Patent Application
20120006035
The subject matter disclosed herein relates to an air turbine starter for a gas turbine engine. More specifically, the subject disclosure relates to a shut-off mechanism for an air turbine starter.
Aircraft engines, for example, gas turbines, are typically equipped with an air turbine starter (ATS) mounted on the engine accessory gearbox. The functional purpose of the ATS is to accelerate the engine up to a desired speed prior to ignition of the engine combustor and to continue assisting the engine start until the engine is capable of operating independently. The ATS is typically driven by pressurized air provided by an air source such as an auxiliary power unit, another operating engine, or an external air cart connected to the ATS. Pressurized air or gas fed into the ATS drives rotation of an ATS rotor causing rotation of a starter shaft. The starter shaft transmits this rotation to the drive shaft of the accessory gearbox. Rotation of the gearbox shaft drives rotation of a high pressure rotor of the engine which induces airflow into the engine and causes rotation of the engine high pressure rotor assembly. When the engine rotation reaches a desired speed, the ATS is turned off by stopping pressurized air flow.
In certain conditions, the ATS may experience operation at a turbine free run speed, in which the turbine rotor operates without a resistive load on the output shaft, causing the turbine to accelerate to high speed. Such conditions may include an engine start system failure or a momentary loss of pressure at the ATS inlet which causes premature disengagement of an internal clutch of the ATS. Operation at turbine free run speed leads to failure of ATS turbine bearings thus allowing axial travel of the ATS rotor. Without quickly halting rotation of the ATS rotor, the ATS rotor will catastrophically fail causing significant damage to the ATS.
According to one aspect of the invention, an air turbine starter includes an air turbine starter rotor rotatably located at a central axis. The rotor includes a plurality of aerodynamic surfaces extending from a platform. At least one cutting element is located in the air turbine starter such that travel of the rotor along the central axis during operation of the air turbine starter results in removal of the platform and aerodynamic surfaces from the rotor via contact between the at least one cutting element and the rotor.
According to another aspect of the invention, a method of stopping operation of an air turbine starter includes locating at least one cutting element in an air turbine starter proximate to an air turbine starter rotor. The rotor is rotatably located at a central axis and includes a plurality of aerodynamic surfaces secured to a platform. The at least one cutting element contacts the rotor as a result of travel of the rotor along the central axis during operation of the air turbine starter and cuts through the rotor via contact between the rotating rotor and the at least one cutting element thereby releasing the platform and the plurality of aerodynamic surfaces from the rotor.
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
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The ATS rotor 12 includes a plurality of aerodynamic surfaces, for example, blades 26 located at a blade platform 28. The blades 26 and platform 28 are typically connected to a rotor disc 30 via a relatively thin-walled section, or web 32. The blades 26 may be shrouded or unshrouded. At least one cutting element 34 extends toward the web 32 from, for example, the exhaust deflector 24. The cutting element 34 includes a tip portion 36. The tip portion 36 may be triangular in shape as shown or may be another shape, for example, rectangular or partially spherical, depending on the shape of cut desired. In some embodiments, the tip portion 36 is formed integral to the cutting element 34, while in others the tip portion 36 is a separate element, such as a commercial machining tool insert affixed to the cutting element 34. The cutting element 34 and/or tip portion 36 can be formed from tungsten carbide or other suitable cutting material.
Under conditions where the ATS 10 operates at turbine free run speed, such as during an engine start system failure or premature disengagement of the clutch, operation at turbine free run speed leads to failure of the bearings 20, thus allowing axial travel of the ATS rotor 12 in the direction of the cutting element 34 due to the air pressure on the blades 26 and the rotor disc 30. When the ATS rotor 12 travels a sufficient distance, the stationary cutting element 34 engages the web 32 of the rotating ATS rotor 12 and proceeds to cut through the web 32 material and release the platform 28 and blades 26 from the ATS rotor 12. With the blades 26 released from the ATS rotor 12, rotation of the ATS rotor 12 stops, thus preventing further damage to the ATS 10.
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While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
