Vane and shroud arrangements for a turbo-machine

Abstract

A turbine for a turbo-machine is proposed in which, at a gas inlet for a turbine wheel, vanes extend from a nozzle ring though slots in a shroud. The vanes are formed with a leading portion which is arranged to contact a leading portion of a corresponding slot, and a trailing portion which is shaped, when the leading portion of the vane and slot are together, to be spaced from a corresponding trailing portion of the slot with a substantially constant spacing at room temperature. The contact may be a point contact, e.g. close to the leading edge of the vane. Alternatively, the vane may include a leading surface portion which conforms closely with the shape of a corresponding leading surface portion of one of the slots.

Claims

1. A turbine comprising: (i) a turbine wheel having an axis, (ii) a turbine housing for defining a chamber for receiving the turbine wheel for rotation of the turbine wheel about an axis, the turbine housing further defining a gas inlet, and an annular inlet passage from the gas inlet to the chamber, (iii) a ring-shaped shroud defining a plurality of slots and encircling the axis, each slot having a slot surface; and (iv) a nozzle ring supporting a plurality of vanes which extend from the nozzle ring parallel to the axis, and project through respective ones of the slots; each of the vanes having: an axially-extending vane surface which includes (i) a vane outer surface facing a radially-outer surface of the corresponding slot, (ii) an opposed vane inner surface facing a radially-inner surface of the corresponding slot, and a median line between the vane inner surface and the vane outer surface extending from a leading end of the vane to a trailing end of the vane; the vane being positioned with a leading surface portion of the vane inner surface contacting a corresponding leading surface portion of the respective slot surface; the vane inner surface further including a trailing surface portion extending along at least 33% of the median line and, which, at room temperature and when the leading surface portions are in contact, is spaced from an opposed trailing surface portion of the slot surface by a distance in the range 10 microns to 250 microns.

2. A turbine according to claim 1 in which, at room temperature and when the leading surface portions are in contact, the trailing surface portions are spaced apart by a distance in the range 25 microns to 100 microns.

3. A turbine according to claim 1 in which the trailing portion of the vane surface extends for at least 50% of the length of the median line.

4. A turbine according to claim 1 in which the leading surface portion of the vane extends along at least 15% of the length of the median line, the respective profiles of the leading surface portion of the vane surface and a corresponding leading surface portion of the respective slot surface diverging from each other by no more than 1 micron to 50 microns.

5. A turbine according to claim 4 in which the leading edge portion includes a point where the median line intercepts the leading edge of the vane.

6. A turbine according to claim 1 in which the leading surface portion of the vane extends along less than 5% of the length of the median line.

7. A turbine according to claim 1 in which, in use, radially inner portions of the surfaces of the vane and slot are at a lower pressure than radially outer portions of the surfaces of the vane and slot.

8. A turbocharger comprising a turbine comprising: (i) a turbine wheel having an axis; (ii) a turbine housing for defining a chamber for receiving the turbine wheel for rotation of the turbine wheel about an axis, the turbine housing further defining a gas inlet, and an annular inlet passage from the gas inlet to the chamber; (iii) a ring-shaped shroud defining a plurality of slots and encircling the axis, each slot having a slot surface; and (iv) a nozzle ring supporting a plurality of vanes which extend from the nozzle ring parallel to the axis, and project through respective ones of the slots; each of the vanes having: an axially-extending vane surface which includes (i) a vane outer surface facing a radially-outer surface of the corresponding slot, (ii) an opposed vane inner surface facing a radially-inner surface of the corresponding slot; and a median line between the vane inner surface and the vane outer surface extending from a leading end of the vane to a trailing end of the vane; the vane being positioned with a leading surface portion of the vane inner surface contacting a corresponding leading surface portion of the respective slot surface; the vane inner surface further including a trailing surface portion extending along at least 33% of the median line and, which, at room temperature and when the leading surface portions are in contact, is spaced from an opposed trailing surface portion of the slot surface by a distance in the range 10 microns to 250 microns.

9. In combination, a ring-shaped shroud and a nozzle ring, the shroud and nozzle ring being for positioning within a turbine including a turbine wheel, and a turbine housing defining a chamber for receiving the turbine wheel for rotation of the turbine wheel, the turbine housing further defining a gas inlet, and an annular inlet passage from the gas inlet to the chamber; the ring-shaped shroud defining a plurality of slots and encircling an axis which in use is the rotational axis of the turbine wheel within the chamber, each slot having a slot surface; and a nozzle ring supporting a plurality of vanes which extend from the nozzle ring parallel to the axis, and project through respective ones of the slots; each of the vanes having: an axially-extending vane surface which includes (i) a vane outer surface facing a radially-outer surface of the corresponding slot, (ii) an opposed vane inner surface facing a radially-inner surface of the corresponding slot, and a median line between the vane inner surface and the vane outer surface extending from a leading end of the vane to a trailing end of the vane; the vane being positioned with a leading surface portion of the vane inner surface contacting a corresponding leading surface portion of the respective slot surface; the vane inner surface further including a trailing surface portion extending along at least 33% of the median line and, which, at room temperature and when the leading surface portions are in contact, is spaced from an opposed trailing surface portion of the slot surface by a distance in the range 10 microns to 250 microns.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Embodiments of the disclosure will now be described for the sake of example only, with reference to the following drawings in which:

(2) FIG. 1 is composed of FIG. 1(a) which is an axial cross-section of a known variable geometry turbine, and FIG. 1(b) which is a cross-section of a part of the turbine of FIG. 1(a);

(3) FIG. 2 is an axial view of a nozzle ring which can be used in the known arrangement of FIG. 1;

(4) FIG. 3 is an axial view of a shroud which can be used in the known arrangement of FIG. 1;

(5) FIG. 4 shows the positional relationship between the nozzle ring of FIG. 2 and the shroud of FIG. 3;

(6) FIG. 5 shows a possible positional relationship between a vane and a respective slot;

(7) FIG. 6 illustrates the formation of foreign object damage on vanes of a turbine;

(8) FIG. 7 illustrates a region of the outer surface of the vane which is not subject to foreign object damage;

(9) FIG. 8 indicates how the positional arrangement of FIG. 5 is modified due to foreign object damage;

(10) FIG. 9 illustrates the positional relationship of a vane and a respective slot in a first embodiment of the disclosure;

(11) FIG. 10, which is composed of FIGS. 10(a) and 10(b), illustrates the profile of a slot in the first embodiment of the disclosure; and

(12) FIG. 11 illustrates the positional relationship of a vane and a respective slot in a second embodiment of the disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(13) Referring to FIG. 2, a nozzle ring is shown which could be used in the known turbocharger of FIG. 1. The nozzle ring is viewed in an axial direction from the right as viewed in FIG. 1(a) (this direction is also referred to here as “from the turbine end” of the turbocharger), from a position between the nozzle ring 5 and the shroud 6.

(14) The axis of the shaft about which the turbine wheel 9 (not shown in FIG. 2, but visible in FIG. 1(a)) and compressor wheel 11 (also not shown in FIG. 2, but visible in FIG. 1(a)) rotate is denoted as 100.

(15) Viewed in this axial direction, the substantially-planar annular nozzle ring 5 encircles the axis 100. From the nozzle ring 5, vanes 7 project in the axial direction. Defining a circle 70 centred on the axis 100 and passing through the centroids of the profiles of the vanes 7, we can define the nozzle radius 71 as the radius of the circle 70. Gas moves radially inwardly in the gap between the nozzle ring 5 and the shroud 6.

(16) The nozzle ring 5 is moved axially by an actuator 16 (not shown in FIG. 2, but visible in FIG. 1(a)) within an annular cavity (also not shown in FIG. 2, but visible in FIG. 1(a)) defined by a portion 60 of the turbine housing. Each vane 7 is optionally longitudinally-symmetric (that is, its profile as viewed in the axial direction, may be same in all axial positions), although in some embodiments only a portion of the vane 7 is longitudinally-symmetric. The profile of the vane (or the longitudinally-symmetric part of it) looking along the longitudinal axis is elongate, having two ends, with a median line extending between those ends. To either side of the median line is major surface of the vane where the profile has relatively low curvature, and at either end of the median line the curvature of the profile is higher.

(17) The actuator exerts a force on the nozzle ring 5 via two axially-extending guide rods. In FIG. 2, a portion 32 of the nozzle ring 5 is omitted, making it possible to view the connection between the nozzle ring 5 and a first of the guide rods. The guide rod is not shown, but its centre is in a position labelled 61. The guide rod is integrally formed with a bracket 33 (commonly called a “foot”) which extends circumferentially from the guide rod to either side. The bracket 33 contains two circular apertures 62, 63. The surface of the nozzle ring 5 which faces away from the shroud 6 is formed with two bosses 34, 64 which project from the nozzle ring 6. Each of the bosses 34, 64 has a circular profile (viewed in the axial direction). The bosses 34, 64 are inserted respectively in the apertures 62, 63, and the bosses 34, 64 are sized such that the boss 34 substantially fills the aperture 62, while the boss 64 is narrower than the aperture 63. The connection between the boss 34 and the aperture 62 fixes the circumferential position of the nozzle ring 5 with respect to the bracket 33 (in typical realizations, the relative circumferential motion of the nozzle ring 5 and the shroud 6 about the axis 100 is no more than 0.05 degrees). However, the clearance between the boss 64 and the aperture 63 permits the bracket 33 to rotate slightly about the boss 34 if the guide rods move apart radially due to thermal expansion. For that reason, the boss 34 is referred to as a “pivot”.

(18) The location, as viewed in the axial direction, at which a second of the guide rods is connected to the nozzle ring 5 is shown as 31. The connection between the nozzle ring 5 and the second guide rod is due to a second bracket (not visible in FIG. 2) integrally attached to the second guide rod. The second bracket is attached to the rear surface of the nozzle ring 5 in the same way as the bracket 33. The pivot for the second bracket is at the location 35.

(19) Holes 24, 25 are balance holes provided in the nozzle ring 5 for pressure equalisation. They are provided to achieve a desirable axial load (or force) on the nozzle ring 5.

(20) Facing the nozzle ring 5, is the shroud 6 illustrated in FIG. 3. FIG. 3 is a view looking towards the shroud 6 from the nozzle ring 5 (i.e. towards the right side of FIG. 1). The shroud defines slots 30 (that is, through-holes) for receiving respective ones of the vanes 7. The edge of each slot is an inwardly-facing slot lateral (i.e. transverse to the axis 100) slot surface. Note that in FIG. 3 the slots 30 are not illustrated as having the same profile as the vanes 7 of FIG. 2, but typically the respective profiles do have substantially the same shape although the slots are of greater size than the vanes.

(21) FIG. 4 is another view looking in the axial direction from the nozzle ring 5 towards the shroud 6 (i.e. towards the right side of FIG. 1(a)), showing a representative vane 7 inserted into a respective representative slot 30. The vane 7 has a generally arcuate (crescent-shaped) profile, although in other forms the vanes are substantially planar.

(22) Specifically, the vane 7 has a vane inner surface 41 which is closer to the wheel. The vane inner surface 41 is typically generally concave as viewed in the axial direction, but may alternatively be planar. The vane 7 also has a vane outer surface 42 which is closer to the exhaust gas inlet of the turbine. Each of the vane inner and outer surfaces 41, 42 is a major surface of the vane. The vane outer surface 42 is typically convex as viewed in the axial direction, but may also be planar. The major surfaces 41, 42 of the vane 7 face in generally opposite directions, and are connected by two axially-extending end surfaces 43, 44 which, as viewed in the axial direction, each have smaller radii of curvature than either of the surfaces 41, 42. The end surfaces 43, 44 are referred to respectively as the leading edge surface 43 and the trailing edge surface 44.

(23) In most arrangements, the vane outer surface 42 is arranged to oppose the motion of the exhaust gas the inlet passageway, i.e. the motion of the exhaust gas in the inlet passageway is such as to direct the exhaust gas against the vane outer surface. Thus, the vane outer surface 42 is typically at a higher pressure than the vane inner surface 41, and is referred to as the “high pressure” (or simply “pressure”) surface, while the vane inner surface 41 is referred to as the “low pressure” (or “suction”) surface. These oppose corresponding portions of the inwardly-facing surface which define the edge of the slot 30, and which are given the same respective names.

(24) As viewed in the axial direction, each vane 7 has a median line 51 which extends from one end of the vane to the other (half way between the vane inner and outer surfaces 41, 42 when viewed in the axial direction), and this median line has both a radial and a circumferential component. We refer to the surface of the slot which the vane inner surface 41 faces as the slot inner surface 46, and the surface of the slot which the vane outer surface 42 faces as the slot outer surface 47. As shown in FIG. 4, there is a gap of substantially constant width between the periphery of the vane 7 and the surface of the slot 30. This gap includes four portions: between the vane inner surface 41 and the slot inner surface 46; between the vane outer surface 42 and the slot outer surface 47; and between the vane's leading and trailing edge surfaces 43, 44, and respective leading and trailing portions 49, 59 of the edge of the slot. The surfaces 46, 47, 49 and 59 together constitute the inwardly-facing slot surface which defines the slot.

(25) FIG. 5 shows a possible positional arrangement between a vane and shroud slot which is proposed in GB 1619347.6. The turbine has the form illustrated in FIGS. 1 and 2, with the difference that the vanes and/or slots in the shroud are differently shaped and sized. In FIG. 5, elements corresponding to elements of FIGS. 1 to 4 are given reference numerals 100 higher. Thus, a representative vane 107 is depicted within a representative slot 130. The vane outer surface 142 faces a slot outer surface 147, and a vane inner surface 141 faces a slot inner surface 146. Optionally, the vane 107 may be longitudinally-symmetric along the whole of its length (i.e. with the same profile, as viewed in the axial direction, in all axial positions). In another possibility, only a part of the vane 107 may be axially symmetric, e.g. including the portion which can be inserted into the slot 130 when the vane 107 is in its most advanced position. In this case, the portion of the vane shown in FIG. 5 is part of this axially symmetric portion of the vane. The vane 107 is integrally formed with the nozzle ring 5, as a one-piece unit, for example by casting and/or machining.

(26) In contrast to the known vanes of FIG. 4, the vane 107 of FIG. 5 has a narrower clearance between the vane inner surface 141 and the opposed slot inner surface 146. By contrast, a much wider gap exists between the vane outer surface 142 and the corresponding portion 147 of the slot outer surface 147. This means that exhaust gas entering the shroud recess 8 between the outer vane surface 142 and the slot outer surface 147 is largely prevented from exiting the shroud recess between the vane inner surface 141 and the slot inner surface 146.

(27) To further this effect, the vane surface and slot surface are formed with a conformal portion 145 which extends along at least about 80% of the length of the median line 151. As illustrated in FIG. 5, the conformal portion 145 of the vane surface in FIG. 5 includes substantially all of the vane inner surface 141. The profile (that is the shape, as viewed in the axial direction) of the vane inner surface 141 and a corresponding portion of the slot inner surface 146 are very similar to each other, so that they can be placed against each other with a very small gap between them along the whole length of the conformal portion 145. Specifically, the profile of the vane inner surface 141 and the corresponding portion of the slot inner surface 146 at room temperature are such that they may be positioned against each other with a gap between them which, e.g. transverse to the median line, is no more than 0.35% of the nozzle radius 71. The vane's leading edge surface 143 is in contact with the corresponding portion 149 of the inner surface of the slot 130.

(28) If there is differential thermal expansion between the vanes 107 and the shroud (for example, because they are formed from different materials and/or experience different temperatures), the conformal portion of the vane 107 may be forced against the against the slot inner surface 146. Fictional force between them may then prevent axial motion of the vane relative to the shroud. However, there is a certain free play in the system (for example, due to the coupling of the nozzle ring 5 to the rods illustrated in FIG. 2, the nozzle ring may have a certain inherent freedom to rotate about the axis 100), which allows the vanes 107 to retract to a certain extent from the conformal surface of the slot.

(29) FIGS. 6 and 7 illustrate the formation of foreign object damage (FOD) during the use of the turbine of FIG. 5. The large arrow indicates the general direction of rotation of the exhaust gas entering the turbine, and rotation of the turbine wheel. FIG. 6 is a view of the shroud in the axial direction, and FIG. 7 is an enlarged portion of FIG. 6. The shroud defines the slots 130 which contain the respective vanes 107. It has been found experimentally that for a given one of the vanes 107 (indicated as 107a) a line 150 exists, extending from the trailing edge of the adjacent vane (indicated as 107b) in the upstream direction (i.e. in the direction from the vane 107a which is opposite to the large arrow in FIG. 6), such that the vane 107b protects the vane 107a from FOD in a “leading surface portion” 160 of the inner surface of the vane 107a which is radially outward of the line 150. The line 150 represents, in fact, the trajectory of a particle of debris which just passes the inner end of the vane 107b, and then impacts on the vane 107a. All FOD damage to the vane 107a is between the interception of the line 150 with the inner surface of the vane 107a and the trailing edge 165 of the vane 107a.

(30) In the case of a nozzle ring of nozzle radius 48.1 mm, and with each of the vanes having a length of 23 mm (i.e. the length of the median line), the undamaged portion of the vane inner surface 141 has been found to extend for at least the first 4 mm of the length of the median line from the end of the median line at the leading edge 167 (i.e. 17% of the length of the vane). Between 4 mm and 5 mm there are some small impact craters and minor pitting. At all points further than 5 mm from the leading edge 167 of the vane 107a, the surface has the same condition. This effect is observed to be equal on all the vanes of the turbine. (Note that a computer simulation suggested at all FOD would be at least 5.5262 mm from the leading end 167, but this was found to be an over-estimate.)

(31) FIG. 8 illustrates a result of the FOD at a trailing part of the vans 107a. Suppose that at a point 161 near the trailing edge 165 of the vane 107a, there is FOD (such as a raised crater) to the inner surface of the vane 107 which causes the portion of the inner surface 141 of the vane 107a near the damage to be spaced from the opposing slot inner surface 146 by a distance of 0.05 mm. It is found that this can result, at a point 162 on the vane inner surface 141 near the leading edge 167, in a larger spacing (such as 0.15 mm) between the vane 107a and the slot inner surface 146. Gas is able to pass through this gap (from the recess behind the shroud) to the low pressure side of the vane 107, reducing the efficiency of the turbine.

(32) Turning to FIG. 9, a portion of a first embodiment of the disclosure is illustrated. The embodiment includes a representative vane 207 having at least a portion which is longitudinally-symmetric parallel to the axis 100, and a representative slot 230 which is longitudinally-symmetric in the direction 100. The view of FIG. 9 is looking parallel to the direction 100, and shows the longitudinally-symmetric (portion of the) vane 207 in cross-section. The vane 207 has opposed major surfaces (an inner surface and an outer surface) with a median line (not shown) half way between them, extending from a leading edge of the vane 207 to a trailing edge. To either side of the median line is a major surface of the vane where the profile has relatively low curvature, and at either end of the median line the curvature of the profile is higher than on the major surfaces.

(33) The embodiment is a turbine with a construction equal to that of the known system of FIGS. 1-3 (and accordingly elements corresponding to respective elements of the vane and slot of FIGS. 5-8 are given reference numerals 100 higher), with the sole difference that the radially inner surface 241 (vane inner surface) of the vane 207 and/or the slot inner surface 246 of the slot 230 have different respective profiles from the known system of FIGS. 1-3.

(34) Firstly, in a leading surface portion 260 of the vane 207, the vane inner surface 241 and slot inner surface 246 closely conform to each other. In particular, they may be designed with exactly the same shape, but in practice diverge from each other by 1 micron to 50 microns, or more preferably 1 micron to 25 microns.

(35) Secondly, when the vane inner surface 241 and slot inner surface 246 are in contact with each other in the leading portion 260, at all positions on the vane inner surface 241 which are closer towards the trailing edge 265 than the leading portion 260 (this set of positions is referred to as a “trailing surface portion” 266 of the vane inner surface 241), the vane inner surface 241 is spaced from the slot inner surface 246. The spacing in substantially all of the trailing surface portion 266 may be at least 0.05 mm, which, in the case of a nozzle ring with a nozzle radius of 48.1 mm, corresponds to about 0.1% of the nozzle radius. In practice, tolerances in the manufacture of the vane 207 or slot 230 can cause this spacing to be reduced. Furthermore, in use this spacing is reduced at isolated positions within the trailing surface portion 266 due to crater damage on the vane inner surface 241.

(36) However, even if there is FOD in the trailing surface portion 266 which causes the surface of the vane inner surface 241 to be raised by a height of 0.05, this will not cause the vane inner surface 241 to impact the slot inner surface 246 in the trailing surface portion 266, and therefore will not cause the vane inner surface 241 to be spaced from the slot inner surface 246 in the leading surface portion 260.

(37) Similarly, if, due to tolerances in the manufacture of the vane 207 and/or the slot 230, the inner surface 241 of the vane 207 in the trailing surface portion 266 happens to be deformed by a distance 0.05 mm in the direction toward the slot inner surface 246, this will not cause the vane inner surface 241 to impact the slot inner surface 246 in the trailing surface portion of the vane inner surface 241, so it will not cause the inner surface 241 to be spaced from the slot inner surface 246 in the leading surface portion 260. In practice the manufacturing tolerance of the vane 207 and slot 230 may be as high as 0.1 mm, so a spacing of 0.05 mm merely reduces the chance of the vane inner surface 241 being spaced from the slot inner surface 246 in the leading surface portion 260. For that reason, it may be preferred to provide a larger spacing between the vane inner surface 241 and the slot inner surface 246 in at least the majority of the trailing surface portion 266, such as a spacing of 0.1 mm.

(38) The spacing between the vane inner surface 241 and the slot inner surface 246 in the trailing surface portion 266 has the further advantage of reducing the risk of the vane 207 becoming stuck to the shroud due to a thermal transient.

(39) In FIG. 9 the range of contact portions of the vane slot and slot surfaces is shown including all the radially inner surface of the vane up to the leading end 267 of the vane 207, but not including any of the radially outer surface of the vane 207. However, in variants of the embodiment, a radially outer portion of the vane 207 proximate the leading edge of the vane 207 may contact the slot surface (in the manner shown in FIG. 7).

(40) FIG. 10(a) shows the profile of the slot 230 with no vane present. The view is parallel to the rotational axis 100, and the slot is longitudinally symmetric in this direction. The slot inner surface 246 includes a leading surface portion 2461, which, when the vane 207 is present, lies along the leading surface portion 260 of the vane inner surface 241. The slot inner surface 246 further includes a trailing surface portion 2462 which, in use, is spaced from a corresponding trailing surface portion of the vane inner surface 241. As illustrated in FIG. 10(b), which is an enlarged view of a portion of FIG. 10(a), there is a transition region 2463 between the leading and trailing surface portions 2461, 2462 of the slot inner surface 246, including a convex portion 2464 of the surface 246, and a concave portion 2465 of the surface 246. The length along the vane of the portion 2461 may be about 5.33 mm, and the radius of curvature of each of the portions 2464, 2465 may be about 0.5 mm (i.e. a factor of about 10 lower).

(41) Turning to FIG. 11, a view is shown of a second embodiment of the disclosure. Elements having the same meaning as elements of the first embodiment are given reference numerals 100 higher. The embodiment includes a vane 307 having at least a portion which is longitudinally-symmetric parallel to the axis 100, and a slot 330 which is longitudinally-symmetric in the direction 100. As in the first embodiment, the vane 307 has opposed major surfaces (an inner surface and an outer surface) with a median line half way between them, extending from a leading edge of the vane to a trailing edge. The view of FIG. 11 is looking parallel to the direction 100, and shows the longitudinally-symmetric (portion of the) vane 307 in cross-section. A trailing surface portion 366 of the vane inner surface 341 (again, the low pressure side of the vane) is conformal with a trailing surface portion 3461 of the slot inner surface 346. The two trailing surface portions are slightly spaced apart, e.g. with a substantially-constant spacing between them. The spacing is typically in the range 10 microns to 250 microns, and more preferably at least 25 microns and/or no more than 100 microns. However, the vane inner surface 341 also includes a leading surface portion 368 opposing a leading surface portion 3462 of the slot inner surface 346 which gradually approaches the vane inner surface 341, until the two contact each other at a contact point 3463. The contact point 3463 may be on a line 3464 which is a tangent to the profile of the vane 307 and passes through the rotational axis 100, which is at the centre of the shroud. This position is chosen to minimise (or substantially eliminate) radial force transmitted between the vane and the shroud.

(42) From the point 3463 towards the leading edge 367 of the vane 307, the vane's leading edge surface 343 is spaced from the opposed corresponding portion 349 of the inner surface 346 of the slot 330. The distance of the contact point 3463 from the leading edge 367 of the vane may be less than 10% of the length of the median line, or even less than 5%. The contact between the vane 307 and the inner surface of the slot 330 extends along much less than 5% of the median line of the vane between its opposed major surfaces, such as along less than 1% of the length of the median line, or even 0.1% of the length of the median line.

(43) Since the trailing surface portion 3461 of the slot inner surface 346 is spaced from the trailing surface portion 366 of the vane inner surface 341, imperfections on the trailing surface portions due to machining tolerances and/or due to FOD to the vane 307, do not cause the trailing surface portions to touch each other. Thus, there is no force developed between the trailing surface portions which separates the slot inner surface 346 and the vane inner surface 341 in their respective leading surface portions 368, 3462, such that contact at the contact point 3463 is lost.

(44) Since all the contact between the vane 307 and the slot 330 is at the narrow contact point 3463, there is little of no risk of the vane 307 becoming locked against the slot 330, such that sliding motion of the vane 307 in the axial direction is impaired.