Seal and hydraulics terminology.
Definitions for the terms that appear in specifications, datasheets, and service reports.
API 6A
American Petroleum Institute Specification 6A. Covers wellhead and Christmas tree equipment. Elastomer compounds certified to API 6A are tested and qualified for use in high-pressure, sour service wellhead applications.
Bore honing
A machining process that uses abrasive stones on a rotating, reciprocating tool to resize and refine the internal bore of a cylinder. Honing corrects dimensional errors, removes surface damage, and produces the cross-hatch finish required for oil retention and seal bedding.
Compression set
The permanent deformation of an elastomer after sustained compression. Expressed as a percentage of the original deflection. A seal with high compression set loses its ability to maintain contact force over time. Measured to ASTM D395 or ISO 815.
Durometer
An instrument that measures the hardness of elastomers and plastics. Shore A scale is used for soft rubbers (0–100). Higher values indicate harder material. Most hydraulic seals are specified in the 70–90 Shore A range.
Dynamic seal
A seal that operates against a moving surface. Examples include rod seals, piston seals, and rotary shaft seals. Dynamic seals must balance sealing effectiveness with acceptable friction and wear rate.
EPDM
Ethylene Propylene Diene Monomer. An elastomer with excellent resistance to water, steam, and brake fluids. Not compatible with petroleum-based hydraulic oils. Specified for water treatment, HVAC, and automotive brake systems.
FKM
Fluorocarbon rubber (Viton is a tradename). Excellent resistance to petroleum fluids, hydraulic oils, fuels, and elevated temperatures (up to approximately 200°C continuous). The standard specification for sour gas and high-temperature sealing.
FFKM
Perfluoroelastomer. Near-universal chemical resistance with temperature capability to 320°C+. Significantly more expensive than FKM. Specified for the most aggressive chemical and thermal environments.
Gland
The housing that contains and retains a seal. In a hydraulic cylinder, the gland nut retains the rod seal pack. Gland geometry determines seal compression and retention.
HNBR
Hydrogenated Nitrile Butadiene Rubber. Superior to NBR in high-temperature service, with better mechanical strength and resistance to sour gas (H₂S). Specified for dynamic seals in oil and gas applications.
Honing tolerance
The dimensional accuracy achieved by the honing process, expressed as a bilateral tolerance (e.g. ±2 µm). Tighter tolerances require more honing passes and precise machine calibration. Santoo achieves ±2 µm dimensional tolerance on honed bores.
NBR
Nitrile Butadiene Rubber (Buna-N). The most widely used elastomer for hydraulic and pneumatic sealing. Compatible with petroleum-based hydraulic oils, mineral oils, and water glycol fluids. Temperature range typically –40°C to +120°C.
NSF
National Sanitation Foundation. NSF certification of elastomer compounds confirms suitability for contact with potable water and food. Required for seals used in water treatment, food processing, and beverage applications.
O-ring
A toric seal with circular cross-section. Seals by compression against mating surfaces. The most widely manufactured seal type. Standard sizes are defined by international standards (ISO 3601, AS568).
Pressure rating
The maximum working pressure a seal or assembly is qualified for. Exceeded pressure causes extrusion of the seal material into the clearance gap, leading to seal failure. Anti-extrusion rings (backup rings) are used to extend pressure capability.
PTFE
Polytetrafluoroethylene (Teflon is a tradename). Near-universal chemical resistance, very low coefficient of friction, and temperature range –200°C to +260°C. Used as seal faces, backup rings, encapsulated O-rings, and as filled compounds for PTFE-lip rotary seals.
Ra
Arithmetic mean surface roughness, measured in micrometres (µm). The primary surface finish parameter for cylinder bores and hydraulic rod surfaces. A rod surface finish of Ra 0.2 µm is the standard specification for hydraulic rod chrome plating. Bore cross-hatch finish is typically Ra 0.4–0.8 µm.
SCADA
Supervisory Control and Data Acquisition. The monitoring system on Santoo's test benches. Records pressure, flow, leakage, and stroke data in real time against the work order. Generates the validated test report issued with each unit.
Shore hardness
See Durometer.
Static seal
A seal between two non-moving surfaces. Examples include O-rings in face seal and boss seal fittings, and flat gaskets on flanged joints. Static seals are not subject to dynamic wear but must maintain contact force over the service life.
WRAS
Water Regulations Advisory Scheme. UK standard for materials in contact with potable water. WRAS-listed elastomers are tested and approved for use in water distribution and supply systems.
Wiper seal
A seal positioned at the external end of a hydraulic cylinder rod gland. Its function is to exclude contamination (dust, dirt, moisture) from entering the cylinder as the rod retracts. Also called a dust seal or scraper seal.
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