US2730840A - Method of treating mounted glass disc to resist breakage - Google Patents

Description

F. s. CRAIG 2,730,840

METHOD OF TREATING MOUNTED GLASS DISC TO RESIST BREAKAGE Jan. 17, 1956 Filed Sept. 28, 1949 F|Ca.4. Fig.5. F|c=.7.

INVENTOR.

Ii'EETl-EQD UF-TREA'HIIG MGUNIEB GlLrKSS DISC Eli} RESET ERIKA-RAGE Frank 5. Craig, Chili, N. Y, assignor to General Raiiway Signal Company, Rochester, N. Y.

ApplicationSeptezhioerlfi 149;SriaiNo 118,308

3 (Ilaims. "(CL 49- -79) This invention relates to ring mounted color glass disc for railway signals, and it'moreparticularly pertains to discs that have been especially heat treated to resist breakage and-themethod of heat treatment involved.

The conventional Searchlight type ofrailway-signals can display a Selected color aspectby'reason of the-selective positioning of small color glass-discs close to the focal point in the light'beam of "the signal. "The color discs are secured within metal rings-which are spun, pressed, or otherwise formed over the edges of the glass discs, and these rings in turn are secured to" an electromagnetically actuated sector so that they maybe selectively positioned within a beam of light obtained by a: suitable light system of the signal. Because. ofthe rapid heating and cooling of the color glass 'di'scs astheylare"selectively positioned within the light beam, and particularly where approach lighting of the signals ise'rnjgloyed'forv selectively energizing and deenergizingthe. signals in accordance with the approach of trains, only a beam of limited intensity can be employed without dangerof breakage of the color' glass discs due toh'eatand'thus permittingium colored light to beemitted by the signal. This'is' more particularly true of the. green color. glass'discs.

An object of the present 'invention is to permit'light beams of higher intensity to beernployed in searchli'ght signals withoutbreakageof thegreen' color'glassdiscs' by reason of high temperature annealing of the discs'subsequent to the rings having been secured thereto.

Another object of the; present invention is'toform a diametric crack in the color glass discs" prior to the mounting of the'discs in rings, and prior tothe high'temperature annealing of the mounted'discs, to further prevent breakage of the l'glassin use within-the lightbeam of a searchlight signal.

Other objects,"purposes' and characteristic fe'atures'of the present invention will be in partob vious from. the

accompanyingdrawing, and in part pointed out asthe de scription progresses.

In describing the invention in'detail, reference ismade to the accompanying drawing'in which:

Fig. l is a view of a ring mounteddiametrically cracked color glass 'disc formed of'h'eat resistance annealed colored glass; and

Fig. 2 is a plan view of a ringmounted color glass disc of the character 'shown in Fig. 1 with shaded portions pro vided to indicate dark portions apparent when the disc is viewed in a polariscope after-high" temperature annealing but prior to being used in a signal;

, Fig. 3 is a plan view of a diametrically cracked-color glass disc as viewed in a; polariscope after the disc has been heat treated'subsequent to mounting within its ring,

and after the disc has been used in alight signal;

Fig. 4 is a' sectional view of a color glass'disc taken along the line 4-4 of Fig. 1 after insertion of the disc in'its mounting ring, but prior'tothe ring being spun over the edge of the disc;

FigfS is a, sectional viewof a color disc' after the assoiii Eatented Jan. 17, 1956 ice elated mounting ring has beenspunover the edge of the disc;

Fig. 6 is afragmentary view illustrating "the'manner in which a color light disc is disposed inthe" beam of a *searchlight signal;

'dated April 22, 1941, comprises'a light system having a lamp 10 secured in a lamp socketlland'disposed with its filament 12 at the focal point of a suitable parabolic refiector'13. The reflector 13 focuses the light emitted by the lamp It) at a point P, and slightly displaced from this focal point P is a color' disc 14 secured in a disc operating sector 15 by suitable spring clips 16 hearing against a ring 17 in which the color glass disc 14 is secured. it is generally provided that three of such color glass discs of the colors red, yellow, and green respectively are disposed adjoining respective openings in this operating sector 15 so that the electromagnetic actuation of the sector 15 about a pivot point. (not shown) is effective to select the aspect to be'displayed by the signal in accordance whichever one of the three color glass discs is disposed within'the beam of light emitted by the light system near the focal point F.

The green color glass employedis of the commercially available heat resistant lime glass character having a low coefficient of expansion to resist breakage. Glass of this heat resistant character'is annealed so as to remove all internal stresses and thus whenexamined in a polariscope, no stresses are indicated. After a color glass disc-has been cut to a desired diameter fromthis material, a diametric crack 18 is formed therein by any suitable means such as the color glass disc being brought to bear against a hot wire; and then the disc is dropped within a flanged ring 17 of soft material such as aluminum or copper as shown in Fig. 4, the open edge 17a of the ring 17 being spun over the edge of the colorglass-disc soas-to-form a'channel shaped ring 17 mounting the colorglass disc 14 as is illustrated in the sectional viewaccording to Fig. 5.

After the color. glass. disc 14 is thus secured within the ring-1'7, subsequent to having its diametric crack 13 formed therein, the mounted diso14 is inserted in a furnace which has been preheated to 1200 F.,- disposed on a suitable rackwhich supports the disc by its ring 17 and does not contact the color. glass.

that internal stresses are set up within the glass, and such stresses have-been somewhat oriented in a manner to form patterns when viewed'under the polariscope substantially parallel to the diametric crack 18.

The ring mounted color glass disc 14 when thus annealed is conditioned tor use-within alight-signal assembled" as schematically illustrated in 'Fig. 6, 'andbecause of this particular treatment, operation of the color glass disc will withstand a much higher temperature, and permit the use of a higher wattage lamp in the signal so as to intensify the light beam emitted, without causing the color disc 14 to be broken by heating within the signal so as to permit pieces of the glass to become disassociated from the disc 14 and thus permit the passage of uncolored light through the disc 14 and also perhaps damage or obstruct the operation of the operating mechanism of the signal by the pieces of glass dropping into such operating mechanism.

After a diametrically cracked and high temperature annealed color glass disc of the character described has been used in a signal, when removed from the signal and viewed in a polariscope, a pattern is apparent as is illustrated in Fig. 3 wherein it is indicated that additional stresses have been set up because of the heat applied to the glass by the light beam of the signal. It will be noted, however, that there are still dark areas indicated when viewed in the polariscope substantially parallel to the diametric crack along the lines of the shaded areas illustrated in Fig. 2 as being indicative of internal stresses that are set up by the high temperature heat treatment subsequent to assembly of the ring and prior to its use in a signal.

Although there may be different theories with respect to the reasons for the improved resistance of the disc 14 to breakage when treated as above described, it is reasonable to believe that the increased resistance of the glass disc 14 to breakage is accomplished by obtaining additional room for expansion of the disc 14- within the mounting ring 17 in two different ways. One way is the forming of the diametric crack 18 which actually expands the disc 14 and thus enlarges the diameter of the ring 17 across the points where it is spun over the edge of the color glass disc 14; and secondly additional room is accomplished within the ring 17 by reason of linear shrinkage of the color glass disc 14 when it is annealcd subsequent to the assembly of the mounting ring 17.

The expansion of the disc 14 when the diametric crack is formed therein is obvious, and it will be readily apparent as the ring 17 is spun over the edge of the color glass disc 14 (or is bent over the edge of the disc 14 by a forming die), that the glass cannot be compressed to occupy as small a space as prior to the forming of the diametric crack.

With respect to the heat treatment, the temperature to which the disc 14 and its mounting ring 17 are heated is substantially above the normal annealed range of the glass, and the cooling of the disc 14 quickly in still air after it has been subjected to this high temperature is eflective to reduce the linear dimensions of the glass to an extent, according to tests made by the United States Bureau of Standards, which may amount to from 15 to 18 microns per centimeter of length. Because of heat resistant glass of this character having a lower coeflicient of expansion than aluminum or copper, it will be apparent that the heating during the high temperature of the disc 14 when mounted within the ring 1'7 does not set up stresses in the disc 14 because of the presence of the ring, as the heat is applied to the ring as well as to the glass. Upon cooling, however, subsequent to the heat treatment, the disc 14 is reduced in its linear dimension in the amount of from 15 to 18 microns per centimeter, while the metal ring merely restores when cooled to its original dimensions. The disc 14 when thus treated undergoes a shrinkage which is permanent, and thus, upon subsequent heating in a signal, it never restores to its original linear dimension. It is therefore provided that because of the shrinkage in the linear dimension of the disc 14, space is obtained for expansion of the disc 14 within its mounting ring 17 when it becomes heated by the light beam within a signal so that the disc 14 can expand without stresses being set up due to the limiting efiect of the ring 17 which has been spun over one edge of the glass. It will thus be seen that if a shrinkage is provided by the heat treatment of from 15 to 18 microns per centimeter in linear dimension, the shrinkage substantially, if not entirely, compensates for the expansion of the color glass disc 14 due to temperature change when heated within a single so that the metal ring 17 in which the disc 14 is mounted, which is not heated by the light beam of the signal, does not limit the expansion of the disc 14 so as to cause stresses to be set up opposing the expansion of the disc 14 due to temperature change.

Because the shrinkage of the disc 14 due to high temperature heat treatment may not always fully compensate for the expansion of the disc 14 when used within a signal, it can be said that the additional space for expansion obtained within the ring 17 for the disc 14 by the diametric crack provides a safety margin insuring that there will be no stresses set up within the disc 14 when used within a signal due to compression. by the ring 17 which fails to expand materially. Therefore the color glass disc 14 may or may not be required to have a diametric crack formed therein in accordance with the extent to which the disc 14 is to be heated in a signal. That is, if the wattage of the lamp of a signal is to be only slightly increased over that which the color glass disc 14, without a diametric crack and without high temperature heat treatment subsequent to assembly of the ring 17 will stand, it may be that the color glass disc 14 need be only heat treated in order to prevent breakage. On the other hand, if the wattage of the signal lamp is to be materially increased over that which the heat resistant glass disc 14 will normally stand without a diametric crack and without being annealed subsequent to the assembly of the ring 17, both the diametric cracking and the high temperature annealing process may be employed to permit maximum light intensity to be provided by the signal in which the glass is to be used without danger of breakage of the color glass discs 14.

If a color glass disc 14 is to be annealed at the 1200 F. temperature after being assembled within a mounting ring 17, without having a diametric crack formed therein prior to mounting, the disc 14 may be identified when viewed in the polariscope by the substantially square dark pattern illustrated in Fig. 7, such pattern being indicative of internal stresses having been set up in the glass due to the high temperature annealing and quick cooling. When this glass disc 14 without the diametric crack is subsequently used in a signal, new stresses are set up so that the cooled glass disc 14 when viewed in a polariscope after having been used in a signal has a pattern as indicated in Fig. 8 wherein four light areas are formed comparable to the respective leaves of a four leaf clover.

As has been indicated in the drawings, the character which the glass discs have assumed due to their heat treatment is distinctively indicated by stress patterns visible in a polariscope. The heat resistant glass from which the discs are cut has been annealed within the proper an nealing range to remove stresses and therefore it is isotropic in character as is indicated by there being no strain pattern visible when viewed in a polariscope. When thus annealed the glass is of a particular character facilitating its being cut into discs 14, and it is also of such a character as to readily form the diametric crack 18 by bringing the respective discs 14 into contact with a hot wire.

After the glass has been annealed at 1200 F., however, and cooled rapidly in still air as has been specified, it is no longer isotropic in character, but instead there are double refractions causing patterns of light and shaded areas to be visible when viewed in a polariscope. As has been pointed out, the pattern formed in the polariscope is indicative of the change in character of the discs 14 due to the particular heat treatment to which the discs 14 have been subjected. it has been found that it is consideraoly more diflicult to form the diametric crack 18 in the disc 14 after the high temperature annealing has been done, and thus in addition to it being desirable that the diametric crack be formed prior to spinning of the ring 17 onto the disc 14, the diametric crack 18 is more readily formed prior to the high temperature annealing process.

In setting the high temperature annealing to be at 1200 F. for two and one-half minutes, there are other considerations aside from the shrinkage of the disc 14. Although higher temperatures could be used it it were only the glass to be considered, the temperature employed approaches the melting point of the aluminum ring 17, and thus the temperature cannot be materially increased above this point. Another consideration is that a longer period of heating could well be employed except that a slight fading of the color of the discs 14 is noted as the time of heat treatment is increased. Therefore, taking these matters into consideration, it has been found that the specified time and temperature of annealin is preferable, although other times and temperatures may be employed Without the character of the discs 14 as indicated by the stress patterns being materially modified.

it thus provided according to the present invention 'nproved resistance to breakage of mounted colored disc 14 can be provided by heat treatment at a erature above the normal annealing range of the ass and that a glass disc 14 so treated can be identified the particular shaded areas indicative of internal stresses which can be seen when the glass is viewed in a polariscope.

Having thus described a mounted glass disc and a method of heat treatment of such disc as one embodiment of crack in the disc, mounting the disc in a circumisr metal channel, heating the mounted disc and its assoc metal channel to approximately 1200 F. mately two and one-half minutes, and cooling the L. ed disc in still air at room temperature.

2. The method of manufacture of a mounted glass disc to render such disc heat resistant comprising forming a diametric crack in the disc, mounting the dis: in a circumferential metal channel, heating the mounted disc and its associated metal channel to a temperature above the normal annealing range of the glass for a relatively short length of time, and cooling the mounted disc in still air at room temperature.

3. The method of manufacture of a mounted glass disc to render it heat resistant comprising mounting the disc in circumferential metal channel, heating the mounted disc and its associated metal channel to approximately 1- for substantially two and one-half minutes, and cooling the mounted disc in still air.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. 3. THE METHOD OF MANUFACTURE OF A MOUNTED GLASS DISC TO RENDER IT HEAT RESISTANT COMPRISING MOUNTING THE DISC IN A CIRCUMFERENTIAL METAL CHANNEL, HEATING THE MOUNTED DISC AND ITS ASSOCIATED METAL CHANNEL TO APPROXIMATELY 1200* F. FOR SUBSTANTIALLY TWO AND ONE-HALF MINUTES, AND COOLING THE MOUNTED DISC IN STILL AIR.

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