About seals

The term "seal" in actuality is a very broad term. In industry today, there is a huge variety of seal types that typically fall into either of two categories: static or dynamic. Furthermore, the application of seals and their varying configurations is vast. The scope of this article will be limited to the most common seal types and will focus primarily on a basic understanding of seals.

The purpose of seals

Regardless of any specific sealing application, the purpose of a seal can be singular in nature or multi-functional. Generally, seals are used for the following purposes:

  • To keep lubrication in
  • To keep contaminates out
  • To isolate regions of dynamic components (ex: pistons of pneumatic cylinders)
  • To wipe surfaces clean

Seal types

Some basic seal types are as follows:

  • Rotating shaft seals
  • Piston seals
  • Rod seals
  • Wipers
  • O-rings

Rotary shaft seals

Common to most machinery components using rotating shafts is the need to prevent the escape of oil or grease from within a gearbox or crank shaft housing. The use of oil in a housing is more typical than grease, but the need to keep the lubrication in is the same. In the case of rotating shaft seals, the purpose is twofold, which is to keep lubrication in and contaminates out.

Rotating shaft seals employ varying designs depending on the specific application. The design differences of these seals depend more specifically on variables such as shaft speed, chemical compatibility, system pressure, temperature and environmental exposure. Any given seal can therefore be made from varying materials and may in fact be made with multiple materials. The most common rotary oil seals used in automotive applications, for example, are made of steel and rubber. The steel functions as the rigid structure of the seal while the rubber is used as a means of sealing. Of course, the term rubber is generic and in itself can be one of many different materials. Elastomeric materials used in seals can be materials other than rubber, such as Teflon or polyester.

Rotating shaft seals must be designed in such a way as to remain secure in the mount. Not only is it necessary for a seal to seal around a shaft, the outside diameter must seal as well. The contact surfaces of the seal against the shaft must be tight enough that the lubrication cannot escape, yet the seal must be able sustain its structure while there is friction between the rotating shaft and the seal contact area.

Piston seals

Not relevant to this discussion are automotive pistons where the seals are spring loaded metallic rings. Piston seals here refers to components such as hydraulic cylinders and pneumatic cylinders. There are many different cylinder designs in use in a variety of industries. For example, hydraulic cylinders in the automotive industry are brake master cylinders, clutch master cylinders, brake slave cylinders and clutch slave cylinders. In the heavy equipment industry (such as excavation), hydraulic cylinders are used to power back hoes, loader buckets, out riggers and more. Of course, these cylinders can be very small (small enough to hold in the palm of your hand) or very large.

No matter the size or purpose, the functional concept is the same for cylinders. The action by which cylinders do their work is by means of the piston. The piston within the cylinder is forced back and forth by means of fluid pressure on one side of the piston or the other. A rod (or rods) connected to the piston protrudes the cylinder end(s), providing a means of attachment to devices requiring displacement. Seals must therefore be used on a piston to prevent fluid (air, gas, oil, water etc.) from leaking around it otherwise the cylinder would not function correctly or would be weak.

Piston seals in the manner being discussed here are not intended for rotation (although incidental slow/limited rotations may be expected), but rather for sliding. Piston seals may be o-rings, x-rings, lip seals, bands or a combination of types. The particular arrangement depends on factors such as pressure, temperature, and travel speed. Piston seals are designed to close very small gaps. Clearances between pistons and cylinder walls are very small, but there must remain clearance to allow movement of the piston within the cylinder, hence the need for seal(s).

Rod seals

A rod seal can be visualized similar to that of a rotary shaft seal. However, a rod seal is not intended for shaft rotation but rather for the in and out motion of a rod extending from a housing. The rod protruding from a hydraulic cylinder is a perfect example. Another good example is the fork seals on a motorcycle. Typically, the primary purpose of a rod seal is to keep lubrication or fluid pressure from escaping to the outside environment.

Wipers

Wipers come in a variety of configurations and are not limited to applications such as cylinders. Wipers are an important part of most cylinders, but they may be designed for applications such as wiping against a machine surface as one part moves across another. The wiping pushes away any accumulated debris that could otherwise find its way under the sliding surfaces, resulting in damage. This is the same for cylinders and components like them. The wiper cleans the rod surface to protect the components behind it such as a seal, a bushing, a rod etc. A wiper is specifically designed to wipe a surface, and is not necessarily meant to seal in the way an oil seal does.

This image shows a set of fork seals, a dust seal and an oil seal. Each of these functions as a wiper. They are not designed the same as rotary oil seals.

O-rings

As the name implies, o-rings are "O" shaped. A basic o-ring is simply a round cord molded into a circle. They are available in numerous cord sizes, diameters and materials (the most prevalent being neoprene). Following the same concept of o-rings, round rubber cording is sometimes molded into complex shapes other than circular.

O-rings are widely used in many sealing applications, mostly in static applications where there are no moving parts between the sealing surfaces. However, it is not uncommon for o-rings to be used as rod and piston seals in applications such as small cylinders, rod seals, valve stems and more. O-rings generally should not be used in rotary sealing applications.

O-rings are a superior sealing choice when sealing is required between two mating surfaces. They can be advantageous over gaskets. For example, parts with o-ring seals can generally be assembled and disassembled easily with little risk of seal damage. They are also excellent for fluid pressure applications. The disadvantage of o-rings is the need for o-ring grooves. With this type of seal, it is necessary to have a groove to keep the seal in place, otherwise it would wander and deform. In a pressure application, an o-ring without a groove will blow out. Additionally, a groove must be properly sized for the seal to be effective.

Other variants of o-rings exist, such as x-rings and quad rings. As the name implies, the cross section is not round. These variant shapes can transition o-rings into applications where standard o-rings are limited, such as rotating shafts.

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