Shaft seals are simple devices that prevent the passage of fluids along a rotating shaft; they are necessary when a shaft extends from a housing that contains fluid. They are mostly seen in hydraulic cylinders, which produce linear motion by using pressurized hydraulic fluid. They are found in pumps and gear boxes, which provide the speed and torque from a rotating power source to another device.
Shaft seals are made of a soft, elastomeric material like rubber in ring form that is bonded to another ring made of a metallic material. These devices provide an effective air and water tight seal in all hydraulic cylinders, which are integral components to any hydraulic system within vehicles and machinery like cars, construction equipment that have multiple moving parts, forestry vehicles, agricultural machinery, military and commercial aircraft, marine vessels, plastic injection molding machinery and hydraulic presses. Shaft seals are radial and include strut seals, hydraulic pump seals, axle seals, power steering seals and valve stem seals, although those in hydraulic systems are the most common type. They must be manufactured from materials that are resistant to a variety of temperatures, chemicals, oil and moisture, since they come in continuous direct contact with the hydraulic fluid.
All shaft seals are considered rotary seals and require critical tolerances and tight precision. Their design is more complex than other types of seals that contain liquids and are composed of an elastomeric ring bonded with a metallic ring that is pressed into a groove in the housing through which the shaft extends. The sealing is done by a lip on the elastomeric ring of the seal that is pressed snugly around the shaft while still allowing it to move back and fourth without letting contaminants in or hydraulic fluids out. They also prevent metal-to-metal contact, since the pump's shaft and housing are usually made of metals like aluminum, bronze, steel and copper. Shaft seals, no matter what type of shaft they are sealing, are very important components in a pump system. Without them, the fluid would leak to the outside and cause major system problems. Although they last for a long time, regular maintenance that includes checking on the seal's wear and aging can prevent seal failure. Many hydraulic systems are equipped with a backup hydraulic seal, which also helps to protect the system.
