Dynamic Seals

Dynamic seals retain or separate fluids, keep out contaminants, and contain pressure. They create a barrier between moving and stationary surfaces in applications such as rotating shafts and pistons rings. Important dimensions for dynamic seals include seal inside diameter (ID), shaft outside diameter (OD), housing bore, axial cross-section, and radial cross-section. Service limits include maximum operating speed and maximum operating pressure. Sealing orientation and direction vary widely. Internal seals fit into a housing bore with the sealing lip in contact with the shaft. By contrast, external seals fit onto the shaft so that the sealing lip contacts the housing bore. Symmetric seals are suitable substitutes for both internal rod seals and external piston seals. Axial seals are used with housing or machine components. For proper sealing, the seal lip on a dynamic seal needs to point towards the medium being contained.

There are many types of dynamic seals. Oil and grease seals have a flexible lip that rubs against a shaft or housing to prevent the leakage of fluids. Some oil and grease seals include a spring to help keep the lip in contact with the shaft. Hydraulic and pneumatic seals are designed for devices that provide reciprocating motion. This category includes piston seals, rod seals, U-cups, and flange packages. Exclusion seals are dynamic seals such as wipers and scrapers that support sliding or reciprocating motion. Clearance or labyrinth seals provide positive clearance and do not rub against the shaft or housing. Many clearance seals consist of a rotor and a stator. For proper sealing, the radial gap between these members must exceed the clearance of the bearing being sealed. Clearance seals, which do not perform well at low operating speeds, include laminar products that contain a series of flat metal washer-like rings of varying diameters. Other ring seals include piston and backup rings. Metallic piston rings are used to seal cylinders and have a higher working temperature than elastomeric, fabric, or polymer materials. Backup rings are washer-like devices that are installed in the downstream side of the gland and are used to prevent a seal from extruding while under pressure. Bearing isolators, another type of dynamic seal, can feature either a labyrinth or O-ring construction.

Dynamic seals are made of natural and synthetic rubbers, polymers and elastomers, metallic compounds, and specialty materials. Ethylene propylene diene monomer (EPDM), a high-density synthetic rubber, provides superior durability for dynamic seals. Commonly used polymers and elastomers include fluoroelastomer, fluorosilicone, nitrile, polyamide, polyacrylate, polyetheretherketone, polyoxymethylene, urethane, polyurethane, and polytetrafluoroethylene. Metallic seals are made of sintered bronze, cast iron, or stainless steel. Sintered bronze is not suitable for heavily loaded applications, but useful when lubrication is inconvenient. Cast iron is a family of materials made mainly of iron, but with important trace amounts of carbon and silicon. Stainless steel is chemical and corrosion resistant and can have relatively high-pressure ratings. Specialty materials for dynamic seals include leather and felt. Felt seals are used primarily with heavy lubricants. The operating temperature range for felt is - 65° F to 200° F (- 54° C to 93° C).

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