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 (usually at a 45° angle) on the surface of the seal.
Some conditions which cause an O-ring to slide and roll at the same time may be:
1. Eccentric components.
2. Wide clearance combined with side loads.
3. Uneven surface fi nishes.
4. Inadequate or improper lubrication.
5. O-ring too soft.
6. Stroke speed (usually too slow).
7. Improper installation (O-ring pinched or rolled).
8. O-ring sizes outside of recommended dimensions for reciprocating pistons (See chart below):
|Gland Dimensions for Industrial Reciprocating O-Ring Seals|
|Groove Diameter (Rod Gland) Tolerance|
|-.000 for all sizes|
|+.002 for sizes 2-006 through 2-324|
|+.004 for sizes 2-325 through 2-460|
|Groove Diameter (Piston) Tolerance|
|+.000 for all sizes|
|-.002 for sizes 2-006 through 2-324|
|-.004 for sizes 2-325 through 2-460|
Some solutions for spiral failure include:
1. Improve surface finish of sealed assembly at dynamic interface (Cylinder Bore, Piston Rod).
2. Check for out-of-round components (Cylinder Bores especially).
3. Provide proper lubrication. Consider the use of internally lubed O-rings.
4. Replace with a harder O-ring.
5. Consider use of alternate seal shapes. for example, the Parker T-seal is specifically designed to prevent spiral failure and its use will allow for increased tolerances because of built-in anti-extrusion back-up rings. Parker T-Seals are available to fit a number of standard AS568 O-ring grooves and may directly interchange with O-rings in most cases. Parker Double Chamfered Precision Cut Seal or can be a replacement for an O-ring and back-up assembly.