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Use the Right Grease for the Job

Peter Haig, Solvent Extract (SX) Consultant with permission of BIN95 Business Industrial Network

Posted 10/10/2024

Grease – use the right one for the job. The classification, selection, and application of greases is included in this article. Greases are designed for specific applications and if used in the wrong service they will not be effective. Compare the properties of greases and check those used in your equipment are suited to the service.

Video: A Brief History of Grease – watch before reading on!

Classification, Selection, and Application of Lubricating Greases

The classification of lubricating greases is not uniformly regulated. Because of the versatility and the variations in their composition, greases are essentially classified on the basis of their base oil or thickener.

oil lubricant chart

Base Oils

The oil present in a lubricating grease is referred to as its base oil. The proportion of base oil can vary depending on the type and quantity of thickener and the intended application of the grease. For most greases, the base oil content is between 85% and 97%. The type of base oil gives a grease some of its typical characteristics.

Thickeners

Thickeners are divided into soaps and non-soaps and give lubricating greases their typical properties as well. Soap greases can be divided into simple and complex soap greases, each of which are referred to by the name of the cation on which the soap is bases (e.g. lithium, sodium, calcium, barium, or aluminum soap greases).

These metal soaps are made from fatty acids, which are products obtained from animal or vegetable oils and fats.

These fatty acids are a mixture of a wide variety of chemically defined fatty acids. They are split into fatty acids and glycerides by hydrolytic decomposition. The fatty acids are then combined with the corresponding metal hydroxides to form the metal soaps used as thickeners for lubricating grease production.

Additives

Additives counteract wear and corrosion, provide additional friction reducing effects, improve the adhesion of the grease, and prevent damage under boundary and mixed friction conditions.

Additives therefore affect the quality, potential applications, and ultimately the practical value of the grease. Additives can be solid, polar, or polymeric.

oil lubrication
grease

Solid Additives

Graphite, Molybdenum Disulfide, Zinc Sulfide, talc, polytetrafluoroethylene, etc. are incorporated into greases in powder form or as pigments. They act in the boundary and mixed friction regions. Solid additives improve running-in and emergency operating characteristics.

Polar Additives

Polar substances are hydrocarbon molecules which behave in an electrically non-neutral way because of their molecular structure (i.e. by incorporating other elements such as oxygen, sulfur, or chlorine), and are retained on metal surfaces as if they were magnetic. The presence of polar substances increases adhesion of the lubricant film, since pure hydrocarbons are “non-polar”.

Polymer Additives

The correlation between temperature and the viscosity of mineral oils can be influenced by additives. These additives consist of organic polymers with molecular weights of between 10,000 and 200,000. At moderate temperatures their chain-like molecules are tangled together, but at high temperatures they extend into elongated threads. By simultaneously switching to a low-viscosity base oil with a higher viscosity index (VI), the viscosity-temperature curve can be flattened. The presence of polymers makes the viscosity of a base oil dependent on the shear rate.

Polymers generally improve the wear protection offered by lubricants. Polymers used as adhesion additives for greases include polyisobutylenes, olefin polymers, and others. They improve the grease’s adhesion to surfaces.

oil lubricant chart 2

Advantages Of Grease Lubrication Over Oil Lubrication

  • Reduced design complexity
  • Less maintenance is often required, since lifetime lubrication is possible.
  • Less risk of leakage and simpler seal design
  • Sealing effect is reinforced by used grease overflow (“grease collaring” or labyrinth).
  • With high-speed greases, metered grease quantities, and a running-in period, low bearing temperatures can be achieved at high revolutions.

Disadvantages Of Grease Lubrication Compared with Oil Lubrication

  • Less heat dissipation possible
  • Contaminants are captured by the grease film and not removed, especially with grease lubrication of minimal quantity.
  • At present, limiting speeds or speed factors are lower than with oil injection and oil/air lubrication.


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Peter Haig

Peter Haig is a Solvent Extraction (SX) Consultant formerly working with both BASF and Shell Chemicals. He is known as one of the most respected engineers in the solvent extraction field. Specialties in this field include Diluent and SX Chemistry, Process Control, Equipment Advice and most importantly, SX Safety. Also has experience with mine visitations, configuration modeling, plant trials, training and arranging laboratory testing when required.