What are Metric Oil Seals?

Oil seals are often called grease, fluid, or dirt seals. These seals close spaces between stationary and moving components in mechanical equipment. Oil seals are designed to prevent the escape of lubricant. They also block contaminants from entering machinery. This is especially important in severe environments. They also prevent the mixing of different mediums like lubricating oil and water.

What is an Oil Seal?

Oil seals come cataloged as metric oil seals or imperial sized oil seals. The seals can also be custom-made to match the bearings in new types of machinery. They are installed in practically every type of machine including vehicles, protecting all kinds of precision, precisely-fitted ball bearings, sleeve bearings and roller bearings.

The oil seal gets its structural stability from an interior metal ring which serves as an inner skeleton. The outer skin is made of a more flexible material like nitrile rubber or other materials based on the physical environment of the seal. A spring on the lip of the seal supports the lip and keeps the lubricant from leaking. The lip construction is what blocks contaminants from outside.

Where loads are light, the outer skin layer can be made of silicone. It can be made of Fluroelastomer (orViton) to protect under high temperatures (more than 120 degrees Celcius). The skin can also be made of Poly Acrylate or Polytetra-FluroEthylene.

The shaft on which the oil seal is mounted has to be ground to a rough surface finish. The shaft also should be hardened to prevent grooves forming on the shaft when the pressure is exerted by the spring on the seal. The area where the seal is fitted also has to be ground to prevent grooves that tend to wear out the lip of the oil seal faster than normal.

Oil seals have a flexible lip that actually rubs against the rotating shaft or housing to prevent leakage. The spring keeps the lip in contact with the shaft. Bearing isolator oil seals are dynamic seals that incorporate a rotor or rotating member and a stator or stationary member. The rotor actually turns with the shaft. Some oil shafts are bearing isolators with a "labyrinth" construction. Others incorporate simpler O-rings.

Types of oil seals include:

• single and double lip 
• metal cased 
• rubber or polymer
• internal, external and axial orientation
• spring loaded

A patent for an oil seal was filed by Nelson Thomas Edward on August 12, 1937 and published a year later. There were two purposes described. The device was to provide an oil seal between a fixed housing and a rotating part. The seal is described as

made of a yieldable resilient material having flat sealing faces and comprising two ring portions connected together by an annular portion substantially v-shaped in cross-section which may yield or expand as the mounting requires.

The oil seal would seal one face against a rotating member and the other face to a stationary housing. It would prevent dirt from coming between the hub or shaft and the seal.

Old School O-Rings

Early engines used O-rings (also called packing rings or toric joints) as seals (first patented in 1896). These are just mechanical gaskets in the shape of a torus (a circular ring--like a life saver), seated in a groove and compressed during assembly between two or more parts. It creates a seal at the interface. However, O-rings require a fluid film to lubricate them. They have limited usefulness in vacuum application and at extremes of temperature. The modern oil seal represents a significant improvement over the simple O-ring because it effectively seals in lubrication and prevents contamination from outside under a wide range of pressures and temperatures.

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