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 both 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.
As stated above, spiral failure is generally caused by an O-ring both sliding and rolling at the same time. Conditions which may cause this to occur are as follows:
- Eccentric components.
- Wide clearance combined with side loads.
- Uneven surface finishes.
- Inadequate or improper lubrication.
- O-ring too soft.
- Stroke speed (usually too slow).
- Improper installation (O-ring pinched or rolled).
Suggested solutions to the causes of Spiral Failure are as follows:
- 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.
- Increase hardness of O-ring material.
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 AS568A O-ring
grooves and may directly interchange with O-rings in most cases.