Heat treating glass sheets on a roller hearth conveyor

The present invention improves the quality of glass sheets with respect to roll ripple distortion by providing criteria for selectively maintaining conveyor rolls in a critical portion of the conveyor which comprises only those rolls located within an area defined by a first location a short distance upstream of the exit of the furnace and a second location a short distance downstream of the furnace exit. Surprisingly, by maintaining adequate alignment and using relatively small diameter, closely spaced conveyor rolls in this critical conveyor portion only, roll ripple distortion has been reduced to a considerable extent. The remaining conveyor rolls in the other portions of the conveyor need not be subjected to as frequent maintenance as the rolls in the critical portion of the conveyor, thereby lessening the cost of maintaining glass sheet heat-treating apparatus in good operating condition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to heat-treating glass sheets while conveyed on a 
roller conveyor. 
It is known to temper flat glass by heating a series of glass sheets while 
conveying the latter in a horizontal plane on spaced, rotating, parallel 
conveyor rolls through a tunnel-type furnace having an entrance end and an 
exit and continuing to convey the heated sheets on additional spaced, 
rotating, parallel conveyor rolls through a cooling station where the 
sheets are chilled rapidly by blasting their opposite surfaces with jets 
of tempering medium such as blasts of cold air. In order to achieve a 
desired degree of temper, it is necessary to heat the glass to a 
temperature sufficiently high (corresponding to a viscosity of 10.sup.13.3 
poises for ordinary soda-lime-silica glass) to permit the tempering medium 
to impart the desired temper. It was difficult in the prior art to raise 
the temperature of the glass in the furnace to a temperature sufficient 
for tempering without inducing in the glass a tendency to develop a defect 
known as roll ripple distortion. Roll misalignment and roll spacing within 
the furnace were considered to be the major factors causing roll ripple 
distortion. 
In the past, it was proposed to have the tempering apparatus as close to 
the exit of the furnace as possible, thus reducing the time in which the 
glass cooled before impinging blasts of tempering medium could be applied 
thereon. However, this proposed solution was accompanied by cold air 
entering the exit end of the furnace where the temperature should be 
highest, thus disrupting the desired temperature gradient of glass sheets 
along the length of the furnace from a minimum at the entrance end of the 
furnace to a maximum at the furnace exit, and also disrupting the desired 
temperature pattern across the width of the furnace, particularly in the 
vicinity of the furnace exit where the temperature pattern in the glass 
should be controlled most closely. One of the solutions to this problem is 
to space the cooling or quenching station from the furnace exit sufficient 
distance to minimize this problem. 
The roller hearth conveyors used in the past have included conveyor rolls 
having their upper surfaces aligned as precisely as possible throughout 
the entire length of the furnace to provide spaced, rotating lines of 
support for moving flat glass sheets in as nearly a perfectly aligned 
plane as possible. In addition, prior art roller hearth conveyors have 
included relatively small diameter rolls relatively closely spaced near 
the exit end of the furnace in order to minimize roll ripple distortion. 
However, prior to the present invention, the portion of the roller 
conveyor beyond the furnace exit was either subjected to less severe 
maintenance than the rolls within the furnace or were ignored altogether. 
Since it was considered necessary to apply tempering medium such as air 
blasts between adjacent conveyor rolls downstream of the furnace exit onto 
the opposite glass sheet surfaces and to provide paths for the impinging 
air to escape between the same rolls, the rolls located downstream of the 
furnace were more widely separated than the rolls within the furnace 
immediately upstream of the furnace exit. An alternative prior art 
structure for the roller conveyor portion beyond the furnace exit provided 
offsetting discs supported on relatively small diameter shafts spaced 
apart sufficient distances to provide space between adjacent disc axially 
of each shaft and between discs on adjacent shafts for the application and 
escape of tempering medium between said discs. 
Unfortunately, the prior art resigned itself to the fact that increasing 
the separation between adjacent rollers or between adjacent discs that 
supported the hot glass immediately beyond the furnace was a cause of 
surface irregularity that worked against the beneficial result of surface 
smoothness that close roll spacing within the hottest portion of the 
furnace was desired to attain. In addition, the prior art did not 
appreciate that some latitude of misalignment is permissible in the 
parallelism of the conveyor rolls located in spaced relation to and 
upstream of the furnace exit where even though one or more misaligned 
rolls imparts a pattern of irregularity in the bottom glass surface, it 
could be removed by rolling the surface over properly aligned rolls at 
higher temperature. 
Furthermore, while the prior art may have ignored altogether the 
maintenance of near-perfect alignment of all rolls located downstream of 
the furnace exit, it did not appreciate fully that only those conveyor 
rolls disposed a sufficient distance downstream of the furnace exit to 
provide rotating support for glass sheets in downstream locations where 
the glass was cooled sufficiently to be rigid enough to avoid distortion 
on engaging said downstream rolls could be ignored from a periodic 
maintenance program. Hence, considerable time and expense was wasted in 
servicing all the conveyor rolls when less time and expense was needed to 
maintain the conveyor in adequate running condition by careful maintenance 
and repair of the conveyor rolls located in a critical portion only of the 
conveyor, the location of the critical portion in the prior art 
erroneously being located entirely within the furnace near the furnace 
exit. 
2. Description of the Prior Art 
U.S. Pat. No. 3,806,331 to Bezombes discloses a system for conveying glass 
sheets through a tunnel and through a quenching station disposed 
immediately outside the exit of the tunnel. Conveyor rolls having 
substantially the same diameter are provided throughout the length of the 
furnace and of the quenching station. The conveyor rolls in the hotter 
part of the furnace are almost contiguous to reduce the space between the 
lines of support and prevent sagging between the adjacent rollers. 
However, the space between the rollers immediately beyond the exit of the 
furnace was increased to provide space for air blasts to be applied 
against the opposite surfaces of the flat glass and to escape after 
impinging on the opposite glass sheet surfaces. The cooling blasts of air 
applied immediately beyond the furnace exit in Bezombes had to be shielded 
from the furnace by several furnace shielding means. Furthermore, the need 
for the provision of relatively wide spacing between rolls immediately 
outside the furnace exit results in a certain amount of roll ripple 
distortion in the resulting tempered flat glass sheets. 
U.S. Pat. No. 2,144,320 to Bailey, U.S. Pat. No. 3,396,000 to Carson et al. 
and U.S. Pat. No. 3,454,338 to Ritter disclose conveyor rolls comprising 
shafts of small diameter that support a series of spaced collars or discs 
which are offset from roll to roll to provide additional space for the 
application and escape of air blasts in a cooling station of glass 
tempering apparatus. Unfortunately, the heat-softened glass sheets tend to 
sag between these supporting collars not only in the direction of glass 
movement, but also between spaced collars mounted on the same shafts in 
the direction transverse to glass movement in the axial direction of the 
rolls. Hence, roll ripple distortion in glass sheets treated on such 
apparatus is complicated because two distortion patterns are possible, one 
parallel to the conveyor rolls or shafts and another transverse to the 
first distortion pattern. 
U.S. Pat. No. 2,140,282 to Drake discloses a roller conveying system for 
flat glass sheets in which the conveyor rolls are relatively widely spaced 
in a primary heating zone of a furnace where the glass is initially 
heated, and relatively closely spaced toward the exit end of the furnace 
where the glass temperature is much hotter than in the first stage of the 
furnace. Immediately beyond the furnace exit, the conveyor rolls comprise 
a series of horizontal shafts of relatively small dimension, each carrying 
a plurality of spaced short cylindrical discs with the discs on each shaft 
disposed in overlapping relation and offset from the discs of adjacent 
shafts. This construction also permits some roll ripple distortion both in 
the longitudinal direction from shaft to shaft and in a transverse 
direction from disc to disc mounted on any shaft. 
Furthermore, none of the patents enumerated appear to recognize the 
economic benefit of concentrating primarily on aligning and maintaining in 
close alignment only those conveyor rolls in a critical portion of the 
conveyor. Neither does any of the prior art patents recognize the position 
of the critical portion of the conveyor. 
Despite the many patents existing in the glass sheet conveying art, there 
still remained need for improving the appearance of the surfaces of glass 
sheets which contain poor optical properties associated with the local 
deformation caused in the glass by spaced, rotating supports that were 
misaligned. 
SUMMARY OF THE INVENTION 
According to the present invention, glass sheets are supported across their 
entire width in a critical portion of a roller hearth conveyor on closely 
spaced, small diameter rolls that have their upper surfaces carefully 
aligned. The conveyor of the present invention extends through the entire 
length of a tunnel furnace and of a quenching or cooling station and 
includes a critical conveyor portion whose location differs from that 
considered critical in the prior art. A range suitable for the diameter of 
conveyor rolls in said critical portion is 4 to 8 centimeters and a 
maximum roll-to-roll spacing of 5 millimeters, preferably about 3 
millimeters is desirable in said critical conveyor portion. In addition, 
the present invention provides the teaching that it is not necessary to 
spend as much attention on aligning or straightening conveyor rolls 
outside the critical portion of the conveyor as is needed in said critical 
portion. Furthermore, the present invention teaches the glass sheet 
tempering art that the critical conveyor portion extends a short portion 
of the conveyor length from a first location within the furnace a short 
distance upstream of the furnace exit to a second location outside the 
furnace a short distance downstream of the furnace exit rather than only 
upstream of the furnace exit as in the prior art. 
The present invention will be understood better in the light of a 
description of a preferred, illustrative embodiment that follows.

Referring to the drawings, a furnace 10 of the tunnel type is shown having 
an exit 12 at its downstream end. The entrance to the furnace near the 
upstream end of the conveyor C is not shown as is most of the furnace 
length, because the present invention relates to improvements in a 
critical portion of the conveyor extending from a short distance upstream 
of the furnace exit 12 to a short distance downstream of said exit and the 
manner that glass sheets are treated in the critical portion of the 
conveyor. 
The portion of the conveyor leading to the critical portion of the conveyor 
within the furnace comprises relatively large conveyor rolls 14, only two 
of which are shown in the drawings. The rolls 14 are supported on bearing 
support housings 15 near one of their longitudinal ends and on additional 
bearing support housings 16 and 17 near their opposite ends. The housings 
15 are supported on a longitudinally extending channel member 18, while 
the housings 16 and 17 are supported, respectively, on inner and outer 
longitudinally extending channel members 19. Each of the longitudinally 
extending channel members 18 and 19 is disposed laterally outside one or 
the other of the respective opposite longitudinal side walls 20 and 21 of 
ceramic material of the furnace 10. 
The large diameter rolls 14 are essentially of asbestos supported on steel 
shafts 22. The longitudinal extremities of the shaft 22 are reduced in 
diameter to provide reduced end portion 24 at one of their ends that are 
received within bearing support housings 15 and reduced end portions 25 at 
their other ends that are received in additional bearing support housings 
16 and 17. The reduced portion 25 of each of the large conveyor rolls 14 
is fixed to a sprocket to rotate therewith in response to movement 
imparted through a first drive chain 26. The shaft 22 for the downstream 
conveyor roll 14 is provided with a second sprocket for engaging a second 
drive chain 27. 
Downstream of the large conveyor rolls 14 begins the critical portion of 
the conveyor according to the present invention, where a number of 
relatively small diameter, relatively closely spaced conveyor rolls 28 of 
suitable high temperature material, such as steel or suitable ceramic, is 
provided. The rolls 28 have reduced end portions 29 aligned with the 
reduced end portions 25 of the steel shafts 22. Each reduced end portion 
29 has a sprocket fixed thereto. The second drive chain 27 meshes with 
these latter sprockets to drive the conveyor rolls 28 in unison. 
Each small diameter conveyor roll 28 is supported at the outer end of its 
reduced end portion 29 in an outer bearing support housing 30, and 
inwardly of said bearing support housing in any inner bearing support 
housing 32. The bearing support housings 30 and 32 are supported along the 
inner and outer longitudinal channel members 19, which are both disposed 
outside furnace wall 20. 
A drive motor 34, supported on the inner channel member 19 is connected, as 
seen in FIG. 3, to one of the reduced end portions 29 through a chain 
drive 36 to rotate the rolls 28 and 14 in unison. 
Since the small diameter rolls are exposed to the highest temperatures in 
the furnace, they are liable to warp. To minimize this warp, the following 
structure is provided within the furnace. 
The inner end of each of the small diameter roll 28 is free to expand 
thermally. In addition, intermediate its ends, each small conveyor roll 28 
is rotatably supported on a pair of open type sleeve bearings 37 supported 
on the upper ends of bearing supports 38 carried by the floor of the 
furnace 10. The open type sleeve bearings 37 enable the rolls 28 to have 
their upper extremity portions exposed along their entire length to 
provide rolling support for conveying flat glass sheets through the exit 
portion of the furnace. 
The conveyor continues beyond the furnace exit 12 with a transfer conveyor 
section forming another part of the critical conveyor portion and 
comprising additional conveyor rolls 40 having the same cross-sectional 
dimensions and spacing as the small diameter rolls 28. Each of these 
additional conveyor rolls 40 has a sprocket fixed to a reduced end portion 
41. The sprockets are entrained by a third conveyor chain drive 42. The 
most downstream conveyor roll 28 also contains a second sprocket aligned 
with the aforesaid sprockets on the end portions 41 of reduced diameter of 
additional conveyor rolls 40 about which is entrained the third conveyor 
drive chain 42 to enable the rolls 28 and 40 to rotate in unison. The 
additional conveyor rolls 40 are provided with intermediate portions 
having the same diameter and spacing as the relatively small, relatively 
closely spaced conveyor rolls 28 within the furnace 10 and are also 
covered with very thin fiber glass sleeves 60. If desired, a clutch may be 
coupled to the most downstream conveyor roll 40 to selectively disengage 
its said second sprocket, thereby disengaging the third conveyor drive 
chain 42 when desired. 
The additional conveyor rolls 40 form a transition conveyor section between 
the furnace exit 12 and a first pair of opposing upper and lower slot type 
nozzles 61 and 62, respectively, of a quenching or cooling station 70. The 
latter has conveyor rolls 64 arranged in spaced pairs with their glass 
sheet supporting portions forming a common upper tangent that is a 
continuation of the common upper tangents of conveyor rolls 14, 28 and 40. 
The centers of the nozzles are spaced from one another approximately 6 
inches (about 15 centimeters) apart and the upper nozzles 61 extend 
downwardly to terminate in slots, each extending transversely in spaced 
relation over the conveyor in alignment with a space between adjacent 
pairs of conveyor rolls 64 with the lower nozzles 62 extending upwardly to 
terminate in transversely extending slots, each extending transversely in 
spaced relation below the conveyor so that air blasts under pressure from 
plenum chambers (not shown) are delivered through the respective nozzles 
and their slots and between adjacent pairs of conveyor rolls 64 toward the 
opposite surfaces of glass sheets as the conveyor rolls 64 rotate to 
convey the glass sheets through the quenching or cooling station 70. 
Another feature of the illustrative embodiment of the apparatus is that the 
pairs of small diameter conveyor rolls 64 replace single conventional 
large diameter rolls which were disposed between adjacent pairs of 
opposing slot type nozzles in the quenching or cooling station 70. Each 
pair of conveyor rolls 64 is separated from the adjacent pair of conveyor 
rolls 64 sufficiently to provide an area for the application of blasts of 
tempering medium such as air blasts. The spaces between the conveyor rolls 
64 of each pair are smaller than the spaces between adjacent pairs, yet 
sufficiently farther apart than the rolls 28 in the exit portion of the 
furnace 10 or the additional conveyor rolls 40 of the transfer conveyor 
section so as to provide escape paths for the removal of spent air blasts 
as the latter impinge upon the bottom surface of the moving glass sheets 
in the quenching or cooling station 70. 
The small diameter conveyor rolls 64 in the quenching or cooling station 70 
have reduced end portions 65 and 66 received in bearing support housings 
67 and 68. Suitable longitudinally extending supports 69 are provided 
throughout the length of the remainder of the conveyor C for supporting 
the bearing support housings 67 and 68. 
Beyond the quenching and cooling station 70, large diameter conveyor rolls 
(only one of which is shown in FIGS. 1 and 2) are provided. By the time 
the glass sheets reach the downstream end of the quenching and cooling 
station 70, they are sufficiently rigid that they can be supported by more 
widely spaced conveyor rolls without fear of roll ripple distortion. 
Larger diameter rolls are used in the non-critical portions of the 
conveyor because a fewer number of large diameter rolls than the number of 
smaller diameter rolls is required to cover a given conveyor distance. 
The illustrative embodiment of the invention contains 16 small diameter 
conveyor rolls 28 within the furnace 10, five additional small diameter 
rolls 40 between the furance exit 12 and the quenching or cooling station 
70, and two conveyor rolls 64 between each adjacent pair of nozzles 61 and 
62 in the quenching or cooling station 70. However, the number of rolls in 
the parts of the critical portion of the conveyor can vary depending on 
the diameter of the rolls, particularly those in the furnace. A suitable 
number for furnace rolls 28 of reduced diameter is between 10 and 20, for 
additional transfer rolls is two to 10 and for the number of rolls 64 that 
can be interposed between adjacent pairs of slot nozzles 61 and 62 is two 
or more. 
The conveyor rolls 64 in the quenching or cooling station 70 have a 
diameter of approximately 2 inches (5.08 centimeters), a center-to-center 
distance of approximately 21/2 inches (6.35 centimeters) between the rolls 
of a pair, and a center-to-center distance of 31/2 inches (8.89 
centimeters) between rolls of adjacent pairs. The conveyor rolls 28 in the 
exit portion of the tunnel 10 and the additional conveyor rolls 40 in the 
transfer portion of the conveyor C have the same diameter as conveyor 
rolls 64, but have a space therebetween of 1/8 inch (approximately 3 
millimeters). The size and spacing of the larger diameter rolls 14 
upstream of rolls 28 and the size and spacing of those beyond quenching 
and cooling station 70 are less critical than the size and spacing of the 
conveyor rolls in the critical conveyor portion for reasons explained 
previously. 
When glass sheets mass produced on a roller hearth apparatus of the type 
just described begin to show roll ripple distortion, such distortion can 
be overcome most readily by correcting any misalignment or departure from 
desired curvature in the small diameter conveyor rolls 28 and additional 
conveyor rolls 40 and sometimes those rolls 64 in the upstream positions 
at the quenching or cooling station 70. In the past, maintenance and 
repair of all the rolls in the furnace was deemed necessary to overcome 
the onset of roll ripple distortion. 
According to the apparatus aspect of the present invention, the conveyor 
rolls 28 and 40 in the critical portion of the conveyor should have a 
diameter of 4 to 6 centimeters, preferably about 4.9 to 5.2 centimeters 
and a space between adjacent conveyor rolls not exceeding about 5 
millimeters, preferably about 3 millimeters. These conveyor rolls and the 
conveyor rolls 64 in at least the upstream portion of the quenching and 
cooling station 70 should be maintained in near perfect alignment. Such 
precise alignment is not as essential in the furnace conveyor rolls 14 in 
the upstream portion of the furnace 10 and in the quench conveyor rolls 64 
downstream of the aforesaid upstream portion. 
The glass sheets treated on the apparatus of the present invention are 
conveyed along a path disposed longitudinally of the conveyor. While the 
conveyor rolls have reduced end portions received in the bearing support 
housings, the cylindrically shaped center portions of the conveyor rolls 
in the critical conveyor portion should have a length at least as long as 
the dimension of glass sheets transverse to said path so that the conveyor 
rolls support the entire dimension of said sheets transverse to said path 
on a plurality of closely spaced, rotating lines of support in the 
critical portion of the conveyor. 
The form of the invention shown and described in this disclosure represents 
an illustrative embodiment thereof. It is understood that various changes 
may be made without departing from the gist of the invention as defined in 
the claimed subject matter that follows.