There are many types of transmissions in power transmission that are filled with grease or oil. For example, a gearbox filled with oil to lubricate the gears and bearings or a hydraulic cylinder filled with oil under high pressure to implement a movement. Seals are required to ensure that the oil or grease stays in the unit. Shaft seals are also used to keep out dirt and water.
There is a wide variety of seals, not only in size but also in materials and types. Many aspects are important in selecting the right seal, such as pressure, temperature, rotary or linear motion, speed, type of oil, radiation, shaft material, etc.