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
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
- 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
Bench Test: A laboratory test approximating product service
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
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
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
Clearance: Dimensional difference between sealing element and
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
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
Cold Resistant: Capable of low temperature operation without loss
Composite Seal: Combines the sealing performance of elastomers
with the physical properties of the bonded material (i.e. metal,
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
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
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
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
- 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
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
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
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
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
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
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
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
- 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
Heat Build-up: The temperature rise in a rubber body resulting
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
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
- 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
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
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
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.
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
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
Mold Cavity: Hollow space of the mold within which the uncured
rubber compound is shaped and cured to the desired finished product
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
- 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
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
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,
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
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
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
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
- 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
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
Reciprocating Seal: Seals used in moving piston and rod
Reinforcing Agent: Fillers, such as Carbon Black, added to the
elastomeric batch mix to improve such physical properties as tensile
Relaxation: Decrease in stress occurring with time under constant
load or deformation.
Rectangular Seal: Solid elastomeric ring seal of rectangular
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
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
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
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
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
- 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
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
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
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
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
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
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
Thermal Expansion: Linear or volumetric expansion caused by
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
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
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
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.