Dependence of the seal property of ConFlattype flanges on the fine dimensions of the
knife edge
Cite as: Journal of Vacuum Science & Technology A 21, 438 (2003); https://doi.org/10.1116/1.1545758
Submitted: 17 October 2002 • Accepted: 16 December 2002 • Published Online: 10 February 2003
Satoshi Kurokouchi, Satsuo Shinoda and Shinsaku Morita
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Journal of Vacuum Science & Technology A 21, 438 (2003); https://doi.org/10.1116/1.1545758
© 2003 American Vacuum Society.
21, 438
Dependence of the seal property of ConFlat-type flanges on the fine
dimensions of the knife edge
Satoshi Kurokouchi,a) Satsuo Shinoda, and Shinsaku Morita
Vacs SEV Company, Limited, 2-23-9 Yaguchi, Ohta, Tokyo, 146-0093, Japan
共Received 17 October 2002; accepted 16 December 2002; published 7 February 2003兲
The shape of the sealing face on ConFlat-type flanges has not been addressed at all in the
International Standards Organization standards, yet it is the distinct shape of the sealing face on the
flange, generally referred to as the ‘‘knife edge’’ that primarily distinguishes the ConFlat sealing
system. Results of profiling on the sealing face of 70 mm diameter ConFlat-type flanges, provided
by ten individual manufacturers, reveal that while all the sealing faces share a common 20° angle
on the taper face, there are variations in the tip radius of the knife edge, and in the angle of the
counterslope against the 20° taper face. In flange tightening tests using the ConFlat flanges, the tip
radius was observed to influence flange penetration into the gasket. It was also determined that the
knife edges of all the test flanges were plastically deformed to some extent in the process of
tightening twice at a torque of 120 kgf cm. The knife-edge shape was neither found to be dependent
on the magnitude of deformation, nor did the deformation have an observable influence on the seal
property. In order to closely investigate the dependence of the ConFlat-type flange seal property on
the fine shape of its sealing face, four types of 152 mm diameter flanges, each with a different
knife-edge shape, were subjected to flange tightening tests. The results indicated that flanges with a
small tip radius and a small counterslope angle maintained a satisfactory seal ability, even when
applied tightening torque was lower, but had difficulty retaining sealing stability under an elevated
temperature. On the other hand, flanges with a large tip radius or large counterslope angle on the
knife edge had an advantage in terms of seal stability in baking conditions, but required relatively
higher tightening torque to attain secure sealing. To evaluate the tip radius and the counterslope
angle at which the flange seal capability and the sealing stability at an elevated temperature are in
good balance, the stress condition in the tightened gasket and the flange penetration into the gasket
was computer simulated at various tip radii and counterslope angles, using the finite-element
method. Based on the results of the computer simulation, a reasonable range for the tip radius and
the counterslope angle was estimated to be 0.1 mm to 0.2 mm, and 20° to 40°, respectively.
© 2003 American Vacuum Society. 关DOI: 10.1116/1.1545758兴
I. INTRODUCTION
The ConFlat sealing system was originally developed by
an American manufacturer in the early 1960’s as a bakable
seal for ultrahigh vacuum 共UHV兲 applications,1,2 and has
been most widely employed in small and midsized UHV
systems. Flanges interchangeable with the ConFlat sealing
system have been available from numerous manufacturers
under various trade names since the patent for the original
ConFlat® 共Ref. 3兲 expired. Almost all of the key dimensions
of these ConFlat-type flanges, such as outside diameter,
bore for pipe joint, thickness, and location of bolt holes,
have been standardized under International Standards
Organization 3669, entitled Vacuum Technology—Bakable
Flanges—Dimensions.4 However, the dimensions of the sealing face have not been defined at all, despite the fact that the
particular shape of the sealing face, which is generally referred to as the ‘‘knife edge’’, is a distinguishing feature of
ConFlat-type flanges. The knife edges of currently available
ConFlat-type flanges are identical in their slope angle of 20°
and the location of tip, which is slightly recessed below the
top face of the flange. However, they differ in the tip radius
a兲
Electronic mail: kurokouchi@sev-vacuum.com
438
J. Vac. Sci. Technol. A 21„2…, MarÕApr 2003
and counterslope angle 共the angle opposite the 20° taper portion兲, as shown in Figs. 1共a兲 and 1共b兲 with a reference schematic and some examples.
It is certainly reasonable to surmise that the fine shape of
the flange knife edge has a substantial influence on the seal
property of the ConFlat-type flange, since the knife edge
functions to form a sealing interface on the gasket. The fine
shape of the knife edge can also affect leak probability, in
turn influencing the performance of current UHV systems,
which employ the ConFlat-type flanges. Therefore, the investigation of the relation between the fine shape of the knife
edge and the seal property should serve to elucidate the seal
mechanism of the ConFlat sealing system, as well as to suggest a method for the stable operation of UHV systems.
Presumably, manufacturers of ConFlat-type flanges possess their own relational data on the shape of the knife edge
and the seal properties of the flange, and have established R
共the tip radius兲 and ␪ 关counterslope angle, see Fig. 1共a兲兴 in
accordance with the data. However, such data and the thinking of the manufacturer about the seal mechanism of the
ConFlat sealing system are proprietary, hardly the type of
information that would be available to users. In the present
study, ten pieces of ConFlat-type flanges 70 mm in diameter,
0734-2101Õ2003Õ21„2…Õ438Õ11Õ$19.00
©2003 American Vacuum Society
438
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Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
439
ening load. Gasket tightening tests using 152 mm ConFlattype flanges fabricated for the purpose were performed,
along with stress analyses of the gaskets tightened with these
flanges, in order to investigate the dependence of seal properties on the knife-edge shape, and to observe the influence
of the knife-edge shape on stress distribution in the tightened
gasket. Finally, reasonable values for the tip radius and the
counterslope angle were estimated based on all of the data
obtained in this study.
II. RELATION BETWEEN THE SEALING FACE
SHAPE AND THE SEAL-FORMING PROPERTY
OF COMMERCIALLY AVAILABLE CONFLAT-TYPE
FLANGES
A. Fine shape of the knife edge in commercially
available ConFlat-type flanges
FIG. 1. 共a兲 Schematic of the ConFlat-type flange and its sealing face. 共b兲
Traced profiles of the sealing faces of commercially available, 70 mm diameter ConFlat-type flanges. The flange on the left-hand side was made by
a European manufacturer, while the right-hand side flange is American
manufactured.
made by American and European manufacturers, were subjected to gasket tightening tests to examine their seal properties. The sealing face of each test flange was profiled before and after the tightening test, to observe the original
dimension of the knife edge and its distortion under the tight-
Ten commercially available ConFlat-type flanges with a
nominal outer diameter of 70 mm were collected for the
study. Among them, four flanges were made in America,
while the other six were produced by European manufacturers. All test flanges were brand new, blank-off type, and all
were made of type 304 or type 304L stainless steel.
The sealing face of each test flange was traced using a
profilometer 共Rank Taylor Hobson, Form Talysurf S5兲. Values of R and ␪ of the knife edges derived from the traced
profiles are shown in Table I. As Table I clearly indicates,
there are many commercially available ConFlat-type flanges,
produced with tips of varying radii and counterslope angles
at the knife edge. They differ in their R and ␪ values, but all
have the same angle of 20° on the taper faces, and the same
location of the knife-edge tip; thus, they are mutually compatible. There is no doubt that three of the European flanges,
flange H, H⬘, and H⬙, though provided by different manufacturers, are produced based on the same drawing, since they
are very similar, not only in the dimensions around the knife
edges, shown in Table I, but also in their roughness profiles
on the 20° tapers. Moreover, the flanges cannot be distinguished from one another by their appearance. If these three
flanges are considered as one, and the European flanges are
counted as four rather than six types all together, the following similarities in the fine shape of the flange knife edge at
each region can be articulated: the American knife edges of
TABLE I. Tip radius and counterslope angle on the knife edge 关R and ␪ in Fig. 1共a兲兴 of 70 mm diameter test
flanges.
Flange
Tip radius R (mm)
Counterslope angle ␪ 共degrees兲
American
A
B
C
D
0.050
0.060
0.065
0.215
5
5
0
0
European
E
F
G
H
H⬘
H⬙
0.105
0.108
0.114
0.097
0.105
0.116
40
40
40
10
10
10
JVST A - Vacuum, Surfaces, and Films
440
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
440
the flange have an acute angle, with a tip radius lower than
0.1 mm for all, except for flange D, whereas the knife-edge
angle of most European flanges is obtuse, and the tip radius
is a little over 0.1 mm. Generally speaking, American manufacturers tend to prefer flanges with a sharper knife edge, and
European manufacturers a rounder, one.
B. Difference in seal surface-forming ability
due to the fine shape of the flange knife edge
The variety in knife-edge shape seen in Table I can be
understood to reflect the diversity of interpretations of the
manufacturer concerning the details of the ConFlat system
seal mechanism. To determine how the knife-edge shape influences seal properties, all test flanges are subjected to two
series of tightening tests. The first consists of simple penetration tests without any vacuum process, to examine the influences of R and ␪ on the penetration of the flange into the
gasket. The second is a series of pumping-down and leak
detection tests to determine the requisite tightening torque
for a complete seal.
The first simple penetration test was performed using the
setup shown in Fig. 2共a兲. Tightening was accomplished by a
torque wrench on six stainless steel M6⫻1.00 mm threadpitch bolts, in increments of 40, 60, 80, 100, and 120 kgf cm
per bolt. The bolt threads are lubricated with boron-nitridebased paste to keep friction on the bolt threads constant
throughout the test series. A cover ring was inserted between
washers and the flange face of the bolting side to prevent the
difference in finish on each test flange face from affecting the
friction among the bolt heads, washer faces, and flange. Test
flanges were not tightened in combination with a matching
flange but with a thick stainless-steel plate, to prohibit
mingled penetration of the test flange and the mating flange
which would make the two indistinguishable. The tightened
gasket was made of oxygen-free copper, with Vickers hardness of 82.5. The series of tests in the experiment were performed at room temperature of 共22⫾3兲 °C. The second series
of tightening tests, including pumping-down and leak detection, was performed on the setup shown in Fig. 2共b兲. Test
flanges were tightened in combination with a reducer flange,
the knife edge of which is in the shape shown in Fig. 2共c兲.
The measurable range of the helium leak detector 共ULVAC,
DLMS-33A兲 for determining the leak rate was 1.3⫻10⫺6 to
1.3⫻10⫺11 Pa m3 /s. All other conditions of the second series
of tests were the same as those of the first series.
Measured test flange penetrations into half sides of the
tightened gaskets at a tightening torque of 120 kgf cm, and
the tightening torque at the point where the leak rate dropped
below the detection limit of the leak detector, are shown in
Table II. The penetration in the first test series was derived
from the end-to-end distance of the flange and the plate,
measured at six points at equal intervals. For the second
series, the penetration was determined from the traced profile
of tightened gaskets. In the first test series, the leak stoppage
torque was virtually the same for every test flange, and was
not influenced by the shape of the flange knife edge. This
result seems natural considering that a tightening torque of
J. Vac. Sci. Technol. A, Vol. 21, No. 2, MarÕApr 2003
FIG. 2. 共a兲 Setup for seal-forming test without evacuation. 共b兲 Construction
of experimental setup for flange tightening tests. Flanges of either 152 mm
or 70 mm diameter are installed in each case. RGA: residual gas analyzer;
TMP: turbomolecular pump; DG: diaphragm gauge; LV: leak valve; and PR:
rotary pump. 共c兲 Traced profile of 70–34 mm reducer flange used in the
second series of tightening tests.
40 or 60 kgf cm produced virtually the same penetration
共0.040–0.060 mm兲, and identical seal areas for each test
flange. Penetration measured at the tightening torque of 120
kgf cm in the second series of tests dropped to a level of
91%–98% of the measurements in the first series for every
test flange. Three reasons can be inferred to explain the shallower measured penetration in the second series of tests.
First, recorded penetration in the second test series was derived from the traced profile of the respective tightened gaskets, which necessarily subtracts the springback of gasket
material from the net penetration. Second, the tightening
441
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
441
TABLE II. Penetration of test flange into half side of gasket at tightening torque of 120 kgf cm, and required
tightening torque for completion of seal.
Penetration 共mm兲
Flange
At first series
At second series
Required torque for seal 共kgf cm兲
American
A
B
C
D
0.106
0.106
0.102
0.100
0.102
0.101
0.100
0.093
60
60
60
60
European
E
F
G
H
H⬘
H⬙
0.104
0.102
0.096
0.103
0.104
0.103
0.095
0.099
0.092
0.101
0.100
0.101
40
60
60
60
60
60
force consumed in shearing gasket faces in the second series
of tests was approximately twice that of the first, because the
reducer flange also penetrated the gasket face in the second
series. Third, flange tightening in the first test series deformed the flange knife edge and thereby decreased the penetrating ability.
To make the relationship between the shape of the flange
knife edge and the penetration more obvious, the penetration
measurements listed in Table II were rearranged by dependence on the tip radius and counterslope angle, and were
shown again in Figs. 3共a兲 and 3共b兲, respectively. Clearly,
penetration was dependent on R to a certain extent, but was
independent of ␪. The reason penetration was not observed to
be dependent on ␪ is that the shallow maximum penetration
of approximately 0.1 mm made the contact length between
the counterslope and the tightened gasket too short to manifest any dependence on the counterslope angle.
C. Deformation of the knife edge caused by tightening
During the gasket tightening process, the formation of the
sealing face is mainly due to the distortion of the softer copper material of the gasket, but the harder knife edge of the
flange is also slightly distorted, because the seal pressure
FIG. 3. Dependence of flange penetration into half sides of test gaskets on
共a兲 tip radius of the knife edge, and 共b兲 counterslope angle. Penetration in
the first and second test series is indicated by circular and rectangular symbols, respectively. Both 共a兲 and 共b兲 are rearrangements of information from
Table II.
JVST A - Vacuum, Surfaces, and Films
generated on the interface between the flange and tightened
gasket frequently surpasses the yielding stress value of the
flange material when higher tightening torque is applied.
Thus, to check the yield property of the knife edge, it is
considered important to foresee what kind of knife-edge
shape will maintain a stable seal property over the long term
in practical use.
In order to observe the deformation of the knife edge that
results from the application of tightening force loads on each
test flange, the knife edges of all test flanges were traced
again by the profilometer after the two series of tightening
tests were finished. Comparison of the traced profiles of the
flange knife edges before and after the two series of tightening tests demonstrates that even with only two tightenings at
a torque of 120 kgf cm, the knife edges deformed in all cases.
The deformation modes can be classified into three patterns:
The first deformation pattern is a dent on part of the 20°
taper face, where the gasket was pressed. Flanges A, B, D, H,
H⬘, and H⬙ all showed this type of deformation. Those
flanges all have a counterslope angle of 5°⭐␪⭐10°, except
for flange D, at ␪⫽0°. Typical examples of this pattern are
shown in Fig. 4共a兲. Some variations in this pattern were observed where a part of the tip arc was nicked, as shown in
Fig. 4共b兲. The second pattern of deformation is a dent of the
knife-edge tip, as shown in Fig. 4共c兲. This pattern was observed on all ␪⫽40° flanges, namely E, F, and G. The third
deformation pattern is a swelling counterslope to compensate
for the dent on the 20° taper, as shown in Fig. 4共d兲. Only
flange C, at ␪⫽0° manifested this pattern. In every pattern,
the maximum dent deformity of the knife-edge tip from its
original height was as little as 0.01 mm.
After completing this observation of the flange knife-edge
deformation patterns, second series tightening tests and
knife-edge profiling were then repeated on all test flanges.
The profiles of the flange knife edges after undergoing a third
round of tightening at the torque of 120 kgf cm were virtually the same as those for the previous test, giving rise to the
conjecture that the magnitude of deformation of the flange
knife edge is determined by the maximum tightening load
which has been applied to it. In addition to this stability in
the shape of the knife edges, the measured penetration and
442
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
442
FIG. 5. Traced profile around the knife-edge tip, tip radius 共R兲, and counterslope angle 共␪兲 of 共a兲 flange J; 共b兲 flange K; 共c兲 flange L; and 共d兲 flange M.
M8 bolts, so that the deep penetration of the flange and,
therefore, distinct manifestation of the dependence of knife
edge shape would be expected.
A. Experiment and simulation
1. Gasket tightening test using 152 mm diameter
flanges
FIG. 4. Deformation of flange knife edge due to tightening carried out twice
at 120 kgf cm. 共a兲 Example of the dent on a 20° taper. 共b兲 Variation on the
case showed in 共a兲, with the arc of the knife-edge tip partly shaved. 共c兲
Example of a pattern in which the top of the knife edge was dented. 共d兲
Deformation pattern observed on flange C.
the required tightening torque to accomplish the sealing also
remained substantially unchanged in all cases. Based on
these results, it can be surmised that brand-new ConFlat-type
flanges generally lose their resilience in the first tightening
process, but maintain stable seal properties over a long term,
assuming that tightening torque is appropriately controlled,
and that no unexpected but critical damage is done to the
knife edge.
III. INFLUENCES OF KNIFE-EDGE SHAPE ON
STRESS DISTRIBUTION WITHIN THE TIGHTENED
GASKET, AND RESULTANT SEAL PROPERTIES
In the two series of flange tightening tests described
herein, using commercially available 70 mm diameter
flanges, the penetration of the test flanges into gaskets was
not deep enough to allow a detailed examination of the effect
of the fine shape of the flange knife edge on the seal properties of the flanges, particularly in terms of the influence of
the counterslope angle. Deeper penetration would be obtained if tightening torque above 120 kgf cm were applied;
however, maximum tightening torque in the experiments was
of necessity limited to 120 kgf cm, because the M6 stainlesssteel bolts for 70 mm diameter flanges would not bear a
larger tightening load. To elucidate the dependence of the
seal property of a ConFlat-type flange on the fine shape of its
sealing face, four types of ConFlat-type flanges, each with an
outer diameter of 152 mm, were prepared for the experiment.
The nominal outer 152 mm ConFlat-type flange has the narrowest interval of bolt holes seen in flanges tightened with
J. Vac. Sci. Technol. A, Vol. 21, No. 2, MarÕApr 2003
Four ConFlat-type flanges, the knife-edge shapes of
which are shown in Figs. 5共a兲–5共d兲, were subjected to gasket
tightening tests. Flange J, K, and L were produced by remachining the original flange M, a commercially available
flange made of type 304L stainless steel. A blank-off flange
and a 152–70 mm reducer were produced with the same
dimension in their respective flange knife edge, and were
always subjected to the tightening tests in combination.
The test flanges were tightened on the setup shown in Fig.
2共b兲, with an oxygen-free copper gasket of Vickers hardness
value 82.8, using a torque wrench on 16 stainless-steel M8
⫻1.25 mm threadpitch bolts, in increments of 50, 100, 150,
and 200 kgf cm per bolt. Flange end-to-end distance and the
leak rate under each increment of tightening torque were
measured. The series of tests in the experiment were performed at room temperature of 共22⫾3兲 °C, except for certain
tests involving baking processes. The other test conditions
were the same as those in the second series of the tightening
tests with commercially available 70 mm outer diameter
flanges, except that no cover ring like the one shown in Fig.
2共a兲 was used.
2. Analyses of stress distribution in tightened gasket
Seal pressure and its distribution on the sealing interface
directly reflect the magnitude and distribution of stress in a
tightened gasket. Therefore, an analysis of the stress in the
tightened gasket is crucial to the discussion of the seal property of each test flange. In order to observe differences in the
stress conditions of the gaskets tightened with each test
flange, the stress distribution in each tightened gasket was
simulated, employing finite-element method structural analysis software 共Finite-Element Analysis, LUSAS Ver. 13.3兲.
The results of a preliminary tensile test on the test gasket
material were input into the simulation software as the basic
parameters for the stress analysis. The initial geometrical arrangement of the flanges and test gasket in the simulation is
shown in Fig. 6. As the default setting, the radial displacement of the upper half of the gasket outer rim was constrained to simulate the typical condition of actual flange
tightening, sometimes referred to as ‘‘half-side capturing’’, in
which one side of the gasket touches the flange counter.5 The
gasket materials were assumed to be homogeneous, with no
443
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
FIG. 6. Initial geometric configuration of the test flanges and gasket in the
simulation. Tip radius, counterslope angle of the flange, and setting of the
X-directional displacement stopper for the gasket were altered for each case.
residual stresses. Flanges were presumed to be rigid-body
types. The result of the stress analysis was considered to be
the stress distribution when the penetration of the flange in
the simulation equaled the penetration obtained by actual
tightening, under respective tightening torque of 50, 100,
150, and 200 kgf cm.
B. Results and discussion
1. Analysis of seal area and seal pressure
on the tightened gasket
The penetration depth of the flange into the gasket face is
an important index of the seal ability of the flange, because
the area of the seal surface, a determinant of seal reliability,
corresponds fairly closely to the penetration. In other words,
penetration is an index of the seal surface-forming ability of
the flange. Measured penetration of the test flanges J, K, L,
and M into the half sides of the test gaskets is shown in Fig.
7. Flanges J and K, with R of 0.05 mm, penetrated deeper
than flanges L and M, with R is 0.20 mm, throughout the
FIG. 7. Flange penetration, tightening torque, and decreased leak rate.
JVST A - Vacuum, Surfaces, and Films
443
range of tightening torque, from 50 through 200 kgf cm. As
expected, this dependence of flange penetration on the tip
radius appeared more clearly than it did in the 70 mm diameter flange experiment discussed herein. Dependence of the
penetration on the counterslope angle was obvious at tightening torque of 150 kgf cm or more for the group of flanges
with R⫽0.05 mm. On the other hand, the dependence appeared only at the tightening torque of 200 kgf cm for flanges
with R⫽0.20 mm, because the flange penetration of approximately 0.12 mm at a tightening torque of 150 kgf cm was too
shallow to put the counterslope of the flanges into contact
with the gasket. Consequently, test flanges can be sorted by
penetration order, as flanges J, K, L, and M, but the penetration measurements for flanges L and M were virtually identical. This result simply indicates that a flange with a sharper
knife edge has an advantage in terms of penetration into
gasket.
The penetration, however, does not necessarily reflect the
seal ability of the flange. In the Fig. 7 bar graph, the points
indicated with triangles and circles show the tightening
torque where the leak rate dropped to the order of
10⫺10 Pa m3 /s, and where the leak rate dropped below the
detection limit of the detector, respectively. Flange K completed sealing at a lower tightening torque than flange J, yet
the penetration of flange K was slightly shallower than that
of flange J at every tightening torque. This discrepancy between the penetration and the leak stoppage torque points to
the insufficiency of penetration as the indicator of flange seal
ability. For a more exact evaluation, seal pressure should be
considered. Results of computer simulations on Y-directional
共axial兲 compressive stress in the tightened gaskets are shown
in Fig. 8. Y-directional compressive stress on the part of the
gasket where the 20° taper face of the knife edge compresses
generally constitutes over 90% of total seal pressure, and is a
valid indicator of seal reliability.5 At every tightening torque,
the maximum value of the Y-directional compressive stress
in the tightened gasket was highest when flange J was penetrated. However, within the gasket tightened with flange J,
the peak of Y-directional compressive stress was concentrated within a very small region, and was not exposed on the
seal surface. The total magnitude and average Y-directional
compressive stress exerted on the seal surface was higher
when flange K was penetrated rather than when flange J was
tightened. This means that the counterslope of flange K, with
an angle of 20°, functions as an equalizer of compressive
stress on and around the seal surface.
Secondary analyses of gasket material displacement under
the tightening load revealed that the gasket material abutting
the flange counterslope is barely displaced at all, in either the
X-direction 共radial direction兲 or the Y-direction, during the
flange J penetration process. This is because the counterslope
of flange J, which is simply vertical to the gasket face, does
not compress the gasket material. On the other hand,
Y-directional displacement of gasket material is most intense
at the region abutting the knife-edge tip. Thus, in the tightening of flange J, large-scale fault movement of the gasket
material occurs at the boundary between the counterslope
444
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
444
FIG. 8. Y-directional compressive stress in the tightened gaskets.
FIG. 9. Distribution of von Mise’s equivalent stress in the tightened gaskets.
side and the 20° taper face side of the gasket material. This
fault movement produces an excessive compression of gasket material at a point on the boundary line and, consequently, intensifies the compressive stress within the limited
region. Where flange K is tightened, by contrast, compression by the flange counterslope displaces the gasket material,
not only in the X-direction, but in the Y-direction as well.
This Y-directional displacement of gasket material at the
counterslope side acts to stabilize the Y-directional material
flow around the region where the knife-edge tip compressed,
and effectively suppresses the unevenness of the compressive stress on the seal surface. Consequently, the total seal
pressure sufficient to complete the sealing is obtained on the
seal surface of the gasket, even at tightening torque as small
as 50 kgf cm.
In the tightening of flange L or M, once again, satisfactory
seal pressure uniformity is realized on the seal surface of the
tightened gasket; however, secure sealing could not be obtained at the tightening torque of 50 kgf cm, because the 50
kgf cm tightening torque provides insufficient seal pressure
to accomplish the sealing. The maximum value of
Y-directional compressive stress on the seal surfaces at the
tightening torque of 50 kgf cm was 5.1⫻108 Pa in the gasket
J. Vac. Sci. Technol. A, Vol. 21, No. 2, MarÕApr 2003
tightened with flange L or M, and 6.1⫻108 Pa in the gasket
tightened with flange K. This difference of 1⫻108 Pa in
Y-directional compressive was the determinant of sealing
success or failure for each flange at the tightening torque of
50 kgf cm. Where the penetration is made by a knife edge
with a larger R, the compressed area on gasket face is large,
but the penetration of the flange and the volume of the deformed gasket material tends to decrease. This, in turn, results in lower Y-directional compressive stress around the
knife-edge tip, and did not enable sufficient seal pressure at
50 kgf cm tightening torque.
2. High equivalent stress yielded by tightening with
a sharper flange knife edge and its disadvantage
under elevated temperature
Figure 9 shows results of equivalent stress analyses on
gaskets tightened with flanges J, K, L, and M. In the gasket
tightened with flange J, the region where equivalent stress
reaches its maximum closely abuts the counterslope, and
stretches narrowly toward the inner portion of the gasket, as
if extending the line of the counterslope. This linear region is
clearly seen only at the captured 共upper兲 side of the gasket,
445
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
445
FIG. 10. Schematic model of gasket banding due to the local capturing of the
gasket outer rim. 共a兲 Local contact between a gasket and the retaining
counter at the beginning of the tightening process. 共b兲 As the diameter of
gasket expands, local compression and bending of the gasket increases.
but it can also be made clearly visible at the uncaptured
共lower兲 side by scale adjustment of the stress contours. The
previously described fault movement of gasket material during the tightening of flange J occurs on this line. Equivalent
stress in the gasket tightened with flange K was not as intense as that in tightening with flange J; however, it was
concentrated in the region compressed by the counterslope,
just as it was with flange J tightening. The intensified equivalent stress in the gasket seems to substantiate seal reliability.
However, a comparison of Figs. 8 and 9 reveals very little
overlap between the region of intensified equivalent stress
and the region where Y-directional compressive stress was
intensified. This location discrepancy between the two types
of stress indicates that equivalent stress in the gaskets tightened with flanges J or K does not reflect the seal pressure,
but rather is an efficient indicator of the gasket material
shearing force.6 In the gasket tightened with flange L or M,
on the other hand, the locations of higher equivalent stress
and higher Y-directional compressive stress overlapped to a
great degree. This overlap proves that tightening force applied on the tightened bolts was effectively used to form a
seal surface and to generate seal pressure.
Equivalent stress also indicates the energy consumed for
gasket bending. During the flange tightening process, the
gasket outer diameter expands as a result of an outward flow
of gasket material caused by applying a higher tightening
torque. In practical use, this expansion of the outer diameter
almost never contributes to improved seal reliability; rather,
it simply causes local bending of the tightened gasket at the
point of contact with the retaining counter of the flange,5 as
is schematically shown in Fig. 10. This bending of the tightened gasket is more aggravated as the gasket outer diameter
expands. Since expansion of the outer diameter is the result
of increased gasket material flow as the flange penetrates
more deeply, tightening using a flange with small R and
small ␪ tends to bend the gasket severely at a higher tightening torque. Figure 9 shows that the equivalent stress was
symmetrically distributed on both sides of the gasket tightened with flange L or M. In the gasket tightened with flange
J or K, by contrast, the region enclosed with a stress contour
line of 9⫻108 Pa is wider on the uncaptured side than on the
captured side at a tightening torque of 200 kgf cm. This
asymmetrical distribution of equivalent stress was undoubtedly brought about by the gasket bending. Since half-side
capturing, where the outer rim of a gasket half side is partly
in contact with the end counter of the flange, is typical of
JVST A - Vacuum, Surfaces, and Films
FIG. 11. Change of the flange end-to-end distance after the baking process.
sealing in the ConFlat system,5 this asymmetric distribution
of equivalent stress seen in the computer simulation should
also occur in almost all actual cases of gasket tightening
using a flange with an acute knife edge.
It seems reasonable to assume that a flange with a sharper
knife-edge tip, which tends to severely intensify stress concentration in the tightened gasket, would be at a disadvantage in terms of maintaining seal stability at an elevated temperature, since higher stress and higher temperature enhance
the creep distortion of metallic gasket material. A gasket
tightening test including a baking process was conducted on
flanges J, K, L, and M to evaluate the influence of this concentrated stress on gasket material creep. The test flanges
were tightened at torque of 200 kgf cm, and were baked at
250 °C for 7 h on the setup shown in Fig. 2共b兲. Measured
changes of the flange end-to-end distance after the completion of cooling-down process, and after the tightening bolts
were released are shown in Fig. 11. The decrease in the endto-end distance seen in Fig. 11 generally corresponds to the
lowered tension on tightening bolts which occurs due to the
creep of gasket material and, consequently, indicates a lost
tightening force. Meanwhile, the springback in flange endto-end distance seen when the tightening bolts are released is
a valid indicator of residual elasticity in the gasket. In the
tightening with flange L or M, the decrease in flange end-toend distance after baking was shorter, and the springback due
to the release of the tightening bolts was greater than that
measured when flanges J and K were used for the tightening.
This result clearly shows that the loss of both the tightening
force and elasticity of the gasket were smaller when flange L
or M was used for the tightening than when flange J or K
was used. On the other hand, the result also implies that
almost all compressive stress in the gasket tightened with
446
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
flange J or K was relaxed due to the creep of gasket material
during the baking process. Relaxation of internal compressive stress in the gasket is very detrimental to the bakable
seal, because it nullifies the tension of the tightening bolts
and, therefore, diminishes the seal pressure. Diminishing the
tightening torque to approximately 130 kgf cm for flanges J
and K limits the magnitude of equivalent stress around the
seal surface of the tightened gaskets to the same level as that
in the gasket tightened with flange L or M at a tightening
torque of 200 kgf cm. While such a restriction of tightening
torque would serve to decrease the creep of gasket material,
it is less effective in terms of seal reliability, because it also
decreases the seal pressure and flange penetration. Where the
tightening is done with flange K, seal pressure and flange
penetration at the tightening torque of 130 kgf cm are estimated at 1.45⫻109 Pa and 0.062 mm, respectively. Both values are smaller than those obtained in the tightening with
flange L or M at 200 kgf cm tightening torque. Tightening
with flange L or M at a higher torque is thought to be more
beneficial to seal reliability than tightening with flange J or K
at a limited tightening torque.
3. Discussion on practical limits of tip radius
and counterslope angle in the flange knife edge
The results of the series of tightening tests conducted with
flanges J, K, L, and M showed that R and ␪ of the flange
knife edge should be somewhat larger to enable better sealing stability in applications including a baking process. At
the same time, the results demonstrated that larger R is disadvantageous in terms of the flange sealing ability when the
applied tightening torque is lower. Taken together, these facts
lead to the conclusion that the R and ␪ values must be within
a reasonable range, in order to retain the practical versatility
of the ConFlat-type flange. To estimate this reasonable range
for R and ␪, the critical values of R and ␪, those at which the
flange seal property will be considerably deteriorated, must
be determined. As the stress analyses on the gasket tightened
with flanges J, K, L, and M illustrated, small R and small ␪
values are advantageous in obtaining a relatively higher seal
pressure, although they also induce equivalent stress concentration around the knife-edge tip in the tightened gasket. Excessive concentration of equivalent stress in the tightened
gasket harms the stability of sealing under elevated temperature. As a reference in determining the lower limits of R and
␪, therefore, top priority should be given to equivalent stress
distribution in the gaskets. On the other hand, upper limits
also need to be established, because while increased R and ␪
generally make for uniform compressive stress on the seal
area, they decrease the maximum and average values of the
seal pressure. This decreased seal pressure in turn deteriorates the seal ability of the flange and, consequently, inhibits
secure sealing at a lower tightening torque. For this reason,
flanges L and M, at R⫽0.20 mm, did not obtain complete
sealing in the tightening tests discussed herein, at the tightening torque of 50 kgf cm. Thus, to ascertain the upper limits
of R and ␪, a determinant factor should be the level of seal
pressure to be generated around the flange knife edge. Only
J. Vac. Sci. Technol. A, Vol. 21, No. 2, MarÕApr 2003
446
TABLE III. Simulated stress conditions in gaskets tightened with the hypothetical flanges. SE 1: Asymmetry of equivalent stress distribution on each
side of the tightened gasket. SE 2: Band of intensified equivalent stress
extending from the counterslope of flange. SE 3: Level of equivalent stress
around the knife-edge tip. SY: Y-directional compressive stress on seal surfaces. For each SE factor, ⫻is applied where the condition is considered
harmful to the sealing; 䉭 where it is difficult to judge whether the condition
is good or bad for sealing; and 䊊 in cases where no problem is predicted.
For SY, the tightening torque 共in kgf cm兲 at which secure sealing is expected
is shown.
R (mm) and ␪共degrees兲 of
hypothetical flanges
R⫽0.05/␪ ⫽0°(J)
R⫽0.05/␪ ⫽2°
R⫽0.05/␪ ⫽5°
R⫽0.05/␪ ⫽10°
R⫽0.05/␪ ⫽20°(K)
R⫽0.05/␪ ⫽40°
R⫽0.05/␪ ⫽60°
R⫽0.10/␪ ⫽0°
R⫽0.10/␪ ⫽20°
R⫽0.10/␪ ⫽40°
R⫽0.15/␪ ⫽0°
R⫽0.15/␪ ⫽20°
R⫽0.15/␪ ⫽40°
R⫽0.20/␪ ⫽0°(L)
R⫽0.20/␪ ⫽20°(M)
R⫽0.25/␪ ⫽0°
R⫽0.30/␪ ⫽0°
R⫽0.40/␪ ⫽0°
SE 1
SE 2
SE 3
SY
⫻
⫻
⫻
⫻
⫻
䉭
䉭
䉭
䉭
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
⫻
⫻
⫻
⫻
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
⫻
⫻
⫻
⫻
⫻
⫻
䊊
䉭
䉭
䉭
䊊
䊊
䊊
䊊
䊊
䊊
䊊
䊊
100
50
50
50
50
50
100
50
50
50
50
50
50
100
100
100
150
150
two values for each R and ␪ 共0.05 mm/0.20 mm for R and
0°/20° for ␪兲 were examined in the tightening test and the
stress analyses. Data from an experiment in which the only
two values were considered for each parameter are insufficient to find the critical values of R and ␪. To make up for
this insufficiency in data quantity, stress distribution in tightened gaskets was simulated for imaginary ConFlat-type
flanges with knife-edge shapes different from those of
flanges J, K, L, and M. The simulation result was considered
to have recreated stress conditions when the volume of plastic work in the imaginary gasket equaled the volume of plastic work in the actual gasket tightened with flange J, K, L, or
M. Because there is not enough space to graphically demonstrate all the stress distribution simulation results in the same
way as Figs. 8 and 9, certain key factors which should be
considered in determining the proper range of R and ␪ are
tabulated in Table III, which serves as a substitute for a display of the stress contours in the simulated gaskets. Data for
the tightening of flange J, K, L and M are also displayed in
Table III as a reference.
In Table III, the influence of ␪ on the flange seal property
can be seen clearly by comparing the stress conditions in the
gasket tightened by flanges of 0.05 mm R. In the ␪ range of
0° to 20°, uniformity of the Y-directional compressive stress
on the seal surfaces increases as ␪ increases. This consequently lowers the tightening torque required to complete
sealing, but does not significantly alleviate the asymmetric
distribution and the excessive concentration of equivalent
stress. An increase of ␪ to 20° or more certainly alleviates the
447
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
447
FIG. 12. Simulated penetration of flanges into half sides of gaskets.
irregularities of equivalent stress distribution and the fault
movement of gasket material, but ␪ over 60° increases the
required tightening torque for complete sealing. However,
the effects of altering ␪ values are insignificant where R is
0.10 mm or more, particularly in the range of a tightening
torque between 50 and 150 kgf cm, because the flange counterslope hardly comes in contact with the gasket. In the gasket tightened with a R⫽0.10 mm flange, equivalent stress
distribution resembles that observed in the gasket tightened
with the R⫽0.05 mm/ ␪ ⫽40° flange. This result suggests
that increasing R has a similar effect on the seal property of
the flange as that of increasing ␪. Improper distribution of
equivalent stress is almost entirely done away with when the
R reaches 0.15 mm, without any deterioration in the flange
seal capability. However, an increase to more than 0.20 mm
does deteriorate the seal ability at a lower tightening torque.
Meanwhile, results of the stress analyses showed that secure
sealing cannot be expected at a tightening torque of 100
kgf cm using a flange with R over 0.30 mm. Since tightening
torque is usually applied in a range from 100 to 150 kgf cm
in practical use, any flange with a doubtful seal ability at 100
kgf cm cannot be classified as a properly designed ConFlattype. Therefore, R values greater than 0.30 mm should be
excluded from what is considered a reasonable R range.
Penetration of the flange into the tightened gasket should
be among the factors considered in establishing lower limits
for R and ␪. In the simulation, there was complete circularity
and flatness with no irregularity observed in the flanges.
Moreover, the tightening force exerted on flange knife edges
was distributed uniformly along the entire circumference of
the flange. In the tightening experiments for the present
JVST A - Vacuum, Surfaces, and Films
study, all test flanges were of the blank-off or reducer type,
which has no welded part and no welding distortion. Additionally, the tested flanges were horizontally oriented to the
floor, as showed in Fig. 2共b兲. This accounts for the uniform
penetration of the flange knife edge along the gasket circumference, without any welding distortion or bending moment
on the flange. However, in actual tightening, ConFlat-type
flanges are typically welded with pipes, and frequently fixed
vertically to the floor. The welding frequently distorts the
flange, disrupting the completeness of circularity and flatness, while the vertical orientation generates a bending moment on the faces of the flange. This welding distortion and
bending moment on the flange contribute to a lessening of
the flange knife-edge penetration, particularly in tightening
with a larger-diameter flange, since both the welding distortion and the bending moment enlarge as the flange diameter
increases. Under such conditions, overly large R and ␪ values
may diminish the seal area and seal pressure and, consequently, ruin the seal ability of the flange. Therefore, the
reasonable range of R and ␪ needs to be reconsidered, both in
terms of maintaining the practical versatility of the ConFlattype flange, and also from the standpoint of flange penetration under actual tightening conditions.
The simulated penetration of hypothetical flanges is
shown in Fig. 12. The penetration of flanges J, K, L, and M,
already shown in Fig. 7, is displayed again in Fig. 12 as a
reference. Comparing the penetration of each flange showed
in Fig. 12 clearly demonstrates that ␪ values of 60° or more,
or R values of 0.25 mm markedly reduce the penetration of
the flange. As has been shown in Table III, R of 0.25 mm or
448
Kurokouchi, Shinoda, and Morita: Dependence of the seal property of ConFlat-type flanges
more and ␪ of 60° or more also provide insufficient seal
pressure.
Factoring in all of the results summarized in Table III and
Fig. 12, it is clear that flanges with R⭓0.25 mm or ␪⭓40°
have a very fragile seal ability in practical use. It is safe to
conclude that the R value for ConFlat-type flanges should be
set in the range of 0.10–0.20 mm to attain a dependable seal
property. The optimal balance of higher seal pressure and
increased sealing stability at an elevated temperature seems
to be obtained at the R value of 0.15 mm. If there is a need to
set the R value under 0.10 mm, the ␪ value should be in the
range of 20°– 40°, in order to avoid the excessive concentration of equivalent stress in the tightened gasket.
IV. CONCLUSION
The tip radius of the flange knife edge and the angle of
counterslope against the 20° taper face undoubtedly influence the seal property of ConFlat-type flanges. The precise
influences of these dimensional parameters on the ConFlattype flange seal property were experimentally evaluated. A
reasonable range for the tip radius and for the counterslope
angle were considered based on results of flange tightening
tests, and stress analyses in the tightened gaskets.
Prior to investigating the dependence of knife-edge shape
on the seal property, the sealing faces of ten commercially
available 70 mm diameter ConFlat-type flanges were profiled. The tip radius and counterslope angle of the profiled
knife edge on each flange fell within the range of 0.05–0.215
mm, and 0°– 40°, respectively. These profiling results demonstrated that currently available ConFlat-type flanges marketed under various trade names are designed with knife
edges that vary widely in tip radius and counterslope angle.
Following the sealing face profiling, all the flanges are
subjected to two series of gasket tightening tests. Little difference in the seal ability between the test flanges was observed; however, a tendency toward shallower flange penetration into the gasket corresponding with increased tip
radius was seen. Penetration dependence on the counterslope
angle was not manifested in the tightening tests, because in
every case the penetration was too shallow to sufficiently
compress the flange counterslope to the gasket. Tightening at
the maximum tightening torque of 120 kgf cm carried out
twice deformed every flange knife-edge to some extent, but
deterioration of the flange seal ability was negligible.
J. Vac. Sci. Technol. A, Vol. 21, No. 2, MarÕApr 2003
448
In a closer investigation of flange seal property dependence on knife-edge shape, four 152 mm diameter ConFlattype flanges with varying tip radii and counterslope angles
were fabricated and subjected to gasket tightening tests.
Computer simulation stress analysis testing was also conducted on the gaskets tightened with each test flange. Results
of the tightening tests and the stress analysis showed that a
combination of small tip radius and small counterslope angle
increases the flange penetration, enabling a large seal area
and sufficient seal pressure, but causing stress concentration
in the tightened gasket. Results of a gasket tightening test
including a baking process illustrated that excessive stress
concentration tends to enhance the creep of gasket material
at an elevated temperature, and increases the probability of a
leak. On the other hand, experimental results also indicated
that a larger tip radius or larger counterslope angle effectively prevent the excessive stress concentration in the vicinity of the knife-edge tip. These effects of the larger tip radius
and the larger counterslope angle, consequently, equalize the
seal pressure; however, as the tip radius or counterslope
angle grow larger, average and peak seal pressure both decrease. Moreover, the larger tip radius and counterslope
angle lessen the penetration of the flange and decrease the
seal area.
In order to determine a reasonable tip radius and the counterslope angle range and retain the practical versatility of
ConFlat-type flanges, computer simulation was performed on
14 flange gasket tightening patterns, using hypothetical
flanges with varying tip radii and counterslope angles. Stress
conditions in the tightened gaskets, and the penetration of the
flange in the simulation indicated that proper tip radius values lie between 0.10 and 0.20 mm, while proper values for
the counterslope angle are from 20° to 40°. It is estimated
that values within these ranges ensure desirable performance
for ConFlat-type flanges.
1
W. R. Wheeler and M. Carlson, Transaction of the 8th National Vacuum
Symposium, 1961 共Pergamon, New York, 1962兲, p. 1309.
2
W. R. Wheeler, Transaction of the 10th National Vacuum Symposium,
1962 共MacMillian, New York, 1963兲, p. 159.
3
Trademark of Varian Associates Inc.
4
International Standard Organization 3669, 1986, Vacuum Technology—
Bakable Flanges—Dimensions.
5
S. Kurokouchi, S. Morita, and M. Okabe, J. Vac. Sci. Technol. A 19, 693
共2001兲.
6
S. Kurokouchi, S. Morita, and M. Okabe, J. Vac. Sci. Technol. A 19, 2963
共2001兲.