Lubricating Greases
Lubricating greases can be defined as solid or semi-fluid products resulting from the dispersion of a thickening agent in a lubricating fluid, with additional components often being used to give them special properties.
Greases are not easily defined as liquids or solids and are therefore often described as plastic solids with viscoelastic properties. Lubricating greases are structural elements, especially if they are used as long-life lubricants or for lifetime lubrication.
Another simple way to describe a grease is to think of it as a sponge. The thickener is the sponge and the base oil is the water. The thickener in the grease creates a matrix that holds the base oil in place in a solid structure and gives the grease its consistency. When the grease is under different operating conditions, such as temperature, load or shear, a viscoelastic flow is created in the grease. The plasticity of the grease, its consistency, is usually given a number according to the NLGI classification system. Semi-liquid (00) to normal (2) are the most common.
Advantages of lubricating greases
There are many advantages to using a grease over an oil, and these can be summarised in terms of their relevance to lubrication practice:
- Better surface adhesion.
- Better sealing and isolation from the medium.
- Excellent wear protection.
- Higher lubrication at high loads and low speeds.
- Higher corrosion protection.
- Wider operating temperature range.
- Reduced lubrication point migration.
However, there are several circumstances where a grease is a more technically constrained choice than an oil: where effective heat dissipation through the lubricant is required, where effective removal of physical particulate contaminants is required, and where a lubricant is required for very high speed regimes where a dynamically very light lubricant is required.
What do lubricating greases consist of?
In their formulation, greases contain 65 to 95% by weight of lubricating oil (mineral and/or synthetic), 5 to 35% of thickener and 0 to 10% of additives (liquid and/or solid). Apart from the percentage of each of these components, the chosen type(s) of each component will be decisive for the properties of the lubricating grease:
The type of base oil and its viscosity are of fundamental importance for certain basic properties of greases. Operating temperature, pumpability, extreme pressure performance, ageing stability, elastomer compatibility, adhesiveness, oil separation and noise suppression are just a few of the characteristics of a grease that are directly determined or influenced by the base oil.
1. Mineral 2. Synthetic
>> Polyalphaolefin
>> Polyglycol
>> Ester
>> Silicone
>> Polyetherperfluorinated (PFPE)
The thickener is a key component in transforming the lubricant from a liquid oil to a consistent compound. Its function is therefore to retain the base oil within its structure and to act as a dispenser of the base oil when the grease is under pressure to lubricate the elements in contact. In the same way, the thickener must also act as a recovery element for the oil when the load ceases, maintaining the consistency with minimal variations for acceptable periods of time.
The nature of the thickener, together with the correct choice of base oil, has a significant influence on the fundamental properties of greases such as water resistance, corrosion protection, sealing ability and resistance to extreme loads and temperatures.
The most used types of thickeners are classified as follows:
| Simple soap | Complex soap | Non-soapy |
| Lithium | Lithium | Polyurea |
| Calcium | Calcium | Bentonite |
| Aluminium | Aluminium | Polytetrafluoroethylene (PTFE) |
| Sodium | Sodium | Silica gel |
| Other | Other |
Additives are added to greases to improve or provide certain characteristics that are not achieved by oil and thickener alone. A grease may contain up to 10 % of additives, of one or more types in combination, of which the following are the most important in terms of their use:
- Extreme Pressure (EP)
- Anti-wear
- Friction modifiers
- Corrosion protectors
- Antioxidants
- Adhesion enhancers
- Solid lubricants
Factors to consider when selecting a grease
When selecting a lubricant, it is necessary to know the mechanical and functional requirements and the operating conditions of the lubricated equipment. These aspects are fundamental for the correct choice of a reference and for the definition of product quantity and relubrication periods.
By interpreting data such as the following, we will be able to define the properties of the product to be recommended (NLGI consistency, thickener, synthetic or mineral nature of the base oil, viscosity of the base oil, need for solid lubricant, penetration capacity of the grease, pumpability of the grease, required additives, etc.):
- Element to be lubricated (bearing, chain, cable...)
- Linear/circular speed
- Workload
- Lubrication System
- Vibrations
- Current lubrication frequency
- Working temperature range
- Environmental conditions (dust, water, aggressive environments...)
- Possible incompatibilities (materials, products in service...)
Do not hesitate to contact our team for advice.
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+34 93 547 58 59
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info.es@fuchs.com
