Also known as oil seals and radial lip seals, shaft seals are widely used in conjunction with rotary, reciprocating and oscillating shafts.

The main functions of shaft seals are to:

•Retain lubricants / liquids•Exclude dirt / contaminants•Separate fluids•Confine pressure

The Functions of a Shaft SealINTRODUCTION

“ A shaft seal is not just a dam or a barrier; it’s also a pump.”

— Rick Hudson

• Rotary Motion

• Oscillating Motion

• Reciprocating Motion

Typical Shaft Seal Applications

Rotary Motion Oscillating Motion Reciprocating Motion

Movement round and round. The direction of

this rotation is very important to seal

design. A shaft may rotate only clockwise

(CW), only counterclockwise (CCW), or be bi-

rotational (variously rotating both CW and

CCW).

The speed at which a shaft rotates is

noted in revolutions per minute,

or RPM.

Rotary MotionTYPICAL APPLICATIONS

Rotation back and forth within an arc defined

by the degrees of rotation. An oscillating

shaft is defined by the degrees of this arc,

and the number of cycles (full movement

through the arc and back again to the starting

point)

per minute.

Oscillating MotionTYPICAL APPLICATIONS

Chiefly defined by two variables: the length of

each stroke (movement in one direction, in or

out of the housing), and the number of full

cycles (movement in and out of the housing) per

minute.

Reciprocating MotionTYPICAL APPLICATIONS

• Leather Strips

• Rope Packings

• Assembled Leather Seals

• Assembled Synthetic Rubber Seals

• Bonded Seals

• Seals with Reduced Bonding Areas

• Unitized Seals

• Composite Seals

• Value Added Seals

The Evolution of Shaft Sealing

As motorized vehicles replaced wagons, leather strips were replaced by

rope packings. These packings were typically made of flax, cotton, or

hemp.

The first “shaft seals” were nothing more than leather strips

attempting to contain the animal fat that lubricated the rotating

axles of horse-drawn wagons.

Leather Strips to Rope PackingsTHE EVOLUTION OF SHAFT SEALING

Rope Packings were superseded by:

• Assembled Leather Seals

• Assembled Synthetic Rubber Seals

Assembled SealsTHE EVOLUTION OF SHAFT SEALING

By the 1950s, technology allowed for the chemical bonding of

rubber to metal. In the 1970s, techniques and materials were

developed allowing for reduced bonding areas.

Bonded SealsTHE EVOLUTION OF SHAFT SEALING

In the 1980s, unitized seals were developed that incorporated sealing

surfaces into seal designs. Later, composite seals were introduced

featuring lips of PTFE bonded to synthetic rubber.

Unitized and Composite SealsTHE EVOLUTION OF SHAFT SEALING

Seal designers are now combining the seal with other

components in the sealing area.

These other components might include filters, reinforcing inserts, and

excluders. The resulting value-added seals make life even easier for the

user by reducing the number of components and simplifying assembly.

Value-Added SealsTHE EVOLUTION OF SHAFT SEALING

The five basic components of a typical shaft seal are:

The Anatomy of a Shaft Seal

• Outer Shell (Case)

• Inner Shell (Case

• Primary Lip (Head Section)

• Secondary Lip (Auxiliary Lip)

• Garter Spring

The cross-sections of typical shaft seals are made up of many variable features.

The most important design feature of a shaft seal is the elastomeric sealing lip.

The beam length is the axial distance from the thinnest part of the lip (the flex thickness) to the point at which the lip contacts the shaft.

Lip DesignTHE ANATOMY OF A SHAFT SEAL

Beam length has a huge impact on lip force, friction, wear and resistance to deformation.

For a given flex thickness, a short lip exerts more force on the shaft (with a corresponding increase in friction and wear) than a long lip.

A short lip also has better resistance to distortion caused by high pressure than a long lip.

Beam LengthTHE ANATOMY OF A SHAFT SEAL

Beam length also affects a lip’s ability to follow any shaft eccentricities.

A long lip is more flexible than a short lip and can thus more easily follow shaft eccentricities, such as shaft-to-bore misalignment (STBM) or dynamic runout (DRO).

Shaft EccentricityTHE ANATOMY OF A SHAFT SEAL

Lip distortion due to high pressure can be a serious concern.

This distortion increases contact between the air side surface of the lip and the shaft, which in turn increases friction and wear. Seal life is shortened. In extreme cases, high pressures have even been known to force the seal out of the bore or to tear the lip away from the case.

PressureTHE ANATOMY OF A SHAFT SEAL

Two other important lip variables are the angles that meet at the head of the lip (point nearest the shaft) to form the contact point.

The angle facing the fluid being sealed is known as the oil side angle, or scraper angle. The angle facing away from the fluid is the air side angle or barrel angle. To prevent leakage, the oil side angle must always be greater (steeper) than the air side angle.

Contact PointTHE ANATOMY OF A SHAFT SEAL

To ensure contact between the sealing lip and shaft, the lip must always have a smaller inside diameter (I.D.) than the diameter of the shaft.

The difference between the seal lip I.D. and the shaft diameter is known as interference. This designed-in interference is what allows the seal to function effectively as a fluid block to prevent leakage.

InterferenceTHE ANATOMY OF A SHAFT SEAL

In many shaft seal designs, lip interference is augmented through use of a garter spring.

A garter spring is a helically coiled spring formed into a ring. If present, the garter spring rests in a radiused groove molded into the head section of the sealing lip. A spring does two things:

1. It contributes to the sealing force (or load) between the lip and shaft, and

2. It helps ensure proper loading even if the lip material is compromised (such as by swelling).

The Garter SpringTHE ANATOMY OF A SHAFT SEAL

The type of fluid being sealed often determines whether a spring is used.

Because thicker fluids, like greases don’t flow readily and thus require a large leak path to escape, a lip without a spring will often suffice as a seal. Thinner fluids, such as oil and water canflow through smaller gaps, so a spring will typically be used to ensure consistent contact between lip and shaft.

Garter Springs and Fluid TypeTHE ANATOMY OF A SHAFT SEAL

In addition to the primary sealing lip, many designs also incorporate a smaller, secondary lip to exclude dust, dirt, and other contaminants.

Unlike the primary lip, a secondary lip typically faces the application’s air side because dirt and other debris may migrate in from outside the assembly.

There are two types of secondary lips: radial andaxial. A radial dirt lip faces the shaft. An axial dirt lip faces away from the shaft and will requirea vertical surface against which to seal. Some designs feature both.

The Secondary Sealing LipTHE ANATOMY OF A SHAFT SEAL

The case does two things for the seal. First, it provides stability, allowing the seal O.D. to press fit snugly into a housing bore. Second, it also provides protection, preventing damage to the lip during installation.Cases come in a variety of shapes. There are L-cup cases, inner cups (double cases), stepped cases, reverse channel cases, and shotgun cases.

In most shaft seals, the elastomeric portion is chemically bonded to a stamped metal case.

Lip BondingTHE ANATOMY OF A SHAFT SEAL

The Outside Diameter (O.D.)THE ANATOMY OF A SHAFT SEAL

Depending on the application, it may be necessary to alter the seal O.D. in order to prevent leakage.

For shaft seal O.D.s, there are three basic categories: metal, rubber, and a combination thereof. Metal O.D. seals may be treated in various ways to improve performance. The entire case is usually coated with adhesive, making the metal resistant to corrosion and helping bore retention.

The Outside Diameter (O.D.)THE ANATOMY OF A SHAFT SEAL

Bore sealant applied to a metal O.D. seal is useful when there are light scratches or marks on the bore surface. Deep scratches necessitate the use of a secondary adhesive such as Permatex®.

Another option is spraying the seal O.D. with a

polyurethane-based bore sealant.

Bore SealantTHE ANATOMY OF A SHAFT SEAL

Giving the seal uniform retention strength after installation, a precision ground O.D. can be very effective if pressed into a bore with good surface finish. If the bore surface is rough, a secondary adhesive / sealant will be needed.

A third option is to grind the metal O.D., resulting in a straight wall with a very accurate outside dimension.

Precision GrindingTHE ANATOMY OF A SHAFT SEAL

For example, metals expand with heat, and an aluminum

housing will expand twice as quickly as a steel seal

O.D. This phenomenon, known as differential thermal

expansion, would allow a leak path to develop between

the seal O.D. and the

housing.

Because of their thermal properties, metal O.D. seals may not be ideal for every application.

Differential Thermal ExpansionTHE ANATOMY OF A SHAFT SEAL

Shaft seals with a rubber coating on the O.D.

are often used in applications where metal O.D.

seals will not work. The rubber coating encapsulates the seal’s metal case and ensures good contact between the seal O.D. and the bore. Rubber and aluminum have comparable thermal expansion rates, and the rubber maintains a tighter, more “reactive” fit during both expansion and subsequent contraction of the housing.

Rubber Coated O.D.THE ANATOMY OF A SHAFT SEAL

Small, round ribs can be molded along the seal O.D. This ribbed rubber O.D. design offers high point-of-contact unit loading, thus increasing sealability and bore retention.

Rubber O.D. seals can be altered to further facilitate sealing and retention.

Ribbed Rubber O.D.THE ANATOMY OF A SHAFT SEAL

The metal provides retention while the rubber provides sealability. In addition to protecting the rubber portion from installation damage, the metal also assists with accurate alignment in the bore and minimizes seal cocking and/or movement during use. The rubber allows a tighter elastic fit in the bore than with metal alone.

Seals featuring both metal and rubber on the O.D. may be needed for truly problematic applications.

Metal and Rubber O.D.THE ANATOMY OF A SHAFT SEAL

S-lip seals have a single, spring-loaded sealing lip. The SBY seal, for example, has a spring-loaded lip and metal O.D.

Standard Designs: S-lipSEAL DESIGNS

T-lip Seals have a spring-loaded primary lip plus a secondary dirt exclusion lip. The TBY seal, for

example, has both a spring-loaded primary lip and a

dirt lip in combination with a metal O.D.

Standard Designs: T-lipSEAL DESIGNS

V-lip Seals have a single, non-spring-loaded sealing lip. The VBY seal, for example, has a non-

spring-loaded lip and a metal O.D.

Standard Designs – V-lipSEAL DESIGNS

K-lip Seals have a non-spring-loaded sealing lip plus a secondary dirt exclusion lip. The KBY seal,

for example, has a non-spring-loaded primary lip

and a dirt lip in combination with a metal O.D.

Standard Designs: K-lipSEAL DESIGNS

Once installed, a typical shaft seal is defined by the two sealing surfaces.The first is a tight static seal between the seal O.D.

and the housing bore. As we mentioned, the seal O.D.

is designed to be slightly larger than the bore to

ensure proper contact, or interference, between the

O.D. and the bore. The second is a dynamic sealing

surface formed between the

elastomeric lip and the moving shaft. The seal lip

I.D.

is designed to be slightly smaller than the shaft

diameter to ensure that the lip will be expanded

(stretched outward) by the shaft upon installation.

The lip thus exerts radial force (load) on the shaft.

Interference and Radial ForceHOW A SHAFT SEAL WORKS

The elastomeric sealing lip is formulated to undergo a degree of wear.

A properly-finished shaft surface abrades away a thin layer of rubber from the seal tip as it contacts the shaft. Microscopic pores known as microasperities form on the lip’s wear path. If the shaft is too smooth, or if the elastomeric lip has not been properly formulated, microasperities will not form.

MicroasperitiesHOW A SHAFT SEAL WORKS

Once formed, microasperities are advantageous for a couple of reasons.

1. They serve as reservoirs to hold lubrication that prevents further lip wear.

2. They contribute to an inherent pumping capability.

MicroaperitiesHOW A SHAFT SEAL WORKS

As microasperities form on the lip’s wear path, the plunge ground surface on the shaft under the seal lip is worn smooth.As the shaft rotates, the contact point of the lip is sheared in the circumferential direction. The microasperities are pulled and elongated, creating tiny helices.

MicroasperitiesHOW A SHAFT SEAL WORKS

Because of the geometry of the sealing lip, the pumping of the helices on the air side is greater than the pumping of the helices on the oil side.

The net result is an in-pumping effect that prevents leakage from the oil reservoir (sump). This in-pumping is sometimes enhanced by molding pumping aids onto the air side of the lip, resulting in a hydrodynamic seal.

Hydrodynamic SealingHOW A SHAFT SEAL WORKS

Due to surface tension between the air, the fluid, and the shaft, a curved meniscus develops at the meeting point between air and fluid, on the air side of the lip.Hydrodynamic theory predicts that this meniscus can shift inward (toward the oil side) as shaft speed increases. But because of the oil held in the lip’s microasperities and a thin film of oil on the shaft, the lip and shaft will not make direct, unlubricated contact.

MeniscusHOW A SHAFT SEAL WORKS

The majority of shaft seal lips are made from one of the following materials:

• Nitrile (NBR)• Hydrogenated Nitrile (HNBR)• Fluoroelastomer (FKM)• Polyacrylate (ACM)• Silicone (VMQ)

Shaft Seal Lip Materials

PTFE (Teflon®) is also used in some specialized shaft seal designs.

Temperature Range: -25° - +250° FRelative Cost: Low

Also known as Buna N, nitrile rubber is the most commonly used elastomer in the manufacture of shaft seals and other sealing devices. Nitrile is a copolymer of butadiene and acrylonitrile.

Nitrile (NBR)SHAFT SEAL LIP MATERIALS

ADVANTAGES•Good oil compatibility•Good abrasion resistance•Good low temperature properties•Relatively low cost

DISADVANTAGES•Poor resistance to EP lubes and synthetic oils•Not recommended for temperatures over 250°F•Vulnerable to ozone and UV

Temperature Range: -25° - +300° FRelative Cost: Medium

As the name suggests, hydrogenated nitrile results from the hydrogenation of standard nitrile. Hydrogenation is the process of adding hydrogen atoms to the butadiene segments, which reduces the number of weak carbon-to-carbon double bonds in the polymer chain.

Hydrogenated Nitrile (HNBR)SHAFT SEAL LIP MATERIALS

ADVANTAGES•Greatly improved tensile strength over NBR•Excellent abrasion resistance•Increased heat resistance•Improved ozone and UV resistance

DISADVANTAGES•Increased cost over NBR

Temperature Range: -15° - +400° FRelative Cost: HighFluoroelastomers are elastomeric compounds that contain fluorine. The most commonly-known brand of FKM is DuPont’s Viton®. Fluoroelastomers make excellent seals due to their exceptional resistance to chemicals, oil, and temperature extremes.

Fluoroelastomer (FKM)SHAFT SEAL LIP MATERIALS

ADVANTAGES•Performs well at elevated temperatures•Excellent resistance to petroleum products•Excellent chemical compatibility

DISADVANTAGES•Poor low temperature performance•Poor resistance to amines in EP lubricants•Relatively high cost

Temperature Range: -25° - +300° FRelative Cost: Medium

Polyacrylate offers good resistance to petroleum fuels and oils, is resistant to flex cracking, and also resists damage from oxygen, sunlight, and ozone.

Polyacrylate (ACM)SHAFT SEAL LIP MATERIALS

ADVANTAGES•Good compatibility with most oils, including EP additives•Good resistance to oxidation and ozone•Tolerates higher operating temperature than NBR

DISADVANTAGES•Poor compatibility with some industrial fluids•Poor compression set resistance•Poor resistance to water•Poor performance at low temperatures

Temperature Range: -65° - +300° FRelative Cost: HighSilicones are primarily based on strong sequences of silicon and oxygen atoms, rather than long chains of carbon atoms as with many hydrocarbons. This silicon-oxygen backbone is much stronger than a carbon-based backbone.

Silicone (VMQ)SHAFT SEAL LIP MATERIALS

ADVANTAGES•Wide temperature range•High lubricant absorbency, minimizing friction•Wide temperature range

Good flexibility•Poor abrasion resistance•Relatively high cost•Swells in petroleum-based fluids

Shaft Seal Manufacturing

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