Also known as oil seals and radial lip seals, shaft seals are widely used in conjunction with rotating, reciprocating, and oscillating shafts to contain fluids and to exclude contaminants. In some applications, shaft seals are designed to contain pressure or to separate fluids. Shaft seals have several key strengths: They are economical, easy to install, and effective in a wide range of environments.

There are many factors to carefully consider as you select or design a shaft seal for a specific application. This Shaft Seal Design & Materials Guide provides detailed information on the many factors that influence the design of an effective shaft seal.

PURPOSE OF ANY SEAL
Any mechanical assembly containing fluids must be designed so that these substances flow only where intended and do not leak out of the assembly. 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 a seal is to block the clearance gap so that nothing passes through it.

EVOLUTION OF SHAFT SEALS
Consumer needs drive the development of most products, and shaft seals are no exception. Technological advances and the ever-more-demanding needs of end-users have spurred the development of increasingly sophisticated radial lip seals over the past century. In actuality, the first “shaft seals” (such as those found on the axles of low-speed frontier wagons) were nothing more than leather strips attempting (typically with very limited success) to contain the animal fat that served as lubrication.

As time wore on and industry revolutionized society, motorized vehicles replaced wagons, and leather strips were replaced by rope packings made of flax, cotton, and hemp (see Figure 1). Though still relatively crude, such packings worked because lubricants tended to be very viscous (thick), operating speeds were still low, and temperatures never got high enough to degrade the lubricants or seal materials.

As the 1920s arrived, application speeds and temperatures further increased. Thinner, more environmentally unfriendly lubricants became common, and sealing them adequately became more difficult. Rope packings were superseded by assembled leather seals (see Figure 2). A leather lip was chemically treated to improve oil resistance, then clamped into a metallic case to facilitate installation and removal. The metal case allowed for a pressfit seal to prevent bore leakage, and the leather lip rode a region of the shaft that had been ground to a prescribed roughness.

Technical improvements to machinery, vehicles, and road surfaces caused shaft speeds and application temperatures to increase. New oils were developed to withstand these higher temperatures. Unfortunately, these higher temperatures and the new lubricants caused swelling and degradation of leather sealing lips. These difficulties were overcome in the 1940s with the development of oil-resistant polymers. Assembled synthetic rubber seals featuring lips made of nitrile (NBR) rather than leather became the norm (see Figure 3).

By the 1950s, technology allowed for the chemical bonding of rubber to metals. This made possible a seal in which the rubber lip was chemically bonded to the case (rather than clamped in place). Seals of the 1960s began to feature lips made of materials other than nitrile. Silicone and polyacrylate materials were developed and used for bonded seals (see Figure 4). Polytetrafluoroethylene (PTFE) has great chemical and temperature resistance in combination with good low frictional properties. As a result, PTFE was used to replace leather and NBR materials in assembled lip seals. Methods of bonding PTFE to rubber or metal did not exist at this time. Fluoroelastomer also started being used in the 1970s. Though all of these “alternative” materials could be useful, they were also more expensive than nitrile, so seal designers sought ways to minimize material usage and reduce costs. This resulted in the production of seals with reduced bonding areas. An example of this is shown in Figure 5.

Seal designers also began to look beyond the seal for ways to further improve performance and to extend reliability. They turned their attention to the sealing surface itself, and by the 1980s, seals that incorporated running surfaces into their designs became common. These unitized (or cassette) seals (see Figure 6 for one example) took some of the worry away from the end user, who no longer had to be concerned about preparation of a proper running surface on the shaft. Should a shaft surface become damaged (for example, severely scratched) during service, replacement of a standard shaft seal with a unitized seal can often prevent (or postpone) the costly alternative of shutting down the application for either re-machining or replacement of the shaft.

Thanks to the development of improved cements, composite seals featuring PTFE bonded to rubber also became possible. An example of a composite seal is shown in Figure 7.

The most recent evolution of seal design has come about in the last decade. Seal designers are now combining the seal with other components from the sealing area (such as filters, reinforcing inserts, and excluders). The resulting value-added seals (such as the one shown in Figure 8) make life easier for the user by reducing the number of components and thus simplifying both purchasing and assembly.