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GLOSSARY
of terms used in sealing
 

- A -

Abrasion Resistance: The ability of a rubber compound to resist surface wearing by mechanical action.

Accelerator: A chemical compound that speeds up the vulcanization of natural or synthetic rubbers.

Aeration: Air (or gas) bubbles entrained or accumulated in a liquid.

Aliphatic: A major group of organic compounds characterized by the presence of straight chain arrangements of carbon atoms. The three subgroups that comprise aliphatic hydrocarbons are: paraffins (alkanes), olefins (alkenes), and acetylenes (alkynes).

Ageing: Change in characteristics of rubbers with time specifically influenced by environmental factors (eg. light, heat etc).

Air Checks / Traps: Surface markings or depressions resulting from the trapping of air between the rubber surfaces being cured and the mold or press surface.

Air Side: The side of a seal which faces outwards or towards atmosphere, as opposed to the fluid being sealed.

Ambient Temperature: The temperature of the environment surrounding a particular object.

Aniline Point: General indication of the aromatic content of an oil fluid determined as the lowest temperature at which the oil is miscible with an equal volume of aniline.

Anti-Extrusion Ring (Device): A washer-like device of a relatively hard, tough material installed in the gland on the downstream side of the seal to prevent seal extrusion into the diametrical gap while under pressure. In short, A ring or similar device assembled with a seal to prevent the seal extruding into the clearance space. (alternative description for Back-up Ring or or Back-up washer).

Antioxidants: Additive in a rubber mix to resist oxidation.

Antiozonant: Additive in a rubber mix to resist degradation caused by ozone.

Automatic Seal: General term applied to describe seal designs that are pressure energized. More specifically it is used to classify certain types of flexible lip seals.

Axial Clearance: Clearance between a sealing element and the inside face of the cover.

Axial Interferences: Clearance or dimensional difference between the ID or OD of a seal and the assembled rod (or shaft) or housing diameter respectively.

- B -

Back (of seal): The side of a seal facing outwards or opposite to that facing the fluid being sealed.

Back-up Ring: A washer-like device of a relatively hard, tough material installed in the gland on the downstream side of the seal to prevent seal extrusion into the diametrical gap while under pressure. In short, A ring or similar device assembled with a seal to prevent the seal extruding into the clearance space. (Alternative name for Anti-Extrusion ring or Back-up washer)

Back-up washer: A washer-like device of a relatively hard, tough material installed in the gland on the downstream side of the seal to prevent seal extrusion into the diametrical gap while under pressure. In short, A ring or similar device assembled with a seal to prevent the seal extruding into the clearance space. (Alternative name for Anti-Extrusion ring or Back-up Ring)

Bellows: Pseudo-static 'boot' type exclusion seal, also known as a garter.

Bench Test: A laboratory test approximating product service conditions.

Bending Modulus: The measure of applied force required to bend a material a given distance around a specified radius. A measure of material elasticity (stiffness).

Bleeding: A film or beads formed by such compound components as plasticizers that have migrated to the surface of rubber products because of incompatibility with the base elastomer and/or the compound ingredients.

Blemish: A surface mark or deformity.

Blisters: A raised spot on the seal’s surface created by an internal void, or air-filled pocket.

Bloom: A milky surface discoloration caused by the migration of certain compound components (such as antiozonants) to the rubber’s surface after molding or storage. The waxy film serves as a protective coating shielding the part from oxidation. This discoloration DOES NOT adversely affect material performance.

Braid: Hollow or solid structure of round, square or polygonal section constructed from interlocking filaments or yarn strand laid obliquely to the axis of the braid.

Braid-over-Braid: A braid produced by more than one pass through a multiple-carrier braiding machine.

Break-out Friction: Frictional force to be overcome to initiate movement. Specifically static friction

Brittler Point: Temperature at which an elastomer becomes brittle.

Bulls Ring: A type of piston ring. A rigid or semi-rigid ring employed at one end or both ends of a acking to exclude extrusion of the packing into the clearance space.

- C -
 

Case: Metal component of a seal to which the sealing element is bounded, clamped or otherwise contained.

Centring Ring (Centering Ring): An extemsion of a gasket designed to locate the gasket centrally on a flange.

Checking: Cracking or crazing of the surface of an elastomer due to the action of sunlight.

Chevron Seal: Seal ring (or ring set) of V-shaped cross-section.

Chrysotile: Fibrous mannesium silicate, or 'white' asbestos mineral.

Clearance: Dimensional difference between sealing element and related component.

Co-axial Seal: A composite seal in the form of two (or more) Co-axial ring members.

Collar: Characteristic type of flexible lip rod sea, also known as a hat seal or hat ring.

Coefficient of Thermal Expansion: Value used to determine the amount of linear dimensional change for a particular elastomer, which is temperature dependent.

Cold Flexibility: Flexibility following elastomer exposure to a specified low temperature for a specified period of time.

Cold Flow: A term describing the tendency of certain materials to continue to deform or “creep” under constant sealing pressure (compressive load).

Cold Resistant: Capable of low temperature operation without loss of serviceability.

Composite Seal: Combines the sealing performance of elastomers with the physical properties of the bonded material (i.e. metal, plastic, etc.).

Compound: An elastomeric material resulting from the combination of a number of individual chemical ingredients into a batch mix. Further processing of the thoroughly mixed ingredients, to induce cross linking of polymer chains (vulcanization), results in the creation of a useful rubber-like product.

Compression Modulus: The ratio of applied compressive force (stress) to the resulting deformation of the test material (strain). Compressive strain is expressed as a fraction of the original height or thickness of the test specimen in the direction of the applied force. In short, Its the ratio of compression stress to resulting compression strain expressed as percentage of the original dimension.

Compression Seal: Seal working on the principle of being compressed to fill the clearance space.

Compression Set: Permanent deformation of rubber after subscription to compression for a period of time. Specifically determined as the ratio of dimensional change to compression strain. OR The amount by which an elastomeric material fails to return to its original size after release from a constant compressive load.

Conductivity: Elastomers are considered conductive when they posses a direct current resistivity of less than 10,00,000 ohm/cm.

Contact Force (Contact Load): Total interface pressure between a seal and the adjacent surface.

Controlled Gap Seal: A seal designed to maintain constant clearance with shaft.

Copolymer: A polymeric material compromising molecules of two or more different kinds.

Cover: Member or casing protecting or strengthening a seal element.

Creep: Movement or deformation of a substance under the effect of prolonged load stress.

Crocidolite: Fibrous iron silicate, or 'blue' asbestos.

Crown Height: The height of a (gasket) sealing element above the surface of the retainer.

Cup: Specific type of piston seal defined by its geometry (ie cup-shaped); but also utilized as a description of a (seal) case.

Cure: Vulcanization process applied to rubbers.

- D -
 

Deflash: A process of removing unwanted, excess material (flash) from a finished product.

Degassing: Intentional, controlled outgassing of the volatile (evaporative) components of elastomeric materials.

Diametrical Clearance: The gap between the two mating metal surfaces forming a gland’s internal cavity. Through slight oversizing and accompanying “squeeze,” the O-Ring seals this gap to prevent system leakage. In short, difference between ID of a seal and the shaft or rod diameter; or the ID of a seal and its housing.

Dielectric Strength: The voltage required to puncture a sample of known thickness and is expressed as volts per mm of thickness.

Die Formed Ring: A packing ring mechanically compacted into an (apparently) homogeneous form.

Double Acting Seal: Seal for reciprocating movements capable of sealing with both directions of movement.

Dry Running: Rubbing contact without any liquid being present at the interface.

Dynamic Friction: Friction generated when relative movement takes place between two contacting surfaces.

Durometer: Durometer Hardness: Arbitrary measurement of hardness of a rubber compound related to the resistance to penetration of an indentor point on a durometer. In a Shore A scale, the resultant numerical rating of hardness runs from lower numbered (30 or 40) softer materials to higher numbered (80 to 90) harder materials. Usually designated with a +/-5 tolerance.

Dynamic Seal: A seal capable of working with relative movement (either reciprocating or rotary) between components being sealed.

- E -

Elasticity: Property inherent in elastomeric materials of readily returning to its original form when released from a deforming load.

Elastomer: A general term used to describe both natural and synthetic polymers possessing the resilience required to return to approximate original shape after major or minor distortion.

Elongation: Generally referred to in terms of tensile (pull apart) testing, elongation is the increase in length of a test specimen, expressed as a percentage of its original (unstretched) length...relative to a given load at the breakpoint. Ultimate elongation is the elongation at the moment rupture.

Extrusion: The forced extension of part of the seal into the diametrical clearance gap of the gland, caused by excessive system pressure.
 

- F -
 

Face: Front surface of a seal (where appropriate).

Face Seal: Squeezed, like a gasket, on both the top and bottom surfaces of the seal’s cross section. An axial seal. Seal embodying two faces in rubbing contact in a plane at right angles to the axis of the seal.

FDA: Food and Drug Administration is a U.S. government agency that regulates the ingredients in rubber compounds that are intended for use in food and medical applications.

Filler: A finely divided material used to reinforce or modify elastomer physical properties, impart certain processing properties, or reduce cost. Typical examples are carbon black, clays, calcium carbonates and silicas.

Filler Ring: Elastic ring assembled with a U-ring or V-ring to consolidate the section.

Finger Ring: A spring form with flexible fingers.

Flash: Excess rubber around a molded part due to cavity overflow and/or parting line of molded surfaces.

Flash Line: Exaggerated degree of flash due to clearance or gap between mould parts.

Flex Cracking: Surface cracks resulting from repeated flexual cycling.

Flex Fatigue Resistance: The ability to withstand fatigue resulting from repeated distortion by bending, extension or compression

Flexual Modulus: Stress at a certain stran - not a ratio and not a constant, but merely the co-ordinate of a point on the stress-strain curve.

Flinger (Ring): Washer-form ring mounted next to a gland or gland follower for directing any leakage away from the shaft.

Flinger Ring: Secondary seal element in the form of a ring generating ‘windback’ action.

Flow Crack: Imperfection in a molding due to imperfect flow of material during molding.

Flow Lines: Molded article surface imperfections caused by failure of the rubber stock to blend with itself during the molding operation.

Flow Mark: Imperfection in a molding due to incomplete flow of material in the mould.

Fluid: A liquid or gas.

Follower: See Gland Follower.

Friction (Break-Out): Friction developed in dynamic seal situations during machine start-up. When machine operation is irregular, O-rings tend to conform (adhere) to the microfine grooves of surrounding glandular surfaces, requiring extra initial force to break them out of these microfine grooves.

Friction (Running): A force which resists objects already in motion.

Front: The side of a seal facing the fluid to be sealed. Specifically applied to rotary shaft in seal descriptions.

 

- G -
 

Garter: Pseudo-static exclusion seal, usually in the form of elastomeric bellows.

Garter Spring: Helical wire spring of circular geometry fitted to a lip seal (specifically an oil seal) to enhance lip contact pressure.

Gasket: Static seal made from deformable sheet material sandwiched and compressed between two mating plane surfaces. Used to retain fluids under pressure or to seal out foreign matter.

Gland: Complete cavity into which the seal is installed. Includes a machined groove and mating metal surfaces. General description of housing or cavity for accommodating compression packings or sealing rings.

Gland Cover: Fixed gland member fitted on the non-pressure side of a gland to retain the seal against the action of pressure.

Gland Follower: Adjustable gland member, which can be tightened to compress and expand radially the packing in a gland.

Groove: The machined glandular recess into which the seal is fitted.

- H -
 

Hardness: Resistance of rubber to forced distortion as measured by the indentor point of a durometer gauge. Resistance to indentation of a rubber, normally measured and specified in IRHD (International Rubber Hardness Degrees).

Hardness, Shore A: Durometer reading in degrees of rubber hardness as measured on a Shore A gauge. Scale is 0-100, with higher numbers indicating greater hardness.

Hard Face: A facing of high hardness applied to softer materials.

Hat Ring: Alternative name for a collar seal.

Head: Portion of a seal carrying the sealing edge.

Header: Ring of hard material used in conjunction with seal ring(s) to locate the seal(s) and eliminate axial movement.

Heart Seal: Solid elastomeric ring seal with a heart shaped cross-section.

Heat Build-up: The temperature rise in a rubber body resulting from hysteresis.

Heel: The part of the seal cross-section adjacent to the clearance gap on the non-pressurized side; or adjacent to the shaft on the back of an oil seal.

Hermetic Seal: An airtight seal.

Housing: Annular recess into which a shaft or rod seal is assembled.

H-Ring: Solid elastomeric seal ring of H-shaped cross-section.

Hydraulic Packing: A packing specifically designed for the sealing of hydraulic fluids in cylinders etc.

Hysteresis: The percent energy lost per cycle of deformation, or 100% minus the resilience percentage. Results from internal friction and is manifest by the conversion of mechanical energy into heat.
 

- I -
 

I.D.: The inside or hole diameter of an O-ring.

Interference: Negative dimensional difference between a seal ID or OD and the final seal assembly diameter.

ISO: International Organization for Standardization model for quality assurance in design, development, production, installation and servicing.

- J -

Junk Ring: Alternative name for an anti-extrusion ring.

- L -
 

Lantern Ring: Ring with radial ports located at an intermediate position in a gland to allow coolant or lubricant to be introduced.

Lead-in (Chamfer): Chamber introduced in component(s) to facilitate assembly of seal on to a rod or shaft, or into a cylinder or housing.

Leakage Rate: The rate at which a fluid (either gas or liquid) passes a barrier.

Life Test: A laboratory test of the amount and duration of product resistance to a set of destructive forces or conditions. Used to compare the relative performance capabilities of various product designs.

LIM: “Liquid Injection Molding” is a closed manufacturing process using LSR in an injection molding machine.

Lip: The part or edge of a seal which forms the sealing surface.

Lip Opening Pressure: Air pressure required to lift a lip seal off its shaft and allow air leakage at the rate of 10 l/mm.

Lip Seal: A seal where the sealing surface is in the form of a flexible lip.

Load: Actual pressure at sealing face of a seal, normally the sum of the interference load and fluid pressure acting on the seal.

Low Temperature Flexibility: The ability of an elastomeric product to be flexed or bent at low temperatures without cracking.

LSR: “Liquid Silicone Rubber” is composed of a two-component, low viscosity, heat-curable rubber system.
 

- M -

Memory: Ability of an elastomeric material to return to its original size and shape after deformation.

Migration: Degradation products removed from an elastomeric sealing element and escaping to other parts of the system.

MIL: Abbreviation for Military.

MIL STD: Military Standard.

Mismatch: Unequal O-ring cross-sectional radii caused by dimensional differences in the mold cavity.

Mix: General description of a rubber compound formulation.

Mod-chip: Has actually to do with seals. Infos can be found here.

Modulus: The tensile stress force in p.s.i. required to produce a specified increase in material length (usually 100% elongation). Specifically the shape of the stress/strain curve for a material at a given elongation.

Modulus of Elasticity: One of several measurements of stiffness or resistance to deformation. The ratio of stress to the strain produced by that stress when stress is proportional to strain. But in rubber, modulus measurements are made in comparison or shear, rather than in tension, and they are only valid for strains up to about 15%.

Mold: Typically made from steel. Product is formed within machined cavity.

Mold Cavity: Hollow space of the mold within which the uncured rubber compound is shaped and cured to the desired finished product form.

Mold Finish: The surface finish of the mold which imparts the desired surface quality to the finished molded product.

Mold Marks: Slight irregularities in the surface of molded articles caused by mold machining marks,or damage to the mold itself.

Mold Release: A lubricant used to assist in the removal of rubber products from the mold.

Moulding Shrinkage: Loss of dimension of a moulded product after removal from the mould and subsequent cooling.

MS: Abbreviation for Military Standard.

Multiple Seal: A seal set comprising two or more seal rings of sealing elements.
 

- N -
 

Neck Bush: Throttle bush fitted at the bottom of a stuffing box or gland.

Nominal Dimension: The mean dimension of a molded article, from which small dimensional (plus and minus) deviations are allowed as manufacturing tolerances.

Non-Fill: A molding condition where the rubber fails to completely fill the mold cavity, resulting in an incomplete part.
 

- O -

Occlusion: The mechanical entrapment of gases, liquids or solids within the folds of a substance.

O.D.: The outside diameter of an O-ring. A dimensional reference.

Off Register: Eccentric O-ring cross-sectional radii caused by lateral shift of one mold cavity relative to the other.

Offset: Step or break in the surface or a moulded product due to faulty register of the mould.

Oil Resistant: Ability of vulcanized rubber to resist swelling and other detrimental effects of exposure to various oils.

Oil-side: The side of a seal facing the fluid being sealed (specifically applied in the case of an oil seal).

O-Ring: Solid elastomer ring seal of circular cross-section. A doughnut-shaped object, or torus, that functions as a seal, blocking the passage of liquids or gases, by being compressed between the two mating surfaces comprising the walls of the cavity (gland) into which the ring is installed.

Oscillating Seal: Most commonly used in faucet valves, in this application the inner or outer member of the gland moves in an arc around the axis of a shaft. Movement is limited to a few turns in one direction and a few turns in the return direction (i.e. faucet on, faucet off).

Outgassing: Primarily occurring in vacuum situations, the volatile (evaporative) components of some rubber compounds may become vaporized in the vacuum and released (outgassed) by the compound into the surrounding environment.

Oxidation: The reaction of oxygen with a rubber compound, typically resulting in surface cracking of the rubber material. As oxidation involves the transfer of electrons, reduction in the physical strength of elastomers may also occur from exposure to the oxidizing agent.

Ozone Cracking: Surface cracking of rubber due to the degrading effect of ozone.

Ozone Resistance: The ability of vulcanized rubber to withstand cracking and physical deterioration from exposure to ozone, a more active oxidizing agent than oxygen itself.
 

- P -

Packing: General name for a compression type dynamic seal housed in a gland. Also applicable to the materials used in this type of seal.

Panting: Movement between sealed surfaces of a static seal due to pressure fluctuations and insufficient clamping or tightening.

Pedestal Ring: Support for a U-ring seal.

Permanent Set: The deformation remaining in a rubber specimen following both stress and relaxation over a period of time. ; The amount of residual displacement in a rubber part after a distorting load has been removed.

Permeability: The rate of gas flow through a particular rubber material. ; A measure of the ease with which a liquid or gas can pass through a rubber film.

Pitting: Surface voids produced by mechanical erosion (wear) or chemical action.

Plasticizer: A chemical agent added to the rubber compound batch mix to soften the elastomer for processing, as well as to improve physical properties of the compound product (i.e., increase elongation, reduce hardness, improve tack). ; Constituent of a rubber mix controlling the hardness and plasticity of the final product.

Polymer: A long molecular chain material formed by the chemical combination of many similarly structured, small molecular units. Eg. natural rubber and synthetic rubbers.

Post Cure: A second step in the vulcanization of certain elastomers, used to drive off residual decomposition products resulting from initial vulcanization.

Pseudo-static Seal: An exclusion seal (eg bellows or garter) for excluding dust, dirt etc, but also capable of accommodating relative movement between the components to which it is attached.

Pusher-Type Seal: A mechanical seal in which the secondary seal is automatically pushed along the shaft or sleeve to compensate for wear.
 

- Q -

QS 9000: Quality System model, used in conjunction with the ISO 9000 standard, for the automotive industry.

Quad Ring: Solid elastomeric ring seal of modified circular cross-section giving four sealing ridges.

Quench: A neutral fluid introduced into a seal cavity to dilute fluid which may have leaked past the seal.
 

- R -

Radial Clearance: Clearance between the shaft and internal diameter of an oil seal cover.

Radial Interference: Negative dimensional difference between the radial dimension of a seal and its housing or space into which it is fitted.

Radial Load: Total load carried by the lip of a rotary shaft seal or rod seal.

Radial Seal: Compression is applied perpendicular to the seal centerline.

Reciprocating Seal: Seals used in moving piston and rod situations.

Reinforcing Agent: Fillers, such as Carbon Black, added to the elastomeric batch mix to improve such physical properties as tensile strength.

Relaxation: Decrease in stress occurring with time under constant load or deformation.

Rectangular Seal: Solid elastomeric ring seal of rectangular cross-section.

Resilience: Energy recovery property of an elastomer under deformation cycles. Specifically the ratio of energy returned to energy input, per cycle of rapid deformation. ; The capability of returning to original size and shape after deformation.

Rider Ring: Wear or load-carrying ring associated with some form of ring seal (usually a metallic ring or piston ring).

Ring: Any circular seal or seal element.

Ring Gasket: A flange gasket which lies wholly within the ring of bolts.

RMA: Rubber Manufacturer’s Association.

RMS: Root Mean Square. A measure of surface roughness typically applied to the machining of metal gland and shaft surfaces. RMS stands for the square root of the sum of the squares of micro-inch deviation from true flat.

Rotary Seal: Seals for rotating shafts, with the turning shaft protruding through the I.D. (hole) of the O-ring. ; Seal type specifically suitable for sealing rotating shafts of rotary motions.

Rotary Seal Ring: Driven or rotating face of a mechanical seal.

Rubber: A common name for both naturally occurring and synthetically made elastomers. ; Elastomeric substance, either natural or synthetic.

Rubber Face: Rubber coating applied to a seal case to provide a sealing surface against the seal housing.

Rubber, Natural: A natural product of the juices of certain tropical plants (latex), improved through heat treating with sulfur (vulcanization).

Rubber, Synthetic: Man-made elastomers such as Nitrile, Fluorocarbon, Silicone, etc.

Running Friction: A force which resists objects already in motion. ; Friction generated by a seal under dynamic operating conditions.

Run-in: Period of initial operation and wear during which a seal becomes properly bedded down.

Runout (Shaft): Same as gyration. When expressed in inches along, or accompanied by abbreviation “TIR” (total indicator reading), it refers to twice the radical distance between shaft axis and axis of rotation.
 

- S -

SAE: Society of Automotive Engineers.

Scorching: Premature curing of compounded rubber stock during processing (Eg. Excess heating) or storage, with the potential for adversely affecting material flow and plasticity during subsequent shaping and curing processes.

Scorch: Premature curing of vulcanized rubber due to excessive heat.

Scraper (ScraperRing): Heavyduty wiperseal to exclude grit and heavier contaminants with reciprocating rod movements.

Scuffing: Surface roughness produced by mechanical wear.

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

Sealing edge: The extreme section of an oil seal which provides the actual seal.

Sealing Element: Portion of the seal section or seal element covering the sealing edge.

Sealing Land: Flat portion of sealing edge of an oil seal after prolonged contact with shaft.

Seal Plate: Alternative description for a gland cover.

Seal Width: The overall axial dimensions of a seal.

Secondary Seal: O-Ring, bellows or similar device which accommodates leakage from the primary seal of a mechanical face seal.

Separator: An intermediate ring of thin, stiff material which allows individual rings in a seal assembly to slide over one another.

Shaft Run-out: A dimension equivalent to twice the displacement of the axis of a shaft from normal at any particular point on the shaft under running conditions.

Shelf-Aging: The potential degradation of seal performance capabilities due to exposure of seal elastomers to stressful environmental factors during storage. Proper packaging and storage conditions help to avoid this problem.

Shell: Case of an oil seal.

Shore A Hardness: Durometer reading in degrees of rubber hardness as measured on a Shore A gauge. Scale is 0-100, with higher numbers indicating greater hardness.

Shrinkage: (1) All rubber materials shrink to some extent during molding. This is normal and should be taken into consideration (by individual polymer shrink rates) when designing rubber parts. (2) Decreased seal volume due to exposure to adverse environmental factors. Can be an indication of plasticizer extraction from system chemicals.

Single Acting Seal: Dynamic seal capable of sealing in one direction of movement only.

Size, Actual: Actual dimensions of a molded article (including manufacturing tolerances).

Slinger Ring: Alternative name for a flinger.

Slipper Seal: Co-axial seal comprising an O-ring and a hard low-friction PTFE bearing ring (slipper ring).

Soft Packing: Gland packing of soft resilient material.

Spew: Excess material forced from a mould during moulding process.

Split-ring Seal: Split rigid section ring (usually metallic) similar in form and principle to a piston ring.

Split Seal: Elastomeric seal ring split to facilitate assembly.

Squeeze: Deformation of a seal produced when assembled with an interference fit.

Static Friction: Instantaneous or 'holding' friction of a seal under static conditions.

Stationary Seal Ring: Static face of a mechanical seal.

Stick-Slip: Jerky or irregular motion when a seal is operating under varying static-dynamic friction.

Stiction: Initial friction or break-out friction when motion is started.

Strain Relaxation or Creep: Is that characteristic of all elastomers to show gradual increase in deformation under constant load with passage of time. It is usually expressed as percent relative creep, which equals total deformation minus intial deformation divided by initial deformation times 100.

Stress relaxation: The loss in stress when an clastomer is held at a constant strain over a period of time.

Stuffing Box: Alternative name for a gland for containing packings or seal rings.

Swell: Increase in volume of a seal or elastomeric material when in contact with a fluid.
 

- T -

Tear Resistance: Resistance to the growth of a cut in the seal when tension is applied. ; The force per unit of thickness required to propagate a nick or cut in a direction normal to the direction of the applied force, or to initiate tearing in a direction normal to the direction of the stress.

Temperature Range: The working range marked by the limits of minimum and maximum operating temperatures for effective seal performance.

Tensile Strength: Pull-apart strength. A measure of the compound’s strength when stretched to the breaking point. ; The force per unit of the original cross-sectional area which is applied at the time of the rupture of a specimen.

Tensile Stress: More commonly called "modulus", is the stress required to produce a certain elongation.

Tension Set: Increase in normal (unstressed) length of an elastomeric specimen after initial stretching and release.

Terpolymer: A polymer resulting from the chemical combination of three monomers.

Thermal Expansion: Linear or volumetric expansion caused by temperature increases.

Thermoplastics: Polymeric materials that soften and can be re-formed when heated, returning to original properties when cooled.

Thermoset: Elastomers that undergo a permanent chemical crosslinking of molecules when processed, heated and molded, and therefore cannot be reprocessed.

Thread Stretch: Loosening of bolted assemblies or unions due to bedding down of threads.

Throttle Bush: Restructive bush fitted at the bottom of a stuffing box or gland; also descriptive of a bushing seal.

TIR (Total Indicator Reading): A measurement of roundness with relationship to a centerline and expressed in total diametric deviation.

Torque: A turning or twisting force, generally associated with the rotation of a shaft.

Toroidal Seal: Alternative name for an O-Ring.

Torsional Strength: Ability of a seal to withstand damage due to twisting.

TPE: “Thermoplastic Elastomer” combines the rubber-like performance of elastomers with the processing advantages of plastic. Scrap material can be recycled without significant loss in physical properties, unlike thermoset materials.

Track: Mark made on a shaft by a rotary seal.

Trapped O-Ring: Static seal using an O-ring in a special groove.

Trim: Removal of excess material (flash) from a molded rubber article.

Trim Cut: Damage to the molded article by trimming too close.

Trim Diameter: Of an oil seal, without spring.

Trim face: Front surface of a rotary oil seal trimmed to an angle to firm the sealing edge.

Triumming Angle: Angle between the trimmed face of a seal lip and the seal axis.
 

- U -

Ultimate Elongation: The % of specimen stretching at the point of breaking. Generally referred to in tensile testing.

Under-Cure: A condition where rubber has not been cured enough, exhibiting poor physical properties and/or tackiness.

Unidirectional Seal: A seal which provides fluid sealing from one side only.

Unirotational Seal: A rotary shaft seal designed for application with one direction of shaft rotation only.

Unit Seal: A seal ring consisting of a single ring, and not normally subject to axial compression.

U-Ring: Flexible lip seal of U-shaped cross-section.
 

- V -

Viscosity: Resistance to flow.

Voids: Empty pockets where not intended.

Volume Change: Increase or decrease in the size of a specimen expressed as a percentage of original volume. Generally associated with immersion of elastomer samples in various chemical agents.

Volume Swell: A term generally used to describe the increase in physical size of a specimen immersed in a particular chemical agent.

V-Ring: Flexible lip seal of V-shaped cross-section: also known as a chevron seal.

Vulcanization: The heat induced cross linking (curing) of polymer chains, converting basic visco-elastic liquids into three-dimensional networks of flexible, elastomeric chains (the molded rubber product). ; Heat process treatment for rubber to stabilize and harden it.
 

- W -

Wear Bond: Mark made by a rotary seal on a shaft.

Wear Ring: A ring of hard material associated with a seal or seal assembly and intended to take rubber wear.

Weathering: The tendency of some O-ring seals to surface crack upon exposure to atmospheres containing ozone and other pollutants.

Weeping: Small amount of leakage from a seal, arbitrarily defined as leakage rate of less than one drop per minute.

Width: 1. The cross-sectional diameter of an O-ring. 2. One half the difference between the I.D. and O.D. of the ring.

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