WELDED CONNECTOR FOR BLADE LINER

Abstract: A connector sleeve (17) secured to the backing plate (7a) of a wear-resistant liner (7) facilitates attachment of the liner (7) to the body (9) of a rotor blade by virtue of welding of a thrust collar (20) to a portion of the connector sleeve (17) projecting past a bore (18) in the blade body (9). Removal of the liner later for replacement is possible by burning off the welds connecting the thrust collar to the connector sleeve. A clamping tool (22) comprising a tubular body (23) and a bolt (24) can be used to hold the thrust collar firmly against the back of the blade body (9) during the welding operation.

Description: WELDED CONNECTOR FOR BLADE LINER The present invention relates to a fluid machine having a bladed rotor. When a fluid machine is used in conjunction with a fluid which has erosion characteristics and is therefore likely to wear the surface of the fluid dynamic blade or other surface of the rotor or impeller, it had been known to provide a liner of erosion- resistant material and to attach it to the blade or other fluid dynamic surface of the rotor in order to lengthen the service life of that rotor. The nature of such wear-resistant liners can be varied and can range from, on the one hand, the use of thin plain mild steel liners attached at localised areas to extensive cover with liners which are coated with an extremely hard wear-resistant material such as tungsten carbide or chromium carbide. The localised plain mild steel type of liner is relatively inexpensive and can be fitted easily. The more extensive liner coated with wear-resistant material tends to be much heavier than the localised thin plain mild steel liner so it can add considerably to the cost and complexity of the rotor or impeller and, due to the unacceptability of welding such hard coated surface material directly to the structural parts of the rotor or impeller, presents additional problems. The hard-surfaced liner of the blade of the rotor or impeller is particularly difficult to fit, especially in the case of centrifugal impellers, due to fact that the attachment system must be capable of both carrying the centrifugal load induced by the rotating liner and of providing a sufficiently well distributed support to ensure that stresses and deflections within the liner are kept to acceptable limits, even when the rotor is rotating at high speed. It is therefore necessary to ensure that any attachment system does not compromise the structural integrity of the blade as a whole. Such a liner is available in a range of thicknesses which will therefore allow the minimising of the increase in the weight of the rotor or impeller due to the presence of the liner. With such a system there is a relatively high incidence of failure due to the inherent difficulty in reliably attaching such materials to the bladed surface and at the same time causing them to adapt to the shape of the blade surface. In order to satisfy both of these requirements it has become accepted practice for such liners to be purpose-made to the shape of the blade, and the resulting increase in cost, weight and complexity has had to be accepted. Additionally the requirement for a purpose made liner gives rise to some problems in procuring an adequate supply of the appropriately shaped liner, due to the specialised nature of the manufacture of the liner and to the limited number of potential supplies of such liner. One prior art form of attachment of a wear-resistant liner relies on the attachment of countersunk bolts into the mild steel backing plate of the purpose made composite liner so as to have their heads entrapped beneath the hard material defining the wear- resistant surface. Such a system is shown in Figure 3 to be described later. Typically such a system would use Ml 6 bolts and would require the mild steel backing plate of the liner to have a thickness of the order of 10 mm in order to accommodate the bolt head. Hence the proprietary types of wear resistant plate stock with a hard surface are not suitable for use in this application. The purpose made liners are often curved and, if so, this curving is carried out prior to depositing the hard surface on the purpose made liner. The pre- formed backing plate is then held in a purpose made jig with all of the embedded countersunk bolts in position, and then the hard surface is laid over the mild steel backing plate to conceal the countersunk heads. In some environments it may be necessary to provide a protection cup to shield the nut, and the protruding end of the bolt shank, from erosion, as shown in Figure 3. Welding of the stud to the pre-formed liner, as shown in Figure 4a, is well proven but because the studs need to be very short in relation to their diameter there is difficulty in assuring adequate alignment of the access of the stud shank, and misalignment of only a small fraction of a degree between the nut and the blade surface could result in bending stresses in the bolt shank which exceed the yield stress of the weld and result in detachment of the bolt from the hard surface. Furthermore, any cracks which might be induced in the weld as a result of this bending stress may not be easily detectable prior to start-up of the rotor and detachment of the entire liner may result. An alternative system (Figure 4b) of connecting the liner to the blade involves directly plug welding into a recess defined by a hole which is formed in the blade and then blanked off by the (mild steel) backing plate of the composite liner. In the case of such a plug weld it is difficult to ensure adequate quality of the attachment because of the limited access for both the welding operation and any subsequent inspection operation. There is thus a high risk of weld defects and lack of adequate fusion of the weld. Although such welds can be commonly used successfully for the attachment of static liners where there is negligible induced service load and where the attachment integrity is not so critical, for a liner attached to a dynamic component such as the impeller or rotor blade the need arises for much more critical attachment and the ability to resist high induced service loads. In order to avoid these disadvantages of the prior art systems it has been necessary to develop alternative methods of attachment of a hard-surfaced liner to a blade of a rotor or impeller so as to ensure that the strength of the attachment is adequate to resist induced stresses arising from both the centrifugal force on the rotating liner and the fluid dynamic forces on the liner/blade combination, and to do so without compromising the structural strength of the blade perse. Accordingly, one aspect of the present invention provides a rotor for a fluid machine comprising a plurality of blades each provided with a wear-resistant liner attached thereto by welding, characterised in that the attachment weld comprises at least two tack welds between a connector sleeve joined to the backing plate of the liner and a thrust sleeve engaging the rear face of the blade body and holding the thrust sleeve firmly against the rear face of the blade body. A second aspect of the invention provides a method of connecting a wear- resistant liner to a rotor blade of a fluid machine comprising:- forming a plurality of bores in said rotor blade; attaching a plurality of connector sleeves to the liner at locations to come into register with said bores in the blade when the liner is attached to the blade; the liner being pre-formed to fit on the fluid dynamic surface of the blade; bringing the liner and the blade into engagement and placing a thrust collar around the projecting portion of the connecting collar at the rear (non- fluid dynamic) face of the blade; presenting a tubular clamping tool to said clamping collar and mounting it with respect to said connector sleeve so that the tool can be actuated to thrust the clamping collar firmly against the rear surface of the blade body; and applying tack welds between the connector sleeve and the thrust collar to hold the thrust collar in place relative to the connecting sleeve, and finally removing the clamping tool from the connector sleeve. In order that the pres ent invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings in which: - FIGURE 1 is a section taken on a plane including the rotation axis and a radius of a rotor with a prior art attachment system used; FIGURE 2 is a view taken on the arrow II of Figure 1; FIGURE3 is adetail of a first prior art method of attaching a hard-surfaced liner to a blade using countersunk headed screws ; FIGURE 4a is a detail showing a second prior art method of attaching a hard- surfaced liner to a blade using welded studs; FIGURE 4b is a detail showing an alternative system of attachment of a hard- surfaced liner to the blade using plug welding; FIGURE 5a is a sectional view of a connector in accordance with the present invention welded to the liner and holding the liner in place by virtue of being welded to a thrust collar bearing against the blade body; FIGURE 5b is a view showing the connector of Figure 5a during the installation process; and FIGURE 6 is a view of the clamping tool for holding the collar firmly against the rear of the blade body during the welding operation. As mentioned above, Figure 1 shows a prior art centrifugal rotor 1 having a hub 2 rotatable about an axis 3 and supporting an annular rotor body 4. A central plate 5 defines the median plane of the rotor and has blades 6 mounted to either side of it in a spiral configuration between it and a respective one of two side plates 5a, 5b, as shown in Figure 2. Thus in operation of the rotor, which in this case is a centrifugal impeller, the air or other pumped fluid can enter from the right and the left flowing through the circular central opening of the respective side plate 5a or 5b, along paths shown by arrows Fj and F 2 , and towards the central plate 5. It is then deflected radially outwardly by the action of the rotating blades 6 (referenced 6a, 6b, 6c ... etc. in Figure 2). When the air or other pumped fluid passing through the impeller exerts abrasive action on the blades 6, the blades need to be protected by erosion-resistant liners 7 which are attached by means of nuts on bolts 8 shown in Figure 2. Figure 3 shows the body 9 of one of the blades 6 and illustrates that the liner 7 comprises a backing plate 7a, in this case of steel, having a wear-resistant layer 7b, in this case of tungsten carbide. The liner 7 wraps around the leading (radially inner) edge ofthe blade ό. The bolt 8 is shown as having a countersunk head 8' which fits in a correspondingly countersunk bore in the backing plate 7a of the liner 7, this bore permitting the shank of the bolt to protrude to the side opposite that at which the liner is attached (the non-aerodynamic or hydrodynamic side of the blade). A nut 10, and optional corrosion protection cup 12 having an upstanding skirt 12a providing erosion protection for the nut 10, can then be attached to the shank in order to allow the bolt 8 to be later released from the blade 9, if desired, for removal of the liner 7. Figure 4a shows a modification of the embodiment of Figure 3, differing in that the bolt 8 is replaced by a stud 8a welded at 8b to the liner backing plate 7a, and in that a washer 11 is in this case used instead of the cup 12 of Figure 3. A further prior art embodiment is shown in Figure 4b in that the liner 7 is attached to the blade body 9 by virtue of the backing plate 7a being placed over the aerodynamic or hydrodynamic surface of the blade and covering a hole 13 which has been formed in the blade 9. With the liner 7 clamped in position, a plug weld 14 is then formed in the base of a recess, which is defined by the hole 13 and blanked off by the backing plate 7a, with sufficient weld material to fuse to both the backing plate 7a and the body 9 of the blade. The prior art embodiment of Figure 4a, using welded studs, has the problem that the short length of stud makes it difficult to guarantee that it is perpendicular to the part of the backing plate 7a on which it is mounted, and any misalignment will place considerable strain on the weld between the stud 8a and the backing plate 7a when the nut 10 is tightened. Likewise, the embodiment of Figure 4b has a disadvantage that there is only a very confined space within the hole 13 which must be at least partially filled with the plug weld material 14 and this both restricts access for the welding operation and hinders access for any quality control inspection of the finished product. Figure 5a shows the connector in accordance with the present invention, already installed to clamp the liner 7 against the blade body 9 on the fluid dynamic (hydrodynamic or aerodynamic) surface of the blade 6. The connector comprises an internally threaded sleeve 17 which fits with adequate clearance inside a bore 18 in the blade body 9 and has previously already been welded to the exposed face of the backing plate 7a of the liner 7. As with the countersunk bolt of the embodiment of Figure 3, this welding of the connector sleeve 17 to the backing plate 7a provides the liner with a plurality of pre- attached connectors which can then serve to hold the liner on the backing plate once those connectors (sleeves 17) have been threaded through the appropriate apertures (bores 18) in the blade body. Figure 5a shows that this welding operation has already been completed in that a weld 19 positions a thrust collar 20 in firm abutment against the rear (non-fluid dynamic) face of the blade body 9 so that the collar abuts the region around the bore 18 in the blade body. Figure 5a also shows the weld 21 which joins the connector sleeve 17 to the liner backing plate 7a and illustrates the fact that the bore 18 is countersunk so as to accommodate the bulk of the strip weld 21. The thrust collar 20 is held firmly against the rear face of the blade body 9 during and after the welding operation. In the present embodiment this is achieved by use of a clamping tool 22 having a tubular body 23 whose end face becomes thrust against the thrust collar 20 when a bolt 24 within the sleeve is screwed into the corresponding internal thread of the connector sleeve 17. Bolt 24 also has a hexagonal head 24a which, together with spherical washers 25 between the head and the adjacent end of the tubular body 23 of the clamping tool 22, is capable of holding the tubular body 23 against the blade body 9 even in the event of there being misalignment of the axis of the connector sleeve 17 relative to the axis of the tubular body 23 when it sits firmly against the thrust sleeve 20. This accommodation of misalignment is as a result of the spherical washer assembly 25. If a flat washer had been used then there would have been an uneven compressive load distribution around the washer when the bolt head 24 is drawn upwardly against it, but the fact that the washers have a spherical surface allows thin re- alignment to ensure uniform distribution of compression load despite any misalignment in the axis of the connector sleeve 17. The tubular body 23 of the clamping tool 22 is shown in perspective in Figure 6 and comprises a through-bore 27 to accommodate the shank of the bolt 24 and which opens out into a widened mouth portion 27a to accommodate the projecting end (lowermost in Figure 5a and 5b) of the connector sleeve 17. At this same end of the tubular body 23 are two cut-away portions or slots 28, diametrically opposite one another, to allow access for welding to provide tack welds either forming part of an eventual continuous strip weld 19 (Figure 5a) or forming welds which are themselves sufficiently strong at these diametrically opposed regions of the connector sleeve 17 to hold the thrust collar 18 under all service loads to which the blade/liner combination is going to be subjected in use. The process of assembling the liner 7 to the blade body 9 is as follows :- Firstly the liner 7 comprising the backing plate 7a and the wear-resistant layer 7b is pre-formed so as to match the curvature of the blade and to have a lip which extends round the leading edge (the radially innermost edge) of the blade 6 on the rotor. Next a plurality of connector sleeves 17 is attached, by strip welds 21, to the backing plate 7 at locations which will correspond to the bores 18 in the blade body 9 when the liner 7 and the blade body 9 are assembled together. As indicated above, any minor misalignment in the axis of the connecting sleeve 17 with respect to the normal to the liner backing plate 7a can be tolerated in the later stages of this assembly operation. Next the clamping tool 22 comprising the tubular body 23, the nut 24 and the spherical washers 25, is presented to the connector and the bolt 24 is screwed into the internally threaded connector sleeve 17 far enough to compress the washers 25 and to draw the tubular body 23 firmly against the thrust collar 20 to provide the required clamping load. At this stage the location of the tack welds 19 (yet to be made) is exposed by the slots 28 so that the tack welds 19 can be made. Once this tack welding operation is complete the bolt 24 can be unscrewed from the connector sleeve 17 and the tool can be removed and used at the next connector to be welded. If desired, the two diametrically opposite tack welds 19 formed in the step just described can be supplemented by the provision of a continuous strip weld around the periphery of the exposed end of the connector sleeve 17 so as to provide a more secure attachment of the thrust collar 20 to the connector sleeve 17. However, this more secure attachment will of course mean more difficult removal when, subsequently, the liner 7 is to be removed for replacement purposes. As compared with the prior art welded stud method shown in Figure 4, the present invention uses a method of connection which eliminates the risk (normally associated with welded studs) of inducing an unacceptable degree of bending stress at the connector to backing plate weld 8b. This is achieved by substituting a plain bore (i.e. non threaded) collar as the main retaining element instead of the nut which is normally used with externally threaded studs (Figure 4a). The relative dimensions of the internal diameter of the collar 20 and the external diameter of the connector 17 are such as to ensure that there is sufficient clearance between these two elements to ensure that the end face of the collar 20 is able to fully abut against the underside of the blade without imposing bending on the connector 17. All that is now required is a means of inducing sufficient tensile load to the connector 17 to ensure an adequate clamping force between liner 7 and blade 9 together with some means of permanently locking this load within the system. The former requirement is achieved by means of the purpose designed tool 22 which is shown in use on Figure 5b. The essential features of this tool are that it provides the necessary clamping force between liner 7 and blade 9 while, simultaneously, locating the collar 20 against the underside of the blade. There is further provision (18) within the tool 22 to allow access for partially welding the collar 20 to the connector 17 whilst it is maintained under full clamping load by means of the tool. When this welding has been done, the tool 22 can be removed and the clamping force will be retained within the connection. Further welding can now be carried out on the collar and the process repeated at each connection point. In the event of the liners 7 requiring to be removed after aperiod of service (this would normally be required only when there is a need to replace excessively worn liners with new liners ), this would be achieved by burning off the collars 20 or gouging out the remaining collar welds. While this may be regarded as more labour intensive than the removal of nuts it will not, in fact, be the case since experience has shown that even such nuts require to be burned off in a similar manner after a period of service. Further advantages of this system over those based on the conventional welded stud (Figure 4a) or countersunk bold (Figure 3) result from the fact that the welded collar 20 is, inherently, less susceptible to mechanical failure due to erosion since it would require most of the weld to be worn away before this could occur. By contrast, a bolted connection could fail when there is sufficient local wear to cause bursting of the nut. For this reason, it is most unlikely that any additional erosion protection (such as the cups shown in Figure 3) would be required. It should be noted that this principle could also be utilised using externally threaded connectors where the required clamping force could be equally well applied by a clamping tool suitably adapted to engage with such a thread. However, this would require the connector length to be increased and would leave the threaded portion protruding beyond the collar, thereby increasing flow interference on the underside of the blade. The thread size would also be significantly larger for the same diameter of connector unless it was stepped down to a smaller diameter at the thread, which would add significantly to the manufacturing cost of the connectors. For these reasons, the internally threaded connector is considered to be the preferred option. However, it could also be achieved with some kind of threadless connector (e.g. a plain bore with an internal groove which could be engaged by a clamping tool). This invention provides a method for attaching proprietary pre-hard surfaced liner material to impeller surfaces in such a way that the risks normally associated with prior art methods of achieving this are eliminated. In particular, it is compared with the welded stud method since that provided a comparable facility for site replacement of liners but had the inherent disadvantage of introducing parasitic bending stresses on to the stud weld, which could lead to failure of the connection. The invention allows the required clamping force to be provided without inducing such bending loads. Although in the above the material of the backing plate 7a of the liner is simply referred to as steel, it is possible for other materials than steel to be used, and also when steel is used there is a whole range of steels, having varying hardness, from which the backing plate material can be chosen.

Claims: C L A I M S 1. A rotor (4) for a fluid machine comprising a plurality of blades (6) each provided with a wear-resistant liner (7) attached thereto by welding, characterised in that the attachment weld comprises at least two tack welds between a connector sleeve (17) j oined to the backing plate (7a) of the liner and a thrust collar (20) engaging the rear face of the blade body (9) and holding the thrust collar firmly against the rear face of the blade body. 2. A rotor according to claim 1 characterised in that the weld comprises a continuous annular strip weld around the perimeter of the connector sleeve. 3. A rotor according to either of the preceding claims, characterised in that the connector sleeve has an internal thread. 4. In combination with the rotor according to claim 3, a clamping tool (22) comprising a tubular body (23) and a bolt (24) having a shank whose thread engages the internal thread of the connector sleeve (17), said tubular body (23) having at one end a recess to receive the end of the connector sleeve (17) and at the other end an abutment face against which clamping thrust can be applied by virtue of the head (24a) of the bolt (24). 5. A combination according to claim 4, characterised in that the said recess to receive the connector sleeve includes slots (28) to provide access to the connector sleeve (17) and the thrust collar (20) for allowing manufacture of said tack welds. 6. A combination according to claim 4 or 5, and including spherical washers (25) to fit between the head (24a) of said bolt (24) and the said abutment face of the tubular body for allowing misalignment of the bolt with respect to the axis of the tubular body during screwing in of the clamping bolt. 7. A method of connecting a wear-resistant liner (7) to a rotor blade (6) of a fluid machine, characterised by:- forming a plurality of bores (18) in said rotor blade; attaching a plurality of connector sleeves (17) to the liner (7) at locations to come into register with said bores ( 18) in the blade when the liner is attached to the blade; the liner being pre-formed to fit on the fluid dynamic surface of the blade; bringing the liner (7) and the blade (6) into engagement and placing a thrust collar (20) around the projecting portion of the connecting sleeve (17) at the rear (non-fluid dynamic) face of the blade; presenting atubular clamping tool (22) to said clamping collar (20) and mounting it with respect to said connector sleeve (17) so that the tool can be actuated to thrust the clamping collar firmly against the rear surface of the blade body; and applying tack welds between the connector sleeve (17) and the thrust collar (20) to hold the thrust collar in place relative to the connecting sleeve, and finally removing the clamping tool (22) from the connector sleeve (17). 8. A method according to claim 7 characterised by including the further step of providing additional weld material between the thrust collar and the connector sleeve to mount the thrust collar more securely relative to the connector sleeve.

Inventors: WALKER ROBERT ALEXANDER, WALKER, ROBERT, ALEXANDER
Applicants: HOWDEN POWER LTD, WALKER ROBERT ALEXANDER, HOWDEN POWER LIMITED, WALKER, ROBERT, ALEXANDER
Application year: 2000
Main classification: F04D29
IPC codes: F04D23, F04D29