Patent Description:
A high pressure turbine drives the high pressure compressor through an outer shaft to form a high spool, and the low pressure turbine drives the low pressure compressor through an inner shaft to form a low spool. The fan section may also be driven by the low pressure turbine through the inner shaft.

The engine is typically started by driving the high spool through a tower shaft through an accessory gearbox. Once the high spool is up to speed, the low spool follows and the engine is brought to an idle condition. When the engine is operating, the accessory gearbox is driven through the same tower shaft to drive accessory components such as hydraulic pumps and electric generators. The loads from the accessory gearbox on the high spool reduce efficiency.

Turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies. <CIT>, <CIT> and <CIT> disclose arrangements of the prior art.

In an aspect of the invention, an auxiliary gearbox drive system for a turbofan engine is provided according to claim <NUM>. In another aspect of the invention, a turbofan engine is provided according to claim <NUM>.

In a further embodiment of the foregoing auxiliary gearbox drive system for a turbofan engine, the output comprises a lay shaft coupled to the carrier.

In a further embodiment of any of the foregoing auxiliary gearbox drive systems, both the first means for selectively coupling the ring gear to the first tower shaft and the second means for selectively coupling the sun gear to the carrier comprise a one-way mechanical clutch.

In a further embodiment of any of the foregoing auxiliary gearbox drive systems, the first means couples the first tower shaft to the ring gear and the second tower shaft drives the sun gear in a first operating condition such that both the first tower shaft and the second tower shaft combine to drive the output.

In a further embodiment of any of the foregoing auxiliary gearbox drive systems, the second means couples the sun gear to the carrier and the lay shaft drives the carrier and thereby the second tower shaft and a high speed spool in a starting operating condition.

Although the different examples have the specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations.

The inner shaft <NUM> is connected to a fan section <NUM> through a speed change mechanism, which in exemplary gas turbine engine <NUM> is illustrated as a geared architecture <NUM> to drive fan blades <NUM> at a lower speed than the low speed spool <NUM>.

For example, gear system <NUM> may be located aft of the low pressure compressor <NUM> and the fan blades <NUM> may be positioned forward or aft of the location of the geared architecture <NUM> or even aft of turbine section <NUM>.

The example gas turbine engine includes the fan section <NUM> that comprises in one non-limiting embodiment less than about <NUM> fan blades <NUM>. In another non-limiting embodiment, the fan section <NUM> includes less than about <NUM> fan blades <NUM>. Moreover, in one disclosed embodiment the low pressure turbine <NUM> includes no more than about <NUM> turbine rotors schematically indicated at <NUM>. In another non-limiting example embodiment, the low pressure turbine <NUM> includes about <NUM> turbine rotors. A ratio between the number of fan blades <NUM> and the number of low pressure turbine rotors is between about <NUM> and about <NUM>. The example low pressure turbine <NUM> provides the driving power to rotate the fan section <NUM> and, therefore, the relationship between the number of turbine rotors <NUM> in the low pressure turbine <NUM> and the number of blades <NUM> in the fan section <NUM> disclose an example gas turbine engine <NUM> with increased power transfer efficiency.

The example engine <NUM> includes an accessory drive system <NUM> that receives power from both the high speed spool <NUM> and the low speed spool <NUM>. The accessory drive system <NUM> drives an accessory gearbox <NUM> that includes accessory component <NUM> and lubricant pump <NUM>. The accessory component <NUM> may include pumps, generators and other devices driven to enable operation of different engine and aircraft systems. The accessory gearbox <NUM> is also coupled to a starter <NUM>. The starter <NUM> is capable of driving the accessory drive system <NUM> to start the engine <NUM>. In this example, a tower shaft assembly <NUM> is coupled to both the low speed spool <NUM> and the high speed spool <NUM> to distribute power extraction between the two spools <NUM>, <NUM>.

Excessive power extraction from a single spool, such as the high speed spool <NUM>, can limit operation and degrade overall performance and engine efficiency. Accordingly, the example accessory drive system <NUM> extracts power from both the low speed spool <NUM> and the high speed spool <NUM> to meet the overall power demands of the engine <NUM> and the aircraft associated with the engine.

Referring to <FIG>, with continued reference to <FIG>, the example accessory drive system <NUM> includes a superposition gearbox <NUM> that is coupled between accessory gearbox <NUM> and the tower shaft assembly <NUM>. The superposition gearbox <NUM> is an epicyclic gearbox that includes a sun gear <NUM> that rotates about an axis <NUM>. A plurality of intermediate gears <NUM> are engaged with the sun gear <NUM> and supported by a carrier <NUM>. A ring gear <NUM> circumscribes and is engaged with the plurality of intermediate gears <NUM>. The example superposition gearbox <NUM> is not coupled to a static structure of the engine <NUM> and, therefore, is operated by various input combinations to provide the desired distribution of power.

In the disclosed example, the tower shaft assembly <NUM> includes a first tower shaft <NUM> that is driven by a gear <NUM> disposed on the low speed spool <NUM>. A first gear <NUM> on the tower shaft <NUM> is coupled to the gear <NUM>. A second gear <NUM> is disposed on a second end of the tower shaft <NUM> and engages a drive gear <NUM> disposed on a ring gear shaft <NUM>.

A second tower shaft <NUM> is coupled to a drive gear <NUM> that is driven by the high speed spool <NUM>. The second tower shaft <NUM> includes a first gear <NUM> driven by a gear <NUM> on the high speed spool <NUM>. A second gear <NUM> of the second tower shaft <NUM> is engaged to drive gear <NUM> disposed on a sun gear shaft <NUM>. In this example, the first tower shaft <NUM> and the second tower shaft <NUM> are disposed concentrically about a common axis <NUM>. Moreover, the axis <NUM> is disposed at an angle relative to the engine longitudinal axis A and an axis <NUM> of the superposition gearbox <NUM>. It should be appreciated that although the specific disclosed embodiment includes concentric tower shafts <NUM>, <NUM>, other configurations and orientations of the tower shafts are within the contemplation and scope of the claims.

First tower shaft <NUM> is coupled to the ring gear shaft <NUM> that is selectively coupled to the ring gear <NUM>. The second tower shaft <NUM> is coupled to the sun gear shaft <NUM> that is coupled to drive the sun gear <NUM>. The sun gear shaft <NUM> is directly coupled to the sun gear <NUM> and is not selectively engaged to the sun gear <NUM>.

The superposition gearbox <NUM>, therefore, has a first input provided by the first tower shaft <NUM> through the ring gear shaft <NUM> to drive the ring gear <NUM> and a second input provided by the second tower shaft <NUM> to drive the sun gear shaft <NUM> and, thereby, the sun gear <NUM>. An output from the superposition gearbox <NUM> is provided by a lay shaft <NUM> that is coupled to the carrier <NUM>. The lay shaft <NUM> drives the accessory gearbox <NUM> in the disclosed example embodiment. The accessory gearbox <NUM> includes another gear system or plurality of gears as is required to the drive accessory components schematically illustrated at <NUM> and <NUM>. Moreover, the accessory gearbox <NUM> is coupled to the starter <NUM> to provide a driving input through the lay shaft <NUM> to the superposition gearbox <NUM> to drive the high speed spool <NUM> during starting operation.

Referring to <FIG>, with continued reference to <FIG>, the example superposition gearbox <NUM> includes a first clutch assembly <NUM> that selectively couples the ring gear shaft <NUM> to drive the ring gear <NUM>. The first clutch <NUM> selectively couples driving input from the low speed spool <NUM> through the first tower shaft <NUM> to the ring gear shaft <NUM> and ring gear <NUM>.

A second clutch assembly <NUM> selectively couples the sun gear <NUM> to the carrier <NUM>. The second clutch assembly <NUM> is shown uncoupled such that the sun gear <NUM> and the carrier <NUM> may rotate at different relative speeds. Coupling of the sun gear <NUM> to the carrier <NUM> enables the second tower shaft <NUM> to directly drive the carrier <NUM>. Accordingly, through a selective coupling of the first clutch assembly <NUM> and the second clutch assembly <NUM> different inputs from the high speed spool <NUM> and the low speed spool <NUM> can be input through the superposition gearbox <NUM> to drive the lay shaft <NUM> and, thereby, the accessory gearbox <NUM>.

The first clutch assembly <NUM> and the second clutch assembly <NUM> are one-way mechanical clutches that do not require control or separate independent actuation. Each of the clutch assemblies <NUM>, <NUM> are automatically engaged and disengaged depending on the speed and direction of torque input. The selective actuation of the mechanical clutches <NUM>, <NUM> enables both the low speed spool <NUM> and the high speed spool <NUM> to provide torque input to drive the lay shaft <NUM>.

In the configuration for operation shown in <FIG>, the first clutch <NUM> is engaged to couple the ring gear shaft <NUM> to the ring gear <NUM>. Accordingly, the first tower shaft <NUM> driven by the low speed spool <NUM> is coupled to drive the ring gear <NUM>. The second clutch <NUM> is not engaged and is free running. The second tower shaft <NUM> drives the sun shaft <NUM> to drive the sun gear <NUM>. Accordingly, the low speed spool <NUM> is driving the ring gear <NUM> and the high speed spool <NUM> is driving the sun gear <NUM>. The driving inputs are both in the same rotational direction and result in an overall output through the carrier <NUM> to drive the lay shaft <NUM>.

Accordingly, a rotational input, schematically indicated at <NUM>, of the low speed spool <NUM> combined with a rotational input <NUM>, schematically shown by the rotational arrows, combined within the superposition gearbox <NUM> generate an output, schematically shown at <NUM>, to drive the accessory gearbox <NUM>.

Differing rotational inputs provided by <NUM> and <NUM> may be of differing speeds and torques. The superposition gearbox <NUM> receives and automatically distributes the torques to provide the driving input <NUM> through the lay shaft <NUM> to drive the accessory gearbox <NUM>.

Referring to <FIG>, another schematic illustration of the disclosed drive system <NUM> is schematically shown configured for a starting operation configuration. In the starting configuration, starter <NUM> provides input torque <NUM> to drive the accessory gearbox <NUM> and the lay shaft <NUM> to rotate high speed spool <NUM>. In this position, the first clutch assembly <NUM> is not coupled because the ring gear <NUM> due to the direction of torque input and rotation. Accordingly, the first tower shaft <NUM> is not back driven by the superposition gearbox <NUM>. The second clutch <NUM> is engaged to couple the carrier <NUM> to the sun gear shaft <NUM>. Accordingly, rotation of the carrier <NUM> by the lay shaft <NUM> drives the sun gear shaft <NUM> and, thereby, the second tower shaft <NUM>. Driving the second tower shaft <NUM> rotates the high speed spool <NUM> to provide a rotational input, schematically shown at <NUM>. None of the torque or rotational input provided by the lay shaft <NUM> is transmitted to the low spool <NUM>. The high speed spool <NUM> is driven until the engine starts and begins spinning under its own power as is known and understood by those skilled in gas turbine engine structure and operation.

Referring to <FIG>, with continued reference to <FIG>, a forward wind milling operating condition is schematically shown and includes an input of torque on the low speed spool <NUM> from the fan section <NUM>. Airflow schematically shown at <NUM> passing through the fan <NUM> when the engine is not operating will cause rotation of the low spool <NUM>. Rotation of the low spool <NUM> in turn causes rotation of various structures in the engine. Forward rotation of the engine requires lubricant to be supplied. In this example embodiment, rotation of the low spool <NUM> causes rotation of the geared architecture <NUM> (<FIG> and <FIG>) and therefore creates a need to provide lubricant to rotating components. Moreover, other components of the engine such as the support bearings as well of the superposition gearbox <NUM> require lubricant when rotated. Accordingly, the disclosed accessory drive system <NUM> drives the lay shaft <NUM> to drive the accessory gearbox <NUM> that in turn drives a lubricant pump <NUM>. The lubricant pump <NUM> drives lubricant through a system of conduits schematically shown at <NUM> of the lubrication system schematically indicated at <NUM> that provides lubricant to engine components as indicated at <NUM> and the superposition gearbox <NUM>. The example lubrication system <NUM> is shown schematically and is contemplated to include features that distribute lubricant throughout the engine <NUM> as would be understood by one skilled in turbine engines.

Operation of the accessory drive system <NUM> in the illustrated forward wind milling operating condition includes rotation of the low speed spool <NUM> caused by airflow <NUM> through the fan section <NUM>. Rotation of the low speed spool <NUM> drives the first tower shaft <NUM>. The first clutch <NUM> is coupled such that rotation of the first tower shaft <NUM> rotates the ring gear shaft <NUM> and the ring gear <NUM>. The high speed spool <NUM> is stationary and does not provide an input to the second tower shaft <NUM> and the sun gear <NUM>. The sun gear <NUM> is therefore held stationary. The sun gear <NUM> rotates with the carrier <NUM> because the second clutch <NUM> does not allow the carrier <NUM> to rotate faster than the sun gear <NUM>. Accordingly, because the high speed spool <NUM> is attached to the sun gear <NUM>, the high speed spool <NUM> becomes a parasitic drag on the system to slow or prevent rotation of the sun gear <NUM>. Rotation of the ring gear <NUM> in combination with the sun gear <NUM> drives the intermediate gears <NUM> about the axis <NUM>. Rotation of the intermediate gears <NUM> drives the carrier <NUM> and thereby the lay shaft <NUM>. Accordingly, the input schematically indicated at <NUM> from the low speed spool <NUM> into the superposition gearbox <NUM> generates an output <NUM> to drive the accessory gearbox <NUM>. Driving of the gearbox <NUM> in turn drives the pump <NUM> to operate the lubrication system <NUM>.

Referring to <FIG>, an aft wind milling operating condition is schematically shown where airflow <NUM> from aft of the engine is driving rotation of the fan <NUM>. Airflow <NUM> from the aft direction causes the fan section <NUM> and thereby the low speed spool <NUM> to rotate in about the axis A in a direction that does not cause coupling between the ring shaft <NUM> and the ring gear <NUM>. Accordingly, neither the torque from the low speed spool <NUM> or the high speed spool <NUM> is transmitted through the superposition gearbox <NUM>. In the reverse wind milling operating condition, rotation of the high speed spool <NUM> is not possible and therefore lubricant flow is not necessary. Accordingly, the first clutch <NUM> does not couple the ring shaft <NUM> to the ring gear <NUM> and no torque is transferred to the accessory gearbox <NUM>.

The example accessory drive system <NUM> includes a superposition gearbox <NUM> that automatically distributes input driving torque between the low speed spool <NUM>, the high speed spool <NUM> and the accessory gearbox <NUM> as required during engine operation. The selective operation of the superposition gearbox <NUM> is enabled by first and second one-way clutches that provide different combinations of inputs and outputs that automatically couple based engine operating conditions.

Claim 1:
An auxiliary gearbox drive system for a turbofan engine, the auxiliary gearbox drive system comprising:
a sun gear (<NUM>), wherein the sun gear (<NUM>) is configured to be coupled to a second tower shaft (<NUM>) of the turbofan engine;
a plurality of intermediate gears (<NUM>) engaged to the sun gear (<NUM>) and supported in a carrier (<NUM>); and
a ring gear (<NUM>) circumscribing the intermediate gears (<NUM>),
characterised in that the system further comprises:
a first means (<NUM>) for selectively coupling the ring gear (<NUM>) to a first tower shaft (<NUM>) of the turbofan engine;
a second means (<NUM>) for selectively coupling the sun gear (<NUM>) to the carrier (<NUM>); and
an output (<NUM>) to an auxiliary gearbox (<NUM>).