In an oil seal environment there is a shaft rotating or sliding inside a housing or bore. To alleviate friction there must be some permissible clearance between the bore and said rotational shaft, the function of the oil seal is to prevent the internal oil escaping between the shaft and its housing, while ensuring external protection from dirt, water, etc. The most common type of oil seals on motorcycle are rotary oil seals – also called shaft seals or lip seal. The selection of the most suitable material, seal configuration, working pressure and temperature resistance for the seal's working environment has a critical impact on its sealing performance and longevity.
Most oil seals consist of three components, the metal case, the sealing element (elastomer) and a garter spring. The metal case gives rigidity and strength while the garter spring provides a constant uniform load to the seal lip on its rotating shaft.
Static seal. This is the relationship between the housing/bore and the outside diameter of the seal. To create a positive seal, the outside diameter of the seal must be slightly larger than the housing/bore.
Dynamic seal. This is the relationship between the rotating shaft and the inside diameter of the sealing element, a garter spring is normally used to increase the radial interference of the seal lip and the point of contact on the shaft.
Oil Seal ApplicationsOil seals are widely used in our motorcycles as bearing seals and rotary seals for fast rotary motion under low pressures, the normal maximum working pressure being seven psi, higher working pressures with conventional seals will force the seal lip against its rotating shaft creating excessive friction and wear to both components. Shafts that have suffered damage in this way or have grooves worn in them created by foreign matter or carbonized oil on the lip seal, can be successful reclaimed by installing a seal designed with a shorter seal lip to run on the unworn part on the shaft therefore avoiding costly shaft repairs.
Grease seals and seals intended for use with very viscous fluids are often of springless construction and should only be used in none pressure applications, although they will normally provide adequate service up to a maximum working pressure of three psi. Medium and high pressure seals are also available but should only be fitted when applications warrant their use.
Oil Seal MaterialsIt's of the utmost importance to use a material that is compatible with the environment in which the seal is operating. The choice is relatively simple for four-stroke engines, as the main consideration is to install a seal that is compatible with engine oils. However the choice for two-stroke engines is more critical, especially with the introduction of ethanol in our fuels. This now means that in particular two-stroke engines must utilize rotary oil seals resistant to petroleum, oil and ethanol.
Most of our older two-strokes will have seals made from Nitrile (Nitrile Butadiene Rubber) which is by far the most commonly used oil seal material in general use.
Sealing MaterialsLeather. The earliest form of sealing material, which has been around as long as cows and is still available today, leather is fairly resistant to the effects of alcohol and petroleum products as opposed to some the early forms of rubber, however it is not impervious to wear, tear or heat. The development of rubber products with their ability to retain shape in arduous condition has made leather seals almost redundant. Don't expect to find a leather oil seal in your modern motorcycle.
Nitrile Butadiene Rubber. Nitrile is the most commonly used elastomer (rubber). Nitrile oil seals offer excellent resistance to petroleum based oils fuels, silicon grease, hydraulic fluid, water and alcohols, although seals used in alcohol and water environments mush have stainless steel garter springs for longevity. Nitrile has a good balance of working properties, in particular low compression set, high abrasion resistance and high tensile strength, combined with low manufacturing costs. Normal working temperature range -40c to +135c degree. Not recommended for use with brake fluid and ketones.
Viton. A brand of synthetic rubber fluoropolymer elastomer. The name is the registered trademark of Dupont performance elastomers. Depending on grade it offers excellent resistance to petroleum products and solvents, good high temperature resistance and low compression set characteristics, for use with a wide range of chemical exposure situations, it offers high resistance to permeation and chemical attact by alcohol, ethanol and blends of bio fuel containing ethanol. Normal working temperature range -40c to +204c degree. Not recommended for products containing acetone.
Other materials used to make oil seals include highly saturated Nitrile, Teflon, Silicone and polyacrylate.
Fitting a Oil SealCorrect fitting of oil seals is important to their performance and longevity. An ill-fitting oil seal can cause damage to bearings or rotating components. Seals should be installed in clean surroundings, keep the seal wrapped until required. The shaft must be clean and any burrs removed, apply a thin coating of oil to the shaft, then to the sealing lip. Take care to install the seal at right angles to the shaft using a bell tool or suitable installation tool to distribute the load evenly when pressing the seal into position, ensuring that the lip is not damaged. If the shaft has a square shoulder a special tapered sleeve should be utilized so that the seal lip can eased over the square edge without damage to the seal lip. When a seal is fitted with the open-side outwards it is sometimes advantageous to carefully remove garter spring in order to position the seal properly then refit the spring after installation.
Persistent leakage is very seldom the fault of the seal itself, the problem is usually due to careless fitting, the wrong seal configuration, wrong material choice or defects on the shaft or housing.
Though relatively cheap, their importance cannot be understated as they play a major part in ensuring satisfactory performance. Remember, the Challenger Space Shuttle disaster was down to incorrect material choice used in one of its seals. So you see, they're a lot more than just black rubber round things!