How do Rubber O-Rings Seal Components?
O-rings and rubber O-rings are employed where the pressure differential between the components of the system are static and/or the pressure differential between components is considerably less than that produced in hydraulic systems. The size of a rubber O-ring is delimited by the internal diameter (where the endpoints are the internal surface of the ring) and the cross-sectional area (where the endpoints are the external surface of the ring). In the UK, all O-rings and rubber O-rings are classified according to British standard sizes which extend from BS001 to BS932.
Fluid dynamics is the study of the behaviour of moving fluids, which in this context refers to gases and liquids. As the pressure of a fluid increases, the degree of contact stress that the O-ring can withstand must also increase. The mathematics of fluid dynamics is applied to each type of O-ring, such that components can be sealed to very high pressures. All O-rings are designed and manufactured for a particular set of processes and if they installed properly leaking cannot occur. Furthermore, these specialist gaskets function by allowing the pressure of a moving fluid to pass through the O-ring itself, which is for all intents and purposes incompressible. The primary parameter to avoid leaking through the region where components join is that the pressure of the fluid is less than the yield stress of the O-ring itself. In turn, the key pre-requisite to obtaining this state of affairs is to ensure that the parts of the transport system itself are of superior quality and that the correct rubber O-ring is mounted into the joining parts.
The potential impact of temperature
Rubber O-rings are an example of a composite material which means they are made from two or more elements or compounds that have different physical and chemical properties. In addition, rubber O-rings are in chemical terms an amorphous solid, which means they do not have a crystalline shape. Short of a major accident, most rubber O-rings will not have to endure a temperature that causes them to melt. There is, however, always a possibility, (no matter how remote) that a progressive decrease in temperature can occur, and this means a phenomenon known as glass transition can present itself. Put simply, glass transition refers to the temperature at which a given composite begins to turn from a tough, durable and reliable substance to one which is more viscous in character. It is not a change of state because it occurs over a range of temperatures, where the range itself is determined by the composition of the O-ring. The phenomenon itself is not fully understood and as such is the subject of a great deal of research in engineering and materials development circles. Overall, glass transition is associated with situations where the temperature of the fluid transport system is rapidly cooled and/or where the fluid pressure is significantly and rapidly increased. Clearly, glass transition is a situation to be avoided.
Overall, Rubber O-rings can only function in manufacturing systems where the pressure of the fluid medium being directed is less the contact stress pressure of the O-ring mounted inside the joint between components.
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