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While small in cost, seals are often one of the most important components in any product. A seal must be carefully designed and produced to ensure superior performance of the product in which they are used. This section provides the technical data necessary for proper seal design and selection, including how to determine groove dimensions, cross sectional squeeze and other necessary criteria.

All sealing applications fall into one of two categories - those in which the seal or sealed surface moves, and those in which the seal is stationary:

A seal that does not move, except for pulsations caused by cycle pressure, is called a static seal. Examples include the face seal in an end cap, seals in a split connector, and seals between two stationary members.

Dynamic seals are those that are subjected to movement. These are further defined as rotary (stationary seals exposed to a rotating shaft) or reciprocating (seals exposed to linear motion). Rotary seals and reciprocating seals require different design, dimensioning and material selection for proper function.

Factors to Consider in Seal Selection

Proper seal design begins with careful consideration of the sealing application. Appropriate material hardness, for example, is determined by the friction and pressure to which the seal will be exposed, as well as the cross-sectional dimensions of the seal. Other key factors include temperature range, adjacent surfaces, and media. (Keep in mind that the environment may differ from one side of the seal to the other.) Established standards may also dictate which materials may be used.

Friction

The functional life of a seal is determined primarily by the level of friction to which it is exposed. Key factors contributing to friction include rubber hardness (the standard compound for most seal applications is 70 durometer Shore A hardness), surface finish, temperature extremes, high pressure and the amount of squeeze placed on the seal.

The use of "slippery rubber" compounds can help lessen friction and improve seal life. Surface coatings and seal treatments such as PTFE and molybdenum disulfide are also used to reduce seal friction.

Please contact us early in the design process. Given the number of variables involved, it is impossible to accurately calculate seal friction in terms of force-per-area. By the same token, there are no quick and easy "standard rules" for specifying the appropriate seal material or coating solution. The close working relationship that exists between our design engineers and staff chemists means we can provide you with the optimum in part design and material selection.

Surface Finish

Shorter than expected seal life is generally the result of too fine a finish on either the rod or the cylinder bore. A highly polished (non-porous) metal surface does not retain the lubricant necessary to control friction, whereas a rough or jagged surface will abrade the seal and lead to early seal failure.

In order to avoid these problems, we recommend an ideal surface finish of 20-24 RMS (.5-.6 +/-m), with an acceptable range of 20-32 RMS (.5-.81 +/-m). The surface finish should never be finer than 16 RMS (.4 +/-m). We further recommend the use of a finishing tool called a Flex Hone, from Brush Research Manufacturing Co., Los Angeles, CA. The Flex Hone creates a cross-hatched finish that provides the ideal combination of smoothness and lubricity.

Roller-burnished and mandrel-drawn finishing, on the other hand, should be avoided wherever possible. While these methods assure a very true diameter, they also result in a surface finish well beyond the recommended range.

Roller burnishing can be employed, however, prior to flex honing to ensure maximum dimensional accuracy and an optimal sealing surface.

Ease of Installation

Many seals are easily damaged during installation. For example, a seal is often inserted onto a shaft by sliding it over a threaded surface. You can avoid seal damage here by reducing the rod diameter in the threaded region. Also, include an entering chamber for the seal, and avoid sharp corners on grooves. Proper groove design is discussed in more detail beginning in section 4-11.

 

 

Recommended Diametrical Clearances

This chart shows recommended maximum total diametrical clearances. Less clearance is always desirable (i.e., move to the left on the chart whenever possible).

Assumptions:

  1. For seal cross-sections of .139 (3.53mm) and larger. Smaller cross-sections require tighter tolerances.
  2. Piston and bore are concentric. For severe side load or eccentric movement, you must reduce the amount of clearance.
  3. For seals without anti-extrusion backups under moderate temperature conditions. Soft rubber compounds under higher temperature conditions would call for smaller clearances than those shown on the chart.
  4. Total diametrical clearance includes cylinder expansion due to pressure.

Seal Extrusion

The o-ring pictured here failed when it was extruded from the groove. Extrusion is a common source of seal failure in both static and dynamic applications. Part or all of the seal is forced from the groove by high continuous or pulsating pressure on the seal. If left uncorrected, the entire cross-section will eventually disintegrate.

The risk of seal extrusion can be minimized by following these simple rules:

  1. Choose a seal configuration and material designed to withstand the anticipated pressure.
  2. Make sure the amount of clearance between adjacent surfaces is appropriate to the hardness of the material. Clearance must not exceed recommended limits for the compound.

 


Information Provided by Minnesota Rubber/Quadion Corporation, copyright 2002
Product names are Registered trademarks of Minnesota Rubber, A Quadion Company.