US2398912A - Rheostat - Google Patents

Description

April 23, 1946. R. ABRAHAMSON RHEOSTAT Filed March 1, 1944 W0 nllillihllil /////Imw llllllullllullllillllilllllll.

man/WM aux KITTOANE) Patented Apr. 23,1946

UNITED STATES PATENT OFFICE rmnos'ra'r Robert Abrahamson, Baldwin, N. Y.

Application March 1, 1944, Serial No. 524,539

2 Claims.

This invention relates to rheostats. It has particular application to rotary rheostats, comprising a substantiall toroidal resistor element relatively to which a rotary contact arm or brush is moved in rder to connect'a variable portion of the resistor element in a circuit,

For the purpose of this description the term rheostats is meant to include any variable resistor regardless of its specific use and purpose, and is understood to include, for example, voltage dividers, also called potentiometers.

' Among its principal objects, the invention provides an improved rheostat of compact size ca pable of carrying particularly heavy currents, and having greater number of steps than conventional rheostats resulting in finer and more accurate adjustment.

The invention further provides numerous features of design resulting in a more dependable rheostat for heavy currents than was heretofore available. Among its features are longer life, less wear, and a particularly low amount of heat developed at the point of contact between the mov. able brush or arm andthe resistor element proper.

Among the principal objects of the invention is further a. novel and improved method of making rotary rheostats resulting in cheaper production and'a smaller and more reliable rheostat than conventional methods were capable of producing.

These and further features, advantages and details of the invention will appear more fully from a consideration of the detailed description which follows accompanied by drawings, showing, for purely illustrative purposes, forms of rheostats made according to the invention, and illustrating steps in the method of making them.

Although the novel features which are believed to be characteristic of the invention will be particularly pointed out in the claims appended hereto, the invention itself, its objectsand advantages, and the manner in which it may be carried out may be better understood by referring to the following description taken in connection with the accompanying drawing forming a part hereof, in which-- Fig. 1 is a perspective illustration of a form of mandrel for winding a helical resistor elementi Fig. 2 is a perspective illustration of a toroidal core to be inserted into the helical resistor element wound on the mandrel shown in Fig. 1;

Fig. 3 is a diagrammatic illustration of the manner of placing the resistor element on the core;

Fig. 4 1s a plan view 01 a rotary rheostat made according to this invention.

Fig. 5 is an elevational cross-section through the rheostat, shown in Fig. 4, a section being taken on line 5-5 of Fig. 4;

Fig. 6 is an elevational cross-section through a base, resistor element, and'core of a rotary rheostat designed for heavy current with and providing a great number of steps of adjustment.

Fig. 'I is a cross-section through the resistor helix as it appears after removal from the mandrel;

Fig. 8 is a perspective view illustrating a method of winding a resistor helix equipped with reenforcing contact pieces;

Fig. 9 is a greatly enlarged cross-section through a reeniorcing contact piece, a section being taken on line 99 of Fig. 8;

Fig. 10 is a perspective view of a resistor element assembled on a core, the resistor element having reenforcing contact pieces secured to alternate convolutions of the helix; and

Fig. 11 is a cross-section through a modified form of contact piece for attachment to a resistor helix.

Similar reference characters refer to similar parts throughout the drawing and the specificatlol'i.

In the drawing accompanying and forming part of this description the invention is explained by reference to specific structure, but it will be understood that the details may be modified in various respects without departure from the broad aspects of the invention.

I As far as I am aware, it is conventional practice to make a rotary rheostat by first preparing a toroidal core of a suitable insulatin and heat resistant material, usually a ceramic substance, on which is then wound the resistor element proper. This is an expensive and time consuming procedure due to the dii'flcuity of winding a coil on a core extending through an arc of nearly 360. Special machines, so-cailed toroid winders .have been developed for this purpose. However these machines can only be operated at relatively slow speeds. Toroid winders usuall consist of a hoopshaped bobbin which is rotatable about an axis normal to its plane and carries a supply 01 wire. The hoop-shaped bobbin is rotated about its axis and deposits the wire from the bobbin on the core. The portion of the core being wound extends substantially through the axis of rotation of the bobbin, and is slowly advanced in order to secure the proper spacing between adjacent turns wound on the core. -Each revolution of the bobbin causes one turn or. convolution of wire to be deposited on the core.

During the winding considerable stress is exerted on the core by reason of the tension under which the wire is necessarily maintained and becaus of the deformation which the wire undergoes it is bent around the core. The stress is particularly great when heavy gauge wire or ribbon of rectangular cross-section is wound as is commonly used in rheostats for heavy loads. This stress often leads to destruction of the highly brittle and fragile ceramic core, and limits the gauge of wire or ribbon, hereinafter referred to for convenience collectively as "resistor wire," which ma be wound on a core of a predetermined thickness.

My improved method avoids these disadvantages. According to the invention I prewind the resistor helix on a straight mandrel on a conventional winding machine or a lathe. The winding operation may be carried out at a much higher rate than can be attained on a toroid winder.

A mandrel ID, as shown in Fig. 1, is equipped with a clamp H for holding one end of the resistor wire l2 to be wound thereon. The mandrel has a cross-section corresponding to that of the core on which the resistor element will be mounted later. The mandrel is rotated in the direction of the arrow i3 causing the wire to be deposited thereon in the form of a helix H,

A core l6 of appropriate cross-section is separately prepared oi ceramic or other suitable insulating and heat resistant material.

The prewound resistor helix [4 is then removed from the straight mandrel i and transferred onto the core I5, as is shown in Fig. 3. The assembly may be provided with the usual terminal clamps shown for example in the embodiment illustrated in Fig. 5, and is secured to the core in any convenient manner such as by applying vitreous enamel thereto and baking it in an oven.

A rotary rheostat so assembled is shown in Figs. 4 and 5. The rheostat comprises a core l6 having a resistor helix l4 mounted thereon as'previously described. Helix l4 and core 15 are cemented with ceramic cement or vitreous enamel on an insulating base it of ceramic orother suitable insulating and heat resistant material. The base has a central aperture ll through which a shaft 18 extends. A knob or hand wheel I8 is secured to one end of the shaft and the other end of the shaft has secured thereto a bracketed hub IS. A contact arm 20 is pivotally secured to the bracketed portion of the hub 18 at 2! and carries a brush element 22. The brush element 22 has a neck 22' extending through a corresponding aperture in the arm 20 and rests on the face 23 of the resistor helix it under action of a spring 24. Terminal clamps 25 and 26 are connected to the ends of the resistor helix l4 and a center terminal 21 leads to the hub [9 of the contact arm 20 through a spring washer 28. The helix and core assembly i4 and I may be secured to the base IS in any suitable manner, such as by fusing the parts together by a ceramic flux in an oven as is indicated at 29.

A base, a resistor element, and a core for particularly heavy currents are illustrated in Fig.

mltting a great number of turns to be wound on a relatively short length of mandrel. The stress exerted on the mandrel during winding is considerable, and would destroy any insulating core. if it were attempted to wind this form of helix on an insulating core directly with a toroid winder. The winding on a straight mandrel, preferably made of high tensile steel, however, presents no difficulties.

The advantages of the improved form of helix shown in Fig. 7 are manifest. A heavy current can be carried by the rheostat due to the large cross-sectional area of the ribbon from which the helix 3! was wound. The heat dissipation of the resistor is excellent since only its narrow edge is secured to the core, and a large surface area is exposed to the air. The number of steps of adjustment provided by the rheostat shown in Fig. 6 is more than twice the number which can be accommodated on a rheostat of equal size wound in conventional manner with a ribbon resting on the core with its wide edge.

The contact surface of the resistor may be increased by machining on the helix a flat surface 33 after completion of the assembly of the base 30, helix 3 I, and core 32. The resulting reduction of the cross-sectional area of the resistor ribbon at the machined portion which theoretically would lead to a corresponding increase in the temperature of the resistor element at the weakened portion is compensated for by the extremely favorable cooling condition for these portions which are exposed to the air and farthest remote from the base 30.

A modified form of resistor helix equipped with reenforcing contact pieces is shown in Figs. 8 to 10. Previous to the winding of the resistor wire 34 into a helix, a plurality of rcenforcing contact pieces are strung on the wire. The contact pieces 36 may be tubular pieces of rectangular cross section and are advanced intermittently as the wire is wound on the mandrel l0, so as to cause the contact pieces to form a contiguous contact surface. The contact surface may either be on the face of the finished rheostat corresponding to surface 33 in Fig. 6, or on the periphery in the event that the contact arm or brush is designed to travel around the periphery of the resistor element. After winding the helix is removed from the mandrel.

The reenforcing contact pieces which are preferably made of copper or bronze or brass are then metallically fused to the resistance wire or ribbon in any convenient manner such as by soldering or brazing, or spot welding.

A greatly enlarged cross-section through a contact portion 35 brazed to the resistor wire 34 at 36 is illustrated in Fig. 9. The helix. with its attached contact pieces, is then transferred onto a toroidal core 3'! and is fused to the core at 38. The contact pieces may then be machined at 39 to produce a flat contact surface.

The rheostat thus made is admirably suited for carrying very heavy loads. The contact heat developed at the point of contact between the resistor element and the contact arm or brush is very low because the contact proper is formed by two elements both of which are good conductors. For example the contact piece 35 may be made of copper, brass, or bronze, and the brush may be made of a copper graphite composition. Were it not for the contact pieces of a material of high electrical conductivity, the contact heat developed at the point of contact would be considerably greater leading to sparking, oxidation and an early destruction of the rheostat.

Reenforcing contact pieces may be applied to a limited number of convolutions of the helix if a limited number of steps of adjustment is desired. Fig. 'shows a resistor and core assembly in which reenforcing contact pieces 35' are secured to alternate turns of the resistor helix 34' mounted on a core 40. The contact pieces are machined to produce a flat contact surface 39'. It is not necessary that the contact pieces be assembled with the resistor wire during the winding of the helix on a mandrel. Fig. 11 shows in cross-section a contact piece 35" adapted for assembly with a prewound helix. The contact piece has a substantially U-shaped cross-section.

providing a space ll fitting over the resistor wire to which it is to be secured. The space I is shaped so as to fit the shape of the wire employed to which it is thereafter brazed.

The features and advantages of the hereinbefore described method and the rheostats produced thereby are manifold. The winding of the resistor helix on a straight mandrel permits of higher production speeds than are obtainable with a toroid winder. Heavy gauge resistor wire may be used for forming the helix without danger of injury to the highly fragile core, no appreciable force or stress being exerted on the core during the transfer of the prewound helix onto the core.

The winding of resistor ribbon in its narrow edge produces a resistor helix providing a far greater number of steps of adjustment, and permits the use of a single rheostat where heretofore two rheostats were used in parallel because of the impossibility of winding in the conventional manner a heavy gauge ribbon with its narrow edge on a ceramic core.

The use of contact pieces of high electric conductivity greatly reduces the contact heat and the consequent contact oxidation commonly occurring in conventional rheostats operated under heavy loads.

Oxidation is the chief source of destruction of rheostats since the oxides are poor conductors leading to a further increase in the development of contact heat, and since the extreme hardness of the oxides causes increased friction and a rapid wear of the resistor element and brush.

The method of prewinding of the resistor helix on a mandrel lends itself particularly well to a convenient manner of securing U-shaped con- The hinged. mounting of the contact arm in a bracketed hub insures a constant contact pressure; which is determined by the force of the spring acting between the arm and the hub. This form of mounting makes the brush element easily replaceable. A worn brush element may be replaced simply by lifting the contact arm off the resistor helix. No tools are necessary for this operation and the exchange does not involve tact pieces thereto after removal of the helix from the mandrel.

the danger of permanent bending or deforming the brush arm as it so frequently occurs in rheostats of conventional construction.

Obviously the present invention is not limited to the specific embodiments herein shown and described. It may be employed with equal benefit in the production of other forms of rheostats. Such application of the invention will be obvious to persons skilled in the art, and is manifestly within the scope and spirit of this invention.

What is claimed is:

1. A rotary rheostat comprising, in combination, a base; a toroidal resistor thereon; a central shaft extending through said base; a hub member on said shaft; 2. brush arm hingedly mounted on said hub member about a hinge axis spaced from and normal to the axis of said shaft; a contact element carried by said brush arm for making contact with said resistor; and a spring acting between said hub member and said brush arm for resiliently urging said arm and contact element against said resistor, while permitting removal of said arm from said resistor by swinging movement of said arm away from said resistor about the hinge axis against the action of said spring, the contact pressure being determined by the stiffness of said spring.

2. A rotary rheostat comprising, in combination, a base; a toroidal resistor thereon; a central shaft extending through said base; a. hub member on said shaft; a brush arm hingedly mounted on said hub member about a hinge axis spaced from and normal to the axis of said shaft; a contact element removably inserted into said brush arm for making contact with said resistor; and a helical spring coaxially arranged with said hinge axis for resiliently urging said arm and contact element against said resistor, while permitting separation of said am and contact element from said resistor by swinging movement of said arm away from said resistor about the hinge axis against the action of said spring, whereby the contact element may be conveniently removed and replaced.

ROBERT ABRAHAMSON.

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