DRIVE SHAFT ASSEMBLY

A drive shaft assembly comprising a first shaft and a second shaft is disclosed. The second shaft is axially translatable from an engaged configuration with the first shaft at one limit of its travel to a disengaged configuration from the first shaft at the opposite limit of its travel. Engagement of the first and second shafts is via cooperating helical splines provided thereon. The helical splines give rise to a disengaging axial force on the second shaft in a fault condition in which a driving torque is applied from one of the first and second shafts to the helical splines in a rotational direction tending to unscrew the helical splines. Consequently when the second shaft is in the engaged configuration the disengaging axial force translates the second shaft to the disengaged configuration.

The present disclosure concerns drive shaft assemblies. The disclosed drive shaft assemblies may have particular application in gas turbine engines where it may be desirable under certain operating conditions to decouple a driven element such as a fan from the remainder of the drive train. While for simplicity the remainder of the background to the disclosure is explained in the context of an aero gas turbine engine having a drive train for powering a fan, this is not intended to be limiting. Indeed it will be appreciated that the invention may have application in any drive shaft assembly where disengagement is desirable under particular operating conditions.

In an aero turbofan engine a fan is provided at the inlet of the engine that in use is driven by a turbine. The fan and turbine are connected by a drive shaft assembly in order that power generated by the turbine is transmitted to the fan. In particular engine architectures the drive shaft assembly comprises a gearbox, typically to gear down the rotational speed of the fan by comparison with the rotational speed of the driving turbine.

Under particular failure conditions (e.g. loss of lubricating oil or failure of a ring or planet gear) the gearbox may seize, consequently preventing rotation of the fan. Under these conditions the fan, unable to windmill in on rushing air from the inlet of the engine, may present an increased drag force by comparison with an engine failure in which the fan is nonetheless free to windmill. This increased drag caused by the static fan may compound the impact of a loss of power caused by the failure of the engine.

According to a first aspect there is provided a drive shaft assembly comprising optionally a first shaft and optionally a second shaft, the second shaft optionally being axially translatable from an engaged configuration with the first shaft at one limit of its travel to a disengaged configuration from the first shaft at an opposite limit of its travel, engagement of the first and second shafts optionally being via cooperating helical splines provided thereon, said helical splines optionally giving rise to a disengaging axial force on the second shaft in a fault condition optionally in which a driving torque is applied from one of the first and second shafts to the helical splines optionally in a rotational direction tending to unscrew the helical splines, such that when the second shaft is in the engaged configuration the disengaging axial force optionally translates the second shaft to the disengaged configuration, the driving torque optionally being a torque that exceeds any torque applied from the other of the first and second shafts to the helical splines. As a consequence of the first aspect, the drive train can be decoupled at the helical splines. This decoupling may occur when a) the direction of the driving torque remains consistent but the shaft supplying it changes, and/or b) the shaft supplying the driving torque remains consistent but its direction reverses. This may place a system into failsafe configuration (particularly in the case of a)) and/or prevent damage to the drivetrain and or components driven by the drive train (particularly in the case of b)).

In some embodiments the helical splines give rise to an engaging axial force on the second shaft in a reset condition in which the driving torque is applied from one of the first and second shafts to the helical splines in a rotational direction tending to screw together the helical splines, such that when the second shaft is in the disengaged configuration the engaging axial force translates the second shaft to the engaged configuration. This may provide a convenient means for returning the second shaft to the engaged configuration following a fault condition event. As will be appreciated the driving torque may be applied manually for instance as part of a maintenance/repair activity, or may arise automatically if the fault condition passes.

In some embodiments the first shaft is substantially axially fixed. In this way the limit of travel for the second shaft as it reaches its engaged configuration may be provided by an axial stop of the axially fixed first shaft reacting any axial force exerted by the second shaft.

In some embodiments the shaft assembly further comprises a third shaft drivingly engaged with the second shaft, the second shaft being intermediate the first and third shafts. The second shaft may therefore be considered to provide a disengageable connection between the first and third shafts for instance in a shaft coupling region.

In some embodiments the second and third shafts have cooperating axial splines which provide the driving engagement between them. The axial splines may allow for both rotational engagement of the second and third shafts and for axial translation of the second shaft with respect to the third shaft. In alternative embodiments however the cooperating axial splines between the second and third shafts may be replaced with a second set of helical cooperating splines.

In some embodiments the third shaft is substantially axially fixed. In this way the limit of travel for the second shaft as it reaches its disengaged configuration may be provided by an axial stop of the axially fixed third shaft reacting any axial force exerted by the second shaft.

In some embodiments the cooperating axial splines slide axially adjacent one another during translation of the second shaft towards the engaged configuration and towards the disengaged configuration. The splines may therefore act as a guide for translation of the second shaft.

In some embodiments the third shaft is axially positioned and the axial splines of the third shaft are of sufficient length such that the cooperating axial splines remain engaged throughout translation of the second shaft between the engaged configuration and the disengaged configuration.

In some embodiments the drive shaft assembly further comprises a gearbox. The drive shaft assembly may have particular utility in mitigating against gearbox seizure.

In some embodiments the third shaft is provided intermediate the gearbox and the second shaft.

In some embodiments in a non-fault condition the driving torque is applied to the first shaft from the second shaft via the helical splines in a rotational direction tending to screw together the helical splines. When operating in the non-fault condition the engaging axial force therefore tends to retain the second shaft in the engaged configuration. As will be appreciated, and where provided torque may be applied to the second shaft via the third shaft and/or gearbox.

In some embodiments a shear pin is provided arranged to provide an axial force against translation of the second shaft away from the engaged configuration towards the disengaged configuration. As will be appreciated the shear pin may be arranged to shear in the event that the fault condition occurs.

In some embodiments a selectively adjustable retaining element is provided arranged to provide an axial force against translation of the second shaft away from the engaged configuration towards the disengaged configuration. The retaining element may be adjusted in the event that the fault condition occurs such that it no longer provides the axial force against translation of the second shaft. Adjustment of the retaining element may be by active or passive control and could for example comprise a positional adjustment or deformation of the retaining element. Adjustment may be controlled by a controller such as a Full-Authority Digital Electronic Control (FADEC).

In some embodiments the driving torque applied in the fault condition is from the first shaft. Thus where the driving torque in the non-fault condition is applied from the second shaft, the rotational direction of the torque applied in the fault condition is the same as the rotational direction of the torque applied in the non-fault condition.

In some embodiments the driving torque applied in the fault condition is from the second shaft. Thus where the driving torque in the non-fault condition is applied from the second shaft, the rotational direction of the torque applied in the fault condition is opposite to the rotational direction of the torque applied in the non-fault condition.

As will be appreciated the handedness of the cooperating helical splines may be selected to complement the rotational direction of the driving torque in the fault and non-fault conditions and the chosen spline configuration. Thus assuming the second and third shafts have cooperating axial splines and that the driving torque in the non-fault condition is in the clockwise direction, the cooperating helical splines may have right handedness. Similarly, assuming the same arrangement, if the driving torque in the non-fault direction is in the anti-clockwise direction, the cooperating helical splines may have left handedness.

In some embodiments a load driven by the drive shaft assembly when operating in the non-fault condition is provided on the first shaft. The load may be a compressor and may be a fan. Decoupling at the helical splines may decouple a prime mover such as a turbine or intermediary drive shaft assembly component such as a gearbox from a load such as a fan.

According to a second aspect there is provided a gas turbine engine comprising the drive shaft assembly according to the first aspect. The gas turbine engine may be an aero gas turbine engine.

In some embodiments a compressor is provided on the first shaft. The compressor may for example be a fan.

In some embodiments when the second shaft is in the engaged configuration the compressor is connected via the drive shaft assembly to a turbine which in use drives the compressor.

In some embodiments the fault condition comprises a seizure of the gearbox. In that case, where for instance in the non-fault condition drive is supplied from the turbine via the drive shaft assembly to the compressor, the loss in drive from the turbine caused by seizure of the gearbox may automatically give rise to the fault condition. In that case the driving torque may no longer be delivered from the second shaft to the first but from the first to the second. The switch in the shaft from which the driving torque is applied occurs because the second shaft is prevented from rotating by the seized gearbox while the first shaft tends to continue rotating primarily in view of the ram air from the inlet of the engine causing a windmilling torque on the first shaft. Because the driving torque from the first shaft is in the same direction as the former driving torque from the second shaft, it tends to unscrew the helical splines, translating the second shaft into its disengaged configuration and disengaging the compressor from the seized gearbox. The compressor may now be free to windmill and therefore may give rise to reduced drag.

With reference toFIG. 1, a gas turbine engine is generally indicated at10, having a principal and rotational axis11. The engine10comprises, in axial flow series, an air intake12, a propulsive fan13, an intermediate pressure compressor14, a high-pressure compressor15, combustion equipment16, a high-pressure turbine17, and intermediate pressure turbine18, a low-pressure turbine19and an exhaust nozzle20. A nacelle21generally surrounds the engine10and defines both the intake12and the exhaust nozzle20.

The compressed air exhausted from the high-pressure compressor15is directed into the combustion equipment16where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines17,18,19before being exhausted through the nozzle20to provide additional propulsive thrust. The high17, intermediate18and low19pressure turbines drive respectively the high pressure compressor15, intermediate pressure compressor14and fan13, each by suitable interconnecting shaft.

Referring now toFIGS. 2 and 3a drive shaft assembly for an aero gas turbine engine is generally shown at30. The drive shaft assembly30comprises a first shaft32, a second shaft34and a third shaft36. The second shaft34is intermediate the first32and third36shafts. The first32and third36shafts are positionally fixed in the axial direction. The second shaft34is axially translatable between limits of travel in which it is respectively in an engaged configuration with the first shaft32and a disengaged configuration with the first shaft32. InFIG. 2the second shaft34is shown in the engaged configuration and inFIG. 3it is shown in the disengaged configuration.

The first shaft32has a low pressure compressor, in this case a fan (not shown) provided thereon.

Referring to view B ofFIG. 2, the first32and second34shafts are coupled by cooperating helical splines38provided thereon in a shaft coupling region40. The helical splines38have right handedness and as shown inFIG. 2are screwed together in the manner of a screw threaded nut and bolt. In the engaged configuration of the second shaft34a forward end42thereof abuts a first shaft axial stop44of the first shaft32which defines an axial limit of travel for the second shaft34. In the disengaged configuration (as shown inFIG. 3) of the second shaft34the helical splines38have been completely unscrewed such that the first shaft32and second shaft34are no longer coupled.

Referring to view A ofFIG. 2, the second34and third36shafts are coupled by cooperating axial splines46provided thereon. The axial splines of the third shaft36are approximately twice the length of the axial splines of the second shaft34. As shown inFIG. 2, with the second shaft34in the engaged configuration, the axial splines of the second shaft34are engaged with the half of the axial splines of the third shaft36nearest to the first shaft32. A shear pin48is provided on and positionally fixed with respect to the third shaft36. In the engaged configuration of the second shaft34a rearward end50thereof abuts the shear pin48. By contrast, in the disengaged configuration (as shown inFIG. 3) the second shaft34, axially displaced away from the first shaft32, has its axial splines engaged with the half of the axial splines of the third shaft36furthest from the first shaft32. With the second shaft34in the disengaged configuration the shear pin48has sheared and is consequently no longer positionally fixed with respect to the third shaft36. Furthermore the rearward end50of the second shaft34abuts a third shaft axial stop54of the third shaft36which defines a further axial limit of travel for the second shaft34.

The drive shaft assembly30further comprises an epicyclic gearbox52comprising a sun gear (not shown), planet gears (not shown) provided on a planet carrier (not shown) and a static ring gear (not shown). The planet carrier is connected to the third shaft36. The third shaft36is intermediate the second shaft34and gearbox52and provides a transmission path from the planet gears to the second shaft34. The sun gear of the gearbox52is connected to an intermediate pressure shaft (not shown). The intermediate pressure shaft connects the sun gear to an intermediate pressure turbine (not shown), providing a transmission path from the intermediate pressure turbine to the sun gear. An intermediate pressure compressor (not shown) is also connected to the intermediate pressure turbine between the gearbox52and intermediate pressure turbine. As will be appreciated the particular planetary gearbox arrangement described, with the input provided to the sun gear and the output provided via the planet carrier is not intended to be limiting. In alternative embodiments an alternative epicyclic gearbox configuration may be provided, for instance a star or differential configuration. Further the gear/carrier to which the intermediate pressure shaft and the third shaft are respectively connected may be varied, particularly to suit a particular gearing ratio desired.

In use the drive shaft assembly30is normally operated under non-fault conditions. In the non-fault condition the second shaft34is engaged with the first shaft32as perFIG. 2. Clockwise rotational drive is provided from the intermediate pressure turbine via the intermediate pressure shaft to both the intermediate pressure compressor and to the sun gear of the gearbox52. Rotation of the sun gear drives clockwise orbiting of the planet gears about the sun gear, in turn rotating the planet carrier in a clockwise direction. Rotation of the planet carrier rotates the third shaft36and so the second shaft34via the cooperating axial splines46. The clockwise rotation of the second shaft34tends to screw together the cooperating helical splines38in view of their right handedness, thereby creating an engaging axial force between the first32and second34shafts tending to maintain the engaged configuration. The sheer pin48also tends to maintain the engaged configuration. The clockwise rotation of the second shaft34also rotates the first shaft32and the fan supported thereon in a clockwise rotation. As will be appreciated in view of the above, when the drive shaft assembly30is operated in the non-fault condition, the driving torque is applied to the first shaft32from the second shaft34. Further that driving torque is applied in a rotational direction tending to screw together the helical splines38.

During operation of the drive shaft assembly30in the non-fault condition it may be that the gearbox52seizes. Seizure of the gearbox52means that driving torque can no longer be applied from the intermediate pressure turbine to the fan via the drive shaft assembly30. Indeed as a consequence of the seizure of the gearbox52and the rotational inertia of the fan and first shaft32, driving torque is instead applied from the first shaft32to the helical splines38. The transmission of torque from the fan rather than from the turbine constitutes operation in a fault condition.

Under operation in the fault condition the first shaft32continues to rotate in the clockwise direction, but because the driving torque is supplied from the first shaft32to the helical splines38rather than from the second shaft34to the helical splines38, it tends to unscrew the helical splines38. This in turn gives rise to a disengaging axial force on the second shaft34, tending to force it axially away from the first shaft32. As the disengaging axial force is applied it shears the shear pin48and causes translation of the second shaft34towards its disengaged configuration. As the translation occurs the cooperating axial splines46slide axially adjacent one another. Once the disengaged configuration is reached the first32and second shafts34are no longer connected and the rearward end50of the second shaft34abuts the third shaft axial stop54. In this disengaged configuration the fan is not prevented from rotating as it otherwise would be as a consequence of its connection to the seized gearbox. The fan is therefore free to windmill in air that may be passing through it as a consequence of forward motion of an associated aircraft. Consequently the fan may provide reduced drag by comparison with a scenario in which it is prevented from rotating.

As will be appreciated the first32and second shafts34may be re-engaged. By way of example a maintenance activity may be undertaken in which the gearbox52is replaced or a fault giving rise to the seizure is repaired. Thereafter a driving torque may be applied in a clockwise direction from the second shaft34to the helical splines38. This will tend to screw together the helical splines38giving rise to an engaging axial force on the second shaft34translating it towards the first shaft32and its engaged configuration. As will be appreciated a similar engaging axial force would be achieved by applying a driving torque in an anticlockwise direction from the first shaft32to the helical splines38. Once the forward end42is abutting the first shaft axial stop44, a replacement shear pin48is provided on and positionally fixed with respect to the third shaft36. Such a maintenance activity constitutes operation under a reset condition. Once the maintenance activity is complete, operation under the non-fault condition may be resumed.

As will be appreciated, aside from the manner in which the second shaft34may be translated from the engaged to the disengaged configuration in the fault condition described above, similar disengagement may be brought about in an alternative fault condition in which the driving torque continues to be applied from the second shaft34to the helical splines38, but its rotational direction is reversed (i.e. in this case anticlockwise). Whilst such functionality may not be required with respect to an aero gas turbine engine, drive shaft assemblies used in alternative applications may benefit from such functionality. Specifically in particular applications it may be that a driving torque rotation reversal might damage components of the drive shaft assembly and/or associated components or systems. In such applications the disengagement may prevent transmission of the driving torque having a reversed rotational direction.