O-rings
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Frequently asked questions
Which dimensions identify an O-ring?
An O-ring is defined by its inside diameter (ID) and cross-section (CS), for example 20 x 2.5 mm. The inch series under AS568 uses dash numbers such as -214, each tied to a fixed ID and CS pair. You also need the material and hardness, usually 70 Shore A.
How do I measure an old O-ring accurately?
A used O-ring has often swollen or taken a set, so measuring the groove is more reliable: groove root diameter, bore diameter and groove depth. If you must measure the ring, check the cross-section at several points and take the largest value, and measure the ID with the ring lying relaxed, not stretched.
When do I need a back-up ring?
At high pressure, rubber tends to be forced into the clearance gap between parts and gets nibbled. With a 70 Shore A O-ring, from roughly 100 bar upwards or where the gap is large, fit a PTFE back-up ring on the low-pressure side, or on both sides if pressure reverses.
NBR O-rings swell in a hot water system. What material should I use?
For hot water, steam or glycol solutions EPDM is the usual choice, since NBR ages quickly there. Note that EPDM must not contact mineral oil or mineral-based grease, including assembly grease; use silicone grease instead.
Which installation mistakes make O-rings fail early?
The most common are a ring twisted in its groove, overstretching it over a shaft, and nicking it on threads or unchamfered bore edges. Lubricate with a grease compatible with the material, cover threads with a thin sleeve and check the ring is not twisted before closing up.
How does O-ring Shore A hardness matter?
70 Shore A is the common grade, balancing sealing ability and extrusion resistance. 90 Shore A is harder and copes better with high pressure and larger gaps, but needs more squeeze to seal at low pressure. Soft grades around 50 to 60 Shore A suit weak flanges or places where clamping force is small.
How much should an O-ring be squeezed in its groove?
For static sealing the squeeze is typically about 15 to 30 percent of the cross-section, and for dynamic sealing about 10 to 20 percent to keep friction down. The groove volume should exceed the O-ring volume by about 15 to 25 percent to leave room for swell and thermal expansion. An overfilled groove forces the ring out into the gap.
Are there O-ring equivalents between metric, AS568 and JIS sizes?
Many sizes nearly coincide, for example the JIS B 2401 P and G series use 1.9, 2.4, 3.1, 3.5 and 5.7 mm sections close to AS568 sizes. Substitute only when the inside diameter differs by no more than a few percent and the section still gives correct squeeze in the groove. When a groove drawing is available, recalculate squeeze rather than just comparing table values.
