1、_ 6$(7HFKQLFDO6WDQGDUGV%RDUG5XOHVSURYLGHWKDW7KLVUHSRUWLVSX EOLVKHGE6$(WRDGYDQFHWKHVWDWHRIW echnical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, LVWKHVROHUHVS
2、RQVLELOLWRIWKHXVHU SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions. Copyright 2016 SAE International All rights reserved. No part of this publication may be reproduced
3、, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970 (outside USA) Fax: 724-776
4、-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AIR5913 AEROSPACE INFORMATION REPORT AIR5913 Issued 2011-02 Reaffirmed 2016-11 Landing Gear Shock Strut Heat
5、Damage RATIONALE AIR5913 has been reaffirmed to comply with the SAE five-year review policy. 1. INTRODUCTION The purpose of this report is to outline the type of damage referred to as “Ladder Cracking”. Discuss how it is being repaired and describe the use of a bearing material that has resolved thi
6、s problem without introducing other problems. TABLE OF CONTENTS 1.INTRODUCTION. 1 2.REFERENCES 2 3.BACKGROUND 2 4.SOLUTION 55.SUMMARY AND CONCLUSIONS 9LIST OF FIGURES FIGURE 1 767 MAIN GEAR . 3FIGURE 2 SHOCK STRUT SKETCH . 3FIGURE 3 LADDER CRACKS 4FIGURE 4 TEST SETUP 5FIGURE 5 BASELINE TEST 6FIGURE
7、6 HEAT DAMAGE COMBINED RESULTS 7FIGURE 7 OTHER MATERIAL TESTING 7FIGURE 8 FRICTION DATA . 8SAE INTERNATIONAL AIR5913 Page 2 of 9 2. REFERENCES The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable issu
8、e of the other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulation
9、s unless a specific exemption has been obtained. 2.1 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323 (inside USA and Canada) or 724-776-4970 (outside USA), www.sae.org. AIR5883 Landing Shock Struts Bearing Selection AIR5885 Land
10、ing Gear Common Repairs ARP4915 Disposition of Landing Gear Components Involved in Accidents/Incidents AIR1594 Plain Bearing Selection for Landing Gear Applications 2.2 Other Publications AD-D127069 CONSIDERATION OF MECHANICAL, PHYSICAL, AND CHEMICAL PROPERTIES IN BEARING SELECTION FOR LANDING GEAR
11、OF LARGE TRANSPORT AIRCRAFT. Author: Fewtrell, H. E. published 1983. http:/ 3. BACKGROUND 3.1 Airline experience has shown that, under certain load conditions, Aluminum nickel bronze bearings operating on bare and chrome plated high strength steel, has resulted in the generation of severe thermal sp
12、ikes over small discrete areas which have caused cracking of the steel with in some cases, resulting in catastrophic failures.1 3.2 Landing Gear Configuration A typical landing gear that encounters this type of damage is shown in Figure 1 these are of the cantilever design. 1 See Reference Para 3.5
13、for other testing. SAE INTERNATIONAL AIR5913 Page 3 of 9 FIGURE 1 767 MAIN GEAR FIGURE 2 SHOCK STRUT SKETCH The type of strut illustrated in Figure 2, comprises of a piston/inner cylinder supported by and able to slide inside the outer cylinder. Drag and side loads exert a considerable force on each
14、 of the sliding bearings which are usually have a designed projected area bearing pressure of around 6000 psi. However, due to deflections the bearing loads are concentrated at the open end of the lower bearing and they can be considerably higher than 6000 psi. There have been many different design
15、concepts to alleviate the problem of high bearing pressures. Self aligning, bell mouthed, non-metallic and others have met with some success. Outer Cylinder Upper BearingLower BearingInner CylinderDrag LoadUP Typical Cantilevered Type Shock Strut SAE INTERNATIONAL AIR5913 Page 4 of 9 3.3 Heat Damage
16、 The type of heat damage that is encountered in landing gear shock struts reveals itself as streaks or travel marks on chrome plated pistons. It is usually not detectable by the naked eye until it is evidenced by plating breakdown in the form of “Ladder Cracks” (see Figure 3). These cracks sometimes
17、 penetrate into the substrate material and have led to a structural failure or rejection of the part. 6OUTER CYLINDERPISTONFIGURE 3 LADDER CRACKS This damage is caused by rapid motion accompanied by high bearing pressure in a localized area. This type of action generates a localized high temperature
18、 sufficient to produce over-tempered and un-tempered martensite in the low alloy steel that most shock strut inner and outer cylinders are made from. The cracking occurs in the un-tempered areas. The cracks do not usually propagate out of this area as in most cases the damage is in the compressive a
19、rea of the inner cylinder. A forward raked gear is a concern as the cracks may propagate in tension areas of the cylinder. 3.4 Discovery The heat damage is usually discovered during overhaul. The general method of detection is Magnetic Particle Inspection and/or Dye Penetrant inspection of the plate
20、d piston. Detection through the chrome is often difficult and the damage is not detected until the chrome plate has been stripped and the part has been Nital Etched. Heat damage has also been found on the inside of steel outer cylinders in the area where the upper bearing slides. 3.5 Repair Repair i
21、s usually accomplished by a general reduction of the piston outside diameter followed by local grinding in the areas of most damage. If, after this operation, the part is still capable of being returned to service, it receives plating. The depressed areas are filled with nickel plate up to the diame
22、ter of the surrounding area. The piston is then chrome plated and ground to the original finished diameter. It has been observed also that parts that are repaired with sulphamate nickel plating and re-chrome plated do not usually exhibit ladder cracking after return to service. The common belief is
23、that the nickel acts as a heat sink, between the chrome and the metal substrate. Engineering usually assesses any removal of damage on the inside of outer cylinders. How it will affect the shock strut performance is considered. It is unfortunate that repairs of this nature have become woven into the
24、 fabric of landing gear overhaul and are taken for granted. Considerable savings can be realized if this type of damage could be eliminated. SAE INTERNATIONAL AIR5913 Page 5 of 9 4. SOLUTION 4.1 Design Almost all cantilever type shock struts use metallic upper and lower bearings such as Aluminum Nic
25、kel Bronze. These bearings slide on chrome plated and ground (inner cylinders or pistons) and bare steel inside diameters of outer cylinders. The use of these materials has been the norm since the 1960s and in some cases airlines specified the use of Al-Ni-Bronze bearings. A non-metallic material ca
26、lled “MICARTA” 2 a phenolic resin laminated material was used but it had a problem in as much as it became swollen when immersed in hydraulic fluid and instead of working like a bearing it became a good brake. Numerous other materials are also used for the upper and lower bearings applications with
27、various degrees of success. It is believed that Dacron lined bearings were less problematic than fiberglass woven bearings. As airplanes got larger and more complex, landing gears got larger and truck beams were added to provide good floatation. Overhauls revealed heat damage. It was apparent that a
28、 bearing material was needed that produced a lower friction value and reduced the friction-generated heat. As well as reducing the friction, the bearings needed to be durable so that it would last at least to the first overhaul. 4.2 Solution Research (The search for a bearing material) A series of t
29、ests were run using low alloy steel plates, with 16 micro-inches finished bare steel, chrome plated or Tungsten Carbide coating on each side. A clamping fixture was used to apply a force to both sides of the plate through bearing pads of a known area. The plate was then pulled through the clamped pa
30、ds at 3-10 fps velocities with pad pressures up to 60 kpsi. This was done with the pads immersed in MIL-PRF-5606 with additives and dry. Figure 4 illustrates the test set-up. FIGURE 4 TEST SETUP 2 www.norplex- SAE INTERNATIONAL AIR5913 Page 6 of 9 The first objective was to reproduce the heat damage
31、 encountered in landing gear shock struts using the same combination of bearing materials and cylinder finishes that was aluminum nickel bronze running on chrome plate and bare low alloy steel. Al Ni Br against Bare 4340M Steel05101520253035404520 30 40 50 60 70 80 90 100 110Velocity - inches per se
32、condBearingPressure -kpsiAl Ni Br - No DamageAl NI Br - Heavy DamageFigure 4.2-2FIGURE 5 BASELINE TEST The heat damage was reproduced using this test method. Ladder Cracking was not produced in this test. Having established the heat damage threshold (see Figure 5), various other bearing materials we
33、re tested at the same settings. The results are shown in Figure 6. Figure 7 shows data obtained from some other materials that were tested. Copper Beryllium did not perform well. Dyflon 3 and Kentlon 4 were non metallic coatings which showed little durability. Heat damage was also produced with chro
34、me plated steel plates. Inspection of the plates for heat damage entailed stripping the chrome plate and nital etching the base material. This, at the time, was time consuming and expensive. Limited funds and time led to concentrating on bare steel plates. There was also very limited testing of tung
35、sten carbide coated steel plates. 3 Southwest Products, AHR Aerospace Bearing Division. 4 Southwest Products, AHR Aerospace Bearing Division. SAE INTERNATIONAL AIR5913 Page 7 of 9 05101520253035404520 30 40 50 60 70 80 90 100 110Velocity - inches per secondBearingPressure -kpsi Al Ni Br - No Damag
36、eAl NI Br - Heavy DamageDU - No DamagePV=12,500,000 psi fpmPV=10,000,000 psi fpmPV=7,500,000 psi fpmPV=5,000,000 psi fpmPV=2,500,000 psi fpmFIGURE 6 HEAT DAMAGE COMBINED RESULTS FIGURE 7 OTHER MATERIAL TESTING Tested in MIL-PRF-5606 + 2.4% Lubrizol 13950510152025303540450 10 20 30 40 50 60 70 80 90
37、100 110VELOCITY - Inches/secondBearing Pressure - KSIBe Cu No Heat Damage Be Cu Heat Damage DYFLON Heat Damage KENTLON No Heat Damage KENTLON Heat DamageSAE INTERNATIONAL AIR5913 Page 8 of 9 Breakout and Running Friction0.000.050.100.150.200.250.300.350.40A1 B1 B2 B3 B4 B5 C1Various Materials in flu
38、id on Bare SteelCoefficient of FrictionBREAKOUTFRICTIONRUNNINGFRICTIONFLUIDS: A = MIL-PRF-5606, B = A + LUBRIZOL, C = SKYDROLPAD MTLS: 1 = AL-NI-BR, 2 = CU-BR, 3 = DU, 4 = KENTLON, 5 = DYFLONFIGURE 8 FRICTION DATA The friction data shown in figure 8 was obtained from the pull test with a bearing pre
39、ssure of 6000 psi. DU 5 appeared to be the more durable material. DU is a sintered bronze coating that is impregnated with a Teflon matrix material. It is produced in sheet form with a steel or aluminum bronze backing material from which bearings of various sizes can be formed. 4.3 Solution Research
40、 (Durability Testing) It was decided to use DU in the 777 landing gear shock struts. A test was set up that simulated the main gear shock strut and used production configured upper and lower bearings complete with the sealing elements. The test article was subjected to flight by flight cycling and c
41、ompleted 14 000 flight cycles. The bearings were examined and found to be in excellent shape. There was still plenty of wear left. The design lent itself to being able to reinstall the bearing elements inverted which would introduce new wearing surfaces into the high wear area. Both the inner and ou
42、ter cylinders were inspected for heat damage. None was found. 5 GGB Bearing Technology, Chadds Ford, PA 19317. SAE INTERNATIONAL AIR5913 Page 9 of 9 5. SUMMARY AND CONCLUSIONS 5.1 Comments Hydraulic fluids and a hydraulic fluids additive were specifically developed to improve “slip stiction” and to
43、 alleviate “ladder cracking” in landing gear struts. However, lower bearings are set below the lower gland seal so they essentially run dry. It is quite clear that the problem of ladder cracking is not well documented. It is assumed that the problem is caused due to high velocity contact point (prob
44、ably in the micro-seconds range). An earlier attempt to recreate this phenomenon in a test lab environment was not successful. In 1976, similar tests were conducted on a DC-10 Center Line Gear piston. Aluminum-Bronze material was tested at high bearing pressures. Although there was evidence of overh
45、eating (mostly on the bearing material), ladder cracking was not reproduced. Another report is listed that examined Al-Ni-Br and Copper Beryllium bearings (Para 3.5). 5.2 Original Equipment The DU bearings were introduced on the B-777 airplane and have been incorporated into the following aircraft:
46、B-737-Next Generation B-747 B-767 B-787 F-22 F-35 A-6 Embraer 190 A380 A400M Note: uses DP4B 6 similar to DU with a different matrix material. 5.3 Service Experience Aircraft known to have experienced ladder cracking: DC-10/KC-10: MLG B-707 / E-3: MLG B-727: MLG DHC-7: MLG B-767: MLG B-757: MLG A-10
47、: MLG Some of the 777s have had their first scheduled overhaul. Examination of one airplanes main and nose gear shock struts showed that the bearings and the inner and outer cylinders were in excellent condition and there was no heat damage found. DU bearings operating on HVOF coated pistons have al
48、so been found in excellent condition at overhaul. 5.4 Cost of Ownership It is expected that this trend will continue and that the airlines and landing gear overhaul facilities will reap the benefit of not having to do costly repairs to the inner and outer cylinders. PREPAIRED BY SAE SUBCOMMITTEE A5-B, GEARS, STRUTS & COUPLINGS OF COMM
