CA1209529A - Extrusion machinery - Google Patents

Abstract

ABSTRACT
In Conform machinery for continuous friction actuated extrusion the wheel is made up of at least three parts, namely two cheeks with a hub between them. Annular coolant passages extend between these members and are fed by ducts which extend at least through the cheek members.
These ducts (and any extending through the hub) are lined with thermally-insulating material such as PTFE. This greatly reduces thermal stresses around the coolant ducts, which can be a cause of premature and catastrophic failure of the wheel.

Description

cS2~31 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 ' ~

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- 2 -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 ., , ., , , ~ .
, 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 .. . . .. . .. .... .. ......... . . . . ....

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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 . . .

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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.

.... . .... _.. .

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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 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

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Machinery for continuous friction-effected extrusion, comprising an arcuate first member, a second member in the form of a wheel having a circumferential groove formed in its peripheral surface into which groove the first member projects to form a passageway between said first member and said wheel, said wheel being rotatable to urge material in said passageway towards an exit end thereof, an abutment member extending across said passageway at said exit end thereof and at least one die orifice through a wall of said passageway selected from said abutment member and an adjacent part of said arcuate first member;
said wheel comprising two cheek members and a central hub forming annular passageways between said cheek members and said hub and means for producing coolant flow in said annular passageways including ducts extending through said cheek members distinguished by the fact that said ducts through said cheek members are lined with thermally-insulating material so that cooling is concentrated at surfaces of said annular passageways between said cheek members and said hub.

2. Machinery in accordance with Claim 1 in which said means for producing coolant flow further includes ducts through said hub characterised in that they are also lined with thermally-insulating material.

3. Machinery as claimed in Claim 1 in said groove is formed by said central hub and two rings separate from said cheek members.

4. Machinery as claimed in Claim 3 in which slip surfaces between said cheek members and said rings are generally parallel to sidewalls of said groove and spaced from them a distance not less than half nor more than twice the width of said groove, subject to a minimum distance of 3 mm.