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How O-rings work

The O-ring is the simplest of the seals, and the most common type is one of a wide variety of static and dynamic applications. The design of the O-ring groove is relatively simple - an economical and reliable seal is obtained when the following rules are developed for the groove shape. The O-rings tend to return to their original shape when the cross-section is compressed due to the basic cause of the O-ring, making a good seal.
Basically, an O-ring seal consists of an O-ring groove designed with a resilient circular cross-section to provide an initial compression.
The force required for a compressed o-ring is the result of hardness and cross-sectional diameter. The stretching of the O-ring passes through a reduced cross-section, which reduces the sealing compression of the O-ring's potentially affected seal.
At zero or very low pressure, the natural elasticity of the rubber compound provides a seal. Sealing performance can be improved by increasing radial extrusion. This increase in extrusion can have a negative effect on the dynamic pressure of the seal.
Radial extrusion provides friction between the O-ring and the groove that holds it in the installed position. Engineered to deform, the rubber compound flows upwards into the extrusion gap and is completely sealed against leaks until the applied pressure is sufficient to overcome the friction and deformation of the O-ring into the small extrusion gap (assuming that the rubber has reached its pressure Limitation of flow, further increase in force will lead to failure through shearing or squeezing).
The purpose of the groove is to provide an initial force in the seal spanning one axis at 7% to 30 percent. This compressive force is usually perpendicular to the force applied within the range. There is a free volume in the slots to other axes.
When pressure is applied, the O-ring moves toward the low pressure side of the groove. The sealing pressure is transmitted to the surface being sealed, which is actually higher than the fluid pressure applied by the amount equal to the initial disturbance pressure.
The stress due to interference between the seal and the mating surface caused by the applied pressure is increased. Although this condition still exists, the O-ring will continue to propagate normally and reliably up to a few hundred pounds of force, assuming that the O-ring is selected to the correct size and the groove is machined to the proper size.
As the pressure increases, the ring deformation will be exaggerated and the ring part will eventually be squeezed into the extrusion gap. If the extrusion gap is too large, then it will fail completely from the high pressure seal.
When the pressure is released, the result of the rubber compound returns to its natural shape in the elasticity of an O-ring, preparing a similar cycle.
These materials, at their normal operating temperatures, are almost impossible to compress and have a very low modulus of elasticity. Changing their shape (rather than their volume) and the radial extrusion applied will result in increasing the length of the seal across the slot.
This increase will be greater as a result of the expanded rubber, due to the compatibility of the sealed fluid and the material. The tank must be correctly sized to allow maximum expansion of the rubber compound, otherwise the assembly will develop very high stresses.
When sufficient force is applied, the O-ring moves toward the low pressure side until its contact surface of the slot. Additional pressure or force directs the deformed O-ring toward the extrusion gap. The O-ring will initially be deformed into a "D" shape. This deformation will increase 70%-80% of the initial cross section of the surface contact area. The surface contact area of the O-ring under high pressure is approximately twice that of the original geometry at zero pressure.
The possibility of sealed extrusion is not limited to dynamic applications. In static axial applications, the tension of the mounting bolts 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 (some data is given in the figure above). In practice, the gap is usually specified for the size and application of a given ring. If working at low temperatures, it may be necessary to reduce the depth of the gland to compensate for the contraction of the ring and provide the desired size of squeeze in the contraction.
At the other end of the scale at that temperature, it may be desirable to slightly increase the groove depth to avoid over-extruding the ring at operating temperature. This effect can be significant at extreme temperatures because the coefficient of thermal expansion of the elastomer is higher than that of metal.

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