O-rings are the most commonly used seals in fluid-power systems. They must deform to function properly. As the name implies, the O-rings have a doughnut shape. (Torus is the geometric term that describes the O-ring.) They are installed in cavities known as grooves and then compressed. The resulting zero clearance within the groove provides the seal that blocks the flow of liquids and gases. This simple arrangement serves many fluid-power systems very well, but success still requires careful design, selection, and installation procedures.
O-rings typically fail in their applications because of the combined adverse effects of several environmental factors. Experience has shown the most common causes of O-ring failure are:
- improper groove design — allowing for too much or too little compression, not enough room for displacement under compression, or tolerance stack-up;
- incorrect O-ring size;
- incompatibility between the seal elastomer and environmental elements it must contact;
- improper seal installation, and
- inadequate seal lubrication.
The combination of stresses that act on O-rings can be complex and difficult to evaluate. Therefore, both the seal compound and size should be tested in the real environment of its service. The following examples describe the common types of O-ring failure that can occur and the possible problem sources.
Failure without Visible Evidence on the O-ring
Failure pattern: Of the all various types of seal failure, this is among the hardest to diagnose because the result of the problem is not visible on the seal. There are no visual clues.
Problem sources:
- Insufficient compression;
- Tolerance stack-up;
- Eccentric-shaped components;
- Parting lines and /or flash left on the seal from the molding process;
- Improper volume relationship between the seal and its groove.
Compression Set
Failure pattern: Common to both static and dynamic sealing applications, compression set failure produces flat surfaces on both sides of the O-ring’s cross section.
Problem sources:
- The selected elastomer has poor compression set properties;
- Low heat resistance of material;
- Excessive swelling of seal material in system fluid;
- Too much squeeze to achieve seal;
- Incomplete curing (vulcanization) of the O-ring material during production.
Extrusion and Nibbling
Failure pattern: Typical of high-pressure systems, this pattern can be identified by the many small bites or nibbles taken out of the O-ring on the low-pressure (downstream) side.
Problem sources:
- Excessive groove clearances;
- Excessive system pressure;
- Seal material too soft;
- Degradation of seal by system fluid;
- Irregular clearance gaps caused by eccentricity of the product;
- Improper machining of O-ring groove (leaving sharp edges);
- O-ring size is too large for groove.
Abrasion
Failure pattern: Occurring primarily in dynamic seals involving reciprocating, oscillating, or rotary motion, this failure pattern can be identified by a flattened surface on one side of the O-ring’s cross section.
Problem sources:
- Metal surfaces of groove are too rough and act as an abrasive;
- Poor lubrication;
- Excessive temperatures;
System fluid contaminated with abrasive particles.
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