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Additional terms can be found in the O-Ring Design and Materials Guide.

A

ABRASION - progressive wearing away of a surface in service (like the sealing lip of a shaft seal) by mechanical action such as scraping, rubbing, or erosion.

ABRASION RESISTANCE - resistance of a rubber compound to wearing away when in dynamic contact with an abrasive surface.

ADDITIVE - material added to an elastomeric compound to alter its properties, e.g. a reinforcing agent to improve strength or a plasticizer to aid flexibility and processibility; also known as filler.

ADHERE - (a) to cling or stick together; or (b) to cause two surfaces to stick together.

ADHESION - tendency of rubber or other material to stick to a contact surface; may result from chemical or physical interlocking.

ADHESIVE - substance used to hold materials together.

AIR SIDE - side of the seal facing away from the fluid being sealed.

AIR SIDE ANGLE - angle between the air side surface of the primary lip of a shaft seal and the shaft; also known as the barrel angle.

ASPERITIES - microscopic pores that develop on a shaft seal’s elastomeric sealing lip at the point of contact with the shaft as a result of wear; asperities can be beneficial in that they help hold lubrication for the lip and facilitate a micro-pumping action that prevents leakage; also known as microasperities.

ASSEMBLY CONE - installation aid that fits over part or all of a shaft and decreases the chances of damaging a shaft seal’s lip on potential hazards such as keyways or splines.

AUXILIARY LIP - non-spring-loaded, optional lip which, if present, extends axially or radially from the heel of the primary sealing lip on the air side of a shaft seal and prevents contaminants from reaching the contact point; also known as a dirt lip, dust lip, or secondary lip.

AXIAL CLEARANCE - space between the end of a shaft seal’s head section and an inner case; also known as lip clearance.

AXIAL DIRT LIP - non-spring-loaded lip that extends axially from the heel of the primary sealing lip on the air side of a shaft seal; impinges on a radial flange to prevent contaminants from reaching the contact point.

B

BARREL ANGLE - angle between the air side surface of the primary lip of a shaft seal and the shaft; also known as the air side angle.

BEAM LENGTH - axial distance from the thinnest portion of a shaft seal’s flexible lip to the point at which the lip contacts the shaft; also known as lip length.

BEDDING-IN - period of initial operation during which wear to a shaft seal’s lip is most pronounced and the contact surface develops; also known as break-in or run-in.

BELL-MOUTHING - condition in which a shaft seal’s elastomeric lip contacts the shaft on the seal’s air side rather than on the tip.

BI-DIRECTIONAL SEAL - seal designed for use with a shaft that rotates in both a clockwise and counterclockwise direction; also known as a bi-rotational seal.

BLISTER - an enclosed cavity that protrudes from, and thus deforms, the sealing surface.

BOND - adhesion between a shaft seal’s elastomeric sealing lip and the metal case.

BONDED SEAL - type of shaft seal with an elastomeric sealing element bonded to a case during molding.

BORE - cylindrical surface machined into the housing to mate with the outside diameter of a shaft seal; also known as the housing bore. (See Figure 256)

BREAK-IN - period of initial operation during which wear to a shaft seal’s lip is most pronounced and the contact surface develops; also known as bedding-in or run-in.

C

CAP - portion of a shaft seal’s head section purposely removed during knife trimming or demolding; sometimes referred to as the “maidenhead.”

CASE - rigid member (typically steel) to which the sealing lip is bonded during molding; protects the lip and provides a surface to be press-fitted into the housing; double case shaft seals feature both an inner case and an outer case; also known as a shell.

CASE WIDTH - axial width of the seal case.

CAVITY - hollow space within a mold in which uncured rubber is shaped and vulcanized; also known as mold cavity. (See Figure 254)

CHAMFER - beveled edge in a component to facilitate assembly of a seal onto a rod or shaft, or into a cylinder or housing; also known as a lead-in chamfer.

CHATTER MARKS - axial tears in a shaft seal’s contact pattern; indicative of stick-slip.

CHECKING - cracking or crazing of an elastomeric surface, such as the lip of a shaft seal.

CIRCUMFERENTIAL SPEED - speed of the moving shaft expressed in feet per second (fps).

CLEARANCE GAP - gap between two mating surfaces, such as the necessary gap between a moving shaft and the housing in which it moves; a shaft seal can block this gap to prevent lubricant leakage.

COCKING - misalignment of a shaft seal such that it is not perpendicular to the bore in which it is supposed to fit and the shaft it is supposed to seal; may be caused by incorrect installation or improper design; also known as seal cocking.

COEFFICIENT OF THERMAL EXPANSION - may be linear or volumetric: (a) the coefficient of linear thermal expansion is the change in length per unit of length for a one degree rise in temperature; and (b) the coefficient of volumetric thermal expansion is the change in volume divided by the product of the original volume and the change in temperature. The coefficient of volumetric thermal expansion is three times the coefficient of linear thermal expansion for a solid material.

COIL - a single turn of a coiled wire garter spring. (See Figure 255)

COINING - process whereby patterns (such as helical ribs) are transferred onto a surface (such as a PTFE sealing lip) during a molding operation.

COMPRESSION SET - (a) the amount, expressed as a percentage of deflection, by which a rubber specimen does not return to its original thickness following release of a compressive load; and (b) the end result of a progressive stress relaxation. In terms of the life of a shaft seal, stress relaxation is like dying, whereas compression set is like death.

CONTACT BAND - a worn path on the shaft surface where it contacts the sealing lip due to the microasperities in the lip; found under the wear path; also known as a wear band or wear track.

CONTACT POINT - point at which a shaft seal’s sealing lip and the shaft touch; also known as the interface. (See Figure 257)

COUNTER-BORE - a bore with a shoulder against which a seal seats; this type of bore aids in installation by effectively stopping the seal at a particular point relative to the shaft.

CRACK - sharp break or fissure in a rubber surface caused by excessive strain and/or exposure to detrimental environmental conditions, such as ozone, weather, or ultraviolet (UV) light.

CURE - heat-induced process whereby the long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. This reaction transforms soft, weak, non-crosslinked materials into strong elastic products; also known as vulcanization.

CUT - slice-like opening in a rubber surface caused by unwanted contact between the surface and a sharp object.

D

DEFORMATION - change in the shape of a seal, or seal component, as a result of compression; also known as deflection.

DIFFERENTIAL THERMAL EXPANSION - variance in the heat-induced rates of expansion for two different materials (such as the metal of a housing bore and the metal case of a shaft seal); this variance may lead to the formation of a gap between the case and the housing, and this gap may allow leakage.

DIRT LIP - non-spring-loaded, optional lip which, if present, extends axially or radially from the heel of the primary sealing lip on the air side of a shaft seal and prevents contaminants from reaching the contact point; also known as an auxiliary lip, dust lip, or secondary lip.

DOUBLE LIP - describes a shaft seal with both a primary sealing lip and a secondary (contaminant exclusion) lip, or that has two sprung lips to separate fluids; also known as dual lip.

DRY-RUNNING - shaft seal operation without lubrication at the contact point between the seal and the shaft; will contribute to the buildup of friction and heat, hastening seal failure.

DUROMETER - (a) an instrument that measures the hardness of rubber by its resistance to surface penetration of an indenter point; and (b) the numerical scale indicating the hardness of rubber.

DUST LIP - non-spring-loaded, optional lip which, if present, extends axially or radially from the heel of the primary sealing lip on the air side of a shaft seal and prevents contaminants from reaching the contact point; also known as an auxiliary lip, dirt lip, or secondary lip.

DYNAMIC RUN-OUT (DRO) - amount (in inches or millimeters) that the shaft’s sealing surface does not rotate around the true center; taken by applying an indicator to the side of the shaft as it slowly rotates; should not exceed 0.010 inch (0.25 mm) TIR; also known as shaft run-out.

DYNAMIC SEAL - seal functioning in an environment in which there is relative motion (e.g. rotary, reciprocating, or oscillating) between the mating surfaces being sealed.

E

ELASTICITY - an elastomer’s inherent ability to readily regain its original size and shape after being released from a deforming load.

ECCENTRICITY - the variations of the shaft surface in reference to the centerline of the shaft.

ELASTOMER - any natural or synthetic material meeting the following requirements: (a) it must not break when stretched 100%; and (b) after being held at 100% stretch for five minutes then released, it must return to within 10% of its original length within five minutes.

ELONGATION - percentage increase in original length (strain) of a specimen produced by a tensile force (stress) applied to the specimen. “Ultimate elongation” is the elongation at the moment the specimen breaks.

F

FATIGUE RESISTANCE - capable of withstanding fatigue caused by repeated bending, extension, or compression; also known as flex resistance.

FILM THICKNESS - in a shaft seal, the tiny distance between the primary sealing lip and the shaft that is typically occupied by a thin film of lubricant.

FLASH - excess rubber remaining on the parting line of a molded rubber product.

FLEX RESISTANCE - capable of withstanding fatigue caused by repeated bending, extension, or compression; also known as fatigue resistance.

FLEX SECTION - part of a shaft seal, typically the area between the head section (sealing lip) and the heel section (point of connection between the lip and the case); thickness of the flex section impacts the seal’s ability to maintain optimal interference with the shaft.

FLEX THICKNESS - thickness of the area on a shaft seal between the head (sealing lip) and the heel (point of connection between the lip and the case); flex thickness should be thick enough to prevent unwanted lip distortion but not so thick as to compromise followability.

FLINGER - washer-like device designed to lend radial momentum to a liquid so as to keep it away from the sealing lip; may be incorporated into a wear sleeve; also known as a slinger.

FLUID SIDE - side of the seal facing the fluid being sealed; also known as the oil side.

FOLLOWABILITY - ability of the sealing lip to maintain contact with the shaft despite vibrations or dynamic run-out.

G

GARTER SPRING - helically coiled spring, typically made of carbon steel or stainless steel wire, formed into a ring and used in a shaft seal to help maintain contact between the sealing lip and the shaft.

GRINDING CHATTER - excessive out-of-roundness of a shaft; defined by the RMA (Rubber Manufacturers Association) as greater than 45 cycles or lobes; also known as waviness.

H

HARDNESS - measure of a rubber’s relative resistance to an indenter point on a testing device. Shore A durometers gauge soft to medium-hard rubber. Shore D durometers are more accurate on samples harder than 90 Shore A.

HEAD SECTION - part of a shaft seal, typically the air and fluid side surfaces of the sealing lip and spring groove (if present).

HEAD THICKNESS - radial distance between the bottom of a shaft seal’s spring groove and the contact point between lip and shaft.

HEAT RESISTANCE - rubber compound’s capacity to undergo exposure to some specified level of elevated temperature and retain a high level of its original properties; also known as heat aging or air aging.

HEEL GASKET - shaft seal featuring a rubber coating on the outside of the case (air side) to help prevent bore leakage and corrosion of the case by external contaminants.

HEEL SECTION - part of a shaft seal, typically the area of attachment for the sealing lip and the (outer) metal case.

HOOP FORCE - the force generated by the spring in the sealing lip when it is stretched around a shaft upon installation.

HOUSING BORE - cylindrical surface machined into the housing to mate with the outside diameter of a shaft seal; also known simply as the bore. (See Figure 256)

HYDRODYNAMIC SEAL - shaft seal utilizing helical ribs, pads, grooves, or sinusoidal patterns molded into the sealing lip on the air side to force fluid weepage back under the lip; correct orientation of the patterns (clockwise, counterclockwise, or bi-directional) depends on the direction of shaft rotation as seen from the air side.

I

I.D. - inside diameter of a seal or component.

INCOMPLETE TRIM - instance in which not all of the material that should have been trimmed from a surface was actually removed.

INITIAL TENSION - the “pre-load” created in a spring by backwinding the coils during the manufacturing process such that force is required to pull the coils apart.

INNER CASE - in shaft seals featuring a double case, the innermost of the two cases; typically supplies increased structural rigidity for demanding applications.

INTERFACE - point at which a shaft seal’s sealing lip and the shaft touch; also known as the contact point.
(See Figure 257)

INTERFERENCE - difference between the diameter of a shaft seal’s sealing lip and the diameter of the shaft to be sealed; interference is designed so that the lip diameter is smaller than the shaft diameter, thus ensuring the formation (and maintenance) of a contact point between the lip and the shaft.

INVERSION - reversal or turning over of a sealing lip that will lead to leakage; can occur during installation.

K

KNIT LINE - imperfection of the seal material due to premature curing (scorching) of the rubber compound.

L

LEAD-IN (CHAMFER) - beveled edge in a component to facilitate assembly of a seal onto a rod or shaft, or into a cylinder or housing; also known simply as a chamfer.

LEAK RATE - rate at which a fluid (liquid or gas) passes a seal or barrier.

LIP CLEARANCE - space between the end of a shaft seal’s head section and an inner case; also known as axial clearance.

LIP DIAMETER - inside diameter of a shaft seal’s primary lip measured with the garter spring (if used) installed.

LIP LENGTH - axial distance from the thinnest portion of a shaft seal’s flexible lip to the point at which the lip contacts the shaft; also known as beam length.

LIP OPENING PRESSURE (LOP) - measure of the pressure required to flow air at 10,000 cm3/minute between a shaft seal’s contact point and a shaft-sized test mandrel; the seal case O.D. must be concentric with the mandrel and air must be applied to the outside lip surface.

LIP SEAL - device utilizing the planned interference between an elastomeric lip and a mating surface (such as a shaft) to prevent leakage.

LOAD - actual pressure at a sealing face; in the case of a shaft seal, the sum of the elastomeric lip’s inherent beam force, the hoop force (as a result of lip stretch upon installation), and the garter spring tension, all of which contribute to shaft loading at the contact point.

LOW TEMPERATURE FLEXIBILITY - ability of an elastomeric product (such as the sealing lip of a shaft seal) to resist cracking or breaking when flexed or bent at low temperatures.

M

MACHINE LEAD - screw threads or spiral grooves seen on a shaft due to improper lathe machining; plunge grinding is recommended to eliminate machine lead; also known as shaft lead.

MATING SURFACES - points where different parts of an assembly meet.

MEMORY - an elastomer’s ability to regain its original size and shape following deformation.

MENISCUS - curved boundary at the meeting point of air and fluid between the sealing lip of a shaft seal and the shaft.

MICROASPERITIES - microscopic pores that develop on a shaft seal’s elastomeric sealing lip at the point of contact with the shaft as a result of wear; microasperities can be beneficial in that they help hold lubrication for the lip and facilitate a micro-pumping action that prevents leakage; also known simply as asperities.

MODULUS - the force (stress) in psi (pounds per square inch) required to produce a certain elongation (strain), usually 100%, in a material sample; a good indication of toughness; also known as tensile modulus or tensile stress, but not the same as shear modulus.

MOLDED LIP SEAL - shaft seal with a sealing lip formed by molding rather than by trimming with a knife.

MOLD IMPRESSION - imperfection molded into the surface of a material; typically due to nicks and other blemishes on the surface of the mold cavity.

N

NIB JOINT - point at which the two ends of an extension spring are joined to form a circular garter spring.

NICK - unwanted void within the sealing material created after molding.

NOMINAL PRESSFIT - the difference between the seal O.D. nominal dimension and the bore I.D. nominal dimension.

NON-FILL - unwanted void within the sealing material created during molding; typically due to improper material flow within the mold.

NON-SPRING-LOADED - used to describe a shaft seal without a garter spring as part of its sealing lip.

NOSE GASKET - shaft seal featuring a rubber coating on the fluid side used to prevent leakage due to improper finishing of the bottom of the bore; the rubber coating also helps prevent corrosion of the case by the sealed fluid.

O

O.D. - outside diameter of a seal or component.

OFFSET - amount (in inches or millimeters) that the shaft center is offset relative to the bore center; taken without the shaft moving; almost always exists to some degree but should be no more than 0.010 inch (0.25 mm) TIR; also known as shaft-to-bore misalignment (STBM).

OIL COKING - the deposition of a hard layer of carbon on a shaft seal’s lip, can result when oil comes in contact with high underlip temperatures; also known as simply coking.

OIL SEAL - specific type of shaft seal designed to retain oil.

OIL SIDE - side of the seal facing the fluid being sealed; also known as the fluid side.

OIL SIDE ANGLE - angle between the fluid side surface and the shaft; also known as the scraper angle.

OSCILLATING SHAFT - rotating shaft with limited, reversing travel, as in an on/off valve.

OUTER CASE - in shaft seals featuring a double case, the outermost of the two cases; typically contains the inner case and provides the point of attachment for the sealing lip.

OUT-OF-ROUNDNESS (OOR) - extent to which a shaft’s cross-section, a seal O.D., or the housing I.D. deviates from a true circle.

P

PASSIVATION - process of reducing the chemical reactivity of a metallic surface through exposure to a nitric acid solution; sometimes used with stainless steel wire springs to accentuate corrosion resistance.

PLUNGE GRINDING - preferred method for finishing the surface of a shaft so as not to leave machine lead that can later contribute to seal leakage.

POOR BOND - inadequate adhesion between two layers of material; can occur between two rubber layers or in a rubber-to-metal bond.

POROSITY - instance in which a material is full of numerous tiny openings.

PRESSFIT - a shaft seal or component that is fitted into place with a hydraulic press or similar mode; the act of fitting a shaft seal or component in place using a hydraulic press or similar mode; see also nominal pressfit.

PRIMARY LIP - typically spring-loaded elastomeric lip of a shaft seal that prevents unwanted movement of fluid by maintaining contact (interference) with the moving shaft.

Q

QS 9000 - Quality System developed by the automotive industry to supplement the ISO 9000 standard.

R

RADIAL DIRT LIP - non-spring-loaded lip that extends radially from the heel of the primary sealing lip on the air side of a shaft seal; impinges on the shaft to prevent contaminants from reaching the contact point.

RADIAL LIP SEAL - dynamic seal designed to retain or contain fluids and/or exclude foreign materials through the exertion of radial pressure (due to interference) on a moving shaft; also known as oil seal or shaft seal.

RADIAL LOAD - sum of all forces (such as seal interference and garter spring tension) that maintain contact between a shaft seal’s lip and the shaft; expressed in ounces per inch of shaft circumference; should be kept to just enough to seal without generating unnecessary friction and seal wear; also known as radial force or lip force.

RADIAL WALL DIMENSION (RWD) - radial distance between the shaft seal O.D. and the lip I.D. (contact point) as measured on a complete but uninstalled seal. (See Figure 259)

RADIAL WALL VARIATION (RWV) - extent to which the radial wall dimension of a shaft seal is not consistent; excessive variation can prevent a seal from seating properly in the bore.

RECIPROCATING SEAL - dynamic seal used to seal pistons or rods that are in linear motion.

RESILIENCE - a compound's ability to rapidly regain original size and shape following deformation; also known as rebound.

RHEOMETER - cure meter which determines and plots a cure curve illustrating the state of cure for a given time and temperature; typically either an Oscillating Disk Rheometer (ODR) or a Moving Die Rheometer (MDR).

ROTARY SHAFT - shaft that rotates clockwise (CW), counterclockwise (CCW), or in both directions (variously CW and CCW).

ROUGHNESS - closely-spaced irregularities on a shaft surface that are the result of manufacturing and/or cutting, as by tools or abrasive materials.

ROUGH TRIM - instance in which the trimming of a sealing surface leaves it with unwanted irregularities on both sides of the contact point.

RUN-IN - period of initial operation during which wear to a shaft seal’s lip is most pronounced and the contact surface develops; also known as bedding-in or break-in.

RUN-OUT (SHAFT) - phenomenon which occurs when the shaft’s axis and the axis of rotation are different, causing the shaft to wobble or gyrate; expressed in inches followed by the abbreviation “TIR” (Total Indicator Reading).

R VALUE - axial distance between the centerline of the garter spring and the contact point; a positive R value means the spring is located toward the air side relative to the contact point, and this is desirable; a negative R value means the spring is located toward the fluid side, which will result in immediate leakage; also known as spring position.

S

SCOOP TRIM - instance in which a seal surface is concave as a result of trimming.

SCORCHING - premature curing of rubber during storage or processing, usually caused by excessive heat.

SCORING - grooving of the shaft’s surface (as by the sealing lip), or scratching of a shaft seal’s O.D. (as by a rough housing bore).

SCRAPER ANGLE - angle between the fluid side surface and the shaft; also known as the oil side angle.

SCRATCH - superficial blemish on the surface of a seal or shaft due to abrasion.

SCUFFING - damage to a shaft seal’s metal surface due to adhesive wear.

SEAL COCKING - misalignment of a shaft seal such that it is not perpendicular to the bore in which it is supposed to fit and the shaft it is supposed to seal; may be caused by incorrect installation or improper design; also known simply as cocking.

SEAL WIDTH - total axial measurement of a seal (including the case, if present).

SECONDARY LIP - non-spring-loaded, optional lip which, if present, extends down from the heel of the primary sealing lip on the air side of a shaft seal and prevents contaminants from reaching the contact point; also known as an auxiliary lip, dirt lip, or dust lip.

SHAFT - rotating, reciprocating, or oscillating component that operates within a cylinder or housing.

SHAFT DIAMETER - diameter of the shaft expressed in inches or millimeters.

SHAFT FINISH - usually meant to be the surface roughness measured in microinches or micrometers Ra; a low finish number is indicative of a smoother surface than a high finish number; rotating shafts need to be finished in accordance with the RMA (Rubber Manufacturers Association) specifications that also define Rz and Rpm; also known as surface finish.

SHAFT RUN-OUT - amount (in inches or millimeters) that the shaft’s sealing surface does not rotate around the true center; taken by applying an indicator to the side of the shaft as it slowly rotates; should not exceed 0.010 inch (0.25 mm) TIR; also known as dynamic run-out.

SHAFT SEAL - dynamic seal designed to retain or contain fluids and/or exclude foreign materials through the exertion of radial pressure (due to interference) on a moving shaft; also known as oil seal or radial lip seal.

SHAFT SPEED - speed of a moving shaft expressed in rotations per minute (rpm).

SHAFT-TO-BORE MISALIGNMENT (STBM) - amount (in inches or millimeters) that the shaft center is offset relative to the bore center; taken without the shaft moving; almost always exists to some degree but should be no more than 0.010 inch (0.25 mm) TIR; also known as offset.

SHELL - rigid member (typically steel) to which the sealing lip is bonded during molding; protects the lip and provides a surface to be press-fitted into the housing; double shell shaft seals feature both an inner shell and an outer shell; also known as a case.

SINGLE LIP - used to describe a shaft seal with one sealing lip.

SLINGER - washer-like device designed to lend radial momentum to a liquid so as to keep it away from the sealing lip; may be incorporated into a wear sleeve; also known as a flinger.

SPARKOUT - point where sparks are visible during a grinding operation.

SPIRAL TRIM - instance in which the trimming of a sealing surface leaves an undesirable deep spiral groove.

SPRINGBACK - tendency of a shaft seal with a rubber-covered O.D. to unseat itself slightly following installation due to shearing stresses between the rubber and the housing bore.

SPRING CLEARANCE - radial distance between the top of a shaft seal’s garter spring and the innermost portion (whether metal or rubber) of the seal’s case.

SPRING GROOVE - radiused opening molded into a shaft seal’s elastomeric sealing lip to hold the garter spring.

SPRING LOAD - total tension generated by a garter spring when stretched to the designed deflected length; calculated as the combination of the spring’s initial tension and its spring rate.

SPRING-LOADED - used to describe a shaft seal with a garter spring as part of its sealing lip. (See Figure 260)

SPRING POSITION - axial distance between the centerline of the garter spring and the contact point; also known as R value; a positive R value means the spring is located toward the air side relative to the contact point, and this is desirable; a negative R value means the spring is located toward the fluid side, which will result in immediate leakage.

SPRING RATE - the additional force required to stretch a spring from position A to position B divided by the amount of stretch (N/mm).

SPRING WIND-UP - tendency of an assembled (but uninstalled) garter spring to deform rather than remain flat; for example, to twist into a figure 8.

SPRUNG INTERFERENCE - amount of interference between a sprung lip of a shaft seal and the shaft; calculated as the difference between the shaft diameter and the sprung lip diameter.

SPRUNG LIP - a shaft seal lip with a garter spring in place.

SPRUNG LIP DIAMETER - inside diameter of a shaft seal’s primary lip measured with the garter spring installed.

STATIC SEAL - seal functioning in an environment in which there is no relative motion between the mating surfaces being sealed.

STICK-SLIP - phenomenon in which a shaft seal’s sealing lip and the shaft surface alternate between adhesion and slipperiness due to insufficient lubrication; may allow leakage or even destroy the seal.

STRESS RELIEVING - process of relieving stresses in an unassembled coiled spring through exposure to heat; intended to help ensure that the spring force will not be adversely affected by heat during actual service.

SUMP - cavity or reservoir within which the fluids of a system are contained.

SUMP TEMPERATURE - temperature of the fluid within an assembly’s sump.

SURFACE CONTAMINATION - unwanted material (such as dust or dirt) on the surface of a seal or shaft.

SURFACE FINISH - usually meant to be the surface roughness measured in microinches or micrometers Ra; a low finish number is indicative of a smoother surface than a high finish number; rotating shafts need to be finished in accordance with the RMA (Rubber Manufacturers Association) specifications that also define Rz and Rpm; also known as shaft finish.

SURFACE SPEED - speed of a moving shaft’s surface expressed in meters per minute (mpm) or feet per minute (fpm).

T

TEAR - a separation or pulling away of part of a sealing structure.

TEAR RESISTANCE - resistance to the growth of a nick or cut in a rubber specimen when tension is applied.

TENSILE STRENGTH - force in pounds per square inch (psi) required to break a rubber specimen.

TIR - Total Indicator Reading; total range of a dial indicator reading when gauging misalignment.

TRIBOLOGICAL - of or relating to friction or frictional build-up due to motion and wear.

TRIMMED LIP SEAL - shaft seal with a sealing lip formed by a knife cut rather than by molding.

TRIMMING - removal of excess material and/or shaping of rubber following vulcanization. (See Figure 261)

U

UNBONDED FLASH - loose rubber that has inadvertently adhered to the seal surface and may impair performance, or flash that does not properly bond to an intended mating material.

UNDERLIP TEMPERATURE - temperature of the oil between a rotating shaft and a shaft seal’s lip at the contact point; measured in test situations using an infrared camera.

UNI-DIRECTIONAL SEAL - seal designed for use with a shaft that rotates in only one direction, either clockwise or counterclockwise; also known as a uni-rotational seal.

UNITIZED SEAL - shaft seal that incorporates a running surface into the seal design; used when the actual shaft surface lacks an acceptable finish; also known as a cassette seal.

UNSPRUNG INTERFERENCE - amount of interference between an unsprung lip of a shaft seal and the shaft; calculated as the difference between the shaft diameter and the unsprung lip diameter.

UNSPRUNG LIP - a shaft seal lip without a garter spring.

UNSPRUNG LIP DIAMETER - inside diameter of a shaft seal’s primary lip measured without the garter spring (if used) installed.

V

VISCOMETER - shearing disk device used to gauge the viscosity of a rubber sample under heat and pressure. Often referred to as the Mooney Viscometer, this device was once the most common tool for determining processing characteristics but has now largely been replaced by the rheometer.

VISCOSITY - resistance to flow; the thicker the substance (such as a liquid), the more viscous it is, i.e. the less it flows.

VULCANIZATION - heat-induced process whereby the long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. This reaction transforms soft, weak, non-crosslinked materials into strong elastic products; also known as cure.

W

WAVINESS - excessive out-of-roundness of a shaft; defined by the RMA (Rubber Manufacturers Association) as greater than 45 cycles or lobes; also known as grinding chatter.

WEAR PATH - a worn path on the sealing lip where it contacts the shaft due to the plunge ground surface of the shaft; the site of the formation of microasperities.

WEAR SLEEVE - replaceable mild steel sheath drawn over a damaged shaft to provide an improved sealing surface or over a shaft made of a soft material (such as cast iron) to provide a harder, more wear-resistant sealing surface; also known as a wear ring.

WEEPAGE - very small amount of seal leakage; may or may not be enough to necessitate replacement of the seal or redesign of the assembly.

WETTING - application of, or formation of, a continuous liquid film on a surface.

Additional terms can be found in the O-Ring Design and Materials Guide.