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Until recently, it was thought that current generations of biodegradable and low- toxicity lubricants could not satisfactorily meet the requirements of rolling bearing applications. The application of biodegradable lubricants and greases has been studied at the SKF Engineering and Research Centre for a number of years. This led to a formulation that can meet the stringent demands of a range of engineering applications without adversely affecting the environment. A new grease, LGGB 2, has been tested and approved for steel-on-steel spherical plain, ball and roller bearings. The grease is based on synthetic ester oil, lithium/calcium thickener and other biodegradable ingredients, and it has low toxicity.
In particular, equipment users in the construction, earthmoving, forestry and farming industries are increasingly seeking “green” solutions because environmental contamination may be an issue. Although only a small portion of lubricants used in these applications may pose an actual threat to the environment, this can add up to several million metric tons worldwide.
Study of Requirements
The search for a suitable biodegradable and low-toxicity lubricant involved a number of key issues such as:
A central issue was to establish a clear definition of “green grease.” Most materials will degrade in some way over time. There are many processes responsible for these changes, most commonly biodegradation, oxidation and photolysis. Chemical and physical changes over time often occur simultaneously.
An understanding of biodegradation is vital to the development of clear parameters on material breakdown in a green grease. In biodegradation, the material is gradually broken down through the metabolic action of such living organisms as bacteria, fungi, yeast and algae. Hydrocarbons, which are the main constituent of biodegradable lubricants, are transformed into carbon dioxide and water by this process.
Naturally, this process is not entirely predictable as it can be influenced, for example, by the mix of living organisms, temperature and humidity. A material that may easily degrade under one set of circumstances may not readily degrade under others.
The minimum basic requirements are sufficient bacteria population, correct oxygen levels and a suitable temperature range. The rate of degradation is also affected by such factors as fluid viscosity, sunlight, mineral salt content, availability of nitrogen, pH levels, solubility and the ability of bacteria to adapt to the source of oil nutrient. Ultimately, of course, the lubricant should be reduced to its simplest natural form while leaving no harmful by-products that could have a detrimental and long-term effect on the local environment.
Traditional lubricants based on mineral and synthetic oils are ultimately biodegradable through the slow rate of decomposition. Under some conditions, they can exist as a contaminant in groundwater for close to a century.
When using any tests to establish the biodegradable performance of a new lubricant, one must bear in mind the often unpredictable and complex processes at work.
With no single universal standard that clearly defines biodegradability, a decision was made to use a number of existing tests. These were used to establish whether toxicity and biodegradability occurred within a reasonable length of time.
Basically, biodegradability means that micro-organisms in the soil, rivers and oceans can break down greases with relative ease.
From exhaustive studies, it was found that ester or vegetable fluids formulated carefully into lubricants perform well in real applications. In particular, it was noted that a combination of synthetic ester oil and a lithium/calcium thickener resulted in good properties in terms of water and corrosion resistance without the need to use toxic additives.
The new grease formulation, based on these constituents, performs well with ball, roller and plain bearings. This is a significant development, as conventional lubricating greases are normally considered suitable for only one bearing type. Each bearing class has a different contact surface, which can affect lubricant performance. For instance, the lubricant film in ball bearings has a point contact; in roller bearings it is a line contact; and for plain bearings it is full contact over the area. To assess performance, four different formulations were subjected to stringent bearing tests. These tests were:
From these tests, it was found that LGGB 2 offered excellent performance over a range of operating conditions and bearing types and established a steady operating temperature set at 90°C.