In industries from aerosol to aerospace, O-rings are the most commonly used seals in the world. Why? Because they are effective, economical, and easy to use.

Although O-ring sealing is a simple concept, there is a lot to consider as you design a seal for a specific application. This guide provides detailed information on the many factors that influence the design of an effective O-ring seal. Here’s a quick overview:

BLOCKING THE GAP
Any mechanical assembly containing fluids must be designed so that these substances flow only where intended and do not leak into other parts of the assembly (or out of the assembly entirely). Seals are incorporated into mechanical designs to prevent such leakage at the points where different parts of an assembly meet. These meeting points are known as mating surfaces, and the space between them is called a clearance gap. The purpose of any seal is to block the clearance gap so that nothing passes through it.

A number of methods may be used to block the gap, including welding, brazing, soldering, or machining lapped fits. You might also simply squeeze a softer material between the two harder materials of the assembly. This last method describes the function of an O-ring.

IN THE GROOVE
An O-ring seal has two essential parts: the O-ring and the gland. The gland consists of the machined groove into which the O-ring is installed and the mating surface to be sealed. The primary components of rod and piston glands are shown in Figure 1.

A seal is effected when an O-ring is squeezed between mating components, thereby creating zero clearance and preventing the escape of fluids through the clearance gap. Figure 2 shows rod and piston O-rings installed. As can be seen, the groove for a rod seal is machined into the housing, whereas the groove for a piston seal is machined into the piston itself. The versatility of O-rings allows them to function effectively in either configuration.

SEALING YOUR FATE
To better understand how an O-ring seals, think of the O-ring as a highly viscous (thick) “fluid” with very high surface tension. When placed under pressure, the O-ring is forced to “flow” within the groove toward the clearance gap. As the O-ring flows against (and slightly into) the gap, it produces zero clearance and prevents the sealed substance from escaping. Figures 3 through 6 illustrate this process.

In Figure 3, the O-ring has been installed but is not under pressure. In Figure 4, the O-ring is under just enough pressure to effect a seal. Figure 5 shows the seal under maximum pressure. The seal is extruding (extending) slightly into the clearance gap but is still functioning effectively. In Figure 6, the pressure has now exceeded the seal’s capabilities, forcing it to extrude severely. A leak path forms, and the seal fails.

STAYING IN SHAPE
An important factor in the effectiveness of any O-ring is its memory, or ability to remember its shape. The molecular properties of the O-ring are such that it is always trying to regain its original shape despite being squeezed and/or distorted by pressure. This memory function allows a properly designed O-ring seal to block the clearance gap and prevent leakage, all the while resisting extrusion into the gap or otherwise losing its shape. Since an O-ring’s memory is directly related to its chemical structure, let’s take a closer look at some basic structural concepts next.