Almost every hydraulic system, however, will face occasions when the equipment is not operating, and the pressure falls to zero. Or, in some applications, the system's pressure may never exceed 100 psi. These are typical of the types of operations defined as low pressure; that is, when the confined fluid media exert little or no pressure force on the sealing element to affect or augment a seal.

fig. 7. simple representation of how squeeze force compresses a combination gasket/o-ring seal during gland assembly.Within the framework of low-pressure sealing, several primary design considerations affect sealability:
seal squeeze,
compression set,
sealing force,
gland surface finish conditions, and
molding flash.

Squeeze

A seal component is generally installed in a groove machined into one of the surfaces to be sealed. As the two surfaces are brought together to form a gland, they squeeze the diametral cross section of the seal. The mechanical squeezing action deforms the seal cross section; the degree of deformation obviously is a function of the squeezing force. In low-pressure applications, the tendency of the squeezed elastomer to maintain its original shape creates a seal. As the elastomer shape is deformed in its gland, it exerts a counteracting (reaction) force against the mating surfaces equal to the force squeezing it, Figure 7, and hence, provides the available sealing force.

Thus, squeeze is a major low-pressure consideration. The recommended squeeze levels are a function of seal cross section, the application conditions and whether the application is dynamic or static.Dynamic compression typically is lower than static compression, due to seal wear and friction considerations. Table 1 summarizes dynamic squeeze levels as defined by MIL-G-5514F — a document that serves as a good guide to those parameters. Static data in the table are summarized from common industrial practice.

Compression set

Compression set reflects the partial loss of memory due to the time effect. In hydraulic systems operating over extreme temperature ranges, it is not uncommon for compression-type seals, such as O-rings, to leak fluid at low pressure because they have deformed permanently or taken a set after used for a period of time. The term compression set refers to the permanent deflection remaining in the seal after complete release of a squeezing load while exposed to a particular temperature level. As related to low-pressure sealing, set-the loss of memory-reduces the compressive sealing force.

Compression set is expressed as a fraction of the initial squeeze. Thus, a 0% compression set value indicates complete recovery from a compressive load, producing the maximum possible compressive sealing force. A 100% set value indicates no recovery or rebound at all. A seal in this condition will no longer provide a sealing force and hence, has no ability to act as a low-pressure seal. The bar graph in Figure 8 depicts the range of typical compression set values for various sealing elastomers. Of course, compression set properties are a major but not the only factor affecting elastomer choice for low-pressure sealing. Compatibility with various hydraulic fluids must be considered as well.

Figure 8. percentage compression set exhibited by typical families of sealing elastomers used in fluid power systems.Sealing force

There are several factors affecting the sealing force:
material hardness,
percentage squeeze, and
seal cross-section size.

For a certain amount of squeeze, the sealing force is directly related to the hardness or elastic moduli of seal materials for low-pressure applications. The harder the material, the larger the initial sealing force. A seal material has a nonlinear stress-strain curve and needs to be described by special material models. For simplicity, linear moduli, such as Young's modulus and shear modulus, are usually used due to their direct relations to the material hardness. The modulus commonly used for specification purpose is tensile stress at a specified elongation. For example, modulus at 100% elongation is the tensile stress corresponding to that elongation.

Hardness generally is measured with a durometer gage - typically using the Shore A scale. The gage measures the force required to deflect the flat surface of a rubber specimen with a pointed indicator. The A scale ranges from 0 to 100; a 90 Shore A compound would be designated as a hard (or high-viscosity) material, and would exhibit much higher compressive force than a 60 Shore A compound, which would be classified as soft.

For a specific material, seal compression force of the elastomeric material increases as the percentage deflection of the seal's diametral cross- section increases. Dynamic squeeze levels typically should be limited to around 12% due to friction and related-wear considerations. Static squeeze levels can be as high as 30%.

fig. 9. plots of compressive load vs. o-ring seal cross-section for three different seal material hardnesses.It generally is recommended that a minimum of 0.009-in. squeeze be induced on radial seal cross sections due to compression set considerations. Maximum radial squeeze should be held to 30% because greater squeeze causes assembly difficulties and elastomer deterioration. Compressive sealing load is also directly related to the size of seal's cross-section, Figure 9.

Gland surface finishes

Two physical characteristics of the seal contact-band areas can affect how well the available sealing force is transmitted. These are:
parting line projection and flash on the seal, and
sealing surface finishes in the gland.

The finish on machined surfaces that come into contact with the seal is a significant factor in achieving optimum seal performance. Finishes can be defined by different systems, which are often misunderstood and sometimes incorrectly specified in hydraulic design. The American Standard Association provides a set of terms and symbols to define basic surface characteristics, such as profile, roughness, waviness, flaws, and lays.

Roughness is the most commonly specified characteristic and is usually expressed in units of µin. Roughness provides a measure of the deviation of the surface irregularities from an average plane through the surface. In most cases, geometric average roughness or root mean square (RMS) is the preferred method. RMS measurement is sensitive to occasional peaks and valleys over a given sample length.

As related to low-pressure sealing, the sealing element must penetrate these micro imperfections and irregularities in order to block the passage of the fluid media across the contact band area. It is generally accepted and recommended that dynamic interfaces should not exceed RMS values of 16 µin. or 0.4 µm. Static interfaces should not exceed RMS values of 32 µin. or 0.8 µm. Special fluid media would benefit from smoother finishes as listed in Table 2.