1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical 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 there
2、from, is the sole responsibility of the user.” 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 2013 SAE International All rights reserved. No part of this p
3、ublication may be reproduced, 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: +1 724-776-497
4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/ARP4912BAEROSPACERECOMMENDED PRACTICEARP4912 REV. BIssued 1996-12 Revised 2007-04
5、Reaffirmed 2013-10 Superseding ARP4912A Design Recommendations for Spare Seals in Landing Gear Shock Struts RATIONALE ARP4912B has been reaffirmed to comply with the SAE five-year review policy. 1. SCOPE This SAE Aerospace Recommended Practice (ARP) provides recommendations on cavity design and the
6、installation of elastomer type spare seals in these cavities. 2. APPLICABLE DOCUMENTS The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on t
7、he 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 regulations unless a specific exemption has been obtained. 2.1 SAE Publ
8、ications 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. AMS-P-83461 Packing, Preformed, Petroleum Hydraulic Fluid Resistant, Improved Performance at 275F (135C) 2.2 U.S. Govern
9、ment Publications Available from the Document Automation and Production Service (DAPS), Building 4/D, 700 Robbins Avenue, Philadelphia, PA 19111-5094, Tel: 215-697-6257, http:/assist.daps.dla.mil/quicksearch/. MIL-PRF-5606 MIL-PRF-83282 MIL-P-25732 (Inactive) Copyright SAE International Provided by
10、IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3. RECOMMENDATIONS 3.1 Spare seal cavities should be designed with maximum bottom radii, both to eliminate stress risers and to distinguish the spare seal cavity from the actual seal glands.
11、Half round cavities may be considered for rod type T-Seals or O-rings. 3.2 All spare seal cavity edges are to be smooth and free of sharp edges and burrs to eliminate seal damage during installation. 3.3 Design considerations must assure that spare seals cannot function as a static seal. Since MIL-S
12、pecs require seal elastomers to swell 10 to 20% in MIL-PRF-5606 and 5 to 15% in MIL-PRF-83282, these increases in volume must be considered. 3.4 Spare seal cavities should be located downstream (nonpressurized side) of the active static seals. This location minimizes fluid contact, reducing seal swe
13、ll which can make seal installation difficult or impossible. Seal swell in excess of 15% increase in circumference has been seen after 500 h in MIL-PRF-5606. 3.5 Seals should be lubricated and packed in grease in the spare seal cavities. A list of recommended greases with typical swell is included i
14、n Appendix A. 3.6 Seals are to be stored in their “as molded“ configuration. When seals are turned inside out and stretched over the lower bearing, excessive stress is applied to the dynamic sealing surfaces. 3.7 If seals are installed in separate grooves, backup rings may be stored with the seals.
15、3.8 Several aircraft have recently been designed with active spare seals. This design as shown in Figure 1 uses two seals in tandem with a normally open valve arrangement between them to eliminate a pressure differential across the inboard or spare seal. When the outboard seal fails, the valve is cl
16、osed and the inboard seal becomes the primary seal. Although the life of this seal is not as long as the outboard seal, it is a spare seal arrangement that should be considered. FIGURE 1 - DOWTY AEROSPACE LG-GLOUCESTER 3.9 This document addresses nitrile type seals used with common mineral oil strut
17、 fluids, such as MIL-PRF-5606 and MIL-PRF-83282. Phosphate ester systems using Skydrol or Hyjet fluids with EPR type elastomers may exhibit different stretch and swell characteristics and require alternate compatible greases for installation. 3.10 Design considerations should include the prevention
18、of pressure trapping between the static seal and spare seals. SAE INTERNATIONAL ARP4912B Page 2 of 8_ Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3.11 Recommended spare seal cavity dimensions are
19、 shown in tabular form in Figure 2. 3.12 The spare seal gland size and location should not allow deformation of the lower bearing created by extended loads of the piston during aircraft take off. 4. TEST RESULTS Several tests have been run to guide the designer in addressing the permanent set and sw
20、ell of stored seals. These tests all utilized an elastomer meeting the MIL-P-25732 specification. 4.1 In the first test typical seals were stretched on mandrels 5, 10, 15, and 20% larger than the standard rod diameter for these seals, then aged 500 h in MIL-PRF-5606 fluid at 160 F. The results are s
21、hown in Figure 3. This test was originally planned to extend to 1000 h but fluid swell expanded the seal diameter beyond the mandrel diameter after 500 h exposure. 4.2 Since seal swell was clearly the predominant factor, several nitrile seal materials were immersed in commercial strut fluids and vol
22、ume swell results of long-term soaks are shown in Figure 4. 4.3 To isolate the effects of long-term tensile set without the effects of fluid, the test described in 4.1 was repeated in air at 160 F. The results of these tests are shown in Figure 5. 4.4 In a further attempt to isolate stored seals fro
23、m fluid swell, a 7441-ft T-Ring was installed with a typical 5% stretch and sealed in the groove with a viscous silicone grease (Dow Corning DC-55). The assembly was then dipped into MIL-PRF-5606 fluid once a week to simulate downstream seal leakage. After six months the seal was removed and measure
24、d seal swell was only 1.5% of the original diameter. 4.5 The seals used in these tests were T-Seals of the following type as shown in Table 1: TABLE 1 Part Number Dimensions Application 7451FT 11.500 OD x .275 Cx 747 MLG Dynamic Strut Seal 7441FT 7.510 OD x .275 Cx DC-10 NLG Dynamic Strut Seal 7339F
25、T 3.640 OD x .210 Cx 737 NLG Dynamic Strut Seal 5. CONCLUSIONS 5.1 Seal swell due to fluid contact is a major problem when attempting to install a seal that has been exposed to hydraulic fluid. 5.2 Tensile set, even if seal is stretched 20%, should not be cause for concern. 5.3 By locating the spare
26、 seal downstream from the static seal and packing it in a low swell grease, minimum swell and less installation difficulty will be encountered. 6. NOTES 6.1 The change bar ( l ) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial c
27、hanges, have been made to the previous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document. PREPARED BY SAE SUBCOMMITTEE A-5B, GEARS, STRUTS, AND COUPLINGS OF COMMITTEE A-5, AEROSPACE LANDING GEAR SYSTEMS SAE INTERNATIONAL ARP4912B Pa
28、ge 3 of 8_ Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-FIGURE 2 - TYPICAL SPARE SEAL STORAGE ARRANGEMENT (SEPARATE CAVITIES) SAE INTERNATIONAL ARP4912B Page 4 of 8_ Copyright SAE International Pr
29、ovided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-FIGURE 3 - EFFECT OF STRETCH AND FLUID ON DIAMETRICAL CHANGE AT 160 F SAE INTERNATIONAL ARP4912B Page 5 of 8_ Copyright SAE International Provided by IHS under license with SAENot f
30、or ResaleNo reproduction or networking permitted without license from IHS-,-,-FIGURE 4 - EFFECT OF FLUID ON VOLUME CHANGE WITH TIME (AT TEMPERATURE = 165 F, WITH -214 O-RINGS) SAE INTERNATIONAL ARP4912B Page 6 of 8_ Copyright SAE International Provided by IHS under license with SAENot for ResaleNo r
31、eproduction or networking permitted without license from IHS-,-,-FIGURE 5 - EFFECT OF EXPOSURE TIME ON STRETCHED T-RINGS AT 160 F (T-RING SIZES: 7339, 7441, AND 7451 FT) SAE INTERNATIONAL ARP4912B Page 7 of 8_ Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reprodu
32、ction or networking permitted without license from IHS-,-,-APPENDIX A Several installation lubricants have been evaluated and found to be compatible with AMS-P-83461 Type nitrile seals. These include, but are not limited to the following as shown in Table A1: TABLE A1 Test Time Test Temperature Swel
33、l (%) 1) Aeroshell-6 6 months Room Temperature 3.46 2) Aeroshell-14 6 months Room Temperature 6.33 3) Aeroshell-23C 6 months Room Temperature 3.88 4) White Petrolatum 6 months Room Temperature 3.10 5) DC-55 Silicone Grease* 6 months Room Temperature 5.53 6) AFS-682 Grease+ 6 months Room Temperature 2.50 * Dow Corning Molykote55M + Aviation Fluid Services SAE INTERNATIONAL ARP4912B Page 8 of 8_ Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
