Four-lobed elastomer seal that fits an O-ring gland but makes two sealing lines per side. Used in reciprocating hydraulic and pneumatic duty where an O-ring would roll in the groove and fail in a spiral.
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An X-ring, also sold as a quad ring, seals on the same principle as an O-ring - initial diametral squeeze, then pressure-energised contact - but its cross-section has four lobes instead of one round profile. Each side of the groove therefore carries two sealing lines rather than one, and the valleys between the lobes hold lubricant at the sealing interface.
The reason to specify one is reciprocating service. An O-ring in a slow reciprocating gland can slide at one part of its circumference and roll at another; the accumulated twist eventually tears the ring in a helical pattern, which is spiral failure. The four-lobe section engages the groove so that the ring slides rather than rolls, which is why an X-ring is the usual remedy where an O-ring has failed in a spiral, particularly at reciprocating speeds below about 0,3 m per minute and on low or balanced-pressure components.
X-ring grooves are cut deeper than the equivalent O-ring gland, so the ring seals at lower diametral squeeze and therefore with less friction and less wear. One consequence is practical: standard O-ring back-up rings do not fit an X-ring groove, and a back-up ring must be machined to the actual groove dimensions.
Sizes follow the AS568 dash numbers, so an X-ring is dimensionally the same as the corresponding O-ring; the trade convention is to prefix a 4 to the O-ring number when ordering. For dynamic duty, choose the largest cross-section the space allows, because a thicker ring absorbs more tolerance variation. State the AS568 number, the elastomer and the hardness, and confirm the groove depth before fitting.
| Cross Section | Four-lobed; two sealing lines on each side of the groove, with lubricant reservoirs in the valleys between the lobes |
|---|---|
| Size System | AS568 dash numbers - dimensionally the same as the corresponding O-ring; trade convention is to prefix a 4 to the O-ring number |
| Cross-Section Sizes | 0.040 in (1,02 mm), 0.050 in (1,27 mm), 0.060 in (1,52 mm), 0.070 in (1,78 mm), 0.103 in (2,62 mm), 0.139 in (3,53 mm), 0.210 in (5,33 mm) and 0.275 in (7,00 mm), covering sizes 4001 to 4475 |
| Pressure Range | Vacuum to 400 bar (6000 psi); back-up rings required above 50 bar (750 psi) |
| Reciprocating Speed | Up to 0,5 m/s (1.6 ft/s) |
| Temperature Range | -50 to +200 °C (-60 to +400 °F), depending on the elastomer or compound |
| Groove | Deeper than the equivalent O-ring gland, giving lower diametral squeeze and lower friction. Standard O-ring back-up rings do not fit; a back-up ring must be machined to the actual groove dimensions. |
| Standard | Not covered by ISO 3601, which applies to circular-section o-rings only |
