1、02FTM5Crack Length and Depth Determinationin an Integrated CarburizedGear/Bearingby: J.S. Kachelries, Materials Engineeringand R.J. Drago, Gear TechnologyTECHNICAL PAPERAmerican Gear Manufacturers AssociationCrackLengthandDepthDeterminationinanIntegrated Carburized Gear/BearingJ.S. Kachelries, Mater
2、ials Engineering and R.J. Drago, Gear TechnologyThestatementsandopinionscontainedhereinarethoseoftheauthorandshouldnotbeconstruedasanofficialactionoropinion of the American Gear Manufacturers Association.AbstractDuring overhaul of a primary drive helicopter Spur Gear, which also serves as the outer
3、race for an integral sphericalbearing,magneticparticleinspectiondetectedanapproximately0.3-inchlongcrackindicationontheracewaysurface.Subsequent metallurgical evaluation disclosed that the indication was a 0.028-inch deep, pre-existing grinding crackthat had not been detected by the manufacturer dur
4、ing final inspection. A sample re-inspection of product that wascurrently in storage that had been shipped from this manufacturer revealed that additional components with grindingcracks. In an effort to determine if the processing cracks posed a safety of flight concern, several gears that contained
5、cracks were designated to undergo a rigorous dynamic bench test. However, in order to insure that the bench testproducedthemaximumamountofrelevanttestdata,itwasnecessarytodocument,nondestructively,allofthegrindingcrackdimensions(location,lengths,anddepths)priortothestartofthetest.Thelocationsandleng
6、thsofthecracksweredetermined using a specially modified magnetic rubber inspection technique, which could reliably document cracklengthsasshortas0.006inch.Afixturewasdevelopedthatmadeitpossibletoproduceamagneticrubbercastingofthegear raceway, while simultaneously performing up to thirty, one second
7、central conductor shots. To nondestructivelyestimate crack depths, a unique,highly sensitive,laboratory eddycurrent inspectiontechnique wasdeveloped. Usingalimitednumberofdatapoints,aresponse-to-depthcurvewasgeneratedthatpredictedthemaximumcrackdepthsto+/-0.002inch.Usingthesetwotechniques,thestatus(
8、i.e.,growth)ofeachcrackwasmonitoredatvariousstagesofthebenchtest.Atthecompletionofthebenchtest,adetaileddestructivemetallurgicalevaluationofeachofthetestgearswasconductedanddocumented.Thepost-testdataconfirmedtheaccuracyofthenondestructiveeddycurrentestimates,andprovided additional data that will ma
9、ke future crack depth predictions more accurate.Copyright 2002American Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 2002ISBN: 1-55589-805-X.Page 1 Crack Length and Depth Determination in an Integrated Carburized Gear/Bearing James S. Kachelries Associa
10、te Technical Fellow Materials Engineering (610) 591-7577 e-mail: Raymond J. DragoSenior Technical FellowGear Technology(610) 591- 2014e-mail: INTRODUCTION During a scheduled overhaul of a military helicopter transmission, magnetic particle inspection revealed an approximately 0.30 inch long crack i
11、ndication on the outer raceway of an Integrated Planetary Gear/Bearing Assembly. In this integral design, the gear manufacturer carburizes an approximately 7-inch pitch diameter spur gear (4 inch tooth width), including an approximately 5.7 inch diameter spherical bore. The gear manufacturer complet
12、es the gear teeth portion of the spur gear, and the spherical bore is subsequently ground by the bearing manufacturer. The raceway of this Gear/Bearing Assembly is carburized to a 50 HRC effective case depth of 0.070-0.090 inch. After finish grinding of the raceway, the bearing manufacturer performs
13、 a 100% magnetic particle inspection and a surface temper inspection (i.e., Nital Etch) of the spherical raceway. If no indications are found, the gear is matched with a through hardened Inner Ring, Rolling Elements, and cage to complete the Integrated Planetary Gear/Bearing Assembly. Both the Forwa
14、rd and Aft Chinook Transmissions contain six of these Planetary/Gear Bearing Assemblies comprising the Second Stage Planetary System. A schematic illustrating the location of the Second Stage Planet Gear/Bearing in the Forward Transmission Assembly is shown in Figure 1. A photograph showing the magn
15、etic particle indication on the spherical raceway of the Planet Gear returned from service is shown in Figure 2. Figure 2. Photograph of Planet Gear/Bearing showing Magnetic Particle Indication on Integral bearing raceway (Insert). Forward Rotor Shaft First Stage Sun Gear First Stage Planetary Assem
16、bly Second Stage Planet Gear inner Ring and Rollers Figure 1. Schematic Illustrating some of the Primary Drive Components of the Helicopter Forward Transmission Assembly. The grinding cracks were located on the ID Raceway of the Second Stage Planet Gear. Input Pinion Second Stage Planet Gear .Page 2
17、 Subsequent metallurgical evaluation of the indication revealed that it was associated with a pre-existing grinding crack that had not been detected during the magnetic particle inspection operation following the spherical bore grinding process. A photograph of the fracture surface of the grinding c
18、rack, after it was mechanically opened in the laboratory, is shown in Figure 3. Surface temper inspection (i.e., Nital Etch) of a portion of the raceway adjacent to the grinding crack disclosed an approximately 0.25 inch wide band of retempering damage; see Figure 4. Residual stress profiles, Figure
19、 5, obtained using x-ray diffraction techniques revealed significant residual tensile stresses in the areas associated with the retempering burn. For reference, Figure 5 also shows a similar x-ray diffraction analysis from a nonfunctional, undamaged portion of the raceway. This profile disclosed sha
20、llow compressive stresses, typical of properly ground, carburized steel. Because the above Gear/Bearing Assembly had two distinct Non-Destructive Evaluation escapements1(i.e., Magnetic particle and surface temper inspections), it was decided to re-inspect the product that was currently in storage th
21、at had been shipped from this manufacturer. 1In the Helicopter industry, the word escapement applies to components that had rejectable flaws that were not detected by the supplier and were subsequently shipped as compliant product. A sample magnetic particle inspection revealed additional escapement
22、s, indicating that the original cracked Gear/Bearing was not an isolated occurrence. Therefore, there was a possibility that other cracked parts could be in service. Consequently, a damage tolerance bench test was proposed. Among the test objectives were: Determine what loading conditions were requi
23、red to initiate and propagate a fatigue crack from a pre-existing grinding crack. Determine if it was necessary to ground the fleet for immediate inspection, or to institute flight restrictions (i.e., limit cargo loads, air speed, maneuvers, etc.). Determine if a propagating crack would initiate spa
24、lling debris so that the chip detectors could discover the crack prior to ultimate failure. The damage tolerance bench test utilized a standard Transmission (Figure 1), which included six Second Stage Planetary Gear/Bearing Assemblies. The Test Plan required that the transmission contain several Gea
25、r/Bearing Assemblies with pre-existing grinding cracks. In order to achieve the above test objectives, it was necessary to determine the locations, lengths, and depths of the pre-existing grinding cracks on the Planetary Gear/Bearing raceway prior to testing. This paper documents the laboratory tech
26、niques used to determine/document these three specific variables. Figure 4. Surface Temper Etch revealed evidence of grinding damage (i.e., Retemper Burn). Raceway Retemper Burn Figure 3. Photograph of fracture surface of grinding crack shown in Figure 2 0.028 inch Fracture Surface of Grinding Crack
27、-200-150-100-500501000.000 0.002 0.004 0.006 0.008 0.010Figure 5. X-ray diffraction residual stress data Depth Below Surface (inch) ResidualStress(ksi) ! Retempered Area Undamaged Area .Page 3 Test Procedure and Results A photograph of crack indications from one of the test Gears using conventional
28、fluorescent magnetic particle inspection is shown in Figure 6. Photographs such as these could have been used to document crack locations, and to monitor changes in crack length during inspection intervals. However, the location of the crack indications on the ID raceway of the Gear made accurate ph
29、otographic documentation impractical. The method selected to determine the exact location and length of the pre-existing grinding cracks was Magnetic Rubber Inspection. Magnetic Rubber is a commercially available product that is traditionally used to facilitate crack detection in ferromagnetic compo
30、nents where line-of-sight is difficult, or impossible. The constituents of the Magnetic Rubber used in this evaluation are: Yellow Base: Vulcanizing silicone rubber with suspended/dispersed iron oxide particles (5-60 micron) Catalyst: Stannous Octoate Curing Agent: Dibutyl Tin Dilaurate Magnetic Rub
31、ber Inspection requires that these three liquid components be thoroughly mixed immediately prior to inspection, and be poured into a prepared cavity that is contiguous with the test component, while simultaneously applying an external magnetic field. In the presence of an applied magnetic field, the
32、 suspended iron oxide particles migrate to the magnetic flux leakage fields associated with the grinding cracks. When the liquid rubber cures, the aligned iron oxide particles become trapped in the hardened rubber. The cast rubber now contains a permanent record of the location and lengths of the de
33、tectable flaws in the inspected component. However, the ratio of base-to-catalyst-to-curing agent, and the ambient temperature and humidity, have a drastic effect on the sensitivity and mechanical integrity of the cast rubber. If the rubber cures too quickly, the high viscosity will impede iron oxid
34、e particle migration, significantly reducing sensitivity. If the rubber cures too slowly, the flux background obscures indications, the inspection time becomes excessively long, and the cast rubber is mechanically fragile. In addition, the strength of the magnetizing field also affects the readabili
35、ty of the indications. If the field is too strong, the indications become bloated, and background interference is excessive. If the magnetization field is too weak, there is insufficient migration of the iron oxide particles, and the crack lengths cannot be accurately identified. By experimentation,
36、 an optimum mixture of base, catalyst, curing agent, and magnetic field strength were developed. The curing time was a reasonable 45 minutes, and the technique had the sensitivity to reliably detect flaws as short as 0.006 inch in length, see Figure 7. A simple test fixture (Figure 8) was developed
37、to create the cavity for the liquid magnetic rubber that also permitted the use of a large central conductor for applying the magnetic field, see Figure 9. The magnetizing technique required the immediate application of thirty, one second duration shots (DC current), applied at two second intervals.
38、 A viewing fixture (Figure 10) was used to hold (and rotate) the rubber casting so that the indication lengths could be documented with a measuring microscope, and their circumferential locations could be recorded relative to an indexing marker that was glued to the gear ID prior to making the rubbe
39、r casting. A sample of the data obtained from one of the test gears, before, during, and after the bench test was completed is contained in Table I. Figure 11 contains photographs illustrating the change in crack lengths captured by magnetic rubber inspection. Comparisons of the estimated crack leng
40、ths using magnetic rubber to the actual crack lengths Figure 6. Example of Fluorescent Magnetic Particle Indications observed on raceway of Planet Gear. Figure 7. Example of Magnetic Rubber Indications of micro-cracks .Page 4 revealed an average maximum error of 0.002 inch for axial cracks less than
41、 0.025 inch in length, and 0.005 inch for axial crack lengths greater than 0.025 inch; see Table I. The maximum total error between the estimated crack length and the actual crack length was 0.011 inch (axial cracks only). Table I - Estimated Crack Length versus Actual Crack Lengths1Bench Test Crack
42、 Start 79 Hours115 Hours Final Actual LengthMax Error 301A 0.143 N/A2N/A2N/A20.148 0.005 301B 0.168 N/A2N/A2N/A20.170 0.002 301C 0.188 N/A2N/A2N/A20.194 0.006 301D 0.103 N/A2N/A2N/A20.101 0.002 301E 0.144 N/A2N/A2N/A20.140 0.004 316A 0.092 0.092 0.093 0.093 0.091 0.002 2272D 0.158 0.159 0.160 0.164
43、0.159 0.005 2272J 0.143 0.144 0.144 0.155 0.144 0.011 2272K 0.119 0.108 0.116 0.121 0.119 0.011 2272L 0.151 0.141 0.150 0.148 0.148 0.007 2272M 0.127 0.126 0.123 0.131 0.127 0.004 2272N 0.100 0.103 0.104 0.107 0.104 0.004 2271G 0.019 0.020 0.021 0.021 0.020 0.001 2271Q 0.024 0.023 0.025 0.025 0.024
44、0.001 2271S 0.020 0.018 0.019 0.019 0.019 0.001 2271AC 0.019 0.017 0.018 0.019 0.018 0.001 2271AG 0.023 0.022 0.025 0.022 0.023 0.002 Notes: 1. All dimensions are in inches. 2. Gear failed and was removed from bench test. Figure 9. Photograph of central conductor magnetization technique for inspecti
45、on of Planet Gear Raceway. Aluminum Central Conductor Planet Gear Tacky Tape Gasket Wax Paper Sleeve Fiberglass Tool Figure 8. Exploded view of magnetic rubber magnetization fixture. Wood Support Fixture Cast Rubber Ring Indications Figure 10. Photograph of viewing fixture for inspection of magnetic
46、 rubber castings .Page 5 Figure 11. Photograph of magnetic rubber crack indications prior to bench test (upper left), and when the failed gear was removed from testing (upper right). The bottom photograph shows fatigue propagation emanating from the base of a grinding crack. It should be noted that
47、the magnetic rubber inspection method detailed above used only one direction of magnetization (i.e., Central conductor, circular field). Therefore, only cracks oriented in the axial direction could be reliably detected. For the purposes of the bench test, this was considered sufficient, because circ
48、umferential cracks were determined to be comparatively benign on a raceway surface. However, in order to fully inspect a component, multiple, perpendicular fields are required. The final crack parameter that needed to be determined was the maximum depth. Unfortunately, the complex gear geometry prev
49、ented the use of ultrasonic and X-ray inspection techniques. Attempts to estimate the crack depths qualitatively using the relative intensities of the fluorescent magnetic particle indications proved to be unreliable. The method that produced the best quantitative data was eddy current inspection2. The use of eddy current inspection to detect cracks in conductive, metallic materials is well established. However, the technique has traditionally been used as a go / no go inspection. Typically, the eddy current equ
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