Gas turbine engine

Abstract

A gas turbine engine for an aircraft. The engine comprising: an engine core comprising a turbine, a compressor, a fan located upstream of the compressor and comprising a plurality of fan blades, and a core shaft connecting the turbine to the compressor; a gearbox which receives an input from the core shaft and outputs drive, via a driveshaft, to the fan so as to drive the fan at a lower rotational speed than the turbine, the drive shaft and core shaft forming a shaft system. The shaft system provides: a first portion which extends forward from a first thrust bearing to the fan, the first thrust bearing supporting the shaft system and being located between the turbine and the gearbox, and a second portion extending rearward from the first thrust bearing to the turbine, such that in the event of a shaft break within the second portion of the shaft system, said shaft break dividing the shaft system into a front portion axially located by the first thrust bearing and a rear portion no longer axially located by the first thrust bearing, the rear portion is free to move axially rearwardly under a gas load; and wherein the engine further comprises a shaft break detector, configured to detect a shaft break in the shaft system.

We claim: 
     
         1 . A gas turbine engine for an aircraft comprising:
 an engine core comprising a turbine, a compressor, a fan located upstream of the compressor and comprising a plurality of fan blades, and a core shaft connecting the turbine to the compressor;   a gearbox which receives an input from the core shaft and outputs drive, via a driveshaft, to the fan so as to drive the fan at a lower rotational speed than the turbine, the drive shaft and core shaft forming a shaft system, wherein the shaft system provides:
 a first portion which extends forward from a first thrust bearing to the fan, the first thrust bearing supporting the shaft system and being located between the turbine and the gearbox, and 
 a second portion extending rearward from the first thrust bearing to the turbine, such that in the event of a shaft break within the second portion of the shaft system, said shaft break dividing the shaft system into a front portion axially located by the first thrust bearing and a rear portion no longer axially located by the first thrust bearing, the rear portion is free to move axially rearwardly under a gas load; and 
   wherein the engine further comprises a shaft break detector, configured to detect a shaft break in the shaft system.   
     
     
         2 . The gas turbine engine as claimed in  claim 1 , wherein the core shaft is supported by one or more non-thrust bearings located rearwards of the first thrust bearing. 
     
     
         3 . The gas turbine engine as claimed in  claim 1 , wherein the first thrust bearing is located such that the pathway for torque transmission from the turbine to the compressor includes the first portion of the shaft system. 
     
     
         4 . The gas turbine engine as claimed in  claim 1 , wherein the drive shaft is supported forward of the first thrust bearing by one or more further thrust bearings. 
     
     
         5 . The gas turbine engine as claimed in  claim 4 , wherein the first thrust bearing and a further thrust bearing are mechanically coupled in that one raceway of the further thrust bearing is rotationally locked relative to a radially opposite raceway of the first thrust bearing. 
     
     
         6 . The gas turbine engine as claimed in  claim 1 , wherein the shaft break detector includes a pair of phonic wheels spaced axially along the shaft system, each configured to sense a rotational speed of a respective portion of the shaft system. 
     
     
         7 . The gas turbine engine as claimed in  claim 1 , wherein the shaft break detector includes a pair of microwave sensors, configured to sense respective rotational speeds of axially spaced portions of the shaft system. 
     
     
         8 . The gas turbine engine as claimed in  claim 1 , wherein the shaft break detector includes a phonic wheel, located at a first portion of the shaft system, and a microwave sensor, configured to sense a rotational speed of the shaft system at a second portion of the shaft system axially spaced form the first portion. 
     
     
         9 . The gas turbine engine as claimed in  claim 1 , further comprising a roller bearing supporting the drive shaft. 
     
     
         10 . The gas turbine engine as claimed in  claim 1 , wherein the shaft break detector is configured to register a shaft break when it detects a twist exceeding a predetermined threshold between two points in the first portion of the shaft system. 
     
     
         11 . The gas turbine engine as claimed in  claim 1 , further comprising an axial movement sensor, configured to register a shaft break when it detects rearward axial movement of the rear portion of the shaft system in the event of a shaft break within the second portion of the shaft system. 
     
     
         12 . The gas turbine engine as claimed in  claim 1 , further including a sealed cavity located rearward of the first thrust bearing, wherein the sealed cavity, during operation of the gas turbine engine, is pressurised to a pressure exceeding that of a cavity containing the first thrust bearings, such that an axially forward force is applied to the core shaft. 
     
     
         13 . The gas turbine engine as claimed in  claim 1 , wherein:
 the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft;   the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor;   the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft; and   wherein the second core shaft is axially located by one or more respective thrust bearings.   
     
     
         14 . The gas turbine engine as claimed in  claim 13 , wherein the first thrust bearing of the first core shaft and a thrust bearing of the second core shaft are mechanically coupled in that a radially outer raceway of the second core shaft thrust bearing is rotationally locked relative to a radially inner raceway of the first thrust bearing. 
     
     
         15 . The gas turbine engine as claimed in  claim 1 , wherein the fan is coupled to an input shaft of the gear box through a fan catcher shaft, which axially locates the fan, and through a fan shaft component which transmits torque from the gearbox but does not axially locate the fan.