orings

Sealing Devices and Materials (O-Rings)

Sealing devices and materials prevent leakage, and contain pressure or exclude contamination between components.

The two most common types of seals are gaskets and packings:

Gaskets:

Mechanical compression needed to provide a positive seal between two stationary joints.

Packings:

Generally, some form of relative motion is involved, occuring between members of the joint.

There are many commercial types and forms of packing and gasket materials. Certain types of seals, for example, the O-ring, may be used either as a gasket or a packing.

Sealing Devices and Materials

The sealing devices and materials used in fluid power systems and components are divided into two general classes:

Static Seals and Dynamic Seals

Static Seals

The static seal is used to prevent leakage in a mechanical joint where there is no relative motion between mating surfaces.

Material used to create a seal between two stationary faces of a mechanical joint are called gaskets.

To create an effective barrier, gasket material will deform to fill the space between imperfect mating surfaces of mechanical joints to prevent fluids from leaking. This compression requires that the joint be tightly bolted or otherwise held together.

Dynamic Seals

The dynamic seal is a seal provided in a mechanical coupling where movement between the sealed surfaces is intended, such as packing material sealing to a rotating or reciprocating shaft.

Material used to provide a seal in a dynamic seal is called packing.

Packing material usually consists of bulk deformable materials which are shaped by manually adjusted compression.

Dynamic Seals are usually Rotating, Reciprocating, or Oscillating Seals.

Seal Materials

Many different materials have been used in the development of sealing devices.

The material used for a particular application depends on several factors:

• fluid compatibility,

• resistance to heat,

• resistance to pressure,

• wear resistance,

• hardness,

• type of motion, and, most importantly,

• flexibility.

Since seal materials largely depend on their flexibility, most viable material choices are limited to elastomers, especially for O-rings.

For certain applications, gaskets and packings can be made from natural materials, such as cork or leather, and metals.

Nitrile Rubber

Nitrile rubber is the most common choice of material for O-rings. It comes in various grades that differ in the composition of acrylonitrile.

It has high abrasion resistance, high mechanical strength, and resistance against gas permeation and compounds like mineral oils and greases.

It is also suitable for a wide temperature range going up to 130°C.

Silicone Rubber

Silicone is another popular material for manufacturing O-rings.

It is resistant to various harmful environments such as ozone, UV, oils, and hot air.

It has great temperature resistance and retains its mechanical properties, including flexibility, up to 230°C.

However, its drawbacks include poor strength and wear/tear resistance. This makes it suitable only for static sealing applications.

Thermoplastic Polyurethane

Polyurethane is an ideal candidate for extreme working conditions like high-pressure sealing.

It boasts high toughness, abrasion resistance, and thermal stability.

On the negative side, it can lose its properties in environments containing acids, chlorinated hydrocarbons, and humidity.

Fluoroelastomer

Fluoroelastomers are good choices for O-rings due to their chemical inactivity against most potentially harmful compounds.

Fluoroelastomers have a smooth surface with self-lubricating qualities, making them perfect for applications where a lubrication system is difficult to set up.

However, they are susceptible to mechanical creep over time.

Other Elastomer O-Ring Materials

Cholorprene

Ethylene Propylene

Perfluoroelastomer

Polyacrylate Rubber

Hydrogenated Nitrile Rubber

PTFE (Teflon)

PEEK

Space Shuttle Challenger Disaster

The failure of an O-ring seal was determined to be the cause of the Space Shuttle Challenger disaster on January 28, 1986.

A crucial factor was cold weather prior to the launch.

This was famously demonstrated on television by Caltech physics professor Richard Feynman, when he placed a small O-ring into ice-cold water, and subsequently showed its loss of flexibility before an investigative committee.

Types of Seals Based on Shape

T-Seal

T-Seals have enhanced sealing pressure profiles and improved stability, particularly in high-pressure systems.

T-Seals are suited for both reciprocating and rotary motion equipment, with specific styles tailored for rod or piston applications.

They serve critical functions in hydraulic and pneumatic cylinders, aerospace components, automotive systems, and heavy machinery, offering low friction, minimal wear, and long-term resilience.

V-Ring

A V-Ring is a solid rubber seal used on shafts to prevent dirt, moisture, and other unwanted material from entering an oil seal, housing, or any other desired area while simultaneously keeping lubricants in.

It works by stretching over the shaft to create a tight seal along the shaft while the flexible lip seals lightly against the counterface at a right angle to the shaft.

The V-Ring extends the life of bearings, oil seals, and shafts.

O-Ring

An O-ring is circular in shape, and its cross section is small in relation to its diameter.

O-rings are usually molded from rubber compounds; however, they can be molded or machined from plastic materials.

The O-ring is usually fitted into a rectangular groove (usually called a gland) machined into the mechanism to be sealed.

An O-ring seal consists of an O-ring mounted in the gland so that the O-ring's cross section is compressed (squeezed) when the gland is assembled.

Advantages of O-Rings

• Simplicity,

• Ruggedness,

• Low cost,

• Ease of installation,

• Ease of maintenance,

• No adjustment required,

• Small space requirement,

• Reliability, and

• Effectiveness over wide pressure and temperature ranges

O-Ring Applications

O-rings are used in both static (as gaskets) and dynamic (as packing) applications.

An O-ring will almost always be the most satisfactory choice of seal in static applications if the fluids, temperatures, pressure, and geometry permit.

Standard O-ring packings are not specifically designed to be used as rotary seals. When infrequent rotary motion or low peripheral velocity is involved, standard O-ring packings may be used.

O-rings are often used as reciprocating seals in hydraulic and pneumatic systems.

Types of Glands/Grooves for O-Rings

Flange/Face Seal Groove

The flange seal groove is the most common and simplest among the O-ring groove types.

The groove is rectangular in shape and has no gap between the mating surface. Thus, there is direct surface-to-surface contact, eliminating any issues related to ring extrusion gaps.

Flange seal groove is a static O-ring groove design suitable for non-moving components.

Recommended O-ring groove dimensions for different sizes are defined by AS568 standards.

Dovetail Groove

The dovetail groove has inclined sidewalls, making it look like a dovetail.

The wall inclination creates a groove with a trapezium cross-section, whose main purpose is to hold the O-ring in during installation and maintenance.

This design is suitable for static sealing.

Moreover, the locking capability makes the dovetail groove ideal for applications with regular disassembly and maintenance, allowing for less downtime and quicker handling.

Triangular Crush Groove

This design has a triangular cross-section with the groove machined into one of the mating components.

It allows for a simple, cheap grooving process with adequate sealing capacity.

The seal is static and also non-reusable.

If it is subjected to extreme, uneven compression can permanently deform the O-ring.

Dynamic Gland Seal

A dynamic gland seal is used when two mating components are moving in relation to each other while maintaining a seal.

There will always be a gap between the two surfaces.

Sizing of O-Rings

The assemblage of both mating surfaces squeezes the O-ring in between them, creating a tight seal.

To facilitate this, the inner mating component has a groove machined into its surface at the spot where sealing is required. Engineers call this groove the O-ring groove (O-ring gland).

The O-ring groove dimensions require the O-ring to slightly stretch to achieve a sturdy O-ring fit in the groove, quite similar to a snap-fit joint.

Moreover, the diameter of the O-ring is intentionally kept slightly larger than the depth of the O-ring groove size, which causes it to extend beyond the mating surface.

This extended part comes in contact with the outer mating surface when it is assembled.

The assembly pressure compresses the O-ring to such an extent that it partially fills up the O-ring groove (O-ring gland) and seals the gap between both surfaces.

This gap is another important design consideration. Engineers use the term ‘clearance gap’ to describe it.