AGMA 98FTM12-1998 Gear Refurbishment an Economical Approach for Aerospace Overhaul & Repair《齿轮翻新.航空大修和维修的一种经济方法》.pdf

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1、98FTM12Gear Refurbishment, An EconomicalApproach for Aerospace Overhaul namely some type of refurbishment program and the gears being replaced were storedwithout scrapping since in many cases the parts were rejected but not damaged beyond- repair. During a visit to the facility, the sub-contractor s

2、aw the rejected gears and entered_ into discussions with overhaul personnel for consideration of repair. A number of gearswere sent back to Arizona where they were inspected for wear and stock availability forrework.A choice had to be made between beginning the refurbishment program with powerreduct

3、ion or accessory gears; The reduction gears were chosen for two reasons: 1. Thecustomer would be delighted since the cost of a refurbished gear would be less than a newone especially if we zero timed it in the Power by the Hour Program. 2. Power gears ofaircraft quality AGMA 11 - 13 were most in nee

4、d as well as being the most rewardingfinancially for the customer. If this proved successful, accessory gears would easily beadded.The Power by the Hour concept was started with the Coast Guard. AlliedSignal owns therepairs and the Coast Guard pays an hourly rate. It was tried for a year and then th

5、e pricewas adjusted for FOD damage and other events that were beyond the realm of normalmaintenance. A seven year contract is now in progress.The first step was to inspect the gears visually to determine which might be repairable;Gear tooth distress was by far the major reason for reject. The main c

6、oncern was pittedand spalled gears. Previous experience had shown that pitting could be up to .007 inchesdeep and that grinding away the surface tended to cover it up as well as making the pitarea smaller. This type of pitting is a subsurface phenomenon with the depth below thesurface of distressed

7、area difficult to ascertain. It was decided to just scrap pitted and orspalled gears. The remainder of surface distress cases including wear, end loading,frosting, and scoring were acceptable candidates for rework.The first gear to be visually inspected was the torquemeter idler which contains both

8、acoarse pitch and the larger diameter, fine pitch gear. Of the fifteen gears reviewed, allfifteen of the coarse pitch gears were considered reworkable and, seven of the fifteen freepitch gears were considered reworkable. The prospects for rework were excellent and wewould start with the coarse pitch

9、 gear since it would have the most stock left for rework.This further reinforced the decision to go with power gears first. Reworking byregrinding the gear teeth was the only solution to the refurbishment problem and minimalstock removal was critical. The sub-contractor assured us that their experie

10、nce in this areawould be equal to the task and the refurbishment program was born.The major steps required to qualify this program were then established as follows:1. Determine the minimum measurement over wires, stock allowance, for rework. Thisrequires knowledge of the following:a. Case depth vers

11、us hardness as manufactured. A surface hardness of 60HRCminimum was required for “zero“ timing to insure adequate operational life.b. Stock allowance for original manufacturing.c. Material Review Board, for blueprint discrepant parts, actions for this partnumber.d. Stock allowance for rework. This w

12、ill be over and above MRB limits.1. Establishment of an acceptable “Rework Procedure“ by Air Gear. This required areview of the overhaul manual to determine all possible rework conditions for eachpart. For example, bearing journ, al repair etc.2. Identification procedure of test parts to pre-empt im

13、proper assembling of these partsinto production engines. (_Repair Development Engineering number is supplied byAlliedSignal)3. Inspection of refurbished parts.b. This required some paperwork changes on the part of the contractor sinceboth companies did not have the same inspection equipment. As an e

14、xample,Holler machines used at the contractors plant contained their own tolerancebands built in for involute and lead charts. The sub-contractor uses Fellowsequipment and overlay templates had to be made for their inspection.1. Test requirements for acceptance. We required a total of 1,000 hours of

15、 testing foracceptance. To expedite the program a 150 hour in-house test was initiated andfollowed by three lead thefleet engines for the Coast Guard to reduce the overalllength of time required. A150 hour engine test is equivalent to approximately 1000hours of field operation because of increased t

16、ime spent at high power levels.AGMA 911-A94, Annex B offers an excellent discussion of Aerospace Gearbox testand Mission requirements.2. A marking procedure to be used at the sub-contractor to keep track of refurbishedhardware and to identify the number of reworks.3. Final acceptance of Air Gear for

17、 licensed refurbishment of AlliedSignal Enginesgears by part number.For the eight or coarse pitch gear the carburized case depth range is 0.028 - 0.040 inch perblueprint. This is the case depth range to reach a hardness of 50HRC. For operationalpurposes, we are interested in the minimum case depth t

18、o 60HRC. Fielding a part at thissurface hardness level was considered satisfactory for operational longevity. A review ofas hardened case depth records showed that a 0.028 inch case depth curve crosses 60HRCat 0.018 inch. Figure 3 shows the hardness versus depth traverse for the test sample gearteet

19、h that accompany the production parts in the furnace. The as manufactured maximumstock allowance per specification is 0.006 inch per side and another 0.002 inch was usedfor MRB action, which is for parts that are not to blueprint but are still acceptable foroperation. This left ( 0.018 - 0.006 - 0.0

20、02 ) = 0.010 inch. Other factors not yetaccounted for include tolerance on the hardness and depth measurements, a desire tominimize operational backlash, and the actual measured data points which start at 63HRCbut could have been 62HRC or 61HRC. The additional stock for refurbishment waslimited to 0

21、.001 inch below the MRB limit.For the torquemeter idler gear the overhaul manual defines areas other than just the gearteeth for inspection and rework, these include the bearing journals, the body fitted bolts,and the adjoining surfaces for fretting wear. Figure 4 shows an isometric view of thetorqu

22、emeter idler gear along with a simplified tabulation of the basic rework procedure_ including numbered circles representing the rework areas on the gear. Some views of_ typical gear tooth patterns are also shown. The actual rework sheet for this gear is showni3in the Appendix A. Table I is a listing

23、 of the gear tooth inspection results both prior toand after refurbishment for a typical gear. Figure 7 shows the Fellows involute charts forcomparison; The Hofler has tolerance bands which are printed in red if the actual trace isnot to blueprint. The Fellows chart has the inspection template super

24、imposed on it forease of visualization.The subcontractors rework procedures were accepted and the first gears were refurbished.The engine test was acceptable and three sets of gears were identified for lead thefleetand installed into Coast Guard aircraft. After 1,000 +hours all gear sets looked fine

25、 andAir Gear was certified for refurbishment of the torquemeter idler gear.The output gear followed since it also was of the same coarse pitch. The fine pitchtorquemeter idler gear followed shortly after because it was in short supply and was theremaining gear from the assembly. Some of these gears

26、were acceptable without reworkbut Greer found them to be noisy during the engine acceptance run. They requested thatthe sub contractor regrind the teeth on these gears even though no rework was required.LT101 gears are now shipped directly to the sub-contractor for inspection and dispositionwhich is

27、 more cost effective and reduces turn around time for the customer.The success of this program led to a search for other candidate gears. The ALF502Turbofan reduction gears were considered because of surface distress caused by high oiltemperature operation. The ALF502 reduction gears were originally

28、 designed in the1970s are made from SAE9310 carburizing steel which is the standard of the Aerospaceindustry. At an operating temperature of 350F, the material starts to lose its hardnesswhich leads to wear and surface distress. For certain applications a higher operatingtemperature material is requ

29、ired. The upgraded ALF507 engine contains reduction gearsmade from EX53 “Pyrowear“ carburizing steel which doesnt soften until 450F. Somecustomers prefer to refurbish the SAE9310 gears for economy of operation. Thereduction gear system is a star type planetary configuration, see Figure 5, comprised

30、of aninput sun gear, which is splined to the power turbine, driving seven planet.gears which inturn drive an internal ring gear. The ring gear is splined to the turbofan. Thetransmission provides approximately 7,500 horsepower at a turbine speed of 17,500 rpmwhich is reduced to a fan speed of 7,600

31、rpm. The sun and planet gears were consideredfor rework while the ring gear did not exhibit signs of surface distress because theconcave curvature of the teeth increases the contact area to reduce the operationalcompressive stress to 90,000 psi as compared with the sun gear mesh of 130,000 psi.The p

32、rocedure for qualifying the ALF502 reduction gears is the same as for the LT101reduction gears and again pitting or spalling was excluded as candidates for rework. Thepitch is 10.8 and the corresponding case depth range is 0.018 - 0.028 inch. The minimumcase depth curve of Figure 6 shows a crossing

33、point of 60HRC at 0.011 inch depth andagain the traverse is of the sample test gear teeth that accompany the production lot in thefurnace. The as manufactured maximum stock allowance for this gear is 0.005 inch.4Consideration of stock allowance for refurbishment is more involved for the planet geart

34、han for the sun gear. There are two different blueprint part numbers for the planet gearwith each one having a different range of tooth thickness. Tighter control was requiredfor the sun-seven planet-ring gear mesh to improve load sharing as compared with asingle gear mesh. It was also d.esired to h

35、ave the same total range of stock allowance onthe planet gears for manufacturing purposes. By having two different planet gear partnumbers, both requirements could be achieved. Table II is a listing of this geometry. Astock allowance of 0.002 inch was established for refurbishment of the sun gear.A

36、requirement of the assembly is that only planets of the same dash number may be builtinto the carder or housing. If a -24 planet is reworked within the blueprint tolerance ontooth thickness, it will remain a -24 gear and if not, it is allowed to become a -23 gear.Creation of a third dash number for

37、the planet gear was not desirable so the smallest valueof blueprint tooth thickness was maintained for the -23 gear. This meant that thestringent requirement for tooth lead, involute, spacing, index and runout had to bemaintained with a minimum amount of stock allowed for repair on both loaded faces

38、 ofthe idler gear. To further complicate matters, the planet gear bores may require a chromeplate repair because of wear in which case the bore must be located from the gear teeth.The planet gear assembly is comprised of a threaded pin onto which two bearings and agear are press fitted. The assembly

39、 is held together by a lock cup and nut. Since the boreis the datum surface, any machining error will be reflected in the tooth characteristics.The sun gear also requires a lengthy repair since the teeth are silver plated and the gearcontains a damper ring that needs replacement. The gear is then ch

40、eck balanced and re-balanced if necessary. The sun and planet gears may then be zero timed and re-identifiedas a new set upon assembly.During development of the rework procedure the inspection equipment needed to be re-calibrated to provide the eight microinch finish required on both the sun and pla

41、net gears;Checks are required at three axial locations to verify uniform honing and a honed gearwas supplied to the sub-contractor for this purpose as a reference. Involute and lead dataalso required inspection machine synchronization between the two companies and Themachines had to then be re-calib

42、rated for acceptance. Long roll involutes are used withnumerous points being identified. Overlay templates for both involutes and the crownedsun gear lead are now the inspection masters used to check charts at the sub-contractorsplant.When these issues were resolved, acceptable gears were submitted

43、and subjected to a1000 cycle engine test run in October 1997. The gears looked fme after test and Air Gearis now qualified to refurbish these gears also.Air Gear International Inc. is an FAA/JAA repair station equipped strictly to repair gears,shafts, splines, and related components for the Aircraft

44、/Aerospace industries. The- restoration of parts is a combination of proprietary development as well as proven repairs5Ilisted in the manufacturers overhaul and repair manuals. Their engineering staff also hasthe capability to develop repairs for FAA approval.Precision gears are difficult to manufac

45、ture because they require thin sections tominimize weight which necessitates careful machining especially after carburizing andhardening. The repair of these gears is also difficult with plating, nital etch and evengrinding contributing to distortion with minimal stock removal. The normal method ofs

46、tock removal for gears includes grinding, honing, lapping and grind-plate-grind. In somecases extra stock could be allowed and this was utilized in the refurbishment process.Once a part is complete, it is subject to all the inspection criteria of a new part.After reviewing a number of parts both vis

47、ually and dimensionally, it was clear thatregrinding the teeth would remove the surface damage in the most economical manner.Refurbishment jobs across production equipment could not be justified. The cost of setup, machine overhead for small quantities, scheduling difficulties, and maintaining areli

48、able delivery schedule for the customer make it very difficult to utilize a productionfacility. It was decided to retrofit one of the specialized gear grinders to do the job. Thisway a machine could be dedicated to a specific part or family of parts and it could be leftsetup for recurring work thus

49、reducing turn around time Changes mustbe improved in writing by the FAA. The repair station and inspection procedure manualgo through a mandatory once a year audit and the FAA may come in anytime for spotcheck audits.Repair approval requires either submitting the applicable FAA approved repair manual orthat section of the manual that applies. Repairs that are developed by the repair station- are approved on form 8110-3 which is controlled on Regulation 14 Part 33. The_ applicable D.E.R., Design Engineering Representative, must sign it off before submittalto the FAA. After approval, the