Optimum sealing performance has proven troublesome in certain sealing environments. Therefore, for the following O-ring applications, specific elastomers and actions are recommended.
In some cases, a variety of elastomers may be "acceptable" in a given application. The final material choice may be guided by the secondary operating conditions of the systems; or in the case of "equal" performance, by considerations of cost and availability. We are using the O-ring as an example, but much of this information applies to other seal types.
Standard methods employed for minimizing the effects of O-ring friction include reducing seal squeeze; increasing compound hardness; specifying a low friction compound, such as Teflon®; surface treatment with a low friction coating; and reducing the O-ring's cross section to reduce the amount of contact area (being conscious of avoiding spiral failure).
The use of internally lubricated compounds has proven especially effective in applications requiring low friction performance without the reduction of squeeze.
To date, homogeneously dispersed lubrication in the form of Erucanides (natural fatty acids), Teflon™, Paraffin waxes, petroleum and molybdenum disulfide have been sucessfully incorporated into Ethylene Propylene, Nitrile, Neoprene™, Fluorocarbon and Silicone.
Surface treatment of O-rings with lubricants helps to protect against abrasion, pinching, or cutting during installation in parts assembly. External lubrication helps seat the O-rings into grooves with minimum twisting and maximum assembly speed.
In hydraulic systems where lubricating fluids are nearly always present, surface treatment of O-rings is less essential.
IN PNEUMATIC OR VACUUM APPLICATIONS WHERE SYSTEM FLUIDS ARE PREDOMINANTLY ABSENT, O-RING SURFACE LUBRICATION IS MANDATORY
FOR EFFECTIVE, LOW FRICTION OPERATION AND THE PREVENTION OF SEAL LEAKAGE. O-ring surface lubricants help prevent leakage from around the seal by filling the micropores of both the O-ring and surrounding metal surfaces.
A final benefit of O-ring lubrication is the protection it offers some elastomers from the degrading effects of exposure to oxygen and ozone. In this regard, O-ring surface lubrication acts as a barrier, helping to prevent premature seal aging and extending O-ring service life.
PRECAUTIONARY NOTES: In ALL cases requiring O-ring lubrication, make certain to select a lubricant that is compatible with both the O-ring compound and the system chemicals being used. The lubricant, or additive which it contains, SHOULD NOT cause excessive shrinkage or swelling of the O-ring compound.
Also, check the recommended temperature range for the lubricant of choice, making certain to operate within stated limits.
Finally, if system filtration is being employed, check to see that the lubricant is capable of passing through filters prior to use with the system O-rings.
As a general guide, Table H (O-ring Lubricant Guide) at the end of this section, lists a number of lubricants used with specific O-ring compounds, in the application shown.