One-piece thermally resistant gimbal device for a replaceable headlamp bulb

A replaceable headlamp bulb for automotive applications comprising a discharge light source, a gimbal device, and a plastic lamp base in which the gimbal device is a one-piece construction that allows the light source to be precisely aligned and locked in a specific position with respect to the lamp base. The gimbal device is a cylindrically shaped member made of glass-mica material with a metallic ring fitted in a castellated circumferential groove at one end of the glass-mica member. The light source is mounted at the opposite end of the gimbal device to the ring-fitted portion. The plastic base contains an internal cylindrical opening into which the ring-fitted end of the glass-mica gimbal is inserted and aligned with respect to the plastic base. The gimbal device is locked inside the plastic base by heating of the metallic ring using Radio-Frequency (RF). During the RF heating the mica material reacts in the vicinity of the metallic ring which is seated inside the castellated groove to lock the gimbal to the ring. Ring sizing tolerance can be relaxed because the gaps between the castellations on each side of the groove walls serve to accommodate an oversized ring so that the ring is contained within the groove.

FIELD OF THE INVENTION 
This invention relates to a replaceable automotive headlamp bulb. More 
particularly, the invention relates to such a replaceable bulb in which a 
device is provided for aligning and locking the light source in a fixed 
position with respect to a lamp base that is a part of the replaceable 
bulb. 
BACKGROUND OF THE INVENTION 
With automotive designs tending to lowered hood lines for better appearance 
of the front end of the vehicle and also better aerodynamic performance 
the need for smaller headlamp packaging is apparent. Smaller headlamp 
packaging requires smaller light bulbs which operate at higher 
temperatures, up to 700 degrees centigrade, due to their smaller size. 
These light bulbs, which are typically of the double-ended type but can be 
of the single-ended type, are made from quartz material. The light bulb is 
typically fixed onto a plastic base inside the headlamp assembly. The 
plastic base and the bulb mounted thereon forms a replaceable bulb. In 
order for the lamp to be acceptable to the average consumer, it must be 
cost competitive relative to other similar headlamps and must also be 
reliable. Therefore the replacement bulb must be of a construction with 
fewer parts for ease of assembly, quality and cost reasons. Proper 
alignment and stability of the light bulb inside the headlamp assembly are 
important requirements for the performance of the headlamp. The light bulb 
must be aligned in a specific position with respect to the plastic base. 
Once aligned and precisely locked in the appropriate position with respect 
to the plastic base, the bulb must remain in that aligned position during 
use, for acceptable performance of the headlamp. In fact, the lamp must 
conform to Federal Motor Vehicle Safety Standard 108 which defines a 
deflection test for replaceable headlamp bulbs. The deflection test 
requires that the light source must be rigidly mounted in the lamp fixture 
in such a manner that the bulb does not permanently deflect beyond a 
specified distance of 0.005 inches when a 4 pound force is applied to the 
bulb perpendicular to its longitudinal axis. 
A gimbal device is one of the mechanisms typically used to align the light 
source with respect to the plastic base. The gimbal, which serves as a 
holder for the bulb may also be required to insulate the plastic base from 
the heated bulb. The gimbal is generally contained within a cylindrical 
opening in the plastic base from which the light bulb can be aligned, 
after which the gimbal is attached to the plastic base by either thermal 
or mechanical means so as to secure the light bulb in the aligned 
position. 
U.S. Pat. No. 4,569,006 describes one type of gimbal device designed to 
align and precisely lock the light source in the aligned position while 
providing a system that is readily adaptable to mass production for cost 
reasons. While serving its intended function, this gimbal design has the 
disadvantage that it comprises three distinct pieces and so does not lend 
itself to mass production as readily as a construction with fewer pieces. 
In addition, the use of multiple parts/processes invariably has a negative 
quality impact resulting in yet additional quality costs. 
U.S. Pat. No. 4,795,939 describes a gimbal device comprising two pieces, a 
ceramic member used as a bulb holder for a single ended quartz bulb and a 
plastic member that joins the ceramic bulb holder to the plastic base. The 
plastic member contains a groove onto which a metallic ring is fitted. The 
ring-fitted end of the plastic member is attached to a cylindrical opening 
of the plastic base by means of radio-frequency (RF) heating of the 
metallic ring to form a plastic-to-plastic weld. The two-piece gimbal does 
not easily lend itself to mass production due to the multiple parts of the 
gimbal. 
From the standpoint of cost, quality, and ease of mass production it would 
be advantageous to eliminate the intermediate plastic member that connects 
the plastic base to the ceramic bulb holder. In this case a precise 
circumferential groove must be made onto the surface of the ceramic holder 
in order to contain the metallic ring which is heated to cause the ceramic 
member to be attached to the plastic base using RF energy. In order for 
this RF thermal attachment method to form a rigid joint, the ceramic must 
either deform or melt with the application of RF energy or the groove must 
be precisely sized to the dimensions and shape of the metallic ring. 
However ceramics will not soften or melt, so that any flowing which serves 
to interlock the ceramic and the plastic must be done by the plastic base 
only. While the plastic base material could melt, there is no guarantee 
that the plastic base will flow into the ceramic groove and act to 
stabilize the joint. If the gimbal material will not flow, the joint may 
be stabilized by manufacturing the gimbal to such tight tolerances that 
the groove in the gimbal surface is very close-fitting to the metal ring 
contained in the groove. In this case, the joint is formed by virtue of 
the protrusion of the metallic ring into the softened cylindrical portion 
of the plastic base and the melting and flowing of the plastic around the 
metal. The joint is rigidified by the locking effect of the ring, 
protruding into the plastic wall while still within the walls of the 
groove. 
This type of joint is dependent on the close-fitting nature of the ring and 
the groove. If there is any gap between the groove and the sides of the 
ring namely the tolerance gap, the gimbal may rock slightly, though still 
locked in the base. The rocking will allow the bulb to move outside the 
deformation limit of the federal specifications. Due to their nature, 
ceramic materials can not be molded to tight tolerances required for this 
type of fit. Ring sizing tolerance could also be tightly controlled for 
close-fitting of the ring into the groove, but this would be a costly 
option. The tapered side walls of the groove allow the tolerance gap to 
increase since the ring expands radially outwards as it melts into the 
plastic. Thus it is apparent that elimination of the tolerance gap would 
provide for a tighter more reliable joint. In addition, it would be 
particularly advantageous from a cost and quality standpoint to achieve 
such a tighter more reliable joint without the need to maintain a tight 
ring sizing tolerance. 
It is the objective of this invention to provide a one-piece gimbal device 
with a superior gimbal device/plastic base joint for improved reliability 
in firmly supporting the light bulb: that contains the fewest parts 
possible for ease of assembly; that can be manufactured to tight 
tolerances to produce a high quality part; that is adaptable to mass 
production at a reasonable cost; and one in which the ring sizing 
tolerance is not critical for obtaining a rigid gimbal device/plastic 
joint. 
SUMMARY OF THE INVENTION 
The present invention provides a means for precisely locking a cylindrical 
glass-mica member, used as a lamp holder, to a plastic base thus providing 
a rigid support for the light source as well as an insulating material 
between the high temperature light source and the plastic base. For 
replaceable bulbs used in automotive applications, the light source, which 
operates at high temperatures, must be aligned and then rigidly held 
precisely in that focused position with respect to the plastic base. A 
one-piece glass-mica gimbal is provided for use as a holder for the light 
bulb, as an aid in focusing the light source, and as a means of firmly 
attaching the gimbal to the plastic base. Essentially, the gimbal device 
is a one-piece construction that allows the light source to be precisely 
aligned and locked in a specific position with respect to the plastic 
base. 
In accordance with the present invention the devised means for aligning and 
precisely locking the light source consists of a cylindrical thermally 
resistant glass-mica gimbal member that is a holder for the light bulb, a 
metallic ring that is fitted onto a castellated groove on one end of the 
gimbal member, and a plastic base that contains an internal opening into 
which the ring-fitted gimbal device is engaged, aligned, and then firmly 
attached to the plastic base by thermal means. This embodiment constitutes 
the replaceable bulb of an automotive headlamp. The lamp holder is of a 
mica filled glass material that is precision manufactured to contain a 
castellated groove by injection molding and hence lends itself to mass 
production of a precision part at a reasonable cost. Upon heating, the 
metallic ring protrudes into the softened plastic, thus locking the 
metallic ring to the plastic base. As a result some free space, namely a 
tolerance gap, is formed between the sides of the groove and the metallic 
ring. The mica filled glass material is desirable because when the 
metallic ring is heated the mica particles expand or bloat in the vicinity 
of the metallic ring to fill up the tolerance gap between the sides of the 
groove and the ring, thus locking the gimbal member to the metallic ring. 
The alternating castellations in the ring-containing groove of the 
glass-mica holder will serve to accommodate an oversized ring inside the 
groove. The oversized ring will fit inside the groove in such a manner 
that upon heating its outward radial expansion will not result in a 
substantial increase in the tolerance gap so as to cause an unreliable 
joint. Hence the ring sizing tolerance can be relaxed while still 
eliminating the tolerance gap to obtain a locking effect between the sides 
of the groove and the metallic ring. The mica filled glass material is 
desirable for this application because of its bloating characteristic, 
high temperature resistance, low thermal conductivity, and ease of 
manufacturability of the castellated lamp holder to tight tolerances. 
The glass-mica bulb holder provides for a reliable gimbal/plastic joint and 
a high quality part manufacturable by mass production at a reasonable 
cost. The glass-mica material has a bloating characteristic which causes 
it to eliminate the tolerance gap between the groove and the ring thus 
ensuring a tight, reliable joint. The glass-mica material can also be 
precision molded to contain alternating castellations in the groove so 
that the ring sizing tolerance is not critical to achieving a tight fit. 
The one-piece ceramic gimbal also reduces the number of parts in the 
replaceable bulb assembly thus contributing to lower overall cost and 
higher quality of the headlamp.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. I is a side view of one embodiment 10 of the present invention 
illustrating the light source 12, the one-piece glass-mica gimbal device 
14, and the plastic base 16. The presented embodiment constitutes a 
replaceable bulb of an automotive headlamp. Thus a new bulb is replaced by 
installing into the headlamp fixture the preferred embodiment of FIG. 1 in 
a manner that is standard for a replaceable headlamp bulb of this type. 
Once installed in the fixture the assembly must conform to Federal Motor 
Vehicle Safety Standard 108. The standard demands that the replaceable 
bulb mounted inside the lamp fixture should not deflect beyond a specified 
distance of 0.005 inches in response to a 4 pound force applied 
perpendicular to its longitudinal axis. A tight reliable joint between the 
one-piece gimbal device 14 and the plastic base 16 is critical to ensure 
that the lamp meets the deflection specification. 
The high temperature light source 12 shown in FIG. 1 is of the double-ended 
type, made of quartz material, and having a pair of lead wires 18 and 20 
emerging from the neck sections on either side of the bulbous portion 22 
of the light source 12. The outer lead wire 18 is connected to an external 
lead wire 24 that is directed into the internal opening 26 in the plastic 
base 16 via an axial groove 28 in the side of the glass-mica lamp holder 
30 of FIG. 4. The inner lead 20 terminates in the internal opening 26 of 
the plastic base 16, whereupon both leads 18 and 20 can be connected to 
the terminals of a conventional electrical source using well known means. 
The neck section 32 of the light source 12 that encapsulates the inner 
lead wire 20 is held in the internal opening 34 of the glass-mica lamp 
holder 30 in such a manner that the ceramic cement within the internal 
opening 34 provides support for the neck section 32 of the light source 
12. The end tip 36 of the supported neck section 32 of the light source 12 
extends into the opening 34 of the glass-mica lamp holder 30 so that it is 
approximately in the same plane as the ring-fitted end opening 48 of the 
lamp holder 30. 
The plastic base 16, constructed from a high temperature plastic, is shown 
in FIG. 1 containing a cylindrical internal opening 26 into which the 
ring-fitted end of the gimbal device 14 is engaged. The plastic base 16 is 
of a configuration that it can be mounted into the headlamp fixture in a 
manner that is consistent with industry practice. 
The gimbal device 14 of FIG. 1 comprises the glass-mica lamp holder 30 with 
a circumferential metallic ring 38 that is contained within a castellated 
groove 40 at one end of the bulb holder 30. The lamp holder 30 of FIG. 1, 
FIG. 2, and FIG. 4 is tapered at the end opposite the ring-fitted end so 
that the edges around the circumference of the holder at this end are 
smoothed out. The lamp holder 30 is of a mica based glass material 
manufacturable to contain a castellated groove 40 and an axial groove 28 
through the castellated groove of the holder 30 by injection molding. The 
metallic ring 38 is contained within an alternating castellated groove 40 
extending around the full circumference of the holder 30 where the 
opposite sides or walls of the groove that support the fitted ring are 
discontinuous in such a manner that resembles the top of a castle. 
Furthermore, the discontinuities in the walls or the castellations 
alternate on either sides of the groove 40 along the full circumference of 
the lamp holder 30. FIG. 2 is a side view of the lamp holder showing a top 
view of alternating castellations 42 and 44 on either sides of the groove 
40. FIG. 4 shows the castellations from a 3-dimensional view. The slot 28 
of FIG. 3 and FIG. 4 is designed to accommodate the external lead wire 24 
used to complete the electrical circuit of the lamp, in accordance with 
the embodiment of the present invention shown in FIG. 1. 
As seen in FIG. 1 the lamp holder 30 contains an internal opening 34 
extending from the tapered end 46 to the ring-fitted end 48 of the gimbal 
14. FIG. 1 shows that the size of the opening is largest at the 
ring-fitted end 48 of the gimbal 14 and gets progressively smaller towards 
the tapered end 46 of the gimbal 14. FIG. 3 is a section of the lamp 
holder viewed from the ring-fitted end 48 of the gimbal 14 illustrating 
the changes in diameter of the internal opening 34 of the glass-mica 
gimbal 14 towards the tapered end 46 as represented by the concentric 
circles 50, 52 and 54 of FIG. 3. The outer circle 50 represents the 
diameter of the opening at the ring-fitted end 48, the middle circle 52 is 
the diameter of the internal opening 34 at the beginning 52 of the tapered 
section 56 of the gimbal 14 and the minimum inside diameter 54 of the 
internal opening 34 at the tapered section 56 of the gimbal 14 is 
represented by the inner circle 54. The tapered section 56 at the end of 
the gimbal 14 onto which the light source 12 is mounted is tapered not 
only for appearance purposes but also to prevent the gimbal 14 from 
blocking light emitted from the bulbous portion 22 of the light source 12 
in accordance with the embodiment of the present invention disclosed in 
FIG. 1. The internal opening 34 of the gimbal 14 is made considerably 
smaller at the tapered section 56 of the gimbal 14 so that it is of a 
suitable diameter to contain the ceramic cement used to support the neck 
section 32 of the light source 12 that is inserted into it, as is 
illustrated in FIG. 1. 
The ring-fitted end 48 of the gimbal 14 is inserted into the opening 26 of 
the plastic base so that the inside walls of the internal opening 26 of 
the plastic base 16 are in contact with the metallic ring 38 and/or the 
spherically shaped outer surfaces 42 and 44 of FIG. 2 on either sides of 
the groove 40, as illustrated in FIG. 1. Such a configuration, shown in 
FIG. 1, gives the gimbal device 14 the freedom to slide axially in one 
plane and to rotate about the center of the spherically shaped outer 
surface of the castellations in the other two planes thus enabling the 
light source 12 to be focused with respect to the plastic base 16. Once 
the light source 12 is focused with respect to the plastic base 16 the 
metallic ring 38 in the gimbal/plastic joint is heated by application of 
Radio-Frequency (RF) energy whereupon the heat causes softening of the 
plastic walls and protrusion of the metallic ring 38 into the softened 
plastic base 16; and expansion of the gimbal material in the groove 40 
thus interlocking the lamp holder to the plastic base. Upon heating of the 
gimbal 14 to high temperatures via the metallic ring 38, the mica 
particles in the glass-mica gimbal 14 expand or "bloat" only in the groove 
40 material in the vicinity of the ring 38 and fill up any voids next to 
the sides of the ring 38 so as to form a tighter joint. In order to form a 
tight joint which is required for a stable and reliable support of the 
light source 12, any gap between the sides of the groove 40 and the ring 
38 must be eliminated. Without correction, this tolerance gap between the 
sides of the groove 40 and the ring 38 which is due to the manufacturing 
tolerance of the groove 40 would result in apparent looseness of the 
plastic/gimbal joint and hence deflection of the light source 12 from the 
focused position outside the tolerance specified by Federal requirements. 
Generally in order to form a rigid fit at the plastic/gimbal interface 
employing the RF method of attachment, both the gimbal material and the 
plastic must melt and flow to fill the tolerance gap between the sides of 
the groove and the ring, or alternatively the ring must fit perfectly into 
the groove thereby eliminating the same tolerance gap. However, ceramics 
do not melt and flow. Furthermore, conventional ceramics can not be molded 
to the tight tolerances required for this type of fit. Ring sizing 
tolerance can be controlled to the level required to eliminate the 
tolerance gap but at a cost. Hence the bloating characteristic of the 
glass-mica material provides for a way to eliminate the tolerance gap 
problem in a cost effective way. An additional advantage of the glass-mica 
material is it can be precision manufactured to contain the castellated 
groove by injection molding and so the part lends itself to mass 
production at a reasonable cost. Since the groove walls alternate on 
either side of the groove, ring tolerance can be relaxed, because the gaps 
between the walls serve to accommodate the ring inside the castellated 
groove opening. Without the castellations in the groove, an oversized ring 
will significantly expand radially to increase the tolerance gap between 
the sides of the groove and the ring. In the extreme case the ring may 
expand outside the groove leaving no opportunity to achieve the locking 
effect between the sides of the groove and the ring. A glass material 
would be unsuitable for this application since it can not be machined to 
contain the castellations. 
The two-piece gimbal disclosed in U.S. Pat. No. 4,795,939 contains an extra 
plastic member between the ceramic lamp holder and the plastic base. This 
member contains a circumferential groove and a metallic ring disposed 
therein for attaching the plastic member to the plastic base. The joint in 
this case is formed by virtue of the melting and/or co-mixing of the 
plastics from the base and the ring containing member. During the RF 
heating process the metallic ring expands radially into the plastic base 
thus locking the metallic ring to the plastic base. In the case of an 
oversized ring, such radial expansion will result in a substantial 
increase in the tolerance gap. To obtain a firm and reliable weld the 
presence of voids in the joint must be minimal. However there is no way to 
ensure that the plastics melt and flow in such a manner that the gaps in 
the joint are eliminated during the RF heating process. Hence the joint 
may not always be reliable. Ring sizing tolerance may be tightly 
controlled to alleviate the tolerance gap problem, but this is costly and 
still does not ensure that all voids are eliminated. The additional 
plastic part naturally adds to the total number of parts and the 
complexity of the assembly process which may have a negative impact on the 
overall quality and cost of the product. Some benefits of a simplified 
design with fewer parts includes cost, quality, and ease of manufacturing 
by mass production. 
The provided glass-mica gimbal is heat resistant and so maintains its 
structural integrity under the intense heat of the RF ring and the light 
source with which it is in contact. It also possesses a low thermal 
conductivity in order to insulate the plastic base from the heated bulb. 
High temperature plastics are both too costly and do not possess the 
thermal stability required at high temperatures to be used in place of the 
ceramics or glass. Due to their lack of thermal stability, plastics may 
exhibit outgassing at elevated temperatures to produce a condensable vapor 
that can affect the appearance of color output from the lamp if deposited 
on glass. On the other hand, a metallic gimbal would result in overheating 
of the plastic base due to the high heat transfer characteristics inherent 
in metals. The present invention has the added benefits that it can be 
used for any high temperature light source such as discharge and halogen 
IR, single-ended or double-ended bulbs with the appropriate modifications, 
and does not require that the gimbal be capable of melting or be made of 
polymeric materials in order to use the RF attachment method. In addition 
the gimbal device presented in this invention consists of a one-piece 
construction of the high temperature glass-mica holding mechanism and 
gimbal/ring attachment portion of the holder which makes it superior to 
prior art since multiple parts constructions invariably have a negative 
effect on quality, cost, and ease of mass production. 
Although the hereinabove described embodiment of the present invention 
constitutes a preferred embodiment of the invention, it should be 
understood that modifications can be made thereto without departing from 
the scope of the invention as set forth in the appended claims. For 
instance, with regards to the RF metallic ring, instead of a single ring, 
it may be possible to use alternate configurations such as a series of two 
or more segments of a ring contained within the groove. In this case RF 
energy could be employed to each segment of the ring separately to achieve 
the desired locking effect between the gimbal and the lamp base that is 
described in this specification.