By Colin Macqueen, Trelleborg Sealing Solutions Americas
| The robustness of new sealing technology is proven by using test rigs. This rig is capable of replicating the most demanding service conditions and is able to capture real time data on leakage, friction, temperature generation, and even the pressure behavior between two seals in a tandem system. |
Successful sealing involves containment of fluid within fluid power systems and components — while excluding contaminants. In a typical reciprocating seal, elastomeric materials accommodate dimensional variations caused by manufacturing tolerances, side loads, and cylinder deformations under pressure. System pressure on the seal surface attempts to compress the seal axially. This compression forces the seal more tightly into the gland and helps improve conformability of the seal with its contacting metal surfaces. The resilience of the rubber material creates a tight, durable seal.
In general, higher pressure improves the sealing. The key to high-pressure sealing is the use of a material or a combination of materials that has sufficient tear strength, hardness, and modulus to prevent extrusion through any gap. At higher pressures, the elastomeric sealing element must be backed by a material with higher stiffness.
As pressures reach 20,000 psi, the extrusion gap must be closed, and the elastomeric seal must be protected by a sequence of progressively harder, higher-modulus materials. Properly designed, this progression of materials prevents extrusion, tearing, cutting, or other destructive deformation of the elastomeric seal and distributes loads more uniformly to the element that bridges the gap.
Elastomeric compounds used in seals are derived from certain base polymers, such as polyurethane, nitrile, fluoroelastomer, and ethylene propylene. Seal manufacturers develop their own special compounds of these base polymers to enhance or suppress different chemical or physical properties to fit specific requirements of an application.
Of all elastomer compound properties, the most critical are how they change. Every property of every compound changes with age, temperature, fluid, pressure, and other factors. Compounds with the least tendency to change properties are the easiest to work with; they produce a seal that is adaptable to more applications. The number of properties evaluated for an application depends on the severity of conditions. Factors typically include resilience and memory, abrasion resistance, coefficient of friction, and fluid compatibility.
Resilience and memory refer to a compound’s ability to return to original shape and dimensions after a deforming force is removed. Resilience implies a rapid return, while memory implies a slow return. In seals, resilience is important because it permits a dynamic seal to follow variations in the sealing surface. Additional attention is required for lowtemperature applications.
Abrasion resistance, the resistance to wear when in contact with a moving surface, is the product of other properties. These include resilience, hardness, thermal stability, fluid compatibility, and tear/cut resistance. It also is influenced by the compound’s ability to hold a film of protective lubricant on its surface. Harder compounds are usually more resistant to wear, so dynamic seals of 85- durometer compounds are common. If the seals encounter high temperatures, it may be good practice to specify an even harder material to compensate for the softening effect of heat. In low-temperature applications, a softer material might be preferred because elastomers tend to harden as temperatures drop.
Coefficient of friction is compound- specific and different for running and break-out. Usually, break-out friction is higher. Break-out friction increases with time between cycles. The coefficient of friction is affected by temperature, lubrication, and surface finish. Aging and the influence of service fluids on the compounds may also affect hardness and, in return, both breakout and running friction. Dynamic contact between elastomer seals and stationary hardware should be avoided where critical and a low friction material such at PTFE should be selected (see box on page 40).
As far as fluid compatibility is concerned, a fluid is considered incompatible with a compound if the fluid causes enough property changes to reduce sealing function and/or shorten the working life of the compound. Dissimilar chemical structure is the key to fluid compatibility. For non-polar liquids — such as hydrocarbon fuels and oils — nitriles, fluorocarbons, or fluorosilicone polymers are normally used. For polar liquids, such as phosphate ester hydraulic fluids, ethylene propylene compounds are most satisfactory.
Many other materials can be suitable for high-pressure applications. Often, the choice of seal materials is dictated by the fluid medium, system operating temperatures, cost, or system pressure. The potentially higher efficiency of high-pressure systems comes at a slight cost premium. Sealing materials for high pressures are more expensive, and seal designs often are more complicated. Higher sealing pressures increase sealing force and friction. Increased friction causes higher wear rates and may require more frequent seal replacement, but frictional force and wear rates typically increase more slowly than pressure.