Electric lamp with improved lamp cap assembly

The electric lamp has a hollow body (20) of synthetic material in its lamp cap (6), which screens a current supply conductor (11) extending to a base contact member (8) of the lamp cap from the shell portion (13) of the lamp cap (6). The body (20) has projections (22) distributed over it, which bear with clamping fit on the shell portion (13).

FIELD OF THE INVENTION 
The invention relates to an electric lamp comprising 
a translucent lamp vessel with an end portion which comprises a sealed 
exhaust tube and a stemtube, 
a light source inside the lamp vessel, 
a lamp cap having an axis and contact members, which cap is fastened to the 
end portion of the lamp vessel by means of an adhesive compound, 
current supply conductors which extend from the light source to respective 
contact members at the lamp cap and are connected thereto, 
the lamp cap having a metal shell portion and a base portion of insulating 
material which carries at least one base contact member provided with an 
opening which issues into a channel through the base portion, through 
which channel a current supply conductor passes to the exterior, 
in the lamp cap a hollow body of synthetic material which extends from a 
first open end near the lamp vessel to the base portion of the lamp cap 
and screens or insulates the current supply conductor extending to the 
base contact member from the shell portion of the lamp cap. 
DESCRIPTION OF RELATED ART 
Such a lamp is known from GB 11 84 300 (1970-3-11). 
It is the purpose of the body of synthetic material in the lamp cap to 
prevent the current supply conductor to the base contact member coming 
into contact with the shell portion of the lamp cap or, alternatively, to 
prevent a discharge arc arising between the two. Such a contact or such a 
discharge arc can arise especially in lamps of relatively high power, when 
the current supply conductor melts through owing to the passage of a too 
high current. It is desirable to avoid these risks because otherwise the 
circuit breaker in the mains through which the lamp is fed can be 
triggered and/or the lamp cap can become welded to its lampholder. 
A disadvantage of the known lamp is that the lamp vessel and the lamp cap 
have to be assembled with the lamp cap in bottom position in order to 
prevent the body of synthetic material dropping from the lamp cap or 
assuming a skew position in the lamp cap. After this assembly operation, 
the lamp must be turned upside down to bring the lamp cap up, so that the 
current supply conductor can be fastened to the base contact member. 
Another disadvantage is that the body of synthetic material, being an 
additional component to be assembled, must first be provided in the lamp 
cap during lamp assembly. 
A similar lamp is known from U.S. Pat. No. 2,141,146 (1935-9-17), as well 
as a lamp in which the shell portion of the lamp cap is coated on the 
inside with an insulating compound. The compound may consist of the same 
material as the insulating material of the base portion, usually glass, 
and be integral with it. It has shown itself to be difficult, however, on 
the one hand to apply sufficient insulating material for ensuring the 
screening effect, while on the other hand not so much insulating material 
is applied that there is no certainty that the lamp cap will fit the end 
portion of the lamp vessel. When lamp caps with aluminium shell portions 
are used, moreover, the use of the relevant insulating material for 
coating the shell portion is not possible, because the lamp cap is heated 
too much during the application of this material, so that the shell 
portion becomes too soft and loses its shape. The same disadvantages hold 
for the lamp caps disclosed in this U.S. Patent which have a coating of 
cured cement. 
GB 1 139 266 (1969-01-08) discloses a lamp obtained with the use of a lamp 
cap in which a ring is pressed of a material which forms a foam when 
heated. This lamp has the disadvantage that the foam may have become 
degenerated and pulverised at the end of lamp life after operation at 
comparatively high temperature. It was also found to be necessary to check 
the lamp cap for the presence of the foam ring before the lamp is 
assembled. Another disadvantage of the lamp is that, after foaming of the 
material and curing of the foam, rifts may be present in the foam which 
still render the shell portion accessible for a discharge arc. 
A general disadvantage of the use of lamp caps which deviate from normal 
lamp caps is that such deviating, more expensive lamp caps must be kept in 
stock specially for the manufacture of lamps of relatively high power 
ratings which involve the risks referred to above. 
U.S. Pat. No. 2 076 582 (1937-04-13) discloses a lamp in which the current 
supply conductor to the base contact member is surrounded by a sleeve of 
insulating material. A disadvantage of this is that an extra component 
must be provided during the assembly of lamp cap and lamp vessel. 
SUMMARY OF THE INVENTION 
The invention has its object to provide a lamp of the type described in the 
opening paragraph which, among other characteristics, is of a simple and 
reliable construction and which is easy to manufacture. 
According to the invention, this object is achieved in that the body of 
synthetic material has outward projections which are distributed over its 
circumference and bear with clamping fit on the shell portion of the lamp 
cap. 
It is achieved through the presence of the projections that the body of 
synthetic material is held by the lamp cap, irrespective of tolerances in 
the dimensions of the lamp cap and the body of synthetic material. The 
lamp cap with the body of synthetic material inside can thus be subjected 
to various manipulations without losing this body. Consequently, it is 
possible not only to provide the lamp cap in the usual manner on a lamp 
vessel with its base portion pointing upwards, but also to provide the 
body of synthetic material in standard lamp caps outside the lamp 
production line, depending on whether the lamp production requires the use 
of protected lamp caps. A favourable effect of the projections is that the 
adhesive compound, for example the cement compound, can make contact with 
the open end of the body of synthetic material at the inside and at the 
outside. The resulting close connection between the adhesive compound and 
the body of synthetic material leads to a reliable screening of the 
current supply conductor from the shell portion of the lamp cap by the 
body of synthetic material. 
In a favourable embodiment the projections are axially extending fins. 
These may become narrower towards the shell portion of the lamp cap so 
that the body of synthetic material adapts itself easily to the lamp cap 
as regards its dimensions when this body is introduced into the lamp cap. 
It is favourable if the body of synthetic material has a portion which 
becomes gradually narrower towards the channel in the base portion of the 
lamp cap. The current supply conductor which is to be passed through this 
channel is then automatically guided towards this channel when the lamp 
cap is being provided. 
In many lamp types, the channel in the base portion and the opening in the 
base contact member are wide relative to the diameter of the current 
supply conductor. One of the causes of this is that the metal plate which 
forms the base contact member is provided with an opening by breaking out 
the material of the plate towards the interior. This leads to a jagged rim 
around the opening, which rim is embedded and anchored in the insulating 
material of the base portion in order to retain the plate. 
When liquid metal is deposited on the base contact member in order to 
secure the current supply conductor to this member, there is a risk of 
liquid metal, for example solder, running through the opening into the 
channel and even into the lamp cap. No good fastening may be obtained then 
for lack of, for example, solder. In lamps without a body of synthetic 
material in the lamp cap, solder leaking away may form a wire of solder 
inside the lamp cap, which wire may short-circuit the lamp. 
In a favourable embodiment, the body of synthetic material has an 
elongation, for example a conical or cylindrical one, which extends into 
the channel in the base portion. The body thus reduces the effective 
channel passage and obstructs the flowing away of liquid metal. 
In lamp caps having two base contact members, the body of synthetic 
material preferably has two cavities which become gradually narrower 
substantially from the open-end portion towards the relevant channel, one 
for each current supply conductor. This renders unnecessary the use of 
hollow needles which are inserted through the respective channels and 
through which respective current supply conductors are guided into the 
interior before the lamp cap is lowered onto the lamp vessel. In this way 
the needles guide the current supply conductors through the channels. 
In a very favourable embodiment, the body of synthetic material has a tube 
which accommodates one current supply conductor together with the exhaust 
tube and extends, for example, from outside the lamp cap to the base 
portion of the lamp cap. 
The advantage of this embodiment lies in the restriction of the possibility 
of a discharge arc striking inside the space delimited by the lamp cap as 
a result of melting-through of a current supply conductor at the end of 
lamp life and leaping over to the other current supply conductor. If that 
risk should arise nevertheless, the tube, which is necessarily narrow 
because of the restricted space around the exhaust tube, quickly ensures 
that the discharge arc extinguishes. 
In a modification of this embodiment, the tube has a tight connection to 
the gradually narrowing portion of the body of synthetic material. More in 
particular, the tube surrounds the exhaust tube substantially completely. 
This can be realised in spite of dimensional tolerances by not providing 
the body of synthetic material into the lamp cap up to the smallest 
possible distance from the base portion, so that during mounting of the 
lamp cap on the lamp vessel the lamp vessel itself, if necessary, presses 
the body of synthetic material more deeply into the lamp cap the moment 
the tube hits against the lamp vessel. This modification can be used 
especially if the body of synthetic material has an elongation which 
extends into the channel. 
In another modification the tube has a mechanically weak, for example, 
flexible or knock-off end portion which easily adapts itself to the lamp 
vessel. 
It will be clear that, if a body of synthetic material has a tube which 
surrounds a current supply conductor from the outside, this current supply 
conductor will be very effectively screened from the shell of the lamp 
cap. The cement compound need not make contact with the body of synthetic 
material for that purpose. 
The lamp according to the invention may be an incandescent lamp or a 
discharge lamp, in which the light source is an incandescent body or a 
discharge arc, respectively, possibly arranged in an inner envelope. 
The body of synthetic material may be made of, for example, thermoplastic 
resins, for example polypropylene oxide, polyphenylene sulphide, polyamide 
imide, polyimide, polyether sulphon and polyether imide, generally, 
synthetic materials which withstand the normal operating temperature of 
the lamp cap.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electric lamp of FIG. 1 has a translucent lamp vessel 1 with an end 
portion 2, which comprises a sealed exhaust tube 3 and a stemtube 4. A 
light source 5, an incandescent body, is positioned in the lamp vessel. 
A lamp cap 6 of the E27 type having an axis 7 and provided with contact 
members 8, 9 is secured to the end portion 2 of the lamp vessel 1 by means 
of an adhesive compound 10, capping cement in the drawing. 
Current supply conductors 11, 12 extend from the light source to respective 
contact members 8, 9 of the lamp cap 6 and are secured to them, in the 
drawing with solder 17 to base contact member 8. 
The lamp cap 6 has a metal shell portion 13, which at the same time forms a 
contact member 9, and a base portion 14 of insulating material, glass in 
the drawing, which carries at least a base contact member 8 which is 
provided with an opening 15 issuing into a channel 16 through the base 
portion 14. A current supply conductor 11 emerges through this opening 15 
and this channel 16 to the exterior. 
A hollow body 20 of synthetic material is present in the lamp cap 6 and 
extends towards the base portion 14 of the lamp cap 6. The body 20 of 
synthetic material screens the current supply conductor 11 extending to 
the base contact member 8 from the shell portion 13 of the lamp cap 6. A 
first open end 21 of this body is in contact with the adhesive compound 
10. 
The body 20 of synthetic material has outward projections 22 (see also FIG. 
4), which are distributed over its circumference and bear with clamping 
fit on the shell portion 13 of the lamp cap 6. In the Figures, the 
projections 22 are axially extending fins, which become narrower towards 
the shell portion 13 of the lamp cap 6. 
The body 20 of synthetic material has a portion 23 which gradually becomes 
narrower towards the channel 16 in the base portion 14, and an elongation 
24 which extends into said channel. 
In FIG. 2, corresponding parts have the same reference numerals as those in 
FIG. 1. The hollow body 30 of synthetic material has a tube 35 which 
accommodates one current supply conductor 11 together with the exhaust 
tube 3. The tube 35 has a tight connection 36 to the gradually narrowing 
portion 23 of the body 30 of synthetic material. The tube 35 completely 
surrounds the exhaust tube 3. Near the lamp vessel 1, the tube 35 has a 
mechanically weakened free-end portion 37, whose wall thickness decreases 
gradually towards the end. The current supply conductor 11 is thus 
completely separated from the other conductor 12. 
The body 20 of synthetic material from FIG. 1 is represented separately in 
FIGS. 3 and 4. The body has three projections 22 distributed over its 
circumference and is suitable for an E27 lamp cap. 
The body 40 of synthetic material in FIGS. 5 and 6 has parts with reference 
numerals which are 20 higher than those of corresponding parts in FIGS. 3 
and 4. the body has four projections 42 and is suitable for use in an E14 
lamp cap. 
In FIGS. 7 and 8, parts corresponding to parts of FIG. 1 have reference 
numerals which are 50 higher. The lamp has a lamp vessel 51 with a B15 
lamp cap 56, which has two contact members 58, 59 at its base portion 64. 
The hollow body 70 of synthetic material inside the lamp cap 56 has two 
gradually narrowing portions 73, one for each of the current supply 
conductors 61, 62. The portions 73 extend from the open end 71 of the body 
70 of synthetic material. The body has an opening 77, through which the 
sealed exhaust tube 53 is passed. 
In FIGS. 9 and 10, parts of the body of synthetic material corresponding to 
parts in FIGS. 7 and 8 have reference numerals which are 20 higher. The 
body 90 of synthetic material is designed for use in a B22 lamp cap. 
The body 110 of synthetic material of FIG. 11, in which reference numerals 
are 40 higher than those of corresponding parts in FIG. 8, differs from 
the body in FIG. 8 in that a transverse wall 128 is present which causes 
the open end 111 to connect to the gradually narrowing portions 113, but 
leaves a cavity 117 free to accommodate an exhaust tube. 
In FIG. 12, the body 130 of synthetic material is a modification of the 
body 110 of FIG. 11 with reference numerals which are 20 higher than those 
of corresponding parts in FIG. 11. A tube 137 extends to outside the open 
end 131 in order to surround an exhaust tube and comprises radial 
partitions 149. The tube 137 with its partitions 149 splits up the space 
inside a stemtube of a lamp vessel and allots to each current supply 
conductor present therein a space separated from that of the other one.