This classification of seals are 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 is found in the Critical Operating Environmental Factors section.
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.
Listed in Table A, under “Gland Design” at the end of this section, are the recommended squeeze values for o-rings employed in reciprocating applications. Lower squeeze than that shown in Table A will reduce friction, but 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.
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.
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 of this guide.