What is the difference between oil and grease?
As with all lubricants, the main function of a grease is to prevent contact between moving surfaces, thereby minimising friction, and wear. In the best case, complete lubrication (hydrodynamic lubrication) is achieved where the surfaces are completely separated by the lubricant film. The Stribeck curve illustrates this (Fig. 1).
This is the ideal situation for the continuous operation of a unit or machine. During start-up and at high loads, contact between the surfaces inevitably occurs, which is known as mixed friction. In some areas, this leads to contact between the roughness of the surfaces. That is where the additives added to the grease react chemically and physically through thermal activation to protect both surfaces from wear. The different states of friction are also clearly shown in Fig. 1.
Other functions of greases are to protect against corrosion and to provide an external seal against the ingress of water and dirt, thereby supporting the seals used. In this respect, compatibility with plastics and elastomers, which can determine the life of a unit or machine, is critical and increasingly important. Lubricating greases can also contribute to noise and vibration damping and, depending on the design, also improve efficiency.
| Properties | Grease lubrication | Oil lubrication |
| Refrigeration | Poor | Good (recirculating lubrication) |
| Fluency | Low | Good |
| Compatibility with seals | Simple | Complex, maintenance required |
| Sealing capacity | High | Low |
| Filterability | Poor | Good |
| Complete lubricant replacement | Complex | Simple |
| Leakage tendency | Low | Medium - high |
| Contamination in case of leakage | Low | High |
| Cost of maintenance and supervision | Low | Medium-high |
| Lifetime lubrication | Good (possible) | Bad (generally not possible) |
Lubricating greases can be classified in many different ways. To give just a few examples, they are classified according to their application or by the type of machine element to be lubricated (rolling bearings, plain bearings, wheel bearings, joints, cardan shafts, gear greases, wire ropes, etc.), according to their temperature range (high or low temperature, or temperature range of use), or according to whether they are high performance or multi-purpose greases. Categories by industry, thickener type or base oil type are also common. Finally, it is also quite common to find them classified by NLGI grade or consistency.
A significant advantage of lubricating greases over oils is their consistency, which allows them to remain at a lubrication point for longer and not flow away due to centrifugal force/gravity. This means applications can be made less complex in terms of supply and sealing by using a grease. The correct choice of thickener is important as it determines the drop point (which in turn affects the maximum operating temperature of the grease) and water resistance. Compatibility with plastics and elastomers will also be a consideration, although the affinity of these materials for the base oil will be the key factor here.
Other requirements for greases include use at extreme temperatures (from -70°C to +280°C), low or, depending on the application, high coefficients of friction, wear protection even at high loads and low sliding speeds, corrosion protection in the presence of aggressive media, high and low speeds and good suitability for use in centralised lubrication systems. Developing reliable and future-ready greases requires a systematic approach to complex tasks. For this, the concept of "tribosystem analysis" is suitable, where the lubrication problem to be analysed is experimentally divided into the relevant subfunctions.
In short, lubricating greases are used wherever lubrication with oils is impractical for technical and economic reasons. Applications range from simple loss lubrication, typically used in agriculture, mining and primary industries such as cement, rubber, and steel, to applications where the grease must remain in good condition for the life of the element it lubricates, such as automotive components where it is a critical design element. Greases account for only a small proportion of total lubricant consumption, but they are especially important for the reliability and performance of the equipment, machinery, and components where they are applied, sometimes even as a safety feature.
