O'Rings

O'rings are one the most common seals in use today. They can be used in both static and dynamic applications and offer an economic and effective sealing solution. By choosing a compound and hardness it is usually possible to find a suitable O'ring for most applications. Rubber O'rings can operate in temperatures from -55 deg C to + 320 deg C and (with backup rings) at pressures of 400 Bar. They are available in a large range of standard imperial and metric sizes The most common international standards for O-Ring dimensions, in addition to ISO 3601/1 are:
USA SAE AS-568
Great Britain : BS 1806 - BS 4518, Germany : DIN 3771, Sweden :SMS 1586, France : NF T 47 501, Japan : JIS B 2401-77

Common metric sections include 1.0mm, 1.5mm, 2.0mm, 2.5mm 3.0mm, 3,5mm, 4.0mm, 4.5mm 5.0mm, 6.0 etc
Common Swedish sections include 1.6mm, 2.4mm, 5.7mm, 8.4mm
Common French sections include 1.8mm
Common Japanese Sections include 1.9mm, 2.7mm, 3.1mm 3.6mm,
Common Imperial sections are 1.78mm, 2.62mm, 3.53mm, 5.33mm and 6.99mm
It can be seen from the above that there is an o ring section to suit almost any requirement, inside dimensions range from less than 1mm to over 800 mm with bigger sizes than that available as " Customer Specials ".
Common rubber compounds are available in a range of hardness grades, the most common being 70 shore ( Standard Hardness), 90 shore (Hard) is also readily available, with 40/50 shore (soft) being the least popular. Fluoroelastomer O'rings tend to be slightly harder with 75/80 Shore being the norm.


BS O'ring ( Imperial) Size Charts , Groove Dimensions

Most Common Elastomers

NBR Nitrile -40 deg C + 110 Deg C Mineral based Hydraulic and lubricating oils
Lpg Gas, Butane, Propane, Acetylene, Petrol, Diesel
EPDM Ethylene-propylene -55 deg C + 150 deg C Fire resistant hydraulic fluids, Brake fluids,Hot water, Steam
Not resistant to mineral oils
MVQ Silicon -55 Deg C + 230 Deg C High Temp Air, Water, Oxygen, alkalis and some acids
Static applications only
FPM Fluoro elastomer -40 deg C + 230 deg C High Temperature , Synthetic and hydraulic
fluids, many acids and alkalis.
AU Polyurethane -30 deg + 80 deg C Good wear and extrusion resistance
Not resistant to gycol based fluids, acids or alkalis
FFPM Perflouro elastomer -30 deg + 320 deg C Best chemical resistance of all elastomers
High performance material
FEP Ptfe Encapsulated -50 deg +200 deg C
depending on elastomer core 		Very good chemical resistance except liquid alkaline metals
Suitable for foodstuffs and pharmaceuticals
PTFE Polytetrafluoroethylene -200 deg +260 deg C Very good chemical resistance except liquid alkaline metals
No rubber elastic properties, static applications only
Above table is only a guide to the most common elastomers, performance would be dependant on the particular variant of elastomer used. Chemical resistance should also be checked before use.
The correct functioning of the o'ring is dependant on both the groove size and the radial clearance. For higher pressures the o ring hardness can be increased , backup rings can be used or radial clearance should be decreased.
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