A power transmission device for a gas turbine engine

FIELD OF THE INVENTION

The present invention relates to a power transmission device for a gas turbine engine comprising a low pressure shaft and a high pressure shaft, wherein the high pressure shaft is arranged concentric to the low pressure shaft. The power transmission device further comprises an accessory unit comprising at least one electric machine adapted for a gas turbine engine starting operation and/or for generating electrical power.

The invention also relates to a gas turbine engine comprising the power transmission device and an aeroplane comprising the gas turbine engine.

The low pressure shaft is adapted to drive a low pressure compressor or fan in the gas turbine engine and the high pressure shaft is adapted to drive a high pressure compressor in the gas turbine engine. The gas turbine engine may be a turbojet or turbofan engine.

The accessory unit comprises an accessory gearbox which is adapted to drive further ancillary services both of the turbine engines themselves and of the aeroplane for which they provide propulsion. Such aeroplane ancillary services comprises ancillary machines such as generators, fuel pumps, fuel flow governors, oil pumps, centrifugal breathers, etc.

The need of electric power to be used for the operation of aeroplane services will most probably drastically increase in the future. Therefore, it will be necessary to extract more power from the turbine engines . However, the high pressure shaft that, by tradition, has been the shaft from which said power is extracted, will not be able to deliver all of said power alone, due to the fact that a large mechanical power extraction has a negative effect on the operation of the high pressure shaft as it is likely to cause pumping of the compressor, in particular when the engine is running at low speed.

Accordingly, the manufacturers of aeroplane engines are considering to use also the low pressure shaft for the purpose of transferring power to the accessory gearbox.

One problem with using the low pressure shaft to extract power is the low speed and large operational speed range, which does not match the requirements of the ancillary machines. Contemporary solutions for the extraction of power from the LP-shaft include an extra system of bevel gears and shafts.

PRIOR ART US 5,103,631 discloses a differential gear assembly suitable for providing power take off from a gas turbine engine, comprising first and second power input gears which drive a plurality of idler gears rotatably mounted on a common cage structure. The idler gears are grouped in meshing pairs so that one idler gear of each pair is additionally in meshing engagement with one of the power input gears and the other idler gear of each

pair is additionally in meshing engagement with the other of the power input gears. Together the first and second power input gears drive the common cage structure at the mean speed of the power input gears and the common cage structure in turn drives a single power output shaft. Accordingly, US 5,103,631 discloses a device that uses a planetary gear for the purpose of retrieving power from both LP-shaft and HP-shaft and transfer it to an accessory gearbox. US 5,103,631 also discloses the possibility of providing a different division of torque split between the two power input shafts, by modifying the gear assembly so that one of the gears is of larger diameter than the other. It is also mentioned that any use of input gears which are not of the same diameter would necessitate the appropriate repositioning of the idler gears .

US patent application No. 2006/0034696 discloses a twin-spool turbojet engine with a differential with a first planetary pinion driven by the shaft of the high pressure spool, and a second planetary pinion driven by the shaft of the low pressure spool, said planetary pinions driving satellite pinions mounted in a cage by which the ancillary machines are driven. By means of a separate transmission, a step down gear mechanism, connected to the high pressure shaft upstream the differential as seen in the power drive train direction, the mechanical transmission ratio between the high pressure shaft and the first planetary pinion of the differential gear, and the transmission ratio between the low pressure shaft and the second planetary pinion of the differential gear are such that the power

extraction distribution between the high pressure spool and the low pressure spool is appropriately distributed for the different operation conditions (rotational speeds) of the engine. The power extraction distribution between the high pressure spool and the low pressure spool at idle speed is between 80%/20% and 20%/80%.

SUMMARY OF THE INVENTION A primary purpose of the invention is to achieve a power transmission device for a gas turbine engine, which creates conditions for extracting a larger proportion of the power and preferably all power required by the ancillary machines from the low pressure shaft.

This purpose is achieved in that the power transmission device comprises a transmission unit operatively connected to the low pressure shaft and adapted for power transmission from the low pressure shaft to the accessory unit, that the transmission unit is positioned non-concentric with the low pressure shaft, that a first transmission means is operatively connected for power transmission from the low pressure shaft to the transmission unit, and that a second transmission means is operatively connected for power transmission from the accessory unit to the high pressure shaft in a manner bypassing the transmission unit.

The wording "bypassing" should be interpreted as functionally (or operationally) bypassing the transmission unit in that the transmission unit does not effect the power transmission path from the accessory unit to the high pressure shaft. Preferably, the second transmission means is configured so that the power

transmission path in its entirety extends externally of the transmission unit. In other words, the second transmission means is arranged in parallel to the first transmission means and the transmission unit.

By virtue of arranging the first transmission means operatively connected to the low pressure shaft and the second transmission means operatively connected to the high pressure shaft, a starting operation may be performed using the high pressure shaft, while power off take may be performed from the low pressure shaft.

Further, the power transmission device creates conditions for a minimum of structural modification and additional parts of the gas turbine engine. More specifically, only one connection is required to the low pressure shaft and only one connection is required to the high pressure shaft.

Preferably, each of the first and second transmission means comprises a tower shaft extending out of a gas turbine engine housing, which houses the low pressure shaft and the high pressure shaft. In this way, the transmission unit may be located externally of the gas turbine engine housing, wherein service is facilitated.

According to a further preferred embodiment of the invention, the device comprises means for operatively connecting and disconnecting, respectively at least one of said electric machines adapted for the gas turbine engine starting operation to the high pressure shaft. In this way, the gas turbine engine may be started via the high pressure shaft. Preferably, the connection means is adapted to automatically disconnect said at least one of

said electric machines from operational connection with the high pressure shaft when a speed of the high pressure shaft exceeds a predetermined limit value. Thus, the connection means is passive in the respect that no control means is required for its activation. This creates conditions for a reliable operation. Especially, the predetermined limit value corresponds to a maximum speed for a gas turbine engine starting operation.

According to a further preferred embodiment of the invention, the second transmission means is operatively connected for power transmission from at least one of said electric machines adapted for the gas turbine engine starting operation to the high pressure shaft and also for power transmission from the high pressure shaft to the accessory unit. Thus, the second transmission means is adapted for a two-way power transmission.

According to a further preferred embodiment of the invention, the transmission unit comprises a gear changing mechanism adapted to change between at least two gears for said power transmission from the low pressure shaft to the accessory unit. This is especially advantageous due to the large operational speed range of the low pressure shaft. Thus, this embodiment creates conditions for power transmission from the low speed shaft over substantially the complete operational speed range of the low pressure shaft.

The complete operational speed range of the low pressure shaft corresponds to a self-sustained operation of the gas turbine engine comprising idling, a medium and higher operational speed range.

Further preferred embodiments will be apparent from the further claims, the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained below, with reference to the embodiments shown on the appended drawings, wherein FIG 1 is a schematic illustration of a gas turbine engine in the form of an aircraft engine, FIG 2 is a schematic illustration of a first embodiment of a power transmission device for the gas turbine engine,

FIG 3 is a schematic illustration of a second embodiment of a power transmission device for the gas turbine engine,

FIG 4 and 5 show an example of a gear set in the power transmission device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 is an oversight view of a gas turbine engine 1 according to the invention provided with a power transmission device 2 according to the invention. The gas turbine engine 1 shown in fig. 1 is of conventional construction and comprises, in axial flow series, an air intake 3, low pressure compressor 4, high pressure compressor 5, combustion equipment 6, high pressure turbine 7, low pressure turbine 8 and an exhaust outlet 9. During operation, the high pressure compressor is driven by the high pressure turbine via a first hollow high pressure shaft 10. Similarly, the low pressure compressor is driven by the low pressure turbine via a

second hollow low pressure shaft 11 which is coaxially disposed within the high pressure shaft 10.

The gas turbine 1 operates in the conventional manner whereby air drawn in through the air intake 3 is compressed by the low pressure compressor before passing into the high pressure compressor where it is compressed further. The compressed air then flows into the combustion equipment 6 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through the high and low pressure turbines 7, 8 before being exhausted to the atmosphere through the exhaust nozzle 9.

The power transmission device 2 comprises a transmission unit 13 (see figure 2) , which is adapted to transfer power from the low pressure shaft 11 to an accessory unit 12 during engine operation, said accessory unit 12 being located on the outside of the engine 1. The accessory unit 12 comprises an accessory gearbox 31 and is adapted to drive different ancillary machines, such as generators and oil or fuel hydraulic pumps . More specifically, the accessory unit 12 comprises at least one electric machine 29,30 adapted for a gas turbine engine starting operation and for generating electrical power. However, during start up of the engine 1, the power transfer direction is reversed, whereby power is transferred from a generator in the accessory unit 12 to the high pressure shaft 10. Once, by definition, the engine 1 has been started and power may be extracted from the low pressure shaft 11, the device according to the invention is set in an operation mode by which power is transferred from the

low pressure shaft 11 to the accessory unit 12 through said transmission unit 13.

Figure 2 shows the power transmission device 2 according to a first embodiment. In the shown embodiment, the high pressure shaft 10 and the low pressure shaft 11 are counter-rotating. The transmission unit 13 is arranged externally of a gas turbine engine housing 15, which accommodates at least part of the low pressure shaft. The transmission unit 13 is operatively connected to the low pressure shaft 11 and adapted for power transmission from the low pressure shaft 11 to the accessory unit 12. The transmission unit 13 is configured for changing gears. More specifically, the transmission unit 13 is a step gear transmission. The transmission unit 13 comprises a gear changing mechanism 14 adapted to change between at least two gears for transmitting power from the low pressure shaft 11 to the accessory unit 12 over substantially the complete operational speed range of the low pressure shaft.

A first transmission means 18 is operatively connected for power transmission from the low pressure shaft 11 to the externally located accessory unit 12 via the externally located transmission unit 13. The first transmission means 18 comprises a first tower shaft 19. More specifically, the first tower shaft 19 is connected to the gear changing mechanism 14 in the transmission unit 13 and the low pressure shaft 11, respectively, via bevel gears 27,28.

A second transmission means 32 is operatively connected for power transmission from the accessory unit 12 to the

high pressure shaft 10. The second transmission means 32 comprises a second tower shaft 38. The second tower shaft 38 is connected to the accessory unit 12 and the high pressure shaft 10, respectively, via bevel gears 34,35. The first and second transmission means 18,32 are arranged at a distance from each other in an axial direction 20 of the gas turbine engine 1. The tower shafts 19,38 extend substantially at right angles with the gas turbine rotational axis 20.

The accessory unit 12 is arranged externally of the gas turbine engine housing 15. Further, the tower shafts 19,38 extends through two separate holes in the casing 15 in order to connect the externally located accessory unit 12 with the low and high pressure shafts 10,11.

The device 2 further comprises means 16 for operatively connecting and disconnecting, respectively the accessory unit 12 to the high pressure shaft 10. The connection means 16 is adapted to automatically disconnect the accessory unit 12 from operational connection with the high pressure shaft 10 when a speed of the high pressure shaft exceeds a predetermined limit value, which corresponds to a maximum speed for a gas turbine engine starting operation. More specifically, according to the first embodiment, the connection means 16 comprises a clutch. The high pressure shaft clutch 16 is formed by a freewheel mechanism. The high pressure shaft clutch 16 is arranged externally of the housing 15 and preferably directly on an output shaft of the accessory unit 12, i.e upstream of the second tower shaft in the power transmission direction during a starting operation.

The accessory unit 12 is operatively connected to the high pressure shaft 10 in a manner bypassing the transmission unit 13. The transmission unit 13 is adapted to transmit at least 50%, preferably at least 80% and especially 100% of the required power for the components/machines in the accessory unit 12 from the low pressure shaft 11 over substantially the complete operational speed range of the low pressure shaft.

In order to transmit all, or at least substantially all power to the accessory unit 12 from the low pressure shaft 11 over substantially the complete operational speed range of the low pressure shaft 11, the transmission unit 13 comprises a gear set 21 arranged between the low pressure shaft 11 and the accessory unit 12. The gear changing mechanism 14 is adapted for connecting and disconnecting, respectively, the gear set 21 from being drivingly connected to the accessory unit 12. More specifically, the transmission unit 13 comprises an intermediate power transmission shaft 17, which is drivingly connected between the bevel gear 27 and the accessory unit 12. The gear set 21 is in direct operational connection with the intermediate power transmission shaft 17. Further, the gear changing mechanism 14 is also in direct operational connection with the intermediate power transmission shaft 17.

The gear changing mechanism 14 comprises a clutch. Preferably, the clutch 14 is a one-way clutch in the form of a free-wheeling clutch.

In a low speed range, the gear changing mechanism 14 is disengaged from the low pressure shaft 11, wherein power is transmitted from the low pressure shaft 11 to the

accessory unit 12 via the gear set 21. In a high speed range, the gear changing mechanism 14 is engaged, wherein power is transmitted from the low pressure shaft 11 to the accessory unit 12 via the tower shaft and the gear changing mechanism 14.

The gear set 21 is a differential gear in the form of an epicyclic gear, see fig. 4-5. Means 23 is arranged for activating the gear set 21. The activation means 23 comprises a clutch. 24, or brake, which is adapted to operationally connect an input gear member 101 to a stationary part 25, or wall, in the transmission unit. More specifically, the clutch is a multi-plate clutch. The clutch 24 is the only active means in the device 2 in the respect that it is the only means that is actively controlled during operation. Preferably, a stationary arranged hydraulic piston may be used for application of the clutch, wherein oil supply and drainage may be arranged easily and reliably. The differential gear member to be locked may be a sun gear in the differential gear 21.

The differential gear 21 comprises at least one pair of idler gears 102,103, see figure 4, which are arranged in meshing engagement with each other. A first idler gear 102 is in meshing engagement with the input gear member 101. A second idler gear 103 is in meshing engagement with an output gear member 104. The idler gears 102,103 are rotatably mounted in a differential gear casing 105. The differential gear casing 105 comprises means 106 for rotationalIy rigidly connecting the casing 105 to the intermediate power transmission shaft 17. Said connection means 106 comprises a plurality of circumferentialIy spaced radial drive spokes 106. The

spokes 106 are arranged axialIy between the input gear member 101 and the output gear member 104.

The accessory unit 12 comprises two electric machines 29,30 adapted for a starting operation and for generating electrical power. These electric machines 29,30 are in the following referred to as a Starter/Generator, or S/G.

In a starting operation, one or both S/G 29,30 is controlled to drive the high pressure shaft 10 via the free-wheeling clutch 16. The low pressure , shaft 11 is however not driven since the clutch 24 for the gear set 21 is not activated and the clutch 14 is free-wheeling. When the high pressure shaft 10 is accelerated to a predetermined speed limit value (a maximum speed for the starting operation) , the high pressure shaft clutch 16 will automatically disconnect from a driving connection and freewheel.

During idling, the S/G is switched to an electric power generating state, the gear set 21 is activated via the clutch 24 and the one-way clutch 14 is free-wheeling.

In a medium operational speed range, the multi-plate clutch 24 is released in a manner with controlled slip, wherein the low pressure shaft 11 drives the accessory unit 12 via the intermediate shaft 17 and the clutch 14. This operation is continued in a higher operational speed range (comprising a maximum speed) .

When the speed of the low pressure shaft is reduced to a predetermined limit value, the multi-plate clutch 24 is actuated in a manner with controlled slip, the one-way-

clutch 14 is automatically released and will freewheel, wherein the low pressure shaft 11 drives the accessory unit 12 via the intermediate shaft 17 and the gear set 21.

The invention is not in any way limited to the above described embodiments, instead a number of alternatives and modifications are possible without departing from the scope of the following claims .

According to an alternative embodiment, the first and second transmission means 18,32 may be arranged in a coaxial (concentric) relationship. More precisely, the first tower shaft 19 may be arranged concentric to the second tower shaft 38. In this way, only one hole is required in the casing 15 for the two tower shafts in order to connect the externally located accessory unit 12 with the low and high pressure shafts 10,11.

According to a further alternative embodiment, the gear set may be of a different type and/or design than the disclosed differential gear. For example, the differential gear may be replaced by a planetary gear. Such a planetary gear may be of a design in which the ring gear is locked in order to activate the gear. Further, the planet carrier may form an input (and is rotationally rigidly connected to the intermediate power transmission shaft) while the sun gear forms an output.

In the shown embodiment, the high pressure shaft 10 and the low pressure shaft 11 are counter-rotating. The invention is however applicable also in gas turbines where the shafts rotate in the same direction.