US 3026443 A
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Description (OCR text may contain errors)
March 20, 1962 w. E. WILSON ELECTRIC LAMP AND BASE THEREFOR Filed NOV. 30, 1959 FIG. 3.
5 INVENTOR WILLIAM E. WILSON.
United States Patent Ofihce 3,026,443 Patented Mar. 20, 1962 3,026,443 ELECTRIC LAMP AND BASE THEREFOR William E. Wilson, Cedar Grove, N.J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 30, 1959, Ser. No. 856,113 Claims. (Cl. 313-318) This invention relates to electrical devices and, more particularly, to a fluorescent lamp and an improved base therefor.
The standard type of base presently employed in the fluorescent lamp industry comprises a generally cupshaped member that is fastened to and encloses the sealed end portions of the lamp envelope. In one widely used base design the entire body portion or base shell is molded from a suitable thermosetting plastic such as phenol formaldehyde or the like which has a higher thermal coefiicient of expansion than the glass from which the lamp envelope and the stem portion of the cathode mount fused thereto are conventionally fabricated. Thus, the collar portion of the base contracts at a much faster rate than the sealed end of the lamp envelope enclosed thereby with resultant compression and, in some cases, cracking of the glass seal when the lamps are subjected to low ambient temperatures during shipment, storage or use.
It is accordingly the general object of this invention to provide an electric lamp or similar device which has a plastic type base but is not deleteriously effected or damaged by temperature changes.
Another end more specific object is the provision of a base for a fluorescent lamp or the like that is Wholly or partly fabricated from plastic but which will not excessively strain or cause the enclosed end portion of the lamp envelope to develop cracks when the lamp is ex posed to subnormal ambient temperatures.
The foregoing objects, and others which will become apparent as the description proceeds, are achieved by purposely weakening the plastic collar portion of the base at one or more preselected points in such a manner that it will fracture thereat when the base is attached to the lamp envelope and the collar stressed beyond a preselected limit by subsequent changes in temperature. In this manner the base collar controllably fractures under subnormal temperature conditions thus not only instantly relieving the pressure on the enclosed sealed portion of the envelope but permitting the collar to freely contract without exerting any additional compressive force.
A better understanding of this invention may be ob tained by referring to the accompanying drawing wherein:
FIG. 1 is a fragmentary view in elevation of a fluorescent lamp incorporating the improved base construction of this invention;
FIG. 2 is an enlarged plan view of the interior of the improved base;
FIG. 3 is a cross-sectional view along the line HIIII of FIG. 2, in the direction of the arrows; and
FIG. 4 is an elevational view of one end of the lamp shown in FIG. 1 after the exposure thereof to a temperature variation sufficient to cause the base collar to fracture in accordance with the invention.
While the improved base construction of this invention may be advantageously employed in various types of electrical devices that are susceptible to damage from thermally-induced compressive strains in the base area, it is especially adapted for use in conjunction with fluorescent lamps and has accordingly been so illustrated and will be so described.
With specific reference to the drawing, in FIG. 1 there is illustrated a fluorescent lamp comprising a sealed light-transmitting envelope 12 that contains the usual quantity of mercury 16 and an inert ionizable gas such as argon or the like. The envelope 12 carries the customary phosphor coating 14 on its inner surface that emits light in response to the UV radiations produced by the gaseous discharge that is sustained between the cathode mounts 18 at each end of the envelope when the lamp 10 is energized. The ends of the envelope 1-2 are tapered inwardly and fused to the flared stems of the cathode mounts 18 to provide the usual constricted seals 13 at each end of the lamp 10 in accordance with standard lamp-making practice. Attached to each of the sealed ends of the envelope 12 by the usual basing cement (not shown) is a base 20 comprising a hollow cup-shaped body of molded thermosett-ing plastic material having a bottom wall 24 and a generally cylindrical and continuous flange portion or collar 26. The collar 26 preferably is of uniform wall thickness al (see FIGS. 2 and 3) and of such length and diameter that it totally and snugly encloses each of the respective seals 13. A pair of pins 22 fastened to and projecting from the bottom wall 24 of the base 20 electrically connect with the lead wires of the cathode mounts 18 and serve as terminals for the lamp 10 in the customary manner. The bottom Wall 24 is also provided with an aperture 25, as shown in FIGS. 2 and 3, to vent the gases produced when the basing cement is cured.
Both the envelope 12 and flared stems of the cathode mounts 18 are fabricated from the usual so-called lead glass which is a well-known glass employed in the lamp industry and has a thermal coeflicient of linear expansion/ C. of about 8.9 or 9.0 10- The base 20, or at least the collar 26 thereof, is fabricated from a moldable thermosetting plastic and, due to the basically different physical properties of such plastics, accordingly has a thermal coeflicient of linear expansion considerably higher than that of the glass seals 13. Specific examples of some of the plastic resins suitable for use in fabricating the base shell or collar 26 thereof together with their expansion coefiicients are given in the following table designated Table I:
As will be obvious from the foregoing, the glass seals 13 have a rate of contraction that is at least one order of magnitude smaller than any of the representative examples of suitable plastic materials given in Table I. Thus, should the completed lamp 10 be exposed to subnormal ambient temperatures (as for example 0 C.) dun'ng shipment, storage or while in use the base collar 26 by virtue of its more rapid rate of contraction will compress the glass seals 13 and induce a compressive strain therein that may, and frequently does, cause the seals to crack and even become completely severed from the lamp envelope 12.
The progressive build-up of such compressive strains in and the possible resultant damage to the seals 13 caused by the faster rate of shrinkage of the plastic collar 26 under low temperature conditions is obviated according to the present invention by intentionally weakening the collar at preselected points around its rim. In the specific form of the invention illustrated in FIGS. 2 and 3, this is accomplished by providing a plurality of circumferentially spaced indentations such as grooves 23 in the inner surface of the plastic collar 26 which extend axially inward from its peripheral edge for a substantial portion of its length, preferably a distance at least equivalent to the axial length of the sealed end segment of the envelope 12 that is enclosed by said collar. As shown in FIGS. 2 and 3 respectively, the grooves 28 are V-shaped in cross-section and taper in'a direction such that the point of the V is located proximate the outer surface of the collar 26. The grooves have a depth d that is just slightly smaller than the Wall thickness d of the collar 26 so that the collar is Weakest in these regions and will fracture thereat when strained beyond a preselected limit well below that at which the seals 13 will crack. As will be obvious, the exact amount of stress required to fracture the weakened portions of the collar 26 will depend upon the wall thickness thereat and on the type of plastic material employed. To keep this figure as low as possible the walls of the collar 26 left by the grooves 28 are made as thin as possible. As a specific example, excellent results have been obtained with bases having collars .045 of an inch thick by employing grooves 28 .030 of an inch in depth. Out of seven lamps fitted with such bases and cooled to a temperature of -40 C. from room temperature in 30 seconds, no cracked seals occurred Whereas in a control group of eight'lamps which were identical in every respect except that previous type bases having no relief grooves were employed, there resulted five cracked seals.
As shown in FIG. 4 when the weakened wall portions of the base collar 26 fractures a crack 30appears in the outer surface of the collar that is of substantially the same length as the grooves 28. However, due to the V-conflguration and inward tapering of the grooves 28 the resultant cracks 30 are extremely fine and hair-like in character and as such are virtually imperceptible and thus not objectionable from an appearance standpoint.
It will be apparent from the foregoing that the objects of the invention have been achieved by providing a lamp and an improved fail-safe plastic type base therefor whereby such lamp can safely withstand temperature variations that would otherwise damage or deleteriously efiect the base-enclosed sealed ends of the lamp envelope.
While a preferred embodiment of this invention has been illustrated and described, it is to be understood that the invention is not limited thereto or thereby.
1. In combination with an electric device having an envelope that is susceptible to breakage when compressively stressed, a base attached to said envelope having a continuous flange portion of thermosetting plastic material that has a higher thermal coeflicient of expansion than said envelope and encircles an end thereof, said flange portion having an indentation therein of a depth considerably greater than half the Wall thickness of said flange portion and of suflicient length to cause said plastic material, when strained beyond a preselected limit, to fracture along a predetermined line that extends inwardly from the peripheral edge of said flange portion and thereby at least partially relieve the compressive stress that would otherwise be imparted to the encircled end of said envelope by said flange portion under subnormal temperature conditions.
2. A base adapted for mounting on the end of'a vitreous envelope that is susceptible to breakage when subjected to a compressive stress, said base comprising a hollow member having a continuous flange portion of thermosetting plastic material that is adapted to encircle the end of and has a higher thermal coefficient of expansion than said envelope, said flange portion having at least one indentation therein that extends inwardly from its peripheral edge and has a depth considerably greater than half the wall thickness of said flange portion such that the latter, when subjected to a predetermined stress, will fracture along said indentation and thus be able to contract without exerting an attendant compressive force on the enclosed part of the envelope.
3. The combination of an electric device and an attached base as set forth in claim 1 wherein, said electric device comprises an electric lamp having a tubular envelope with a seal at one end that is susceptible to breakage when co'mpressibly stressed and is encircled by the plastic flange portion of said base, and the indentation in said flange portion comprises a groove that extends axially of said flange portion from its peripheral edge a distance approximately equal to the axial length of the enclosed seal.
'4. A base as set forth in claim 2 wherein, said plastic flange portion comprises a cylindrical collar of substantially uniform wall thickness, and said indentation comprises a groove that extends axially of said collar from its peripheral edge.
5. A base as set forth in claim 4 wherein, said plastic collar is provided with a plurality of circumferentially spaced grooves, and said grooves are V-shaped in crosssection and taper in a direction such that the point of the V is located proximate the outer surface of said collar.
References Cited in the file of this patent UNITED STATES PATENTS 2,451,900 Zambakian Oct. 19, 1948 2,799,801 Freeman July 16, 1957 2,883,577 Manfredi Apr. 21, 1959