GB2406532A

GB2406532A - An apparatus for forming a compressively stressed layer in a root of a blade

Info

Publication number: GB2406532A
Application number: GB0323234A
Authority: GB
Inventors: Richard Patrick George Coghlan
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2003-10-03
Filing date: 2003-10-03
Publication date: 2005-04-06
Also published as: GB0323234D0

Abstract

An apparatus 30 for forming a compressively stressed layer in a root 14 of a blade 10 of a gas turbine engine. The apparatus 30 comprising: a body 32 having a channel 34 for receiving the root 14 and defining at least one constriction 40 of the channel 34, wherein movement of the root 14 through the channel 34 results in the temporary compression of at least a portion of the root 14 by the constriction 40.

Description

AN APPARATUS FOR FORMING A COMPRESSIVELY STRESSED LAYER

IN A ROOT OF A BLADE

Embodiments of the present invention relate to an apparatus for forming a compressively stressed layer in a root of a blade. In particular, they relate to an apparatus for forming a compressively stressed layer in a root of a blade of a gas turbine engine.

Blades are used in several compartments of a gas turbine engine (for example, the turbine assembly) to assist the passage of gas. They generally comprise at least an aerofoil and a root. The root is held in a disc groove of the gas turbine engine. Oxidation of the surface of the root may occur and get rubbed off if the root moves within the groove.

The oxide may remain within the groove, trapped between the blade root and disc groove contact faces, where it promotes the formation of cracks in the surface of the root and may reduce the operational life time of a blade. This is known as 'frettage'.

One method currently employed to reduce the effects of 'frettage' is 'shot peening'.

In this process, metallic balls (typically 0.25mm diameter), are shot at the root thereby producing compressive stresses at the surface of the root. This helps to reduce the formation of cracks. However, the depth of the compressively stressed layer is small, typically 0.15mm. This is often insufficient in preventing the formation of cracks, if there is significant vibration of the root.

It would be desirable to provide an alternative method of forming a compressively stressed layer in a root of a blade of a gas turbine engine.

According to one aspect of the present invention there is provided an apparatus for forming a compressively stressed layer in a root of a blade of a gas turbine engine, said apparatus comprising: a body having a channel for receiving the root and defining at least - 2 one constriction of the channel, wherein movement of the root through the channel results in the temporary compression of at least a portion of the root by the constriction.

The root may be moved through the channel by any suitable means and may be moved through the channel by a mechanical ram.

The root may be compressed when it is cold. The root is cold when there is no or insignificant re-crystallization or creep of the material of the root.

The, at least one, constriction may comprise at least one die for temporarily compressing at least a portion of the root. The, at least one constriction, may comprise a first die for compressing a first side wall portion of the root, a second die for compressing a second side wall portion of the root and a third die for compressing a base portion of the root. The advantage provided by this geometry of dies allows the regions of plastic flow in the blade root to be limited and controlled in extent, avoiding a through-thickness shearing effect, which would not result in compressive residual stress, and avoiding an undesirable bulk extrusion effect, which would change the cross sectional area of the root.

When the root leaves the channel, it may return, at least partially, to its prior shape This is known as 'spring back'. Some elastic relaxation of the deformation imparted by the dies may be expected and allowance for this spring back may be made in the shape of the dies, so that the deformed root may match the geometry of the pre-existing engine grooves.

The third die may be curved in a direction substantially perpendicular to the direction of movement of the root and may have a width that is less than the width of the base portion of the root. This may help to reduce the possibility of through-thickness shearing of the root by the first die and the second die. - 3

The first die, the second die and the third die may comprise sloped portions.

Therefore, when the root is passed through the channel, it may not be cut by contact with the dies.

The compressively stressed layer may have a depth several times deeper than that achievable by shot-peening on a given component. The compressively stressed layer may have a depth greater than 1mm. To reduce the effects of frettage, it is desirable to have a layer of compressive stress at the surface of the root, having a depth greater than 1 mm.

The channel may be curved to accept a curved root. This feature may allow for a large number of different shaped roots to be compressed. Spacer blocks may be inserted between a ram and the root to allow for movement of the root through the channel. As a result, the root may be moved through the channel in a series of steps.

A lubricator may be inserted into the channel to aid formation of the compressively stressed layer.

At least one cutter may be used to remove material from the root. The, at least one cutter, may be located within the channel. The, at least one cutter, may be cutting teeth.

The cutting teeth may be used to remove material from old or used blade roots to shape the root prior to compression.

According to another aspect of the present invention there is provided a method for forming a compressively stressed layer in a root of a blade of a gas turbine engine, said method comprising the steps of: a) providing a body having a channel for receiving the root and defining at least one constriction of the channel, and b) temporarily compressing at least a portion of the root by moving it through the channel past the constriction.

This method may be suitable for replacing 'shot peening'. It may further reduce the cost in forming a compressively stressed layer in a root. The cost may also be reduced due to the deletion of the use of advanced antifrettage coating systems, such as plasma sprayed Copper-Nickel-lndium. The operational lifetime of a root may be increased.

For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings in which: Fig. 1 illustrates, in sectional front view, a dovetail root of a blade of a gas turbine engine; Fig.2 illustrates, in perspective view, one embodiment of an apparatus for forming a compressively stressed layer in a root of a blade; Fig.3 illustrates, in sectional front view, the apparatus illustrated in Fig. 2; and Fig. 4 illustrates, in plan view, a second embodiment of an apparatus for forming a compressively stressed layer in a root of a blade.

The figures illustrate an apparatus 30 for forming a compressively stressed layer in a root 14 of a blade 10 of a gas turbine engine. The apparatus 30 comprises: a body 32 having a channel 34 for receiving the root 14 and defining at least one constriction 40 of the channel 34, wherein movement of the root 14 through the channel 34 results in the temporary compression of at least a portion of the root 14 by the constriction 40.

In more detail, Fig.1 illustrates, in sectional front view, a dovetail root 14 of a blade of a gas turbine engine. The dovetail root 14 may be a root 14 for any blade 10 within a gas turbine engine and may be a root for a fan blade or a compressor blade. The blade 10 comprises an aerofoil 12 and a root 14. The root 14 includes blade shank portions 16a and 16b, fillet radii portions 18a and 18b, dovetail flank portions 20a and 20b and a base portion 22.

The dovetail root has a wide base portion 22, that narrows through the opposing dovetail flank portions 20a and 20b towards the fillet radii portions 18a and 18b. The fillet - 5 radii portions 18a and 18b connect the dovetail root 14 to the aerofoil 12 through the blade shank portions 16a and 16b.

The dovetail flanks 20a and 20b are generally straight and converge from the base portion 22 to the curved fillet radii 18a and 18b. The base portion 22 is generally flat and substantially perpendicular to the blade shank portions 16a and 16b. An angle is defined by the angle formed between tangents of the dovetail flanks 20a and 20b. may be any suitable angle and may, for example, be equal to 90 .

Fig. 2 illustrates one embodiment of an apparatus 30 for forming a compressively stressed layer in a root 14 of a blade 10 of a gas turbine engine. The apparatus comprises a body 32 having an upper surface 33 in which a channel 34 is defined. The channel 34 extends between opposing sides of the body 32, substantially perpendicular to the upper surface 33 of the body 32. The channel 34 comprises an input opening 36 in one side of the body 32 for receiving a root 14, a constriction 40 within the channel 34 for temporarily compressing at least a portion of a root 14 when it is moved through the channel 34 and an output opening 38 in another side wall of the body for releasing the root 14. The channel 34 has a dovetail shaped profile when viewed from the front (see Fig. 3) similar to the dovetail shaped profile of the dovetail root 14.

The constriction 40 within the channel 34 is, in this example, formed from a first die 42, a second die 44 and a third die 46. The dies 42, 44, 46 are situated adjacent the output opening 38 of the channel 34. The dies 42, 44, 46, in this embodiment, extend from the output opening 38 into the channel 34 for one quarter of the length of the channel 34.

The dies 42, 44, 46 comprise sloping portions 42a, 44a, 46a respectively such that they plastically deform but do not cut the root 14, as it is moved through the channel 34. The sloping portions 42a, 44a, 46a may, for example, be ramps.

Fig. 3 illustrates a sectional front view of the apparatus 30 illustrated in Fig. 2.

Where the component is the same as that illustrated in Fig.2, the same reference numeral - 6 has been given to that component. In more detail, Fig. 3 illustrates that the angle formed between tangents to the first die 42 and the second die 44 is denoted by 0, which may be less than to allow for spring back. may, for example, be less than 90 , if equals 90 .

Reference will now be made to Figs. 1, 2 and 3 to describe the process of forming a compressively stressed layer in a root 14 of a blade 10 of a gas turbine engine.

A dovetail root 14 is received by the input opening 36 of the channel 34. The channel 34 has approximately the same dovetail shape as the dovetail root 14, thereby allowing the root 14 to pass through the channel. The dovetail root 14 may be moved through the channel 34 by any suitable means and possibly by mechanical means, such as a mechanised ram (not illustrated in the Figs). The dovetail root 14 is passed through the channel 34 in a cold state. A cold state is one in which there is no or insignificant re- crystallisation or creep of the root 14.

The first die 42 is positioned and sized so that it temporarily compresses, at least, the fillet radii portion 18a and the second die 44 is positioned and sized so that it temporarily compresses, at least, the fillet radii portion 18b. The third die 46 is positioned and sized so that it temporarily compresses the base portion 22. The width of the third die 46 is less than the width of the base portion 22. The third die 46 is curved in a direction substantially perpendicular to the direction of movement of the dovetail blade such that it has a greater thickness in the middle than at the edges. The third die 46 is curved so that the root 14 is plastically deformed such that flanks 20a and 20b are bent downwards rather than sheared downwards by the first die 42 and the second die 44. As the root 14 is moved through the channel 34, it is increasingly compressed by the sloped portions 42a, 44a, 46a of the dies 42, 44 and 46 respectively, whose relief increases gradually to form the constriction 40.

The angle formed between the tangents of the dovetail flank portions 20a and 20b is changed from to (3 by dies 42, 44 and 46. When the root 14 has passed through the - 7 channel 34, the intersecting angle of the dovetail flanks 20 changes from to , or close to +. This is known as 'spring back' in the art. may be calculated so that a desired value for is obtained after 'spring back'. This aids location of the root 14 in a groove within a gas turbine engine. The plastic deformation of the root produces a layer of compressive stress, having a depth several times greater than that achievable by shot peening. The layer of compressive stress may be typically in the range of 1 mm to 3mm from the surface.

The root 14 comprises a region of tensile stress, underneath the layer of compressive stress so as to balance the forces within the root 14.

A lubricant is provided within the channel 34 while the root 14 is being moved through. Such lubricants are well known within the art and shall not be discussed in detail.

One example of a suitable lubricant is Molybdenum Disulphide, MoS2.

Fig. 4 illustrates an alternative embodiment of the apparatus 30. Where a component is the same or has a similar function as that illustrated in Figs. 2 & 3, the same reference numeral has been given to that component. The embodiment illustrated in Fig.4 differs from the embodiment illustrated in Figs. 2 & 3 in that it further comprises a first side cutter 48, a second base cutter 50 and a third side cutter 52. The cutters 48, 50 and 52 remove material from the root 14. The cutters 48, 50 and 52 are, in this embodiment, cutting teeth 48, 50 and 52.

The cutting teeth 48, 50 and 52 are each situated prior to each of the respective dies 42, 44 and 46 within the channel 34. Therefore, if a root 14 is passed through the channel 34, material may be removed from the root 14 prior to its plastic deformation. The cutting teeth 48, 50 and 52 also help to ensure that the angle between the dovetail flanks is after plastic deformation has taken place. This embodiment may be used for recycling old roots that do not have the correct dimensions prior to plastic deformation. - 8

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example the channel 34 may straight or may be curved to receive a curved root 14. If the ram is of a linear motion type, a series of curved spacer blocks may be placed between the ram and the root as it progresses through the curved channel. Therefore, the root 14 is pushed through in a series of steps. Additionally, any number of dies or cutting teeth may be used within the channel 34.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of an patentable feature of combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon. - 9 -

Claims

1. An apparatus for forming a compressively stressed layer in a root of a blade of a gas turbine engine, said apparatus comprising: a body having a channel for receiving the root and defining at least one constriction of the channel, wherein movement of the root through the channel results in the temporary compression of at least a portion of the root by the constriction.

2. An apparatus as claimed in claim 1, wherein the, at least one constriction is at least partially defined by at least one die for temporarily compressing a portion of the root.

3. An apparatus as claimed in any one of claims 1 or 2, wherein the, at least one constriction within the channel is at least partially defined by a first die for compressing a first side wall portion of the root, a second die for compressing a second side wall portion of the root and a third die for compressing a base portion of the root.

4. An apparatus as claimed in claim 3, wherein the angle formed between tangents to the first die and the second die is less than 90 .

5. An apparatus as claimed in any one of claims 3 or 4, wherein the third die is curved.

6. An apparatus as claimed in claim 3, 4 or 5, wherein the width of the third die is less than the width of the base portion of the root.

7. An apparatus as claimed in any one of claims 3 to 6, wherein the first die, the second die and the third die are sloped in the direction of movement of the root.

8. An apparatus as claimed in any one of the preceding claims, wherein the compressively stressed layer has a depth greater than 1 mm. -

9. An apparatus as claimed in any one of the preceding claims, wherein the channel is curved to accept a curved root.

10. An apparatus as claimed in any one of the preceding claims, wherein, in use, a lubricator is inserted into the channel to aid formation of the compressively stressed layer.

11. An apparatus as claimed in any one of the preceding claims, further comprising at least one cutter for removing material from the root.

12. A method for forming a compressively stressed layer in a root of a blade of a gas turbine engine, said method comprising the steps of: a) providing a body having a channel for receiving the root and defining at least one constriction of the channel, and b) temporarily compressing at least a portion of the root by moving it through the channel past the constriction.

13. A method as claimed in claim 12, wherein the root is cold.

14. An apparatus substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

15. A method substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

16. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as the preceding claims.