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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to supporting glass sheets and specifically refers
to an improved method of employing self-closing metal tongs that grip
glass sheets during thermal treatment. Tongs have been used for supporting
glass sheets in vertical planes for thermal treatment by gripping the
opposite glass sheet surfaces near their upper edges. The tongs are
supported on an overhead monorail which extends through thermal treatment
apparatus by means of a supporting clevis mounted to a carriage that rides
on the monorail.
Glass sheets are heated to substantially the softening point before being
pressed to shape, cooled for tempering or annealing, or coated. Tongs for
gripping glass sheets are provided with tong points or discs that
penetrate the glass sheet surfaces and mar the latter and cause dents and
scratches in the vicinity of the gripping areas and even deposit oxides of
the glass engaging members in the surface of the glass during the
temperature cycle. The glass sheet so treated is weakened in the vicinity
of the locations where the glass engaging elements of the tongs engage the
glass sheet. Hence, a long felt need existed in the glass handling art to
provide tongs that do not weaken the glass appreciably in the vicinity of
the glass areas that are gripped by the glass engaging elements of the
tongs.
DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 3,184,259 to Almdale discloses a glass engaging element
composed of a heavy metal carbide which may be composed of a carbide of
one or more of the heavy metals such as tungsten, titanium, vanadium,
columbium, tantalum, and including other hard carbides such as chromium
carbide, boron carbide, and the like, cemented by sintering into a recess
of a cylindrical bore in a ferrous body member by a metal such as cobalt,
nickel or mixtures thereof. The cylindrical bore in the body member and
the cemented carbide member are so constructed and arranged relative to
one another as to avoid stresses in the brazing material which brazes the
carbide glass engaging member to the cylindrical bore in the ferrous body
member so as to avoid as much as possible stresses resulting from the
difference in contraction of the ferrous compositions of the body member
and the carbide composition of the carbide glass engaging member.
A typical glass engaging member of tungsten carbide once sold by the
assignee of the aforesaid Almdale U.S. Pat. No. 3,184,259 consists
essentially of 88 percent by weight of tungsten, 6 percent by weight of
cobalt and 6 percent by weight of carbon. This composition, which is no
longer available in the commercial market, because titanium carbide is
believed to be a superior product, was originally developed to reduce
surface damage to glass sheets suspended by glass engaging members of
self-closing tongs composed of various metal alloys previously used.
Previously, glass gripping tongs included glass gripping elements composed
of tungsten metal alloys of 40 to 80 percent by weight of cobalt, 20 to 35
percent by weight of chromium, 0 to 25 percent by weight of tungsten,
0.075 to 2.5 percent by weight of carbon and 0 to 3 percent by weight of
silicon, which family of alloys are available under the trademark
STELLITE, and various alloys of iron with carbon, chromium, tungsten or
cobalt, tungsten and carbon, which have been used previous to the
development of the tungsten carbide glass engaging elements.
Other glass engaging elements in tongs are disclosed in U.S. Pat. No.
3,456,985 to Bullock and 3,655,233 to Twist. These include silicon
carbide, aluminum oxide, zirconium oxide, chromic oxide or diamond, and
combinations of two or more of the aforesaid refractory abrasive
materials. U.S. Pat. No. 3,363,930 to Webb discloses glass engaging
elements of an aluminum oxide composition that include minor proportions
of other ingredients including at least one of the following: magnesium
oxide, calcium oxide, silicon carbide.
U.S. Pat. No. 3,830,540 to Sperry discloses the use of curved segments of
wire, preferably of a rigid, non-stricking material such as tungsten, for
the glass engaging elements of tongs that grip glass sheets during
tempering wherein hot glass sheets are cooled rapidly by flooding the
glass surfaces with continuous flows of cold liquid. Tungsten metal is
difficult to machine and is extremely costly. Hence, tungsten metal is not
a logical condidate for tongs used in mass production required for
commercial purposes because of the need for large numbers of tongs for
each furnace in operation. Also, tungsten metal tends to oxidize when
subjected to continuous use and forms a powder on oxidation, thereby
tending to deteriorate and limit its effective life under mass production
conditions.
U.S. Pat. No. 1,583,541 to Ferngren teaches the use of a member composed of
an alloy of tungsten and nickel in the forming of a molten glass ribbon.
The member is located relative to the glass forming apparatus so that it
is normally completely surrounded by molten glass so that there is no
appreciable exposure to the atmosphere and, under such circumstances,
there is practically no oxidation or chemical destruction of its surfaces.
The Ferngren patent is mentioned in passing because it discloses the use
of tungsten-nickel alloy in contact with molten glass at a relatively
constant, high temperature. However, despite the issuance of the Ferngren
patent in 1926, the use of tungsten-nickel alloy for glass engaging
elements of glass sheet gripping tongs had never been used in glass sheet
tempering operations, probably because glass sheet tempering involves
exposing the tongs to repeated temperature cycling and oxidizing
conditions at least during the heating stage (although oxidizing
conditions are also present in the cooling stage of tempering cycles that
comprise air quenching), which conditions are minimized or avoided
entirely, respectively, in the use of titanium-nickel alloy recited in the
Ferngren patent.
Despite the existence of these prior art teachings previously enumerated,
there still existed a need for glass engaging elements for self-closing
tongs that combined all of the following characteristics, namely, minimum
marring of the glass in the glass contacting area, minimum weakening of
the glass sheet in the vicinity of the areas of engagement, and life of
adequate duration under conditions encountered in mass production use
before requiring replacement.
SUMMARY OF THE INVENTION
When glass sheets are thermally treated using tongs having glass engaging
members composed of a composition taken from the class consisting of
tunsten nickel composites essentially free of cobalt and iron during the
thermal treatment, the resulting tempered glass sheet is much stronger in
the vicinity of the areas engaged by said members than those produced
using tongs having glass engaging members of the prior art compositions
available for comparative testing other than tungsten metal. Furthermore,
the tungsten nickel composites are easier to work into the shapes desired
for the glass engaging members than tungsten metal and combine the best
features of tungsten metal in contact with hot glass with sufficient
alloying metals to render the resulting alloy more readily workable than
pure tungsten metal, thereby reducing the cost of fabricating glass
gripping tongs to the point where they are commercially feasible. A
particularly preferred embodiment consists of a glass engaging member of
an alloy consisting essentially of at least 90 percent by weight of
tungsten and the balance consisting essentially of nickel and copper, such
as an alloy containing 90 percent by weight of tungsten, 6 percent by
weight of nickel and 4 percent by weight of copper, sold commercially
under the trademark KU-112.
The present invention will be understood better in the light of a
description of illustrative embodiments which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings which form part of the description of the present invention
and wherein like reference numbers refer to like structural elements,
FIG. 1 is a fragmentary elevation view of a pair of self-closing tongs
provided with glass engaging members of the rotatable disc type;
FIG. 2 is a corresponding view of a pair of self-closing tongs having glass
engaging members of a pointed type having rounded ends;
FIGS. 3, 4 and 5 are views of alternate embodiments of glass engaging
element similar to the disc type of FIG. 1, and
FIG. 6 is an enlarged sectional view of a glass engaging element similar to
that of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, FIG. 1 shows one embodiment of tongs improved by
the present invention and FIG. 2 shows another embodiment. In the
embodiment of FIG. 1, an apertured clevis 11 is carried by a carriage (not
shown) supported on a monorail (not shown). The latter extends through a
heating furnace of tunnel-like configuration and a fluid imparting
apparatus, neither of which is shown but which are well known in the art.
The fluid imparting apparatus may be either an air blasting station for air
tempering apparatus as in U.S. Pat. No. 3,849,100 to Luppino, a station to
apply a sublimable cooling medium as in U.S. Pat. No. 3,929,442 to Neely,
a flood quenching station as in U.S. Pat. No. 3,830,540 to Sperry, a
liquid quenching station as in U.S. Pat. No. 3,794,476 to Michalik and
Gorman, or a fluid spraying station for apparatus to impart a coating to a
glass sheet gripped by tongs as in U.S. Pat. No. 3,019,136 to Auffenorde
and Penone. The present invention may also be used for press bending
apparatus as in U.S. Pat. No. 3,290,133 to Thomas. Reference is made to
the various patents enumerated for a detailed description of environmental
apparatus.
The apertured clevis 11 has an apertured bottom portion that supports a
tong support pin 12. The latter also extends through apertures at the
upper end portion of a pair of upper links 16 and 18, to pivot the links
to the tong support pin 12 at their upper ends. Upper link 16 is apertured
at its lower end to receive a link pin 20 whereas upper link 18 is
apertured at its lower end to receive a similar link pin 22. Tong arms 24
and 26 are provided with apertured upper ends to receive link pins 20 and
22, respectively, to pivotally attach the upper portion of tong arm 24 to
the lower portion of upper link 16 and the upper portion of tong arm 26 to
the lower portion of upper link 18. The tong arms 24 and 26 are apertured
to receive a common hinge pin 28 to secure the tong arms to one another
near their lower portion.
Beyond the apertured portion interconnected by the common hinge pin 28,
tong arm 24 forms a horizontally extending apertured extension 30 and tong
arm 26 forms a horizontally extending apertured extension 32. Each of the
horizontal extensions 30 and 32 is vertically apertured. A rod 34 extends
through the vertical aperture in apertured extension 30 and a similar rod
36 extends through a vertical aperture in apertured extension 32. Each rod
has a diametrically extending hole that matches with a cross hole
extending generally horizontally through the apertured extensions 30 and
32 to permit cotter pins 38 and 40 to secure the rods 36 and 34 to the
respective apertured extensions 32 and 30.
The lower end of rod 34 has an enlarged head 42 and the lower end of rod 36
has an enlarged head 44. A disc 46 is rotatably mounted in concentric
relation around rod 34 and makes bearing contact with the upper surface of
head 42. Another disc 48 is similarly mounted around rod 36 and makes
bearing contact with the upper surface of head 44. Sleeves 50 and 52 are
rotatably mounted about rods 34 and 36, respectively, above discs 46 and
48. The discs 46 and 48 have appreciable axial extent (for example, about
3 millimeters thick) and make free rotating contact with the opposite
major surfaces, of a glass sheet G that is gripped between said discs in
pressurized engagement and whose upper edge only is disclosed in the
drawings. The tongs structure depicted in FIG. 1 is similar to that of the
prior art exemplified by using U.S. Pat. No. 3,089,727 to William J. Hay,
Jr., except for the fact that at the time of the Hay patent the glass
engaging elements of the tongs were made of a steel alloy.
In the FIG. 2 embodiment, the tongs are provided with the clevis 11, the
tong support pin 12, the upper links 16 and 18, and the link pins 20 and
22 as in the first embodiment. However, the tong arms 54 and 56 which are
apertured at their lower portion to receive a common hinge pin 58 are
further provided with L-shaped members having downwardly extending portion
60 and 62 respectively. These lower portions extend vertically downward
and have essentially horizontally extending apertures which receive glass
engaging members 66 and 68. The latter are either rounded inner
extremities of threaded bolts 76 and 78 or rounded tips welded to the
inner extremities of the threaded bolts. The glass engaging members 66 and
68 have rounded glass engaging ends.
The freely rotating glass engaging elements 46 and 48 of the first
embodiment or the rounded glass engaging elements 66 and 68 of the second
embodiment are composed of a novel glass engaging composition according to
the present invention. The composition may be a tungsten nickel composite
essentially free of cobalt and iron such as may be machined to form an
externally threaded bolt from an elongated rod of said composite or
tungsten composite tip welded to the inner end of a metal bolt.
FIGS. 3 to 6 illustrate alternate embodiments of the present invention.
FIGS. 3 and 4 disclose only elevational views of glass engaging members
and their supports, FIG. 4 showing the assembly of a ring type glass
engaging element and its support and FIG. 3 only the support. FIG. 5 is a
view along the lines 5--5 of FIG. 4 showing how a wire may be used instead
of a ring with the support of FIG. 3.
Since the glass engaging elements are composed of a composition that is
more expensive than the steel alloy customarily used for glass gripping
tongs, the present invention includes embodiments in which the superior
glass engaging composition is used in limited amounts. For example, in the
embodiment of FIGS. 3, 4 and 5, the disc 46 is made of stainless steel and
is provided with a peripheral groove 47 that receives either a ring 49
(FIG. 4) that is heated for expansion and contracts by cooling to fit into
the peripheral groove 47. A wire 149 having ends 151 and 153 fitting
within radially extending holes in the disc 46 may be wound around the
peripheral groove 47. The wire 149 or ring 49 is made of the special glass
engaging material according to the present invention and is supported like
a tire on a rim, the disc 46 and its peripheral groove 47 being similar to
a wheel and its rim to support either the ring 49 of FIG. 4 or the wire
149 of FIG. 5.
The embodiment of FIG. 6 is similar to that of FIG. 2, except that instead
of making the entire threaded bolt 78 of expensive material, only a small
insert 168 is made of the glass engaging composition and the bolt 78 is
provided with a recess 170 to receive the insert 168. To insure that the
insert 168 fits snugly within the recess 170, the insert 168 is provided
with a peripheral groove 172 and the inner end of the threaded bolt 78 is
provided with a peripheral lip 174. The insert 168 is forced into the
recess 170 of the threaded bolt 78 until the peripheral groove 172 and
peripheral lip 174 are engaged. The insert 168 can be made slightly
smaller than the recess 170 and the recess provided with shoulders to
receive the insert in the manner disclosed in U.S. Pat. No. 3,184,259 to
Almdale, the description of said construction in said patent being
incorporated herein by reference.
While FIGS. 3 to 6 illustrate only one glass engaging member composed of
the preferred composition as taught by the present invention, it is
understood that these glass engaging elements are one of two elements
similar to those illustrated in the tongs of FIGS. 1 and 2. The tongs and
the support for the glass engaging elements are composed of a steel alloy
of the type used in tongs prior to the present invention.
In order to assess the benefits of the present invention compared to the
prior art glass engaging elements available for testing, six groups of
glass sheets were tempered using tongs with pointed glass engaging
elements. Each group of glass sheets was supported by tongs provided with
glass engaging elements of a unique composition for the group as
enumerated in the Table 1. In addition, a seventh group of six glass
sheets was tempered using tongs provided with disc shaped glass engaging
members of the compositions as in group 5.
TABLE 1
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Group Composition of Glass Engaging Element of Tongs
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1 90 percent tungsten, 6 percent nickel, 4 percent
copper (Tungsten Alloy KU-112)
2 95 percent tungsten, 3 percent nickel, 2 percent
iron (Tungsten Alloy KUA-5)
3 80 percent titanium, 10 percent nickel, 8 per-
cent molybdenum, 2 percent iron (referred to as
titanium carbide)
4 12 percent tungsten, 50 percent cobalt, 30 per-
cent chromium, 2.5 percent carbon, balance
others (STELLITE No. 1)
5 90 percent iron, 5 percent chromium, 1.2 percent
tungsten 0.35 percent carbon, balance others
(Pressurdie No. 2)
6 70 percent iron, 18.5 percent tungsten, 7.6 per-
cent cobalt, 0.8 percent carbon, balance others
(Bonded Carbide)
7 Same composition as Group #5 except in the form
of disc tongs
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The tests for each set involved suspending each of six rectangular glass
sheets 25 inches by 12 inches by 7/32 inch nominal thickness (304.8
millimeters by 635 millimeters by 5.6 millimeters) by three tongs, each of
whose glass engaging members engaged the glass sheet at a distance of
about 0.3 inch (7.6 millimeters) below the upper longest edge of the sheet
at distances of about 3 inches (76.2 millimeters), 12.5 inches (317.5
millimeters) and 22 inches (559.8 millimeters) from a corner of the
longest edge while each sheet was heated and cooled according to a
conventional production cycle of temperatures for tempering glass sheets
of said thickness.
Three additional glass sheets were tempered as a group of control samples
using only two tongs of the Group 7 type to grip these latter glass sheets
at about 3 inches (76.2 millimeters) from each longitudinal end.
After completing the tempering, the six sets of glass sheets tempered by
pointed tongs were inspected under a 60 power microscope at the tong marks
with the results reported in Table II for the center tong marks:
TABLE II
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APPEARANCE OF TONG MARKS
Group No.
Remarks
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1 No vents visible. Smooth appearance. Light
red-brown deposits of oxide at marks.
2 No vents visible. Shows scattered, heavier
brownish deposits at marks.
3 Small and large vents (3 to 12 per sample) on
edge of marks. Brown scaly fused deposit.
Vents .01 to .03 inch (.25 to .76 millimeter)
long; .02 to .03 inch (.5 to .76 millimeter) deep.
4 Small and large vents (4 to 15 per sample). Edge
and inside marks. Brown scaly fused deposits.
Vents .01 to .04 inch (.25 to 1.02 millimeters)
long; .01 to .02 inch (.25 to .5 millimeter) deep. -5 Small and
large vents (2 to 8 per sample) on edge
and inside marks. Heavy brown scaly fused
deposits. Vents .01 to .03 inch (.25 to .76
millimeter) long; .01 to .02 inch (.25 to .5
millimeter) deep
6 Small and large vents (4 to 9 per sample) at edge
of marks. Heavy brown scaly fused deposit. Vents
.01 to .025 inch (.25 to .63 millimeter) long; .01
to 0.15 inch (.25 to .38 millimeter) deep. -7 Small and large
vents (4 to 10 per sample) on edge
and inside marks. Heavy brown scaly fused deposits
Vents .01 to .035 inch (.25 to .89 millimeter) long;
.01 to .02 inch (.25 to .5 millimeter) deep
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After the aforesaid visual inspection of the center tong marks under a 60
power microscope, the samples including the control samples were subjected
to a beam load test in which each test sample of each group and the three
control samples 43 to 45 were supported over a horizontal table with each
sample in turn resting on a pair of beams extending across the width of
the glass sheet. The beams were 22 inches (558.8 millimeters) apart and
each glass sheet mounted with its center line disposed at the center of
the space between the glass sheet supporting beams. An additional pair of
beams 10 inches (254 millimeters) apart rested on the upper surface of
each glass sheet sample tested with the center of the glass sheet sample
arranged at the center of the space between the additional pair of beams.
A glass plate having a width equal to that of the glass sheet specimen
rested on the upper pair of beams with its side edges aligned with the
side edges of the glass sheet specimen and an additional force applying
beam was arranged over the entire width of the glass sheet along a line
disposed over the longitudinal center of each glass sheet sample. Force
was applied at a constantly increasing rate toward the force applying beam
and the stress at failure calculated from the formula S=18W/bt 2, where S
is the stress at failure, W is the load at failure, b is the width of the
glass sheet specimen, and t is the thickness of the specimen.
The force was increased at a rate of 169 pounds (76.7 kilograms) per
minute, according to a standard test used at the research laboratories of
PPG Industries, Inc., to test the strength of glass specimens. The stress
at failure was so determined for each sheet in each set and the average
stress at failure value determined for each set. The control samples had
the largest stress at failure values, so that the average of their three
values was set at 100 percent and the average stress at failure values of
other sets were calculated for each set and compared to the stress at
failure average value for the set of control samples.
The results tabulated in Table III show the relative values of the average
stress at failure of each group relative to the average stress at failure
of the control group expressed as percentage of strength relative to that
of the average strength of the control samples. All test specimens of the
seven test groups failed in the vicinity of the central tong mark, whereas
the control specimen #43 failed at a weakened portion of an edge seam and
control specimens #44 and #45 failed at surface scatches near a line
aligned below the line of force application (which would have been in the
vicinity of a center tong mark if the control specimens were gripped by a
third tong in the center as were the test specimens of Groups 1 to 7).
TABLE III
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Comparison of Average Stress at Failure Values
Average Loss of
Strength Compared
Group No. Strength at Failure
to Control
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Control 100 percent --
1 92 percent 8 percent
2 72 percent 28 percent
3 61 percent 39 percent
4 49 percent 51 percent
5 55 percent 45 percent
6 63 percent 37 percent
7 55 percent 45 percent
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After the results of the tests were evaluated, production tongs having
glass engaging elements of the Group 4 composition, previously used in
commercial production, were replaced by tongs having glass engaging
elements of the Group 1 composition in commercial production at one plant.
For the first six month period of using tongs with the replaced glass
engaging members at the plant, a $5,000 savings per annum due to reduction
of glass breakage during production of one pattern alone was experienced,
compared to the breakage resulting from the use of tongs provided with the
prior art glass engaging elements. Significant additional savings from the
production of other patterns at this plant and other plants are expected
as the long conversion program continues.
Further mass production operations have been conducted using the FIG. 2
embodiment of tongs having threaded bolts of the type of Group 1. The
bolts were found to be durable for several thousand production cycles
before they were reworked, except for cases where tongs were dropped or
otherwise mishandled. Reworking the tungsten alloy elements represented no
greater problem than reworking the stainless steel elements of prior art
tongs that caused surface damage in glass sheets gripped thereby for
thermal treatment. Furthermore, similar reworking was required for glass
engaging elements of prior art tongs.
The tongs containing glass engaging elements of the type containing
tungsten based composites of the type of Group 1 were also used in
tempering glass sheets using as a cooling medium sublimable soft particles
of carbon dioxide. Venting, which existed with tongs provided with prior
art glass engaging elements, was virtually eliminated when the glass
contacting elements were changed to the tungsten based composites of the
present invention.
In other tests, tongs containing pure tungsten wire as the glass engaging
elements were used to replace tongs having glass engaging elements of
STELLITE No. 1 in tempering experiments of the dip quenching type when
considerable breakage was experienced under high heat transfer conditions
when glass sheets 0.090 inch (2.3 millimeters) and 0.125 inch (3
millimeters) thick gripped by such tongs were immersed after heating in
polyoxyalkylene glycols. Breakage was reduced considerably in glass sheets
of such thickness, when tongs having glass engaging elements of tungsten
metal were used as the glass engaging elements instead of the STELLITE No.
1 elements under the same conditions otherwise. An additonal test of more
than 60 glass sheets tempered by dip quenching using tungsten as the glass
engaging elements of tongs tested showed no breakage and no vents or metal
deposits at the tong marks on the glass. These tongs with tungsten metal
glass engaging elements were used in more than 1,300 glass sheet tempering
experiments involving cooling by air blasts, cooling by blasts containing
sublimable CO.sub.2 particles and cooling by exposure to liquids over a
period of several years. The tungsten elements that engaged the glass
showed considerable wear after such use, leaving doubts as to their
practicality for mass production purposes.
In still other tests, tongs having disc shaped glass engaging elements of
the composition of Group 1 were compared with those having disc shaped
glass engaging elements of the composition of Group 5 in the production of
parts for Chrysler and General Motors which were gripped by tongs during a
commercial production of these parts by press bending followed by
tempering by quenching with air blasts. Two tongs were used to support the
Chrysler part and four tongs to support the General Motors parts during
processing. After stress measurements using laser beam technology to
determine center tension stress and a DSR refractometer described in an
article by R. W. Ansevin published in ISA Transactions, Volume 4, October
1965, had determined that the parts had adequate temper stresses, they
were subjected to beam loading tests by applying a load to the center of a
steel bar aligned over the worst tong mark for that part using a 2 inch
(50.8 millimeters) span over an aligned with the worst tong mark and the
worst tong mark facing down so as to be in tension when the load was
applied to the top surface. The load was increased by 50 pounds (22
kilograms) per minute. The average breaking load for the glass parts was
401 pounds (182 kilograms) for the glass parts engaged by Group 5 glass
engaging members and 566 pounds (257 kilograms) for the glass parts
engaged by Group 1 glass engaging members, thus indicating a 29 percent
improvement in strength at the long mark in a direct comparison of tongs
provided with disc-shaped glass engaging members.
As a result of these lattermost tests, new tongs for use during press
bending and tempering certain specialty parts provided with disc shaped
glass engaging elements of the tungsten-nickel composition of Group 1 were
substituted for prior art tongs having disc shaped engaging elements of
the composition of Group 5 to suspend these specialty parts during press
bending and tempering when considerable breakage of glass was experienced
with the prior art tongs. Breakage was reduced considerably after
substituting the new tongs.
While the exact reason for the improvement in tempering operations using
either air or soft sublimable particles or liquid immersion for the
cooling step is not known, it has been theorized that the fusion of a low
temperature glass and a low carbon steel develops adherence promoted by
the presence of a layer of iron oxide on the glass engaging element that
contains iron as a component. The iron oxide that forms reacts chemically
with the glass that is being gripped. This iron oxide surface has a
propensity to react and fuse with the glass surface at elevated
temperatures and during cooling, the metal contracts more than the glass,
thus supplying a tensile stress in the outer regions of the contact zone
as a result of the geometry of contact between the glass engaging metal
member and the glass. Therefore, venting is experienced in the region of
the contact zone between an iron alloy glass contacting member and soda
lime glass. Similar conditions exist when a significant amount of cobalt
rather than iron is present in the glass engaging members of the tongs.
Therefore, the glass contacting elements of tongs according to the present
invention are essentially free of iron and cobalt (that is, if any iron or
cobalt is present, it is only in insignificant amounts resulting from
impurities).
In using glass engaging elements composed of tungsten metal or tungsten
metal composites essentially free of iron or cobalt, a tungsten oxide film
forms at elevated temperatures and this film is not easily assimilated
into the glass structure. Therefore, it is believed that the tong does not
adhere to the glass, so that there is no fusion with the glass, which
causes the tensile stress which gives rise to venting. It should also be
noted that for pure tungsten or composites containing at least 90 percent
tungsten, the expansion coefficient to the metal is nearly the same as
some glass compositions.
In the case of tungsten carbide, which is a ceramic material rather than a
metal composite, the material has a high oxidation resistance and thus
avoids the formation of a tungsten oxide film on the interfacial surface
between the glass contacting element and the glass. Furthermore, tungsten
carbide is believed to be very abrasive so that contact of the material
with cold glass under the pressure of the glass contacting elements is
believed to cause surface damage of the glass due to friction,
particularly if there is any motion of the glass contacting element of the
tongs relative to the glass during the tempering operation.
The form of the invention shown and described in this disclosure represents
illustrative preferred embodiments 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 which follows.
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