Effects of Grease on Rotary Shaft Seals

Less than 20% of seals and gaskets ever reach their full expected ‘operating life’ in any given application. Of the 80% that that are replaced due to scheduled maintenance, ineffective performance, or failure, 40% of the ineffective performance or failures 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 of increasing the life of seals and their related mechanical products.  Maximizing the life of seals and related components increases machine up-time, reduces labor costs for repair technicians, and reduces 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.  Understanding the types and function 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 lip seal will help in maximizing the life of your seal in a rotary shaft seal application.

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 lubricating oil would not stay in position. They also act as sealants to prevent ingress of water and in-compressible 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 ideal 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 your system creates a fluid film between the seal lip and shaft which minimizes frictional forces as well as protects against these foreign contaminants. The proper film thickness to achieve optimum service life from your 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. To achieve maximum seal life, 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 your 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 degradation 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, swelling and leakage.

Proper grease application to the seal prior to installation is merely the first step in achieving full service life of your 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 breakdown 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 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 noises as well as spikes in an assembly’s internal temperature are other key warning signs that should alarm the monitoring technician that lubricant levels are dangerously low.

With a full understanding 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 into the assembly.  These contaminants will cause excessive wear on the internal components of your system.  This results in replacements required for parts beyond just the sealing elements of your system as well as the need to replace the contaminated lubricant. 

Understanding the relationship between the lubrication and sealing element of your assembly is vital to ensure your company’s resources are not being unnecessarily wasted. Selecting the proper seal for your application is as equally important as selecting the proper lubrication for your system. Colonial Seal can review your unique application and is able to recommend the best sealing solution for your particular requirement.  We all know that time is money. Do not allow your costs to increase and profits to diminish because a decision was made without first consulting an expert in proper sealing procedures.