Troubleshooting o-ring failures.

As most engineers and plant managers realize, o-ring failures can be critical. The ability to troubleshoot these problems can save precious time in getting production equipment back on line, or redesigning the sealing system of a company's product.

An o-ring is a "simple" device often specified or installed as an afterthought - simple, that is, until it fails. The apparent simplicity of an o-ring should never be confused with the complexity of its function: a device that in deformation must balance the many effects of chemical attack, friction, pressure and temperature while maintaining the fluid integrity of the system. Clearly, then, the modes of failure are case specific and varied across the range of o-ring applications.

Both mechanical and chemical causes of failure of an o-ring seal are revealed in the visual patterns of the failed o-ring. The following is a basic guide to identification of the visual evidence of o-ring failure and the interpretation of that evidence in terms of possible causes.

Seals affected by extrusion and nibbling appear as if many small bites have been taken from the o-ring on the downstream or low pressure side. These tiny bites (figure 1) are commonly found on the ring circumference and are typical of the failures that occur in high pressure systems. The biting is often caused by pressure spikes which force the elastomer into the extrusion gap and shear off small pieces of the seal wall. Materials with low tear strength and low durometer (hardness) such as silicones and fluorosilicones are more susceptible to failures of this type, while higher durometer and high tear-strength materials such as carboxylated nitrile and urethane are less prone to extrusion and nibbling.

Seal damage can also occur during installation. Some of the visual clues of this type of o-ring failure are short cuts, notches and a skinned or peripherally peeled surface. Causes of this type of damage are sharp edges on mating metal components of the o-ring gland, sharp threads over which the o-ring must pass during assembly, an insufficient lead in chamfer, blind grooves in multi-port valves, twisting or pinching of the o-ring during installation, an o-ring volume greater than the groove void (gland) and lack of lubrication during installation.

Another mode of o-ring failure is excessive abrasion. This will be seen as a rough, slightly flattened surface on one side of the o-ring (figure 2). This pattern occurs primarily in dynamic seals involving reciprocating, oscillating or rotary motion. The cause may be a metal surface that is too rough or system contaminants. However, if a metal contact surface is too smooth, the lubricant will not be retained between the seal and the gland walls.

Flat surfaces on the bottom and top of the o-ring cross section (figure 3) indicate compression set. This condition is generally caused by the o-ring material exceeding its high temperature range. Heat hardening (figure 4) has the appearance of a pitted or cracked surface and is often accompanied by the flatness of compression set. Oxidation is similar in appearance to heat hardening.

In high pressure gas applications, a common problem is explosive decompression. This problem is indicated by the random short splits or ruptures going deep into the o-ring cross section. When the o-ring is first removed, the surface may also be covered by small blisters. Explosive decompression typically occurs when a gas is absorbed by an o-ring while operating in high pressure; the gas is trapped within the o-ring when the system pressure drops, causing the surface of the o-ring to blister and rupture as the trapped gas expands. In this situation, an o-ring with the smallest possible cross section should be used, as well as a rubber compound with low permeability.

O-ring used as seals on long stroke hydraulic pistons are susceptible to "spiral failure." Easily diagnosed, spiral failure exhibits a series of deep, spiral 45 degree angle cuts. Lipped seals are typically better suited to piston applications in which this failure often occurs.

Significant changes in o-ring size, such as swelling or shrinking, rapid and extreme seal deterioration and softening are all signs of incompatibility of the o-ring and the chemicals to which it is exposed. Seal volume will increase in size if it absorbs the fluid, or it may shrink if the fluid extracts the components of the o-ring compound. Plasticizer extraction frequently occurs in fuel systems in both static and dynamic applications. The obvious solution to chemically related o-ring failure is a careful material selection that matches the elastomer compatibility to the environment of the application.

The effects of the environment on an o-ring seal such as heat, friction or chemical attack may be difficult to evaluate separately. For example, extreme seal friction will generate heat which may cause thermal expansion which will make the seal more susceptible to chemical attack. Therefore the choice of the dimensional and material specifications of the o-ring is often a compromise that best balances the overall forces acting on the seal. All seals should be tested in their actual application before a final determination of o-ring size and material is made.

Seal failure prevention strategies

Trouble-shooting seal failures through visual inspection can be very effective. However, the next step, selecting a replacement seal, involves technical decisions related to o-ring sizing and material selection.

Based on its customer communications, Apple Rubber estimates that nearly 60% of o-ring failures are caused by improper sizing, 30% of o-ring failures occur because a custom seal design is needed, and another 10% of failures are caused by chemical incompatibility.

While the factors leading to an o-ring failure in a given system may be numerous, it may be useful to keep in mind the following preventative guidelines.

Sizing equations typically determine the size of o-rings selected for static dynamic or unique applications. The most common error in sizing o-rings, however, is failure to take into account the true tolerances of the parts to be sealed by the o-ring. When 5-15% of the seals in a system leak, the problem is most often traced back to the fact that the o-rings were not matched to the actual tolerances of the system, and the gap between the machined parts was not filled by the seal.

In one example, typical of problems related to poor tolerance stack-up, the cylinders in a plastic syringe designed to add nutrients and pesticides to trees leaked when the syringe was operated and even then it was stored in the trunk of a car. When the tolerances of the syringe materials were analyzed, it was found that the expansion of the plastic syringe material at higher temperatures had not been taken into account when selecting o-rings. Apple Rubber recommended installation of thicker o-rings of a higher durometer material, and a doubling of the wall thickness of the plastic syringe. The leaks were eliminated.

It's also important to select o-ring materials that are appropriate to the temperature levels of the operating environment. Excessive heat, over time, degrades o-ring materials physically and sometimes chemically, rendering them non-functional as seals. Excessive heat may cause o-ring materials to swell and harden in a deformed shape. Excessive cold may affect the material properties of a seal, preventing proper sealing.

On rare occasions it may be possible for a given elastomer to sustain or exceed its maximum and minimum temperatures. However, repeated temperature spikes and drops will lead to o-ring failure.

A final caution: the most critical step in preventing o-ring failures is when plant managers and engineers pre-test o-rings in actual applications to confirm whether these "simple" devices will be trouble-free and thus cost-effective.

References

Seal Design Catalog, 1989, Apple Rubber Products, Inc., 310 Erie Street, Lancaster, NY 14086, [phone].

M.W. Brown, Seals and Sealing Handbook, Elsevier Science Publishers Limited, 1990.

American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

E.I. du Pont de Nemours & Co., Inc., Polymer Products Department, Read Building, Concord Plaza, Wilmington, DE 19898. References for Hypalon, Neoprene, Viton, Vamac, Teflon and Kalrez.

Rubber Technology, Third Edition, Edited by Maurice Morton, copyright 1987 by Van Nostrand Reinhold Company Inc., 115 Fifth Avenue, New York, NY 10003.

Society of Automotive Engineers Inc., 400 Commonwealth Drive, Warrendale, PA 15096. Publication AIR 1707.