Electric lamp and discharge devices – With support and/or spacing structure for electrode and/or... – Electrode supported by envelope
Reexamination Certificate
1999-02-08
2001-12-04
Font, Frank G. (Department: 2877)
Electric lamp and discharge devices
With support and/or spacing structure for electrode and/or...
Electrode supported by envelope
C313S025000, C313S634000, C313S239000, C313S292000
Reexamination Certificate
active
06326721
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a high intensity discharge (HID) lamp, of the type having a ceramic metal halide arc tube having opposed axial leads connected to current carrying support members embedded in a glass stem, and a glass envelope having a closed end opposite from the stem.
FIG. 1
shows a known ceramic metal halide discharge lamp having a metal base
10
and a center contact
12
soldered to respective current inputs
16
,
17
embedded in a glass stem
14
, where they are welded to support members
60
,
61
. The first support member
60
has an upper end provided with a loop
64
positioned around a protrusion formed by dimple
24
in closed end
22
of outer envelope
20
, and a lateral arm
62
welded to a first axial lead
38
of arc tube
30
. This is known as an elliptical dimple (ED-type) lamp. The second support member
61
is connected to the second axial lead
39
of arc tube
30
by means of a flexible niobium C hook
66
welded in place. The hook relieves stress that would otherwise be experienced by the arc tube due to expansion and contraction along its axis. The extended sleeves
36
,
37
of the arc tube are particularly sensitive to shearing stresses (torque perpendicular to the long axis of the arc tube). However the hook has an inherent material cost as well as the cost of labor to weld it in place. Further, both the arm
62
and the hook
66
can bend if the lamp is dropped, putting the arc tube
30
off center in the envelope, and likewise off center with respect to a reflector in a lighting fixture. This is especially troublesome with a parabolic reflector, which requires that the light source be at the focal point of the parabola.
U.S. Pat. No. 3,906,275 discloses an ED-type lamp with a quartz metal-halide discharge tube held by straps clamping the opposed pinches and fixed to respective mounting members embedded in the stem. Thermal stresses are not a problem for this type of lamp due to the robust character of the quartz envelope, and the relatively low coefficient of thermal expansion of quartz, 5.5×10
−7
per ° C.
U.S. Pat. No. 4,401,913 discloses high pressure sodium ED-type lamp having a polycrystalline alumina (PCA) ceramic arc tube. The lower terminal is secured to a rod in the stem, and the upper terminal is secured to a rod having an upper end with a helical spring engaging the dimple. The spring compresses to absorb linear expansion of the arc tube, which can be significant due to its length (11 cm), its coefficient expansion (80×10
−7
per ° C.), and high temperatures. Electrical current is provided to the upper terminal by a fine helically bent wire, known as a flying lead, which does not provide any support. The upper end of the arc tube is supported exclusively by the spring engaging the dimple, so that expansion relative to a support frame is not a problem.
SUMMARY OF THE INVENTION
An object of the invention is to relieve thermal stress on an arc tube having opposed axial leads mounted to current carrying support members. A further object is to provide shock absorption to prevent bending of support elements in the event the lamp is dropped.
The lamp according to the invention has a wire frame member with a first end fixed to a first current carrying support member embedded in the stem, a second end fixed to a first axial lead which is adjacent the closed end of the glass envelope, and a coil between the ends, the coil having windings with gaps therebetween.
The wire frame member is especially useful in an ED (elliptical dimple) type lamp, wherein the coil surrounds the protrusion formed by the dimple. A first intermediate portion of the frame member extends from the first end to the coil, and a second intermediate portion extends from the coil to the second end. Welds join the ends to the first support member and the first axial lead, and the second axial lead is substantially fixed with respect to the second support member. The coil therefore compresses to shrink the gaps when the arc tube expands relative to the frame member.
Note that the coil is preferably fitted loosely about the dimple so that it positions the arc tube concentrically within the elliptical outer envelope, but does not press against the closed end of the envelope. Rather, the first intermediate portion is in tension when the coil is in compression.
The coiled spring design has the advantage that no perpendicular torques are generated when the spring is compressed. In contrast, when the C hook is compressed, part of the reaction force generates a torque perpendicular to the long axis of the arc tube, which can lead to cracking the PCA and shortened lamp life. So the stress relief of the invention is superior to that of the prior art in FIG.
1
.
Since the glass envelope does provide a support function (lateral positioning), the wire frame member can be lighter gauge than a fully self supporting frame. In a preferred embodiment, the frame member is 0.037 in. diameter spring temper stainless steel, which is lighter and easier to form than the 0.060 in. support member of the prior art, which also forms the truncated support member of the present invention.
The wire frame member with integral stress relief is especially suitable for a ceramic metal halide discharge tube with a polycrystalline alumina envelope forming a central barrel defining an arc space, and opposed sleeves which seal the axial leads to the electrodes. While not especially long—typically about 36 mm—its fragility necessitates relieving stress imposed by expansion due to the high operating temperature.
In addition to relieving thermal stress, the coil also provides good shock absorption in all directions in the event the lamp is dropped or otherwise jarred. This assures that mounting components do not readily bend and throw the light source off center in the outer envelope.
The wire frame member also facilitates automated assembly of the lamp, since it is not necessary to manually handle a niobium C hook. Rather, the frame member and arc tube are jigged and welded to the support members embedded in the stem.
These and other advantages of the invention will be apparent with reference to the drawings and the detailed description.
REFERENCES:
patent: 3898507 (1975-08-01), Bolt et al.
patent: 4401913 (1983-08-01), Koza et al.
patent: 5698947 (1997-12-01), Choi
patent: 440138 (1934-10-01), None
Gensert James M.
Nelson Gregory J.
Provagna Kevin D.
Shippee Daniel P.
Van Lierop Franciscus H.
Faller F. Brice
Font Frank G.
Lee Andrew
Philips Electronics North America Corp.
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