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 2013 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/AIR1243C AEROSPACE INFORMATION REPORT AIR1243 REV. C Issued 1978-03 Reaffirmed 2005-04 Revise
5、d 2013-02 Superseding AIR1243B Anti Blow-By Design Practice for Cap Seals RATIONALE Revision C to AIR1243 is a general update of the document with added Caution 2 in Section 8. Drawings have been added to Appendix A. 1. SCOPE This SAE Aerospace Information Report (AIR) provides information on anti b
6、low-by design practice for cap seals. Suggestions for piston cap seal sidewall notch design and other anti blow-by design details are also described. It also includes information on two key investigations based on the XC-142 as part of the text and as Appendix A. 1.1 Purpose The purpose of this docu
7、ment is to provide adequate information to the designer so that the blow-by problem will not reoccur. 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 public
8、ations 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 regulations unless a specific e
9、xemption 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. AMS3678 Polytetrafluoroethylene (PTFE) Moldings and Extrusions, Unfilled, Pigmen
10、ted, and Filled AS568 Aerospace Size Standard for O-rings ARP1233 Gland Design, Elastomeric O-Ring Seals, Dynamic Radial, 1500 psi Max AS4716 Gland Design, O-ring and Other Elastomeric Seals SAE AIR1243C Page 2 of 10 3. INTRODUCTION The mechanism of blow-by (see Figure 1) was described in an Air For
11、ce report in December 1965 which was issued following an XC-142 accident. This report has been updated and is attached (see Appendix A). Tests conducted by Vought Aeronautics Division, LTV Aerospace Corporation established that blow-by was a contributing factor to the accident. Following is an excer
12、pt from a magazine article prepared by J. R. Crissey of Vought entitled “How to Avoid the Blow-by Phenomenon“ which addresses the XC-142 accident and contributes further to an understanding of blow-by. The text has been edited to include modern practices and references to current SAE standards and s
13、pecifications. “WHAT CAUSES BLOW-BY?“ “Blow-by occurs when pressure on top of a cap becomes greater than pressure underneath. Thus, the cap is compressed into the seal groove allowing a relatively unrestricted flow past the seal. When the cap is compressed into the groove, seal energizing pressure i
14、s momentarily unable to get under the cap. This allows system pressure to “wedge-in“ across the top of the seal. Pressure reversals across the seal were necessary to trigger a blow-by. For example, a XC-142 power actuator normally has 1500 psi on both sides of the seal with no load applied to the ac
15、tuator. When the actuator is loaded, a differential pressure must be maintained to hold the load. A pressure reversal occurs when the differential pressure is in the direction to aid the load momentarily. The pressure reversal occurred on the XC-142 aircraft because of the high frequency sinusoidal
16、inputs to the servovalve. The seal malfunctions invariably “healed“ themselves, usually within 1.5 s. Some blow-bys lasted as long as 9 s; others were as short as 0.2 s. Healing was instantaneous and the seal functioned normally until the next blow-by occurred. Further testing showed that grooves (n
17、otches) cut in the edges of the PTFE cap permitted instant pressurization under the cap regardless of pressure condition, thus preventing blow-by. Seals which had readily malfunctioned in the test jigs could no longer be made to malfunction after the caps were reworked with grooves. As a result of t
18、his investigation of the XC-142 propeller pitch actuator and system, it is now required that capseals be grooved for all LTV-designed components which use cap seals.“ The following recommendations and design details outline current practice to provide rapid pressure response of piston (OD) cap seals
19、, hence avoid piston blow-by. Although the blow-by phenomenon is thought to be generally understood, information is fragmentary and documented service experience limited. Testing is recommended to confirm performance. SAE AIR1243C Page 3 of 10 4. CONTROL OF DIAMETER CLEARANCE (SEE FIGURE 1) Excessiv
20、e diametral clearance may cause deformation (feathering into the extrusion gap) of the cap seal. This deformation may close off the seal side wall clearance, hence reduce the rate of pressure response of the seal assembly during rapid increase in hydraulic pressure. Diametral clearance per ARP1233 a
21、nd AS4716 will serve to reduce the relative cap-strip seal deformation but will not in itself prevent blow-by. Also, the cylinder designer should consider the increase in clearance due to cylinder pressures. A reinforced PTFE cap seal material per AMS3678 is recommended to reduce seal deformation un
22、der conditions of long or repeated exposure to high temperature over 275 F (135 C) and pressure over 2500 psi (17.25 MPa) Low O-ring squeeze due to high clearance and/or groove diameter can also contribute to blow-by. AS4716 requires a minimum of 0.005 in (0.13 mm) squeeze which is an improvement ov
23、er previous standards. FIGURE 1 - EFFECT OF EXCESSIVE CLEARANCE OF PISTON IN CYLINDER BORE 5. USE OF CAP-STRIP SEALS WITH ADEQUATE SIDE WALL CLEARANCE (SEE FIGURE 2) The difference between the maximum axial length of the cap seal and the minimum length of the groove should not be less than shown in
24、Table 1 to ensure rapid pressure response. TABLE 1 - MINIMUM AXIAL CAP CLEARANCE IN RELATION TO O-RING CROSS SECTION O-Ring Cross-Section W Minimum Side Wall Clearance A inches millimeters inches millimeters 0.070 1.78 0.010 0.25 0.103 2.62 0.010 0.25 0.139 3.53 0.012 0.30 0.210 5.33 0.014 0.35 0.27
25、5 7.0 0.016 0.41 FIGURE 2 - CAP SEAL SIDEWALL CLEARANCE SAE AIR1243C Page 4 of 10 6. USE OF SIDE WALL NOTCHES ON PISTON (OD) CAP-STRIP SEALS (SEE FIGURES 3 AND 4) In addition to the recommended diametral clearance and side wall clearance, notches in the seal side walls should be used to provide rapi
26、d pressure response and prevent blow-by. Recommended notch detail is shown in Figure 3. Notches may not be practical on cap seals with less than 0.030 in (0.76 mm) radial thickness and less than 0.060 in (1.52 mm) axial length. Anti blow-by design recommendations for small cross-section seals are av
27、ailable from seal manufacturers and will vary with installation and service requirements. Procurement activity approval or testing is recommended. Time in use, particularly at high temperature or high pressure or both, may cause distortion of the cap seal sufficient to restrict the effectiveness of
28、the side wall notches. Chamfers can be used to improve the effectiveness of notches and also to extend the service life of the notch (see Section 7). FIGURE 3 - RECOMMENDATIONS FOR SIDEWALL NOTCHES IN PISTON CAP SEAL 7. USE OF CHAMFERS ON PISTON (OD) CAP SEALS (SEE FIGURES 4 AND 5) The effective lif
29、e of side wall notches can be extended by incorporating chamfers in the cap seal that reduce the tendency of the notches to close off with seal use. The chamfers may be local at each notch on the outside diameter of the cap seal (see Figure 4) or circumferential chamfers may be used along the outsid
30、e corners of the seals (see Figure 5). Circumferential chamfers are also effective in reducing the cap seal deformation shown in Figures 1 and 5. Testing is recommended. SAE AIR1243C Page 5 of 10 FIGURE 4 - CAP SEAL WITH CHAMFERED NOTCH SAE AIR1243C Page 6 of 10 FIGURE 5 - RECOMMENDATIONS FOR SIDEWA
31、LL NOTCHES IN PISTON CAP SEAL WITH CHAMFERED EDGES SAE AIR1243C Page 7 of 10 8. USE OF HIGH MODULUS MATERIAL BACKUP RINGS WITH PISTON (OD) CAP SEALS (SEE FIGURE 6) Backup rings made from high modulus material such as polyamide (note temperature limitations), polyimide, polyetheretherketone (PEEK) or
32、 filled PTFE compatible with system operating parameters, may be located in the groove adjacent to the cap seal to protect the seal against extrusion. Adequate side wall clearance (0.010 in (0.25 mm) min) will ensure rapid pressure response of the cap seal. Notches in the backup rings on the radial
33、surface facing the groove side wall can also be used advantageously to increase the fluid pressure response. Notches should be on both sides of backup ring to prevent misassembly. FIGURE 6 - ARRANGEMENT OF PISTON CAP SEAL WITH HIGH MODULUS BACKUP RINGS CAUTION 1: The O-ring can be damaged in the gap
34、 below the high modulus backup ring. This can be minimized by 45 degrees x T/4 chamfers on the backup ring at both the edge that contacts the O-ring and the edge which tends to ride up on the groove radius (not necessary for filled PTFE backup rings). A PTFE or filled PTFE ring next to the O-ring wi
35、th a higher modulus ring outboard will also provide the necessary O-ring protection. Testing is recommended. CAUTION 2: Scarf-cut backup rings with notches in high modulus materials such as polyimide or PEEK (Polyetheretherketone) as used in system pressures up to 3000 psi (20.7 MPa) can lead to fai
36、lure of piston type cap seals at low temperature and under breathing conditions of the cylinder. This is particularly applicable to cap seals manufactured from filled PTFE as these materials have lower elongation values than virgin, pigmented or very low fill PTFEs. The mechanism is as follows. At h
37、igh temperature (e.g., 275 F (135 C) and under high pressure (e.g., 3000 psi (20.7 MPa) the cap tends to flow into the notches in the backup ring, providing a mechanical lock between the cap and the backup ring in the circumferential direction. After this has occurred, and when the backup ring accom
38、modates cylinder breathing the circumferential increase occurs solely at the scarf cut location. When this occurs at low temperature such as -65 F (-54 C) consider that the elongation property of the cap is much lower than at ambient temperature and this can lead to local tensile failure of the cap.
39、 Testing with notches in scarf-cut backup rings is recommended. 9. USE OF PISTON HEAD CHANNELS AND DRILLED HOLES (SEE FIGURE 7) Channels in the piston head which admit fluid pressure into the seal groove will ensure rapid pressure response. A 0.010 in (0.25 mm) wide channel is effective. Channel dep
40、th should not exceed the minimum side wall height of the cap seal to avoid extrusion of the elastomer into the channel. Two to four channels are recommended on each side of the seal groove. Testing is recommended. Drilled holes through the piston head can be used as an alternative to the piston head
41、 channel. Holes must be located in the seal groove so that the cap seal prevents O-ring extrusion into the hole. The number of holes and hole size will vary depending upon design requirements and limitations. CAUTION: Experience with the design practice of using channel and drilled hole techniques t
42、o ensure rapid pressure response is limited. Testing is recommended. SAE AIR1243C Page 8 of 10 FIGURE 7 - PISTON CAP SEAL INSTALLED WITH CHANNELS AND DRILLED HOLES IN PISTON 10. TESTING FOR BLOW-BY Blow-by can be demonstrated in the laboratory using a servo-actuator having a cap-strip-type seal on t
43、he piston. Use a motion transducer attached to the piston rod of the actuator, a pressure transducer located at each pressure port of a manual input servovalve, and a recorder. Motion of the actuator piston must be restricted by an external load sufficient to create a pressure differential applicabl
44、e to the service conditions being evaluated. When a rapid reversal of pressure is applied to the piston, a delay in the piston rod translation coincident in time with a loss in pressure is evidence of blow-by. The duration of the phenomenon may be momentary or for several seconds. Blow-by may be a r
45、andom occurrence as the actuator is cycled. Typically, you must experiment with different applied loads and different step inputs of reversal to the valve. Blow-by has been known to occur as the input was reversed when the actuator was going through a no load point (changing from tension to compress
46、ion load). Blow-by is more likely to occur after considerable cycling at the maximum elevated temperature expected. 11. NOTES A change bar (l) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previ
47、ous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document, including technical revisions. Change bars and (R) are not used in original publications, nor in documents that contain editorial changes only. PREPARED BY SAE PANEL A-6C2, SEAL
48、S OF COMMITTEE A-6, AEROSPACE ACTUATION, CONTROL AND FLUID POWER SYSTEMS SAE AIR1243C Page 9 of 10 APPENDIX A WARNING REGARDING THE USE OF POLYTETRAFLUOROETHYLENE SOLID RING AND ELASTOMERIC RING COMBINATIONS AS PISTON SEALS Appendix A is a slightly revised version of SEJPF Report 65-2, 15 December 1
49、965, issued by the Systems Engineering Group, Research and Technology Division, Air Force Systems Command, Wright Patterson Air Force Base, Ohio. A.1 INTRODUCTION Engineers and component designers responsible for aircraft fluid power systems are continually seeking efficient, simplified and reliable sealing configurations. Seals are essential for confining selected fluid media to systems as required and for isolating fluids under pres
