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:

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):

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.