Note: Descriptions are shown in the official language in which they were submitted.
cS2~31
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EXTRUSION MACHINERY
This invention relates to machinery for
continuous friction-effected extrusion, primarily but not
excluslvely of metal. More particularly it relates to
machinery of the kind in which a passageway is formed
between an arcuate first member and a second member in the
form of a wheel having a circumferential groove form0d in
its peripheral surface into which groove the first member
projscts, the wheel being rota~able to urge material in the
passageway towards one end (the exit end) ~hereof, an
abutment member extending across the passageway at the exit
end thereof and at least one die orifice ~hrough the abutment
member ~r through an adjacent part of the arcuate first
member.
The abutment member may be large enough to block
the end of the passageway completely (as descr ibed in the
specification of UK Patent 1370894) but especially when the
material to be extruded is a rela~ively hard metal, such as
copper, we prefer.that the abutment member is of
substantially smaller cross-section than the passageway and
leaves a substantial gap between the abutment member and the
groove surface and that the material b~ing extruded is
allowed to adhere to the groove surface, whereby a
substantial proportion of the metal (as distinct frcm the
inevitable leakage of flash through a working clearance)
extends through the clea.rance and rsmains as a lining in the
groove to re-enter the passageway while the remainder of the
metal extrudes through the die orifics(s), as described in
' ~
s~
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our U~ Patent ~o. 2069389B.
Such machinery is commonly known as "Conform"
machinery, and will be referred to as such hereinafter
The wheel of Conform machinery is ~ubject to
very high, and cyclic, stresses and is liable to premature
failure though atigue cracking, which adversely affects the
operation of the machinery through high down-time and
considerable replacement cost.
The fatigue cracking problem has led to the
adoption, in place of a monolithic wheel construction, of a
wheel comprising two cheek members and a central hwb which
forms the base of the passageway. Hitherto the cheek
members have usually formed the sidewalls of ~he passageway,
but it has been suggested the sidewalls of the wheel groove
15 should be formed by separate rings; we have expeximented
with such arrangements and found it desirable to provide
~lip surfaces between the cheek members and the rings which
are generally parallel to the sidewalls and spaced from them
a distance not les~ than half nor more than twice the width
~0 of the wheel groove, subjact to a minimum distance of 3 mm.
These two forms of wheel (for brevity
hereinafter called "three-part" and "five-part" wheels
respectively, though either may and will usually have
further, auxiliary, parts) are customarily cooled by
water or other fluid coolant flowing in annular passageways
between ~he parts, and it is a practical necessity for
coolant to be fed to and received from the annular
passageways by ducts extending through the cheek members,
usually there are ducts through the hub as well, the most
S2~
u~ual arrangement being fGr the flow to be inward~ through
each of four equally spaced entry ducts in one cheek member,
around one eighth of the wheel circumference, through a
transfer duct extending through the hub, back around the
circumference (subject to the e~fect of mixing with 10w
from the next entry duct) and out through an exit duct
through the other cheek member axially aliyned with the
respective entry ducto
Inevitably the walls of these coolant ducts are
at considerably lower temperatures than the remainder of the
respective wheel member in which the ducts are formed, so
producing s~ress concentrations around the ducts that
frequently lead to cracking and catastrophic failure of the
wheel.
The present invention substantially reduces this
effect and so enhances average wheel life.
In accordance with the invention the coolant
ducts through the cheek members at least (and preferably
through the hub also when applicable) are lined with
thermally insulating material so that cooling is
concentrated at the surfaces of the annular passageways
between the parts.
Any adequately heat- and fluid-resistant
thermally insulating material can be used, but we prefer a
heat-resistant plastics ma'cerial such as PTFE
(polytetrafluoroethylene). Either a coating or a pre-formed
close-fitting sleeve can be used; a thickness of around
0.05 mm gives an appreciable benefit bu'c a thickness of
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,
3l~`1~$5;~
1-1.5 mm is recommended. When a pre-formed sleeve is used
it is preferably flanged at the upstream end tD secure it
against movement in the direction of coolant flow.
To avoid another source of weakness, preferably
no keyways are used to transmit drive between parts of the
wheel.
The invention will be further described, by way
of example, wi~h reference to the accompanying drawings in
which: Figures 1 and 2 are cross-sections through the
significant components of the wheel of a Conform machine in
accordance with the invention at two places spaced round the
circumferen~e of the wheel by 45; and Figure 3 is a diagram
illustrating the distribution of coolant flow thxough the
wheel.
The wheel comprises two cheek members 1, a hub 2
and a pair of rings 3. The rings 3 and the hub 2 bound the
working groove 4 and all these members are e~posed to a pair
of annular coolant passayes 5. Entry and exit ducts 6, 8
through the cheek members and transfer ducts 7 through the
hub provide for ~hrough flow of fluid and, in accordance
with the invention, these ducts 6, 7, 8 are lined with PTFE
tubes 9 which have flanges 10 at the end at which coolant is
to enter them.
As be~t understood from Figure 3, the coolant
enters from the wheel member 11 in a conventional manner and
passes through any one of the inlet ducts 6 in the right
hand cheek member 6 which conveys it to the first (right
hand) annular passage 5. Here the flow divides to pass in
both directions around the annular passageway 5. After
.. . . .. . .. .... .. ......... . . . . ....
~ 2'~4~,`5 ~ ~
flowing round about 45 (relative to the axis of the wheel)
the flow encounters oppositely-flowing coolant which entered
at the next of the inlet ducts 6, mixes with it, and flows
through the duct 7 to the second (left hand) annular
passageway. Here the mixed flow divides again, flowing in
both directions around the passageway to leave by the exit
ducts 8 which are aligned with the entry ducts 6 through
which it first came. (In Figure 3~ I-I and II-II each
indicate one o the four Pquivalent positions corresponding
to Figures 1 and 2 respectively).
In a practical example, a Conform machine had a
wheel of the design shown in Figures 1 and 2 with a
cir-cumference of one metre and a groove gubstantially nine
millimetres square. The coolant ducts (6, 7, 8) were 8 mm
in diameter, and the PTFE sle~ves 10 had an internal
diameter of 6 mm and a wall thickness of 1 mm, so as to fit
the ducts without nominal clearance. The flanges 10 were 2
mm thick and had an outside diameter of 10 mmO
The quantitative effect of these ~hermally-
insulating tubes may be estimated as follows:--
For an infinite hollow circular cylinder with internal and
external radii of a and b respectively that has a
temperature Ta at radius r = a and Tb at r = b, the
temperature distributions T(r) is given by Conduction of
Heat in Solids, H. S. Carslow & J. C. Jaeger, Oxford
University Press 1959
(b) (r)
T(r) = Ta ln r ~ b ln a (1)
(b)
ln a
. . .
~2~5~
The circwnferential component if stress a~3 due to a
temperature distribution T(r) is given by Theory of
Elasticity, S. Timoshenko & JO ~. Goodier, McGraw ~ill
1 -v r2 ~ r-~--z)JrT(r) dr + ~ T(r) dr - r2T(r)
~here a is the thermal expansion coe~ficient, E the Youngs
Modulu3 and O is a constant.
Then, since
o J r T(~) dr = ~ ~Ta 1~ (r) ~ Tb 1~ (a) ~ ~ 2
substituting in (2) and evaluating at r = a, gives (3)
a E (Tb - Ta) L. ~ ~ - b ~
Cr~3 1 - v 1 ~ (b) 21n ( a)) at r = a
For very large b ( such that b may be neglected)
a 21n a
a E (Tb ~ Ta)' (4)
15 we have a~ = 1 - v
but even with _ as small aæ 5,
~ E tTb ~ Ta)~
(3) gives aQ - 0.73 1 - v
Thus for largs (~) tl~e stress given b~ ( 3 ) is not
a (b)
20 critically dependent on and (4) can be u~ed as a fair
approximation to the hoop stress around the small hole in
an irregularly shaped solid.
.... . .... _.. .
5~
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Taking values for BH13 steel
E = 2.16 x 1011~/m2
Ol a 1~25 x lo C
v = 0.3 , and assuming Tb ~ Ta = 50C
gives, u~ing (4), ~ = 190 MN/m2
The effect of introducing a sleeve uf thermal conductivity
K, and internal radius c into the hole in Figure 1 and
~hen taking the internal radius to be at temperature Ta~
iR to modify the temperature at r = a to be
~1 = K1 Ta ln (a) + K2 Tb ln (~)
K1 ln (b) + K~ ln ~c)
a
Taking K2 ~ thermal conductivity of H13 = 25 Wm loC 1
Kl - thermal conductivity of PTFE = 0.015 Wm loC 1
and if ~ is of the order of 1,5 then unless b were to
c c
e~ceed say 103,
then Tal ~ Tb and the thermal stress is almost entirely
removed.
Because our re~earches revealed a number of
sources o weakness which were dealt wi~h together, a
strict experimental comparison is not available. However,
when a wheel o~ the same major dimensions bu~ with the
groove formed directly by the cheek members and a flat hub
was used to extrude particulate copper, seven failures of
the cheek members occurred by the time 170 tonnes of
copper had been extruded ~mean 24 tonnes per failure).
~5 Examination showed that two of these failures had been
initiated at coolant bores (once such failure per 112
5'2
-- 8 --
tonnes). ~he others were initiated at keyway ~3), at a
sharply machined internal corner (1~ and at a groove
corner (1) and are not relevant to the present invention.
The elimination of keyways and use o separate
rings 3 is cons.idered unlikely to have had any significant
effect on the ra~e of failure at coolant duct~; the
machine described herein by way of example has so ar
extruded 260 tonneæ of copper unaer the 9ame conditions
without any failures of the cheek members whatsoeverO
