Factors Affecting Seal Selection
When lubricant begins leaking from hydraulic equipment, sumps or pumping systems, the chances are that the incorrect hydraulic seals were selected, installed poorly, maintained improperly or the wrong seals are being used for the application at hand.
However, the good news is that with an understanding of the factors that make for long seal life, most of these issues can be resolved. The other part of this is the importance of redefining the procedure for the selection of hydraulic seals.
Evaluating Factors when Selecting Seals
There are several variables to consider when oil seals need to be chosen. All need to be considered by maintenance engineers and designers.
Factors Involving the Shaft
The surface finish of the shaft onto which it will be installed will determine how effectively a seal can do its job. The best results in terms of sealing can be achieved when the shaft is ground or polished with concentric finish marks. The efficiency of the seal will be affected by the direction of finish marks, as well as the spiral lead. Any use of shafts with spiral leads should lead toward the fluid during rotation of the shaft.
The hardness of the shaft is another important factor. Shafts with RC hardness of 30 or more should expect longer seal life. Shaft hardness should be increased to RC 60 where the seal will be exposed to contamination considered to be abrasive.
Shaft speed is determined by the finish of the shaft and the amount of runout, as well as the concentricity of the shaft, housing bore and type of oil seal material as well as the type of fluid that the seal is preventing from leaking out of the equipment it’s installed on.
Another factor is the temperature of the location where the seal is going to be installed. Care should be taken to ensure that the temperature at the site of installation will not exceed the range of the seal’s elastomer.
The pressure the seal will be placed under also affects which type of seal is chosen. Typically, most oil seals have been designed for very low-pressure applications of 8 psi or less. Any pressure higher than this will require the installation and use of some kind of pressure relief solution.
The centres of the bore and shaft need to be aligned. If they are not, this will result in the loss of seal life due to more wear on one side of the lip than on the other.
Continuous lubrication is one of the key factors to ensuring long seal life. Seals should be lubricated continuously using oil of the right viscosity for the application. But viscosity is not enough; the oil also needs to be compatible with the elastomer material of the seal’s lip. This is one of the most important ways to ensure a seal works as it should, and also involves the consideration of what kinds of additives will be used and whether or not a synthetic lubricant is being applied.
Where runout is concerned, as little as possible is the ideal if the goal is to extend seal life. When movement exists and the centre of rotation, shaft whip or bearing rotation is usually the cause. If misalignment is also present, this exacerbates the problem. There is also a myth that misalignment can be compensated for by installing flexible couplings. This will not fix the issue.
The presence of close tolerances of shaft and bore will ensure optimal seal performance. However, these are not the only factors; the vibration, eccentricity and end play of the shaft will also affect the performance of the seal.
When it comes time to purchase new seals, the importance of selecting the correct one cannot be underestimated, but this is what tends to occur more often than not, and what ultimately leads to premature and frequent seal replacement.