How Do O-rings Work

How do O-rings work

O-ring is one of the simplest and most common types of seals for a wide range of static and dynamic applications. The design of O-ring groove is relatively simple - an economical and reliable seal is obtained by following the rules of developed groove shape. The tendency of the O-ring to return to its original shape when the cross-section is compressed is the basic reason for the O-ring to make a good seal.

Basically, an O-ring seal consists of an elastic circular cross-section into a designed O-ring groove, providing an initial compression.

The force required for a compressed O-ring is the result of hardness and cross-sectional diameter. The tension of the O-ring passes through a reduced cross-section, which reduces the seal compression potential of the O-ring seal.

The natural elasticity of rubber compounds provides a seal at zero or very low pressure. The sealing performance can be improved by increasing radial extrusion. This increase in extrusion can have a higher pressure dynamic seal adverse effect.

Radial extrusion provides friction between the O-ring and the groove that holds it in place. Engineered to deform, the rubber compound flows upward into the extrusion gap, completely sealing it against leakage until the applied pressure is sufficient to overcome friction and deformation of the O-ring into the small extrusion gap (assuming that the rubber has reached its limit of flow under pressure, further increase in force will result in failure through shear or extrusion).

The groove is designed to provide an initial force across a shaft between 7% and 30% at the seal percent.This The compression force is usually perpendicular to the range of the force applied. There is a free volume of slots on other axes.

When pressure is applied, the O-ring moves toward the low pressure side of the groove. The sealing pressure is transferred to the surface to be sealed, which is actually higher than the fluid pressure exerted by an amount equal to the initial interference pressure.

Increase the stress of interference between the seal and the mating surface caused by the applied pressure. Although this situation still exists, the O-ring will continue to propagate normally and reliably up to hundreds of pounds of force, assuming that the O-ring is selected to the correct size and the groove is machined to the appropriate size.

With the increase of pressure, the ring deformation will be exaggerated, and finally squeeze the part of the ring to the extrusion gap. If the extrusion clearance is too large, then the seal which is completely extruded from high pressure will fail.

When the pressure is released on the rubber compound, the elasticity of an O-ring returns to its natural shape, preparing for a similar cycle.

These materials, at their normal operating temperature, are almost impossible to compress and have very low elastic modulus. Their shape can be changed (not their volume) and the radial squeeze applied will result in an increase in the length of the seal across the groove.

This increase will be greater as a result of the expanded rubber and heated due to the compatibility of the sealing fluid and the material. The tank must be properly sized to allow maximum expansion of the rubber compound or the component will develop very high stresses.

When sufficient force is applied, the O-ring will move toward the low pressure side until its contact surface of the groove. Additional pressure or force will squeeze the deformed O-ring toward the gap. The O-ring will initially be deformed to a "d" shape. This deformation increases the initial cross-section of the surface contact area by 70% - 80%. The surface contact area of the O-ring under high pressure is about twice that of the original geometry at zero pressure.

The possibility of sealing extrusion is not limited to dynamic applications. In static axial applications, the tension of the assembly bolt under high pressure can open the extrusion gap enough to allow leakage.

The internal pressure limit is determined by the clearance and the hardness of the seal ring (some data are shown in the figure above). In practice, gaps are usually specified for a given ring size and application. If working at low temperatures, it may be necessary to reduce gland depth to compensate for ring contraction and to provide the required squeeze in contraction size.

At this temperature on the other end of the balance, it may be desirable to increase the groove depth slightly to avoid over extrusion of the ring at the operating temperature. This effect can be significant at extreme temperatures because the coefficient of thermal expansion of elastomers is higher than that of metals.