Note: Descriptions are shown in the official language in which they were submitted.
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This invention deals with the installation of sanitary sewers and
is particularly concerned with a method of constructing a gasket for sealing
the space between a sewer pipe and an opening in the wall of a manhole
through which that pipe passes.
The new form oE gasket represents an improvement in the structure
shown in my Canadian Patent No. 996,150, issued August 31, 1976. That patent
shows a gasket made from an extruded rubber strip, having a cross section
in the shape of a capital A. When properly positioned in the manhole opening,
the apex of the A is exposed, and extends radially inwardly to make firm con-
tact with the surface of the pipe. The legs of the A extend outwardly, and
are embedded in cementitious material which lines the opening in the man-
hole wall, as is shown in my Canadian Patent No. 971,997, issued July 29,
1975. The method of constructing such a seal is described in my U.S. Patent
No. 3,832,438, dated August 27, 1974.
In these earlier patents, in which the elastomeric gasket was made
from an extrusion in the cross-sectional form of a capital A, it required ~ -
some distortion of the gasket, as explained at page 6, lines 16 et seq., of
Canadian Patent No. 996,150, to fit this gasket onto the mold rings, but the
webs or skirts (corresponding to the lower legs of the A) were fairly
flexible, so the difficulty of installation proved to be slight. In spite
of the provision of concentric serrations in the web or skirt portions of
the gasket, however, there seemed to be a possibility that, under severe
conditions, the pipe might impose so great a stress on the gasket as to
cause the rubber in the skirt portion under tension to thin out and pull
away from the surface of the concrete in which it was embedded. In order to
reduce this potential area of vulnerability, experimentation with other
shapes of extrusions was
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carried out.
The basic problem was to provide an improved technique for
positively and pernanently anchoring the gasket in the cementitious mater-
ial which defines the inner surface of the opening. Preferably the gasket
has a double-walled head portion, which has a continuous circular cavity
therein. This cavity is walled with rubber or similar elastomer, and its
walls are air-tight, so that it affords a pneumatic cushion.
Ihe problem did not appear to be solvable by the use of individually
molded unitary gasket washers~, since they lacked sufficient resistance to
compression in their lower, inner portion, which carries the major load,
and were too subject to peripheral separation from the pipe in their upper
portions. Furthermore, such washers would have to be stocked in m~ny sizes.
Nor did it appear to be feasible or practical to mold a unitary gasket ring
in the form of a washer with a continuous circular cavity, such as could be
made by using the A-shaped extrusion.
Ihe endless cavity of the A-section gasket provided an answer to
the compressibility problem in the pipe-contacting part, but the difficulty
of installation, because the diameter of the inner opening is so much less
than the diameter at the extreme ends of the webs or "legs" of the A, dis-
couraged any attempt to use cross-sectional forms which would have to accom-
modate an even greater difference between the inner and the outer diameter
of the washer-like gasket, in order to complete installation.
Ihe cross-sectional pattern of the anchoring member of the molded
washer shown in the Netherlands patent to Raatjes, No. 290,612 of 1963,
seemed to be likely to remain embedded in its surrounding concrete, no matter
how severe the stress upon it might become. But there seemed to be no way to
convert a lineal extrusion having this cross-sectional shape into a disk-
like washer, and no practical way to provide the endless cavity unless it
could be formed by extrusion.
*"washers" as used herein, refers to
centrally-apertured articles of disk-like shape.
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Combining an extruded triangular head with a T-shaped anchoring
part seemed to be quite out of the question, for this would greatly increase
the overall height of the extrusion, and thus create an even greater differ-
ence between inner and outer diameter of the finished washer than the A-shaped
gasket entailed. It would apparently be practically impossible to roll the
gasket into place in the molding rings.
It has now been discovered that a seal having all of the desired
properties can be made by utilizing an extrusion having a pipe-contacting
part which, instead of being A-shaped, is pear-shaped in section and is
unitarily provided with a T-shaped anchoring part, the stem of the T being
united with the bottom of the pear-shaped part and affording, when longi-
tudinally considered, a web of substantial height and thickness. Such an
extrusion can be used effectively if handled in the manner hereinafter set
forth.
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The method of the invention consists of a method of constructing a
gasket which comprises the following procedures: ta) providing a linear~xtr~ion
of elastomeric material having, in cross section, a pear-shaped head integrated
in a mid-region of its lower part with a flange in the shape of an inverted
"T", the height of said extrusion being greater than its width, ~b) cutting
the extrusion to a length approximating the circumference of the desired
gasket opening, (c) vulcanizing the ends of the extrusion toge~her to form a
cylindrical ring around a central axis and wherein the height of the extrusion
generally parallels the central axis, and (d) deflecting the head and flange of
the cylindrical ring to form a washer-like gasket, wherein the height of the
extrusion extends transversely of the central axis, with its head portion in-
ward and its flange portion outward, thus placing the inner portion of the
gasket in compression and the outer portion in tension.
The cylindrical ring is preferably positioned on the inner portion of
a mold, and is rolled inwardly, so that the pear-shaped pipe-contacting portion
is deflected radially inwardly and the outer flange (corresponding to the cross-
bar of the T in section) is pulled outwardly, in a manner analogous to that
employed in fitting the bead of a tire to the metal rim of an automobile wheel.
When the inward deflection of the pipe-contacting part has progressed sufficient-
ly to allow it, the complementary part of the mold is slid into place, and itsclamp is drawn up, applying pressure as needed, to force the tubular part
of the ring into the groove which extends peripherally of the mold.
A suitable mold ring for utilizing the novel gasket of this invention ~ -
most effectively is the two-part mold which is described and claimed in
Canadian Patent No. 971,997. As there seen, the mold consists of inner and
outer shells of frustoconical shape which are so configured that when assembled
they define between them a peripheral groove which enclose~ the apex of the A,
leaving the legs of the A free. In the present invention, the pear~shaped head
of the extrusion is received by the peripheral groove formed by the mold rings,
and the T-shaped flange projects radially outwardly therefrom. Concrete or
other cementîtious material is then applied to enclose the flange and define
the peripheral wall of the opening in the manhole wall. Upon separation and
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removal of the mold parts, the cementitious material lining the opening covers
the peripheral flange and embeds it, but the tubular pear-shaped portion is
exposed, and yields to receive the pipe inserted therein.
Cne of the advantages of the present invention is the unexpected
discovery that extrusions having the cross-sectional form described can often
be used with pipes of different sizes, without altering their cross-sectional
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dimensions~ All that is necessary is to change the length of the extrus- -
ion and the diameter of the mold rings. This capacity to coact with pipe
of m~ny different sizes is believed to be due, in part at least, to the
fact that the manner of use places considerable compression on the inner port-
ion of the gasket, and causes tension on its periphery. As a consequence,
the gasket of the present invention hugs the pipe more tightly than would
be the case if the gasket were initially molded as an apertured disk or wash-
er.
It must be realized that the part of the gasket which lies at the
bottom of the opening when in use will have to sustain most of the load of
the pipe and its contents, and in addition will have to resist environmental
stresses, which may be imposed upon the pipe outside of the manhole by the
weight of the fill~ or by impacts due to traffic shock, or by the dead
weight of passing vehicles. If an ordinary washer is used, it may yield too
- much under these stresses, and become overly compressed at the bottom of the
opening. The pipe may then pull away from the gasket at the upper part of
the opening, thereby allowing fluid to pass the seal. Since the stresses
imposed may sometimes be measured in thousands of pounds, particularly where -~
the pipe is large - say 24" or 30" in diameter, it is difficult to employ ~ -
a molded gasket ring which will manifest both sufficient resistance to comp-
ression at the bottom of the opening and sufficient capacity for expansion
at the top.
The pear-shaped head of the gasket described herein is remarkably
adapted to meet this problem. Its walls are of substantial thickness, which
means that a sufficient mass of elastomeric material will be present to carry
the weight imposed. Ihe volume of the preferred circular cavity is sufficient
to afford increased flexibility while the pipe is being installed. At the
same time, elimination of the legs (or webs) of the A of the prior device
avoids for~ation of a thin edge at the interface with the concrete, and the
rounded edges of the pear-shaped bottom permit a self-accommodating rolling
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movement under axial stress.
The other major need is to ensure that the gasket ring is so
firmly anchored in the concrete as to resist any stress which might tend to
dislodge it. This is ensured by the deep embedding of the T-section flange
in the surrounding concrete.
The entire gasket is subjected to substantial distortion as it is
deflected from the cylindrical form which is first produced into an apertured
disk or wa~her. Because the inner diameter of the opening formed by deflect-
ing the cylinder walls is substantially less than the cut length of the
extrusion, the inner edge of the gasket will be under compression, and con-
versely, because the outer diameter of the washer-shaped finished gasket is
substantially greater than the length of the cut extrusion, it will be under
considerable tension. These twin forces cause the gasket to hug very tightly
the pipe which it surrounds.
How these and other objectives, which will occur to those skilled
in the art, are to be attained will be apparent from a consideration of the
description which follows, and from reference to the accompanying drawings,
which illustrate the preferred embodiment of the invention, and in which:
Figure 1 is an elevation, partly in section, of a manhole structure,
with the gasket seal in position;
Figure 2 is a perspective view of an extrusion used in the method
of the invention;
Figure 3 is a transverse section on the line 3-3 of Figure 2;
Figure 4 is a perspective view, showing the extrusion being pre-
pared for use as a gasket;
Figure 5 is a perspective view of the completed gasket;
Figure 6 is a perspective view showing upper and lower mold forms,
with the gasket ring of Figure 5 in process of installation between them;
Figure 7 is an elevational view, partly in section, showing the
gasket of Figure 5 in place in its mold; and
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Fig. 8 is a vertical sectional view of the completed installat-
ion, with the sewer pipe in place.
I~rning now to Fig. 1: I~he mQnhole structure 10 is shown with the
sewer pipe 11 passing through it by way of the openings 12,12. Ihe gasket
13 of the present invention is shown in the distorted position which it
assumes upon insertion of the pipe 11.
The gasket 13 is formed from a continuous extrusion 14 of rubber
or the like, illustrated in Fig. 2. Preferably, it conforms to ASTM Spec-
ification C 443-63T. As seen in the cross-section of Fig. 3, the general
configuration of the extrusion is one in which there is a head portion 15
which is pear-shaped and preferably encloses a sy~metrical hollow or cav-
ity 16. Extending from a mid-part of the outer surface of the pear-shaped
head is a web 17 which interconnects the head 15 with a base flange 18 which
is normal to the vertical center line CL of the extrusion. As seen in Fig. 2,
the form of the extrusion in lengthwise elevation is that of a tubular head
15 connected by a vertical web 17 to the longitudinal flange 18.
The vertical center line CL defines a plane x containing the long
axis of the head portion 15 and extending lengthwise with respect to the
extrusion 14. The base flange or flange structure 18 extends laterally of
the plane x and is symmetrical with respect thereto. The web means 17 lies
generally in the plane x.
It is the apex 20 of the head 15 which makes first contact with a
pipe 11 being inserted in the manhole opening 12. To facilitate insertion
of the pipe, the apex 20 is rounded, as at 21. During installation, the base
or anchoring portion, which consists of the web 17 and the flange 18, are
embedded in concrete or other cementitious material. The finish line of the
concrete which will ultimately be present has been suggested by the dashed
line 19-19, added to Fig. 3. This illustrates the fact that the outer angles
of the pear-shaped part are rounded and the concrete is emplaced in such a
way as to avoid a sharp edge of contact with the lower portions of the pear-
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shaped tube. Thus a possibility of acco~modating some rolling movementof the tubular portion of the gasket is provided, to reduce stress on
insertion of the pipe. The cavity 16 within the tube is preferably sym-
metrical with its outer contour, and is so dimensioned as to provide a
fairly substantial wall thickness. The web 17 spaces the base of the
head from the top of the flange sufficiently to afford a substantial channel
to be filled with concrete.
The proportions of the various parts of the extrusion are, in the
preferred embodiment of this invention, substantially as follows: if the wall ~ -
thickness be taken as 2x, then 3x is the thickness of the flange 18 and also
the thickness of the web 17; furthermore, the radius of the rounded corners
of the pear shape is also 3x; 4x is the height of the web, and 6x is the
widest dimension of the cavity 16. The total height of the base (web plus
flange thickness) is 7x, which is also the height of the cavity 16. The width
of the base flange is 8x, and the widest part of the pear-shaped head is llx.
Ihe height of the pear-shaped portion is 14x. The exterior surfaces of the
pear-shaped portion diverge at an angle of about 40 (20 on each side of the
vertical) and the base angles are 70 each. The area of the rubber in the
walls of the pear-shaped head is preferably about two and a half times the
area of the cavity.
It is, of course, possible to vary these proportions within limits,
but they have yielded excellent results for most installation. Abnormal cond-
itions, such as severe traffic load, may justify some changes, such as in-
creasing wall thickness without altering other dimensions.
In order to convert the rectilinear extrusion into a ring gasket,
it is first necessary to cut a piece of it to the desired length. Experience
has shown that handling of the completed ring is easier if the length of the
extrusion is 3.26 times the diameter of the pipe to be fitted.
The cut section of the extrusion is then curled into the form of a
right cylinder, and its ends are brought together and vulcanized, as seen in
Fig. 4. In order to avoid distortion of the tubular portion of the extrusion
during vulcanization~
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tubular, pear-shaped part of the gasket.
Returning to Fig. 6, it will be seen that the gasket
ring shown in Fig. 5 has been stretched to fit the mold ring
25, and is being deflected radially inwardly, with the tubular
pear-shaped part about to be forced still further down into
contact witA the mold flange 26 in the mold ring 25. A truss 27
spans the ring 25 diametrally, being welded to a lug 28 which
is long enough to slip through and beyond the aperture 29 in
the corresponding truss 30 which spans the mold ring 24. A
quoin key 31 is used to urge the mGld rings together, being
inserted into the keyway 32 in the lug 28 after the rings have
been brought together.
Fig. 7, in addition to showing the gasket ring in its
final position in the assembled mold, also shows the base
flange 18 and the web 17 fully embedded in the concrete. The
tubular portion o~ the gasket, being within the mold, will be
free in space after the mold is removed. This figure illus-
trates the relationship of the concrete to the pear-shaped
tube, indicated by the dashed lines 19-19 in Fig. 3. The con- -
crete should reach fairly well up on the lower rounded corners
of the tube, but not so far as to permit the formation of a
"feather-edge". Indeed, it is believed to be desirable to so
form the mold as to limit the concrete to a level approximately
the same as the upper surface 16a of the base of the cavity,
so that the gasket may roll a little under the stress imposed
by insertion of the pipe 11.
In Fig. 8, the mold rings have been separated and
removed and the pipe 11 has been inserted. This causes a
fairly extensive distortion of the tubular part of the gasket,
more or less schematically indicated at 13a. The dot-and-dash
outline 13 in Fig. 8 indicates the position of the gasket
before the pipe 11 is inserted.
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The sloping conical faces 33,34 are provided for the purpose
of accommodating misalignment of the pipe 11. The curvature of the dash
lines 35,36 indicates the projected circumference of the manhole wall which
lies above and below the locus of the opening 12.
For convenience it may be desirable to conduct much of the work
involved in the shop rather than in the field. If this is desired, the mold
may be assembled within an outer form, so that the concrete in which the
flange is embedded is in the shape of a tubular concrete plug, preferably
cylindrical, of a size to fit the opening in the manhole wall. It is then
carried to the site, and cemented into place within the opening. --
In any event, after the gasket is in place and the concrete is
sufficiently cured, the pipe 11 is inserted through the gasket 13, causing
substantial distortion of the tubular portion, as suggested at 13_. This
greatly extends the contact area, so that there is very little chance that
minor pits or defects in the pipe will escape the sealing action.
Ihe wall thickness of the head and the size of the cavity within
it are so related as to ensure the presence of enough rubber or rubber-like
material to sustain the weight imposed and yet cushion the load to protect
against shocks and momentary stresses. At the same time, the tension imposed
on the circumference of the gasket ensures that tight contact will be main-
tained at the upper surface of the pipe, no matter how extensively the tub-
ular head may be compressed at its lower portion.
This combination of factors, including the expanded area of contact
and the cushioning action, protect against minor defects in the pipe surface.
Should there be a situation in which the pipe at the locus of the gasket is
seriously out of round, or locally flattened, it may be desirable to increase
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the tightness of the seal by injecting a non-hardening, self-
sealing material into the circular cavity, using a syringe
with a fine point for this purpose. The material injected is
placed under sufficient pressure to expand the tubular head
into close contact with the pipe. The pressure needed nay be
supplied mechanically by the syringe, or chemically, as by
using urethane and a foaming agent, in controlled proportions.
When the syringe is withdrawn, the self-sealing material inser-
ted closes the opening made by the point of the syringe.
Needless to say, this expansion-producing material is not
employed until the pipe is in place.
It is noted that the contours of the A-shaped gasket
previously used are significantly less well adapted to the need -
than is the new contour. The inverted "T" which forms the base
element of the extrusion is so firmly embedded in concrete as
to be practically incapable of removal, since the thrust
imposed by insertion of the pipe is substantially normal to
the web 17, and is transmitted through a very substantial body
of rubber-like material, whereas the thrust, when using the
A-shaped washer, is almost parallel to the leg of the A which
is first contacted by the entering pipe, thus incurring the
likelihood that the leg (or skirt portion) will be stretched
and thinned out, and possibly torn out by the roots, so to
speak. The rounding of the lower corners of the pear-shaped
contour, in contrast, permits the gasket to roll a little to
accommodate stress, while the embedding of the T-shaped
anchoring part of the gasket positively prevents dislodgement.
The avoidance of any feather-edge at the interface between gas-
ket and concrete eliminates any tendency of the concrete to
break away or chip out in that location.
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