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 reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. Copyright 2010 SAE International All rights reserved. No part of this publication m
3、ay 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: +1 724-776-4970 (outside U
4、SA) Fax: 724-776-0790 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/AIR6079 AEROSPACE INFORMATION REPORT AIR6079 Issued 2010-12 Selection of Metallic Spring Ener
5、gized Seals for Aerospace RATIONALE A document was needed to review the basics of spring energized seals. This AIR serves to fulfill that need. 1. SCOPE The purpose of this report is to provide design, application and maintenance engineers with basic information on the use of metallic Spring Energiz
6、ed sealing devices when used as piston (OD) and rod (ID) seals in aircraft fluid power components such as actuators, valves, and swivel glands. The Spring Energized seal is defined and the basic types in current use are described. Guidelines for selecting the type of Spring Energized seal for a give
7、n design requirement are covered in terms of friction, leakage, service life, installation characteristics, and interchangeability. Spring Energized seals can also be made in various forms and types, including face seals (internal and external pressure sealing types), and rotary variants too. These
8、further types will not be discussed in this document, but many of the same principles apply for them as well. 2. REFERENCES 2.1 Applicable Documents The following publications form a part of this standard to the extent specified herein. The latest issue of the SAE documents shall apply. The applicab
9、le issue of the other documents shall be the issue in effect at the date of the purchase order. In the event of conflict between the text of this document and the references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and re
10、gulations unless a specific exemption has been obtained. 2.1.1 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. ARP1281 Actuators: Aircraft Flight Controls, Powe
11、r Operated, Hydraulics, General Specification for AS568 Aerospace Size Standard for O-rings AMS3678 Polytetrafluoroethylene (PTFE) Moldings and Extrusions Unfilled, Pigmented, and Filled Components AS4716 Gland Design, O-Ring and Other Elastomeric Seals AS5857 Gland Design, O-ring and Other Elastome
12、ric Seals, Static Applications SAE AIR6079 Page 2 of 14 2.1.2 US Government Publications Available from the Document Automation and Production Service (DAPS) DODSSP, subscription Service desk, Building 4/D, 700 Robinson Avenue, Philadelphia, PA 19111 5094, Tel: 215 697 6257, http:/assist.daps.dla.mi
13、l/quicksearch/. MIL-PRF-5503 Actuators, Aeronautical Actuating Linear Actuating, General Specifications for 3. GENERAL INFORMATION 3.1 Features of Spring Energized Seals A Spring Energized seal is a circumferential band of polymer material energized by a metal spring energizer. The cross section of
14、the Spring Energized seal configuration can be a variety of symmetrical or asymmetrical shapes (See Figure 1), and these can be manufactured in different sizes to suit AS4716 and AS5857 “Inch” or metric size housings. FIGURE 1 TYPICAL SPRING ENERGIZED SEALS Spring Energized seals are normally compri
15、sed of a machined polymer material body (or jacket), containing a spring energizer. The polymer most commonly used is Polytetrafluoroethylene (PTFE), as this provides excellent characteristics in terms of high and low temperature capability, low friction, long life, and wide chemical resistance. Oth
16、er materials can be used for the seal jacket according to the application requirements. The seal relies on the elastic characteristics of the spring to provide sealing force during low pressure (and vacuum) situations and to compensate for any seal material wear. Also the “U” shaped seal jacket can
17、be energized by the system media pressure applied to the chamber to be sealed, and these further increase the sealing force that the seal provides. There are various types of metal spring energizers that can be used with the spring energized seals. These are shown in the seal images in Figure 1 abov
18、e. The three main types of springs are shown in fig 2 SAE AIR6079 Page 3 of 14 a. Cantilever / V spring. ( Good dynamic performance, medium to high deflection, medium spring force.) b. Helical spring ( Good static performance, high spring force, low deflection ) c. Wire / Coil Spring ( Good dynamic
19、performance, high deflection, low/medium spring force and easier for installation purposes.) FIGURE 2 - SPRING TYPES Each of these spring types provide different compression and load deflection characteristics, which combined with the appropriate seal jacket body design, can be used to optimize seal
20、ing performance for different circumstances. It is imperative to select the optimum seal and spring design, for any given application, and for the sealing performance characteristics desired. Consult the seal manufacturer for more information in this respect. The metal Spring Energizer elements are
21、often made in a stainless steel material, but other (more corrosion resistant) alloys are also commonly available. The combination of a metallic spring and a machined PTFE jacket, when fitted in a gland, acts to control fluid leakage. When used in a dynamic application metallic Spring Energized seal
22、s generally, but not always, reduce breakout and running friction and enhance extrusion and wear resistance when compared to PTFE slipper seals. The first PTFE spring energized seals were used successfully in the “nineteen fifties”. PTFE is a chemically-inert, temperature-resistant (from cryogenic l
23、imits to 500F (260 C), low-friction, fluorocarbon thermoplastic, compatible with all industrial and military fuels and hydraulic fluids in military and commercial use. Although PTFE is not elastic in the same sense as an elastomeric material, it is deformable for good sealing and can be stretched fo
24、r ease of installation. Because PTFE is a relatively soft plastic, sealing surfaces must be protected from scratches and nicks during handling and installation. Installation tools for both OD and ID metallic Spring Energized seals are generally recommended for use where a (solid) one-piece gland is
25、involved. Installation tools are not always necessary, depending upon the diameter and cross section sizes used, and upon the exact seal design used. Metallic Spring Energized seals can be limited in installation capability due to the inability of some springs to stretch (for O.D installation) and c
26、ollapse (for I.D. installation). The metal spring can suffer due to permanent deformation or yielding when applied outside design parameters. This characteristic can vary by spring type and seal design. Consult the manufacturer for design limits. b a c SAE AIR6079 Page 4 of 14 Many standard metallic
27、 Spring Energized seals are designed for use in seal hydraulic glands per AS4716 and AS5857. Metallic Spring Energized seals are also used in higher pressure hydraulic systems. Additional extrusion resistance may be provided by reduced clearances and/or the addition of high-modulus backup rings. Con
28、sult the manufacturer for design limits. 3.2 Description of the Working Mechanism of the Spring Energized Seals The basic metallic Spring-Energized seal design has two elements; a. A pressure actuated, “U-shaped” polymer jacket b. A metal spring During low-pressure (or vacuum) operation the sealing
29、force is supplied only by the metallic spring-energizer, which maintains interference with the rod or bore and the groove I.D. or O.D. depending on the configuration. FIGURE 3 - SEAL ENERGIZING WITHOUT PRESSURE As media pressure is applied to the seal, and as this pressure increases, the PTFE jacket
30、 is pressure assisted and acts like a PTFE U-Cup controlling the potential leak paths. FIGURE 4 - SEAL ENERGIZING WITH PRESSURE There are two possible leak paths between the Spring Energized seal lips and the hardware interfaces. The rate of leakage is a function of the fluid pressure, fluid viscosi
31、ty, and the quality of surface conditions at each leakage path. Under dynamic conditions, the film thickness passing by the metallic Spring Energized seal is an additional potential leakage path. Sealing contact area Sealing contact area SAE AIR6079 Page 5 of 14 If the quality of the hardware surfac
32、e finish at each leakage path is substandard, that is, rough, nicked, scratched, or contaminated with foreign particles, then minute flow paths can exist that permit leakage. Assuming constant viscosity, leakage through a tiny orifice is proportional to the P across the orifice. If the surfaces of p
33、otential leakage paths, such as those present in a typical metal Spring Energized seal configuration, could be matched perfectly, leakage would be zero regardless of the pressure, since no passage would exist. There are, of course, practical limits to the achievement of this ideal sealing condition.
34、 It can be approached very closely, however, by selecting seal materials that are deformable under stress, such as an elastomer or PTFE. Although PTFE is deformable under compressive load, it has a higher compressive modulus than rubber and is, therefore, less forgiving of surface imperfections and
35、contamination. This is particularly true at low fluid pressure when deformation due to compressive stress is low. When imperfections on the PTFE surfaces cause some leakage, a brief running-in or application of high pressure will often result in improved surface-to-surface conformity and, therefore,
36、 reduced leakage. Fillers added to PTFE ( See 8.1) improve wear, extrusion resistance and further decrease deformation under load. These can have an adverse effect on leakage if they are not accommodated in design concept, detail, and workmanship. For consistent sealing performance, the lips of a me
37、tallic Spring Energized seal must have a diametric interference fit on the bore and rod. Greater diametric interference is often used to provide greater sealing tightness, but this also increases the sealing load and hence the friction provided. Additionally, stronger spring load designs can also pr
38、ovide similar increases in the sealing effectiveness, again with the same increased friction characteristics. Consult the manufacturer for recommendations. Spring Energized seal performance may be influenced by spring compression or radial squeeze. Optimum squeeze will vary with the PTFE jacket and
39、metallic spring configuration and the friction, leakage, and service life requirements of the application. Consult the manufacturer for recommendations. 3.3 Reasons for Spring Energized Seals to be Used Spring Energized seals are specified in hydraulic and pneumatic fluid power components as alterna
40、tives to O-rings, slipper seals and other types of seals to provide: Very low friction. High-speed service. Almost universal chemical compatibility. Cryogenic service to below 390 F.( - 199 C) High-temperature service to +500 F (+ 260 C), with higher temperatures possible. Permanent elasticity with
41、immunity to aging, embrittlement and compression set. No lubrication required. Not susceptible to explosive decompression damage. No out-gassing (from seal). No vulcanization during long storage time. SAE AIR6079 Page 6 of 14 4. SPRING ENERGIZED SEALS RELATED STANDARDS There are many commercial sour
42、ces for PTFE and filled PTFE metallic spring-energized seals. Usually the trade names used for metal spring-energized seals are proprietary. Many of the commercial standards in use are interchangeable with respect to gland fit but vary in design detail. Most metal spring-energized seal designs are p
43、roprietary and available from only one source. Most commercial spring energized seal standards, except for some proprietary designs, are classified for size with the same dash number as the corresponding ORings per AS568 for the applicable nominal rod or bore size. Radial spring energized seals are
44、further identified as ID (rod) seals or OD (piston) seals. Additionally the groove width is specified to be a no backup-width, one backup-width, or two backup-width gland in accordance AS4716 and AS5857. The current specification for O-ring dynamic glands is AS4716 and for static glands is AS5857. M
45、etallic spring energized seal configurations that are suitable for SAE AS4716 and AS5857 glands may require different back-up rings due to differences in gland depth. The seal supplier is the best source of information regarding the appropriate back-up rings to use in the appropriate glands. Spring
46、energized seals are limited in their ability for installation; O.D. (piston) type and I.D. (rod) type. This is driven by the geometry of the PTFE seal jacket, cross-section to be installed versus diameter, spring type and deflection or yield characteristics and to some extent applied materials. The
47、spring energized seal manufacturer is the best source of information on the installation capability of their products. Where spring energized seals are used in rotary applications, special considerations may be necessary, consult the seal manufacturer for advice. 4.1 Spring Energized Seals for No Ba
48、ckup-Width Glands per AS4716 and AS5857 Spring energized seals rely on the seal jacket to provide extrusion protection without the extra groove length necessary to accommodate backup rings. Hence, extrusion resistance for these seals can be limited, depending upon the operating conditions applied. E
49、xtrusion is a direct function of the temperatures, pressures applied, size of the extrusion gap. Some uses include spool-valve sleeves and swivel glands as well as rod and piston seals for actuators. 4.2 Spring Energized Seals for One Backup-Width Glands per AS4716 and AS5857 When specifying spring energized seals for One Backup Width glands, there are two options. As a common practice the one bac
