Turbine disc

Abstract: A turbine disc 2 for carrying turbine blades is rotatable about a central axis and has a circumferentially extending peripheral face 4 divided into two side face strips by a circumferentially extending root slot 6. For the greater part of its circumferential length opposite sides 16, 18 of the root slot 6 are parallel. But at at least one location arcuate bights 22 and 24 of opposite curvature are formed opposite to one another to form a load slot 22, 24 through which a root of a turbine blade can be introduced into the radially innermost part of the root slot 6. The arcuate bights 22 and 24 have the same radius of curvature and may be arcs of the same circle. On each of two opposite sides of the load slot 22, 24 is a respective shadow slot. One shadow slot is formed by oppositely disposed arcuate bights 32 and 34, and the other shadow slot is formed by oppositely disposed arcuate bights 36 and 38. The radius of curvature of each bight 32, 34, 36 or 38 is greater than that of the bights 22 and 24 and can be at least twice that of the bights 22 and 24.

Description: This invention concerns a turbine disc. It also concerns a turbine wheel comprising the disc, and a turbine engine or machine comprising the turbine wheel. With reference to the accompanying drawings a prior art turbine disc is illustrated in Figures 1A and 1B in which: Figure 1A is a fragmentary and diagrammatic view, partly in section, of the periphery of a known turbine disc, and Figure 1B is a section in line IB - IB in Figure 1A. With reference to Figures 1A and 1B a known turbine disc is shown at A rotatable about an axis and having a peripheral face B extending circumferentially about the axis. A root slot C to engage roots of turbine blades to be held in the turbine disc is formed in the peripheral face B. The root slot C extends circumferentially around the turbine disc and as shown in Figure 1B has a cross-sectional shape which is adapted to receive and securely hold the blade roots. It will be noticed that root slot C has a re-entrant or undercut shape at the deepest and widest part of its cross-section having opposed concave recesses E1 and E2, which are spaced more widely apart than the convex ribs F1, F2 immediately above. Consequently, the root slot C includes in its side-walls cut-away portions J1, J2 comprising opposite arcuate bights in the convex ribs F1, F2. These bights together form a load slot J1, J2 for allowing entry and exit, or "loading" and "unloading", of the blade roots into the root slot C of the turbine disc A. To attenuate the in-service hoop stress concentrations experienced in the perimeter of the turbine disc as a result of the presence of the load slot J1, J2, F, the root slot C also includes so-called "shadow slots" G1 and G2. Conveniently, each shadow slot G1, G2 comprises opposite arcuate bights each having a radius of curvature which is considerably less than that of the bights J1, J2 forming load slot. However, each bight of a shadow slot G1, G2 extends, through the convex ribs F1, F2 towards the respective concave recess E1 or E2, similarly to the bights J1, J2 forming the load slot. Turbine discs are known in which the bights constituting the shadow slots have indentical radii of curvature, that radius being less than the radius of curvature of the bights constituting the load slot. Furthermore, the bights constituting the shadow slots cause substantial widening of the root slot in the convex ribs above the undercuts. US-A-5584658 (Fig. 1A) and US-A-4482297 (Fig. 5) illustrate turbine wheels having a plurality of identically sized load slots comprising arcuate bights for enabling the insertion of turbine blade roots into undercut root slots in the wheels. GB-A-916001 discloses the use of a single load slot comprising axially opposed relatively shallow bights. US-A-3216700 (Fig. 1) shows a turbine wheel having a plurality of identically sized load slots comprising square-cut bights. According to the invention there is provided a turbine disc rotatable about an axis, said turbine disc comprising a peripheral face extending circumferentially about the axis, the peripheral face having a root slot formed therein for engaging roots of turbine blades to be held in the turbine disc, the root slot having a longitudinal extent extending circumferentially around the turbine disc, a depth extending radially of the disc and a width defined between axially opposed side walls of the root slot, the root slot further having an undercut portion of its cross-section for accommodating the blade roots, at least one of the side-walls of the root slot above the undercut portion having cut-away portions including (a) at least one arcuate bight defining a load slot for allowing entry and exit of the blade roots into the undercut portion of the root slot, (b) on circumferentially opposed sides of the load slot, a further at least one arcuate bight defining a shadow slot for reducing hoop stress concentrations in the perimeter of the turbine disc, wherein at least one of the bights constituting the shadow slots has a radius of curvature greater than the radius of curvature of the bight or bights constituting the load slot. It is possible to construct a turbine disc in accordance with the invention above so that when the disc is in use in combination with turbine blades the following advantages may be derived over known turbine discs with conventional shadow slot formation:- (i) hoop stress concentration in the vicinity of the load slot (and shadow slots) can be reduced, thus increasing the life of the turbine disc; (ii) crush stress of the turbine blades in the vicinity of the shadow slots is reduced (minimised) by means of increased contact between blade roots and flanks of said ribs at undersides of the ribs; and (iii) the chance of detrimental blade vibration is reduced or avoided because the increased contact mentioned at (ii) above means the natural frequency of the turbine blades is not significantly affected. The load slot may be constituted by a pair of opposite arcuate bights each in an opposed said side wall. Each or a said shadow slot may be constituted by a pair of opposite arcuate bights each in an opposed said side wall. At least one of the bights constituting the shadow slots may have a radius of curvature at least twice as great as the radius of curvature of the bight or at least one of the bights constituting the load slot. For example, at least one of the bights constituting the shadow slots may have a radius of curvature at least substantially eight times greater than the radius of curvature of the bight or at least one of the bights constituting the load slot. The bights constituting a said shadow slot may have substantially the same radius of curvature. The side walls of the root slot may comprise axially opposed, circumferentially extending first and second ribs having side faces facing across the root slot, wherein a first maximum distance which said bight or one of the bights constituting the load slot extends axially of the disc into the respective first or second rib from a locus of the side face of the rib is greater than a second maximum distance which said bight or one of the bights constituting the shadow slots extends axially of the disc into the respective first or second rib from the locus of the side face of the rib. Preferably the first maximum distance is at least a plurality of times greater than the second distance. The first maximum distance may be at least substantially twice the second maximum distance. For example, the first maximum distance may be between substantially 2.0 and substantially 2.5 times greater than the second maximum distance. With respect to at least one of the shadow slots an end of said bight or at least one of the bights constituting the load slot may be circumferentially spaced from an adjacent end of said bight or at least one of the bights constituting the shadow slot. Alternatively, with respect to at least one of the shadow slots an end of said bight or at least one of the bights constituting the load slot may substantially coincide with an end of said bight or at least one of the bights constituting the shadow slot. Said ends may be substantially on a said locus. At least one of the shadow slots may be adjacent to the load slot. A turbine wheel may comprise a turbine disc formed according to the invention and turbine blades. This turbine wheel may be included in a turbine engine or machine, for example in a gas turbine. The invention will now be further described, by way of example, with reference to the accompanying drawings in which:- Figure 2 is a fragmentary and diagrammatic view, partly in section, of a peripheral side of a fragment of a turbine disc for a turbine wheel of a turbine engine or machine, said turbine disc being formed according to the invention; Figure 3 is a section on line III - III in Figure 2; Figure 4 is a section on line IV - IV in Figure 2 through a shadow slot; Figure 5 is a section similar to Figure 4 including a fragment of a turbine blade arrangement engaged within the root slot at the shadow slot; Figure 6 is a section on line VI - VI in Figure 2 through the load slot and including a fragment of a turbine blade arrangement within the load slot; Figure 7 is a section on line VII - VII in Figure 2 through the root slot and including a fragment of a turbine blade arrangement engaged in the root slot; Figure 8 shows on an enlarged scale a fragment of the view in Figure 2 in which roots of turbine blade arrangements are shown in relation to a said shadow slot; Figure 9 shows on an enlarged scale a fragment of the view in Figure 2, and Figure 10 shows graphically relationships in which a value of a ratio k t of stresses in the turbine disc in Figure 9 varies with respect to a variation in a circumferential distance x from a centre line of the load slot, said centre line being axial or extending parallel to the rotation axis of the disc. In the drawings like references identify like or comparable parts. The turbine disc 2 has a peripheral face 4 extending circumferentially about the disc axis. A root slot 6 to engage roots of turbine blades to be held in the turbine disc 2 is formed in the peripheral face 4. The root slot 6 extends circumferntially around the turbine disc 2 and as shown in Figure 4 has a cross-section shape which is adapted to receive and securely hold the turbine blade roots. Root slot 6 has a re-entrant or undercut shape at the deepest and widest part of its cross-section, having opposed concave recesses 8 and 10, which are spaced more widely apart than convex ribs 14, 20 immediately above. Each rib 14, 20 has an underside or flank 12. The root slot 6 includes in its side-walls cut-away portions 22, 24 comprising opposite arcuate bights in the convex ribs 14, 20. These bights together form a load slot 22, 24 for allowing entry and exit, or "loading" and "unloading", of turbine blade roots into the root slot 6 of the turbine disc 2 (see root 26 of turbine blade 30 in Figures 5 to 7). To attenuate the in-service hoop stress concentrations experienced in the perimeter of the turbine disc as a result of the presence of the load slot 22, 24, the root slot 6 also includes "shadow slots" 32, 34 and 36, 38. Each bight 32, 34, 36 or 38 of a said shadow slot extends, through the convex ribs 16, 20 towards the respective concave recess 8 or 10, similarly to the bights 22, 24 forming the load slot. For the greater part of the circumferential length of the slot 6 its opposite sides 16, 18 are substantially parallel except at the locations of the arcuate bights 22, 24, 32, 34, 36 and 38 formed in the ribs 14 and 20. The arcuate bights 22, 24 have substantially the same radius of curvature, and may have the same centre of curvature and thus be based on different arcs of the same circle which may have a diameter substantially equal to a maximum axial width of the root slot 6. On each of the two opposite circumferential sides of the load slot 22,24 is a respective said shadow slot 32, 34 or 36, 38, as indicated, one shadow slot being formed by the oppositely disposed arcuate bights 32 and 34 respectively formed in the ribs 14 and 20, the other shadow slot being formed by the oppositely disposed arcuate bights 36 and 40 also in the respective ribs 14 and 20. The radius of curvature of each of the bights 32, 34, 36 or 38 is substantially the same. Dotted line 42 or 44 each represents the locus of a respective side face 16 and 18 (see Figures 2 and 9) i.e. the position where the side faces would be were the bights 22, 24, 32, 34, 36 and 40 not present. Whilst either or each shadow slot 32,34 or 36,40 may be circumferentially spaced from the load slot 22, 24, in the arrangement shown in Figures 2 and 9 an end of each arcuate bight 32 and 36 coincides at 46 and 50 on the locus 42 with a respective opposite end of the arcuate bight 22 whilst an end of each arcuate bight 34 and 40 coincides at 52 and 54 on the locus 44 with a respective opposite end of the arcuate bight 24. The radius of curvature of each bight 32, 34, 36 or 38 is greater than that of the bights 22 and 24, for example, it may be at least twice that of the radius of curvature of the bights 22 and 24. Radially with respect to the disc 2, the bights 32, 34, 36 and 38 extend through the ribs 14, 20 from the face 4 (see bight 32,34 in Figure 4). A maximum distance d of axial penetration into a rib 14 or 20 by a bight 32 or 36 from the respective locus 42 or 44 is shown in Figures 4 and 9 with respect to the bight 32. A maximum distance of axial penetration D into a rib 14 or 20 by a bight 22 or 24 from the respective locus 42 or 44 is shown in Figures 6 and 9. Dimension D can be plurality of times greater than dimension d. For example, dimension D may be substantially at least twice dimension d, dimension D may be substantially 2.0 to substantially 2.5 times greater than dimension d. The bights 32, 34, 36, 38 forming the shadow slots axially penetrate the ribs 14 and 20 to a significantly lesser extent than the bights 22, 24 forming the load slot. The radius of curvature of each bight 32, 34, 36, 38 forming the shadow slots may be at least substantially eight times greater than the radius of curvature of each bight 22, 24 forming the load slot. In one example, the width of the root slot 6 between the parallel side faces 16,18 may be substantially 5.351mm (0.211 inches); the radius of curvature of each bight 32, 34, 36, 38 forming the shadow slots may be substantially 32.512mm (1.280 inches); the radius of curvature of each bight 22,24 forming the load slot may be substantially 4.001mm (0.157 inches); dimension D may be substantially 1.271mm (O.051 inches); dimension d may be substantially 0.530mm (0.0209 inches), and axially of the disc 4 distance L between centre line 56 of the load slot 22,24 and centre line 58 of a said shadow slot (say the shadow slot 32,34 in Figure 9) may have a dimension of substantially 8.966mm (0.353 inches. In this example, the radius of curvature of the bights forming the shadow slots 32,34 and 36, 38 may be substantially 8.13 times greater than the radius of curvature of the bights forming the load slot 22,24, and the ratio D/d has a numerical value of substantially 2.39. It will be seen from Figure 4,5, and 6 that in a direction radially of the turbine disc 2, the depth of penetration through the ribs 14,20 of the bights 32,34, 36, 38 forming the load slots is considerably less than the depth of penetration of the bights 22,24 forming the load slot, thus at the shadow slots considerable portions of the flanks 12 of the ribs 14, 20 remain. With reference to Figure 8, it can be seen that substantial areas (of the roots 26) between the dotted lines 60,62 continue to make contact with the flanks 12 (see Figures 4 and 5) at the shadow slots. Thus in comparison with prior art provision of shadow slots the arrangement described with reference to Figures 2 and 9 means that the crush stress on the roots 26 or turbine blades 30, when the turbine wheel is rotated, is reduced in comparison with the crush stress experienced in prior art turbine wheels. With reference to Figure 9, if the dimension d is kept substantially constant but the radius of curvature of the bights 32,34 forming that shadow slot is increased whilst the intersection 46 and 50 remain on the loci 42,44, then the distance L between centre lines 56 and 58 will increase with increasing radius of curvature of the bight 32,34 due to this causing the centre line 58 to move to the right in Figure 9. With reference to Figure 10 values of k t are represented on the ordinate where: k t = the peak hoop stress at centre line 56 of the load slot 22,24 the average hoop stress around the disc 2 , and values of x are represented along the abscissa, where x is a circumferential distance measured from the centre line 56 (Figure 9 along the face 4 and the position of the centre line 56 is represented as zero or the origin of the x - axis. With reference to Figures 9 and 10, when the respective values of D and d are each kept constant and the radius of curvature of the load slot bights 22,24 is also kept constant but the radius of curvature of the shadow slot bights 32,34 is increased, then in Figure 10 graph 70 corresponds to a shadow slot bight 32,34 radius of curvature which is less than shadow slot bight radius of curvature to which graph 72 corresponds which in turn is less than the shadow slot bight radius of curvature to which graph 74 corresponds. By increasing the radius of curvature of the shadow slot bights 32,34 from that corresponding to curve 70 to those corresponding to curves 72 and 74, the distance between the centre lines 56,58 of the load slot and shadow slot increases from L1 to L2 to L3. Thus from Figure 10 it will be understood that for a given shadow slot, k t has a minimum value at the centre line 58 of that shadow slot and by increasing the radius of curvature of the shadow slot bights the value of k t with respect to a given value of x is generally lowered, and in particular the minimum value of k t becomes less the greater the aforesaid radius of curvature. With regard to the turbine disc 2 described above with reference to Figures 2 to 10 it has the following advantages over prior art discs with conventional shadow slots. The advantages are:- (i) hoop stress concentrations in the vicinity of the load slot and shadow slots is reduced and results in increased life of the turbine disc, this is because the increased radius of the shadow slot bights has a greater effect in deflecting lines of equal hoop stress than the depth of the shadow slots; (ii) crush stress on the turbine blades 30 in the vicinity of the shadow slots is reduced or minimised by reason of the contact between the root 26 and the flanks 12 of the root slot, and (iii) because the contact between the roots 26 and flanks 12 at the shadow slots is still quite large, the natural frequency of the turbine blades is not significantly affected.

Claims: A turbine disc (2) rotatable about an axis, said turbine disc comprising a peripheral face (4) extending circumferentially about the axis, the peripheral face having a root slot (6) formed therein for engaging roots of turbine blades to be held in the turbine disc, the root slot (6) having a longitudinal extent extending circumferentially around the turbine disc, a depth extending radially of the disc and a width defined between axially opposed side walls (16, 18) of the root slot, the root slot further having an undercut portion (8, 10) of its cross-section for accommodating the blade roots, at least one of the side-walls of the root slot above the undercut portion having cut-away portions including (a) at least one arcuate bight (22, 24) defining a load slot for allowing entry and exit of the blade roots into the undercut portion of the root slot, (b) on circumferentially opposed sides of the load slot, a further at least one arcuate bight (32, 34) defining a shadow slot for reducing hoop stress concentrations in the perimeter of the turbine disc, characterised by at least one of the bights (32, 34) constituting the shadow slots having a radius of curvature greater than the radius of curvature of the bight or bights (22, 24) constituting the load slot. A turbine disc as claimed in Claim 1, in which the load slot is constituted by a pair of opposite arcuate bights (22, 24) each in an opposed said side wall (16, 18). A turbine disc as claimed in Claim 1, or Claim 2, in which each or a said shadow slot is constituted by a pair of opposite arcuate bights (32, 34, 36, 38) each in an opposed said side wall (16, 18). A turbine disc as claimed in any one preceding Claim, in which at least one of the bights constituting the shadow slots (32, 34) has a radius of curvature at least substantially twice as great as the radius of curvature of the bight or at least one of the bights constituting the load slot (22, 24). A turbine disc as claimed in any one preceding claim, in which at least one of the bights constituting the shadow slots (32, 34) has a radius of curvature at least substantially eight times greater than the radius of curvature of the bight or at least one of the bights constituting the load slot (22, 24). A turbine disc as claimed in any one preceding claim, in which the bights constituting a said shadow slot (32, 34) have substantially the same radius of curvature. A turbine disc as claimed in any one preceding claim, in which the side walls (16, 18) of the root slot comprise opposed, circumferentially extending first and second ribs (14, 20) having side faces facing across the root slot, wherein a first maximum distance (D) which said bight or one of the bights constituting the load slot (22, 24) extends axially of the disc into the respective first or second rib (14, 20) from a locus of the side face of the rib is greater than a second maximum distance (d) which said bight or one of the bights constituting the shadow slots (32, 34) extends axially of the disc into the respective first or second rib (14, 20) from the locus of the side face of the rib. A turbine disc as claimed in Claim 7, in which said first maximum distance (D) is at least a plurality of times greater than the second distance (d). A turbine disc as claimed in Claim 7, in which the first maximum distance (D) is at least substantially twice the second maximum distance (d). A turbine disc as claimed in Claim 7, in which the first maximum distance (D) is between substantially 2.0 and substantially 2.5 times greater than the second maximum distance (d). A turbine disc as claimed in any one preceding claim, in which with respect to at least one of the shadow slots (32, 34) an end of said bight or at least one of the bights constituting the load slot (22, 24) is circumferentially spaced from an adjacent end of said bight or at least one of the bights constituting the shadow slot. A turbine disc as claimed in any one of Claims 1 to 10, in which with respect to at least one of the shadow slots (32, 34) an end of said bight or at least one of the bights constituting the load slot (22, 24) substantially coincides with an end of said bight or at least one of the bights constituting the shadow slot. A turbine disc as claimed in Claim 7 and Claim 11 or in Claim 7 and Claim 12, in which said ends substantially lie on a said locus. A turbine disc as claimed in any one of Claims 1 to 11 or in Claim 13 when appended to Claim 11, in which at least one of said shadow slots is adjacent to the load slot. A turbine wheel comprising a turbine disc as claimed in any one preceding claim, and turbine blades. A turbine engine or machine comprising a turbine wheel as claimed in claim 15. A gas turbine engine comprising a turbine wheel as claimed in Claim 15.

Inventors: LONDON RICHARD ALLAN, LONDON, RICHARD ALLAN
Applicants: ALSTOM TECHNOLOGY LTD
Application year: 2001
Main classification: F01D5
IPC codes: F01D5