Patent Publication Number: US-9851055-B2

Title: Lighting device with mechanical fastening part

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2015/064475, filed on Jun. 26, 2015, which claims the benefit of European Patents Applications Nos. 14196703.4, filed on Dec. 8, 2014, and 14174837.6, filed on Jun. 27, 2014. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of lighting devices. In particular, the present invention relates to lighting devices comprising a light generating unit and a stem for supporting the light generating unit in an envelope of the lighting device. 
     BACKGROUND OF THE INVENTION 
     Traditional incandescent lighting devices are currently being replaced by more energy efficient alternatives, such as halogen lighting devices and LED lighting devices. When designing new lighting devices, it is desirable to resemble the traditional design of incandescent lighting devices in order to enable use of existing manufacturing equipment. 
     In traditional incandescent light bulbs, a tungsten filament is normally supported in a glass envelope by means of a glass stem. An LED based lighting device comprising a light generating unit supported by a core column in an envelope is shown in CN 203115641 U. The light generating unit is connected to the core column by means of a steel needle and a chuck. A drawback with such a lighting device is that the structure for fastening the light generating unit to the core column is rather complex, thereby rendering manufacturing of the lighting device cumbersome. 
     SUMMARY OF THE INVENTION 
     It would be advantageous to achieve a lighting device and a method of manufacturing a lighting device overcoming, or at least alleviating, the above mentioned drawback. In particular, it would be desirable to enable facilitated manufacturing of a lighting device. 
     To better address one or more of these concerns, a lighting device and a method of manufacturing a lighting device having the features defined in the independent claims are provided. Preferable embodiments are defined in the dependent claims. 
     Hence, according to a first aspect, a lighting device is provided. The lighting device comprises an envelope, a light generating unit, a stem arranged to support the light generating unit inside the envelope, and a mechanical fastening part arranged to fasten the light generating unit at the stem. The mechanical fastening part comprises at least one protrusion and the light generating unit comprises at least one hole adapted to mate with the at least one protrusion so as to fasten the light generating unit to the mechanical fastening part. 
     According to a second aspect, a method of manufacturing a lighting device is provided. The method comprises arranging a mechanical fastening part on a stem, the mechanical fastening part comprising at least one protrusion, and arranging a light generating unit comprising at least one hole on the mechanical fastening part such that the at least one hole mates with the at least one protrusion, thereby fastening the light generating unit to the mechanical fastening part. The method further comprises arranging the stem at least partially inside an envelope such that the light generating unit is supported by the stem in the envelope. 
     The present invention is based on an idea of using a stem for supporting the light generating unit in the envelope, which enables making use of existing manufacturing equipment previously used for incandescent lighting devices having a similar stem. By using a mechanical fastening part in order to connect the light generating unit to the stem, it is possible to make use of a stem designed similar to a traditional glass stem of an incandescent lighting device. As the light generating unit is coupled to the stem by mating the protrusion of the mechanical fastening part with the hole in the light generating unit, the structure of the mechanical fastening part can be less complex and manufacturing of the lighting device is facilitated. For example, the present aspect allows making the mechanical fastening part in a single piece of material. Further, the mechanical fastening part may provide a more rigid connection between the light generating unit and the stem, which may better cope with high g-forces that may occur during handling in the factory or by the end user. Moreover, the at least one hole of the light generating unit may be used to position and hold the light generating unit in an assembly tool, which further facilitates manufacturing. 
     The mechanical fastening part may alternatively be referred to as mechanical fixation means. 
     The stem may be an elongated part, preferably extending in a direction along an optical axis of the lighting device. 
     Further, the at least one protrusion mating with the at least one hole may include that the at least one protrusion extends into (optionally through) the hole such that the at least one protrusion may engage with a portion of the light generating unit at the hole. The light generating unit being fastened to the stem may not necessarily mean that it is completely fixed to the stem, (at least some) movement of the light generating unit relative to the stem may be allowed in one or more degrees of freedom as long as the light generating unit is supported by the mechanical fastening part on the stem in the envelope. 
     According to an embodiment, the at least one hole may comprise a through hole. Hence, the at least one protrusion may be mated with the at least one through hole by being inserted in the through hole, which facilitates fastening the light generating unit to the mechanical fastening part. A through hole allows inserting the protrusion in the hole such that the protrusion projects out of the hole, whereby the tip of the projecting portion of the protrusion may be deformed so as to hinder the light generating unit to come off from the protrusion. Alternatively (or additionally), the at least one hole may comprise a blind hole (such as a recess or indent). It will be appreciated that the at least one hole may have any suitable shape, such as elongated, circular or polygonal. 
     According to an embodiment, the mechanical fastening part may comprise a plurality of protrusions arranged (circumferentially) around a longitudinal axis of the stem. Preferably, the light generating unit may comprise a plurality of holes adapted to mate with the plurality of protrusions. Thus, the plurality of holes of the light generating unit may also be arranged (circumferentially) around the stem. According to an embodiment, the light generating unit may be arranged to at least partially surround the mechanical fastening part. Further, the light generating unit may be arranged to at least partially surround the stem. Each one of the present embodiments allow using a chimney-like light generating unit adapted to emit light in radial directions from the lighting device while providing a more rigid fastening of the light generating unit to the stem. Further, such chimney-like shape of the light generating unit may be advantageous in that it facilitates heat dissipation the light generating unit, as it enables heat convention through the chimney-like light generating unit. Manufacturing of the lighting device is facilitated since the light generating unit may be wound onto (or rolled upon) the mechanical fastening part around the longitudinal axis of the stem. 
     According to an embodiment, the light generating unit may comprise one or more carriers and one or more solid state light sources arranged at the one or more carriers. The one or more carriers may e.g. comprise one or more rigid or flexible circuit boards (such as printed circuit boards, PCBs). The one or more solid state light sources may e.g. comprise one or more light emitting elements, LEDs. In an embodiment, a plurality of carriers may be interlinked so as to form a partially flexible structure able to be wound around the mechanical fastening part and the stem, which facilitates manufacturing of the lighting device. According to an embodiment, the at least one hole may be arranged in the carrier, thereby allowing fastening the carrier to the mechanical fastening part. 
     According to an embodiment, the mechanical fastening part may comprise a metal sheet. Since the structure for fastening the light generating unit to the stem is simplified by using the mechanical fastening part with one or more protrusions, a metal sheet (optionally a single metal sheet) can be used for forming the mechanical fastening part. A metal sheet is relatively easy to form into a desired shape, which facilitates manufacturing of the lighting device. In an embodiment, the metal sheet may be folded and/or deep drawn so as to define at least one geometrical feature adapted to fasten the mechanical fastening part to the stem. Thus, no additional part further to the metal sheet is necessary for fastening the mechanical fastening part to the stem, whereby the structure of the lighting device is less complex and manufacturing is facilitated. It will be appreciated that in embodiments wherein the mechanical fastening part is made of a material other than a metal sheet, the mechanical fastening part may comprise a geometrical feature adapted to fasten the mechanical fastening part to the stem. The geometrical feature may then be formed in a way suitable for processing the material, such as folding or casting. 
     According to an embodiment, an edge (such as a rim) of the metal sheet may be shaped so as to define the at least one protrusion. For example, the metal sheet may be cut so as to form the edge with one or more protrusions, whereby manufacturing of the lighting device is further facilitated. 
     For example, the geometrical feature of the mechanical fastening part may be arranged at a center portion of the mechanical fastening part, and the one or more protrusions may be arranged circumferentially around the center part, whereby the light generating unit may be wound onto the mechanical fastening part. 
     According to an embodiment, the stem may comprise a geometrical feature arranged to mate with a geometrical feature of the mechanical fastening part so as to fasten the mechanical fastening part to the stem. For example, the mechanical fastening part may be snapped onto the stem and/or the geometrical feature of the mechanical fastening part may be deformed after having been applied to the stem so as to mate with the geometrical feature of the stem for locking the mechanical fastening part to the stem. The present embodiment facilitates manufacturing of the lighting device. 
     In general, in the present specification, the term “geometrical feature” means a formation going inwards or outwards in a structure, such as a protrusion, an indent, a recess, a blind-hole a through-hole, a corner or edge. Further, a geometrical feature of a part may be adapted to mate with a geometrical feature of another part so as to hinder movement between the two parts in at least one degree of freedom. 
     According to an embodiment, the stem may be light transmissive, whereby the affection of the stem on the light distribution of the lighting device is reduced. For example, the stem may comprise glass, whereby the stem may resemble a standard stem used in traditional incandescent light bulbs for supporting the filament, which in turn facilitates use of manufacturing equipment previously used for such traditional incandescent light bulbs. 
     According to an embodiment, the lighting device may further comprise a cap connected to the envelope and supporting the stem in the envelope. The cap may be arranged so as to electrically, and preferably also mechanically, couple the lighting device to an outside power supply. The cap may be arranged at the lower side of the stem. The lighting device may further comprise a driver for driving the light generating unit and being electrically coupled to the outside power supply via the cap. 
     According to an embodiment, the at least one protrusion may be shaped so as to lock the light generating unit to the mechanical fastening part. For example, the at least one protrusion may be deformed, such as folded or twisted, so as to prevent itself from slipping out of the hole. With the present embodiment, manufacturing of the lighting device is facilitated and a rigid fastening of the light generating unit to the mechanical fastening part is provided since the at least one hole of light generating unit may first be threaded on to the at least one protrusion of the mechanical fastening part and then the at least one protrusion (such as the end portion of the protrusion) may be deformed so as to lock the light generating unit to the mechanical fastening part. 
     It is noted that embodiments of the invention relates to all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the lighting device are all combinable with embodiments of the method as defined in accordance with the second aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects will now be described in more detail with reference to the appended drawings showing embodiments. 
         FIG. 1  is an exploded view of a lighting device according to an embodiment. 
         FIG. 2  shows a light generating unit mounted to a stem of the lighting device shown in  FIG. 1 . 
         FIGS. 3 a -3 c    show a mechanical fastening part connected to a stem according to an embodiment. 
         FIGS. 4 a  to 4 c    show a mechanical fastening part connected to a stem according to another embodiment. 
         FIGS. 5 a  to 5 c    show a mechanical fastening part connected to a stem according to yet another embodiment. 
         FIGS. 6 a  to 6 d    show a mechanical fastening part connected to a stem according to yet another embodiment. 
         FIGS. 7 a  to 7 c    show a mechanical fastening part connected to a stem according to yet another embodiment. 
         FIGS. 8 a  to 8 c    show a mechanical fastening part connected to a stem according to yet another embodiment. 
         FIGS. 9 a  to 9 e    show a mechanical fastening part connected to a stem according to another embodiment. 
         FIGS. 10 a  to 10 c    show a mechanical fastening part connected to a stem according to yet another embodiment. 
         FIG. 11  is a schematic illustration of a method of manufacturing a lighting device according to an embodiment. 
         FIGS. 12 a  to 12 e    show a part of a method of manufacturing a lighting device according to an embodiment. 
     
    
    
     All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the embodiments, wherein other parts may be omitted or merely suggested. Like reference numerals refer to like elements throughout the description. 
     DETAILED DESCRIPTION 
     The present aspects will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the present aspects to the skilled person. 
     A lighting device according to an embodiment will be described with reference to  FIG. 1  showing an exploded view of the lighting device. It will be appreciated that the examples of various features of the lighting device described with reference to  FIG. 1  are combinable with other embodiments. 
     The lighting device  1  may comprise an envelope  100 , e.g. shaped as a bulb, a light generating unit  120 , a stem  130  arranged to support the light generating unit in the envelope  100  and a mechanical fastening part  110  arranged to fasten the light generating unit  120  to the stem  130 . The stem  130  may extend along the optical axis of the lighting device  1 . The stem  130  may be light transmissive. For example, the stem  130  may be made of glass or any other transparent or translucent material. Optionally, the stem  130  may comprise a base portion  131  and a pump tube portion  132  having a smaller diameter than the base portion  131 . Wires  135  may be arranged at the stem  130  for electrically connecting the light generating unit  120  to a driver  140  of the lighting device  1 . The lighting device  1  may further comprise a cap  160  for electrically connecting the driver  140  to a power supply. For example, the cap  160  may be a screw base. The cap  160  may be arranged at the lower end of the stem  160 . The lighting device  1  may further comprise an isolation part  150  arranged to electrically isolate the driver  140  from the cap  160 . 
     According to an embodiment, the envelope  100  may be filled with a Helium-Oxygen mixture. In lighting devices with such a gas mixture, the internal thermal resistance may be relatively high. Therefore, the light generating unit  120  may preferably have a chimney-like shape, as illustrated in more detail in  FIG. 2 , so as to function as a thermal chimney, which enhances heat dissipation from the light generating unit  120 . For example, the light generating unit  120  may comprise one or more carriers  121  arranged around the stem  130  so as to form a chimney-like structure. The one or more carriers  121  may e.g. be one or more circuit boards, such as one or more printed circuit boards, PCBs. The carriers  121  may be interconnected by means of connecting elements  122 . One or more light sources  123  may be arranged on the one or more carriers  121 . 
     The mechanical fastening part  110  may comprise one or more protrusions  115  and the light generating unit  120  may comprise one or more holes  126  adapted to mate with the protrusions  115  so as to fasten the light generating unit  120  to the mechanical fastening part  110 . For example, the holes  126  may be located in the one or more carriers  121 , as illustrated in  FIG. 2 . The protrusions  115  may e.g. extend through the holes  126  of the light generating unit  120 , such that the light generating unit  120  is supported by the protrusions  115 . Further, the protrusions  115  may be slightly deformed, such as bent or twisted, so as to reduce the risk of the light generating unit  120  coming off the protrusions  115  and to reduce possible movement between the light generating unit  120  and the stem  130 . The mechanical fastening part  110  may be formed by a single piece of material, such as metal. Preferably, the mechanical fastening part  110  may be formed by a piece of sheet metal. The desired shape of the mechanical fastening part  110  may e.g. be achieved by cutting, folding and/or deep drawing. For example, the metal sheet may be cut such that the rim of the metal sheet forms the protrusions  115 . 
     In the following, mechanical fastening parts and stems according to different embodiments will be described in more detail. The mechanical fastening parts and stems according to the embodiments described in the following may in general be similarly configured as the mechanical fastening part  110  and the stem  130  as described with reference to  FIGS. 1 and 2 . 
       FIGS. 3 a  to 3 c    show a mechanical fastening part  310  connected to a stem  330  according to an embodiment.  FIG. 3 a    is a perspective view,  FIG. 3 b    is a cross-section taken along line A-A in  FIG. 3 a   , and  FIG. 3 c    is a top view. 
     The mechanical fastening part  310  may be arranged around a longitudinal axis of the stem  330 , such that the protrusions  315  extend radially away from the stem  330  to be able to mate with the holes of the light generating unit arranged around a longitudinal axis of the stem  330  (e.g. as illustrated in  FIG. 2 ). Further, a void (such as blind hole or a through hole) may be located at a center portion of the mechanical fastening part  310  for enabling threading the mechanical fastening part  310  onto the stem  330 . 
     The mechanical fastening part  310  may further comprise a geometrical feature  317  adapted to mate with the stem  330 , such as with a geometrical feature  337  of the stem  330 . In the present example, the geometrical feature  337  of the stem  330  comprises a protrusion in the shape of a rim extending around the stem  330  and the geometrical feature  317  of the mechanical fastening part  310  comprises an indent  318  adapted to mate with the rim. The geometrical feature  317  of the mechanical fastening part  310  may further comprise one or more flanges  319  or the like arranged around the void so as to support the mechanical fastening part  310  around the stem  330 , whereby motion of the mechanical fastening part  310  relative to the stem  330  in the radial direction of the stem  330  is limited. Preferably, the geometrical features  317  and  337  of the mechanical fastening part  310  and the stem  330  may be arranged so as to limit motion of the mechanical fastening part  310  relative to the stem  330  in the longitudinal direction of the stem  330  and preferably also any tilt motion of the mechanical fastening part  310  relative to the stem  330 . 
     The mechanical fastening part  330  may comprise one or more voids  312  arranged somewhere between the protrusions  315  and the geometrical feature  317  of the mechanical fastening part  330 . The voids  312  enhance heat convention through the light generating unit, as they allow heat to flow along the stem  330 . In the present example, the metal sheet of the mechanical fastening part  330  may be folded so as to form the geometrical feature  317 . Thus, the metal sheet may be cut and folded so as to form the flanges  319  and the indent  318 . As the flanges  319  are folded towards the stem  330 , the voids  312  are provided between the geometrical feature  317  and the protrusions  315 . 
     Optionally, the mechanical fastening part  310  may further comprise one or more holes  316  arranged for facilitating fixing the mechanical fastening part  310  in an assembly tool. Further, in the present example, the general extension of the metal sheet forming the mechanical fastening part  310  may be transverse (such as substantially perpendicular) to the longitudinal direction of the stem  330 . 
       FIGS. 4 a  to 4 c    show a mechanical fastening part  410  connected to a stem  430  according to another embodiment.  FIG. 4 a    is a perspective view,  FIG. 4 b    is a cross-section taken along line A-A in  FIG. 4 a   , and  FIG. 4 c    is a top view. 
     The mechanical fastening part  410  and the stem  430  according to the present embodiment may be similarly configured as the mechanical fastening part and the stem according to the embodiment described with reference to  FIGS. 3 a  to 3 c   , except that the geometrical feature  417  of the mechanical fastening part  410  may comprise one or more end stop features  418  arranged to engage with an end portion  437 , such as a rim, of the stem  430  so as to limit motion of the mechanical fastening  410  part relative to the stem  430  in a direction along the longitudinal axis of the stem  430  and towards the cap (i.e. downwards in  FIGS. 4 a  and 4 b   ). For example, the end stop features  418  may be formed by folding or bending a portion of the metal sheet. In the present embodiment, no particular geometrical feature of the stem  430  is necessary and the stem  430  may optionally be free from such geometrical feature, thereby being easier to manufacture. Optionally, the mechanical fastening part  410  may further comprise guiding features  414  for facilitating threading the mechanical fastening part  410  onto the stem  430 . The guiding features  418  may e.g. be arranged at the lower end of the flanges  419 . 
       FIGS. 5 a  to 5 c    show a mechanical fastening part  510  connected to a stem  530  according to yet another embodiment.  FIG. 5 a    is a perspective view,  FIG. 5 b    is a cross-section taken along line A-A in  FIG. 5 a   , and  FIG. 5 c    is a top view. 
     The mechanical fastening part  510  and the stem  530  according to the present embodiment may be similarly configured as the mechanical fastening parts and the stems according to any of the previously described embodiments, except that the geometrical feature  517  of the mechanical fastening part  510  may comprise one or more (such as two) flanges  518 , which may be referred to as hooks, shaped so as to mate with an indent  537 , such as a circumferential indent  537  of the stem  530 , thereby limiting motion of the mechanical fastening part  510  relative to the stem  530 . The flanges  518  may preferably be arranged on opposite sides of the stem  530  so as to support the stem  530  from opposite sides. For example, the flanges  518  may be formed by cutting and folding the metal sheet. Further, the void in the center portion of the mechanical fastening part  510  may be a hole  535  cut out in the center portion of the metal sheet and may enable threading the mechanical fastening part  510  onto the stem  530 . The present embodiment is advantageous in that it is more robust and has less folds and bends, which facilitates manufacturing of the mechanical fastening part  510 . 
       FIGS. 6 a  to 6 d    show a mechanical fastening part  610  connected to a stem  630  according to yet another embodiment.  FIG. 6 a    is a perspective view,  FIG. 6 b    is a cross-section taken along line A-A in  FIG. 6 a   ,  FIG. 6 c    is a top view illustrating when the stem  630  has been inserted in the void at the center portion of the mechanical fastening part  610 , and  FIG. 6 d    is a top view illustrating when the stem  630  is being inserted in the void at the center portion of the mechanical fastening part  610 . 
     The mechanical fastening part  610  and the stem  630  according to the present embodiment may be similarly configured as the mechanical fastening part and the stem according to the embodiment described with reference to  FIGS. 5 a  to 5 c   , except that the geometrical feature  617  of the mechanical fastening part  610  may comprise a single flange  618 , which may be referred to as a hook, shaped so as to mate with an indent  637  of the stem. In the present embodiment, the indent  637  may not necessarily extend circumferentially around the whole stem  630 , but merely at one side of the stem  630 . The geometrical feature  617  of the mechanical fastening part  610  may further comprise one or more (such as two) locking features  619  (which may be referred to as locking fingers) arranged to hold the stem  630  in the void at the center portion of the mechanical fastening part  610 . The locking features  619  may also be arranged to guide the stem  630  into place at the center of the mechanical fastening part  610  upon assembly, as illustrated in  FIG. 6 d   . The stem  630  may be placed eccentrically with the stem  630  and then moved in direction towards the center of the mechanical fastening part  610 , as illustrated by arrow  650  in  FIG. 6 d   , and pressed to pass the locking features  619 , thereby being snapped into the right position at the center of the mechanical fastening part  610 . 
       FIGS. 7 a  to 7 c    show a mechanical fastening part  710  connected to a stem  730  according to yet another embodiment.  FIG. 7 a    is a perspective view,  FIG. 7 b    is a cross-section taken along line A-A in  FIG. 7 a   , and  FIG. 7 c    is a top view. 
     The mechanical fastening part  710  and the stem  730  according to the present embodiment may be similarly configured as the mechanical fastening part and the stem according to the embodiment described with reference to  FIGS. 6 a  to 6 d   , except that the geometrical feature  717  of the mechanical fastening part  710  may comprise a flange  718  with an edge extending along the longitudinal direction of the stem  730  having a protrusion  719  (or dent) adapted to mate with the indent  737  of the stem  730 . The flange  718  may further comprise folded portions  711 , which together with the locking features  719  may be adapted to hold the stem  730  in position at the center of the mechanical fastening part  710 . Optionally, the edges  712  of the locking features  719  may be long enough (and preferably straight) to act as aligning features for making the light generating unit better aligned with the stem  630 . 
       FIGS. 8 a  to 8 c    show a mechanical fastening part  810  connected to a stem  830  according to yet another embodiment.  FIG. 8 a    is a perspective view,  FIG. 8 b    is a cross-section taken along line A-A in  FIG. 8 a   , and  FIG. 8 c    is a side view. 
     The mechanical fastening part  810  and the stem  830  according to the present embodiment may be similarly configured as the mechanical fastening part and the stem according to the embodiment described with reference to  FIGS. 7 a  to 7 c   , except that the edge of the flange  818  of the geometrical feature  818  of the mechanical fastening part  810  may comprise an indent  813  adapted to mate with a flare  837  at the end of the stem  830 , thereby limiting movement of the mechanical fastening part  810  relative to the stem  830  in the longitudinal direction of the stem  830 . 
       FIGS. 9 a  to 9 e    show a mechanical fastening part  910  connected to a stem  930  according to another embodiment.  FIG. 9 a    is a perspective view,  FIG. 9 b    is a cross-section taken along line A-A in  FIG. 9 a   ,  FIG. 9 c    is a top view illustrating the mechanical fastening part  910  in a closed position,  FIG. 9 d    is a top view illustrating the mechanical fastening part  910  in an open position, and  FIG. 9 e    shows a metal sheet  911  before it is folded to form the mechanical fastening part  910 . 
     In the present example, the general extension of the metal sheet  911  forming the mechanical fastening part  910  may be directed along (such as substantially parallel with) the longitudinal direction of the stem  930 . For example, the metal sheet  911  may be formed as a strip (as illustrated in  FIG. 9 e   ) folded into a cylinder-like (annular) shape (as illustrated in  FIGS. 9 a  to 9 d   ), the longitudinal axis of which may be directed along the longitudinal axis of the stem  930 . The protrusions  915  may be cut in the metal sheet  911  and folded so as to extend in a radial direction of the mechanical fastening part  910 . A portion of the metal sheet strip  911  may be folded so as to form a geometrical feature  917  of the mechanical fastening part  910  and so as to extend inwards in the cylinder-like shape of the mechanical fastening part  910 . The geometrical feature  917  may comprise a cut out (or hole)  918  adapted to mate with a geometrical feature  937 , such as a rim (or other protrusion), of the stem  930 . Further, the end portions  912  of the metal sheet strip  911  may be arranged to engage with each other so as to lock the geometrical feature around the stem  930  such that the cut out  918  of the mechanical fastening part  930  mates with the rim  937  of the stem  930 . Hence, the geometrical feature  917  of the mechanical fastening part  910  may be in an open position, as illustrated in  FIG. 9 d   , when inserting the stem  930  at the center portion of the mechanical fastening part  910 . Then the geometrical feature  917  may be closed such that the end portions  912  of the sheet metal strip  911  engages with each other, thereby fixing the mechanical fastening part  910  to the stem  930 . The present embodiment is advantageous in that the connection between the stem  930  and the mechanical fastening part  910  is stiffer. 
       FIGS. 10 a  to 10 c    show a mechanical fastening part  1010  connected to a stem  1030  according to yet another embodiment.  FIG. 10 a    is a perspective view,  FIG. 10 b    is a cross-section taken along line A-A in  FIG. 10 a   , and  FIG. 10 c    is a top view. 
     In the present embodiment, the geometrical feature  1017  of the mechanical fastening part  1010  may comprise a cylindrical feature  1018  adapted to mate with an end portion  1037  of the stem  1030 . Preferably, the end portion  1014  of the cylindrical feature  1018  may be at least partially closed, thereby preventing motion of the mechanical fastening part  1010  relative to the stem  1030  along the longitudinal direction of the stem  1030  towards the cap (i.e. downwards in  FIGS. 10 a  and 10 b   ). Preferably, an opening  1013  may be provided in the cylindrical feature for preventing a gas pocket to form in the stem  1030  during usage of the lighting device. For example, the geometrical feature  1017  of the mechanical fastening part  1010  may be formed by means of deep drawing the metal sheet. 
     A method  1100  of manufacturing a lighting device according to an embodiment will be described with reference to  FIG. 11 . The method  1100  may comprise arranging  1101  a mechanical fastening part on a stem, the mechanical fastening part comprising at least one protrusion, arranging  1102  a light generating unit comprising at least one hole on the mechanical fastening part such that the at least one hole mates with the at least one protrusion, thereby fastening the light generating unit to the mechanical fastening part, and arranging  1103  the stem at least partially inside an envelope such that the light generating unit is supported by the stem in the envelope. The lighting device may e.g. be a lighting device according to any one of the previously described embodiments. 
     The step of arranging  1102  the light generating unit on the mechanical fastening part according to an embodiment will be described in more detail with reference to  FIGS. 12 a  to 12 e    schematically illustrating the light generating unit being wound (or rolled) onto the mechanical fastening part. 
     Firstly, a manufacturing tool  180  may be open, the light generating unit  120  may be inserted in the tool  180  and the mechanical fastening part  110  may be put in position at the manufacturing tool  180 , as illustrated in  FIG. 12 a   . Then, a first one of the carriers  121   a  (such as the second carrier seen from one of the ends of the light generating unit) is positioned such that the hole of the carrier  121   a  mates with a first one of the protrusions  115   a  of the mechanical fastening part  110 , as illustrated in  FIG. 12 b   . A clamping tool  181  of the manufacturing tool  180  may then hold (or clamp) the connecting element  122  connecting the first carrier  121   a  to the neighboring carrier  121   b , as illustrated in  FIG. 12 c   . Further, a bending tool  182  of the manufacturing tool  180  may press down the neighboring carrier  121   b  such that the hole of the neighboring carrier  121   b  mates with the neighboring protrusion  115   b . Preferably, the bending tool may press the neighboring carrier  121   b  at the position of the connecting element  122 . Similarly, another neighboring carrier  121   c  may be pressed such that the hole of that carrier  121   c  mates with another neighboring protrusion  115   c  of the mechanical fastening part  110 , as illustrated in  FIG. 12 e   . This process may continue until the complete light generating unit  120  has been wound onto the mechanical fastening part  110 . Further, a crimping tool  190  may deform, such as twist or fold, some or all of the protrusions  115   a ,  115   b ,  115   c  of the mechanical fastening part  110  so as to further secure the light generating unit  120  onto the mechanical fastening part  110 . 
     In the following, further non-limiting examples of embodiments of the invention will be described. 
     To be able for LED lamps to leverage on the high volume production capabilities of cheap GLS lines, new assembly and fixations means for LED substrates may be needed to replace those used for filament placement and halogen burner placement. To reduce cost and to increase the production speed, only small modifications can be made to both glass lamp parts and production lines. In the following, it is described how a 3D structure of multiple L2 boards can be mounted to a standard stem of a glass (incandescent) bulb with small changes to said stem and consequently the production line. This allows the GLS productions lines to be used for the production of LED lamps. 
     Incandescent lamps incorporate supporting structures for their filaments. With advent of halogen lamps based on GLS designs, new fixation means were introduced that did not require the basic design of the stem to be changed. Halogen lamp production was thus able to leverage on the incandescent production. For LED lamps to be able to leverage on the GLS production, a similar solution is needed. 
     Over the past decades, GLS production lines have been improved for speed and efficiency. With the accelerating transition from conventional light bulb to LED bulbs, the demand for LED bulbs is putting a lot of pressure on the slow (1000 pcs/hr) production lines. LED bulb production lines are slowly moving towards (semi)automation, but it would be advantageous to utilize the existing high speed (4000 pcs/hr) lines for GLS. As the GLS lines are so well optimized, they have limited flexibility wrt design of the bulbs. Therefore, to reduce cost and keep the production speed high, only small modifications can be made to both glass lamp parts and production lines. Below, it is described an example of how a 3D structure of L2 boards (forming a thermal chimney) can be mounted to a standard stem with only small changes to said stem and consequently the production line. 
     Generally, an LED bulb may comprise an optical part and a thermal part that each roughly takes up 50% of the outer surface. Usually, the thermal part may comprise mostly metal for high thermal conductivity, thus lowering the internal thermal resistance of the lamp. 
     The LED lamp described in the following example may rely on an outer surface that is similar to a GLS lamp, thus fully made of glass. To cope with the intrinsically high internal thermal resistance of a gas filled glass bulb, a smart design is desired to effectively spread the heat inside the lamp. In order to do so, a LED structure was conceived that functions as a thermal chimney. Such a 3D structure of multiple L2s, may pose a problem for assembly. 
     A solution to this problem may be a simple (sheet metal) part that may have several functions: it may function as a ratchet/guide for ease of assembly of a flat L2 shape to a 3D L2 shape; it may support and position the 3D L2 assembly inside the lamp to the stem; it may orientate the L2 assembly to the stem to assist welding of L2 wires to stem wires; it may have an open structure to allow gas to flow through, for thermal dissipation and heat spreading; it may allow the L2 assembly to stay fixed to the stem at high g-forces (typically 850 g) that can occur during handling both in the factory as by end users. 
     The sheet metal supporting structure may preferably: fixate some or all six degrees of freedom of the L2 assembly; not degas (preventing lumen decay); imply reduced material use; have a reduced number of bends and be easy to cut (to save cost). 
     The (extended) pump tube of the stem may have a flare, bump, rim or indentation for mechanical fixation. 
     According to an example, a lighting device may be provided, which can be manufactured on the traditional lines for GLS lamps, that is the well-known incandescent lamp. The lamp may comprise a glass bulb, a glass stem and a lamp cap which may be similar to the GLS lamp. In the lamp, the light generating part may comprise a plurality of carriers each provided with one or more solid state light sources, like for instance LEDs. These carriers may be arranged around the glass stem and may be connected to the mechanical fixation part. The mechanical fixation part may be connected with the glass stem. 
     Further, the bulb may be filled with a He—O2 gas mixture for improving the thermal performance of the lamp. A LED driver may be mounted in the cap of the lighting device, said driver being electrically connected to the outside power supply via the cap, and further electrically connected to the light generation part via the wires that are positioned along the stem. 
     Itemized List of Embodiments 
     1. A lighting device comprising an envelope, a stem positioned inside the envelope and a cap arranged at the lower side of the stem and connected to said envelope, characterized in that the lighting device further comprises a light generating unit connected to the stem via mechanical fixation means.
 
2. The lighting device according to item 1, characterized in that the light generation means comprises at least one carrier with at least one solid state light source.
 
3. The lighting device according to item 1, characterized in that the light generation means comprises a plurality of carriers with at least one solid state light source.
 
4. The lighting device according to item 1, characterized in that the mechanical fixation means are mechanically coupled to the stem.
 
5. The lighting device according to item 4, characterized in that the mechanical fixation means are a deep drawn part or a folded metal sheet.
 
6. The lighting device according to item 1, characterized in that the stem is made from glass.
 
7. The lighting device according to item 1, characterized in that the stem is further provided with an indent, a rim, a bump or a flare for the fixation of the mechanical fixation means.
 
8. The lighting device according to item 1, characterized in that the envelope is made from glass.
 
9. The lighting device according to item 1, characterized in that the envelope is at least partly transparent or translucent.
 
10. The lighting device according to item 1, characterized in that the lighting device further comprises a driver unit.
 
11. The lighting device according to item 10, characterized in that said driver unit is electrically connected to the cap for connecting it to an outside power source.
 
12. The lighting device according to item 1, characterized in that the envelope is filled with a Helium-Oxygen mixture.
 
13. A method for manufacturing a lighting device, comprising the following steps;
         mounting the LEDs on a carrier   arranging the one or more carriers on mechanical fixation means   connecting the mechanical fixation means to the stem   inserting the sub-assembly of carriers with LEDs, mechanical fixation means and stem in the envelope   connecting the stem to the envelope   creating the helium-oxygen mixture in the envelope   closing the envelope   mounting the cap       

     The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the mechanical fastening part may be made of another material than sheet metal, such as cast metal, plastic or ceramic. 
     Additionally, variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.