Do you know what keeps oil or grease in an application? Typically, it’s seals. You may be familiar with seals, but did you know that they perform multiple functions that have an impact on the reliability and performance of industrial equipment. For example, seals not only keep lubricants in, but they may also prevent contaminants from entering the system.
This two-part tip series on seals will focus on their type, selection, wear modes and their compatibility with various oils and greases.
By definition, a seal is a device that prevents the passage of a fluid or semi-fluid between two mating surfaces. As long as the seal is unbroken, it will stop fluid movement from an area of high pressure to low pressure. Anything that breaks a seal, such as expansion and contraction, vibration and contamination, will allow the fluid (oil) or semi-fluid (grease) to pass.
Types of seals
There are two general types of seals: static and dynamic.
Static seal: A static seal is one where there is no movement between the seal and the mating surfaces. Examples are o-rings and gaskets. A washer is an example of a static “o-ring” type seal. Another example is the simple rubber ring between a garden hose and spray nozzle. The seal placed between the mating surfaces of a valve cover and engine head would be an example of a “gasket.” O-rings and gaskets are often pressure activated, meaning their shape deforms as installation pressure is applied. This allows the material to take the shape of the surface, closing or eliminating the gap. Some o-rings and gaskets are also temperature sensitive and will expand and contract with changes in operating or environmental temperatures. This phenomena impacts the choice of materials used in o-rings and gaskets, as well as the installation pressure applied.
Dynamic seal: A dynamic or mechanical seal is one used where there is linear or rotary motion applied between the mating surfaces. Examples include the rings of a piston engine, or the shaft seal in a centrifugal pump. In the latter, the shaft rotates against the seal which is mounted in the stationary pump housing.
While both static and dynamic seals are designed to keep fluids in, dynamic seals must be able to withstand wear and contamination related friction in the area where the seal contacts the rotating shaft.
Seal design and choice
A static seal has a very simple design, as it takes the shape of the two mating services. These are usually elastomers which absorb the entire load across the mating surface, and must be re-torqued occasionally, to prevent leakage.
Mechanical or dynamic seals are much more complex. They contain a primary sealing ring, a closing mechanism (spring), secondary sealing elements, and mounting hardware. Mechanical seals are installed perpendicular to the shaft and the spring force must be sufficient to maintain contact pressure to avoid leakage.
Mechanical seals may be configured in single or multiple arrangements. A single mechanical “lip” seal is designed solely to keep the lubricant in the housing. A dual or double “lip” seal is arranged to not only keep the oil in, but also to keep contaminants out. Other factors affecting seal design include:
Next week, we’ll take an even closer look at seal design and explore seal wear. In the meantime, please “Like” this article if you found it helpful and leave a comment below if you have any questions.