US3541882A - Electrically insulating knobs - Google Patents

Description

Nov. 24, 1970 R. TESTA 3,541,882

ILECTRICALLY INSULATING KNOBS I I Filed Dec. 51, 1968 INVENTOR. Rafa/1o Z 570 BY I bf v IITTOR/VFX United States Patent US. Cl. 74553 6 Uaims ABSTRACT OF THE DISCLOSURE An instrument knob assembly includes an external surface of electrically conducting material, which is grasped by the operator. To insure electrical isolation between this external surface and the metallic shaft of the instrument, which may be at an elevated electrical potential, an intermediate insulating member is positioned between the shaft and the external knob. Where the main knob is itself in the form of a generally cylindrical main body and removable end cap, the intermediate insulating member is preferably also composed of an insulating sleeve and a detachable insulating disc. The insulating sleeve and disc may be molded on the interior surfaces of the cylinder and removable end plate, respectively, of the knob itself.

This invention relates to an instrument knob of the type utilized to turn a rotatable shaft, which in turn is connected to an adjustable element or assembly of the instrument. More particularly the knob of the invention is so constructed as to provide a substantial amount of electrical insulation between the exterior of the knob (even though the exterior is electrically conducting itself) and the shaft (which may intentionally or inadvertently be at an elevated electrically potential) with Which the knob is utilized.

In many different types of instruments, it is often desired to provide a manually turnable control knob, connected to a shaft which moves an adjustable element or assembly of an instrument. Since the shaft is typically metallic, it may be at an elevated electrical voltage, particularly when it is the control shaft for an electrical element or assembly (e.g., a potentiometer, a variable transformer, an electrical switch or the like). Even when the element or assembly to which the shaft is connected contains no electrical element intended to be at such an elevated voltage, such a shaft may still carry an electrical potential from other parts of the instrument through various electrically conducting paths. Therefore, any such shaft may be at a voltage different from true ground if any part of the apparatus has or is connected to a voltage source. Thus, any electrically operated device may involve a certain degree of safety hazard whenever the human operator must manually adjust such a shaft through an electrically conducting knob.

Although knobs, which are themselves composed entirely of electrically insulating material (for example, artificial resin) may be designed in such a configuration as to substantially totally enclose the manually adjustable end of such a metallic shaft, such a solution to the problem has certain disadvantages. First, since such elec- 3,541,882 Patented Nov. 24, 1970 trically insulating knobs are of relatively soft material, special provision must be made to attach these knobs firmly (as to relative rotation) to a conventional metal shaft. Thus, the knob and shaft normally have keyways or other non-symmetrical (about the axis of the shaft) mating surfaces to insure against slippage of the knob on the shaft. In those applications where the knob is integrally connected to an index or scale, relative adjustment of the knob on the shaft (for calibration, for example) is made impractical. Additional knobs made wholly of insulating materials of this type are relatively unattractive. While this drawback may not be of great significance on relatively inexpensive consumer products, it may cause serious problems in marketing of relatively expensive apparatus, for example, scientific instruments.

To avoid the relative cheapness in appearance of wholly plastic knobs, such plastic knobs have been coated with a metal (e.g., chrome-plated) on their exterior surface. If such metallic coated plastic knobs are to be fastened to the shaft both firmly and adjustably, they must typically contain metallic elements in their interior so as to afford metal to metal fastening surfaces. The combination of a metallic coating on the outside and metallic fastening means on the inside of such knobs usually results in leakage paths of relatively poor electrically insulating properties, thereby rendering the knobs unsafe if the shaft carries (usually inadvertently) any substantial electrical potential.

The present invention provides a knob of this general type (that is, one having a body portion of plastic, with metallic fastening means inside and a metallic coating on the outside) which is so constructed as to insure a high degree of electrical insulation between the interior and metallic exterior of the knob. In particular, the present invention provides a knob of this type in which an intermediate insulating sleeve plus other constructional features insures that the metallic coating on the outside of the knob is completely electrically insulated from the interior metallic elements (and therefore the instrument shaft).

An object of the invention is the provision of an instrumental control knob of the general type just discussed, which provides a high degree of electrical insulation between the metallic exterior and the metallic shaft to which the knob is attached.

A more specific object of the invention is the provision of such a control knob, including an insulating sleeve generally positioned intermediate the metallically coated exterior body of the knob and its interior metal parts.

A related object is the provision of such a knob in which the majority of the parts may be molded from synthetic resins, thereby reducing the cost thereof.

Other objects, advantages and features of the invention will become obvious to one skilled in the art upon reading the following specification in conjunction with the accompanying drawing in which:

FIG. 1 is a vertical cross section through an exemplary embodiment of the invention, showing the knob as fully assembled and attached to the shaft;

FIG. 2 is an end elevation, as seen from the right in FIG. 1, of the main body assembly of the knob, with the end cap and attaching screw removed;

FIG. 3 is an end elevation, as seen from the left in FIG.l, of the removable end cap, as disassembled from the rest of the knob;

FIG. 4 is an end elevation, as seen from the left in FIG. 1, of the fully assembled knob; and

FIG. 5 is a perspective view of the split collet (also shown in FIG. 1) to attach the knob to the shaft.

In FIG. 1, a completed knob assembly is shown attached to a shaft 8, which is operatively connected in turn to the element or assembly which is desired to be adjusted (e.g., potentiometer). The exemplary knob assembly illustrated comprises a main body subassembly 10, comprising a generally cylindrical outer sleeve component 12. This outer component may optionally include (as by being intricately formed therewith) a skirt portion 14, which typically would have on its visible (right-hand in FIG. 1) surface 16 an index, such as an arrow, or a scale. Most of the cylindrical outermost surface of sleeve component 12 will typically be knurled, as indicated at 18.

The other component of the knob assembly which is visible during use is a removable end cap 20, comprising a flattened conical main component 22 and a protruding cylindrical flange 24. This cylindrical flange is cut substantially through at a series of spaced radial positions, as indicated at 26 (see FIG. 3) so as to form a series of arcuate individual flange portions 24a, 2411, etc. (four such cuts at 26 and four such flange portions being illustrated in FIG. 3). The outer peripheral surface of the flange portions and the inner adjacent surface of the main body sleeve component 12 are substantially the same diameter at their meeting interface 28, so as to cause the individual flange portions to bend slightly inwardly, thereby forming a tight pressfit between the end cap and main body components.

In the preferred exemplary embodiment, both the main body outer sleeve component 12 and the main component 22 and cylindrical flange 24 of the removable end cap subassembly are composed of a synthetic resin, all the external surfaces of which have been metal plated, as indicated generally at 30. For example, the sleeve component 12 of the main body and the component 22 and flange 24 (which typically are integrally formed) may each be made (as by molding) from either an electroplating grade of acrylonitrile-butadiene-styrene (usually designated ABS) or of the polypropylene plastic suitable for electroplating, marketed by the Shell Chemical Company. These types of artificial resin are preferably utilized since they may be readily metal coated by available commercial techniques. The metal coating preferably utilized is chromium, so as to provide a relatively untarnishable, attractive appearing surface. Such chrome plating of elements constituted of either of the two types of artificial resins mentioned is a commercially available service rendered for example by Madan Plastics Company of Cranford, N.J., for A.B.S.; and Miami Plating Inc., of Dayton, Ohio, and MacDermid Inc. of Waterbury, Conn., for polypropylene.

Since such metal plating of plastic parts (12, 22) will cause all of their surfaces to have a metallic coating, each of the two subassemblies (10, is electrically conductive, not only on the outer surfaces but also on their inner surfaces (32, 34, respectively). In fact when the knob is assembled, all of the metallically coated elements 1034 form one continuous electrical path. For this reason all of these parts (10-34) must be electrically insulated from both shaft 8 and any metal parts that may be physically (and therefore electrically) connected to this shaft. For this purpose the invention provides intermediate insulating members between parts 10-34 and the shaft and the metallic parts connected thereto, as will now be described.

An intermediate insulating liner or sleeve member 40, generally in the shape of a hollow cylinder, is rigidly attached to the interior surface 32 of the main body or outer sleeve component 12. This intermediate insulating liner member may be of an artificial resin (i.e., a plastic) which may conveniently be formed inside outer body component 12 by a second molding operation (after component 12 has been molded and plated with metal). Preferably member 40 is made of a moldable artificial resin having at least moderately high structural strength, in order to successfully resist the force that is applied to its internal (generally cylindrical) surface 42 by the metallic collet utilized to fasten the knob to the shaft (as will hereinafter be described). As may be seen in FIG. 1, the generally cylindrical bore defined by internal surface 42 starts to widen gradually near its left-hand end, as indicated at 44. The right-hand terminal portion of internal surface 42 is re-entrant, as generally indicated at 46 so as to form an inwardly extending flange portion 48 of insulating member 40. The right-hand edge surface 50 of member 40 is generally flat, except for an upstanding cylindrical flange 52, which may be formed integrally with the member 40 during the (second) molding operation.

In an analogous manner, the removable end cap 20 may have molded to its interior surface 34 (after the cap has itself been molded and then plated) a generally disc shaped insulating member 60. Member includes a rela tively long (in the horizontal direction in FIG. 1) cylindrical flange 62, which may be integrally formed with member 60 during the same (second) molding operation. As may best be seen in FIG. 1, the adjacent surfaces of the upstanding cylindrical flange 52 (of member 40) and the long cylindrical flange 62 (of member 60) are in substan tial contact, when the removable end cap 20 is positioned on the main body assembly 10. Thus, when the knob is fully assembled, elements 4062 form an continuous generally cup-shaped insulating layer between elements 1034 and the elements along the central axis of the knob (e.g., shaft 8 and the metal elements attached thereto, about to be described). Preferably the long cylindrical flange 62 extends into a groove 54 in the right-hand edge surface 50 of member 40, thereby forming a rabbet joint therewith, so as to insure an effectively continuous insulation between the elements inside and outside members 40 and 60 when the knob is assembled.

Between the shaft 8 and the internal surface 42 of member 40 is positioned a generally cylindrical split collet 70, which may be made of brass. For most of its (left-hand) length, collet has an inner cylindrical surface 72 slightly larger than the outside diameter of shaft 8, so as to be slideable thereon when the collet is in its unstressed condition. The outer surface 74 of the collet is substantially cylindrical except at its left-hand end where it gradually flares so as to increase in diameter substantially in the same manner as the gradually widening left-hand end 44 of internal surface 42 of sleeve member 40. Because of the presence of slots 78 cut completely through (in the radial direction) collet 70 for about /3 of its length (from the left-hand edge in FIG. 1), the left-hand part of the collet may be readily compressed in the radial direction so as to decrease the effective diame ter of its inner surface 72. Such compression will be caused by the wedging action between the tapered internal surface 44 of insulating member 40 and the similarly flaring surface 76 of collet 70, as the collet is drawn to the right in FIG. 1. To so draw the collet to the right relative to the shaft 8 and the knob, the right-hand portion 80 of collet 70 has internal screw threads 82, so as to mate with the threads 84 on the external surface of shank portion of a binding head screw 86.

ASSEMBLY AND OPERATION As noted during the description of the structure, the outer sleeve component 12 is separately molded (for example, of ABS or polypropylene artificial resin), and then coated with a metallic (e.g., chromium) dress surface 30. In an analogous manner the removable end cap component 22 is similarly molded and subsequently chromed. The intermediate insulating sleeve member 40 is then molded (in a separate second molding operation) into the interior surface of component 12. As is well known in such molding operations, the various contours of the generally inner surface 42, 44 and 46 of insulating member 40 may be readily formed since all of the interior surface is readily accessible to molding forms or dies. Similarly flange 52 and groove 54 may be readily molded into the member 40 during the same operation since they are on an easily reachable surface thereof. In a closely analogous manner, the disc-shaped insulating member 60 and its long cylindrical flange 62 may be molded into the generally cup-shaped interior of the removable end cap 20 (defined by its inside surface 34 and the interior cylindrical surface of the protruding cylindrical flange 24). This forming of member 60 will of course be a secondstage molding operation, after the main end cap component 22 has been both molded and metal plated.

As previously noted, at least the material of the intermediate insulating member 40 should have suflicient structural strength to resist the force applied at its left-hand internal surface 44 by the wedging action of collet 70 upon subsequent attaching of the knob to shaft 8. For this purpose, member 40 may be molded of ABS resin specifically chosen for its structural strength. Such ABS material is available commercially, for example, from Uniroyal Chemical, Division of Uniroyal, Inc., Naugatuck, Conn. (A suitable electroplating grade of polypropylene is available from Shell Chemical Company, Scarsdale, New York.)

A collet 70 will be placed inside the completed main body subassembly (i.e., comprising elements 12-18, 30, 32 and 40-54) and loosely attached thereto by mak ing a few turns on binding head screw 86 (which will enter through the aperture defined by inwardly extending radial flange portion 48). The thus assembled main body 10, collet 70 and screw 86 (the removable end cap 20 still being unattached) will then be slid onto the end of shaft 8. Tightening of screw 86 will then draw collet 70 (to the right in FIG. 1) along the internal surfaces 42, 44 of member 40, thereby causing tapered surface 44 to wedge the flared left-hand end 76 of the collet radially inwardly toward shaft 8. Because of the split nature of the collet (i.e., slots 78), substantially the entire left-hand half of collet 70 will close about shaft 8 with increasing tension as screw 86 is tightened. After completion of this fixing of the main body subassembly 10 on the shaft 8, the removable end cap 20 is simply pushed onto the righthand open end of main body component 12. Since the surfaces mating at interface 28 (of the interior surface of component 12 and the outer surface of the protruding cylindrical flange 24 of end cap component 22) have? slight negative clearance, end cap 20 will be firmly held by friction on the right-hand end of the main body subassembly 10. In particular, the individual arcuate flange portions 24a, 24b, etc. will be sprung inwardly slightly (as allowed by radial cuts 26) so as to form a firm friction fit between elements 12 and 22. At the same time the rabbeted or interlocking parts (52, 54 and 62) of the two insulating members (40 and 60) will mate so as to insure complete insulation of the interior metal parts (shaft 8, collet 70 and screw 86) from the metal coated exterior knob elements (12 and 22). Although the left hand part 56 of insulating member 40 does not actually enclose the metal parts (shaft 8 and collet 70), there is no electrical path between these elements and (the lefthand parts of) the main knob body component 12, the left-hand end 56 of the insulating sleeve member 40 further shielding any tendency for an electrical lea to occur in this location.

Insulated knobs conforming substantially to the illustrated embodiment have provided sufficient electrical insulation between the shaft (and other internal metal parts) and the exterior parts that there was no substantial current leakage to the external parts (10, 20) even when electrical potentials in excess of 4,000 R.M.S. volts were applied to shaft 8. Specifically, the knobs passed the following rigid test. The entire assembly was maintained at high relative humidity (83 i-3%) at room temperature for 24 hours. The assembly was then subjected to an increasing AC. voltage (across the shaft to the outside of the knob), starting at about 2,000 volts. The voltage was gradually increased in moderate (about 200 volts) steps until the upper test limit of 4,000 volts was reached. No breakdown or sparkover occurred, even though the 4,000 volt potential was continued for over one minute and the voltage then brought back to 2,000 volts in the same stepped manner.

Although a single exemplary embodiment of the invention (which has been the successfully tested in prototype form) has been disclosed in detail, it will be obvious to one skilled in the art that many changes may be made therein without departing in principle therefrom. For this reason the invention is not limited to any of the details of this single exemplary embodiment, but rather is defined solely by the scope of the appended claims.

What is claimed is:

1. An electrically insulating knob assembly having a manually graspable knob comprising a generally cylindrical main portion and a generally flat end plate, both having an electrically conducting external surface, which knob is adapted to be rigidly attached to and partially surround one end of a metallic shaft, comprising:

a generally cylindrical intermediate insulating sleeve,

positioned between said electrically conducting cylindrical main portion and said metallic shaft, so as to insulate at least said cylindrical main portion from said shaft;

a generally disk-shaped insulating member, positioned between said generally flat end plate and the end of said metallic shaft, and in generally overlying relationship to the adjacent open end of said intermediate insulating sleeve;

means for sealingly connecting said adjacent open end of said intermediate insulating sleeve and said diskshaped insulating member;

whereby said sleeve and said member form a generally cup-shaped, effectively continuous intermediate insulating assembly, completely isolating all parts of the electrically conducting external surfaces of both said main portion and said end plate from said metallic shaft.

2. An electrically insulating knob assembly according to claim 1, in which:

said end plate is removably connected to said cylindrical main portion of said knob;

said intermediate insulating sleeve is rigidly attached to said main portion;

said disk-shaped insulating member is rigidly attached to said end plate;

and said connecting means comprises releasably connectable parts of said intermediate insulating sleeve and of said disk-shape insulating member;

whereby said end plate may be removed along with said disk-shaped insulating member.

3. An electrically insulating knob assembly according to claim 2, in which:

said releasably connectable parts comprise rabbeted flanges extending, respectively, from said intermediate insulating sleeve and said disk-shaped insulating member;

whereby a releasable but electrically insulated connection between said sleeve and said member is obtained.

4. An electrically insulating knob assembly according to claim 2, in which:

said intermediate insulating sleeve comprises synthetic resin, molded onto the interior cylindrical surface of said main portion.

5. An electrically insulating knob assembly according member completely isolate all parts of such electricalto claim 4, in which: 1y conducting material from said metallic shaft.

said disk-shaped insulating member also comprises synthetic resin, molded onto the interior surface of said References Cited end plate. 5 UNITED STATES PATENTS 6. An electrically insulating knob assembly according 3 061 869 11 19 2 Scalo et 1 237 53 XR to claim 1, in which: 3,313,057 4/1967 Leddy 16-121 XR said generally cylindrical main portion and said generally flat end plate both are of such construction that FRED MATTERN, Pflmafy EXamlIler all of their surfaces comprise electrically conducting 10 SHOEMAKER, Assistant Examiner material;

whereby said generally cylindrical intermediate insulat- US. Cl. X.R.

ing sleeve and said generally disk-shaped insulating 23753

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