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Automated Gear Prognostics for Helicopter Transmissions Avinash V. Sarlashkar Kenneth Barlow Wangming Lu, Ph. D Michael J. Roemer, Ph. D Impact Technologies, LLC Naval Air Systems Command 200 Canal View Blvd., Suite 300 AIR 4.4.3.3 Rochester, NY 14623 Patuxent River, MD ABSTRACT Gear failures in helicopter power transmissions are very critical, both from the view point of the safety of the occupants as well as the ability of the asset to complete the mission. Two prominent failure modes exist for spiral bevel gears commonly used in helicopter transmissions. These failure modes are a) gear tooth separation from a high-cycle fatigue crack originating in the tooth fillet and b) surface damage resulting from scuffing damage on gear tooth contacting surfaces. An automated process to perform gear prognostics has been developed that encompasses the critical elements necessary for reliable estimation of failure progression rates in spiral bevel gears and provide associated confidence bounds. The Spiral Bevel Gear Prognostic System (GPS) described herein automates the generation of gear geometries and associated finite element models, estimation of tooth contact loads, 3-D fracture mechanic calculations, leading to prediction of remaining useful life. The software system is designed to run on a PC running Microsoft Windows and seamlessly uses the finite element program ANSYS and the 3-D fracture mechanics program Franc3D. Also presented are the results of scuffing tests for the gear material AISI 9310. In particular, test results are presented with respect to degree of scuffing damage and the associated features such as oil debris concentration and changes to coefficient of traction. Keywords: Prognostics, HCF, Spiral Bevel Gears, Fracture Mechanics, Franc3D, ANSYS INTRODUCTION Spiral bevel gears are used extensively in helicopter power transmissions. Therefore, overall reliability and safety of helicopters is dictated by the reliability of these individual spiral bevel gears. There are two primary modes of gear failures. First mode is the High Cycle Fatigue (HCF) driven crack initiation followed by crack propagation. Cracks typically initiate at highly stressed fillet region at the base of a gear tooth. It is also possible that a crack could initiate from a tooth surface as a result of surface damage such as pitting, or, alternately, a crack could also initiate from a machining mark or other surface damage that could occur during assembly. The worst outcome involves crack propagation that ultimately leads to tooth separation and subsequent loss of functionality of the gearbox. The second failure mode involves surface deterioration that can be the result of one or more of the following: wear, pitting and scuffing. Typical symptoms associated with the surface failure are increase in vibration and noise level. In the spiral bevel gear prognostic system (GPS), scuffing and subsequent wear were evaluated since, compared to other forms of surface damage, it is more severe and therefore can rapidly reduce the remaining useful life (RUL). Scuffing is abnormal wear of tooth surfaces due to combination of high contact pressures and sliding velocities at the interface. High contact pressures along with high sliding velocities between contacting surfaces can result in repeated localized welding and fracture leading to severe surface deterioration. Calculation of remaining useful life of an in-service spiral bevel gear system depends on the mode of failure. For the first failure mode described above, the total useful life is composed of two parts: crack initiation life and crack propagation fatigue life. For the second failure mode, service life is defined by surface durability, i.e., the time required to accumulate certain amount of surface damage such as, for example, depth of wear on the contact surfaces. Surface durability depends on a variety of factors such as rolling and sliding velocities, contact pressure, type of lubricant and type of lubrication, material and associated material heat treatment etc. Developed by Impact Technologies, LLC and its contractors under a Phase II SBIR effort, this prototype GPS is aimed to demonstrate a robust physics-based tool that models physics of different failure modes for spiral bevel gears and at the same time has the ability to use vibration and other system-level features to provide enhanced prognostics for the spiral bevel gear systems. This prototype system includes several modules for: a) Automated generation of spiral bevel gear geometries and the associated finite element models, b) Gear tooth contact analysis, c) Crack initiation and 3-D crack propagation calculations, d) Gear surface scuffing model, and e) An example of integration with Health and Usage Monitoring System (HUMS) data. At the heart GPS is the gear prognostic kernel that interacts with different modules. These individual modules are responsible for specific automation tasks. Figure 1 shows overall organization of GPS. Figure 1 Architecture of the spiral bevel gear prognostic system (GPS) The program is developed to run on a PC running Microsoft Windows and seamlessly interacts with the commercial finite element analysis program ANSYS, 3-D fracture mechanics program Franc3D, and Gleasons gear tooth contact analysis program T900. ANSYS is primarily used as the main structural analysis and post-processing module, Franc3D is used for 3-D crack propagation analysis and the T900 program is used for gear finite element model generation and gear tooth contact analysis. Output of GPS is the estimated remaining useful life, measured as time in operating hours. Basically, input data includes the spiral bevel gear geometry, torque, material properties, and other system-level feature data that can be used to “tune” the predicted analytical results. Mainly, there are two types of test data: vibration signal based features, and experimentally obtained scuffing progression information. Fusion of numerically predicted results (ANSYS, Franc3D, T900) with those from test and real-time measurement data, if processed properly, can significantly improve the robustness and accuracy of RUL predictions. In the following sections, basic concepts behind the development of GPS are presented on a module-by-module basis. In the next section, gear geometry and finite element model generation modules are presented. This is followed by presentation of the gear tooth contact analysis module. Both modules use the T900 program developed by Gleason Corporation. A modified version of the 3-D fracture mechanic program, Franc3D, developed by Fracture Analysis Consultant, Inc (FAC) is used to perform 3-D crack simulation. This modified version of Franc3D is tightly coupled with ANSYS to improve computational efficiency. The GPS kernel collects information from all sources and then “fuses” this information to generate best RUL estimate for the in-service asset. The basic idea in fusion is to use as much available evidence, such as inspection data, to improve on the quality of the results that would be produced by analytical models alone. AUTOMATIC GENERATION OF GEAR GEOMTRY AND FE MODELS Generation of gear geometry and associated finite element (FE) models serves as the starting point for GPS. A custom version of Gleasons T900 program is used to generate this information in an automated fashion. Figure 2 shows gear / pinion finite element models generated using Gleasons program. The finite element models are imported in ANSYS for subsequent analysis purpose. Figure 2 Finite element model of a pinion In GPS, the gear / pinion finite element models shown above are further improved by using precise gear blank and shaft geometry. With the geometry data provided by end users through friendly Graphical User Interface (GUI), the shaft finite element models are created using ANSYS macros. The shaft generation module allows definition of boundary conditions (different bearing configurations) and meshing control parameters. The partial gear / pinion models shown above are expanded into models containing full shaft and bearing support definitions. One example of the finite element model for the whole pinion shaft is shown in Figure 3. A more precise definition of gear blank geometry along with shaft geometry and bearing support conditions provide improved stress definition, especially for aerospace gears. Figure 3 Pinion FE model complete with associated shaft geometry. GEAR TOOTH CONTACT ANALYSIS Besides automatic generation of gear geometry and associated finite element models, gear tooth contact analysis is performed using the T900 program. The T900 program generates the theoretical conjugate lines of contact. A sample plot of conjugate contact lines projected on to tooth surface is shown in Figure 4. The T900 program discretizes the otherwise continuous contact process into a number of conjugate contact lines. Each contact line is represented by a number of contact points. Tooth contact analysis provides contact pressure and sliding velocity at each of these contact points. Both contact pressure and sliding velocity will be used to predict gear surface wear rate and/or scuffing potential. Figure 4 Conjugate lines of contact computed by the T900 program. In reality, each contact line is an elongated contact ellipse. CRACK PROPAGATION LIFE ANALYSIS If the failure mode is tooth separation, then the fatigue life can be considered to be composed of two parts: crack initiation life and crack propagation life. The first part can be calculated using the local strain approach 1. The second portion is discussed in this section. Two variables are used to characterize crack propagation: propagation rate and direction. Both these parameters significantly affect remaining useful life of the in-service component. Further, the initial crack can be defined by a number of characterizing parameters, i.e., crack location, crack orientation, crack shape and size are required parameters to define initial crack or flaw. Under this SBIR Phase II effort, enhancements were made to the Franc3D program. FAC and Impact Technologies worked jointly on these enhancements where Franc3D was tightly coupled with the commercial ANSYS program. Some of these enhancements included: a) Developing a batch mode to perform a series of fracture simulations b) Capability to generate quarter-point wedge elements along a crack front to accurately model crack tip stress singularity, c) Providing the option to use M integral to calculate stress intensity factor solutions, and d) Adding the interface that combines best of Franc3D and ANSYS for fracture simulations. In this new approach, crack simulations are performed using finite elements instead of boundary elements. This allows the use of ANSYS solvers for efficient solutions. However, crack front prediction / propagation and remeshing tasks are carried out within Franc3D. Figure 5 shows a graphical tool within GPS to define initial flaw shape / size / location and orientation. Figure 5 Graphical tool within the GPS interface to facilitate definition of shape / size / location and orientation of initial flaw. Flowchart in Figure 6Error! Reference source not found. shows Franc3D with “ANSYS in the loop” to predict crack evolution. To support features specific to 3-D fracture analysis on gears, the following additional enhancement were made to the Franc3D program: 1. Variation of material properties as a function of the distance into the gear material can be defined. The purpose of this feature is to allow the variation of the material properties, at least those related with fracture mechanics, within the case-hardened layer. This variation is assumed to be one-dimensional. 2. Residual compressive stresses as a function of the distance into the gear can also be defined. The purpose of this feature is to model the presence of compressive stress within the case-hardened layer. Again, the variation of compressive stress is assumed to be one-dimensional. The effective stress field after accounting for the compressive stress will be used for fracture mechanics analysis. Presence of compressive residual field will reduce the rate of crack propagation (at least when the crack is within the case hardened layer). Figure 6 Integrated Franc3D / ANSYS approach for 3-D crack simulation During 3-D crack growth simulation, at each step of propagation, the direction and extent of crack growth at each point along the crack front must be determined. This is complicated for crack growth in gears because the moving nature of the load means that the mode I stress-intensity factors and the ratio between the mode II and mode I stress-intensity factors change continually as the teeth go through mesh. In this program, a procedure developed by Spievak 2 was adopted to determine the extent and direction of crack growth for all points along the crack front at each crack growth step. Output of the Franc3D program is crack evolution. Figure 7 compares the computed and observed surface trajectory of the crack 3 as viewed from the heel of the pinion. The computed and observed trajectories appear very similar. Crack1 cm Figure 7 Computed (left) and observed (right) crack surface trajectory view from the heel of the pinion 3. Crack propagation life predictions are first done on deterministic basis. Using a Monte Carlo wrapper, these deterministic predictions are converted to statistical distributions. For this purpose, parameters defining the selected crack propagation models are considered as statistical variables. Currently, two crack propagation models are supported in GPS: Paris model and Walker model. There are two material constants defining a Paris model: C and m. For Walker model, besides C and m, another parameter, n, is needed. Theoretically, the statistical distribution of these parameters can be of any type. From an event occurrence probability viewpoint, the total probability of failure at any given time is the combination of two independent events: the crack initiation probability and the probability of crack propagation to failure. The total probability is: )(*)(pPiPPtotal= (2) where: P(i) = Crack initiation probability, P(p) = Probability of crack propagation to failure GEAR SURFACE LIFE MODULE This module is responsible for RUL estimation in the event of a gear tooth scuffing event. Scuffing model specific to AISI 9310 case hardened material was developed based on experimental data. For these tests, lubricant commonly specified by the Navy was used. Different gear meshes for the H-60 helicopter drive system were considered to identify the range of tooth surface contact conditions such sliding velocities, rolling velocities, contact stresses, flash temperatures and elasto-hydrodynamic (EHD) lubricant film thickness. Surface tests using selected contact conditions were conducted on a ball-on-disc machine, known as the WAM machine, which is capable of simulating simultaneous rolling and sliding. Figure 8 shows the test configuration of a 13/16 in diameter ball loaded against a flat disc. Both ball and disc were made of AISI 9310 steel case carburized to a hardness of 62.5-63.5 RC. Both disc and ball are driven independently to achieve desired sliding and rolling velocities. The intent was to initiate the wear process by loading the contact until a macro-scuff failure event occurred and then to continue running at that scuff load and measure the wear progression. After a test development phase was completed, wear, traction coefficient, acoustic, wear particle and temperature data were collected and analyzed for five different sliding conditions that represented the range of operational conditions found in the H-60 helicopter drive system and the Ryder Gear test machine. The data provides a basis for developing models for failure progression of a scuff-initiated wear failure. Figure 8 WAM ball-on-disk surface durability test machine A simplified wear model, applicable to spur and helical gears, was developed using Archards wear model as the basis 4-5. The model will be adapted to spiral bevel gear contact conditions and will serve as an initial attempt for developing RUL estimates. Analysis of wear particle indicates that the severity level of the wear coefficient is associated with the increase and size distribution of the generated wear particles. This can form the basis of developing a prognostic model by: 1) monitoring the increase and size distribution of the generation of wear particles, and 2) then applying the appropriate wear coefficient to the wear model at the onset of a sharp increase in wear particle generation. In the tests, a rotating ball was loaded in increments until a macro-scuffing event had occurred after which the test was continued for additional 300 seconds while the load was held constant. Wear particle generation was monitored throughout each test using a Pall Corporation model PFC400 particle counter. Oil was circulated for 900 seconds prior to applying a load in order to obtain a baseline reading on the particle counter. If no macro-scuffing event occurred, the loading was increased to the capability of the test equipment (140 lbs) and held there for 300 seconds before the test was ended. At the end of the test, the ball and disc wear track were measured and an estimate of the weight of wear was arrived at. The test protocol is illustrated graphically in Figure 9 in which a macro-scuff occurred. Figure 10 shows the associated wear particle measurement. The oil used in the test program was Royco 555 that conforms to military specification DOD-PRF-85734. This is a high gear load carrying oil currently in use in Navy helicopter transmissions. Initial calibration testing on the WAM was conducted using Herco A which is a synthetic base stock oil used for blending fully formulated oil. Run Time (seconds)0400800120016002000240028003200Traction Coefficient-0.020.000.020.040.060.080.10Load (lbs.), Temperature (C)020406080100120140160180200220Vertical LoadTraction CoefficientTesting/Aera.jnbBall TemperatureWAM High Speed Load Capacity Testw In-Line Particle DetectionDisc TemperatureTest: AERA18Lube: 555Ball: aera18-9a, 9310, Ra=10-12 in.Disc: 9-78a, 9310, Ra=6 in.Entraining Velocity: 158 in/sec.Sliding Velocity: 450 in/sec.Temperature: Ambient Velocity Vector Angle (Z): 110 Figure 9 Typical variation of system parameters during surface durability test Run Time (seconds)0400800120016002000240028003200Traction Coefficient-0.020.000.020.040.060.080.10Load (lbs.), Temperature (C), Particle Count020406080100120140160180200220# of Particlesper 100ml1e+31e+41e+51e+6Vertical LoadTraction CoefficientTesting/Aera.jnbBall Temperature 6um4 mWAM High Speed Load Capacity Testw In-Line Particle DetectionDisc Temperature 10 m 14m 21 m 38 m 70 mTest: AERA18Lube: 555Ball: aera18-9a, 9310, Ra=10-12 in.Disc: 9-78a, 9310, Ra=6 in.Entraining Velocity: 158 in/sec.Sliding Velocity: 450 in/sec.Temperature: Ambient Velocity Vector Angle (Z): 110 30 m Figure 10 Wear particle distribution as function of surface damage level HUMS DATA There is on-going research that attempts to use vibratory and other types of data available through HUMS boxes to assess current health of an asset. A variety of information is monitored / recorded. Important pieces of information from the view point using GPS with HUMS data are: 1. “Health Information” such as vibration levels at key locations on the gearbox and possibly real-time oil analysis. The health information such as the gearbox vibration levels is expected to provide indications as to the health of the gears inside the gearbox. The raw vibration data will be processed, probably on a real-time basis, to extract certain features such as residual kurtosis, residual peak-to-peak, residual skewness, narrowband kurtosis, and energy operator kurtosis. 2. Both, past and future “Usage Profiles” (torque, speed, duration) are important. The past usage information will allow the estimation of consumed life and therefore, knowing the future usage profile will allow estimation of RUL. During RUL prediction process, integration of feature data is of paramount importance it can assist in calibrating the model-based aspects and reduce uncertainty levels. This means that tighter confidence bounds can be calculated for RUL. GRAPHICAL USER INTERFACE (GUI) DESIGN For GPS, an intuitive GUI has been developed to facilitate the whole process from gear geometry generation to RUL estimation. Access to each of the modules is provided under individual drop-down menus. Examples of the GUI are shown in Figure 11 and Figure 12. Figure 11 shows high-level menus. Figure 12 shows the interface to define gear / pinion shaft geometry, bearing configuration, and boundary conditions. Figure 11 High-level user interface menus for GPS Figure 12 Gear /