Heating apparatus for drawing glass rod

A heating apparatus for drawing a glass rod having a tapered portion coupled, at its upper end, with a feed bar of an outside diameter smaller than that of the glass rod, including an apparatus housing into which the glass rod is inserted, a first sealing cover which is formed with a bore having such a dimension as to seal a clearance between the glass rod and the bore and is hermetically mounted on an inlet opening of the apparatus housing and a second sealing cover which is movably attached to the feed bar and is formed with an aperture having such a dimension as to seal a clearance between the feed bar and the aperture. The second sealing cover has a hollow portion which is formed with a space for accommodating the tapered portion and is hermetically placed, at its lower end, on the inlet opening.

BACKGROUND OF THE INVENTION 
The present invention relates to a compact heating furnace for drawing a 
long glass rod, which can be effectively used for drawing base material of 
optical fiber. 
Conventionally, such a heating furnace as shown in FIG. 1 has been used for 
drawing a glass rod, for example, base material of optical fiber. This 
known heating furnace has a furnace body 21. A cylindrical inlet opening 
22 for inserting base material A of optical fiber into the furnace body 21 
is provided at a central portion of a top wall of the furnace body 21, 
while a cylindrical outlet opening 23 for dragging a drawn glass portion D 
out of the furnace body 21 is provided at a central portion of a bottom 
wall of the furnace body 21. A feed bar E for feeding the base material A 
to a heating element 26 in the furnace body 21 and a drag bar C for 
dragging the drawn glass portion D out of the furnace body 21 are, 
respectively, coupled with upper and lower ends of the base material A of 
optical fiber beforehand. A sealing cover 24, which is formed with a bore 
having such a dimension as to seal a clearance between the feed bar E and 
the bore, is hermetically placed on an upper end of the inlet opening 22. 
Meanwhile, supply ports 28 and 29 for supplying inert gas into the furnace 
body 21 are, respectively, provided at the inlet opening 22 and the outlet 
opening 23. 
In the case where the base material A of optical fiber is drawn by using 
the known heating furnace of FIG. 1, a blind cover 24a is initially placed 
on the inlet opening 22. Then, in an open state of a gas exhaust vent 30 
of a lower cover 25, inert gas is supplied into the furnace body 21 from 
the supply ports 28 and 29 by a supply device (not shown) for supplying 
inert gas so as to occupy atmosphere in the furnace body 21 such that 
excessive inert gas flows out of the gas exhaust vent 30. Subsequently, 
the heating element 26 in the furnace body 21 is heated to a predetermined 
temperature for drawing the base material A of optical fiber so as to be 
maintained at the predetermined temperature. Thereafter, in order to 
insert the base material A of optical fiber into the furnace body 21, the 
gas exhaust vent 30 of the lower cover 25 is closed. Then, after the blind 
cover 24a has been removed from the inlet opening 22, the base material A 
of optical fiber is inserted into the furnace body 21 and a lower end of 
the base material A is heated by the heating element 26. Subsequently, 
when the lower end of the base material A has been heated so as to be 
softened, the drag bar C is dragged downwardly so as to form the drawn 
glass portion D. Since the base material A of optical fiber is held at 
high temperatures during and after heating of the base material A, it is 
necessary to maintain atmosphere of inert gas in the furnace body 21. 
However, when long base material of optical fiber is drawn by using the 
known heating furnace of FIG. 1, an upper end of the base material 
projects out of the inlet opening 22 provided at the top wall of the 
furnace body 21 and thus, it becomes impossible to hermetically seal the 
upper end of the inlet opening 22 by the sealing cover 24. Therefore, the 
cylindrical inlet opening 22 of the heating furnace is required to have a 
length necessary for accommodating therein the base material of optical 
fiber. Meanwhile, the feed bar E is also required to have a length 
sufficient for feeding the base material of optical fiber from the upper 
end of the inlet opening 22 towards the heating element 26, thereby 
resulting in difficult operation. Thus, the heating furnace as a whole 
becomes larger in height, thereby resulting in deterioration of its space 
factor. Furthermore, the known heating furnace has such a drawback that 
the long feed bar is required to be used, thus resulting in extreme 
deterioration of its working efficiency. 
SUMMARY OF THE INVENTION 
Accordingly, an essential object of the present invention is to provide a 
heating apparatus which is capable of drawing a long glass rod without the 
needs not only for making an apparatus housing larger in size but for 
making a feed bar larger in length, with substantial elimination of the 
disadvantages inherent in conventional heating apparatuses of this kind. 
In order to accomplish this object of the present invention, a heating 
apparatus for drawing a glass rod, according to one preferred embodiment 
of the present invention comprises: an apparatus housing having an inlet 
opening, into which said glass rod is axially downwardly inserted from 
said inlet opening; said glass rod having a tapered portion formed at its 
upper end portion such that a diameter of said tapered portion decreases 
in an upward direction of said glass rod; said tapered portion being 
coupled, at its upper end, with a feed bar for feeding said glass rod into 
said apparatus housing; said glass rod having an outside diameter larger 
than that of said feed bar; a first sealing cover which is formed with a 
bore having such a dimension as to seal a clearance between an outer 
periphery of said glass rod and said bore and is hermetically mounted on 
said inlet opening; and a second sealing cover which is movably attached 
to said feed bar and is formed with an aperture having such a dimension as 
to seal a clearance between an outer periphery of said feed bar and said 
aperture; said second sealing cover having a hollow portion; said hollow 
portion being formed with a space for accommodating said tapered portion 
of said glass rod and being hermetically placed, at its lower end, on said 
first sealing cover. 
In accordance with the present invention, when the glass rod is fed into 
the apparatus housing from the inlet opening and projects above the inlet 
opening, the first sealing cover having the glass rod passing therethrough 
maintains hermetic state in the apparatus housing by covering the inlet 
opening. Subsequently, upon progress of drawing of the glass rod through 
heating thereof, when the tapered portion disposed at the upper end of the 
glass rod passes through the first sealing cover and thereafter, hermetic 
state in the apparatus housing is maintained by the second sealing cover.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, there is shown in FIG. 2, a heating furnace 
K1 for drawing a glass rod according to a first embodiment of the present 
invention, which is applied to base material of optical fiber. The heating 
furnace K1 includes a furnace body 1 having a top wall 1a, a bottom wall 
1b and a side wall 1c. An inlet opening 2 for inserting base material A of 
optical fiber into the furnace body 1 is provided at a central portion of 
the top wall 1a, while an outlet opening 3 for dragging a drawn portion 14 
of the base material A of optical fiber out of the furnace body 1 is 
provided at a central portion of the bottom wall 1b. A heating element 6 
for heating and softening the base material A of optical fiber is provided 
in a space of the furnace body 1, which is enclosed by the top wall 1a, 
the bottom wall 1b and the side wall 1c. 
In the present invention, since the base material A of optical fiber to be 
drawn is especially large in length, an upper portion of the base material 
A is so disposed as to project out of the inlet opening 2 when a distal 
end of the base material A is disposed at a central portion of the heating 
element 6 at the time of start of drawing the base material A. Therefore, 
in order to hermetically seal the inlet opening 2, a first sealing cover 
7, which is formed with a bore having a such a dimension as to seal a 
clearance between an outer periphery of the base material A and the bore, 
is placed on the inlet opening 2. A feed bar 8 for feeding the base 
material A into the furnace body 1 and a drag bar 13 for dragging the 
drawn portion 14 of the base material A out of the furnace body 1 are 
beforehand coupled with upper and lower ends of the cylindrical base 
material A, respectively coaxially with the base material A. The base 
material A is formed, at its upper end, with a tapered portion A' having a 
diameter decreasing towards a coupling point between the base material A 
and the feed bar 8. 
A second sealing cover 9 having a bell-like shape is preliminarily mounted, 
by using a mounting device 10, on the feed bar 8 of the base material A 
projecting above the inlet opening 2 so as to cover the tapered portion A' 
of the base material A. The second sealing cover 9 has a cylindrical 
portion 9a and a flange portion 9b provided at a lower end of the 
cylindrical portion 9a. The second sealing cover 9 is formed with an 
aperture having such a dimension as to seal a clearance between an outer 
periphery of the feed bar 8 and the aperture. 
The base material A of optical fiber is heated so as to be drawn by the 
heating furnace K1 as follows. Initially, atmosphere in the furnace body 1 
is occupied by inert gas and the heating element 6 is heated to a drawing 
temperature. 
To this end, the inlet opening 2 is closed by a blind cover 7a and a lower 
cover 11 is mounted on the outlet opening 3. Then, inert gas, for example, 
nitrogen, argon or the like is supplied into the furnace body 1 from a 
supply port 4 of the inlet opening 2 and a supply port 5 of the outlet 
opening 3. In an open state of a gas exhaust vent 12 provided at the lower 
cover 11, atmosphere in the furnace body 1 is occupied by inert gas and 
supply of inert gas from the supply ports 4 and 5 into the furnace body 1 
is continued. 
Subsequently, in this state, the heating element 6 is heated to a 
predetermined drawing temperature and then, is maintained at the 
predetermined drawing temperature. Then, the blind cover 7a is removed 
from the inlet opening 2 while the outlet opening 3 is kept closed by the 
lower cover 11. Thereafter, the base material A having the feed bar 8 and 
the drag bar 13 coupled beforehand with its upper and lower ends, 
respectively is inserted from the inlet opening 2 into the furnace body 1 
coaxially with the furnace body 1 and the heating element 6 so as to be 
set such that the lower end of the base material A is disposed at a 
central position of the heating element 6. In the present invention, since 
the base material A of optical fiber is especially large in length, the 
upper portion of the base material A projects out of the inlet opening 2 
when the base material A is set in the furnace body 1. Thus, the first 
sealing cover 7, which passes the base material A therethrough so as to 
seal the clearance between the outer periphery of the base material A and 
the bore of the first sealing cover 7, is placed on the inlet opening 2 so 
as to maintain hermetic state in the furnace body 1. In this state, the 
base material A is heated so as to be softened and is drawn by dragging 
the drag bar 13 downwardly. When the base material A is supplied into the 
furnace body 1 in response to progress of drawing of the base material A, 
the flange portion 9b of the second sealing cover 9 having a bell-like 
shape is placed on an upper face of the first sealing cover 7 into close 
contact therewith. Hence, the second sealing cover 9 having a bell-like 
shape, which passes the feed bar 8 therethrough so as to seal the 
clearance between the outer periphery of the feed bar 8 and the aperture 
of the second sealing cover 9, maintains hermetic state in the furnace 
body 1. In this state, hermetic state in the furnace body 1 is maintained 
by the second sealing cover 9 even when the tapered portion A' of the base 
material A is being passed through the first sealing cover 7 upon further 
supply of the base material A into the furnace body 1. Upon further 
progress of drawing of the base material A, the feed bar 8 proceeds by 
passing through an upper end portion of the second sealing cover 9 such 
that the clearance between the outer periphery of the feed bar 8 and the 
aperture of the second sealing cover 9 is sealed by the second sealing 
cover 9, whereby hermetic state in the furnace body 1 is maintained. 
The second sealing cover 9 is integrally formed with the cylindrical 
portion 9a in the heating furnace K1. However, it can also be so arranged 
that the first sealing cover 7 is integrally formed with the cylindrical 
portion 9a as shown in FIG. 3 depicting a modification K1' of the heating 
furnace K1. 
FIG. 4 shows a portion of a heating furnace K2 according to a second 
embodiment of the present invention. The heating furnace K2 includes a 
first sealing cover 7, a cylindrical member 15 and a second sealing cover 
9 which are formed separately from each other. The first sealing cover 7 
is hermetically placed on the inlet opening 2 of the furnace body 1. The 
cylindrical member 15 defines therein a space for accommodating the 
tapered portion A' of the base material A and is longitudinally split into 
two halves. Meanwhile, the second sealing member 9 is preliminarily 
retained at a lower end of the feed bar 8 and then, is hermetically placed 
on an upper end of the cylindrical member 15 so as to seal a clearance 
between the outer periphery of the feed bar 8 and an aperture of the 
second sealing member 9. 
Since the cylindrical member 15 is of split construction in the heating 
furnace K2, mounting and removal of the cylindrical member 15 can be 
performed easily. Meanwhile, the cylindrical member 15 is not necessarily 
required to be exactly cylindrical and can be replaced by a hollow member 
formed with a space not only for passing the base material A of optical 
fiber therethrough but also for accommodating the tapered portion A' of 
the base material A. Furthermore, the cylindrical member 15 is not 
required to be of split construction. 
Meanwhile, in the heating furnace K2, the cylindrical member 15 is formed 
separately from the first sealing cover 7 and the second sealing cover 9. 
However, it can also be so arranged that the cylindrical member 15 is 
formed integrally with the first sealing cover 7 in the same manner as in 
the heating furnace K1' of FIG. 3. Furthermore, if a side wall of the 
cylindrical member 15 is formed with a cavity, it becomes possible to cool 
the cylindrical member 15 by supplying cooling medium into the cavity. 
Furthermore, in the heating furnace K2, the second sealing cover 9 is 
formed by a single hollow disc but can be replaced by a set of a small 
disc 9c and a large disc 9d as shown in FIGS. 5, 6a and 6b depicting a 
modification K2' of the heating furnace K2. If the second sealing cover 9 
is formed by a single hollow disc as in the heating furnace K2, such an 
undesirable phenomenon may take place that since the aperture of the 
second sealing cover 9 is of such a diameter as to seal the clearance 
between the outer periphery of the feed bar 8 and the aperture, feed of 
the second sealing cover 9 is stopped upon frictional contact between a 
side face of the feed bar 8 and the face of the aperture of the second 
sealing cover 9 even if the second sealing cover 9 is slightly inclined 
relative to the feed bar 8. If the second sealing cover 9 is not disposed 
in contact with the tapered portion A' of the base material A, for 
example, the second sealing cover 9 is retained in the course of the feed 
bar 8, hermetic state of the furnace body 1 cannot be maintained 
undesirably when hermetic state of the furnace body 1 is required to be 
maintained by the second sealing cover 9 and the cylindrical member 9 
after the tapered portion A' of the base material A has passed through the 
cylindrical member 15. 
Therefore, it is desirable as shown in FIGS. 5, 6a and 6b that the second 
sealing cover 9 is constituted by the small disc 9c having an inside 
diameter d1 and an outside diameter d2 and the large disc 9d having a 
large inside diameter D. The inside diameter d1 of the small disc 9c is of 
such a dimension as to seal the clearance between the outer periphery of 
the feed bar 8 and the inside diameter d1 for fitting the feed bar 8 
thereinto. The inside diameter D of the large disc 9d is of such a 
dimension as to allow the large disc 9d to be retained at the tapered 
portion A' of the base material A. The large disc 9d has such an outside 
diameter as to cover an upper end of the cylindrical member 15. 
Meanwhile, the diameters of the small disc 9c and the large disc 9d have 
the following relation. 
EQU (d2-d1)/2&gt;D-d1 
By the above described relation, when the small disc 9c and the large disc 
9d are overlapped each other so as to cover the upper end of the 
cylindrical member 15, such a phenomenon does not take place that a 
lateral clearance is formed between the small disc 9c and the large disc 
9d due to positional deviation therebetween. 
Furthermore, in the heating furnace K1 of FIG. 2, it can also be so 
arranged that the through-hole portion of the second sealing cover 9 
having a bell-like shape is of such a two-piece construction as described 
above. 
Moreover, the through-hole of the small disc 9c is of such a dimension as 
to form a quite small sealing clearance between the feed bar 8 and the 
through-hole. Thus, if the through-hole of the small disc 9c is formed 
angularly, such an undesirable phenomenon frequently takes place that the 
feed bar 8 is frictionally retained by the through-hole of the small disc 
9c with the result that the small disc 9c does not seal the upper end of 
the cylindrical member 15. Accordingly, it is desirable that each of 
opposite edges of the through-hole of the small disc 9c is formed into a 
round shape having no angular portion as shown in FIG. 7. In the same 
manner as described above, it is desirable that each of opposite edges of 
the through-hole of the second sealing cover 9 of the heating furnace K1 
of FIG. 2 is formed into a round shape. 
Meanwhile, as illustrated in FIG. 2, if a pipe 16 shown by one-dot chain 
lines is provided at a center of the furnace body 1 so as to enclose the 
base material A of optical fiber and a gas inlet port 17 and a gas exhaust 
vent 18 for introducing into and discharging out of the furnace body 1 
inert gas, respectively, which are shown by dotted lines, are provided at 
the side wall 1c of the furnace body 1, service life of the heating member 
6 can be increased further. 
As is clear from the foregoing description, when a long glass rod is drawn 
by the heating furnace of the present invention, hermetic state in the 
furnace body can be maintained in two steps by using the first sealing 
cover, the second sealing cover and the hollow member. Therefore, the long 
glass rod can be drawn through heating thereof by the compact heating 
furnace without the need for using a heating furnace having a large height 
in accordance with the length of the long glass rod. Furthermore, in the 
heating furnace of the present invention, since the feed bar for feeding 
the glass rod into the furnace body can be made short in length, working 
efficiency of the heating furnace has been improved remarkably. 
Consequently, in accordance with the present invention, production cost of 
the heating furnace as a whole is reduced drastically, thereby resulting 
in reduction of production cost of its products. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.