Material Selection of O-Rings Used in Static and Dynamic Sealing Applications

O-Ring Sealing Application Types:

O-rings as mechanical seals are ubiquitous and have found adoption in a wide variety of industries and applications. Typically, O-rings are differentiated based on the materials that they are manufactured from. However, another major factor that must be considered while selecting O-ring materials is their application, whether it is static or dynamic. In static sealing applications, as the name suggests, there is no relative motion between the O-ring and the mating surface in contact with the seal. This type of sealing action can be further divided into static axial and static radial. While in static radial, the squeezing action acts on the ID and OD on the O-ring, in static axial the squeezing action acts on the top and bottom of the ring.

The other type of sealing application is dynamic sealing, in which there is relative motion between the O-ring and the surface with which it is in contact. This relative motion between the O-ring and the surface leads to dynamic friction between these planes, which play a role in selecting the O-ring material for a particular application.

Material Requirements for Dynamic Sealing Applications:

While selecting O-rings for use in dynamic sealing applications, a few specific properties of the O-ring material must be considered. The main factors include material hardness and resistance to abrasion. When we consider the hardness of a material, it plays a key role in O-rings used for dynamic sealing, as the hardness of the material determines the coefficient of friction of the material as well. A material that is soft, i.e. it has a low durometer reading, has a high co-efficient of friction. The higher the coefficient of friction, the more difficult it is for the sealing surface to move. This makes soft elastomers, with consequently higher coefficients of friction, less ideal for dynamic sealing applications.

The other factor is the abrasion resistance of elastomers. In dynamic sealing applications, the constant motion and rubbing of the O-ring against the surface can cause abrasion of the O-rings. This abrasion wears out the affected section of the O-ring, negating any sealing effect facilitated by the O-ring. This makes it imperative that the elastomers that are used in O-rings for dynamic sealing applications have good resistance to abrasion.

Given below is a list of common elastomers used to manufacture O-rings, along with their Shore A hardness range and resistance to abrasion. The Shore A hardness is provided as a range because variations in certain variables in the manufacturing process can produce the same elastomer with differing hardness.

(click column header to sort results)
MATERIAL  
SHORE A HARDNESS RANGE  
ABRASION RESISTANCE  
Buna N
30-95
Excellent
Butyl
40-90
Good
Natural Rubber
20-90
Good
Silicone
30-90
Poor
FKM (Viton)
60-90
Excellent
EPR
30-90
Excellent
Neoprene
50-90
Fair

Material Requirements for Static Sealing Applications:

The main physical properties to be taken into consideration while selecting an O-ring for static sealing applications are the hardness of the elastomers and their resistance to compression set. The hardness comes into play for static sealing applications, especially in cases where the sealing surfaces are imperfect. A softer elastomeric O-ring would flow better into the pits and voids present on the surfaces and provide a good sealing action. However, the softness of the elastomers also makes it susceptible to extrusion caused by high pressure. In static sealing applications, where the fluids are of high pressure, it is ideal to select an O-ring manufactured from an elastomer with a high Shore A hardness rating.

Compression set occurs when the O-ring is permanently deformed, i.e. it retains the deformation even after the compressive force is removed from the seal. This is caused by excessive compression and can significantly reduce the seal life of the O-ring. Different materials have varying affinities to compression set and an elastomer must be selected to avoid such compression sets.

(click column header to sort results)
MATERIAL  
SHORE A HARDNESS RANGE  
RESISTANCE TO COMPRESSION SET  
Buna N
30-95
Good
Butyl
40-90
Fair
Natural Rubber
20-90
Excellent
Silicone
30-90
Good
FKM (Viton)
60-90
Good
EPR
30-90
Good
Neoprene
50-90
Good

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