposed in and substantially filling the annular space, attached to and in electric contact with one of the plates and in rubbing electric contact with the other of the plates.

References Cited in the file of this patent UNITED STATES PATENTS 1,797,545 Churher Mar. 24, 1931 1,896,264 Weichsel Feb. 7, 1933 2,690,514 Phelan Sept. 28, 19-54

Images