WHAT IS AN O-RING?
An O-ring is a simple and versatile ring-shaped packing or sealing device with a circular
cross section, from which the "O" in its name is derived. 0-rings function as compact and reliable sealing devices by absorbing the tolerance stackup between closely mated surfaces in both dynamic (moving) and static (stationary) applications. Although 0-rings can be made from a variety of materials, they are most commonly molded in one piece from an elastomeric (rubber-like) material. This manual is concerned with elastomeric 0-rings only.
The basic O-ring design has been in use for many years. Prior to World War 11, O-ring
availability and usage was very limited. However, after World War II, O-ring usage
increased dramatically, primarily due to the standardization of sizes and extensive aircraft applications.
Today, billions of 0-rings are used for almost every conceivable sealing application,
including fluid power, fluid containment, and a wide variety of non-sealing applications.
They are used to seal pumps, valves, cylinders, and fittings in passenger cars, trucks,
aircraft, farm equipment, industrial hydraulic equipment, home appliances and plumbing.
0-rings can also be used in a variety of non-sealing applications, such as bumpers, drive
belts, tension bands, conveyors, spacers, and friction drives.
HOW DO O-RINGS SEAL?
An O-ring seals by blocking any potential leak path of a fluid (liquid or gas) between two
closely spaced surfaces. The O-ring is generally installed in a machined groove in one of
the surfaces to be sealed. As the two surfaces are brought together, forming a gland, they squeeze the cross section of the O-ring. This squeezing action results in a deformation of the O-ring cross section. With O-rings, the greater the squeeze, the larger the deformation. It is the unique characteristics of the elastomer material used in 0-rings that makes the Oring such a good seal. The elastomer, a highly viscous, incompressible fluid with high surface tension, has a capacity for remembering its original shape for a long time. In low-pressure applications (that is, where the confined fluid exerts little or no pressure on the O-ring), the tendency of the elastomer to maintain its original shape creates the seal. As the O-ring is deformed when the mating surfaces are brought together, it exerts a force against the mating surfaces equal to the force it takes to squeeze it. The areas of contact between the O-ring and the mating surfaces (contact bands) act as a barrier to block the passage of the fluid.
In applications where higher pressure is exerted by the confined fluid, the sealing action
of the O-ring caused by the squeeze of its cross-section is augmented by fluid pressure,
transmitted through the elastomer. The O-ring is forced to the side of the gland, away
from the pressure. As it is pressed against the side, the O-ring, cross section is deformed into a "p" configuration. The Elastomer exerts equal force in all directions and is forced up to (but not into) the gap between the mating surfaces.
When pressure is released, the O-ring returns to approximately its original installed
shape, ready for the next application of pressure. The O-ring is also able to seal in both
directions. In a double acting system where the pressure application changes from one
side of the O-ring to the other, the O-ring moves, seating itself in the opposite side of the
gland.
If pressure exceeds the limits of the O-ring, or if the gap that the O-ring must block is too
large, the elastomer will enter the narrow gap between the inner and outer members of the
gland. This may result in extrusion failure, causing the fluid to leak.
An O-ring is a simple and versatile ring-shaped packing or sealing device with a circular
cross section, from which the "O" in its name is derived. 0-rings function as compact and reliable sealing devices by absorbing the tolerance stackup between closely mated surfaces in both dynamic (moving) and static (stationary) applications. Although 0-rings can be made from a variety of materials, they are most commonly molded in one piece from an elastomeric (rubber-like) material. This manual is concerned with elastomeric 0-rings only.
The basic O-ring design has been in use for many years. Prior to World War 11, O-ring
availability and usage was very limited. However, after World War II, O-ring usage
increased dramatically, primarily due to the standardization of sizes and extensive aircraft applications.
Today, billions of 0-rings are used for almost every conceivable sealing application,
including fluid power, fluid containment, and a wide variety of non-sealing applications.
They are used to seal pumps, valves, cylinders, and fittings in passenger cars, trucks,
aircraft, farm equipment, industrial hydraulic equipment, home appliances and plumbing.
0-rings can also be used in a variety of non-sealing applications, such as bumpers, drive
belts, tension bands, conveyors, spacers, and friction drives.
HOW DO O-RINGS SEAL?
An O-ring seals by blocking any potential leak path of a fluid (liquid or gas) between two
closely spaced surfaces. The O-ring is generally installed in a machined groove in one of
the surfaces to be sealed. As the two surfaces are brought together, forming a gland, they squeeze the cross section of the O-ring. This squeezing action results in a deformation of the O-ring cross section. With O-rings, the greater the squeeze, the larger the deformation. It is the unique characteristics of the elastomer material used in 0-rings that makes the Oring such a good seal. The elastomer, a highly viscous, incompressible fluid with high surface tension, has a capacity for remembering its original shape for a long time. In low-pressure applications (that is, where the confined fluid exerts little or no pressure on the O-ring), the tendency of the elastomer to maintain its original shape creates the seal. As the O-ring is deformed when the mating surfaces are brought together, it exerts a force against the mating surfaces equal to the force it takes to squeeze it. The areas of contact between the O-ring and the mating surfaces (contact bands) act as a barrier to block the passage of the fluid.
In applications where higher pressure is exerted by the confined fluid, the sealing action
of the O-ring caused by the squeeze of its cross-section is augmented by fluid pressure,
transmitted through the elastomer. The O-ring is forced to the side of the gland, away
from the pressure. As it is pressed against the side, the O-ring, cross section is deformed into a "p" configuration. The Elastomer exerts equal force in all directions and is forced up to (but not into) the gap between the mating surfaces.
When pressure is released, the O-ring returns to approximately its original installed
shape, ready for the next application of pressure. The O-ring is also able to seal in both
directions. In a double acting system where the pressure application changes from one
side of the O-ring to the other, the O-ring moves, seating itself in the opposite side of the
gland.
If pressure exceeds the limits of the O-ring, or if the gap that the O-ring must block is too
large, the elastomer will enter the narrow gap between the inner and outer members of the
gland. This may result in extrusion failure, causing the fluid to leak.
O-RINGS, SQUARE RINGS,
QUAD SHAPED RINGS
QUAD SHAPED RINGS
V-RINGS
WIPPER-RINGS
POLY -SEALS
U-CUPS
WHAT MAKES THE O-RING SUCH A GOOD SEAL?
Listed below are some of the outstanding characteristics that make 0-rings one of the
most versatile, dependable, yet Inexpensive seals available, with sealing capability from
hard vacuum to high pressure.
• The circular cross section provides minimum surface area, which enhances
resistance to abrasion, fluids, adverse environments, and arid mechanical damage.
• The 0-ring's circular cross section is also appealing because it is adaptable to
axial, radial, or angular squeeze.
• 0-rings seal in both directions
• 0-rings will fit into confined spaces without the need for bulky, adjustable, or
expensive support structures.
• O-ring seals are easy to maintain. They do not require retightening of bolts.
• 0-rings do not require adhesives or other special methods for assembly.
• The O-ring is flexible and absorbs metal tolerance stack-up.
• O-ring compounds can be selected to resist most environmental extremes.
TYPES OF O-RING SEAL APPLICATIONS
This section provides an overview of the most common types of O-ring sealing
applications. It is not intended to be a comprehensive study, but rather to offer
background information to those unfamiliar with the uses of 0-rings.
Although there are many uses of 0-rings, all O-ring sealing applications can be classified
as either static or dynamic.
Static Seal
In a static sea application, there is no relative motion between parts of the gland that
contact to the O-ring. (Small amounts of movement, such as might be caused by thermal
expansion, vibration, bolt stretch, or O-ring response to fluid pressure, do not alter the
static definition.) Static seals are often categorized according to the direction in which
squeeze is applied to the O-ring cross section. There are two basic directions of squeeze:
axial and radial. There are also applications which combine both axial and radial squeeze;
however, for beat results. It is recommended that squeeze be applied in one direction
only, to allow for O-ring expansion.
Static Axial Seals
In this application, the squeeze is on the top and bottom of the O-ring, similar to a flat
gasket. Static axial seals are typically utilized in face seal applications.
Another type of static, axially squeezed seal is a seat seal application in which the O-ring
Is located on the face of one part of a structure that closes against another structure. The
sealing action is created when closing compresses the O-ring between the two mating
surfaces.
Although there IS relative motion between the mating surfaces, sealing occurs only in the
closed position; therefore, the seat seal is generally considered a static application. Seat
seals are often used in poppet valves.
Listed below are some of the outstanding characteristics that make 0-rings one of the
most versatile, dependable, yet Inexpensive seals available, with sealing capability from
hard vacuum to high pressure.
• The circular cross section provides minimum surface area, which enhances
resistance to abrasion, fluids, adverse environments, and arid mechanical damage.
• The 0-ring's circular cross section is also appealing because it is adaptable to
axial, radial, or angular squeeze.
• 0-rings seal in both directions
• 0-rings will fit into confined spaces without the need for bulky, adjustable, or
expensive support structures.
• O-ring seals are easy to maintain. They do not require retightening of bolts.
• 0-rings do not require adhesives or other special methods for assembly.
• The O-ring is flexible and absorbs metal tolerance stack-up.
• O-ring compounds can be selected to resist most environmental extremes.
TYPES OF O-RING SEAL APPLICATIONS
This section provides an overview of the most common types of O-ring sealing
applications. It is not intended to be a comprehensive study, but rather to offer
background information to those unfamiliar with the uses of 0-rings.
Although there are many uses of 0-rings, all O-ring sealing applications can be classified
as either static or dynamic.
Static Seal
In a static sea application, there is no relative motion between parts of the gland that
contact to the O-ring. (Small amounts of movement, such as might be caused by thermal
expansion, vibration, bolt stretch, or O-ring response to fluid pressure, do not alter the
static definition.) Static seals are often categorized according to the direction in which
squeeze is applied to the O-ring cross section. There are two basic directions of squeeze:
axial and radial. There are also applications which combine both axial and radial squeeze;
however, for beat results. It is recommended that squeeze be applied in one direction
only, to allow for O-ring expansion.
Static Axial Seals
In this application, the squeeze is on the top and bottom of the O-ring, similar to a flat
gasket. Static axial seals are typically utilized in face seal applications.
Another type of static, axially squeezed seal is a seat seal application in which the O-ring
Is located on the face of one part of a structure that closes against another structure. The
sealing action is created when closing compresses the O-ring between the two mating
surfaces.
Although there IS relative motion between the mating surfaces, sealing occurs only in the
closed position; therefore, the seat seal is generally considered a static application. Seat
seals are often used in poppet valves.
Static Radial Seals
With this application, the squeeze is between the Inside Diameter (I.D.) and Outside
Diameter (O.D.) of the O-ring. Typical static radial seals are cap seals and plug seals.
Although uncompromised radial or axial squeeze is the preferred approach, 0-rings are
extremely flexible and tend to work in grooves of many shapes and various directions of
squeeze. For example, with crush seals the squeeze exerted on the O-ring is angular
(diagonal) through the O-ring cross section. In a crush seal application, the O-ring is
confined in a triangular gland having little, if any, volume greater than the O-ring.
This lack of gland void is a disadvantage of the crush seal, The O-ring is more
susceptible to damage during installation and upon high temperature excursions where
the relatively higher coefficient of thermal expansion of the O-ring may cause the metal
parts of the gland to warp. Therefore, the crush seal is most frequently used where cost
and ease of machining are important.
With this application, the squeeze is between the Inside Diameter (I.D.) and Outside
Diameter (O.D.) of the O-ring. Typical static radial seals are cap seals and plug seals.
Although uncompromised radial or axial squeeze is the preferred approach, 0-rings are
extremely flexible and tend to work in grooves of many shapes and various directions of
squeeze. For example, with crush seals the squeeze exerted on the O-ring is angular
(diagonal) through the O-ring cross section. In a crush seal application, the O-ring is
confined in a triangular gland having little, if any, volume greater than the O-ring.
This lack of gland void is a disadvantage of the crush seal, The O-ring is more
susceptible to damage during installation and upon high temperature excursions where
the relatively higher coefficient of thermal expansion of the O-ring may cause the metal
parts of the gland to warp. Therefore, the crush seal is most frequently used where cost
and ease of machining are important.
Dynamic Seals
In dynamic sealing applications there is relative movement between the parts of the
gland, thus, O-rings are subject to a sliding action against the gland. This motion
introduces friction, which creates design problems different from those of static seals. For the great majority of dynamic applications, 0-rings are squeezed radically and are
subjected to reciprocating motion, either intermittent or continuous, or, less frequently, to
an Intermittent rotary or oscillating motion.
Reciprocating Seals
In this type application, there is a relative reciprocating (back and forth) motion along the shaft axis between the inner and outer elements of the gland 0-rings used in reciprocating applications are called piston and rod seals. Reciprocating service in aircraft hydraulic systems has been the basis for most of the developmental work and improvements in Oring sealing technology.
In dynamic sealing applications there is relative movement between the parts of the
gland, thus, O-rings are subject to a sliding action against the gland. This motion
introduces friction, which creates design problems different from those of static seals. For the great majority of dynamic applications, 0-rings are squeezed radically and are
subjected to reciprocating motion, either intermittent or continuous, or, less frequently, to
an Intermittent rotary or oscillating motion.
Reciprocating Seals
In this type application, there is a relative reciprocating (back and forth) motion along the shaft axis between the inner and outer elements of the gland 0-rings used in reciprocating applications are called piston and rod seals. Reciprocating service in aircraft hydraulic systems has been the basis for most of the developmental work and improvements in Oring sealing technology.
Oscillating Seals
In this application, the inner or outer member of the gland moves in an arc around the
axis of the shaft - first in one direction and then in the opposite direction, generally
intermittently with nn more than a few turns in each direction. The most common
application for oscillating O-ring seals is in faucet valves. However, because 0-rings help
to simplify design and reduce costs, they are used in valves of all types.
Other Dynamic Seals
0-rings are extremely versatile and may be used in applications where data for ordinary
reciprocating or oscillating seals does not apply. For example, 0-rings have proven to be
practical seals in many rotary applications where either the inner or outer member of the
gland elements continuously turn around the axis of the shaft. However, developmental
engineering work may be required in these applications.
In this application, the inner or outer member of the gland moves in an arc around the
axis of the shaft - first in one direction and then in the opposite direction, generally
intermittently with nn more than a few turns in each direction. The most common
application for oscillating O-ring seals is in faucet valves. However, because 0-rings help
to simplify design and reduce costs, they are used in valves of all types.
Other Dynamic Seals
0-rings are extremely versatile and may be used in applications where data for ordinary
reciprocating or oscillating seals does not apply. For example, 0-rings have proven to be
practical seals in many rotary applications where either the inner or outer member of the
gland elements continuously turn around the axis of the shaft. However, developmental
engineering work may be required in these applications.
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