What makes up grease
Not all greases are the same, and not all applications requiring grease are the same. For example, one grease is designed for very hot environments, such as steel mills, and must be mechanically stable, while another grease is designed for sub-zero temperatures in refrigeration equipment. Changing grease for a purpose other than its intended use can lead to failure.
Greases vary in color, viscosity, pumpability and efficiency.
What's in my grease?
What does grease consist of?
List of grease ingredients
Lubricants typically consist of three main ingredients:
- Base oil
- Thickener
- Extras
Lubricant base oil
The base oil (base) is the largest component of a lubricant. Two main properties of the base oil affect the behavior of the lubricant.
Naphthenic oils have excellent solubility in soap thickeners. More refined white mineral base oils typically provide better oxidative stability and help achieve NSF H1 registration.
Synthetic base oils, most often polyalphaolefins (PAO), improve thermal stability and low-temperature performance.
Other types of base oils in lubricants include biodegradable esters, which can be used in environmentally friendly lubricants.
The second important property of base oils is viscosity.
Low viscosity base oils flow easily and are suitable for high-speed applications. Low viscosity is also important in low-temperature applications, where it provides sufficient and adequate lubrication, although viscosity increases at low temperatures due to the slowing down of the fluid's molecular motion.
On the other hand, high-viscosity base oils are typically used in slow-speed, heavy-duty, high-temperature applications. In such cases, sufficient oil film thickness on metal surfaces is important for effective lubrication.
Grease thickeners
Thickeners in greases control the viscosity of the grease. The higher the concentration of thickener, the higher the consistency (NLGI classification) of the grease.
Grease with a higher thickener concentration and higher NLGI classification is usually better suited to high temperature operation and results in less oil separation.
A thinner grease with a lower NLGI classification has better pumpability and is better suited for low temperature operation.
There are many types of thickeners used in lubricants.
The four most commonly used thickeners are discussed below and their properties that affect grease behavior are explained.
Lithium stearate and lithium complex thickeners
They have been the most widely used thickeners in greases for over 60 years. These soap thickeners provide greases with excellent natural water resistance and pumpability.
Composite Aluminum Thickeners
They also provide excellent water resistance, improved adhesion and viscosity. Aluminum thickeners can also be certified as NSF H1 greases.
Polyurea thickeners
They are formed by the reaction of isocyanates with amines. Because the thickener is metal-free, the grease has excellent natural resistance to oxidation. Polyurea thickeners have a very small particle size, which reduces noise during application.
Calcium sulfonate based thickeners
They have a very large particle size and a shell-like shape. Calcium sulfonate thickeners are characterized by exceptional resistance to heat, wear, stress and corrosion. Calcium sulfonate greases are also very resistant to mechanical stress and do not soften/harden over time during application.
Lubricant additives
In addition to the performance properties provided by the base oil and thickener, a grease may be further modified with additives to meet the requirements of a specific application.
- Additives include antioxidants that increase the grease's resistance to degradation.
- Corrosion inhibitors provide increased protection against rust.
- Additives for high-pressure greases can be in liquid or solid form to increase the load-bearing capacity of the grease.
- Wear-resistant additives provide increased protection against pitting and scratching of metal surfaces, extending service life.
- Dyes and other solid additives may be used in the lubricant to obtain any desired color.
Examples of grease applications
Based on the above, let's consider some applications and their suitable lubricants. In the example given at the beginning, a suitable lubricant for a very hot, highly loaded, slow-speed steel mill would be a high viscosity refined mineral oil that would provide sufficient film thickness at high temperatures and loads.
The target thickener would be calcium sulfonate , not only because of its high temperature and mechanical stability, but also because of its inherent thickening properties and wear resistance.
Another example of application is bearings for electric motors . Such bearings are usually high-speed ball bearings with many metal contact points in the bearing. Suitable greases may contain low-viscosity base oils due to the high speed of application. Suitable thickeners are polyureas due to the small size of the thickener.
Small particle size thickeners do not remain in numerous ball bearing contact areas and can reduce operating noise.
Summary
Lubricants can have different performance characteristics depending on their basic composition.
With millions of applications requiring lubrication in today's world, it is important not to assume that one or two types of lubricant will meet all of your equipment's needs.
It is important to consider the many aspects of lubrication needs and their impact on the appropriate lubricant when selecting a lubricant for the following applications
