Dynamic Seal Types
This classification of seals is used in situations involving reciprocating, rotating or oscillating motion. Dynamic seal performance may be substantially affected by a number of operating environmental factors.
Such factors include seal swell in fluids, surface finish of metal parts, lubrication, system pressure, thermal cycling, O-ring squeeze, O-ring stretch, and friction. Since many of these factors are interrelated, it is important to consider ALL of them in dynamic sealing situations.
In discussions of individual dynamic seal types, therefore, mention will be made of the most critical operating environmental factors to consider. More detailed information on Critical Operating Environmental Factors is found in section 5.
Reciprocating Seals
Reciprocating seals, as depicted in Illustration 4.6, are used in situations involving a moving piston and a rod. These seals constitute the predominant dynamic application for O-rings.
For optimum performance of reciprocating seals, careful consideration of the following factors is required:
Compound Selection for Thermal Cycling
Thermal cycling from high (100°F and above) to low (-65°F and below) temperatures may cause O-rings to take a compression set at elevated temperatures and fail to rebound enough at low temperatures to provide a leak-proof seal. Such O-ring leaks are especially prone to occur in low pressure, reciprocating applications. Therefore, when extreme operating thermal cycles are anticipated, it is recommended that you specify a seal compound that exceeds, rather than merely meets, desired temperature range, compression set, and resilience needs.
Control Over Pressure Shocks
With sudden stopping and holding of heavy loads, hydraulic components can create system pressures far in excess of seal extrusion resistance capabilities. To prevent extrusion and eventual O-ring failure, pressure shocks should be anticipated and effectively dealt with in both seal selection and system design. As required, mechanical brakes or pressure relief valves may have to be built into the hydraulic system. The use of back-up rings or increased seal durometer may also be necessary to prevent O-ring extrusion. For more information on the effects of pressure, see Illustration 5.1 in Section 5 .
Squeeze
Listed in Table A1, under "Gland Design" at the beginning of this section, are the recommended squeeze values for O-rings employed in reciprocating situations.
Lower squeeze than that shown in Table A2 will reduce friction, at a cost of possible leakage in low pressure situations. Greater squeeze than that shown will increase friction and sealing capability, at a cost of difficult assembly, faster seal wear, and the increased potential for spiral failure.
Stretch
When the I.D. of an O-ring is stretched, the O-ring's cross section is reduced. In such instances, be sure to consider that the O-ring's reduced cross section maintains the correct percentage of seal squeeze. The percentage of stretch should not exceed 5% in most applications (Stretch Formula)
Rotary Seals
As shown in Illustration 4.7, O-rings may be used as seals for rotating shafts, with the turning shaft protruding through the ID of the O-ring.
The most important factors to consider in designing rotary seal glands are frictional heat buildup, O-ring stretch, squeeze, application temperature limits, and shaft and glandular machining.
Application Temperature Limits
Rotary shaft seals are not recommended for applications with operating temperatures lower than -40°F or higher than +250°F. The closer the application to room temperature, the longer the O-ring can be expected to effectively seal.
Frictional Heat Buildup
As the generation of frictional heat is inevitable with rotary seal applications, it is suggested that O-rings be composed of compounds featuring maximum heat resistance and minimum friction generating properties. Internally lubricated compounds are typically used for rotary applications.
Stretch
As the rotating shaft protrudes through the center hole of the O-ring, it stands to reason that any tightness (stretch) of the O-ring's I.D. to accommodate the shaft will cause the ring to grab the turning shaft and spin, which in turn causes seal failure. In this application I.D. stretch must be eliminated by using shaft diameters no larger than the free state (relaxed) I.D. of the O-ring.
Squeeze
In most rotational shaft applications, O-ring squeeze should be kept to as little as 0.002" by using an O-ring with an O.D. of about 5% larger than the accompanying gland. Once installed, peripheral compression puts the O-ring's I.D. in light contact with the turning shaft. This design minimizes frictional heat buildup and prolongs seal life.
Rotary Seal Gland Dimensions
Table G lists the recommended dimensions for rotary seal glands.
Oscillating Seals
In an oscillating O-ring application, the shaft moves in an arc within the gland, and in contact with the ID of the seal. Because there is a tendency for the shaft to twist, self-lubricated O-rings with a hardness of 80 to 90 durometers are most often employed. Caution should be used, however, with graphite--containing compounds as they tend to pit stainless steel alloys.
Oscillating Seal Gland Dimensions
Oscillating seal gland dimensions are the same as those used for reciprocating applications. (See Table F)