SAE AIR 1707B-2014 Patterns of O-ring Failures《O型密封圈失效型式》.pdf

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5、Superseding AIR1707A Patterns of O-ring Failures RATIONALE Document was updated for 5-year review and clearer, more legible images were included. 1. SCOPE The information presented herein describes the commonly observed patterns of O-ring failure by means of both text and illustration. Possible caus

6、es and corrective actions are indicated for alleviating the problem. 1.1 Purpose This document is intended to provide a guide for analyzing O-rings which have failed in order to correct the circumstances which have caused the failure. A great deal can be learned toward solving a sealing problem invo

7、lving O-rings by close observation of the failed O-ring. 2. REFERENCES There are no referenced publications specified herein. 3. FAILURE PATTERNS 3.1 General Failures are usually due to a combination of causes. The patterns of O-ring failure described herein, then, represent the modes most often enc

8、ountered. Subtle variations will occur, but the major cause generally predominates. 3.1.1 Note that this document does not advise on rubber compound selections or design details, although some general guidance comments are made. Consult an applications engineer for material selection, design details

9、, and the proper use of antiextrusion devices. Also, consider the substitution of other sealing types for the O-ring. SAE INTERNATIONAL AIR1707B Page 2 of 20 3.1.2 The failures discussed herein have been classified under the following headings: Extrusion and Nibbling 3.2 Spiral Failure 3.3 Abrasion

10、3.4 Compression Set 3.5 Weather and Ozone Cracking 3.6 Heat Aging and Oxidation 3.7 Plasticizer Extraction 3.8 Installation Damage 3.9 Gas Expansion Rupture 3.10 Failure Due to Backup Ring 3.11 3.2 Extrusion and Nibbling 3.2.1 Occurrence Usually associated with dynamic rod or piston seals but can oc

11、cur in a static seal with pressure pulsations, especially where it opens and closes the clearance. 3.2.2 Appearance Exhibits many small nibbles removed from the O-ring adjacent to the downstream clearance area, or a small section of the O-ring may be extruded. Typical of high pressure systems, this

12、process sometimes erodes half of the O-ring cross-section before major leakage occurs. 3.2.3 Cause Clearance is too large, pressure is too high, O-ring is too soft, O-ring is softened and swelled by fluid, clearance increases under pressure, eccentricity causes irregular gap, and corners of O-ring g

13、land are too sharp. 3.2.4 Corrective Action Closer metal fits (reduce diametral clearance or reduce plate gap,) use of backup rings or other antiextrusion devices such as cap strips, harder O-ring material, use of O-ring more compatible with system fluid, more rigid or concentric metal components, a

14、nd break edges of gland to a radius of about 0.001 to 0.002 inch (0.025 to 0.050 mm). T-seals that fit into the same gland may also be substituted. 3.2.5 Examples FIGURE 1 - EXTRUSION AND NIBBLINGSAE INTERNATIONAL AIR1707B Page 3 of 20 FIGURE 2 - EXTRUSION AND NIBBLING (USED WITH PERMISSION FROM R.L

15、. HUDSON)FIGURE 3 - NIBBLING AND SKIVINGSAE INTERNATIONAL AIR1707B Page 4 of 20 FIGURE 4 - NIBBLING AND ABRADINGFIGURE 5 - NIBBLING AND EXTRUSIONSAE INTERNATIONAL AIR1707B Page 5 of 20 FIGURE 6 - NIBBLING, EXTRUSION, PITTING, AND SPIRAL FAILURE3.3 Spiral Failure 3.3.1 Occurrence Usually associated w

16、ith long stroke piston seals but can occur with dynamic rod seals. Has been observed even on short stroke pneumatic piston seals. 3.3.2 Appearance Exhibits a deep spiral cut (usually at about 45 degree) into the O-ring cross-section. 3.3.3 Cause Conditions which cause segments of the O-ring to slide

17、 and others to roll, simultaneously. The O-ring generally gets pinched or “hung up“ at one point of its periphery. Side loads causing the O-ring to get caught in an eccentric component, uneven surface finishes, poor or uneven dispersion of lubricant and stroke speeds contribute. 3.3.4 Corrective Act

18、ion Improve metal surface finish to the 10 to 20 microinches (0.25 to 0.50 m) range, improve lubrication, use backups or other antiextrusion devices if possible, reduce gap, and consider seals other than O-rings such as T-seals. SAE INTERNATIONAL AIR1707B Page 6 of 20 3.3.5 Examples FIGURE 7 - SPIRA

19、L FAILUREFIGURE 8 - SPIRAL FAILURE (USED WITH PERMISSION FROM R.L. HUDSON)SAE INTERNATIONAL AIR1707B Page 7 of 20 FIGURE 9 - SPIRAL FAILURE (PHOTO COURTESY OF PARKER HANNIFIN)3.4 Abrasion 3.4.1 Occurrence Dynamic seals, either reciprocating or rotary. 3.4.2 Appearance Flat area on one side of cross-

20、section of O-ring where moving contact is made with metal component. Frequently shows wear lines on the cross-section parallel to motion of the seal. 3.4.3 Cause Too rough or too smooth metal surfaces, poor lubrication, high temperatures, or fluid contamination. 3.4.4 Corrective Action Use recommend

21、ed metal finishes, provide adequate lubrication, use O-ring material compatible with service temperature, and eliminate fluid contamination. Rapid wear can be expected with finishes below 5 microinches (0.13 m). SAE INTERNATIONAL AIR1707B Page 8 of 20 3.4.5 Examples FIGURE 10 - ABRASIONFIGURE 11 - A

22、BRASION ON ONE SIDESAE INTERNATIONAL AIR1707B Page 9 of 20 FIGURE 12 - ABRASION (USED WITH PERMISSION FROM R.L. HUDSON)3.5 Compression Set 3.5.1 Occurrence Dynamic and static seals. 3.5.2 Appearance Flat area on both sides of O-ring cross-section in the area being squeezed. 3.5.3 Cause Temperature t

23、oo high (either environmental or frictional) causing hardening, excessive volume increase from fluid causing overfill, excessive squeeze to achieve seal, O-rings not completely vulcanized, or use of compound with poor set properties. In extremely rare cases, this can also be caused if a ring is over

24、-cured (reversion.) 3.5.4 Corrective Action Use O-ring material compatible with fluid and temperature service, minimize conditions which increase service temperature, review squeeze to be sure it is proper, and use an improved, low-set compound if available. A good quality, well-inspected O-ring is

25、important. 3.5.5 Examples FIGURE 13 - COMPRESSION SETSAE INTERNATIONAL AIR1707B Page 10 of 20 FIGURE 14 - COMPRESSION SET ON THREE SIDES (CROSS-SECTION)3.6 Weather or Ozone Cracking 3.6.1 Occurrence Dynamic or static seals under stress and exposed to atmosphere containing ozone and air pollutants. 3

26、.6.2 Appearance Many small cracks perpendicular to the direction of stress may be small enough to be unseen with unaided eye or large enough to progress entirely through the cross-section (also see 4.8.3). Crazing of the outside surface of the O-ring is the most common. 3.6.3 Cause Ozone attacks uns

27、aturated or double bond points in polymer chain of some polymers causing chain scission. The damage is usually seen on the outside surface, exposed to the atmosphere. 3.6.4 Corrective Action If conditions of service cannot be remedied, the rubber material must be changed. Use rubber materials that a

28、re saturated and inherently resistant to ozone attack. Change storage conditions to reduce exposure time to detrimental environment. Sufficient improvement can, at times, be obtained by using rubber compounds with antiozonant additives. SAE INTERNATIONAL AIR1707B Page 11 of 20 3.6.5 Examples FIGURE

29、15 - WEATHER OR OZONE CRACKING FIGURE 16 - CRACKING FROM OZONESAE INTERNATIONAL AIR1707B Page 12 of 20 FIGURE 17 - CRACKING FROM OZONE3.7 Heat Hardening and Oxidation 3.7.1 Occurrence Dynamic or static seals subjected to elevated temperatures and atmosphere. Common in pneumatic or air service. 3.7.2

30、 Appearance Hardening of the rubber begins at the surface and progresses through the entire O-ring cross-section. Hardening is accompanied by high compression set if O-ring is under compression. 3.7.3 Cause Environmental temperature and oxygen exposure are too high for the selected rubber material,

31、causing hardening due to additional cross-linking in the rubber, evaporation of rubber plasticizer, or oxidation. It may be a combination of these. 3.7.4 Corrective Action Decrease service temperature, select a rubber material with better resistance to high-temperature and/or oxygen exposure, or pro

32、tect surface from oxygen exposure (as wiper seals). SAE INTERNATIONAL AIR1707B Page 13 of 20 3.7.5 Examples FIGURE 18 - HARDENING AND OXIDATIONFIGURE 19 - WEATHER CRACKING (USED WITH PERMISSION FROM R.L. HUDSON)SAE INTERNATIONAL AIR1707B Page 14 of 20 FIGURE 20 - HARDENING AND PITTING 3.8 Plasticize

33、r Extraction 3.8.1 Occurrence Dynamic and static seals usually in fuel systems subject to dry-out periods, but occasionally in other systems. 3.8.2 Appearance Small cracks in stressed area of O-ring cross-section, accompanied by loss in volume. This is often difficult to detect with a visual inspect

34、ion. 3.8.3 Cause Extraction of plasticizer by service fluid followed by evaporation of service fluid during dry-out period. Results are hardening of the rubber material and decrease in elongation to the extent that cracks appear where stressed. This condition is accelerated in the presence of ozone.

35、 3.8.4 Corrective Action Assuming service conditions cannot be improved, change to a compatible rubber material with low or no extractable plasticizer content. SAE INTERNATIONAL AIR1707B Page 15 of 20 3.8.5 Examples FIGURE 21 - PLASTICIZER EXTRACTION3.9 Installation Damage 3.9.1 Occurrence Dynamic a

36、nd static seals usually installed with squeeze between inside and outside diameter of the O-ring, or stretching an O-ring over tube ends or threads. 3.9.2 Appearance Short cuts or notches or a “shinned“ surface or a “skived“ cut that can extend at a uniform thickness clear around the periphery of th

37、e O-ring and can continue through the cross-section as if it were being peeled. May also present as spiraling or twisting in certain applications. 3.9.3 Cause Sharp corners on mating metal parts, threads, or insufficient lead-in chamfer; inside diameter of O-ring too small on rod seal or too large o

38、n piston seal installation. Blind grooves in multiple port valves are especially troublesome. 3.9.4 Corrective Action Adjust dimensions of metal components and O-rings to eliminate cutting or pinching of seals when possible. Lead-in chamfers should be 20 degrees maximum and “break“ sharp corners of

39、chamfer and groove edges. Use tubular installation tools to cover threads and sharp corners during O-ring installation. If feasible in the application, use an elastomer that is more resistant to mechanical damage (e.g., EPDM instead of silicone). SAE INTERNATIONAL AIR1707B Page 16 of 20 3.9.5 Exampl

40、es FIGURE 22 - INSTALLATION DAMAGE (USED WITH PERMISSION FROM R.L. HUDSON)FIGURE 23 - TWISTED AND PUNCTURED INSTALLATION DAMAGEFIGURE 24 - SKIVED INSTALLATION DAMAGESAE INTERNATIONAL AIR1707B Page 17 of 20 3.10 Gas Expansion Rupture 3.10.1 Occurrence High pressure gas seals subject to fast decompres

41、sion periods, sometimes called explosive decompression. 3.10.2 Appearance Short splits or ruptures which may be deep into the O-ring cross-section. When O-ring is first removed, the surface may have little blisters. They either split or go away. 3.10.3 Cause Absorption of gas or liquefied gas under

42、high pressure with subsequent lack of escape on decompression causing blistering and ruptures. 3.10.4 Corrective Action Increase time lapse for decompression or use higher modulus rubber material or special compounds that will resist rupture. 3.10.5 Examples FIGURE 25 - GAS EXPANSION RUPTURESAE INTE

43、RNATIONAL AIR1707B Page 18 of 20 FIGURE 26 - RAPID GAS EXPANSION/EXPLOSIVE DECOMPRESSION RUPTURES (CROSS-SECTIONS)FIGURE 27 - GAS EXPANSION RUPTURESSAE INTERNATIONAL AIR1707B Page 19 of 20 FIGURE 28 - RAPID GAS RUPTURE/EXPLOSIVE DECOMPRESSION3.11 O-ring Failure Due to Backup Ring Failure or Scarf-Cu

44、t End 3.11.1 Occurrence More commonly seen in dynamic seals, but may also be present in static seals. 3.11.2 Appearance Similar to 4.2 except that damaged sections of the O-ring correspond to damage to the backup ring (see Figure 29) or to the scarf cut or overlap of the backup ring (not shown). 3.1

45、1.3 Cause Thermal changes, pressure surges, installation damage of the backup ring, dimensionally poor-fitting backup ring, use of hard scarf-cut backup ring with soft elastomer, and extrusion of backup ring into the clearance gap. 3.11.4 Corrective Action When O-ring damage corresponds to backup ri

46、ng damage, check design details and installation procedures. When O-ring damage corresponds to the location of the backup ring scarf-cut, consider the uncut type of backup ring. Consider other seal types. SAE INTERNATIONAL AIR1707B Page 20 of 20 3.11.5 Examples FIGURE 29 - O-RING FAILURE DUE TO BACK

47、UP RING DAMAGE 4. NOTES 4.1 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 previous 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 AMS COMMITTEE “CE“