9 Factors for Selecting Oil Seals

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?

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:

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 type of 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

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.

Shaft Hardness

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.

Shaft Surface Finish

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.

Concentricity

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.

Shaft and Bore Tolerances

The best seal performance is achieved when close shaft and bore tolerances are present. Other factors include shaft eccentricity, end play and vibration.

Runout

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.

Lubricant

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.

Effects of Grease on Rotary Shaft Seals

Effects of Grease on Rotary Shaft Seals

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Less than 20% of seals and gaskets ever reach their expected “operating life” in any given application. Of the 80% that are replaced due to scheduled maintenance, ineffective performance, or failure, 40% can be attributed to improper lubrication. With the steady rise in the price of raw materials, it is imperative that companies continue to examine ways to increase the life of seals and related mechanical products. This can increase machine uptime, reduce labor costs for repair technicians, and reduce the cost of replacing seals, gaskets, and related mechanical components. Effective use of lubricating grease is one variable that will contribute to maximizing the life of a seal in a rotating shaft application.  This article will discuss understanding the types and functions of grease; the interaction of the shaft seal lip in a dynamic rotating application; effective greasing requirements; and indications of failure or reduced performance between the grease and a seal lip.

Lubricating grease is made by adding a thickening agent to either mineral-based or synthetic oil. These agents include metallic soap, urea compound, organophilic modified clay, and carbon black. Additives such as rust inhibitors, extreme pressure additives, and oxidation preventatives are also used to increase performance. Greases are applied to mechanisms that can only be lubricated infrequently and where a lubricating oil would not stay in position. They also act as sealants to prevent ingress of water and incompressible materials. Grease-lubricated bearings have greater frictional characteristics due to their high viscosity. 

Sealing against foreign contaminants such as moisture and solid airborne particles is vital for maintaining equipment performance. Contaminants that pass by the seal and enter the sump cavity can severely damage bearings, gears, and other machine components. Properly applying grease to a system creates a fluid film between the seal lip and shaft that minimizes frictional forces as well as protects against these foreign contaminants. The proper film thickness to achieve optimum service life from a seal is unique for each system. Microscopic projections from the surface of the shaft, referred to as “asperities,” dictate the ideal thickness of the lubricating film. The fluid film between the surface of the shaft and seal lip should be the same thickness as the average asperity height. 

The reason moisture and solid particles will have significant adverse effects on a system is because water has a very low load-bearing capability, and if it is able to penetrate into the system, it will cause a degredation of the lubricating film. This breakdown will cause corrosive, adhesive, abrasive, and hydrogen-induced wear to the seal lip and casing. Solid particles allowed to enter the system will scratch and abrade surfaces, causing an increase in the amount of solid contaminants in the application. These “wear areas” provide ideal conditions for oxidation to occur. When the grease oxidizes, it typically darkens due to the build-up of acidic oxidation byproducts. These byproducts have a destructive effect on the grease’s thickening agent, as well as the seal lip material, causing softening, oil bleeding, and leakage.

Proper grease application to the seal prior to installation is merely the first step in achieving full service life of a seal. Maintaining proper grease levels is equally important and is typically the area that is most neglected. A good rule to live by is no lip seal should run without lubrication for any prolonged period of time. This “dry-running” will cause the remaining grease to thin and eventually break down, resulting in direct contact between the shaft and seal lip. Overfilling is as detrimental to a system as dry-running. Overfilling will often cause a rapid rise in the internal temperature of a system caused by the increase in the amount of work the system must perform in order to push the excess grease out of the way. The most common method followed for proper greasing states the fill level should be approximately one quarter to one third of a seal casing’s free volume.

As is the case with seals, grease also has an optimum operating life. As the grease approaches the end of its ideal operating life, there are tell-tale signs that will warn an observant machinist that purging of the lubricant in the system is required. As we have previously discussed, when grease degrades, the lubricating film preventing surface-to-surface contact between the seal and shaft begins to deteriorate. As direct contact between the shaft and seal lip increases, a frictional phenomenon known as “Stick-Slip” begins to occur. “Stick-Slip” is a jerky motion between two surfaces due to alternating gripping and slipping of contact areas due to the lack of lubricating fluid. This action produces a surface wave oscillation that will literally tear a seal apart or, in a best-case scenario, allow leakage. Increased noise, as well as spikes in the internal temperature of an assembly are other key warning signs that should alarm the monitoring technician that lubricant levels are dangerously low.

With a full undersatnding of the relationship between the seal, shaft, and grease of an assembly, we are now able to center our attention on the risks and resulting costs of improper lubrication. Beyond the cost of having to replace a prematurely failed seal, we have also learned that this failed seal will often times allow foreign contaminants to enter the assembly. These contaminants will cause excessive wear on the internal components of a system, resulting in replacements needed for parts beyond just the sealing elements, as well as a need to replace the contaminated lubricant.

Understanding the relationship between the lubrication and sealing element of an assembly is vital to ensure resources are not being unnecessarily wasted. Selecting the proper seal for an application is as equally important as selecting the proper lubrication. Time is money, so do not allow costs to increase and profits to diminish because a decision was made without first consulting an expert in proper sealing procedures.

Colonial Seal Company is an ISO 9001:2008 Certified specialty distributor of rotary shaft seals, oil seals, hydraulic seals, custom large diameter metal cased seals and non-metallic seals. For more information visit their website: www.colonialseal.com.

Oil lubricated seals

About our seals

We believe a seal should deliver top performance, keep you going and offer you Peace of Mind. This idea led to the development of the oil lubricated Supreme® seal. Introduced in the 1980s and further developed ever since, Supreme oil seals offer a ‘plus’ to many propulsion systems and vessels worldwide.

Our oil-lubricated Supreme shaft seals are used in stern tubes, thrusters, bulkheads, fin stabilisers, rudder stocks and gearboxes. The Supreme oil seals are produced with high quality housing parts, rubber lip seals and liners.

How oil-lubricated seals work

The oil-lubricated Supreme seals closes the gap between stationary and moving components of rotating shafts. It prevents the escape of lubricants (oil leakage) and ingress of harmful materials such as dirt or sand. The lube oil system circulates the oil and controls pressure, flow and temperature in order to build up an appropriate lubrication film for the white metal slide bearing.