[0001] The present invention relates to a method and apparatus for forming ducting, notably
to a method for forming ducting of a polygonal cross-section.
BACKGROUND TO THE INVENTION:
[0002] Polygonal ducting has hitherto been made by axially joining sheets of a suitable
wall material along their edges by flanges or the like. However, this is cumbersome
and difficult where the cross-section is to be other than a simple triangular, square
or rectangular shape Furthermore, problems are often encountered in sealing the axial
joints in such a construction. Where a circular cross-section duct is required, this
can be made by winding a strip of the wall material outside or inside a suitable former
to form a spirally wound duct with the turns of the spiral overlapping or crimped
together to form the joint along the adjacent edges of the material.
[0003] It has been proposed to form a polygonal duct by spirally winding a strip of the
wall material and forming the duct by bending the strip at each apex of the cross-section
of the duct. However, such a method suffers from problems in ensuring accurate register
of the bends with each other and the duct produced often has twists in it which are
unacceptable. In order to reduce this problem, it has been proposed to increase the
radius of the bends so that the duct has a more rounded appearance. However, this
is not wholly successful and gives rise to problems in using the duct.
[0004] We have devised a method and apparatus which reduces the above problems and which
produces a polygonal duct having sharp bends at its apexes which assist use of the
formed duct.
SUMMARY OF THE INVENTION
[0005] Accordingly, the present invention provides a method for forming a duct having a
polygonal cross-section with at least three apexes thereto, which method comprises
subjecting a generally circular cross-section duct to radial stretching along the
line of the intended apexes of the desired cross-section to be formed whereby the
duct is deformed in at least three directions to adopt the desired cross-section.
[0006] Preferably, the initial duct is a generally circular cross-section duct which has
been formed by any suitable process, for example by forming a sheet of wall material
into a circular cross-section duct by drawing the sheet over a suitable mandrel or
through a suitable die or other forming tool and welding the axial butt or overlap
joint in the duct so produced. However, it is preferred that the initial duct be a
spirally wound duct fabricated using conventional spiral winding techniques. Furthermore,
the invention can be applied as the last stages of the duct forming process so that
the initial circular cross-section duct is produced only as a transient intermediate
in an overall process. For convenience, the invention will be described hereinafter
in terms of a separate process for converting a length of a spirally wound circular
cross-section duct to the desired polygonal cross-section.
[0007] The intial duct will usually have a substantially circular cross-section. However,
for some shapes of duct, notably for large diameter ducts, it may be preferred to
subject the initial duct to some pre-forming to reduce the extent of shaping of the
duct. Thus, the intial duct may be subject to initial shaping into an oval cross-section
duct by applying external pressure to squeeze the duct along one diameter, or partial
forming of the duct into a polygonal shape may be carried out by drawing the duct
through an external former, for example to produce a generally triangular cross-section
with rounded apexes which are to be finished using the method of the invention. For
convenience, the invention will be described hereinafter in terms of the forming of
a duct which has not been subjected to such pre-forming.
[0008] In the method of the invention, the duct wall is stretched from within the duct to
adopt the desired configuration. The stretching causes the curved wall of the duct
to deform into a series of generally planar surfaces and the overall circumference
of the duct is increased, typically by from 0.5 to 5%, during this operation so as
to reach or slightly exceed the yield point of the wall material so that the stretch
is permanently induced.
[0009] The stretching can be achieved by a wide range of devices and can be carried out
progressively both radially and axially. However, we prefer that the duct stretching
mechanism act against opposed internal faces of the duct simultaneously so that the
wall of the duct itself acts as the plattern against which the mechanism acts and
the desired increase in circumference is achieved. Thus, the stretching is preferably
achieved by means of a radially expanding mandrel carrying axially extending former
bars which are radially extended to bear against the wall of the duct and simultaneously
deform the sections of the duct at the apexes of the desired cross-section. Such a
mandrel can extend the full length of the duct to be stretched so that the desired
cross-section is achieved in a single operation. However, we prefer that the mandrel
extend for only part of the axial length of the duct and that the stretching is carried
out progressivly along the length of the duct as the mandrel moves axially within
the duct. If desired, the mandrel can act progressively radially as well as axially,
for example as when the leading portion of the mandrel does not extend to the full
radial extent of the desired cross-section to be formed.
[0010] Thus, a suitable stretching mechanism for present use comprises a series of axial
bars each having a radially outward face conforming to the internal shape of the apex
to be achieved and carried on a radially expansible central traveller. The free ends
of the bars are inserted through the duct and are supported both axially and radially
in an end plate having radial tracks which allow the ends of the bars to move radially
outward to the desired position of the apexes in the cross-section of the stretched
duct. The traveller is intially located at the other end of the bars and is set to
have a radial size which, with the radial depth of the bars, corresponds to the desired
shape of the duct. The traveller is then drawn or pushed through the duct, for example
by an hydraulic ram, and causes the bars to move radially outwardly to stretch the
walls of the duct.
[0011] In a preferred form of the stretching mechanism, a mandrel carrying a series of radially
extensible short axial former bars substantially parallel to one another and to the
wall of the duct is moved progressivly along the duct to form the desired shape of
the duct in a series of stages. The mandrel comprises a central axial support having
the axial former bars attached thereto at the desired positions circumferentially
by radially expansible means. The expansible means may be, for example, radially acting
hydraulic rams or screw mechanisms. However, a preferred expansible means comprisies
a series of radial arms pivotting about transverse axes which are pivotted from a
position at which they lie adjacent the axis of the mandrel to one in which they adopt
a more radial orientation. The arms carry at or adjacent their radially outward ends
the axial former bars. Axial movement of an operating rod or rotation of a cammed
rod or the like causes the former bars to be moved radially and thus stretch the wall
of the duct. Preferably, the former bars have an axially tapered configuration whereby
the forward portion of the mandrel has a smaller extended radius that the rearward
portion of the mandrel, thus aiding forward motion of the mandrel into un-stretched
sections of the duct. Furthermore, the forward portion can be configured so that only
partial stretching of the wall of the duct is carried out by this portion of the mandrel,
thus achieving a measure of pre-forming of the duct before complete forming of the
duct is achieved by the rearward portion of the former bars.
[0012] The invention provides a simple method and apparatus whereby polygonal cross-section
ducts can be formed having any desired configuration and with sharp radius apexes.
The configuration of the mandrel for use in the method of the invention can readily
be adjusted so that it can be used on a wide range of sizes of duct. Furthermore,
by appropriate design of the mandrel, for example by having one of the former bars
fixed, it is possible to use the method of the invention to form asymmetric ducts
and not solely ducts which are radially symmetrical. If desired, the apexes can be
formed with large radii of curvature to give a duct with rounded corners.
DESCRIPTION OF THE DRAWINGS:
[0013] The mandrel of the invention will now be described with respect to a preferred form
thereof as shown in the accompanying drawings in which Figure 1 in an axial view of
the mandrel in the radially collpased configuration; Figure 2 is a transverse cross-section
along the line X-X through the mandrel of Figure 1; Figure 3 is an axial view of an
alternative form of the mandrel of Figure 1 in the radially extended configuration;
Figure 4 is a transverse section through an alternative form of mandrel; Figure 5
is an axial view of a further alternative form of mandrel; and Figure 6 shows typical
cross-sections of the ducts which can be formed using the method of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] The mandrel of Figure 1 comprises a central support shaft 1, which can be of any
suitable size and shape, having journalled within it for axial movement an operating
rod 2. At the end of shaft 1 are mounted three axial former bars 3, connected to the
end of the shaft by pivot linkages 4 which allow the bars 3 to move radially with
respect to the longitudinal axis of shaft 1. The operating rod extends beyond the
end of the shaft 1 to lie substantially centrally between the three formed bars. The
bars are connected to the operating rod by pivotted radial arms 5. When operating
rod 2 is moved axially rearwardly, the arms 5 pivot to lie adjacent the rod 2, thus
moving the former bars 3 radially inwardly. When rod 2 is moved axially forward, the
arms 5 pivot to move the bars 3 radially outward to bear against the wall of a duct
within which the mandrel has been inserted.
[0015] As shown in Figure 1, the leading portion of the former bars can be tapered to aid
insertion of the mandrel into the duct and forward maovement of the mandrel within
the duct.
[0016] In place of the multiple fishplate linkage used in the mandrel of Figure 1, the former
bars 3 can be directly pivotted on the end of shaft 1 and a single pivotted fishplate
linkage 6 used to expand or contract the mandrel radially may be used as shown in
Figure 3.
[0017] In place of the push/pull action of the operating rod 2 shown in Figures 1 to 3,
the former bars 3 can be mounted on radially acting hydraulic rams 10 housed within
an operating head at the end of shaft 1 which are fed with hydraulic fluid by, for
example a bore 11 within shaft 1.
[0018] Alternatively, as shown in Figure 5, the radial movement of the former bars 35 can
be achieved by axial movement of a tapered plug 20 carried by operating rod 2; or
the plug shown in Figure 5 could have a cammed profile so that rotation of the plug
causes the bars 3 to move radially in radial support channels in an operating head
similar to that shown in Figure 4.
[0019] As stated above, the above forms of mandrel can be used to form symmetric triangular
ducts as shown in Figure 6d. Where the mandrel carries four or six former bars, it
can be used to form the square or hexagonal cross-section ducts shown in Figures 6c
and 6a. In all these cross-sections, the former bars have acted symmetrically and
radially outwardly from the longitudinal axis of the mendrel. In the case of rectangular
cross-section ducts, the former arms may either act along radii which are not symmetrically
directed about the axis of the mandrel (as shown dotted in the bottom section of Figure
6b) or may act from points removed from the axis of the mandrel (as shown dotted in
the upper section of Figure 6b) and the optimum design of the mandrel may vary having
regard to the size of the duct to be deformed using the mandrel.
[0020] With the asymmetric triangular section duct shown in Figure 6e, the lower former
bar 3 has remained stationary, for example because it is fixed within the operating
head or because no radial drive thrust has been applied to it, whereas the upper two
former bars have been moved radially to stretch the wall of the duct.
[0021] By varying the movement, the location and direction of thrust of the former bars
in the mandrel, it is possible to produce a wide variety of cross-sections in the
formed ducts.
[0022] Accordingly, the present invention provides a radially acting mandrel adapted for
axial travel within a duct, which mandrel comprises axially orientated former bars
having a radially outward surface conforming to at least part of the desired internal
shape of the duct, at least some of which former bars are adapted to be urged radially
outwardly by a drive mechanism so as to contact and deform the walls of the duct in
at least three directions to shape and stretch the walls of the duct to form a polygonal
cross-section duct.
1. A method for forming a duct having a polygonal cross-section with at least three
apexes thereto, which method is characterised in that it comprises subjecting a generally
circular cross-section duct to radial stretching along the line of the intended apexes
of the desired cross-section to be formed whereby the duct is deformed in at least
three directions to adopt the desired cross-section.
2. A method as claimed in claim 1, characterised in that the initial duct is a generally
circular cross-section duct which is radially deformed by drawing a radially expanding
mandrel through the duct.
3. A method as claimed in either of claims 1 or 2, characterised in that the radial
deformation of the duct is achieved by means of a mandrel which extends for only part
of the length of the duct and is moved axially so that radial deformation of the duct
is carried out progressively as the mandrel is moved axially with respect to the duct.
4. A method as claimed in any one of the preceding claims, characterised in that the
duct is subjected to an initial radial deforming stage before being deformed fully
to its desired radial configuration.
5. A method as claimed in any one of the preceding claims, characterised in that a
mandrel within the duct carries radially extensible axial arms which are caused to
extend radially to bear against opposed areas of the wall of the duct simultaneously.
6. A method as claimed in any one of the preceding claims, characterised in that the
circumferential dimension of the wall of the duct is increased during the radial deformation
of the duct wall so as to reach or exceed the yield point of the wall material.
7. A method as claimed in any one of the preceding claims, characterised in that the
circumferential dimension of the duct wall is increased by from 0.5 to 5% during the
radial deformation of the duct wall.
8. A radially acting mandrel adapted for axial travel within a duct and for use in
the method of claim 1, which mandrel comprises axially orientated former bars 3 having
a radially outward surface conforming to at least part of the desired internal shape
of the duct, at least some of which former bars 3 are adapted to be urged radially
outwardly by a drive mechanism 2 so as to contact and deform the walls of the duct
in at least three directions to shape and stretch the walls of the duct to form a
polygonal cross-section duct.
9. A mandrel as claimed in claim 8, characterised in that the leading portion of the
mandrel does not extend to the full radial extent of the desired cross-section to
be formed.
10. A mandrel as claimed in either of claims 8 or 9, characterised in that at least
some of the axial former bars 3 are carried by a pivot mechanism 4, 5, 6 upon an axially
extending support member 1 whereby the pivot mechanism 4, 5, 6 causes the former bars
3 to move radially with respect to the support member 1; and in that it carries a
mechanism 2 for urging the former bars 3 radially with respect to the support member
1.