O-ring sets
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Frequently asked questions
How do metric and inch O-ring sets differ?
A metric set holds millimetre sizes and suits European, Japanese and Chinese machines. An inch set follows AS568 dash numbers and suits American equipment and many SAE hydraulic fittings. Sizes in the two series are close but not identical, and mixing them leads to leaks.
Should I choose an NBR or FKM set?
An NBR 70 Shore A set covers most replacements in mineral oil and compressed air systems. An FKM set, usually brown or green, is needed for engines, compressors and lines with heat or chemicals. Many workshops keep a large NBR box and a smaller FKM one.
How do I take rings from the box without mixing sizes?
Read the compartment label and check with an O-ring gauge or callipers before fitting, because neighbouring sizes look almost identical. Return unused rings to the right compartment; mixing them is the most common reason a set stops being useful.
How long do O-rings in a spares set keep?
Rubber ages with light, ozone and heat. Kept in a closed box in a dark, cool place away from electric motors and welders, NBR lasts for years and FKM longer still. Discard any ring that has hardened, has surface cracks or feels tacky.
What tools make fitting O-rings from a set safer?
A set of plastic or brass picks removes old O-rings from grooves without scratching them. A tapered guide or thin sleeve covers threads and sharp edges when the ring is passed over a shaft. Apply a film of material-compatible grease before fitting to cut friction and prevent twisting.
What should I do when one size in the box runs out?
Record the used up size by inside diameter and cross-section, along with material and hardness, so the right refill can be bought. Do not substitute a similar size with a different section, since the squeeze in the groove will be wrong. Tracking which compartments empty fastest shows which sizes deserve separate stock in larger quantities.
Can O-rings from a set be used for dynamic sealing?
Most O-rings in a set are general purpose and best suited to static sealing such as flanges, covers and fittings. For slow reciprocating motion at low pressure, an O-ring can still work if the groove is designed correctly and lubricated. Cylinder rods or high-pressure pistons call for dedicated seals rather than O-rings from a set.
How can I tell that O-rings in a set have expired?
Stretch a ring gently and look: fine surface cracks, a tacky or chalky surface, or a ring that does not return to shape all indicate ageing. A ring that has hardened and lost elasticity when squeezed should not be used either. Discard the whole compartment if one ring is bad, since rings from the same batch age alike.
