Like any device subject to judgment in design or human error during installation, O-ring seals can fail.
While there are a number of different types and causes of seal failure, this article highlights three common reasons.
1. Extrusion and nibbling of the O-ring is a primary cause of seal failure in dynamic applications such as hydraulic rod and piston seals. This form of failure may also be found in static applications subject to high pressure pulsing, which causes the clearance gap of the mating flanges to open and close, trapping the O-ring between the mating surfaces. Solutions include: decrease clearance by reducing machining tolerances; use back-up devices; check O-ring material compatibility with system fluid; increase rigidity of metal components; replace current O-ring with a harder O-ring; break sharp edges of gland to a minimum radius of 0.002 inches; and ensure installation of proper size O-rings.
2. Probably the most common cause of O-ring failure is compression set. An effective O-ring seal requires a continuous “seal line” between the sealed surfaces. The establishment of this line is a function of gland design and seal cross-section, which determines the correct amount of squeeze (compression) on the O-ring to maintain seal integrity without excessive deformation of the seal element. Solutions include: use “low-set” O-ring material whenever possible; select O-ring material compatible with intended service conditions; reduce system operating temperature; and check frictional heat build-up at seal interface and reduce if excessive.
3. Spiral failure of an O-ring is often found on long stroke hydraulic piston seals and to a lesser degree on rod seals. This type of O-ring failure is caused when the seal becomes “hung-up’’ at one point on its diameter (against the cylinder wall) and slides and rolls at the same time. The resultant twisting of the O-ring as the sealed device is cycled finally causes the seal to develop a series of deep spiral cuts on the surface of the seal. Solutions include: improve surface finish of sealed assembly at dynamic interface; check for out-of-round components; provide proper lubrication; replace with a harder O-ring; and consider use of alternate seal shapes.
Part 2 will cover three more common causes of O-ring failure.