The O-Ring seal, as reminded numerous times, is based on the elastic reaction to the exerted compression by the system elements. The loss of the elastic properties bring to a decaying of the seal capacity, the ring is no longer able to exert pressure onto the housing walls which allows the fluid to flow from one part to another. The actual elasticity of the O-Ring compound is influenced by many factors such as the exercise temperature, the contact fluids and the mechanical solicitation. These factors intervene modifying the molecular links and as a consequence, the compound's chemical and physical characteristics. The loss of elasticity can be permanent or reversible, as happens for the due hardening at low temperatures, hardening which disappears when the temperature returns above the minimum limit.
To evaluate the elastic seal characteristics of the compounds, an valuation parameter was introduced indicated as Compression Set. The Compression Set represents the percentage of decrease of the test piece thickness, subjected to a compression test with test cycles defined, with respect to the starting thickness. The compound to be used must present high compatibility with the exercise conditions to the end to avoid a quick decrease in the elastic characteristics.
The O-Rings that are permanently deformed present an evident pressing of the diameter in the direction of the seal surfaces (figure 9.1).
O-Ring with permanent deformation