REDUCING O-RING FRICTION
The need to reduce O-ring friction and wear is neither new nor unique. Looking for ways to minimize breakout and running friction (as well as the wear that results from both) is a major factor in many product designs. This is especially true for pneumatic tools, plumbing devices, hydraulic valves and cylinders, or any application requiring manual operation.
Generally speaking, friction is reduced in two main ways. The first method involves the application of an external surface treatment, such as a molybdenum disulfide (MoS2) coating. Because "moly" coatings rub off, however, they don’t last much beyond installation. Baked-on PTFE coatings also wear away quickly and can cause contamination. The most permanent surface treatment is chlorination, in which the O-ring elastomer (typically nitrile) is subjected to concentrated amounts of chlorine gas. As a side effect of chlorination, the O-ring surface develops numerous minute cracks. These tiny cracks are actually advantageous in that they act as reservoirs for secondary external lubricants, such as oils and waxes. The additional step of treating the seal’s surface with oil or wax facilitates installation and helps reduce breakout friction (otherwise known as static friction, or stiction).
Stiction is also reduced via the second method of friction reduction: internal lubrication. Internally-lubricated compounds have a friction-reducing agent dispersed directly into their chemical structure. The lubricant is added when the elastomeric compound is initially mixed. Though the lubricant does alter the compound, the elastomer’s basic properties remain largely unchanged. An internally-lubricated nitrile is still nitrile; it is simply a special formulation designed to minimize friction. It may help you to think of internal lubrication as the end result of a planned incompatibility. By design, the added friction-reducing agent will not be chemically compatible with the base elastomer. This conflict means that the agent will separate itself out and "bloom" up onto the O-ring’s surface. Continual blooming of the agent keeps the seal’s exterior coated with lubricant, making the O-ring slippery and less inclined to stick during start-up.
AGENTS FOR CHANGE
It is worth noting here that lubrication can be either organic or inorganic. Widely-used organic lubricants include amides (in both flake and pellet form), waxes, esters, powdered PTFE, and mineral oils. Inorganic agents include graphite (powdered and flake) and MoS2. Whether organic or inorganic, the lubricant in use must be compatible with system fluids to avoid leaching (removal) of the agent, which can lead to dangerous degrees of seal shrinkage. The lubricant must also be compatible with all adjacent surfaces to avoid structural damage. The chart (above) shows the results of both abrasion and friction testing conducted on hydrogenated nitrile samples containing a number of widely-used lubricants. As you can see, internal lubrication using organic oleamides produces an outstanding blend of abrasion and friction properties.