Compact fluorescent tube

Abstract

 A compact fluorescent tube comprises one or more U-shaped main parts (14) which together form a closed discharge chamber (28), which is coated internally with fluorescent material (18) and is provided with electrodes (16) at each end thereof. Each main part includes two straight and parallel tubular legs (19) of circular cross-section and a bridging or connecting tube (20) provided at one end of the tubular legs. At the junction with respective straight tubular legs, the bridging tube forms corner regions (24) whose cross-sectional area (A) is greater than the cross-sectional area of remaining parts of the bridging tube and of the straight tubular legs. The bridging tube (20) is slightly arcuate between the corner regions (24). The cross-section at the apex (27) centrally between the corner regions has an area (B) which is smaller than the cross-sectional area (C) of each of the straight tubular legs (19). A keyhole-shaped bending centre (21) is formed between the tubular legs (19), the diameter (d) of this bending centre in the circular-arcuate part being greater than the distance (e) between the legs.

Description

[0001] The present invention relates to a compact fluorescent tube, or lamp, which comprises one or more U-shaped main parts which together form a closed discharge chamber, electrodes mounted at each end of the discharge chamber, and an inner surface coating of fluorescent material, wherein each main part is formed by bending an originally straight glass tube and includes two straight and mutually parallel tubular legs of circular cross-sect ion and a bridging or connecting tube at one end of the tubular legs, and wherein the bridging tube forms at its junctions with the straight tubular legs corner regions whose cross-sectional areas are larger than the cross-sectional area of remaining parts of the bridging tube and the straight tubular legs. The invention also relates to a method for producing such a compact fluorescent tube.

[0002] Compact fluorescent tubes of this kind have been available commercially for several years without being able to compete seriously with typical incandescent bulbs. Neither can any serious breakthrough be expected while the price of such tubes is at its present relatively high level. In normal cases, the considerably longer burning time of conventional compact fluorescent tubes does not compensate for the higher price of these tubes. When setting the price of a compact fluorescent tube, one essential factor is the cost entailed by the manufacture of the U-shaped main tube part and particularly its top configuration. The manufacturing cost of this part is governed primarily by the fact that the top configuration of the tube is created with excessive emphasis on the willingness of the tube to ignite and the efficiency of the tube, while manufacturing and handling matters have taken secondary importance.

[0003] EP 61758 discloses one such known compact fluorescent tube in which the bridging tube in the tube U-bend is greatly enlarged in relation to the straight glass tubes, so as to enable the discharge column or stream to pass easily therethrough. The pointed and outwardly extended corner regions of the bridging tube are formed in a manner to provide cooling zones in which the mercury vapour is cooled effectively as the tube burns, so that the mercury vapour is able to condensate and maintain a given partial pressure which is adapted to the best possible light yield. This construction provides a compact fluorescent tube which functions well, although at the cost of a somewhat troublesome manufacturing process and also primarily at the cost of a fragile end product. Blow-moulding of the sharp, outwardly drawn corners requires a relatively high blow-moulding pressure and is both time and energy demanding. Furthermore, the method results in a very thin glass wall throughout the whole of the upper part of the tube, and particularly in the corner regions thereof, the glass thickness of which is only about 0.2 mm, meaning that there is a high injury risk in the manufacturing process and subsequent distribution and also when fitting the tube into different lamp fittings. Another drawback with the known tube resides in the very abrupt bending of the originally straight glass tube, which although being imparted the desired compact form with two closely adjacent tubular legs also has a very large and uneven glass-thickening at the centre of the bend. This uneven glass distribution across the cross-section of the tube results in the introduction of stresses in the glass, which can cause cracks to form either immediately in the manufacturing process or at some later time.

[0004] One object of the present invention is therefore to provide a compact fluorescent tube which has greater strength in manufacture, in transportation and in installation than the earlier known tubes of the aforementioned kind, while retaining or improving the operational properties of the tube. Another object of the invention is to provide a harmonically and naturally curved compact fluorescent tube which will enable manufacture to be effected at a high rate of manufacture and at a cost which is lower than the cost of the earlier known tubes. Other objects of the invention and advantages afforded thereby will be apparent from the following description. These objects are achieved with a compact fluorescent tube having the characteristic features set forth in the following Claims.

[0005] The invention is based on the realization that from a functional aspect, the top configuration of a compact fluorescent tube need not include pointed or outwardly drawn corner regions, outwardly projecting ridges, extreme constrictions or like features. The marginal improvement of the starting and operational properties of such tubes that can be achieved with different special configurations is not in reasonable proportion to the additional costs caused by such special configurations. The invention thus take the natural U-shape as its starting point, although the U-shape is adapted to create marked cooling zones in the tube corner regions and to avoid uneven glass distribution. According to the invention, the corner regions have a larger cross-sectional area than remaining parts of the discharge chamber while the centre part of the bridging tube has a smaller cross-sectional area than said remaining parts of the discharge chamber while being curved slightly upwards from the corner regions, resulting in a harmonic and relatively easy to produce basic form. The inner glass or mantle surface at the centre of the bend is conveniently enlarged and given a circular-arcuate shape, which together with the inner glass or mantle surfaces of the straight tubular legs define a keyhole-like space between the legs. This enables the glass to be distributed more evenly in the bend while avoiding the formation of puckers or folds, therewith reducing the risk of internal stresses and cracks.

[0006] The slightly upwardly curved bridging tube and the delimited sharp corner regions provide for gentle bending of the tube, which can therewith more readily be given a greater glass thickness in the outer periphery of the tube, i.e. a thickness of 0.35-0.5 mm, compared with a thickness of about 0.2 mm of the earlier tubes. This together with the keyhole-shaped aperture in the centre of the bend also provides a small difference in glass thickness between the outer and the inner parts of the U-shaped tube. This enables the glass to be freed more readily from stresses in the process of manufacture, which in turn results in less risk of cracks forming in the glass. The gentle and more natural curved form of the tube enables the top configuration of the tube to be produced at a lower blow-moulding pressure in order to fill-out all parts of the configuration, therewith enabling manufacture to proceed at a greater speed. The mechanically strong U-form is much easier to handle during all parts of the tube manufacturing process, which is preferably fully automated.

[0007] The invention will now be described in more detail with reference to the accompanying drawings, in which

Fig. 1 is a side view, partly in section, of an inventive compact fluorescent tube fitted to a fluorescent lamp;

Fig. 2 illustrates the lamp of Fig. 1 from above;

Fig. 3 is a cross-sectional view of a U-shaped top part, taken on the line 3-3 in Fig. 1; and

Fig. 4 illustrates a number of cross-sectional views of the U-shaped top, taken on the degree lines shown in Fig. 3.

[0008] The compact fluorescent tube 11 illustrated in Fig. 1 forms part of a so-called fluorescent tube lamp, which also includes a base housing 12 having a typical standardized screw base 13. The base housing accommodates all the series impedence devices required to operate the lamp. Alternatively, the compact fluorescent tube 11 may be fitted with a conventional compact fluorescent tube base which lacks such series impedence devices, in which case it is necessary to use lamp fittings which are intended particularly for such fluorescent tubes. The illustrated compact fluorescent tube includes two U-shaped main parts 14 which are mutually joined by a connecting or bridging tube 15. Each main part has an electrode 16 mounted at one of the free ends 17 of the tube. Together with the bridging tube 15, the U-shaped main parts 14 form a continuous, air-tight discharge chamber with the electrode 16 mounted at each end. The inner surface of respective main parts 14 is coated with a fluorescent material 18 which converts the released UV-radiation to visible light. Each main part 14 includes two straight tubular legs 19 of constant circular cross-section and a connecting or bridging tube 20 therebetween, this bridging tube having a slightly varying circular cross-section.

[0009] The main parts 14 are produced from originally straight tubes which have the same cross-section as the tubular legs 19. The tubes are heat-treated and curved in a known manner about an inner bending centre 21, whereafter the U-shaped top is blow- moulded. The originally straight tube is bent or curved through an angle of 180° around the bending centre 21, to a desired compact shape with a very small space e between the tubular legs 19. The space between the inner proximal mantle surfaces 20 of the tubular legs, i.e. the distance e, will preferably not be greater than 2-3 mm, thereby producing a very sharply curved inner wall surface 23 at the bending centre 21. In order to avoid an excessively small inner radius of curvature, and therewith thickening and/or puckering of the glass, the inner glass surface 23 is given an arcuate shape whose diameter d is greater than the distance e between the tubular legs. The diameter d will conveniently be 3-4 mm.

[0010] The cross-sectional shape of the bridging tube 20 between the tubular legs 19 is shown in Figs. 3 and 4. Marked or pronounced corner regions 24 are thus formed between the bridging tube and each tubular leg, whereas the major part of the bridging tube is comprised of a slightly arcuate top part 25. The corner regions 24 have an extension f of 10°-20° of the total tube bend of 180°. Each corner part 24 is suitably curved through an angle of between 30 and 45 of the bend angle and has a radius of curvature from the bending centre 21 of from 50-75% of the radius r of the straight tubular legs. The slightly curved outer wall surface 26 of the top part 25 extends between the corner regions in a section of 80°-100° of the bend of the U-shaped main part and has a radius of curvature twhich is 3-8 times larger than the outer diameter of the straight tubular legs 19. The values of the radii of curvature h and t are chosen to give the corner regions an outwardly projecting cold zone which is located slightly to one side of the main propagation path of the discharge column or stream, this propagation path being governed in this section of the tube by the cross-sectional area B of the top point or apex 27 centrally between the two corner regions 24. The radius h will not be chosen at such a small value and the radius t will not be chosen at such a large value as to prevent a top form of desired strength and glass thickness being produced in a simple produc- tion/technical manner.

[0011] The flattened top part 25 creates together with the enlarged circular-arcuate wall surface 23 in the bending centre 21 a top form having an inner cross-sectional area A at the corner regions 24 which exceeds the cross-sectional area of remaining parts of the discharge chamber 28. The smallest cross-sectional area B of the discharge chamber is formed at the same time in the apex 27 between the corner regions 24. Sufficiently effective cooling zones can be created without requiring extreme variations in the cross-sectional area of the discharge chamber. Accordingly, the area A at the corner regions 24 will preferably be 10-20% greater than the corresponding inner cross-sectional area C of the straight tubular legs 19, and the cross-sectional area B at the apex 27 will preferably be 10-50% smaller than the cross-sectional area C. A particularly suitable top form is obtained when the cross-sectional area B is 70-80% of the inner cross-sectional area C. From a handling and aesthetic aspect, each U-shaped main part will preferably have one and the same width across both legs and top region of the tube. Thus, the outerdiam- eter k perpendicular to the plane of the U-bend in Fig. 3 will be constant along the entire bridging tube, whereas the perpendicular outer diameter m will vary along the bridging tube and will have a largest value ml immediately over a 30° bend and a smallest value m2 at the centre position or 90° bend. The constant width k simplifies handling of the glass tubes in the blow-moulding process, while enabling gripping devices, etc., used in the automated manufacturing process to be adapted to one single external measurement. The ratio between the diameters k and m also contributes towards a more uniform glass distribution over the section when bending and moulding the glass tube. The thickness of the glass in the bridging tube will thus always be at least 0.35 mm and will not exceed 2 mm at the bending centre 21.

Claims

1. A compact fluorescent tube which includes one or more U-shaped main parts (14) which form a closed discharge chamber (28) having electrodes (16) mounted at each end of the chamber and being coated internally with a fluorescent material (18), wherein each main part is shaped by bending an originally straight glass tube and includes two straight and mutually parallel tubular legs (19) of circular cross-section and a connecting or bridging tube (20) at one end of the tubular legs, and wherein the bridging tube forms at the junction with respective straight tubular legs corner regions (24) whose cross-sectional area (A) is larger than the cross-sectional area of remaining parts of the bridging tube and of the straight tubular legs, characterized in that in longitudinal section, the bridging tube (20) has an outer, gently curved mantle surface (26) between the corner regions (24), and an inner circular-arcuate mantle surface (23) between the mutually proximal mantle surfaces (22) of the straight tubular legs; in that the inner circular-arcuate mantle surface (23) and the mutually proximal mantle surfaces (22) of the straight tubular legs together define a keyhole-shaped bending centre (21) whose diameter (d) in the circular-arcuate part is greater than the distance (c) between the mutually proximal mantle surfaces; and in that the cross-section of the bridging tube in the centre position (27) between the corner regions (24) has an area (B) which is smaller than the cross-sectional area (C) of each of the straight tubular legs (19).

2. A compact fluorescent tube according to Claim 1, characterized in that in longitudinal section the corner regions (24) have a radius of curvature (h) which is 50-75% of the radius (r) of the straight tubular legs.

3. A compact fluorescent tube according to any one of Claims 1 or 2, characterized in that the outer slightly curved wall surface has a radius of curvature (t) which is 3-8 times greater than the outer diameter of the straight tubular legs.

4. A compact fluorescent tube according to any one of the preceding Claims, characterized in that the inner cross-sectional area (B) of the bridging tube (20) in the centre position (27) between the corner regions (24) is 50-90%, preferably 70-80%, of the inner cross-sectional area (C) of each of the straight tubular legs (19).

5. A compact fluorescent tube according to any one of the preceding Claims, characterized in that the glass thickness in the bridging tube including the corner regions is at least 0.35 mm.

6. A compact fluorescent tube according to any one of the preceding Claims, characterized in that the inner circular-arcuate mantle surface (23) between the straight tubular legs (19) has an evenly distributed glass mass with a maximum glass thickness of 2 mm.

7. A compact fluorescent tube according to any one of the preceding Claims, characterized in that the circular-arcuate part (23) of the keyhole-shaped bending centre (21) has a diameter (D) of 3-4 mm.

8. A compact fluorescent tube according to any one of the preceding Claims, characterized in that the outer slightly curved mantle surface (26) has, in section, an extension of 80°-100° of the bend of the U-shaped main part about the bending centre (21) and each corner region (24) has an extension equal to 10°-20° of said bend.

9. A method for producing compact fluorescent tubes of the kind which include one or more U-shaped main parts (14) which form a closed discharge chamber (28) in which electrodes (16) are mounted at each end thereof and which is coated internally with fluorescent material (18), wherein each main part includes two straight and parallel tubular legs (19) of circular cross-section and a bridging or connecting tube (20) at one end of the tubular legs, said method comprising heating and bending straight glass tubes to form the U-shaped main parts (14) and blow-moulding to form the bridging or connecting tube (20), wherein the bridging tube forms at the junction with respective straight tubular legs corner regions (24) whose cross-sectional area (A) is greater than the cross-sectional area of remaining parts of the bridging tube and the straight tubular legs, characterized by forming the bridging tube (20) so that, in longitudinal section, it presents an outer, slightly curved mantle surface (26) between the corner regions (24), and an inner circular-arcuate mantle surface (23) between the mutually proximal mantle surfaces (22) of the straight tubular legs, wherein there is formed a keyhole-shaped bending centre (21) whose diameter (d) in the circular-arcuate part is greater than the distance (c) between the mutually proximal mantle surfaces; and giving the cross-section of the bridging tube in the centre position (27) between the corner regions (24) an area (B) which is smaller than the cross-sectional area (C) of each of the straight tubular legs (19).

Drawing