The Anatomy and Functions of Shaft Seals

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 Seal
INTRODUCTION
“ 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 Motion
TYPICAL 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 Motion

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 Motion

• 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 Packings
THE EVOLUTION OF SHAFT SEALING

Rope Packings were superseded by:
• Assembled Leather Seals
• Assembled Synthetic Rubber Seals
Assembled Seals

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 Seals

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 Seals

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 Seals

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 Design
THE 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 Length

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 Eccentricity

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.
Pressure

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 Point

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.
Interference

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 Spring

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 can
flow through smaller gaps, so a spring
will typically be used to ensure
consistent contact between lip and shaft.
Garter Springs and Fluid Type

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 and
axial. A radial dirt lip faces the shaft. An axial
dirt lip faces away from the shaft and will require
a vertical surface against which to seal. Some
designs feature both.
The Secondary Sealing Lip

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 Bonding

The Outside Diameter (O.D.)

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.)

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 Sealant

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 Grinding

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 Expansion

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.

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 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.

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-lip
SEAL 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-lip
SEAL 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-lip
SEAL 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-lip
SEAL 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 Force
HOW 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.
Microasperities

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.
Microaperities

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.
Microasperities

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 Sealing

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.
Meniscus

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° F
Relative 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

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

Relative Cost: High
Fluoroelastomers 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)
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

Relative 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)
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

Relative Cost: High
Silicones 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)
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

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The Anatomy and Functions of Shaft Seals

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