Apparatus is provided comprising first and second coaxial shafts, the first and second shafts extending along, and being rotatable about, a central axis. The first and second shafts are coupled together by a splined major coupling at a coupling region of the apparatus, which major coupling enables the shafts to co-rotate about the central axis. A torque element is connected to the first and second shafts, the torque element being operable to force the first and second shafts to relatively rotate about the central axis in order to apply a preload to the major coupling. The apparatus may be comprised in a gas turbine engine, whereupon the first shaft may be a fan shaft and the second shaft may be a LP turbine shaft.

The present invention relates to splined couplings, in particular, although not necessarily exclusively, splined couplings in gas turbine engines.

Splined couplings are routinely used to transmit torque between coaxial shafts in a variety of industrial applications. For example, in gas turbine engines, a splined coupling is provided between the low pressure (LP) turbine shaft and the fan shaft to transmit torque between the turbines and the compressors/fan. The splined coupling enables the shafts to co-rotate.

Splined couplings are often subject to a high degree of stress and are therefore features likely to limit the working lifetime of coupled shafts. To increase lifetime, it is known to increase the strength of the shafts and splined coupling. One way to increase strength is to increase the size of these components. However, as a result, the minimum achievable size of the coupled shafts can be significantly limited. In a gas turbine engine, the diameter of the LP turbine shaft is important as it can determine the minimum diameter of other components of the engine, such as the intermediate pressure (IP) turbine shaft (in 3-shaft applications), fan shaft, compressor bores, bearings and gears.

When a specific minimum diameter is an absolute requirement, it is known to alter the materials of the shafts and/or splined coupling to increase strength. However, such ‘more capable’ materials can be prohibitively expensive.

At its most general, the present invention provides apparatus comprising two shafts coupled together by a splined coupling so that the two shafts can co-rotate, and a means to preload the splined coupling to reduce the stress range on the coupling during rotation of the shafts in normal use. The apparatus may be comprised in, for example, a gas turbine engine, where the two shafts may be an LP turbine shaft and a fan shaft.

According to a first aspect of the present invention, there is provided first and second coaxial shafts, the first and second shafts extending along, and being rotatable about, a central axis,

the first and second shafts being coupled together by a splined major coupling at a coupling region of the apparatus, the major coupling enabling the shafts to co-rotate about the central axis,

a torque element connected to the first and second shafts via two splined minor couplings, the torque element being operable to force the first and second shafts to relatively rotate about the central axis in order to apply a preload to the major coupling,

wherein the two splined minor couplings are angled relative to one another.

Preferably, the splined major coupling comprises a helical angle about the shafts and the angle between two splined minor couplings is less than the helical angle.

By preloading the splined coupling, the stress range on the splined coupling during use may be lower, increasing the lifetime of the apparatus. To achieve a lower stress range, the preloading causes the minimum stress on the spline to be higher such that, when a maximum stress is applied to the splined coupling, the overall stress range experienced by the splined coupling is reduced.

Preferably, the preload is applied in the same direction as the load (force) applied to the coupling during normal use, i.e. normal co-rotation of the shafts, although the preload is of a lower magnitude. By pre-loading the splined coupling, the force on the coupling prior to the initiation of co-rotation may be higher meaning that, upon initiation of the co-rotation, a lower stress range is experienced by the components. If the apparatus is employed in an engine, e.g. a gas turbine engine, initiation of co-rotation of the first and second shafts may occur on engine start-up.

By applying the preload, for shafts having a given outer diameter, material and maximum operating torque, an increase in life may be obtained. Accordingly, the size of the splined coupling may be reduced, reducing the size of the apparatus, and/or ‘less capable’ material may be used, reducing manufacturing costs.

Preferably, the torque element is configured to force relative rotation of the shafts upon moving in a direction parallel to the central axis of the torque element (referred to hereinafter as the “axial direction”). To achieve this, the torque element may be further configured to rotate about the central axis as it moves in the axial direction. The torque element may be substantially annular, with inner and outer sides/surfaces, and may be rotatable about one of the shafts.

Preferably, at the coupling region, the first shaft has a central bore providing an opening that receives the second shaft, an outer surface of the second shaft being adjacent an inner surface of the first shaft. The splined coupling may be provided between a portion of the outer surface of the second shaft and the inner surface of the first shaft. The torque element may be provided between another portion of the outer surface of the second shaft and the inner surface of the first shaft.

Preferably the torque element is coupled on one side to one of the first and second shafts via a helical splined coupling, the splines of the helical splined coupling being arranged to twist about the central axis; and the torque element is coupled on the other side to the other of the first and second shafts via a straight-cut splined minor coupling, the splines of the straight-cut splined coupling extending substantially parallel to the central axis (or at least more parallel to the central axis than the helical splined coupling). The helical splined coupling and the straight-cut splined coupling are referred to hereinafter as “minor” couplings, to distinguish them from the splined coupling between the first and second shafts, which is referred to hereinafter as a “major” coupling. If a force is applied to the torque element in the axial direction, the torque element is guided in the axial direction along the splines of the minor couplings. However, due to the difference in orientation of the splines of the minor couplings, the torque element will rotate about the central axis, forcing the first and second shafts coupled either side to relatively rotate, resulting in the preload being applied to the major coupling.

Preferably the apparatus comprises a nut, e.g. a ring nut, engaged to the torque element, the nut being operable to move the torque element in the axial direction. The nut may be annular. Preferably, the nut is arranged to rotate about a screw-thread that twists about the central axis. The angle of the screw-thread may be adjusted to allow more precise control and/or achieve greater mechanical advantage when moving the nut, and thus the torque element, in the axial direction. The screw-thread may be provided on one of the first and second shafts.

Upon exercising sufficient movement of the torque element to preload the splined coupling, the nut may remain in a fixed position on the screw-thread, maintaining the preload. Additionally or alternatively, the apparatus may comprise a chocking pad or spring element, for holding the ring nut and torque element in a position to maintain the required preload. This may be particularly useful to control the preload during a range of conditions and prevent vibration of the apparatus.

However, for increased strength, e.g. if the strength of the screw-thread proves inadequate, a collar fastener may be provided instead of the ring nut. The collar fastener may be operable to move the torque element, and may be fixable to one or both of the shafts, e.g. using bolts.

The major coupling may be a helical splined coupling. Preferably the helix angle of the helical splined minor coupling is less (e.g. one quarter) of the helix angle of the helical splined major coupling. Accordingly, preloading of the major coupling can be achieved in a controlled manner. Furthermore, by providing the two helical splined couplings, assembly of the apparatus may be easier.

Preferably, the apparatus is comprised in a gas turbine engine. The first shaft and the second shaft may be the fan shaft and the LP turbine shaft. Nevertheless, it is considered that the apparatus could be used in a variety of different applications where the life of a splined coupled shaft arrangement and/or the size or material of the arrangement is an issue. Applications may be in the aerospace, marine, industrial and automotive fields, for example.

In another aspect of the present invention there is provided a method of assembling first and second coaxial shafts, the first and second shafts extending along, and being rotatable about, a central axis,

the first and second shafts being coupled together by a splined major coupling at a coupling region of the apparatus, the major coupling enabling the shafts to co-rotate about the central axis,

a torque element connected to the first and second shafts, the torque element being operable to force the first and second shafts to relatively rotate about the central axis in order to apply a preload to the major coupling,

each shaft having opposing abutment features defining a pre-determined axial position between shafts;

the method comprises the steps ofa) rotating the torque element to relatively rotate the two shafts until the abutment features abut andb) further rotating the torque element to a pre-determined torque to introduce a pre-load into the splined major coupling.

Apparatus according to a first embodiment of the present invention is shown inFIGS. 1A,1B and1C. The apparatus forms part of a gas turbine engine and includes an LP turbine shaft2coupled to a fan shaft3via a splined major coupling1. The splined major coupling1is a helical splined coupling.

The turbine and fan shafts2,3are each substantially tubular, having a central bore21,31extending therethrough. The shafts2,3each extend along a central axis (indicated by broken line100) and are concentric about the central axis100.

For the purposes of the following description, the turbine shaft2is considered to extend rearwardly from the coupling region10, in the axial direction of the apparatus (i.e. a direction parallel to the central axis100), and the fan shaft3is considered to extend forwardly from the coupling region10, in the axial direction of the apparatus.

At the coupling region10, the outer surface diameter of an end region20of the turbine shaft2is smaller than an inner surface diameter of an end region30of the fan shaft3. The end region20of the turbine shaft2can therefore locate within the central bore31at the end region30of the fan shaft3. The major splined coupling is located between the inner surface32of the fan shaft3and the outer surface22of the turbine shaft2, at a first portion101of the coupling region10. The major coupling1comprises helical splines11(seeFIG. 2), machined into the outer and inner surfaces22,32of the turbine shaft2and the fan shaft3respectively, the splines11being interleaved with one another.

When a force is applied to the turbine shaft2, causing it to rotate, torque is transmitted from the turbine shaft2, through the major splined coupling1, to the fan shaft3, causing the fan shaft3to rotate at the same time as the turbine shaft2. To reduce stress range on the major coupling1, the apparatus comprises a means to preload the major coupling1.

The means to preload the major coupling1includes a preload ring4. The preload ring4is provided at a second portion102of the coupling region, to the front of the first portion101. At the second portion102, although the inner surface diameter of the fan shaft3is substantially the same as it is at the first portion101, the outer surface diameter of the turbine shaft2is substantially smaller than it is at the first portion101, resulting in a gap being provided between the outer surface22of the turbine shaft2and the inner surface32of the fan shaft3at the second portion102. The preload ring4is received in this gap and extends around the turbine shaft2, between the turbine shaft2and the fan shaft3. At the transition between the two different inner surface diameters of the turbine shaft2mentioned above, the turbine shaft comprises a shoulder portion25, the shoulder having a forward-facing surface251. It is considered that the gap in which the preload ring4is received may be provided by a number of different shaft configurations.

An outer surface41of the preload ring4is coupled to the inner surface32of the fan shaft3via an axially in-line or straight-cut splined minor coupling5and an inner surface42of the preload ring is coupled to the outer surface22of the turbine shaft2via a helical splined minor coupling6. This preferred configuration is shown schematically inFIG. 1B. An angle θ is defined between the two minor couplings5and6. Splines51aof the straight-cut splined minor coupling5extend parallel to the central axis100, whereas splines61aof the helical splined minor coupling6twist round the central axis100. The splines51a,61aof the minor couplings5,6are machined into the respective surfaces of the shafts2,3and the preload ring4. In alternative embodiments, a second helical splined minor coupling may be provided instead of the straight-cut splined minor coupling, but with splines that are straighter (i.e. twist less round the central axis100) than the splines of the other helical splined minor coupling.

In yet a further embodiment, shownFIG. 1C, splines51bare angled relative to the axis100, i.e. they are helical, and splines61B are generally parallel to the axis100. An angle θ is defined between splines51b,61b. In any of these embodiments it is preferable for the angle θ to be less than the helical angle of the major splined coupling1. The angle θ will be dependent on the degree of pre-loading required and the configuration of each particular application of the present invention.

Upon an application of a force on the preload ring4in the rearward axial direction, the preload ring4is guided by the splines51a,61aor51b,61bof the two minor couplings5,6, to the position shown inFIG. 1A. However, due to the differences in orientation (θ) of the splines51a,61aor51b,61b, relative to the central axis100, as the preload ring4is guided rearward, the preload ring4is forced to twist about the central axis100, forcing the turbine shaft2and the fan shaft3coupled on either of its sides to relatively rotate about the central axis100. The relative rotation forces the splines11of the major splined coupling1to press against one another, thus preloading the major splined coupling1.

To force the preload ring4in the rearward axial direction, in this embodiment a ring nut7ais provided. The ring nut7ais located to the front of the preload ring4, at a third portion103of the coupling region10. A rear surface71of the ring nut7abears against a front surface43of the preload ring4. The ring nut7ais rotatably mounted to the outer surface22of the turbine shaft2via a screw-thread23machined into the outer surface22. The screw-thread23twists around the central axis100such that, upon rotation of the ring nut7aabout the central axis100, the ring nut7aadvances in the rearward axial direction of the apparatus, whilst bearing against the preload ring4, thus forcing the preload ring4in the rearward axial direction, achieving the preloading discussed above.

The rearward axial movement of the preload ring4, and the resultant relative rotation of the turbine and fan shafts2,3, is limited by an axial stop33projecting radially inward from the fan shaft3, to the front side of the preload ring4, which stop33is arranged to engage the front surface251of the shoulder portion25of the turbine shaft2. In more detail, since the major splined coupling is a helical splined coupling, as the preload ring4forces the turbine shaft2and the fan shaft3to relatively rotate, the turbine shaft and the fan shaft are also forced to move relative to each other in the axial direction. As the shafts2,3move relative to each other in the axial direction, the axial stop33moves towards the shoulder25. The axial stop33and the front surface251are configured to abut each other after a predetermined degree of axial movement, and therefore after a predetermined degree of relative rotation, between the turbine shaft2and the fan shaft3. This enables a pre-determined amount of preloading of the major coupling1to be achieved.

Apparatus according to a second embodiment of the present invention is shown inFIG. 3. The apparatus is similar to the apparatus of the first embodiment except for the configuration of the ring nut7′. As in the first embodiment, the ring nut7′ is mounted to the turbine shaft2via a screw-thread23′; however, the screw-thread23′ is provided on the inner surface24of the turbine shaft2, rather than the outer surface22. So that the nut7′ can reach the front surface43of the preload ring4, at the same time as contacting the inner surface24of the turbine shaft2, the nut7′ is substantially L-shaped. It has a main section71, extending in the axial direction of the apparatus and mounted to the screw-thread23′, and a flange section72, extending radially outwards from the rear of the main section71, toward the front surface43of the preload ring4. This alternative arrangement of the nut7′ may provide a more convenient, compact, apparatus configuration.

Apparatus according to a third embodiment of the present invention is shown inFIG. 4. The apparatus is similar to the apparatus of the first embodiment; however, the relative rotation and relative axial movement of the fan shaft3and turbine shaft2are limited by an axial stop26that projects radially outwardly from the turbine shaft2, at the rear of the fan shaft3. As the preload ring4forces the turbine shaft2and the fan shaft3to relatively rotate, the turbine shaft2and the fan shaft3are forced to move in the axial direction, relative to each other, as previously described (the fan shaft3moves rearward relative to the turbine shaft2). During this relative axial movement, the rear end surface34of the fan shaft3moves toward the axial stop26. The apparatus is configured such that the rear end surface34and the axial stop26abut each other after a predetermined degree of relative axial movement between the turbine and fan shafts2,3, and therefore after a predetermined degree of relative rotation between the turbine and fan shafts2,3. This arrangement provides an alternative approach to applying a predetermined amount of preload to the major splined coupling.

The present invention also lends itself to a method of assembling first and second coaxial shafts and which is depicted inFIGS. 5 to 7. These figures comprise the same components as shown inFIGS. 1A,1B and1C and therefore require no further introduction.

InFIG. 5the shafts2,3have been engaged with one another via the splined major coupling1through a relative axial displacement. The preload ring or torque element4is then presented to the shafts so that the two splined minor couplings5and6just engage the shafts' respective splines. The nut7ais then screwed onto the thread23and rotated so that it travels towards and engages the preload ring or torque element4. Corresponding abutments features, in the form of the axial stop33and the front surface251of the turbine shaft2, define an axial gap45therebetween at this stage.

InFIG. 6, the nut7ahas been rotated and has travelled axially rearwardly part way along the screw thread23forcing the preload ring or torque element4along the splines5and6. As the preload ring or torque element4is forced axially rearwardly, the two shafts2and3are rotated relatively to one another by virtue of the angle between splines5and6. As can be seen the axial gap45has closed.

InFIG. 7, the torque element has been rotated to relatively rotate the two shafts until the axial stop33has contacted the front face251. This abutment stops further relative axial movement between the two shafts2,3. The method of assembly, then includes further rotating the preload ring or torque element4to a pre-determined torque to introduce a pre-load into the splined major coupling1.