Definition : -
An O-Ring is a doughnut-shaped object, or torus. The opposite sides of an O-Ring are squeezed between the walls of the cavity or "gland" into which the O-Ring is installed. The resulting zero clearance within the gland provides an effective seal, blocking the flow of liquids or gases through the gland's internal passage.

An O-Ring can be defined by its dimensions (I.D. inner diameter i.e. hole and C.S. cross section), durometer (Shore A hardness), and material composition.

Dimensional Considerations :-

Inside Diameter :
To provide an effective seal, the O-Ring's inside diameter (I.D.) must be smaller than the piston groove diameter, so that the O-Ring is slightly stretched, fitting snugly in the groove. This stretch should be between 1%-4% with 2% as the ideal in most applications. A stretch greater than 4% is not recommended. The stress due to excessive stressing of the O-Ring will cause accelerated aging, cross section reduction and ultimately result in premature seal failure..

Calculate the O-Ring I.D. according to the following formula:
O-Ring I.D. =      Groove Diameter   
                          % of Stretch Desired

Cross Section :
When calculating the cross section (C.S.) of an O-Ring, you need to consider the size of the gland to be filled as well as the amount of squeeze needed to create a good seal.

Factors :-

Before purchasing a seal, the following factors are to be taken into consideration.

Chemical Attack
Its important to study the reactivity of the O-Ring material to the chemicals its exposed to. Specific elastomers degrade on exposure to certain chemicals. Therefore, the first step while designign a seal is to match the application's chemicals with the O-Ring material.

Temperature
The range of temperature to which the O-ring is exposed to is an important factor when considering the efficiency of the Seal. Its important to measure temperature in the immediate O-Ring environment, not just the system temperature. The time of exposure to any high temperature must also be considered. It may involve short bursts, long bursts, sustained levels. To view the temperature resistance of various Sealing Materials view the General Elastomer Properties section.

Friction
There are two types of friction, both of which are important considerations in dynamic (moving) applications. When part movement is intermittent, the effects of BREAKOUT FRICTION can cause excessively high pressures to develop. This pressure can tear portions of the seal that adhered to the gland wall causing seal failure.

In continuously moving applications, excessive O-Ring RUNNING FRICTION can cause heat to build up within the O-Ring material itself. This causes swelling, which causes more heat to develop, and eventually results in seal extrusion and failure. This phenomenon is known as the "Joule" effect.

Pressure
Presence of High Pressure on the Seal can hamper its ability to seal. Therefore, while selecting the right kind of sealing material, its important to keep in mind the pressure under which the O-rings would perform.

However, low pressure can be a problem as well. If the system pressure is below 100 psi, it is classified as low pressure. Because system pressure is not great enough to "activate" the seal, the design must rely solely on the resiliency of the elastomer to retain its original profile under compression. Over time, the elastomer will not resist compression as much and take a compression "set", resulting in possible seal failure. However, by proper component design which may include lowering the seal durometer or cross section, maximum seal utility is achieved. By lowering the durometer and/or cross section, the force required to compress a given cross section decreases, resulting in an effective seal at lower applied force per unit length of seal.

Durometer
Durometer (Shore A) is a measurement of the hardness of an elastomeric compound. The numerical ratings for hardness run from lower numbered (less than 70) softer materials to higher numbered (greater than 70) harder materials, noting that fluorocarbon has a base rating of 75. This classification system is designed to work within a ±5 point range. All materials are not available in all hardnesses.