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9 Factors for Selecting Oil Seals

We just discovered that one of the seals on our pumpingsystem is leaking. Do you haveany ideas as to what could have caused this, and can youoffer some advice for selectinga good seal?

The main causes of externallubricant leakage frompumping systems, hydraulicmachines, gearcases andsumps are the wrong selection,improper application, poorinstallation and inadequate

maintenance practices that are applied to sealing systems. These problems

can be overcome through a better understanding of the types of sealingmaterials available, redefined selection procedures and the consistentapplication of sound replacement and maintenance practices.

A number of variables must be considered when selecting oil seals. There arenine factors that designers and maintenance engineers must evaluate whenoil seals are specified:

Shaft Speed

The maximum allowable shaft speed is a function of the shaft finish, runout,housing bore and shaft concentricity, type of fluid being sealed and the typeof oil seal material.

Temperature

The temperature range of the mechanism in which the seal is installed must

not exceed the temperature range of the seal elastomer.

Pressure

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Most conventional oil seals are designed only to withstand very low-pressureapplications (about 8 psi or less). If additional internal pressure is present oranticipated, pressure relief is necessary.

Shaft Hardness

Longer seal life can be expected with shafts having a Rockwell (RC) hardnessof 30 or more. When exposed to abrasive contamination, the hardnessshould be increased to RC 60.

Shaft Surface Finish

Most effective sealing is obtained with optimum shaft surface finishes. Thesealing efficiency is affected by the direction of the finish tool marks and thespiral lead. Best sealing results are obtained with polished or ground shaftswith concentric (no spiral lead) finish marks. If you must use shafts withspiral finish leads, they should lead toward the fluid when the shaft rotates.

Concentricity

When the bore and shaft centers are misaligned, seal life will be shortenedbecause the wear will be concentrated on one side of the sealing lip.

Shaft and Bore TolerancesThe best seal performance is achieved when close shaft and bore tolerancesare present. Other factors include shaft eccentricity, end play and vibration.

Runout

Runout must be kept to a minimum. Movement of the center of rotation isusually caused by bearing wobble or shaft whip. When coupled withmisalignment, this problem is compounded. Contrary to popular belief andcommon practice, the installation of flexible couplings cannot correct orcompensate for misalignment.

Lubricant

Seals perform much better and longer when they are continuously lubricatedwith an oil that has the correct viscosity for the application and that is

compatible with the seal lip elastomer material. The consideration of sealincompatibility, particularly with certain additives and some syntheticlubricants, should not be ignored, but unfortunately very often is.

ctors for Selecting Oil Seals http://www.machinerylubrication.com/Articles/Print/28997

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