Abstract:
A method and system for ultrasonically inspecting a ring gear of a gearbox is provided. The method and system can be used for the detection, characterization and/or sizing of defects in the ring gear. The ring gear has a plurality of teeth, which extend radially inward with respect to an outer surface of the gearbox. The method includes the steps of selecting an ultrasonic transducer productive of a test signal, positioning the ultrasonic transducer at an outer surface of the ring gear, orienting the ultrasonic transducer so as to direct the test signal to propagate through the ring gear, and activating the ultrasonic transducer so as to test the ring gear for defects therein.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present disclosure relates generally to a method for inspecting the ring gear in a gearbox, and particularly to a method for inspecting a ring gear in a gearbox for the detection, characterization and/or sizing of defects therein. 
         [0002]    Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient. 
         [0003]    Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is typically fed into a utility grid. 
         [0004]    Wind turbine gearboxes are subject to a large amount of loads. Most of these loads are from the rotor and can include main shaft torque, weight of the rotor, axial thrust forces and other dynamic loads. Some of these loads will be steady, some may be transient, while others may vary randomly or periodically. All of the loads contribute to fatigue damage and wear on the various components of the gearbox. Because the gearbox is difficult to remove and replace, failure of any gearbox component can result in a lengthy and expensive repair process. For example, to remove a gearbox from an existing wind turbine, a large crane must be transported to and erected on site. The wind turbine will be off-line and out of energy production during the repair process. 
         [0005]    One component of a gearbox that can fail is the ring gear. One known inspection method for the ring gear is a visual inspection using borescope-type equipment. Visual testing can be used to find large defects such as missing sections of the gear teeth but the capability of the inspection for finding small crack-like defects is not as high as possible. Improving the capability of the visual inspection would result in a time-consuming evaluation resulting in increased downtime and expense. Any inspection method which uses the borescope holes in the gearcase for access will have limited capability for finding small crack-like defects. For instance, any visual surface inspection method would need to interrogate each gear tooth surface. This process would be very difficult, if possible, due to the limited amount of access ports. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one aspect of the present invention, a method for ultrasonically inspecting a ring gear of a gearbox is provided. The method can be used for the detection, characterization and/or sizing of defects in the ring gear. The ring gear has a plurality of teeth, which extend radially inward with respect to an outer surface of the gearbox. The method includes the steps of selecting an ultrasonic transducer productive of a test signal, positioning the ultrasonic transducer at an outer surface of the ring gear, orienting the ultrasonic transducer so as to direct the test signal to propagate through the ring gear, and activating the ultrasonic transducer so as to test the ring gear for defects. 
         [0007]    In another aspect of the present invention, a method for inspecting a component of a wind turbine is provided. The method can be used for the detection, characterization and/or sizing of defects in the component. The component has a plurality of teeth, which extend radially inward with respect to an outer surface of the component. The method includes the steps of selecting a transducer productive of a test signal, positioning the transducer at an outer surface of the component, orienting the transducer so as to direct the test signal to propagate through at least a portion of the component, and activating the transducer so as to test the component for defects. 
         [0008]    In yet another aspect of the present invention, a system for inspecting a component of a wind turbine is provided. The system can be used for the detection, characterization and/or sizing of defects in the component. The component has a plurality of teeth extending radially inward with respect to an outer surface of the component. The system includes a transducer productive of a test signal, and the transducer is positioned at an outer surface of the component. The transducer is oriented so as to direct the test signal to propagate through at least a portion of the component, and is employed to test the component for defects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures: 
           [0010]      FIG. 1  is a schematic illustration of an exemplary wind turbine generator; 
           [0011]      FIG. 2  is a fragmentary cross-sectional schematic illustration of a nacelle that may be used with the exemplary wind turbine generator shown in  FIG. 1 ; 
           [0012]      FIG. 3  is a simplified, cut-away illustration of a wind turbine that includes a gearbox; 
           [0013]      FIG. 4  is a perspective illustration of the input end of the gearbox of Figure. 
           [0014]      FIG. 5  is a partial, cross-sectional illustration of a ring gear shown undergoing an inspection procedure, according to an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  is a schematic illustration of an exemplary wind turbine  100 . In the exemplary embodiment, wind turbine  100  is a horizontal axis wind turbine. Alternatively, wind turbine  100  may be a vertical axis wind turbine. Wind turbine  100  has a tower  102  extending from a supporting surface  104 , a nacelle  106  mounted on tower  102 , and a rotor  108  coupled to nacelle  106 . Rotor  108  has a rotatable hub  110  and a plurality of rotor blades  112  coupled to hub  110 . In the exemplary embodiment, rotor  108  has three rotor blades  112 . In an alternative embodiment, rotor  108  may have more or less than three rotor blades  112 . In the exemplary embodiment, tower  102  is fabricated from tubular steel and has a cavity (not shown in  FIG. 1 ) extending between supporting surface  104  and nacelle  106 . In an alternate embodiment, tower  102  is a lattice tower. The height of tower  102  is selected based upon factors and conditions known in the art. 
         [0016]    Blades  112  are positioned about rotor hub  110  to facilitate rotating rotor  108  to transfer kinetic energy from the wind into usable mechanical energy, and subsequently, into electrical energy. Blades  112  are mated to hub  110  by coupling a blade root portion  120  to hub  110  at a plurality of load transfer regions  122 . Load transfer regions  122  have a hub load transfer region and a blade load transfer region (both not shown in  FIG. 1 ). Loads induced in blades  112  are transferred to hub  110  via load transfer regions  122 . 
         [0017]    In the exemplary embodiment, blades  112  have a length between about 50 meters (m) (164 feet (ft)) and about 100 m (328 ft). Alternatively, blades  112  may have any length. As the wind strikes blades  112 , rotor  108  is rotated about rotation axis  114 . As blades  112  are rotated and subjected to centrifugal forces, blades  112  are subjected to various bending moments and other operational stresses. As such, blades  112  may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position and associated stresses, or loads, may be induced in blades  112 . Moreover, a pitch angle of blades  112 , i.e., the angle that determines blades  112  perspective with respect to the direction of the wind, may be changed by a pitch adjustment mechanism (not shown in  FIG. 1 ) to facilitate increasing or decreasing blade  112  speed by adjusting the surface area of blades  112  exposed to the wind force vectors. Pitch axes  118  for blades  112  are illustrated. In the exemplary embodiment, the pitches of blades  112  are controlled individually. Alternatively, the pitches of blades  112  may be controlled as a group. 
         [0018]    In some configurations, one or more microcontrollers in a control system (not shown in  FIG. 1 ) are used for overall system monitoring and control including pitch and rotor speed regulation, yaw drive and yaw brake application, and fault monitoring. Alternatively, distributed or centralized control architectures are used in alternate embodiments of wind turbine  100 . 
         [0019]      FIG. 2  is a fragmentary cross-sectional schematic view of nacelle  106  of exemplary wind turbine  100 . Various components of wind turbine  100  are housed in nacelle  106  atop tower  102  of wind turbine  100 . Pitch drive mechanisms  130  (only one illustrated in  FIG. 2 ) modulate the pitch of blades  112  along pitch axis  118  (both shown in  FIG. 1 ). 
         [0020]    Rotor  108  is rotatably coupled to an electric generator  132  positioned within nacelle  106  via rotor shaft  134 , sometimes referred to as low speed shaft  134 , a gearbox  136 , a high speed shaft  138 , and a coupling  140 . Forward and aft support bearings  152  and  154 , respectively, facilitate radial support and alignment of shaft  134 . Some wind turbines may omit the forward or aft support bearings, or have bearings integrated within the gearbox. Rotation of shaft  134  rotatably drives gearbox  136  that subsequently rotatably drives shaft  138 . Typically, a lubricating oil is used within gearbox  136 . Shaft  138  rotatably drives generator  132  via coupling  140  and shaft  138  rotation facilitates generator  132  production of electrical power. Gearbox  136  and generator  132  are supported by supports  142  and  144 , respectively. 
         [0021]    Also positioned in nacelle  106  is a yaw adjustment mechanism  146  that may be used to rotate nacelle  106  and rotor  108  on axis  116  (shown in  FIG. 1 ) to control the perspective of blades  112  with respect to the direction of the wind. Mechanism  146  is coupled to nacelle  106 . Meteorological mast  148  includes a wind vane and anemometer (neither shown in  FIG. 2 ). Mast  148  is positioned on nacelle  106  and provides information to the turbine control system that may include wind direction and/or wind speed. In alternative embodiments, mast  148  can be mounted on hub  110  and extend in a direction in front of rotor  108 . 
         [0022]    A portion of the turbine control system resides within control panel  150 . The turbine control system (TCS) controls and monitors various systems and components of wind turbine  100 . A plurality of sensors are distributed throughout wind turbine  100  and the status of various conditions (e.g., vibration level, temperature, etc.) are monitored. The sensed conditions are utilized by the TCS to control various subsystems of wind turbine  100 . 
         [0023]    Gearboxes in wind turbines are large components and require great effort and expense to remove and repair. It would be highly beneficial to detect flaws at an early stage to properly schedule gearbox repair and/or replacement. One such flaw can occur in the ring gear of the gearbox. A ring gear failure or defect (e.g., a cracked or missing gear tooth) can result in the shutdown of the wind turbine requiring repair and/or replacement of the gearbox. 
         [0024]    According to aspects of the present invention, a method is provided for nondestructively inspecting ring gears that uses ultrasonic waves transmitted from the outer diameter of the ring gear. The ultrasonic waves can be used to evaluate the volume of the ring gear and the gear teeth located on the inner diameter. The method can use a combination of beam angles to interrogate the ring gear. The method may use fixed-angle monolithic ultrasonic probes and/or one or more phased-array probes operating at multiple angles generating a sector scan. The method can detect and identify defects in the volume of the ring gear including defects in the gear teeth. 
         [0025]    One primary area of interest for on-site inspection of wind turbine gearboxes is the evaluation of the ring gear teeth. Aspects of the present invention can incorporate a damage tolerance design methodology, which can help detect small flaws in the gear teeth. If cracks can be detected at an early stage, before serious damage has occurred, then a scheduled outage can be planned to minimize down-time, expense and loss of energy production. 
         [0026]    The method, according to aspects of the present invention, is an inspection method performed from the outer diameter of the ring gear. The inspection method can use ultrasonic inspection technology where one or more ultrasonic transducers are situated on the outer diameter of the ring gear. On many wind turbine gearboxes, the outer diameter of the ring gear is one of the outer surfaces of the gearbox. This surface can be easily accessed for inspection of the ring gear. 
         [0027]    The ultrasonic inspection method can include ultrasonic scans performed at one or several beam angles. The angles can be chosen to optimize the detection of crack-like defects located in the gear teeth or volume of the ring gear. Other angles can be chosen to optimize detection of flaws generated from the manufacturing process, flaws initiated during service use, or any other desired flaw of interest. 
         [0028]    The ultrasonic transducer may be either a monolithic (single-crystal) operated at a 0-degree beam angle or mounted to a wedge to generate ultrasonic beams at angles other than 0 degrees. A phased-array transducer may also be used to generate a range of beam angles or multiple beam angles used for the ring gear evaluation. Other transducers could also be used, such as an electromagnetic acoustic transducer (EMAT), piezoelectric transducer, or monolithic piezoelectric transducer (MPT). A piezoelectric transducer may be comprised of a monolithic device or an array of separate elements. The transducer may be of the pulse-echo type, where one transducer produces and transmits the signal and the same transducer receives the reflected signal for subsequent analysis. However, the method may use a “pitch-catch” diagnostic technique, where at least one transmitting device transmits a signal and at least one receiving device receives the signal. The pitch-catch mode can include a single device that transmits and receives, or multiple devices where one or more transmitting devices and one or more receiving devices are used. The transmitting and receiving devices may be embodied in one assembly such as a “dual-element” transducer configuration. 
         [0029]    Multiple beam angles can be used to optimize detection of flaws or to provide complete coverage of the volume of the ring gear. Some ring gears such as those used in wind turbine gearboxes have machined holes in the volume of the gear. Scanning the transducer using several beam angles will allow complete evaluation of the volume of the ring gear. 
         [0030]    The ultrasonic transducer, according to one aspect of the present invention, can be scanned on the outside diameter of the ring gear. The transducer generates ultrasonic wave packets that are transmitted into the ring gear. The wave packets propagate through the ring gear material. If flaws are present in the ring gear, the sound will reflect from the flaws. The reflected sound waves propagating from the flaws will either follow a direct path back to the transducer or propagate to a surface of the ring gear where they will reflect and propagate towards the transducer. The transducer will then receive the reflected ultrasonic waves and convert the acoustic energy into electrical energy. The electrical signals will then be analyzed using ultrasonic test equipment to measure the time-of-flight for the wave reflections, the amplitude of the reflected signals, and other characteristics of the reflections such as phase, frequency, and waveform shape. This information along with the transducer beam angle and location of the transducer on the periphery of the ring gear can be used to determine the size and location of the defect or flaw in the ring gear. 
         [0031]    One advantage to the method of the present invention is that there is no requirement to open the borescope holes for access, as required by the known prior art inspection methods. This test is also performed from the outside surfaces of the gearbox, where access is facilitated. The evaluation of the full ring gear can be performed quickly with the ultrasonic inspection method according to the present invention, and is faster than the known visual inspection methods, which require visually reviewing all gear tooth surfaces for small flaws. 
         [0032]      FIG. 3  illustrates a simplified view of gearbox  136  in accordance with one embodiment of the invention. The silhouette  302  of the person standing inside the wind turbine housing indicates the approximate size of the gearbox  136  in comparison to service personnel. However, gearbox  136  can be of a variety of sizes, shapes, types and configurations. As can be seen, service personnel have good access around the entire outer casing of gearbox  136 . 
         [0033]      FIG. 4  illustrates an enlarged perspective view of the gearbox  136 . The gearbox  136  includes a gearbox housing that as illustrated is made of several components or covers that are detachably secured together. The gearbox housing, as illustrated, includes an input end cover  402 , a ring gear  404 , and a final stage cover  406 . The ring gear teeth (not shown) extend along the interior surface of ring gear  404  as shown by the area  410  between the dotted lines as illustrated in  FIG. 4 . In many gearboxes, the ring gear is assembled to the gearbox without a ring gear cover, and the external surfaces of the ring gear form part of the outside of the gearbox. As described previously, the ring gear has been difficult to inspect. However, the method, according to aspects of the present invention, provides an improved method for inspecting the ring gear in a gearbox. 
         [0034]      FIG. 5  illustrates a partial, cross-sectional view of the ring gear  404 . Defects  520 , such as cracks or fissures, can occur in the ring gear  404  or the ring gear teeth  512 . 
         [0035]    The ultrasonic transducer  530 , according to one aspect of the present invention, can be scanned on the outside diameter of the ring gear. The wave packets propagate through the ring gear material. If defects are present in the ring gear, such as cracks  520 , the sound will reflect from the cracks  520 . The reflected sound waves propagating from the defects will either follow a direct path back to the transducer or propagate to a surface of the ring gear where they will reflect and propagate towards the transducer. The transducer  530  will then receive the reflected ultrasonic waves and convert the acoustic energy into electrical energy. The electrical signals will then be analyzed using ultrasonic test equipment to measure the time-of-flight for the wave reflections, the amplitude of the reflected signals, and other characteristics of the reflections such as phase, frequency, and waveform shape. This information along with the transducer beam angle and location of the transducer on the periphery of the ring gear can be used to determine the size and location of the defect or flaw in the ring gear. 
         [0036]    An exemplary method of testing for defects in the ring gear  404  is accomplished by the appropriate selection of a signal generator, such as an ultrasonic, EMAT or a monolithic piezoelectric transducer, the appropriate positioning of the signal generator, the appropriate orientation of the signal generator, and then the activation of the signal generator. 
         [0037]      FIG. 5  depicts an exemplary signal generator  530  for practicing the exemplary methods disclosed herein, where the dashed lines  535  represent the direction of an outgoing ultrasonic signal, to/from a region of interest (such as a crack for example) in a ring Dear tooth  512 . 
         [0038]    In view of the foregoing description, exemplary methods for testing for cracks will now be discussed. A signal generator  530  productive of a test signal  535  is selected. The signal generator  530  is positioned at an outer surface of ring gear  404 . The signal generator  530  is oriented so as to direct the test signal  535 , to propagate through the ring gear  404  and towards ring gear teeth  512 . Once oriented, the signal generator  530  is activated so as to test the ring gear  404 , which includes the ring gear teeth  512 . To assure a thorough inspection of the ring gear, the signal generator  530  may be moved circumferentially and/or axially to achieve full coverage across the outer surface of the ring gear  404 . Alternatively, the signal generator  530  may be moved across outer surface of ring gear  404  in any suitable pattern. 
         [0039]    In addition to the ring gear of the gearbox, there are other ring gears or gears in general that may be inspected with the method according to the present invention. In wind turbines, the yaw gear and pitch gear also contain ring gears that could benefit from the inspection method according to the present invention. The method of inspection was described in relation to wind turbine components; however, the inspection method could be applied to any component in any machine or device having ring-type gearing. 
         [0040]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.