Failure Modes
SPIRAL FAILURE
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:
- Eccentric components.
- Wide clearance combined with side loads.
- Uneven surface fi nishes.
- Inadequate or improper lubrication.
- O-ring too soft.
- Stroke speed (usually too slow).
- Improper installation (O-ring pinched or rolled).
- 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 |
Groove Diameter (Piston) Tolerance |
-.000 for all sizes |
+.000 for all sizes |
+.002 for sizes 2-006 through 2-324 |
-.002 for sizes 2-006 through 2-324 |
+.004 for sizes 2-325 through 2-460 |
-.004 for sizes 2-325 through 2-460 |
Some solutions for spiral failure include:
- Improve surface finish of sealed assembly at dynamic interface (Cylinder Bore, Piston Rod).
- Check for out-of-round components (Cylinder Bores especially).
- Provide proper lubrication. Consider the use of internally lubed O-rings.
- Replace with a harder O-ring.
- 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.