US 3673401 A
A fluorescent lamp jacket assembly is disclosed employing an expansible or dilatable plastic jacket which substantially encloses the entire length of the glass envelope of a fluorescent lamp and is held to the lamp by means of a pair of plastic end caps so that rupturing of the glass lamp envelope will not be accompanied by bursting or separation of the jacket or its end caps, thereby containing all the debris ensuing from the implosion reaction accompanying breaking of the fluorescent lamp.
Description (OCR text may contain errors)
United States Patent Du Pont 1 June 27, 1972 [54} FLUORESCENT LAMP PROTECTION 3,136,489 6/1964 Oharenko ..240/! 1.4 APPARATUS 3,179,792 4/1965 Weiss .240/ 1 1.4 3,358,167 12/1967 Shanks ..313/25  Inventor: Paul R. Du Pont, Gillette, NJ.
731 Assignee: Thermoplastic Processes Inc., Stirling, NJ. P P Queisser Assistant Exammer-Elhs J. Koch  Filed: Oct. 29, 1969 Attorney-H0ward R. Popper  App]. No.: 872,321 ABSTRACT A fluorescent lamp jacket assembly is disclosed employing an 'i 340/112 expansible or dilatable plastic jacket which substantially en. 58] Fieid y 2 P 51 11 closes the entire length of the glass envelope of a fluorescent 240/1 3 i lamp and is held to the lamp by means ofa pair ofplastic end caps so that rupturing of the glass lamp envelope will not be  References Cited accompanied by bursting or separation of the jacket or its end caps, thereby containing all the debris ensuing from the implosion reaction accompanying breaking of the fluorescent lamp.
4 Claims, 6 Drawing Figures PATENTEDJum I972 3, 678 401 sum 1 or 2 FIG.
INVENTOR R R. DUPONT 8V A 7'7'ORNEV PATENTEUJUNZ? I972 SHEET 20F 2 FIG. 4
FLUORESCENT LAMP PROTECTION APPARATUS BACKGROUND OF THE INVENTION This invention relates to a shielding arrangement for a fluorescent lamp, and more particularly, to a structure for reducing the explosion, shrapnel, and contamination hazard that may exist where such lamps must be used in locations exposing the lamps to vibration, severe variations in temperature, splashing liquids and the like.
It has been appreciated for some time that the performance of fluorescent lamps is enhanced when the lamp is insulated from low temperatures and severe winds. In an article in the Feb. l96l issue of Illuminating Engineering, for example, the authors reported that jacketing the fluorescent lamp allowed the lamp to produce a light output over a temperature range that compared favorably with that obtained from lamps enclosed in weatherproof luminaires. For fluorescent lamps, generally, it is known that light output, as affected by internal mercury vapor pressure, is related to the temperature of the coldest spot of the glass bulb envelope, maximum light output being obtained at a cold-spot temperature of approximately 100 F.
The above-referenced article described a glass-jacketed fluorescent lamp which has since become quite popular despite the fact that its cost is appreciably more than that of the standard, unshielded lamp. The glass jacket of this prior art arrangement had a diameter approximately half an inch greater than the nominal 1% inch diameter of the standard fluorescent lamp and its length was almost equal to that of the exposed portion of the lamp envelope lying between the terminal ferrules. The glass jacket was fastened to the lamp by a pair of rubber bushings each of which grasped not only the terminal ferrule but also the adjacent surfaces of the lamp en velope. A somewhat earlier version of a jacketed fluorescent lamp is shown in L. R. Keiffer US. Pat. No. 2,363,109 issued Nov. 21, 1944 and a somewhat later version in H. Weiss US. Pat. No. 3,179,792 issued Apr. 20, I965.
While the jacketed fluorescent lamp is thus known to stabilize light output over a greater temperature range than is possible with a bare lamp, present day jacketed arrangements are expensive and do not fully protect against the hazards of cuts from broken tubes. Such cuts are considered potentially serious not only because the glass may penetrate deeply because of the high velocity imparted to the shards by the implosion reaction, but also because the phosphors and oxide powders contained in the lamp may lead to infection and slow healing. Since most fluorescent lamps are used in overhead fixtures, a lamp which falls is almost certain to do so from a height sufficient to insure its breaking.
Of course, plastic-jacketed lamps have been introduced which are far less susceptible to shattering than glass jacketed lamps. However, even the plastic-jacketed lamps have thus far permitted the plastic jacket to separate from the fluorescent lamp on impact and have therefore not been reliable in containing the shrapnel if the impact was severe enough to break the lamp envelope within. While the jacket could be cemented to the end bushings, it is desirable to reduce the cost of employing jacketed fluorescent lamps and, obviously, cementing the jacket precludes the re-use of the jacket on a new lamp thereby preventing the cost of the jacket from being amortised over the life of several lamps.
Accordingly, it is an object of the present invention to pro vide an improved jacketed fluorescent lamp which is reusable and economical.
It is another object of the present invention to provide a fluorescent lamp jacket which will remain intact and contain the debris even after an impact strong enough completely to shatter the lamp envelope within.
SUMMARY OF THE INVENTION The foregoing and other objects are achieved according to the principles of the present invention, in one illustrative embodiment thereof, by providing a jacket assembly comprising an expansible wall cylinder of polycarbonate or similar nonbrittle plastic having good heat resistance which is concentrically mounted over the lamp envelope and firmly held at each of its ends by an end cap'of high heat stabilized polypropylene which covers the terminal ends of the lamp. Advantageously, the end caps grip only the terminal ferrule portions of the fluorescent lamp rather than the lamp envelope itself. The plastic jacket is made of a material having a stiffness modulus and wall thickness such that the sudden change in pressure, which may be occasioned by an impact of sufficient force to fracture the glass fluorescent tube envelope within the jacket,
will be dissipated by causing flexure in the jacket walls,
thereby preventing the force of the implosion reaction from being directed to the end caps. l have found polycarbonate tubing, having a nominal wall thickness of 0.020 inches and an impact strength of 14 foot-pounds on the Izod scale, to be satisfactory for this purpose, in one illustrative embodiment.
The ends of the plastic cylinder or jacket of my invention are held in a conically-tapered recess in each end cap. The recess holds the jacket so that a twisting motion is required to remove the cap from the jacket and so that the longitudinal or radial forces that may be exerted in an implosion reaction or on impact with an object in a fall will be wholly incapable of causing separation of the cap from the jacket. Each conicallytapered recess is further provided with a subgated vent through which air that would otherwise be entrapped during assembly of the jacket and end cap may escape, thereby permitting a tighter assembly of cap and jacket. Conversely, the vent acts as a vacuum break or air entry point to facilitate disassembly when it is desired to re-use the jacket with another fluorescent lamp. The subgated vent is in each case operated by the torsional flexure imparted manually in assembling or disassembling the jacket and end caps but is not operated by the radial or longitudinal forces that may be encountered on impact. The polypropylene end cap is closed by a webbed membrane which protects the end of the fluorescent lamp and, in the event that the lamp envelope is broken by a severe impact, provides a vacuum seal so that the jacket and its end caps will remain together, thus containing the debris of the implosion.
BRIEF DESCRIPTION OF DRAWING The foregoing and other objects and features may become more apparent by referring now to the drawing in which:
FIG. 1 shows a portion of the jacket and one version of the end cap of the present invention assembled on a standard fluorescent lamp;
FIGS. 2 and 3, respectively, show an end view and a sectional view through the rim and vent of the cap of FIG. 1;
FIGS. 4 and 5, respectively, show end and sectional views through an alternative version of the end cap of the present invention; and
FIG. 6 shows a stop-action view of the end cap and jacket assembly of the present invention at the instant of implosion of the fluorescent lamp within the assembly.
GENERAL DESCRIPTION Referring now to FIG. 1, there is shown one-half of a fluorescent lamp and one-half of one version of a jacket assembly according to the present invention, it being understood that the other half of the illustrative lamp and jacket assembly, which are not shown, are entirely symmetrical. The conventional fluorescent lamp comprises a glass envelope 9 of transparent or translucent glass containing the usual phosphors and oxides and enclosing a thermionic filament (not shown). Each end of the glass envelope 9 is provided with a terminal ferrule 5 which provides for supporting an end plate (not shown), which plate carries the electrical terminals 4. The construction of a fluorescent lamp being well known, it is not necessary to describe the details thereof herein. It will, of course, be appreciated that fluorescent lamps possess different types of end terminals, the most common, two-pin type terminal 4, being shown by way of example in FIG. 1. Gripping the terminal ferrule about its circumference and sealing the end of the fluorescent lamp, in the region of its terminals 4, is the end cap 2, which forms part of the assembly of the present invention. Inserted into and slightly distorting the tapered annular recess 6 provided in end cap 2, is the expansible plastic shield 10, the details of the end cap construction being more particularly revealed in FIGS. 2 and 3.
Referring now to FIG. 3, there is shown a section of the end cap 2 comprising part of the assembly of the invention, which section is taken along the lines 3-3 depicted in FIG. 2. The end cap is closed at one end by webbed portion or membrane 3 which is pierced by apertures 3a and 3b to accommodate the electrical terminals 4 (FIG. 1) of the conventional fluorescent lamp.
The inside cylindrical portion 2a of the cap 2 nearest the membrane 3 has a diameter sufficient to insure a snug fit about the metal lamp ferrule 5. To the left of portion 2a, the inside of end cap 2 is enlarged to define a slightly tapered or bell-mouthed surface 2b which dimensioned to be large enough not to bind on the glass envelope 9 of the fluorescent lamp. Surface 2b defines the interior portion of an extended, tapered shell section of the main body of cap 2, the exterior portion 20 of which forms one wall for the recess 6.
Radially external to the cylindrical portion 2a, cap 2 is provided with an annular ring 7 which supports the outer lip section 8. The interior surface 8a of outer lip 8 and the exterior surface 2c of the extended, tapered shell section 2b-2c define a tapered annular recess 6 (FIG. 3) into which the tubular shield 10 fits. The facing tapered surfaces 20 and 8a which define recess 6 are each free to flex during the installation of jacket 10 and to remain somewhat deformed by the presence of the plastic jacket while retained in recess 6. The tapered surfaces 2c and 8a provide for tightly engaging the plastic shield 10 despite variations in its wall thickness or of the end cap 2 which are likely to be encountered in production.
To facilitate assembly and disassembly of the end cap 2 with the jacket 10, the end cap is provided with at least one vent 12, which is cut into the annular recess 6 at one portion thereof in such a manner that the recess 6 is both deepened or subgated as well as widened by the vent. The vent 12 thereby permits air that would be entrapped in recess 6 during the assembly of jacket 10 to the end cap 2, to escape, part of the entrapped air being expelled to the atmosphere and part being admitted to the region between the jacket 10 and the lamp envelope 9. In preventing the entrapment of air which would otherwise tend to limit the axial engagement of end cap 2 with cylinder 10, the inner surface 3c of the webbed portion 3 of the end cap is brought into intimate contact with the flat surface a of ferrule 5.
In one important aspect of the present invention, the materials out of which the dilatable, cylindrical jacket and the end cap 2 are made prevent the end cap 2 from becoming disengaged from jacket 10, even when the jacketed and end capped fluorescent lamp is deliberately struck or dropped with sufficient force to break the glass tube envelope 9. In accordance with this aspect of the invention, in one illustrative embodiment thereof designed to protect a standard 40 watt fluorescent lamp, the end cap 2 was made of high heat stabilized polypropylene, the web member 3 had a thickness of 0.018 inch and the annular recess 6 had an axial depth of 0.4375 inch. While a somewhat different thickness webmay' be employed it may not be made so thick as to interfere with the insertion of the lamp into the conventional socket or fixture nor so thin as to constitute a mere mold flashing. The subgated vent in the recess portion of the end cap had an axial depth of 0.4531 inch so that even with the cylindrical jacket 10 bottomed in the recess 6, a gap 10a exists through which air may escape. v 1
The cylindrical jacket 10 is advantageously made of polycarbonate transparent tubing having a nominal wall thickness of 0.020 inch and an impact strength of 14 footpounds on the Izod scale. In the approximate 4-foot length of the 40 watt bulb, a jacket of this material has sufficient flexibility so that even if glass envelope 9 is broken, thereby causing a violent change in the internal pressure within the assembled jacket and end cap, the end cap will not come loose from the jacket. Maintaining the integrity of the assembly prevents the shards of glass from the broken tube 9 from injuring personnel and also protects such personnel from the danger of infectious cuts.
FIGS. 4and 5 show end and sectional views of an alternative end cap which is suitable for protecting a conventional single-terminal fluorescent lamp in accordance with my invention. FIG. 6 shows this end cap assembled together with a plastic shield 10' and a single-terminal lamp. For convenience, reference numerals in FIGS. 4-6 which are similar except for the inclusion of the prime symbol to those employed in FIGS. 1-3 are intended to denote corresponding features. The shield-receiving recess 6' of the single-aperture end cap differs from that of the double-aperture end cap in that it is provided with a wide-mouth shallow channel 15 to facilitate alignment of and assembly to cylindrical shield 10'. In addition, the recess 6' has a somewhat less pronounced taper (as little as 6 illustratively) than the recess 6 of the embodiment shown in FIGS. 1-3. Further, shell section 2b and outer lip section 8' are somewhat heavier than the corresponding features in FIGS. 1-3. Also different in detail is the subgated vent 12' which is now symmetrically faired into recess 6' and does not cut so much into the thickness of the outer lip 8' as did the vent of the first depicted embodiment. It should be understood however that the design of the single-aperture and doubleaperture end caps may interchangeably be used it merely v being necessary to provide the correct diameter and number of apertures appropriate to the type of fluorescent lamp desired to be protected. As was true of the version of FIGS. l-3, the aperture 3ab' for the terminal of the lamp should have a diameter which is the same or even slightly smaller than the lamp terminal in order to maintain the integrity of the enclosure formed by the end caps and jacket after the lamp is broken.
In FIG. 6 there is depicted a stop-action" view of a jacketed fluorescent lamp embodying my invention at the instant that the lamp envelope has been broken through some mishap. The breaking of the glass envelope of the lamp causes a sudden drop in pressure within the jacket 10' as the air which is normally in the space between the lamp envelope and the jacket 10' rushes to occupy the vacuum within the now destroyed glass envelope 9 of the broken fluorescent lamp. However, because the jacket 10 of my invention is made of polycarbonate tubing or other flexible material having similar properties, it is able to dissipate the pressure change through itself collapsing and later re-expanding when the momentum of the air in-rush momentarily causes a temporary overpressure within the confines of the jacket and end cap. The pressure drop will normally be great enough with most present-day lamps to cause the terminals, whether of the single or double pin type, to be sucked into the interior of the jacket. As is wellknown the withdrawal of the terminal pins will cause the lamp to fall from its socket or fixture if it has not already done so because of the mishap which occasioned the breaking of the lamp envelope inasmuch as the conventional fluorescent lamp depends on its terminals not only as the means for supplying electrical power but as the mechanical support within the fixture as well. In accordance with one aspect of my invention, even if the jacketed fluorescent lamp should fall from its socket the end cap which I have described herein will not separate from the jacket thus efiectively containing the shards of glass, the fluorescent powders and oxides and preventing them from coming into contact with personnel.
While I have described two illustrative embodiments of my invention it will be apparent to those skilled in the art that modifications to the recesses 6 and 6 may be made as to the shape and size of the subgated vent so long as the dimensions of the recesses and vents are not changed to make the fit of the jacket too loose.
What is claimed is:
1. A reusable protective assembly for a glass envelope fluorescent lamp having terminal-bearing end-portions, comprising a cylindrical translucent plastic jacket having resilient walls,
said jacket having an axial length slightly shorter than the envelope of said lamp, and
a plastic end cap disposed at each end of said jacket, each said end cap having:
an internal periphery portion for frictional engagement with the periphery of a respective one of said end-portions of said lamp,
an annular recess for receiving and providing a substantially air-tight seal with a respective end of said cylindrical jacket,
and a thin webbed section opposite said annular recess for closing the end of said cylindrical jacket;
said resilient walls of said jacket having sufficient flexure during an implosion of said lamp envelope to absorb the accompanying pressure change thereby preventing separation of said end caps from said jacket.
2. A reusable assembly according to claim 1 wherein said annular recess of each of said endcaps is defined by a pair of concentrically positioned, tapered wall sections, each of said pair of sections being free over a substantial portion of their length axial to said jacket to flex radially toward and away from said glass envelope of said lamp.
3. A reusable assembly according to claim 1 wherein said annular recess is provided with a subgated vent along a predetermined portion of its periphery to prevent entrapment of air in said recess thereby facilitating assembly of said jacket with said end cap.
4. A reusable assembly according to claim 1 wherein said jacket is made of poly carbonate tubing having an impact strength of approximately 14 foot-pounds, lzod and wherein said end caps are made of high heat stabilized polypropylene.
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