ABRASION:
The wearing away of a surface by mechanical action such as rubbing, scraping or erosion.

Abrasion Resistance:
The ability of a rubber compound to resist mechanical wear.

Accelerated Life Test:
Any set of test conditions designed to reproduce in a short time the deteriorating effect created under normal service conditions.

Adhesion:
The tendency of rubber to bond or cling to a contact surface.

Ambient Temperature:
The surrounding temperature. Note that ambient temperature is not necessarily the same as atmospheric temperature.

Atmospheric Cracking:
These are cracks that are produced in the surface of rubber articles by exposure to atmospheric conditions.

Auxiliary (Dust) Lip:
A short, non-spring-loaded lip located on the outside seal face of a radial lip seal to prevent ingress of atmospheric contaminants.

Axial Clearance:
The gap between the toe face of the head section and the inside surface of the inner case.

Bench Test:
A modified service test in which service conditions are approximated using conventional laboratory equipment, not necessarily application-identical equipment.

Blister:
A raised spot in the surface or a separation between layers that forms a void or an air-filled space in vulcanized seals.

Bloom:
A dusty-or milky-looking deposit that sometimes appears on the surface of a seal after molding and storage. It is caused by migration of liquid or solid to the surface.

Bond:
This term is commonly used to denote the attachment of an elastomer to another surface.

Brittleness:
The tendency of a seal to crack when deformed.

Buna-N:
This material is the same as Nitrile rubber.

Buna-S:
A general term for the copolymers of butadiene and styrene. It is also known as SBR and GRS.

Cold Bond:
An adhesion of a previously vulcanized elastomer to another surface through contact cements.

Cold Flexibility:
The flexibility of a seal following exposure to a predetermined low temperature for a predetermined time.

Compound:
A term applied to a mixture of polymers and other ingredients that produce a usable rubber material.

Compression Set:
The amount by which a rubber specimen fails to return to its original shape after the release of a compressive load.

Contact Line Height:
The axial distance from the outside seal face to the lip contact line.

Contact Point:
The line of intersection between the outside and inside lip surfaces of a radial lip seal. In a cross-sectional view, this intersection is illustrated as a point.

Cross-Section:
A view of a seal as if it were cut at right angles to the mold parting line, showing the internal structure.

Durometer:
An instrument used to measure the hardness of rubber. It measures the resistance of the penetration of an indentor point into the rubber surface.

Dynamic:
An application in which the seal is subject to movement or when moving parts contact the seal.

Dynamic Seal:
A seal required to prevent leakage past parts that are in relative motion.

Eccentricity:
This is determined by measuring the shaft runout, TIR and the shaft-to-bore misalignment.

Elastomer:
Any synthetic or natural material with resilience or memory that is sufficient enough to return to its original shape after major or minor distortion.

Elongation:
This generally means “ultimate elongation” or the percent increase in the original length of a specimen when it breaks.

Filler:
Chemically inert, finely divided material that is added to the elastomer to help with processing and to improve physical properties, such as abrasion resistance and strength.

Flash:
Excess rubber that is left around a rubber part after molding due to the space between mating mold surfaces. It is removed by trimming.

Flex Cracking:
A surface cracking induced by repeated bending or flexing.

Flex Resistance:
The relative ability of a rubber seal to withstand dynamic bending stresses.

Flex Section:
The portion of a seal lip that is bound by the head and heel section of a lip seal. Its primary function is to permit relative motion between the seal lip and case.

FPM:
Feet per minute.

Friction:
Resistance to motion due to the contact of surfaces.

Friction Break-Out:
Friction developed during initial or starting motion.

Friction Running:
Constant friction created during the operation of a dynamic seal.

Garter Spring:
A helically coiled wire with connected ends. It is used in tension for maintaining a radial sealing force between the sealing element of a radial lip seal and a shaft.

Hardness:
The resistance to indentation. This is measured by the relative resistance of the material to an indentor point of any one of a number of standard hardness testing instruments.

Hardness Shore A:
The rubber durometer hardness as measured on a Shore “A” gauge. Higher numbers indicate harder material. A “35” Shore “A” durometer reading is considered soft and a “90” is considered hard.

HDLTM:
Timken Hydrodynamic Labyrinth (HDLTM) Seal.

Head Section:
The portion of a lip seal that is generally defined by the inside and outside lip surfaces and the spring groove.

Heel Section:
The portion of a lip seal that is attached to the seal case and bound by the flex section and the outside face.

Hermetic Seal:
An airtight seal that shows no detectable leakage.

Homogeneous:
A material of uniform composition. In seals, it means a rubber seal without fabric or metal reinforcement.

Housing:
A rigid structure that supports and locates the seal assembly with respect to the shaft.

Immersion:
Placing an article into a fluid so it is completely covered.

Inner Case:
A rigid, cup-shaped component of a seal assembly that is placed inside the outer seal case. It has one or more of the following devices: reinforcing member, shield, spring retainer or a lip-clamping component.

Inside Case Inner Diameter:
The inner diameter of the inner case of a radial-lip seal.

Inside Face:
The surface of the inner case that faces and usually is in contact with the sealed fluid.

Inside Lip Angle:
The angle between the inside lip surface and the axis of the seal case.

Inside Lip Surface:
The inside truncated conical surface of the lip.

Leakage Rate:
The rate at which a fluid, either gas or liquid, passes a barrier. The Total Leakage Rate includes the amount that diffuses or permeates through the material of the barrier as well as the amount that escapes through it.

Life Test:
A laboratory procedure that is used to determine the amount and duration of resistance of an article to a specific set of destructive forces or conditions.

Lip Height:
The axial distance from the outside seal face to the toe face.

Lip Length:
The axial distance between the thinnest part of the flex section and the contact line.

Mechanical Bond:
A purely physical attachment accomplished by “through” holes, interlocking fingers, envelope design, riveting, etc.

Memory:
The tendency of a material to return to its original shape after deformation.

Metal (Outer) Case:
The outer, thin-wall, rigid structure of the lip-seal assembly that contains the primary sealing ring and, if present, the inner case, spring and secondary seal.

Molded Toe Angle:
The angle between the toe face of a seal lip and the seal axis.

Nitrile (Buna-N):
The most commonly used elastomer for seals because of its resistance to petroleum fluids. It has good physical properties and a useful temperature range.

Oil Resistant:
The ability of a vulcanized rubber to resist the swelling and deteriorating effects of various oils.

Oil Swell:
This is the change in volume of a rubber article due to the absorption of oil or other fluid.

Outside Case Inner Diameter:
The inside or smallest diameter of the outer case of a lip-seal assembly.

Outside Face:
The surface of the seal case perpendicular to the shaft axis that is not in contact with the fluid being sealed.

Outside Lip Angle:
The angle between the outside lip surface and the axis of the seal case.

Outside Lip Surface:
The outside truncated conical surface of the lip.

Ozone Resistance:
The ability to withstand the deteriorating effect of ozone, which generally causes cracking.

Plasticizer:
A substance, usually a heavy liquid, added to an elastomer to decrease stiffness, improve low temperature properties and enhance processing.

Plunge Ground:
The surface texture of a shaft or wear sleeve produced by placing the grinding wheel perpendicular to the rotating shaft without axial motion.

Polymer:
A material formed by the joining together of many (poly) individual units (mer) of one or more monomers; synonymous with elastomer.

Post Cure:
The second step in the vulcanizing process for more exotic elastomers. It stabilizes parts and drives off decomposition products resulting from the vulcanization process.

Radial Wall Dimension:
The distance between the seal lip contact line and the seal outside diameter measured in a radial direction on a finished seal prior to installation.

Rib (Helix):
A long, narrow projection that is normally triangular in the cross section. It is molded into the outside lip surface of a helix seal. The rib is oriented at an angle to the shaft axis. One end of the rib forms part of the seal-lip contact surface.

RMS:
Root mean square.

Rubber:
It is the same as elastomer.

Runout (Shaft):
This is the same as gyration. When it is expressed in inches alone or accompanied by the abbreviation TIR (total indicator reading), it refers to twice the radial distance between the shaft axis and axis of rotation.

Seal:
Any device used to prevent the passage of a fluid (gas or liquid).

Seal Outer Diameter:
The external diameter of a lip-seal assembly, which normally corresponds to the outer diameter of the outer-seal case.

Seal Width:
The total axial width of the seal.

Shaft:
The reciprocating or rotating member that is usually within the equipment housing but not in contact with the housing bore.

Shaft Hardness:
The resistance to indentation. At minimum, it should be Rockwell C45.

Shaft Out-Of-Round:
The deviation of the shaft cross section from a true click. Out-of-round is measured as the radial distance, on a polar chart recording, between concentric, circumscribed and inscribed circles that contain the trace and are centered to minimize the radial distance.

Shaft Tolerance:
This is the allowable variation in the shaft diameter.

Spring Groove:
A depression formed in the head section of the seal. It is generally semicircular and serves to accommodate and locate the garter spring.

Spring Position:
The axial distance between the seal contact line and the centerline of the spring groove of a radial lip seal. It is commonly referred to as the “R” value.

Spring Retainer Lip:
The portion of the primary lip that restricts the axial movement of the extension spring from a predetermined position.

Spring Set Lip Diameter:
The inner diameter of the seal lip measured with the spring installed.

Static Point:
The section of the helix seal lip incorporating the contact line.

Static Seal:
The part designed to seal between parts that have no relative motion, or a device used to retain fluids under pressure or seal out foreign matter.

Swell:
The increased volume of a specimen caused by immersion in a fluid.

Tensile Strength:
The force in pounds per square inch required to rupture a piece of rubber.

Thermal Expansion:
The expansion caused by an increase in temperature. This may be linear or volumetric.

Toe Face:
The annular surface of the spring-retaining lip.

Torque:
The turning force of a shaft.

Trapped Air:
The air that is trapped in a product or a mold during the curing process that usually causes a loose ply or cover or a surface mark, depression or void.

Trim:
The process that removes mold flash.

Void:
The absence of material or an area devoid of materials where not intended.

Volume Swell:
The increase in physical size caused by the swelling action of a liquid.

Vulcanization:
A thermo-setting reaction that uses heat and pressure resulting in greatly increased strength and elasticity of a rubber-like material.

Vulcanized Bond:
An adhesion of an elastomer to a previously primed surface using heat and pressure that vulcanizes the elastomer at the same time.