Jul. 14, 2025
We just discovered that one of the seals on our pumping system is leaking. Do you have any ideas as to what could have caused this, and can you offer some advice for selecting a good seal?
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The main causes of external lubricant leakage from pumping systems, hydraulic machines, gearcases and sumps are the wrong selection, improper application, poor installation and inadequate maintenance practices that are applied to sealing systems.
These problems can be overcome through a better understanding of the types of sealing materials available, redefined selection procedures and the consistent application of sound replacement and maintenance practices.
A number of variables must be considered when selecting oil seals. There are nine factors that designers and maintenance engineers must evaluate when oil seals are specified:
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 type of oil seal material.
The temperature range of the mechanism in which the seal is installed must not exceed the temperature range of the seal elastomer.
Most conventional oil seals are designed only to withstand very low-pressure applications (about 8 psi or less). If additional internal pressure is present or anticipated, pressure relief is necessary.
Longer seal life can be expected with shafts having a Rockwell (RC) hardness of 30 or more. When exposed to abrasive contamination, the hardness should be increased to RC 60.
Most effective sealing is obtained with optimum shaft surface finishes. The sealing efficiency is 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.
When the bore and shaft centers are misaligned, seal life will be shortened because the wear will be concentrated on one side of the sealing lip.
The best seal performance is achieved when close shaft and bore tolerances are present. Other factors include shaft eccentricity, end play and vibration.
Runout must be kept to a minimum. Movement of the center 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 practice, the installation of flexible couplings cannot correct or compensate for misalignment.
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Seals perform much better and longer when they are continuously lubricated with an oil that has the correct viscosity for the application and that is compatible with the seal lip elastomer material. The consideration of seal incompatibility, particularly with certain additives and some synthetic lubricants, should not be ignored, but unfortunately very often is.
Mechanical seals are used mainly in pumping applications (although the principles are the same for mixers, blowers and compressors). In essence, the pump has two main elements; the first being a rotating shaft and the second being the pump casing. Without a seal, the pumped medium (water, waste, oil, etc) would leak and the pump would be ineffective. The mechanical pump therefore creates the seal between the moving part (dynamic) and the stationary part (static). Because of this Mechanical seals are often called many things including shaft seals, pump seals, rotary shaft seals, water pump seal and combinations of these. Whatever you call it, the product is essentially the same.
Mechanical seals or more technically “end face mechanical seals” were designed in the early s but really came to prominence during the post-war boom (late s) in both industrial and domestic equipment sales.
The elements of a mechanical seal are what make it unique. They are:
The seal between the stationary component and the rotating component is called the ‘Primary Seal’ and is the most important. However there is a ‘Secondary Seal’, which is typically an O Ring and sits between the rotating component sealing member and the shaft of the pump.
The spring is used to create just the right pressure in the primary seal between the stationary and rotating surfaces.
The primary seal surface in various mechanical seal types come in a vast array of materials, such as Carbon-graphite, Silicon carbide, Tungsten carbide and Alumina ceramic. The choice of material is dependent on the medium (the liquid to be sealed), the pressures the seal must withstand and the temperatures of the medium. Speak with your mechanical seals supplier to determine the best materials for your application.
Before the invention of mechanical seals, mechanical packings or gland packings were used. These worked well in principle, but had a higher leakage rate (not ideal for some applications) and can handle higher temperatures and pressures (thanks to the sprint component).
Additionally, the use of packing in earlier designs caused shaft and sleeve wear that reduced the life of the pump and increased costs. Mechanical seals therefore improve equipment lifespan.
The other benefit is that they are self-lubricating in that their design specifically allows a very small amount of leakage onto the sealing surface, which lubricates and keeps the primary seal from eroding. Finally, mechanical seals require virtually zero maintenance as they do not require periodic tightening as Gland packing would.
Ultimately therefore, mechanical seals are a simple, elegant and cost effective solution to creating a long-life seal in dynamic applications.
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