Note: Descriptions are shown in the official language in which they were submitted.
W~ ~2/16~82
PCT/G~92/00478
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A PIPE FITTING
This invention relates to a pipe fitting.
P]astics pipes are widely used in, for example, fuel gas and
water distribution networks where it is important that all connections
between pipes and between pipes and various appliances are very
reliable. The connections must be leak proof and strong enough to
prevent them from being pulled apart as a result of any end load on
a pipe. Such an end load may result from thermal expansion or
contraction in the pipe's length.
Medium density polyethylene pipes have been widely used in
BAtain, particularly for gas and water distAbution, and such pipes
have often been interconnected using couplings of the type described
in British Patent ~umber GB B1596112. These couplings essentially
comprise a sleeve which is slid over the end of the pipe to be
connected and a circumferentially grooved and ribbed tubular spigot
which is pushed into the pipe end. The ribs on the spigot have an
external diameter slightly greater than the internal diameter of the
pipe, whereas the sleeve has an internal diameter approximately the
same as the external diameter of the pipe. The coupling is assembled
by pulling the sleeve on to the end of the pipe containing the spigot,
thus deforming the pipe which is compressed between the sleeve and
spigot. Some of the plastics material is forced into the
circumferential grooves in the spigot, thereby forming a very strong
and leak proof connection. The spigot may be integral with an
appliance to which the pipe is to be connected or integral with a
second spigot thus facilitating pipe to pipe connections.
Pipe couplings of this sort have been in successful use for many
years, but it is necessary to exercise strict control over the
manufacturing tolerances of both the pipe to be connected and the
spigot and sleeve components. If, for instance, the pipe is produced
with a wall thickness which is too large, or a spigot is produced with
an outside diameter too large, or a sleeve produced with the inside
diameter too sma~, then the annular gap between the insert and the
sleeve may be too small to accommodate the pipe material. Thus when
the sleeve is slid over the end of the pipe containing the spigot to
WO 92/16782
PCT/G B92/00478
assemble the coupling a relatively large amount of the pipe material
is forced ahead of the sleeve and high compressive forces are included
in the pipe material. This results in a very large stress being applied
by the sleeve, in which circumstances it has been found that the
ability of a coupling to resist end loads is reduced. It appears. from
tests, that when the stress on the pipe inside the coupling is large,
substantially all of the end load is taken by the material adjacent to
the rib nearest the end of the spigot, there then being a tendency for
the pipe to break in the region of high stress adjacent the end of the
spigot. In contrast, however, if the pipe and coupling component
tolerances are maintained strictly within predetermined limits, then
end load tests result in the pipe failing outside the coupling.
It is possible to manufacture pipes and coupling components
within very tight tolerances but only at significant cost. Furthermore,
the tolerances are more critical for the high density materials which
are now being more widely used in preference to medium density
polyethylene. Accordingly relaxing pipe and/or coupling component
tolerances, if this could be achieved without risking coupling
reliability, would result in substantial economic benefits.
It is an object of the present invention to obviate or mitigate
the above problems.
According to the present invention, there is provided a pipe
coupling comprising a tubular spigot for insertion into the end of a
pipe, and a sleeve for positioning over the end of a pipe into which
the spigot has been inserted to thereby trap the pipe end between
the spigot and sleeve, the outer surface of the spigot defining a
plurality of circumferential ribs separated by circumferential grooves,
wherein the mean cross-sectional area of the space defined between
the radially outer surface of the spigot and the radially inner surface
of the sleeve in the assembled coupling reduces from the end of the
spigot which is first inserted into the pipe.
As a result of the reducing cross-sectional area, the space
available to accommodate the pipe end reduces progressively with
distance from the free end of the spigot. Thus thick-walled pipes are
securely gripped adjacent the free end of the spigot, and thin walled
pipes are securely gripped away from the free end of the spigot.
Preferably the mean cross-sectional area of the space radially
V4^ 92/16782 21~ 5 ~ ~ 6 PCT/GB92/0047X
outside the groove and rib closest to the said first inserted end is
greater than the mean cross-sectional area of the space radially
outside the groove and rib next closest to the said first inserted end,
the mean cross-sectional area reducing for each adjacent groove and
rib pair along the length of the spigot. The reduction in cross-
sectional area may be achieved for example by tapering the spigot, or
by "stepping" adjacent rib and groove pairs along the length of the
spigot. Alternatively the ribs may be of constant width and the
grooves of progressively varying width along the length of the spigot.
or the grooves could be of constant width and the ribs of
progressively varying width along the length of the spigot.
A specific embodiment of the invention will now be described,
by way of example, with reference to the accompanying drawings, in
which;
Figure 1 is a part sectional view of a spigot of a coupling
according to the present invention;
Figure 2 is an enlarged cross sectional view of part of the
spigot of Figure l;
Figure 3 is a part sectional view of a coupling according to the
present invention incorporating the spigot of Figure l prior to
assembly;
Figure 4 shows in section part of the coupling of Figure l fully
assembled; and
Figure 5 is an enlarged cross sectional view of part of an
alternative spigot of a coupling in accordance with the present
invention.
Referring to Figures l and 2, the spigot consists of a tapered
tubular body l with an additionally tapered narrow end 2 and an
external annular flange 3 at its wider end. The tubular body l is
formed with circumferential ribs 4 separated by circumferential
grooves 5, on its external surface. In axial cross-section, the
grooves 5 have straight sides and a straight base with obtuse angles
being formed between the base and sides, and the ribs have a
flattened crest. All the ribs 4 and grooves 5 are of the same height
and depth respectively, the bases of the grooves 4 and the crests of
the ribs 4 having the same angle of taper. This angle of taper is
indicated by the two broken lines, one of which is parallel to the main
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spigot axis and the other of which is parallel to the crests of the
ribs and the base of the grooves. The flange 3 is provided with bolt
holes 6 so that the spigot may be connected to another device (not
shown), e.g. another spigot to facilitate pipe to pipe connections.
Alternatively, a conventional loose flange will be provided, the loose
flange acting on a stub projecting from the spigot.
The spigot of Figure l is used with a sleeve 7 shown in Figure
3. In use, the sleeve 7 is first slipped over the end of a pipe 8. The
internal diameter of the sleeve 7 is substantially the same as the
external diameter of the pipe 8, whereas the external diameter of the
ribs 4 is greater than the internal diameter of the pipe 8. A hydraulic
or mechanical press (not shown) is first used to force the spigot into
the pipe end, causing the pipe end to radially expand, and is then
used to force the sleeve 7 over the end of the pipe 8 surrounding the
spigot body l. The end of the pipe 8 is thus compressed between the
sleeve and the spigot.
As shown in Figure 4, material from the inner surface of the
pipe is forced to flow over the ribs 4 and into the grooves 5. The
degree to which each groove 5 is filled by the pipe material forced
into it depends on the thickness of the pipe wall and the difference
between the diameter of the base of the groove 5 and the internal
diameter of the sleeve 7. Thus, because the spigot body is tapered,
the first groove 5 near the end of the spigot is only slightly filled,
each successive groove 5 then being filled to a greater degree as the
diameter of the spigot body increases. This continues along the
length of the spigot. Beyond this point material from the pipe is
forced to flow along the length of the coupling as the sleeve is
pushed home. However, before this point is reached, there will be a
number of grooves 5 which are not completely filled. Thus it can be
seen that by tapering the spigot body l, there will always be at least
some grooves 5 only partially filled. If an end load was applied to
the pipe, that load would be distributed between the ribs 4 rather
than being concentrated adjacent the rib nearest the spigot end. If
a coupling is used with a pipe having a relatively thin wal], the
grooves and ribs remote from the spigot end will provide the required
engagement with the pipe. If used with a relatively thick walled pipe,
the grooves and ribs adjacent the spigot end will provide the
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necessary engagement. Thus pipe couplings of this type do not have
to be manufac~ured to the very tight tolerances required of existing
couplings, and also have a greater toierance to accommodate variations
in pipe dimensions, which is outside the control of coupling
manuf acturers .
Figure 5 shows part of ar. alternative spigot in which the
tapering of the spigot body l is achieved by "stepping" successive rib
and groove pairs. Figure 5 also shows an alternative configuration
for the end of the spigot body l, which is stepped as opposed to
tapered.
As a further alternative, the axial dimensions of successive
ribs and/or grooves may vary. For example, given constant rib and
groove diameters the ribs may be of constant width and the grooves
of progressively reducing width, or the grooves could be of constant
width and the ribs of progressively increasing width.
It will be understood that any of the aforementioned
embodiments of the spigot could be combined in any combination, for
example a tapered insert with successively wider grooves, taper angles
which vary along the length of the spigot, or different step heights
between adjacent grooves and ribs. As a further alternative, a
combination of a parallel spigot with a tapered sleeve bore could
achieve the desired loud distinction.
The taper of the spigot and/or the varying dimensions of the
grooves, can be chosen to suit the characteristics of any particular
plastics material and the required compression along the length of the
coupling.
The present invention provides a range of possible advantages,
for example:
l. Reduction of maximum pipe compression at the groove nearest
the spigot end, thus removing the problem of premature yielding
adjacent that groove.
2. Distribution of end load along the length of the coupling.
3. Ability to vary the compression along the length of the coupling
to suit the characteristics- of different types of plastics
material.
4. Ability to increase the manufacturing tolerances of the spigot
and sleeve, therefore allowing more economic methods of
WO 92tl6782 210 6 6 ~ ~ PCr/GB92/00478
manufacture to be used.
5. Ability to reduce the number of grooves required to give desired
levels of mechanical performance, and therefore to decrease the
length of the coupling and its cost.
6. Ability to reduce the required maximum coupling assembly loads,
and thus the size of assembly equipment, therefore reducing
equipment costs and making jointing easier.
7. Ability to accommodate larger tolerances on plastics pipe
diameters and wall thickness.
SUBSTITUTE SHEET
