Also known shaft seals, the oil seals are devices widely used to contain fluids, to exclude pollutants and in some cases to conquer pressure or to separate fluids.
Any mechanical assembly that contains fluids must be designed in such a way that these substances flow only where it is necessary and do not escape out of the assembly. The oil seals are incorporated in mechanical designs to avoid such a leak in the points where different partsof the assembly come together. These points of meeting are known as surfaces of contact, and the space betweenthem is called a space of roominess. The intention of an oil seal is to block the space of roominess so that nothing passes across it.
Sealing Lip: The most important characteristic of an oil sealis the sealing lip in elastomer. To assure the contact between the lip and the axle, the lip must present always an inside diameter smaller than the diameter of the axle. The difference between the diameter of the axle and the inside diameter of the lip is known as interference. To increase the interference (for example modifying the length of the spring) increases the quantity of force on the axle, increasing this way the friction and the wear. To diminish the interference (for example, enlarging the inside diameter of the lip) it reduces the force on the axle but also it reduces the capacity of the lip to followaxles in movement.
Metal Case: Anyone that is the way, the case does two things for the oil seal: First, it provides stability, allowing to the outside diameter to enter to pressure and fitted in the housing avoiding the leak for this route. Second, the metal case also provides protection, avoiding damage of the lip during the installation.
Spring: The spring fulfills two principal functions: first it contributes by radial force or load, betweenthe lip and the axle. Second, the spring also helps to assure that the wished quantity of load should be kept even when the material of the lip is filled and running-in due to the exhibition to the lubricant to high temperature. A lip that has swelled up itself is less capable of supporting a consistent contact with the axle without the help of a spring. A weak contact causes the development of a route of leak. The spring hardens artificially to the lip, and this helps to support to the lip in its place.
Secondary Lip: Besides the primarysealing lip, there exist oil seals which incorporate the smaller one, the secondary lip. The function of this one is to exclude dust, dirt and other pollutants. Unlike the primary lip, this secondary lip faces the air side of the application (provided that the dirty and other unwanted matters can try to migrate towards the interior of the assembly).
There are various the factors that influence the suitable functioning of an oil seal. The most influential are outlined the compound, temperature, conditions of the assembly, method of installation, fluid, chemical compatibility betweencompound and fluid, and others.
Compound and Temperature of Operation.
The materials commonly used for sealing lip lips nitrile, silicone, polyacrylic and fluoroelastomer. The material is important because itdetermines the expansion of the lip with the temperature. The properties of the material and its changes for the contact with the oil and the temperature must be known to bear them in mind in the design. The following graph shows the general limits of temperature for the principal compounds used in oil seals. It is necessary to bear in mind that these temperatures can be modified by factors as the type of fluid and speed of rotation of the axle, which will give additional temperature to the lip due to the friction of the system.
The selection of the fluid is one of the most important considerations in the life of the oil seal. The reaction with the element of sealing of rubber often can be harmful to the oil seal.
The majority of the oil seal work better in oil of average viscosity such as oil for engine SAE 10. More light oils do not offer so good characteristics of superficial tension and oil of higher viscosity creates a dragging of bigger friction causing a more short life. As for the pressure of the fluid, there is recommended that the pressure should not exceed the 5 psi (35 KPa) in a standard oil seal.
The environmental conditions found in any application are numerous and change with every design. The level of the effect that these variables will have in the oil seal from the point of view of the useful life must be considered, in such a way that the seal could be designed correctly. A suitable list of conditions to recognizing and including in the preliminary evaluation are:
1. Temperature and range of temperatures.
2. Dust and residues or waste.
3. Dampness and water.
4. Ozone and infrared effects
Flexible lip: The sealing lip of must possessthe necessary flexibility that should allow it at all time, support the contact with the axle or with the seal sleeve. The lip must be capable of supporting the contact during normal conditions of offset.
Distribution of the pressure of contact of the lip: The pressure of contact of the lip on the axle or seal sleeve must expire with the specifications of sealing so much in conditions of movement asof rest.
Resistance to the Vibration: The design of the oil seal must resist the trend to the vibration and to the resonance, which they can cause that appears leakage of fluid, because of loss of contact betweenthe sealing lip of and the axle or seal sleeve.
Resistance of the oil seal to the deformation: The oil seal must support small increases and decreases of pressure without leakage.
Capacity of static sealing: The oil seal must seal several levels of lubricants in a static condition in a wide range of temperatures even when present normal misalignment.
Capacity of Installation: The oil seal must be easily installable to avoid damages to the same one.
Radialload: The principal lip must have the appropriate quantity of pressure on the axle or seal sleeve. Too much pressure causes that the oil seal gets worn out rapidly. Very little pressure causes leak of fluid.
Interference betweenthe primary lip and the axle or seal sleeve: The suitable quantity of interference betweenthe principal lip and the axle or seal sleeve must offer an appropriate static sealing.
As the time passes, the oil seals and other products of rubber can experience changes their physical properties, even they can become useless due to an excessive hardening, softening, cracked or other degradations of the surface. These changes can be the result of a unique factor or of a combination of such factors as the addition of oxygen, ozone, light, heat, dampness, oils, water, or other solvents. Nevertheless, the harmful effects of these factors can be minimized by means of conditions of storage.
The ideal temperature of storage is between 5 ºC and 26 ºC. The high temperatures accelerate the deterioration of the products of rubber. The heat sources in the places of storage must be had so that the temperature of the product stored never exceeds 49 ºC. The effects of the low temperatures are neither destructive nor permanent, but under these conditions, the articles of rubber like the oil seals tend to harden.
Expressed as a percentage, the relative dampness is the relation of the water steam quantity present in the air to the biggest quantity that might be present to a given temperature. Ideally, the relative dampness in the area of storage must be below 75 %. There must be avoided very humid or dry conditions. Where ventilation is needed, this one must be kept to the minimum. It cannot be allowed that the process of condensation should happen. Some materials, such as the polyurethanes based on the polyester, are hygroscopes and absorb the dampness of the air.
The oil seals and other products of rubber always must be protected of the light, especially of the solar light. The strong artificial light with high contentof ultraviolet(UV) is dangerous. Without bearing the sourcein mind, the beams UVs can cause break of the chain. It is recommended recommends the use of polyethylene bagsstored inside big containers of carton.
Oxygen and Ozone:
Both the oxygen (O2) and ozone (O3) are quite harmful to rubber products. When possible, retainers and other molded articles should be stored in airtight containers to protect the circulating air. Oxygen (especially in combination with the heat) causes the rubber goods crossing form additional unwanted causing hardening of the retainer. As with water and UV light, ozone can cause rupture of the chain. Rubber products should be kept away from ozone generators such as electric motors, mercury vapor lamps, welding equipment, and high voltage electrical equipment.
Rubber items should be stored in conditions of relaxation, free of tension, compression or other deformation, all of which can cause cracking or change permanently. The fasteners must not be stored on pins or wires.
Depending on the application therearedifferent types of oil seals, which differ principally for presenting variations in their constitutive parts or coverings that offer them major resistance in the assembly. The oil seals lines offered to the market are: Standard, OBS (Oil Bath Seal) and SS (Seal Sleeve).
The shaft seals can present secondary lip or dust collector and the applications that demandmajor force of the seal can use inner case that is inserted in the metal case. The function of the inner case is to protect the lip and the spring during the manipulation and the installation. The outside diameters in metal have proved much efficiency when they are located in housings of cast iron and steel.
The oil seals with outside diameter in rubber are used often in applications where the outside diameters in metal would not work adequately, such it is the case of the housings in aluminium, where the latter expands approximately two times the proportion of the steel. A progressive expansion of the aluminium as result of the thermal cycle reduces the interference (force of retention) betweenthe outside diameter of steel and housing in aluminium.
Oil seals with diameter in metal and rubber are needed for more demanding applications. The metal offers retention while the rubber provides sealing capacity. The metallic part protects the portion of rubber of the damage in the installation. The metal also represents with a precise alignment in the housing and minimizes the deviation of the oil seal and / or the movement during the use. The element of rubber allows a narrower elastic adjustment in the housing.