Any hydraulic system requires seals to prevent fluid from escaping and causing a leak. Hydraulic seals can be classified into two general categories based on their application: static seals, used where no motion occurs between the parts, and dynamic seals, used to seal moving parts. Dynamic seals prevent or control leakage between surfaces that move past each other. Since these seals contact moving surfaces, they will eventually wear out or fail. Three types of dynamic seals commonly used are lip seals, face seals, and packings. The mechanical face seal is one of the most effective devices in preventing leakage along a rotating shaft which passes in or out of an area of pressurized oil. Two ultra-flat sealing faces are mounted perpendicular to the shaft. A gasket is an installation-activated seal made of relatively soft material. It must be deformed or compressed to fill surface irregularities and close the gasket structure to fluid leakage.