SHAFT MONITORING SYSTEM

Abstract

A monitoring system for monitoring the axial position of a rotating shaft includes a phonic wheel which is mounted coaxially to the shaft for rotation therewith. The phonic wheel includes a circumferential row of teeth. The system further includes a sensor configured to detect the passage of the row of teeth by generating an alternating measurement signal. The teeth are configured around the row such that the teeth contribute a component to the alternating measurement signal. The teeth are tapered in an axial direction of the wheel and the sensor is positioned relative to the phonic wheel such that, in use, axial displacement of the shaft causes the signal generated by the sensor to vary either or both of the amplitude and the pulse width of the component, whereby the axial position of the shaft can be monitored.

Claims

1. A monitoring system for monitoring the axial position of a rotating shaft, the system comprising: a phonic wheel which is mounted coaxially to the shaft for rotation therewith, the phonic wheel comprising a circumferential row of teeth; and a sensor configured to detect the passage of the row of teeth by generating an alternating measurement signal; wherein the teeth are configured around the row such that the teeth contribute a component to the alternating measurement signal; and, wherein the teeth are tapered in an axial direction of the wheel such that the relative proportions of the teeth vary with axial distance across the phonic wheel, and the sensor is positioned relative to the phonic wheel such that, in use, axial displacement of the shaft causes the signal generated by the sensor to vary either or both of an amplitude and a pulse width of the component due to the variation in relative proportions, whereby the axial position of the shaft can be monitored.

2. The monitoring system as claimed in claim 1, wherein the teeth taper smoothly in the axial direction of the wheel.

3. The monitoring system as claimed in claim 2, wherein the teeth are tapered in height such that they are radially higher at one side of the phonic wheel than at an axially spaced other side of the phonic wheel.

4. The monitoring system as claimed in claim 2, wherein the teeth are tapered in width such that they are circumferentially wider at one side of the phonic wheel than at an axially spaced other side of the phonic wheel.

5. The monitoring system as claimed in claim 2, wherein the amount of taper varies linearly with axial distance across the phonic wheel.

6. The monitoring system as claimed in claim 2, wherein the amount of taper varies non-linearly with axial distance across the phonic wheel.

7. The monitoring system as claimed in claim 1, wherein the sensor is an electromagnetic or magnetic sensor that detects a varying distance caused by the passage of the row of teeth.

8. The monitoring system as claimed in claim 7, wherein the sensor detects absolute magnetic field strength between the sensor and the tooth, or wherein the sensor is a laser sensor that detects distance between the sensor and the teeth.

9. The monitoring system as claimed in claim 1, wherein in a normal operating mode of the shaft, the amplitude of the component is substantially equal.

10. The monitoring system as claimed in claim 1, wherein the circumferential row of teeth comprises one or more first teeth and one or more second teeth.

11. The monitoring system as claimed in claim 10, wherein in a normal operating mode of the shaft, either or both of the amplitude and the pulse width of the first teeth is substantially different to either or both of the amplitude and the pulse width of the second teeth.

12. The monitoring system as claimed in claim 1, wherein the circumferential row of teeth is interrupted by a special tooth from which the sensor generates a once-per-revolution signal, the special tooth being configured such that either or both of the amplitude and pulse width of the once-per-revolution signal is different from either or both of the amplitude and pulse width of the measurement signals of the further teeth over the entire axial range of the row.

13. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; and a monitoring system as claimed in claim 1 for monitoring the axial position of the core shaft, the phonic wheel being mounted coaxially to the core shaft for rotation therewith.

14. The gas turbine engine as claimed in claim 13, wherein: the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft; the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

15. The gas turbine engine as claimed in claim 13, further comprising: an engine electronic controller which is operatively connected to the monitoring system to receive the overall signal and is configured to monitor therefrom the axial position of the shaft to which the phonic wheel is mounted.

16. The gas turbine engine as claimed in claim 15, wherein the engine electronic controller converts the signal into a shaft speed based on the frequency of the signal.

17. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and a gearbox that receives an input from the core shaft and outputs drive to the fan via an output shaft so as to drive the fan at a lower rotational speed than the core shaft; a thrust shaft that extends through the gearbox to connect the fan to an axial location bearing mounted on the core shaft, thereby relieving the output shaft of responsibility for axially locating the fan relative to the core shaft; and a monitoring system as claimed in claim 1 for monitoring the axial position of the thrust shaft, the phonic wheel being mounted coaxially to the thrust shaft for rotation therewith.

18. The gas turbine engine as claimed in claim 17, wherein: the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft; the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

19. The gas turbine engine as claimed in claim 17, further comprising: an engine electronic controller which is operatively connected to the monitoring system to receive the overall signal and is configured to monitor therefrom the axial position of the shaft to which the phonic wheel is mounted.

20. The gas turbine engine as claimed in claim 19, wherein the engine electronic controller converts the signal into a shaft speed based on the frequency of the signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0057] FIG. 1 is a sectional side view of a gas turbine engine;

[0058] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0059] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0060] FIG. 4 is a perspective view of an example of a phonic wheel of a shaft monitoring system;

[0061] FIG. 5 is a perspective view of a further example of a phonic wheel of a shaft monitoring system;

[0062] FIG. 6 shows schematically different views A-C and G on the circumference, and views E-F on a radial perspective, of a phonic wheel of a shaft monitoring system;

[0063] FIG. 7 shows an output signal generated by a probe of the monitoring system when located at nominal axial position;

[0064] FIG. 8 shows an output signal generated by the probe of the monitoring system when located at nominal axial position;

[0065] FIG. 9 shows schematically the front end of an interconnecting shaft joining the low pressure turbine to the fan of a non-geared turbofan; and,

[0066] FIG. 10 shows schematically a conventional phonic wheel having a circumferential row of teeth, and a variable reluctance sensor.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0067] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0068] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0069] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0070] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0071] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0072] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0073] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0074] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0075] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0076] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0077] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0078] In the exemplary arrangement for a geared fan gas turbine engine 10 as shown in FIG. 2, the output shaft 50 from the epicyclic gear arrangement 30 is radially located at each end by sets of roller bearings 52. These allow the shaft to transmit torque to the fan 23, but not provide significant axial retention functionality. To axially retain the output shaft 50 and the fan 23, a separate thrust shaft 54 extends from a set of ball bearings 56 attached to the interconnecting shaft 26, through the centre of the sun gear 28 to join to the output shaft 50. Thus torque transmission and axial retention responsibilities are split between the output shaft 50 and the thrust shaft 54.

[0079] Failure of the thrust shaft 54 or the ball bearings 56 can endanger the engine, and thus it is desirable to have early detection of any axial displacement of the shaft 54. Accordingly, the engine also has a monitoring system for monitoring the axial position of the shaft. This system comprises a phonic wheel 56. Conveniently this can be mounted at the front of the output shaft 50 coaxially with the thrust shaft 54. At this location it co-rotates with the entire assembly of the fan 23, output shaft 50 and thrust shaft 54.

[0080] FIGS. 4 and 5 show respective examples of a phonic wheel 56 of a shaft monitoring system, each of the phonic wheels comprising a circumferential row of respective teeth. FIG. 4 shows an example of a phonic wheel 56 comprising a circumferential row of teeth which are smoothly tapered in the axial direction, so as to vary in relative proportions. In the particular example of FIG. 4, the circumferential row of teeth are smoothly tapered in the axial direction, so as to vary in radial thickness. A schematic view of the teeth of FIG. 4 is shown in FIG. 6, Example (A). FIG. 5 shows an example of a phonic wheel 56 comprising a circumferential row of teeth which are smoothly tapered in the axial direction, so as to vary in relative proportions. In the particular example of FIG. 5, the circumferential row of teeth are smoothly tapered in the axial direction, so as to vary in circumferential thickness. A schematic view of the teeth of FIG. 5 is shown in FIG. 6, Example (E). In particular, the phonic wheel 56 comprises a circumferential row of teeth whose passage is detected by a change in the signal of a variable reluctance or distance speed probe mounted to a stationary structure of the engine. In particular, passage of such teeth may be detected by a change in one or more of the amplitude, duration of peak, or pulse width, in the signal of a variable reluctance or distance speed probe.

[0081] FIG. 6 shows schematically a view on the circumference of the wheel 56 of (A) a first example of a tooth from the circumferential row of teeth, (B) a second example of a tooth from the circumferential row of teeth, and (C) a third example of a tooth from the circumferential row of teeth. Furthermore, FIG. 6 shows schematically a radial plan view of: (D) a first example of a tooth from the circumferential row of teeth, (E) a second example of a tooth from the circumferential row of teeth, and (F) a third example of a tooth from the circumferential row of teeth. Finally, FIG. 6 shows schematically, in (G), a view on the circumference of the wheel of the wheel and the speed probe 60 when the wheel 56 is located at a first axial position 57. As shown in the first example (A), the teeth are tapered in the axial direction such that they reduce in radial height from the axially front side to the axially rear side of the wheel. As shown in the second example (B), the teeth are oppositely tapered in the axial direction such that they increase in radial height from the axially front side to the axially rear side of the wheel. In the first and second examples (A and B), the teeth are not tapered in the circumferential direction. In example (D), the teeth do not vary in width from the front to the rear side of the wheel.

[0082] As shown in the third example (C), unlike the examples shown in (A) and (B), the teeth are not tapered in the axial direction. Thus, they do not vary in height from the front to the rear sides of the wheel. In such examples, the teeth are tapered in the circumferential direction, as shown in examples (E) and (F). As shown example (E), the teeth are tapered in the circumferential direction such that they reduce in circumferential width from the axially front side to the axially rear side of the wheel. As shown in example (F), the teeth are oppositely tapered in the circumferential direction such that they increase in circumferential width from the axially front side to the axially rear side of the wheel.

[0083] It will be appreciated that in further examples (not shown), the teeth may be tapered in both of the radial and the circumferential direction. Thus, in some examples, the teeth may vary in both radial height and circumferential width from the front to the rear sides of the wheel. It will be appreciated that in such examples, any such permutation or combination of circumferential width and radial height, selected from the examples shown in (A-F), may be combined to configure a tooth which is tapered in both the radial and the circumferential direction. In such examples, the teeth are configured around the row such that the probe passes over the teeth to provide a component of the signal. In this way, the teeth contribute a component to the total alternating measurement signal generated by the probe.

[0084] In use, an axial shift of the shaft 54 causes the probe 60 to monitor a second section of the tooth, termed the second axial position 58. Examples (A) to (F) show some examples of the first axial position 57 and the second axial position 58 on the wheel 56. Thus, following an axial shift of the shaft, it will be appreciated that the probe 60 monitors the tooth at a second axial position 58, rather than the first axial position 57. In some examples, such as the example shown in Examples (A) to (F), the first axial position 57 of the wheel 56 may be at a midpoint position of the wheel 56 (i.e. between the front side and the axially rear side of the wheel 56). In further examples, the first axial position 57 of the wheel 56 may be axially displaced from the midpoint position of the wheel 56. In some examples, such as those shown in Examples (A) to (F), the second axial position 58 may be axially displaced from the first axial position 57 towards the axially rear side of the wheel 56. In some examples (not shown), the second axial position 58 may be axially displaced from the first axial position 57 towards the axially front side of the wheel 56. In each case however, it will be appreciated that the second axial position 58 is axially displaced from the first axial position 57. The effect of such an axial shift, such that the probe 60 monitors the tooth at a second axial position 58 rather than the first axial position 57, is explained in further detail below.

[0085] In further examples, the circumferential row of teeth may comprise a first tooth comprising a first profile, and a second tooth comprising a second profile which is different to the first profile. Thus, in some examples, the circumferential row of teeth may comprise alternating teeth. In further examples, the circumferential row of teeth may comprise a third or further tooth comprising a third or further profile. In some examples, the first and second (or further) teeth may alternate around the row such that the probe passes alternatingly over the first and second (or further) teeth. In this way, the first and second (or further) teeth contribute respective first and second (or further) components to the total alternating measurement signal generated by the probe. In yet further examples, the circumferential row of teeth may comprise a further tooth of unique axial and/or circumferential proportions (not shown) for the purpose of identifying a complete revolution of the wheel. Thus, the unique tooth may be configured such that one or more of the amplitude, duration of peak, or pulse width of the once-per-revolution signal is different from one or more of the amplitude, duration of peak, or pulse width of the alternating measurement signals of the remaining teeth over the entire axial range of the row.

[0086] FIG. 7 shows a standard voltage signal generated by the variable reluctance speed probe 60 when monitoring one or more teeth at a first axial position 57 of the wheel 56. In the specific example shown, the teeth comprise the axial profile of the type shown in example (C) of FIG. 6, and the circumferential profile of the type shown in example (D). Thus, there will be no observable difference in voltage signal when monitoring one or more teeth at the first axial position 57 relative to a second axially displaced position 58 of the wheel 56.

[0087] In further examples, the teeth may comprise either of the axial profiles shown in example (A) or (B) of FIG. 6, and the circumferential/radial profile of the type shown in examples (D), (E), or (F). In particular, FIG. 7 shows a schematic example of a first voltage signal 71 and a second voltage signal 73, both of which are overlaid over the standard voltage signal and indicate the relative amplitude of the signal at their respective axial positions (57, 58). The voltage signal contains a negative peak when the tooth enters the probe measurement area, and a positive peak when the tooth leaves the probe measurement area, forming the pulse width of the signal. The pulse width may also be known as the phase width. The amplitude may thus depend on several factors including, for example, the proximity of the probe 60 relative to the tooth of the wheel 56.

[0088] In the example shown, the axial profile of the teeth varies, i.e. in either or both of radial height or circumferential width, as shown in examples (A) to (F) and described in relation to FIG. 6. In the specific examples shown in (A) and (B) of FIG. 6, following an axial shift of the shaft 54, the probe 60 monitors the teeth at a second axial position 58 rather than the first axial position 57, this causes the probe 60 to monitor a section of the teeth at a second axial position 58 which is axially displaced from the first axial position 57. Thus, by the probe 60 monitoring the tooth a second axial position 58 rather than the first axial position 57, the airgap between the probe 60 and the teeth is modified. In the specific examples shown in examples (A) and (B) of FIG. 6, this causes the amplitude component of the signal at the second axial position 58 (i.e. at the second axial position 58 of the wheel 56) to have a relatively different amplitude component to that of the first axial position 57 (i.e. at the first axial position 57 of the wheel 56).

[0089] In the specific example shown in FIG. 7, in order to provide the first voltage signal 71 and the second voltage signal 73, the teeth comprise, for example, the axial profile of the type shown in example (A) of FIG. 6, and the circumferential profile of the type shown in example (D) of FIG. 6, as shown schematically in FIG. 4. The first voltage signal 71 is generated by the probe 60 when monitoring the teeth at the first axial position 57 of the wheel 56. The second voltage signal 73 is generated by the probe 60 when monitoring the teeth at the second axial position 58 of the wheel 56. The first voltage signal 71 and second voltage signal 73 therefore show a relative change in amplitude with varying axial position of the wheel.

[0090] As shown in FIG. 7, such a shift from the first axial position 57 towards the second axial position 58, in the manner shown in example (A) of FIG. 6, results in the airgap increasing, and the amplitude decreasing from the first voltage signal 71 to the second voltage signal 73. In further examples, a shift from the first axial position 57 towards the second axial position 58, in the manner shown in example (B) of FIG. 6, results in the airgap decreasing, and the amplitude increasing accordingly. Thus, by means of a shift from the first axial position 57 towards the second axial position 58, a signal of modified amplitude is generated, which indicates that there has been a displacement from the first axial position 57 of the wheel 56. With tooth profile definition, the magnitude of the axial shift relative to the speed probe 60 can be determined. In this way, an identifiable change in one or more components of the signal can be monitored and used to determine the magnitude of the displacement. Thus, the magnitude of the axial shift of the phonic wheel 56, relative to the speed probe 60, can be determined. Thus, the axial position of the phonic wheel 56 can be determined.

[0091] FIG. 8 shows the standard voltage signal previously shown in FIG. 7, and generated by the variable reluctance speed probe 60 when monitoring one or more teeth at a first axial position 57 of the wheel 56. As described in relation to FIG. 7, the teeth contributing towards the standard signal comprise the axial profile of the type shown in example (C) of FIG. 6, and the circumferential profile of the type shown in example (D). Thus, there will be no observable difference in voltage signal when monitoring one or more teeth at the first axial position 57 relative to a second axially displaced position 58 of the wheel 56.

[0092] In further examples, the teeth may comprise either of the axial profiles shown in example (A), (B), or (C) of FIG. 6, and the circumferential/radial profile of the type shown in examples (E), or (F). In particular, FIG. 7 shows a schematic example of a first voltage signal 71 and a second voltage signal 73, both of which are overlaid over the standard voltage signal and indicate the relative amplitude of the signal at their respective axial positions (57, 58). The voltage signal contains a negative peak when the tooth enters the probe measurement area, and a positive peak when the tooth leaves the probe measurement area, forming the pulse width of the signal. The pulse width may also be known as the phase width. The pulse width may thus depend on several factors including, for example, the specific width of the tooth at the point of measurementi.e. at the first axial position 57 and the second axial position 58.

[0093] In the example shown, the axial profile of the teeth varies, i.e. in either or both of radial height or circumferential width, as shown in examples (A) to (F) and described in relation to FIG. 6. In the specific examples shown in (E) and (F) of FIG. 6, following an axial shift of the shaft 54, the probe 60 monitors the teeth at a second axial position 58 rather than the first axial position 57, this causes the probe 60 to monitor a section of the teeth at a second axial position 58 which is axially displaced from the first axial position 57. Thus, by the probe 60 monitoring the tooth a second axial position 58 rather than the first axial position 57, the specific width of the tooth is varied, providing a corresponding change in pulse width. In the specific examples shown in examples (E) and (F) of FIG. 6, this causes the pulse width component of the signal at the second axial position 58 (i.e. at the second axial position 58 of the wheel 56) to have a relatively different pulse width component to that of the first axial position 57 (i.e. at the first axial position 57 of the wheel 56).

[0094] In the specific example shown in FIG. 8, in order to provide the first voltage signal 81 and the second voltage signal 83, the teeth comprise, for example, the axial profile of the type shown in example (C) of FIG. 6, and the circumferential profile of the type shown in example (E) of FIG. 6, as shown schematically in FIG. 5. The first voltage signal 81 is generated by the probe 60 when monitoring the teeth at the first axial position 57 of the wheel 56. The second voltage signal 83 is generated by the probe 60 when monitoring the teeth at the second axial position 58 of the wheel 56. The first voltage signal 81 and second voltage signal 83 therefore show a relative change in pulse width with varying axial position of the wheel.

[0095] As shown in FIG. 8, such a shift from the first axial position 57 towards the second axial position 58, in the manner shown in example (E) of FIG. 6, results in the specific width of the tooth decreasing, and the pulse width decreasing from the first voltage signal 81 to the second voltage signal 83. In further examples, a shift from the first axial position 57 towards the second axial position 58, in the manner shown in example (F) of FIG. 6, results in the results in the specific width of the tooth increasing, and the pulse width increasing accordingly. Thus, by means of a shift from the first axial position 57 towards the second axial position 58, a signal of modified pulse width is generated, which indicates that there has been a displacement from the first axial position 57 of the wheel 56. With tooth profile definition, the magnitude of the axial shift relative to the speed probe 60 can be determined. In this way, an identifiable change in one or more components of the signal can be monitored and used to determine the magnitude of the displacement. Thus, the magnitude of the axial shift of the phonic wheel 56, relative to the speed probe 60, can be determined. Thus, the axial position of the phonic wheel 56 can be determined.

[0096] In further examples, it will be appreciated that the effects described in relation to FIG. 7 and FIG. 8 may be combined according to any such specific tooth geometry used (i.e. combining either of examples (A) and (B) with any one of examples (D), (E), or (F); or example (C) with either of examples (E) or (F)). Thus, such combinations ensure that an aspect of the profile of the teeth is varied over at least a portion of the axial length of the teeth, at least between the first axial position 57 and the second axial position 58.

[0097] In the specific examples highlighted in FIGS. 7 and 8, the change in output signals generated by the probe 60 are as indicated as arrows in FIG. 7 or 8, with the probe being located, during normal use, at the first axial position 57 of the wheel 56. In practice, assembly tolerances can cause the probe to be located off-centre. However, the high/low amplitude ratio of the alternating amplitude signal which such an off-centre location produces can be monitored and normalised. Significant change in the ratio can still be detected and used to indicate axial displacement of the probe relative to the wheel 56.

[0098] The signal is typically received by an engine electronic controller (EEC) of the engine, which compares the heights of the alternating amplitudes and/or the pulse widths to determine whether an axial shift of the thrust shaft 54 has occurred, and the magnitude of the shift. The detection of such an event by the EEC can be used to alert the crew of thrust shaft failure or to initiate an automatic response so that appropriate engine management actions are taken. The EEC can also use the signal as a measure of the rotational speed of the assembly of the fan 23, output shaft 50 and shaft 54.

[0099] In FIG. 6 (A-G), the teeth are shown with linearly varying specific heights or specific widths with axial distance. However, non-linear variation is also possible. For example, an exponentially changing specific height and/or specific width can allow the displacement detection sensitivity of the system varies with axial position. It will be appreciated that the heights referred to in the described examples relate, when installed within the engine, the radial height of the teeth relative to the rotational axis 9 of the gas turbine engine 10.

[0100] As previously described, the circumferential row of teeth may be interrupted by a special tooth from which the sensor generates a once-per-revolution signal, which can be used e.g. for engine health monitoring purposes. For all positions of the probe 60 relative to the wheel 56, this signal should have either or both of an amplitude or pulse width that is different from those of the amplitudes of the alternating measurement signals of the components produced by the teeth discussed above. For example the special tooth can be a short tooth having an inverted V shaped height profile. Thus the peak of the inverted V can coincide with the midpoint position of the wheel where the teeth are of equal height, but the peak being shorter than this equal height, and the short tooth can then reduce in height to both the axially front and rear sides of the wheel, the height always being less than that of the lower portion of the teeth.

[0101] Although described above for monitoring the axial position of the assembly of the fan 23, output shaft 50 and thrust shaft 54, the monitoring system has wider applicability in gas turbine engines, and is not limited to use in geared fan gas turbine engine. For example, it can be used to monitoring the axial position of any interconnecting shaft by which a turbine drives a compressor (e.g. interconnecting shafts 26, 27 in FIG. 1). Typically these interconnecting shafts have a set of ball bearings and a set of roller bearings. FIG. 9 shows schematically the front end of the interconnecting shaft 62 joining the low pressure turbine to the fan of a non-geared turbofan. A front set of roller bearings 64 radially locates the shaft and a rear set of ball bearings (off to the right and thus not shown in FIG. 9) axially locates the shaft. The phonic wheel 56 of the monitoring system is mounted coaxially to the shaft from an extension to the inner case of the roller bearings, and the reluctance probe 60 is mounted to a stationary structure of the engine above the wheel 56. The thrust load on the ball bearings can be rearwards or forwards, depending on engine design, and failure or wear of the bearings causes the shaft 62 to move in the direction of that thrust. As a result, the change in the output signal of the reluctance probe 60 is detected by the engine's EEC.

[0102] A suitably adapted monitoring system can also be used to detect failure of the interconnecting shaft 62, although that may not be necessary if the engine has other systems for detecting loss of torque-transmission capability in the shaft.

[0103] Although described above in relation to monitoring a thrust shaft (FIGS. 1 to 8) and the fan/compressor end of an interconnecting shaft (FIG. 9), the monitoring system may also be used at other locations in the engine, e.g. for monitoring the axial position of an interconnecting shaft at its turbine section end.

[0104] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.