Abrasion in O-rings: avoiding wear

Damage pattern: excessive abrasion in dynamic use

Friction from contact with harder components releases particles from the surface of O-rings. The sealing ring therefore loses mass locally or around its entire circumference. If this wear mechanism is pronounced, it can quickly lead to failure of the seal.

The missing material causes leaks. Friction can indeed lead to excessive abrasion, but it cannot be eliminated entirely. It is essential for creating the sealing effect. Only when the pressure conditions press the O-ring against the sealing faces does it close off both the sealing gap and small irregularities in the component surfaces.

In dynamic applications in particular, abrasion poses a major problem for the durability of the seal. However, there are several starting points for reducing mechanical wear. We address these after describing the damage pattern and its causes.

Abrasion means material loss

If the O-ring is permanently deformed, it has adapted to a static position in the sealing system. This position is defined by the compression of the cross-section between the sealing faces. In addition, a medium such as cooling water or hydraulic oil can exert pressure on the sealing element. This too determines the positioning of the O-ring in the sealing gap and can cause further deformation of its cross-section.

With a dynamic seal, friction adds a further decisive component. It is primarily here that the damage pattern of excessive abrasion arises.

During failure analysis, it is noticeable that the O-ring has lost considerable mass compared with its initial state. As a rule, the cross-sectional area is reduced around the entire circumference. This is also how abrasion can be distinguished from permanent deformation. Although the cross-sections take on similar shapes in both cases, the pronounced loss of mass appears only with abrasion.

The central role of friction

Abrasion arises from the contact of the O-ring with the sealing faces. The effect can be intensified by abrasive particles in the media. The compression required for the sealing effect ensures that the O-ring rests against the sealing faces.

If it seals a pressurized medium against the atmosphere, that medium additionally pushes the O-ring against the pressure-averted flank of the groove. In this way, friction can occur on the O-ring even when sealing machine parts that are actually stationary.

In dynamic applications, by contrast, the components regularly perform a relative movement with respect to one another. This is the case, for example, with a piston and cylinder. Here the O-ring sits in a groove that is cut into the piston. An excess of the O-ring's outer diameter over the cylinder's inner diameter provides the compression of the cross-section. Every time the piston slides up and down in the cylinder, the O-ring rubs over the sealing face. This generates heat, but also mechanical material removal.

The material removal affects the O-ring, since it is by nature softer than the material from which the cylinder and piston are made. Only in this way can it perform its sealing task. This consists of compensating for small manufacturing- or wear-related irregularities in the metallic component surfaces. A minimum of friction is therefore always necessary to achieve the sealing effect.

Causes of abrasion on the O-ring

Because of their surface properties, elastomers tend to stick to one another and to machine surfaces. They generally have a high coefficient of friction. If very heavy abrasion occurs on the O-ring, this is usually attributable to use as a dynamic seal.

A certain amount of material removal through abrasion can occur as early as during installation. However, this is a clear installation error that can be prevented by appropriate training of the technical staff.

Abrasion in static applications

Abrasion can occur even with seals that at first glance appear stationary. Here the machine parts to be sealed do not perform a regular relative movement with respect to one another. Nevertheless, the O-ring can move across the sealing faces. Depending on the surface finish of the sealing faces, more or less pronounced abrasion occurs.

The cause is changing pressure conditions. They cause the sealing ring to migrate across the entire width of the groove. High abrasion arises above all with the cyclic sequence of extreme pressure conditions. This happens despite the short distance the O-ring travels in the axial direction within the groove.

Abrasion in dynamic applications

O-rings used as a dynamic seal are most frequently affected by excessive abrasion. Here they constantly slide over the running surfaces of the components.

Friction losses can never be entirely avoided, but they reduce the efficiency of a technical system. This is because part of the energy input is lost to friction and converted into heat. This heat, firstly, often cannot be reused and, secondly, causes localized heating of the O-ring. That is equally undesirable, since the effect of temperature can alter the material properties of the O-ring.

With dynamic seals, particular attention must be paid to a high surface finish of the running surfaces. The smoother they are, the less friction occurs. In addition, hard particles in the media pose a problem. For example, small metal chips in lubricating oil can damage the running surface and in turn cause more friction between it and the seal.

Another cause of increased abrasion can be a lack of lubrication. If the seals run dry, friction and thermal load rise.

Preventing excessive abrasion: possible strategies

Anyone wishing to prevent excessive abrasion on the sealing element can start above all with the material and the installation space. The operating conditions in terms of pressure and plant operating mode, by contrast, are difficult to influence.

Design improvements

The degree of compression also determines the friction. However, the engineer can reduce this only within narrow limits. A minimum compression must be maintained so that the O-ring has enough reserve for elastic recovery. With dynamic seals in particular, the compression is already at the lower limit in order to keep friction losses within bounds.

It is more promising to specify higher surface finishes for the running surfaces. Grinding and polishing, or chrome plating of the components, are processes that can significantly reduce friction.

Choosing the right O-ring material

Reducing the coefficient of friction of the O-ring surface is a suitable way to prevent abrasion. This can be achieved through the right material selection and additional coatings. Manufacturers add fillers and plasticizers to the O-ring materials, which help determine the friction properties. But even the base polymers differ in their abrasion resistance.

For silicone, for example, it is so low that dynamic use should generally be avoided here. The material NBR, like silicone, is suitable for food applications, but is considerably more abrasion-resistant.

Coatings for dynamic use

The market offers various processes for the surface treatment of elastomers. Reducing the coefficient of friction improves the sliding properties of the O-ring. This is relevant not only for later use in the sealing system. Fully or partially automated assembly also benefits from it, because if the seals do not stick to one another, they can be separated more easily.

Here, however, we are concerned with coatings for dynamic use, not those that primarily serve to ease assembly. Modern coating processes cover O-rings and moulded seals with a nanoscale dry-film lubricant layer. The additional material therefore affects the material properties of the O-ring only very slightly. The core materials EPDM, NBR, HNBR or silicone can thus continue to play to their individual strengths.

It should be noted that the coating must be stable against the influences encountered in operation. These include, for example, sterilization with hot steam or contact with acids and alkalis. Materials used for the dry-film lubricant include PTFE or polysiloxane, for example.

Here too, engineers are called upon to select, from the multitude of solutions, coating systems that comply with the respective requirements. We provide expert support in doing so. Our application specialists know the technical possibilities as well as industry-specific requirements. In this way they identify the most suitable option.

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