Dynamic Seals Help Ease Stress in Hydraulic and Pneumatic Systems
by Rebekah Fuller, IQS editor
In the most general terms, the function of any kind of seal is to block the passage of liquids or gases. Rubber seals can be either static or dynamic. A static seal does not move and simply contains pressure or maintains a vacuum. The purpose of dynamic seals, however, is to reciprocate a give-and-take with mechanical motion, like for pistons and cylinders or rotating shafts. Mechanical seals are essential components of hydraulic and pneumatic systems, in which constantly moving mechanisms can be under extreme stress. Along with preventing leakage and protecting against contaminants, the mechanical seal helps maintain pressure levels in high temperature, pressure and speed applications.
Diagram Provided by Allied Metrics Seals & Fasteners, Inc.
Hydraulic seals are used as rod seals and piston seals. Placed in a groove around the outside diameter of the piston, a piston ring works within a reciprocating engine to seal the combustion/expansion chamber, support heat transfer, and regulate oil consumption. Oil seals retain oil, grease and other lubricants while keeping out air, dust, dirt and other contaminants, and a flexible lip design provides a tighter seal. A multiple lip seal can offer excellent response to changes in system pressure, making the seals exceptionally reliable for rods, rams, pistons and plungers.
The sealing direction is instrumental for hydraulic seals and pneumatic seals in mechanisms that use axial and radial movement. Axial means along the axis or centerline of a part or component – not necessarily up and down depending on the mechanism. Radial means perpendicular, or approximately 90 degrees, to the axis, so radial seals are used with rotary mechanisms such as shafts or rotating bores and include strut seals, hydraulic pump seals, axle seals, power steering seals and valve stem seals. Because the action may be single or double, a seal can be used for one or two directions.
Hydraulic seal manufacturers are experienced with many types of elastomeric rubber and poly materials, such as Nitrile Rubber (NBR), Viton®, Silicone, EPDM, polyurethanes and Teflon® (PTFE), including FDA and sanitary grades of all these. They know how the properties of these materials are suited for specific applications. Some examples: Viton® has excellent resistance to heat and aggressive fuels and chemicals; polyurethanes are some of the toughest, most abrasion resistant engineered “rubbers”; AFLAS® has excellent heat, chemical & oil resistance and high electrical resistivity; Hydrogenated Nitrile Butadiene Rubber (HNBR) is strong and withstands exposure to heat, oil and chemicals in automotive, industrial and other demanding applications. Teflon®, or PTFE, has so many desirable qualities, too many to list here, that make it highly versatile for seals and sealing components in chemical plants, petroleum refineries, oil and gas drillers, refrigeration valves to furnace seals, rod and piston seals, wear bands, bushings, and seals used in food & pharmaceutical processing.
Seals can be custom engineered, and product lines include: o-rings, u-cups, wipers, v-rings, bonded seals (metal washer with vulcanized rubber insert used in high pressure situations where other seals just wouldn’t work), x-rings, square rings, wearbands and guide rings.