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 with a starter 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.

Turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.

<CIT> discloses a multi-shaft power extraction from a gas turbine engine.

<CIT> discloses a power transmission device for a gas turbine engine.

From one aspect, an accessory gearbox for a turbofan engine according to an exemplary embodiment of this disclosure is as claimed in claim <NUM>. Embodiments of this aspect of the invention are provided in claims dependent from claim <NUM>.

From another aspect, a method of operating an accessory gearbox for a turbofan engine according to an exemplary embodiment of this disclosure is as claimed in claim <NUM>. Embodiments of this aspect of the invention are provided in claims dependent from claim <NUM>.

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>.

In a further example, the engine <NUM> bypass ratio is greater than about six (<NUM>:<NUM>), with an example embodiment being greater than about ten (<NUM>:<NUM>), the geared architecture <NUM> is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about <NUM> and the low pressure turbine <NUM> has a pressure ratio that is greater than about five (<NUM>:<NUM>). In one disclosed embodiment, the engine <NUM> bypass ratio is greater than about ten (<NUM>:<NUM>), the fan diameter is significantly larger than that of the low pressure compressor <NUM>, and the low pressure turbine <NUM> has a pressure ratio that is greater than about five (<NUM>:<NUM>).

"Low corrected fan tip speed" is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R) / (<NUM> °R)]<NUM> (where °R = K*(<NUM>/<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> includes an accessory gearbox <NUM> for driving a plurality of accessory components <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. The accessory components <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> support operation of the gas turbine engine <NUM> and 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> including first and second tower shafts <NUM>, <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. The power extracted from the spools <NUM>, <NUM> is combined in a superposition gear system <NUM> disposed within the accessory gearbox <NUM>.

Referring to <FIG>, with continued reference to <FIG>, the superposition gear system <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>.

In the disclosed example, the tower shaft assembly <NUM> includes the 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 the 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 the axis <NUM> of the superposition gear system <NUM>. It should be appreciated that although the specific disclosed embodiment includes concentric tower shafts <NUM>, <NUM>, other configurations and orientations of tower shafts are within the contemplation and scope of this disclosure.

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 extends past the sun gear <NUM> to the starter <NUM>.

The superposition gear system <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>. A first output from the superposition gear system <NUM> is provided by a carrier shaft <NUM> that is coupled to the carrier <NUM>. The carrier shaft <NUM> drives a first group of the accessories in the disclosed example embodiment. The ring gear shaft <NUM> provides a second output to drive a second group of accessories.

The sun gear shaft <NUM> provides both another input into the gear system <NUM> by being driven by the starter <NUM> and a third output to drive at least one accessory component as is schematically indicated at <NUM>. The starter <NUM> provides a driving input to the sun gear <NUM> through the sun gear shaft <NUM>.

Referring to <FIG>, with continued reference to <FIG>, the example superposition gear system <NUM> includes a direct connection between the starter <NUM> and the sun gear shaft <NUM> to provide for direct driving of the high speed spool <NUM>. The sun gear shaft <NUM> is coupled to the starter <NUM> through a starter clutch <NUM>. The starter clutch <NUM> in this example is a mechanical one-way clutch that enables direct driving of the high speed spool <NUM> during starting operations. Once the high speed spool <NUM> is operating, the starter clutch <NUM> prevents back driving or over driving of the starter <NUM>. The sun gear shaft <NUM> is directly connected to the starter rather than being driven through a gear system. The direct drive of the high speed spool <NUM> through the direct connection simplifies operation and the mechanical connections.

Moreover, the example superposition gear system <NUM> provides the first output through the carrier shaft <NUM> that drives a first group of accessory components <NUM> through a first gear system <NUM>. In this example the first group of accessory components <NUM>, <NUM>, <NUM> and <NUM> are driven at a first speed. Once the engine is started, the first output through the carrier shaft <NUM> provides the driving input required to power the first group <NUM> of accessory components <NUM>, <NUM>, <NUM> and <NUM> through the first gear system <NUM>.

A second group <NUM> of accessory components <NUM> is driven by the ring gear shaft <NUM> through a second gear system <NUM>. In this example, only one accessory component <NUM> is shown as being driven by the second gear system <NUM>, however, other components could be included.

A third group <NUM> of accessory components <NUM> is driven by a third gear system <NUM> coupled to the sun gear shaft <NUM>. The third gear system <NUM> is operable to rotate at the speed of the sun gear shaft <NUM> during engine operation and when driven by the starter <NUM> during starting operations.

The superposition gear system <NUM> includes a first clutch <NUM> that couples the ring gear shaft <NUM> to the ring gear <NUM>. A second clutch <NUM> couples the ring gear <NUM> to a static engine structure <NUM>. In this example, both the first clutch <NUM> and second clutch <NUM> are mechanical one-way clutches. Moreover, in this example, the first and second mechanical one-way clutches <NUM>, <NUM> are sprag clutches. It should be appreciated that although sprag clutches are disclosed by way of example, other mechanical clutch systems could be utilized and are within the contemplation of this disclosure.

The second clutch <NUM> couples the ring gear <NUM> to the engine static structure <NUM> during a starting operation to prevent rotation of the ring gear <NUM> and thereby the first tower shaft <NUM> and the low speed spool <NUM>. When the ring gear <NUM> is fixed, the starter <NUM> will drive the sun gear shaft <NUM> such that it will be the only driving output back to the high speed spool <NUM>.

During a starting operation, the starter <NUM> will drive the sun gear shaft in a first direction. The starter clutch <NUM> will engage and enable driving of the sun gear shaft <NUM> by the starter <NUM>. The same rotation provided by the starter <NUM> will engage the ring gear <NUM> such that the second clutch <NUM> will lock the ring gear <NUM> to the static structure to prevent rotation of the ring gear <NUM>. The first clutch <NUM> is not locked in this direction, but does not receive a driving input and therefore does not rotate the corresponding first tower shaft <NUM>. The starter <NUM> thereby directly drives the high speed spool <NUM> to start the engine.

Once the engine has started and the high speed spool <NUM> is rotating at speed, the starter <NUM> will be stopped. The high speed spool <NUM> will begin driving the second tower shaft <NUM> and thereby the sun gear shaft <NUM>. The higher speed provided by the driving of the high speed spool <NUM> will disengage the starter <NUM>. Additionally, once the low speed spool <NUM> begins operation, the first tower shaft <NUM> will being rotating. The first clutch <NUM> will then engage to couple the ring gear shaft <NUM> to the ring gear <NUM>. In this operating condition, both the low speed spool <NUM> and the high speed spool <NUM> will drive portions of the superposition gear system <NUM>.

The first clutch <NUM> will couple power from the first tower shaft <NUM> to the ring gear <NUM>. The second clutch <NUM> will free wheel and allow rotation of the ring gear <NUM>. The second tower shaft <NUM> will drive the sun gear <NUM> and thereby the intermediate gears <NUM> and the carrier <NUM>. The carrier <NUM> will in turn drive carrier shaft <NUM> to drive the first gear system <NUM>, within the accessory gearbox <NUM>. Power from each of the high spool <NUM> and the low spool <NUM> will be split to drive the carrier shaft <NUM> and power the first group <NUM> of accessory components <NUM>, <NUM>, <NUM>, and <NUM> throughout engine operation.

The second group <NUM> includes the accessory component <NUM> and is driven by a gear coupling <NUM> to the ring gear shaft <NUM>. The third group <NUM> of accessory components <NUM> is driven by a gear coupling <NUM> to the sun gear shaft <NUM>. Each of the ring gear shaft <NUM>, sun gear shaft <NUM> and carrier shaft <NUM> rotate at different speeds and therefore the accessory components <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are driven by the shaft <NUM>, <NUM> and <NUM> that best corresponds with a desired speed and operation of each accessory component.

In the disclosed example embodiment, the fuel pump <NUM> is driven by the sun gear shaft <NUM> that is in turn driven by the high speed spool <NUM>. Fuel flow requirements are tied closely to the speed of the high speed spool <NUM>. Coupling the fuel pump <NUM> to the sun gear shaft <NUM> enables driving of the fuel pump <NUM> by only the high speed spool <NUM> at a speed that corresponds and changes proportionally with operation of the engine <NUM> to provide a more desirable corresponding variation in operation. Moreover, the gear coupling <NUM> can be set to generate the desired proportional speed of the fuel pump in direct relationship to the speed of the high speed spool <NUM>.

The first group <NUM> including the main oil pump <NUM>, generator <NUM>, deoiler <NUM> and hydraulic pump <NUM> all demand significant power without a strong tie or correlation to a speed of either of the spools <NUM>, <NUM>. Accordingly, the first group <NUM> of accessories is driven through the gear coupling <NUM> by the carrier shaft <NUM>. The carrier shaft <NUM> is driven by power extracted from both spools <NUM>, <NUM> to provide a steady consistent power output.

The third group <NUM> of accessory components in this example includes the auxiliary oil pump <NUM> that is driven through the gear coupling <NUM> by the ring gear shaft <NUM>. The ring gear shaft <NUM> is coupled to the low speed spool <NUM>. The auxiliary oil pump <NUM> is desired to operate during low and negative G environments as well as operate when the engine is not operating but the fan is rotating to drive the low speed spool <NUM>. In configurations where the accessory components are driven only by the high speed spool <NUM>, the auxiliary oil pump <NUM> is not engaged and therefore does not produce an output flow of oil <NUM>. However, the third output from the superposition gear system <NUM> enables direct driving by the low speed spool <NUM> as needed to provide the desired operation.

The gear couplings <NUM>, <NUM> and <NUM> are each defined in consideration of the accessory component driven. Each of the gear couplings drive corresponding gear systems <NUM>, <NUM> and <NUM> that are tailored to provide the most efficient speeds for each accessory component <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

According, the example accessory drive system <NUM> includes the superposition gear system <NUM> that automatically distributes input driving torque between the low speed spool <NUM>, the high speed spool <NUM> as required during engine operation. The selective operation of the superposition gear system <NUM> is enabled by one-way mechanical clutches that provide different combinations of inputs and outputs that are automatically couple based on engine operating conditions. Moreover, the direct starting input is enabled by coupling of the starter directly to the sun gear shaft <NUM> along with fixing of the ring gear by a one-way mechanical clutch. Additionally, the superposition gear system <NUM> enables driving of different accessory components through different outputs at differing speeds to tailor each individual speed to each accessory component.

Claim 1:
An accessory gearbox (<NUM>) for a turbofan engine (<NUM>), the accessory gearbox (<NUM>) comprising:
a superposition gear system (<NUM>) including a sun gear (<NUM>);
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 accessory gearbox further comprises:
a starter (<NUM>) selectively coupled to the sun gear (<NUM>) through a starter clutch (<NUM>);
a first means for selectively coupling the ring gear (<NUM>) to a first tower shaft (<NUM>) of the turbofan engine (<NUM>);
a second means for selectively coupling the ring gear (<NUM>) to a fixed structure (<NUM>);
a first output coupled to a carrier shaft (<NUM>) for driving a first group (<NUM>) of accessory components (<NUM>, <NUM>, <NUM>, <NUM>); and
a second output coupled to the first tower shaft (<NUM>) for driving a second group (<NUM>) of accessory components (<NUM>, <NUM>, <NUM>, <NUM>),
and in that:
the sun gear (<NUM>) is coupled to a second tower shaft (<NUM>) of the turbofan engine (<NUM>);
the second output comprises a ring gear shaft (<NUM>) driven by the first tower shaft (<NUM>) that is coupled to a second gear system (<NUM>) for driving the second group (<NUM>) of accessory components (<NUM>, <NUM>, <NUM>, <NUM>).