CASTING MOULD AND CASTING METHOD

Abstract: A method and a mould for casting a half steam diaphragm comprising a plurality of aerofoil section blades (1) spaced around and aligned lengthways radially of the diaphragm and fixed at their inner and outer extremities by cast semicircular rings (6, 7), in which the mould comprises an outer fixed portion and an inner movable portion and where said inner portion is constructed by assembling said blades in a jig, disposing casting sand (5) within spaces in said inner portion left by said blades and said jig, disassembling and removing said jig, and then lifting said inner portion into said outer portion.

Description: CAS ING MOULD AND CASTING METHOD This invention relates to a casting mould method of making a casting mould, and in particular moulds for the casting of what are termed steam diaphragms. Steam diaphragms are to be found in steam turbines, where they are a stationary ring of blades interposed between the rotating blades of the turbine itself. Their function is to control the steam flow between the turbine rings themselves, so as to optimise the aerodynamics and fluid flow of the steam. Thus a steam diaphragm usually consists of a number of radially extending aerofoil-sectioned blades, arranged in a circular manner, and fixed at their inner and outer extremities by annular rings. Traditionally, one method of manufacture is to fabricate the aerofoil blades in, e.g. stainless steel, and to assemble the complete diaphragm by casting the inner and outer annular rings in cast iron around the blades. It is the general practice to cast such diaphragms as identical generally semicircular halves, subsequently connected together. It is to be appreciated that the flow of steam through the aerofoil blades in the finished article is highly critical and therefore the manufacturing tolerances for positioning of the blades before casting are similarly very critical . Furthermore the work that can be done on the casting is limited as it is important not to damage or work the blades except to a very minimal extent. Thus imperfections in and from the casting process adjacent the aerofoil blades are extremely undesirable. Such imperfections include line defects which are usually found if the casting sand is assembled region by region, rather than being set as a whole. Existing methods of creating the mould from which to cast such steam diaphragms consist of creating the outer circumference and then building up the inner shapes using a jig mounted between a central pivot and a central pin and a position on the circumferential ring, and an individual blade is lined up on the jig and a segment of sand is cast around the blade. The jig is then moved one position around the circumferential ring and the process is repeated. The disadvantages of this method are that the sand is formed separately, so as to leave casting lines, and that in practice until four or five blades have been assembled it is not possible to check the accuracy of their alignment so that considerable lost work ensues if changes are needed. Other methods have been proposed, and used, but these have also involved the piece by piece assembly of the mould, with the attendant problems of alignment accuracy and casting imperfections. It is accordingly an object of the present invention to provide an improved mould for and method of casting steam diaphragms. According to a first aspect of the present invention there is provided a mould for casting a half steam diaphragm comprising a plurality of aerofoil section blades spaced around and aligned lengthways radially of the diaphragm and fixed at their inner and outer extremities by cast semicircular rings, wherein the mould comprises an outer fixed portion and an inner movable portion, said inner portion being constructed by assembling said blades in a jig, disposing casting sand within spaces in said inner portion left by said blades and said jig, and disassembling and removing said jig and then lifting said inner portion into said outer portion. Said jig may include locating pieces to locate the trailing edge of each said blade. Further, the jig may include inner and outer circumferential support members, and shaped blade supports locatable on said circumferential support members individually to support each blade in accurate alignment. In a preferred embodiment of the invention the inner portion of the mould is lifted by a three point lift, upon a semicircular lifting member moulded within the casting sand. The three point lift may be applied to points proximate the ends of said semicircular lifting member, and to a point generally midway between said ends of said semicircular lifting member. Said midway lifting point may be constructed by forming within said semicircular lifting member a radially extending hole, and inserting therein so as to extend beyond the outer periphery of said inner portion a bar adapted to receive the lift. After said bar has been used to lift the inner portion within said outer portion that it may be capable of being withdrawn into said lifting member. According to a further aspect of the invention there is provided a method of casting a half steam diaphragm comprising a plurality of aerofoil-section blades spaced around and aligned lengthways radially of the diaphragm and fixed at their inner and outer extremities by cast semicircular rings, in a mould comprising an outer fixed portion and an inner movable portion, including the steps of constructing said inner portion by assembling said blades in a jig, disposing casting sand within spaces in said inner portion left by said blades and said jig, disassembling and removing said jig, lifting said inner portion into said outer portion, and casting metal into the mould including into spaces left by the disassembled jig- One embodiment of the invention will now be described by way of example with reference to the figures of which:- Figure 1 shows a front view of a steam diaphragm as used in a steam turbine; Figure 2 is a detail sketch in section showing the arrangement of aerofoil blades in a steam diaphragm; Figure 3 shows the outer portion of a mould for casting a steam diaphragm in semicircular parts; Figure 4 shows a jig for forming the inner portion of a mould partly assembled; Figure 5 shows a detail in section of the assembly of the aerofoil blades within a jig; Figure 6 shows a detail of a blade support for said jig; Figure 7 shows an inner mould portion assembled within an outer mould portion; Figure 8 shows a cast iron lifting member in plan; Figure 9 shows the member of Figure 8 in side section; and Figure 10 shows a cast iron lifting ring profile. Turning now to Figure 1, as has been explained, steam diaphragms are used in steam turbines, although similar items are used elsewhere, and the method described here is not restricted to the manufacture of steam diaphragms. The diaphragm consists of a number of aerofoil sectioned steel blades 1 arranged radially and supported by inner and outer circumferential cast iron rings 2 and 3. The detailed shape of these rings will of course depend on the precise engineering of the turbine into which the steam diaphragm is to fit, and therefor has not been shown in detail. It is conventional to cast such steam diaphragms in two halves, so that there are two semicircular half rings which are delivered to the customer. Turning to Figure 2 this shows the section along the circular line AA of Figure 1, the view being inward along the local radius at the point. It will be seen that the blades are of aerofoil section, and they are carefully aligned in the designed manner. The manufacture of these diaphragms, by casting the inner and outer circumferential rings on to the aerofoil blades is commonly undertaken, as has been explained, and the requirement is to create a mould of sand of semicircular form, the sand occupying the volume within the mould where no metal is to be cast, and there being provided spaces within the mould without sand where the metal is to be cast. Turning now to Figure 3 the initial stage of manufacture is to create a generally semicircular outer portion of the mould. This is shown in plan. Moulding sand 5 has a semicircular annulus 6 formed in it by use of a wooden pattern in a known manner. The precise form of the bottom of the mould will depend upon the shape to be cast, and this is standard foundry practice. Turning now to Figure 4 the inner portion of the mould is assembled by means of a jig. This jig will hold all the blades for a half turbine diaphragm in place and will provide the annular space about the blades into which the casting sand is to be formed. It will be appreciated that the figure is schematic as the details of pattern and mould making common within the foundry industry have not been included. The jig contains inner and outer circumferential supports 6 and 7 which are set to receive the sharp trailing edge of the aerofoil section blades and provide an alignment surface upon which these edges may be rested to be held in precise alignment. Metal blocks 8 are screwed on to the bearing surfaces to provide a stop against which the rear edge of the aerofoil section blades are located. The inner end of the blade is aligned against an inner wall 9. The blade needs to be supported in this alignment and for this purpose blade supports shaped to the profile of the blade, made out of cast resin are used. The method is similar at the inner and outer ends of the blade, and will therefore be described with reference to the outer ends only. Turning to Figure 5 each blade 1 is aligned in turn upon the lower bearing surface 7 and against the metal block 8 so that the trailing edge on the aerofoil sits in vertical and horizontal alignment, . The blade is pushed against the inner wall 9 to provide longitudinal alignment in the radial direction. A cast resin block 10 is then lowered on top of the aerofoil and is received on pins 11 set into the lower bearing surface. This it will be seen then provides a firm surface against which the next blade 1 may be accurately placed in position. The blades are assembled, and it will be appreciated that they can be easily adjusted for imperfections in their manufacture or minor imperfections in the jig by wooden shims, and measurement of their position is easy and relatively uncomplicated. When the blades have been assembled fully and accurately the whole jig is locked together by semicircular inner and outer pinning pieces 12 (of which only the outer is shown) which are pinned to each individual resin blade support by pins 13 which fit into metal blocks 14 cast into the resin when they are manufactured as is shown at 14 in Figure 6. Referring back briefly to Figure 4, in the light of the description of the assembly of the jig it will be seen that the semi-annular space 15 now contains aerodynamic blades held in accurate alignment which with walls defined by the ends of the resin supports, form a space in which an accurate mould may be made. The process of forming the sand into this space is well known in the foundry trade and will not be described further. When the sand, which may be ordinary casting sand, and not a special sand as is sometimes required in alternative methods of making steam diaphragms, has been formed and is set, the jig is dissembled. The pinning pieces are removed and the resin support pieces are individually withdrawn. The pattern for these has been made so that they can be withdrawn either in a radial direction or in case of difficulty upwardly by attaching a pulling eye to the metal insert 14 in Figure 6. It is now possible to lift the aerodynamic blades held in an annular sand mould and lower it within the outer portion of the mould so as to form a complete mould for the casting of the half steam diaphragm. It will be appreciated however, that the lifting of this inner portion of the mould is not straight forward. Referring now to Figures 8, 9 and 10 there is shown a cast iron lifting bar. This is manufactured specifically to lift the inner portion of the mould, and is shaped to sit within the jig and below the blades; it will be appreciated that there is no problem about making the jig deep enough to accommodate this as the depth of the sand is not critical . The lifting bar has three lifting points attached to it. Two lifting points 20 and 21 are, as might be expected, at the ends of the semicircular bar and are provided by ring eyes 22 and 23 that are screwed into 1ifting points provided in the lifting bar when it is cast. These eyes are inserted as the jig is assembled and when lifting is complete the ring eyes are unscrewed and the holes left filled with casting sand. However, two lifting points are insufficient to lift a semicircular inner portion. It is necessary to provide a lifting point in the middle of the semicircular member. This is done by providing with the lifting bar (as shown in Figures 8, 9 and 10,) a hole 16. A threaded lifting point such as at 20 or 21 to which ring eyes 22, 23 are attached is impracticable at this point as the aerofoil blades prevent the ring eye emerging vertically. Into hole 16 therefore is slid a bar 25 of slightly smaller diameter on to the end of which is attached a further lifting eye 24. By this means the inner portion can be lifted using the three lifting points of the lifting bar and safely transferred into accurate alignment with the outer portion of the mould. This assembled state is shown in Figure 7, where it would be seen that the ends of the lifting bar are fitted into the overall shape of the outer mould at 17 and 18 and the inner portion of the mould 29 sites within the outer portion of the mould 3. The lifting bar which fitted into hole 16 cannot be removed now that the inner portion 29 is in place because there is insufficient room to do so. Instead it is pushed back into the hole so that it is completely enclosed within the lifting bar and the small aperture through which it protruded is filled with casting sand and the space that had to be left for it on the floor of the outer portion of the mould is also filled with casting sand. The imperfections from this hole will of course only appear on the outer periphery of the outer ring of the steam diaphragm and can be easily machined away if needed. The casting now proceeds in the normal way, and will not therefore be further described.

Claims: CLAIMS : 1. A mould for casting a half steam diaphragm comprising a plurality of aerofoil section blades spaced around and aligned lengthways radially of the diaphragm and fixed at their inner and outer extremities by cast semicircular rings, wherein the mould comprises an outer fixed portion and an inner movable portion, said inner portion being constructed by assembling said blades in a jig, disposing casting sand within spaces in said inner portion left by said blades and said jig, and disassembling and removing said jig and then lifting said inner portion into said outer portion. 2. A mould as claimed in Claim 1 wherein the jig includes inner and outer circumferential support members. 3. A mould as claimed in Claim 2 wherein the jig includes locating pieces associated with the inner and outer support members and arranged to locate accurately the trailing edge of each said blade. 4. A mould as claimed in Claim 3 wherein blade supports are locatable on said circumferential support members individually to support the blades in accurate alignment. 5. A mould as claimed in Claim 4 including inner and outer circumferential pinning pieces arranged for location upon the shaped blade supports when located on said inner and outer circumferential support members. 6. A mould as claimed in Claim 5 wherein the pinning pieces, the blade supports, the locating pieces, and the support members define space within the mould to be filled, during casting, by casting metal. 7. A mould as claimed in any one of the preceding Claims wherein the inner portion of the mould is carried upon a generally semicircular lifting member contained, in use, within casting sand, the lifting member being provided with a three point lift arrangement for moving the inner portion of the mould into the outer portion of the mould. 8. A mould as claimed in Claim 7 wherein said three point lift comprises two lift points one at each of the ends of the semicircular lifting member, and a third lifting point generally midway between the said ends. 9. A mould as claimed in Claim 8 wherein said generally midway lifting point is constructed by forming within the semicircular lifting member a radially extending bore carrying a bar adapted to receive the lifting point, capable of extending beyond the outer periphery of the inner portion. 10. A mould as claimed in Claim 9 wherein the said bar is capable of withdrawal into the bore in the lifting member after use in the lifting of the inner portion of the mould into the outer portion of the mould. 11. A method of casting a half steam diaphragm comprising a plurality of aerofoil-section blades spaced around and aligned lengthways radially of the diaphragm and fixed at their inner and outer extremities by cast semicircular rings, in a mould comprising an outer fixed portion and an inner movable portion, including the steps of constructing said inner portion by assembling said blades in a jig, disposing casting sand within spaces in said inner portion left by said blades and said jig, disassembling and removing said jig, lifting said inner portion into said outer portion, and casting metal into the mould including into spaces left by the disassembled jig.

Inventors: COCHRANE ANTHONY GORDON, COWLEY DENNIS, KING ROY, WHITTON JOHN ROWLAND, COCHRANE, ANTHONY, GORDON, COWLEY, DENNIS, KING, ROY, WHITTON, JOHN, ROWLAND
Applicants: BRITISH STEEL PLC
Application year: 1991
Main classification: B22C9
IPC codes: B22C9