Considerations in the selection and maintenance of oil seals

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The concept of sealing is the sealing line, an unbroken continuous band or area of contact between the seal and it’s mating surfaces, that will effectively and continually stop fluid movement from a pressurized area to an area of lower pressure. Anything that interrupts the sealing line, such as expansion, contraction, motion, shock, vibration, or contaminant migration across the sealing line, will create or increase leakage.

Static seals are those used in any application where there is no movement between mating surfaces. Common static seals are O-rings and gaskets. O-rings are effective seals because they will return to their original shape after deformation; however, because they are elastomers, O-rings will shrink up to 16 times as much as metals when the temperature drops. Therefore, some connections leak constantly at temperatures that are well below zero. Solutions to this problem are to use an O-ring of a larger diameter or to reduce the clearance gap between the mating surfaces.

Gaskets are the oldest sealing method still in use. Gaskets absorb the entire load across the mating surfaces, and the force required to crush the gasket and establish the sealing line can be very high. Under this constant preload, the gasket material will relax and creep, causing the entire joint to loosen.

Dynamic seals are those used in systems where linear or reciprocating motion is part of the operation. Typical applications are hydraulic cylinder use. Every dynamic seal design is a compromise between the prevention of leakage (past the rod end) and the reduction of wear (in the area where the seal edges and rod surfaces meet). When selecting dynamic seals for these applications, friction, wear, system temperature, fluid viscosity, system pressure and fluid compatibility with the seal material, must be considered.

Contaminant particles in the fluid, or those carried into the system during cylinder rod retraction can cause serious sealing problems. Abrasive or embedded particles trapped between the seal and the reciprocating surface can gouge leakage paths in the rod or cylinder wall.

Several variables must be considered when selecting oil seals. There are nine factors that designers and maintenance engineers must evaluate when oil seals are specified.

Q | Does the maintenance group know the speed of the shaft to be sealed?
Logic: Shaft speed and effective sealing is a function of the shaft finish, run out, housing bore and shaft concentricity, the type of fluid being sealed and the type of oil seal material.

Q | Does the maintenance group know the operating temperature?
Logic: The temperature range of the equipment in which the seal is installed must not exceed the temperature range of the seal elastomer material. See Chart 1 (next page).

Q | Is the maintenance staff aware of the pressures that the seals must withstand?
Logic: Most conventional oil seals are designed only to withstand very low-pressure applications, about eight PSI or less. If additional internal pressures are present or anticipated, pressure relief is necessary.

Q | Is the maintenance staff aware of the importance of the material hardness of sealed shafts?
Logic: Longer seal life can be expected with shafts having a hardness of Rc 30 or more. When exposed to abrasive contamination, the hardness should be increased to Rc 60.

Q | Is the maintenance group aware of the importance of sealed shaft surface finish?
Logic: The most effective sealing is obtained with optimum shaft surface finishes. The sealing effectiveness is directly affected by the direction of the finish tool marks and the spiral lead. Best sealing results are obtained with polished or ground shafts with concentric (no spiral lead) finish marks. If you must use shafts with spiral finish leads, they should lead toward the fluid when the shaft rotates, otherwise leaks will occur.

Q | Is the maintenance staff aware of the importance of bore and shaft concentricity?
Logic: When the bore and shaft centres are misaligned, seal life will be shortened, because the wear will be concentrated on one side of the sealing lip.

Q | Is the maintenance group aware of the importance of shaft and bore tolerances?
Logic: Best seal performance is achieved when close shaft and bore tolerance are present. Other factors include shaft eccentricity, endplay, and vibration.

Q | Is the maintenance group aware of the importance of shaft run out?
Logic: Run out must be kept to a minimum. Movement of the centre of rotation is usually caused by bearing wobble or shaft whip. When coupled with misalignment, this problem is compounded. Contrary to popular belief and common practise, the installation of flexible couplings cannot correct or compensate for misalignment.

Q | Is the maintenance group completely familiar with and knowledgeable of the lubricants used in all equipment using seals?
Logic: Seals perform much better and longer when they are continuously lubricated with oil that has the correct viscosity for the application and that is compatible with the seal lip elastomer or O-ring material. Seal incompatibility, particularly with certain additives and some synthetic lubricants is important. It is also important to determine if the synthetic fluid being considered is compatible with any machine coatings or paint.

Synthetic polyglycols, polyalphaolefins, alkylated aromatics and diesters are generally compatible with the seal materials in See Chart 2 (above). One exception is Diester fluid, which is not compatible with neoprene or low nitrile content Buna N seals. Another exception is polyalphaolefins, which are not compatible with EPDM seal materials. If in doubt, contact the seal and synthetic lubricant manufacturers.
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L. (Tex) Leugner, author of Practical Handbook of Machinery Lubrication, is a 15-year veteran of the Royal Canadian Electrical Mechanical Engineers, where he served as a technical specialist. He was the founder and operations manager of Maintenance Technology International Inc. for 30 years. Tex holds an STLE lubricant specialist certification and is a millwright and heavy-duty mechanic. He can be reached at texleug@shaw.ca.