Note: Descriptions are shown in the official language in which they were submitted.
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EXTERNAL SLEEVE ASSISTED TUBE BENDING
Field and Background of lnvention
[001] - The present invention relates generally to the field of bending tubes
for
use in a furnace, among other things, and in particular to a new and useful
apparatus and method for bending a boiler tube to permit tighter radius tube
bends.
[002] Tubes for steam generating equipment, such as fossil-fueled boilers,
must
be bent to form the various shaped components within them. Rotary draw bending
machines are currently used to bends much of the 1 to 3 inch O.D. (outside
diameter) tubing used in high pressure steam boilers for electric power
generation.
U.S. Patent 5,315,852, issued to The Babcock & Wilcox Company, is an example
of
a current rotary draw bending apparatus and method, which is incorporated
herein
by reference as though fully set forth.
[003] In rotary draw tube bending, a tube is located in the semi-circular
groove
of a bend die having an overall radius approximately equal to the desired tube
bend
radius. Next, a pressure die and clamp die are moved up against the opposite
side
of the tube with the clamp die clamping the front part of the tube to the bend
die.
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The bend die and clamp die are then rotated about a bending axis while the
pressure die moves forward in a line carrying the tube tangentially to the
bend point.
The pressure die holds back the reaction force to create the bend.
[004] When hollow tubes are bent, there is a very decided tendency for the
circular cross section to become oval in the curved or bent part of the tube.
In
particular, the inner radius of the bent portion of the tubing (the intrados)
will wrinkle,
or the outer radius (the extrados) will pull in short of a full circular arc
and thin below
the original tube wall thickness. These results are undesirable since they
reduce
cross sectional area of the tube through the bend, produce a tube which is not
as
strong as a tube with a circular cross section, and may resuit in thinner tube
wall
thickness along the extrados of the tube bend.
[005] In some applications a wiper die, a die having a tangential groove with
a
knife edge that confonns to the bend.die groove, is located adjacent the
intrados
opposite the pressure die to prevent wrinkling of the tube.
(0061. When thin walled tubes are bent to small bend radii, for example when
tubes with a wall thickness of 0.095" - 0.280" are bent to a bend radius
between 1X -
2X times the tube outer diameter, intemal mandrels must typically be used to
support the tubes during bending to improve and maintain the tube shape. The
ASME Pressure Vessel Code Sections I and Viii permit engineering design of
certain elements using tubes with thinner walls, provided they can be bent
successfully. However, the need for an internal mandrel limits the practical
length of
tube that can be bent, since the tube must be slipped over the mandrel bars.
The
use of an intemal mandrel also increases the amount of wali thinning that
occurs on
the extrados of the tube. Bending without the use of a mandrel is generally
reserved
for bends that are less than 180 degrees, or with tubing that has relatively
thick
walls, e.g. tubes with wall thicknesses greater than about 10% of the tube
outer
diameter.
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[007] Wiper dies and intemal mandrels also frequently wear out or break, and
are expensive to replace, which are further disadvantages of current tube
bending
apparatus and methods.
Summary of lnventlon
[008] The present invention is drawn to an apparatus and method for bending
thin walled tubes without the use of a wiper die or an intemal mandrel. An
external
sleeve is used during the bending process which maintains the tube cross
sectional
area and tube wall thickness when making tight radius bends. In one embodiment
the sleeve is split along its longitudinal axis.
[009] It is an object of the present Invention to provide a method and
apparatus
for making tubes with thin walls having tight radius bends.
[0010] It is another objective of this invention to make tight radius bends in
tubes
with thin walls.
[0011] It is a further object of the invention to provide a method and
apparatus for
maintaining tube cross sectional area while making tight radius tube bends.
[0012] It is yet another object of the invention to provide a method and
apparatus
for making tight radius tube bends which reduces the amount of thinning that
occurs
on the extrados of the tube.
[0013] Accordingly, the invention comprises an apparatus for bending a tube in
a
tube bend plane. The apparatus includes a sleeve for receiving a tube therein
and a
bend die mounted for rotation about a bending axis. A clamp die is directed
toward
the bend die for clamping the sleeve to the bend die. The sleeve has an inner
surface engaged by the bend die and an outer surface engaged by the clamp die.
The clamp die holds the sleeve to be bent as the bend die and clamp die rotate
about the bending axis. A pressure die engages the outer surface of the sleeve
for
restraining a portion of the sleeve spaced away from the bend to be formed in
the
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tube. The apparatus also includes means for rotating the bend die and clamp
die to
bend the sleeve and tube about the bend die.
[0014] In an altemate embodiment, the invention comprises a method for-
bending
a tube, the tube having an outer diameter and the bend defining a tube bend
plane.
The steps of the method indude inserting the tube into an extemal sleeve
having an
inner surface and an outer surface. A bend die mounted for rotation about a
bending axis engages the inner surface of the sleeve. A clamp die engages the
outer surface of the sleeve, clamping the sleeve and tube to the bend die. A
pressure die is directed against the outer surFace- of the sleeve adjacent the
clamp
die. The clamp die and the bend die are then rotated to bend the sleeve and
tube
around the bend die.
[00151 In yet another embodiment, the invention comprises a method for bending
a tube having a tube wall thickness less than about 10% of the tube outer
diameter
into a tight radius bend where the bend defines a tube bend plane. The steps
of the
method include inserting the tube into an external sleeve. The sleeve has an
inner
surface and an outer surface, and a first longitudinal slit located parallel
to the tube
bend plane between the inner surface and the outer surface. The inner surface
of
the sleeve engages with a bend die mounted for rotation about a bending axis.
The
outer surface of the sleeve engages with a clamp die to clamp the tube to the
bend
die. A pressure die is directed against the outer surface of the sleeve
adjacent the
clamp die. Rotating the clamp die and the bend die bends the tube and sleeve
around the bend die.
[0016] The various features of noverty which characterize the invention are
pointed out with particularity in the claims annexed to and forming part of
this
disclosure. For a better understanding of the present invention, and the
operating
advantages attained by its use, reference is made to the accompanying drawings
and descriptive matter, forming a part of this disclosure, in which a
preferred
embodiment of the invention is illustrated.
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Brief Description of the Drawings
[0017] In the accompanying drawings, forming a part of this specification,
and in which reference numerals shown in the drawings designate like or
corresponding parts throughout the same:
[0018] FIG. 1 is a schematic top view of an apparatus embodying the
present invention.
[0019] FIG. 2 is a schematic cross sectional view of an apparatus
embodying the present invention taken along line 2-2 of FIG. 1.
Description of the Preferred Embodiments
[0020] Referring to FIGS. I and 2, tube 40 is inserted into an external
sleeve 30. The external sleeve 30 is generally cylindrical, and is split along
its
longitudinal axis in a direction parallel to the tube bend plane to create an
inner
sleeve portion 30', located adjacent the tube intrados (the inside radius of
the
tube bend), and an outer sleeve portion 30" adjacent the tube extrados (the
external radius of the bend). The sleeve length is based on the arc length of
the
neutral axis of the bend plus a nominal clamping allowance. The outside
diameter of the sleeve is selected to match the next standard O.D. size of
tubes.
For example if a 2" O.D. tube is being bent, a 2-1/4" O.D. sleeve would
generally
be selected, or if a 2-1/4" O.D. tube is being bent a 2-1/2" O.D. sleeve would
generally be selected. It is understood that one skilled in the art may
utilize
other tube and sleeve O.D. combinations in a manner allowing existing tooling
to
be used.
[0021] The inside diameter (I.D.) of the sleeve is generally selected to
ensure that it matches the O.D. of the tube being bent within approximately
+/-0.020". However, this is not critical as the shape will change and nest to
the
tube during bending as a result of the clearance at the two slots between the
inner and outer sleeve.
[0022] In one embodiment, the sleeve is made of carbon steel, however
the sleeve material is not limited to carbon steel as advantages from using
other
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materials could also be realized. The use of a higher alloy material may also
be used
as the sleeve material. This will have the effect of moving the neutral axis
towards
the Extrados which will reduce wall thinning and ovality. Low alloy and
stainless
steel sleeves may also be used. In some cases the boiler environment is such
that
erosion/corrosion happens most at these bends and the bends have to be
"shielded
in use" using stainless steel sleeves fitted on over the bends. The bending
method
allows the sleeve that was placed over the tubes to aid bending to be left in
place to
act as a shield in service.
[0023] As shown in FIGS. A and 2, the tube and sleeve are clamped between
bend tooling comprised of bend die 10; clamp die 20 and pressure die 50. The
tube
and sleeve are bent together around bend die 10 using methods known in the
art.
The use of extemal sleeve 30 locally thickens tube 40 in the bend arc area,
thereby
allowing thinner-walled tubes to be bent to tighter bend radii.
[0024] In another embodiment the sleeve may contain one or more splits along
its longitudinal axis. A longitudinal split in the sleeve allows the intrados
portion of
the sleeve to move independently from the extrados portion of the sieeve upon
bending, thus lowering the compressive stresses within in the intrados,
resulting in
reduced intrados wall thickening. Further, the independent movement of the
extrados portion of the sleeve lowers the tensile load applied to the extrados
of the
bend upon bending, thereby reducing the occurrence of wall thinning. These two
advantages of the invention combine to reduce or eliminate the need for a
wiper die
and internal mandrel.
[0025] When an internal mandrel is no longer needed, tube 40 is not restricted
to
a length that would fit over a mandrel bar. The apparatus and method of the
present
invention thus allow thinner wall thickness tubes to be used with the
resultant
material savings. Cross section flow area reduction is also minimized.
[0026] The following examples are provided for the purpose of further
illustrating
the invention but are in no way to be taken as limiting as they are merely
exernplary.
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[0027] A sleeve of 2'/< inch O.D. with 0.110 inch minimum wall thickness was
utilized in bending a 2 inch O.D. tubing with 0.125 inch minimum wall
thickness
utilizing the present invention. The sleeve posses an axial slit extending
down it
length such that when the sleeve is place over the 2 inch O.D. tubing the
sleeve
circumferentially encompasses the tubing with the exception of slit. During
bending
the slit was kept substantially vertical to allow the extrados and intrados of
the 2 inch
O.D. tubing to move independently of one another, therefore reducing the
compressive loads that build up in the intrados. The above tube and sleeve
configuration was then bent on a 2'/ inch radius bend die for 180 degrees of
bend.
Bending was conducted without a mandrel at 500 PSI boost. No wrinkles were
observed in the bent tube which when measured possessed and ovality of 4%.
[00281 While specific embodiments and/or details of the invention have been
shown and described above to illustrate the application of the principles of
the
invention, it is understood that this invention may be embodied as more fully
described in the claims, or as otherwise known by those skilled in the art
(including
any and all equivalents), without departing from such principles. For example,
on
thicker materials, where the wrinkling of the intrados and wall thinning at
the
extrados is not as great, the sleeve need not be split at all if the bend is
near the end
of the tube and/or the sleeve is to remain on the tube after bending. Various
thicknesses of sleeves can also be used to change the bending configuration to
achieve optimum bending conditions. A single sleeve only on the intrados or a
single sleeve only on the extrados could also be considered to achieve optimum
bending conditions.
