Patent Application
20110000183
This invention relates to an assembly comprising first and second components, with provision of contaminant removal from a contact zone between the components. The invention is particularly, although not exclusively, concerned with such an assembly in a gas turbine engine.
Air flowing through a gas turbine engine contains small particles of debris such as soot, dust, sand and grit. These particles are small enough to penetrate contacting regions between components of the engine located in, or forming part of, the flow passages of the engine. When these components are in contact with each other, small movements, particularly repeated reciprocating movements, of one of the components with respect to the other allow the particles to move across the contacting regions.
Such movements can also cause fretting, particularly where the contacting region between the components is under heavy stress. Fretting results in erosion of the contact surfaces and therefore creates debris particles between the respective contacting surfaces of the components.
Particles of debris generated by fretting and particles entrained in the flow through the engine are often abrasive and so increase the rate of wear of the respective contact surfaces of the components. This increase in the rate of wear shortens the useful life of a component.
An example is the dovetail attachment in a gas turbine engine used to attach fan, compressor or turbine blades to their respective discs. Each dovetail attachment comprises a slot into which the root of a blade can be inserted. Each blade has flanks provided on the root. During engine operation, the walls of the dovetail slots act on the blade flanks to resist the centrifugal forces generated by each blade. Cracking of the contacting surfaces can occur, leading to failure of the attachment; if not detected early enough, this may eventually result in the shedding of the blade.
Factors which contribute to cracking include high coefficients of friction at the contact surfaces, high contact stresses, high frequency blade excitation and fretting due to movement of the contact surfaces of the dovetail attachment.
A dry film lubricant is commonly applied to the contact surfaces of the dovetail attachment, principally to reduce fretting but also to reduce the coefficient of friction at the contact surfaces. Dry film lubricants have a tendency to degrade relatively quickly in gas turbine engine applications due to heavy loading and wear and have to be replaced on a periodic basis before substantive damage occurs.
Small particles from the surrounding flow which penetrate the contact region between the flanks of the blade root and the walls of the slot, as well as particles generated as a result of fretting, can migrate into and through the contact region between the flanks and the slot walls as a result of the relative movement between the parts. The relative movement causes the particles to break up, and to abrade the disc and scratch the low friction strip. The process forms an abrasive paste which is forced out of the contact area.
On lower temperature components, a (replaceable) strip containing a low friction wear coating can also be applied to the contact zone to reduce the coefficient of friction at the contact surfaces. Air blown debris can become embedded in these strips making them abrade more like sandpaper rather than acting like a low friction slider.
Another example is that of a unison ring of a gas turbine engine which is used to control the rotational angle of guide vanes located within an annular flow passage of the engine. The unison ring is supported by guide pads that allow the ring to be rotated about its axis, which is coaxial with the engine axis, to increase and decrease the inclination angle of the vanes. The unison ring thus has a reciprocating action about its axis of rotation. Particles caught between the guide pads and the unison ring increase wear of the contacting surfaces of the pads and the ring.
According to the present invention there is provided an assembly comprising first and second components having respective contact surfaces which contact each other over a contact zone, at least one of the contact surfaces being provided with a groove which extends through the contact zone such that, in operation, a pressure difference across the contact zone causes contaminants entering the groove to be expelled along the groove from the contact zone.
The groove may be one of a plurality of grooves in the respective contact surface in which case the grooves may be inclined to one another. The grooves may intersect one another.
The groove, or at least one of the grooves, may have a side wall which is inclined to the depth direction of the groove. The groove may have a ‘V’ or ‘U’ shaped cross section and may have straight or curved walls. The edges of the groove may also be curved or angled and one edge or side of each groove may differ from the other. In particular, the edges of the groove may be shaped in such a manner as to assist removal of contaminants from a contact surface, for example by a “squeegee” effect.
At least one of the components may comprise a substrate provided with a low friction element providing the contact surface and having the groove.
The low friction element may be made from a polyimide material.
The low friction element may be provided with a wear indicator layer in which the groove, or at least one of the grooves, may extend from the contact surface to the wear indicator layer. The low friction element may have a single wear indicator layer, or the wear indicator layer may be one of a plurality of separate layers at different depths below the contact surface. The colour of each layer may differ from that of the other layers, so that the exposure of one of the layers indicates the severity of wear. A graduated indicator layer comprising diffused colour may also be used.
The substrate may comprise a composite material. The low friction element may be integral with the substrate or may be cast into a composite substrate during manufacture.
The low friction element may contact a metallic surface of the other component.
One of the components may be an aerofoil component having a root portion accommodated in a slot of the other component, the contact surfaces comprising a surface of the root portion and a surface bounding the slot. The aerofoil component may be provided with a low friction element in the form of a strip provided on the root portion, the strip extending in the lengthwise direction of the slot.
One of the components may be a unison ring of a gas turbine engine and the other component may be a support structure for the unison ring. The support structure may be provided with a low friction element in the form of a pad provided on the support structure. The pad may be one of a plurality of pads distributed around the support structure. The pad may be in the form of a strip covering a substantial part of the support structure contact zone.
The pad may be larger than the contact zone, the same size as the contact zone or smaller than the contact zone.
For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:
A low friction strip 10 may be provided on one side or on both sides of the blade root 2.
When the engine is running, rotation of the disk 5 creates a centrifugal outwards force on any particles in the contact zone. However, this may not be sufficient to drive them from the contact zone. As the blades 1 of the engine rotate, a pressure difference is created across the blades 1 in the axial direction of the engine. There is thus a pressure difference across the chord of each blade 1. This pressure difference can be used to create an air flow between the main flow path through the engine and the cavity 14. It can be arranged that part of this air flow is through the grooves 12. This air flow can then assist in the removal of particles and debris.
In some embodiments for example if the blade 1 is a turbine blade operating in the flow of hot combustion gases, cooling air may be supplied to the cavity 14. The difference in pressure between the cooling air and the flow along the main flow path creates a pressure differential between the respective ends of the grooves 12 causing cooling air to flow through the grooves 12 from the cavity 14 into the main flow.
In some embodiments the grooves 12 may be shaped and extend into either the main flow path or the cavity 14. Such grooves will act as scoops as the blades 1 and disk 5 rotate, thereby generating a pressure drop to drive air flow through the grooves.
Alternatively or additionally, the groove 12 may diverge along its length to create a pressure drop between one end of the groove and the other. Such a pressure drop would also drive air flow through the groove.
Rotation of the fan blade 1 about the engine axis causes a centrifugal force to act on the fan blade 1 and the blade root 2. The centrifugal force holds the low friction strip 10 in contact with the slot wall 8 at a very high contact pressure. Various factors in operation of the engine, such as high cycle blade excitations, cause the blade root 2 to move within the slot 4. The movement of the blade root 2 with respect to the slot 4 may be a rocking movement or small oscillatory displacements.
Any particles reaching the contact zone between the low friction strip 10 and the slot wall 8, or particles created in the contact zone by movement of the blade 1 in the slot 4, migrate across the contact zone under the action of the relative movement between the low friction strip 10 and the slot wall 4, the particles eventually reaching the edge of the strip 10 or one of the grooves 12.
Particles entering the grooves 12 are carried by air flow along the grooves 12 and are expelled from the contact zone through the respective low pressure ends of the grooves 12. Removal of the particles from the contact zone reduces the amount of wear of the low friction strip 10 and the slot walls 8.
The alignment of at least some of the grooves 12 may be biased in the direction of particle migration. The grooves 12 may be provided in areas of the contact zone under lower contact stress.
An alternative embodiment of a low friction strip 202 is shown in
During operation of the engine, particles entering the grooves 204 are carried by the flow along the grooves 204 and are expelled from the sides of the low friction strip 202.
During operation, as the top layer 304 becomes worn the depth of the groove 312 decreases. Once the top layer 304 has been worn away the indicator layer 306 becomes visible. Where the top layer 304 has been worn away in the vicinity of the groove 312, the groove 312 no longer exists thereby reducing the effectiveness of particle removal from the contact zone. The appearance of the indicator layer 306 indicates that the low friction strip 302 needs to be replaced.
As the top layer 404 becomes worn, the depth and width of the groove 412 decreases. The part of the first indicator layer 406 which is visible in the groove 412, allows the amount of wear to be determined. Once the top layer 404 has worn away the remainder of the first indicator layer 406 becomes visible. At this point, because the groove 412 is V-shaped, its width and depth have been significantly reduced, thereby reducing the amount of flow along the groove 412. As an alternative, the groove 412 may be U-shaped with substantially parallel sides to maintain groove width, and therefore flow, for longer. The first indicator layer 406 thus provides indication that the low friction strip 402 is nearing the end of its operational life. Continued wear results in the first indicator layer 406 being worn away so that the second indicator layer 408 becomes visible. At this point the groove 412 no longer exists and particle removal from the contact zone is reduced. The appearance of the second indicator layer 408 thus indicates that the low friction strip 402 needs to be replaced.
In the embodiments of
The guide pad 104 is located with respect to the engine casing 106 in a recess defined by a surrounding wall 108. The guide pad 104 is one of a plurality of guide pads which are distributed around the axis of the engine. The guide pad 104 is in contact with a radial end face of the unison ring 102 to resist radial movement and warping of the unison ring 102 during operation.
The thickness of the guide pad 104 in the axial direction of the engine is greater than the axial thickness of the unison ring 102. Consequently, when viewed in the direction of the arrow A′, as shown in
The guide pad 104 is provided with two grooves 112 which extend across the contact surface 110 of the guide pad 104 which is in contact with the unison ring 102. The grooves 112 extend axially forwards and rearwards from the contact zone. The grooves 112 have a substantially V-shaped cross section, as shown in
During operation of the engine, flow over the guide pad 104 and the unison ring 102 is provided in a generally axial direction with respect to the unison ring axis. This may be flow ducted from the main flow through the engine, cooling flow or flow from outside the engine. This flow generates a pressure difference between the ends of the grooves 112, causing flow to take place through them across the unison ring 102.
Rotation of the unison ring 102 causes the unison ring 102 to rub against the guide pad 104.
The unison ring 102 may also flex or become displaced in an axial or radial direction so that it moves with respect to the guide pad 104. The unison ring 102 may not, therefore, always remain in contact with the guide pad 104 and may instead be intermittently in contact with the guide pad 104. This relative movement of the guide pad 104 with respect to the unison ring 102 causes particles which have penetrated the contact zone to migrate across the contact zone. As the particles move across the contact zone they wear the guide pad 104 and the unison ring 102. The particles continue to move across the contact surface until they enter one of the grooves 112 or move outside the contact zone. Those particles which enter the grooves 112 are entrained in the flow through the grooves 112 and are expelled from the contact zone. Removal of particles from the contact zone reduces wear of the guide pad 104 and the unison ring 102.
The embodiment of
The embodiment of
The low friction strip 10, the top layers 304, 404 and wear indicator layers 306, 406, 408 of the low friction strips 302, 402, and the guide pads 104 may be made from any suitable low friction material that can withstand the ambient conditions and contact pressures which prevail in use. Suitable materials comprise polymer materials such as aromatic polyimides capable of withstanding elevated temperatures, for example temperatures in excess of 200° C., and possibly 260° C. A suitable material is that available under the name Vespel®.
It will be appreciated that the present invention is not limited to use with the embodiments described above, but can be used in other applications in which debris enters or is generated within a contact zone between two surfaces.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0911459.6 | Jul 2009 | GB | national |
