The surface can either be static or dynamic, in motion or without movement. Static seals are usually used when there is no relative motion between mating surfaces. Dynamic seals are the opposite. They are used when there is a motion between surfaces.

Hydraulic seals can be made from a variety of materials such as polyurethane, rubber or PTFE. The type of material is determined by the specific operating conditions or limits due to fluid type, pressure, fluid chemical compatibility or temperature.

1. Oil and Grease 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. The seal helps to prevent lubricant from escaping and harmful contaminants from entering the machine. They are a critical component in almost every type of machine and vehicle.

2. Hydraulic and Pneumatic Seals

   Hydraulic and pneumatic seals are designed for devices that provide reciprocating motion. They can be used in high pressure, dynamic applications to restrict the leakage of fluid and the entry of foreign materials. Hydraulic seals are important in converting fluid power to linear motion. Pneumatic seals have a similar design but typically operate in air or other gaseous mediums. This category includes piston seals, rod seals, U-cups, and flange packages.

3. Exclusion seals

   Exclusion seals are dynamic seals such as wipers and scrapers that support sliding or reciprocating motion. They clean the surface by scraping abrasive particles such as dirt, mud, and ice.