O-rings are critical components in hydraulic and pneumatic systems, serving as reliable seals that prevent fluid leakage and maintain system pressure. Understanding these fundamental terms is crucial for proper O-ring selection and application in their systems.

Despite their simple appearance, these circular elastomeric seals involve complex engineering considerations and specific terminology that every engineer and technician should understand.

This comprehensive guide breaks down the essential terminology related to O-rings, covering everything from basic dimensional specifications to common failure modes. Whether you’re a seasoned engineer or new to hydraulic systems, understanding these terms is crucial for proper O-ring selection, installation, and maintenance.

Basic Components and Dimensions

O-ring are defined by their ‘O’ shape and have a circular cross section.

• Inside Diameter (ID): The measurement across the inner circle of the O-ring when laid flat
• Outside Diameter (OD): The measurement across the outer circle of the O-ring when laid flat
• Cross-Section Diameter (CS): The thickness of the O-ring material when cut through
• O-Ring Groove: The channel or cavity where the O-ring sits in the assembly

Material Properties

• Shore Hardness: A measure of the O-ring’s firmness, typically measured on the Shore A scale
• Compression Set: The degree to which an O-ring maintains its original shape after being compressed
• Operating Temperature Range: The minimum and maximum temperatures at which the O-ring remains functional
• Quality: ISO 3601 is most common worldwide standard

Operating Parameters

• Compression Ratio: The percentage by which the O-ring is squeezed in its installed position
• Initial Compression or Squeeze: Initial compression refers to the amount of deformation an O-ring experiences when it is first installed in its groove, before any system pressure is applied. This is typically expressed as a percentage of the original cross-section and is crucial for proper sealing performance, most applications require 15-30% initial compression for effective sealing
• Total Sealing Force: The total sealing force of an o-ring is the total of the initial compression plus system pressure force. As system pressure increases, it enhances the sealing effect by forcing the O-ring more firmly against the sealing surfaces. This is often referred to as the “self-energizing”
• Axial Sealing: In axial sealing, the O-ring is compressed between two parallel surfaces that are perpendicular to the centerline of the O-ring. The sealing occurs as these surfaces come together, compressing the O-ring between them.
• Radial Sealing: Radial sealing occurs when an O-ring is compressed between an inner and outer cylindrical surface, such as in a piston or rod assembly. The O-ring creates a seal between these concentric surfaces, with compression occurring in the radial direction (perpendicular to the axis of the cylinder)
• Static Sealing: Static sealing refers to applications where there is no relative motion between the O-ring and the mating surfaces. This means the O-ring remains in a fixed, compressed position throughout its service life.
• Dynamic Sealing: Dynamic sealing refers to applications where there is relative motion between the O-ring and one or more of the mating surfaces. Dynamic sealing applications require special consideration for: Friction and wear characteristics of the O-ring material, Heat generation, lubrication requirements.
• Energising Element: An energising element in an O-ring system refers to a component or mechanism that enhances the sealing capability of the O-ring. Common types of energising elements are spring-energised seals or backup rings. The energising element helps ensure consistent sealing performance across varying operating conditions and can extend the life of the sealing system.
• Stretch: The percentage increase in the O-ring’s ID when installed

Common Issues

• Extrusion: When the O-ring material is forced into the clearance gap under pressure
• Spiral Failure: A twisting failure that occurs during dynamic applications
• Over Stretching: Over stretching occurs when an O-ring is stretched beyond its recommended limits during installation or use. Permanent damage can occur in the o-ring when stretched beyond 50% of its original size or during rapid changes in size. O-ring must always be stretched gradually and evenly.
• Chemical Attack: Degradation of the O-ring material due to exposure to incompatible fluids
• Lubrication: Lubrication improves an O-ring performance and longevity. It helps prevent damage to the o-ring during installation and reduces friction and heat generation in dynamic sealing applications. Chemical compatibility of the o-ring, lubricant and system fluid is critical to prevent seal degradation.
• Assembly Tools: Proper installation tools are crucial when fitting O-rings for several important reasons: preventing O-ring damage, ensuring correct positioning, maintaining cleanliness and installation time efficiency. Most common installation tools are: installation cones/bullets and respective pusher for gradually stretching the o-ring into an external groove and specialist lubrication applicators.