The main function of lubricants is to keep friction surfaces separate from each other. The formation of a lubricant film prevents direct material contact between the frictional partner surfaces. In that however, the lubricants must ensure the greatest possible force transfer and at the same time reduce the friction and the resulting material wear. Lubricants also have a series of additional functions such as protection against rust formation, sealing of the lubrication point against dirt and environmental influences and removal of heat that occurs.
Lubricants can be divided into three main groups, depending on their aggregate state. Lubricating oils belong to the liquid lubricants, greases and pastes to the semi-solid lubricants and solid lubricants to the class of solid lubricants.
The application areas for lubricants-are very extensive. Lubricants can generally be used where two contact surfaces move relative to each other. But lubricants are no longer just used to reduce frictional resistance and avoid wear effects. Lubricants are also used to eliminate squeak and motion noises or for the tactile enhancement of operating elements such as switches and knobs.
Lubricants defined as aids in reducing friction and the easing of working processes are as old as friction itself and the mastering working processes. 5000 years ago, the Egyptians discovered that their wooden sledges, with which they moved large stone blocks across the desert, slid better over the sand if they poured water on the surface in front of the sledges. Murals in tombs dated about 2000 BC show that even then efforts were made to reduce friction between moving parts using animal fats. The chemical investigation of chariot remains of around 1400 BC were able to show that sheep and cow fats were already being used in the chariot axle hubs. These animal fats and oils were a common lubricant for many applications even up to the middle of the 19th century. The philosophers Aristotle and Plato as well as the universal genius Leonardo da Vinci also concerned themselves with the subject of friction.
The mineral oils began their triumphal march at the end of the 19th century before the start of research and manufacture of synthetic oils in the early part of the 20th century. In the last few decades new and further developments have followed in quick succession that are now peaking in today’s high-performance and specialised lubricants.
Types of lubricants
The actual lubricating performance is due to base oil in almost all liquid and semi-solid lubricants. The most important factors for the efficiency of the oils are the viscosity and the temperature stability. It is only seldom that “pure” oils are used as lubricants. Modern lubricants consist mostly of a base oil or a mixture of several oils and additives. This improves the capabilities of the oils as well as their quality and functionality.
Lubricant greases consist mainly of a base oil and a thickener. The thickener functions as a carrier for the oil and ensures the binding of the oil. The thickener functions like a sponge that exudes oil when subjected to a load and binds the oil again when the load is removed. It generally applies that a lubrication system is only as stable as its least stable component. This means, for example, that the thermal stability of the thickener is also important for the temperature stability of the grease. Both mineral and synthetic substances are provided as common thickeners.
Lubricant pastes are very similar in structure to the lubricant greases, have however a much higher solid proportion than greases. Lubricant pastes can be formulated with a pure thickener or with solid lubricants or with a combination of both. Lubricant pastes with a high proportion of thickener thus give solid lubricants that are often used as an assembly paste. A lubricating paste that has been formulated from oil or grease and solid lubricants as main components combines the benefits of a solid lubricant and a pure grease lubricant.
Solid lubricants are those that form closed and extremely thin lubricant films on treated workpieces. They can be applied in the form of powder or in combination with other substances such as oil or grease. Solid lubricants provide very good protection against wear. Solid lubricants are also the main components of anti-friction paints. With these anti-friction paints, also known as dry lubricants or anti-friction coatings, the solid lubricants are not in the oil or grease but are dissolved in an organic or inorganic bonding agent.
Selection criteria and distinguishing features
Most manufacturers do not develop their lubricants on the basis of abstract requirements but rather more on real conditions in an application – matched to customer requirement. The multitude of individual components, base oil, thickener, solid lubricant and additives, also make a multitude of combinations of the properties of a lubricant possible. The selection of the optimum lubricant for an application is always based on extensive analyses including various specific factors. The main properties of the lubricants must also be in line, just like its secondary properties, with the requirements and claims from users and application.
While the final selection of a lubricant is always to be based on successful trials and tests, a meaningful preselection can already be made based on characteristic values and calculations. The most relevant benchmarks in this are the type of application and the operating conditions as well as the period of use of the lubricant. This means whether a lubricant is only to provide a once-off service, in assembly for example, or whether it has to perform over a longer period – possibly up to the total lifetime of the treated component. The question of whether the lubricant used is to perform continuously or whether it is a matter of so-called interval lubrication is just as important. The compatibility with the materials treated and with the surrounding materials such as water vapour or brine is also of great importance. A lubricant must also meet visual and tactile requirements. If an application lies in an area in which the consumer sees or touches the treated component in normal use, the lubricant may not be seen or touched as far as possible. Furthermore, the environmental compliance, the health-related harmlessness, the handling and the user-friendliness are of great importance. The following applies to all selection criteria: The closer these can be differentiated and specified, the nearer is the approach to the best suitable lubricant for the application.
The type of application includes, among other things, the quality, form, size, bearing and housing structure and which motion sequences are concerned in the application. Many manufacturers already advertise their lubricants as user-typical. These “labels” are often however not meaningful for typical application fields or industrial sectors. On the other hand, these product typifications imply, in part, performance characteristics and functionalities which the lubricants cannot satisfy in practice. For example, lubricants specially intended for bearing applications give very good results in a ball bearing application but fail under certain circumstances in a needle roller bearing and result in costly repair work. It generally applies that each application-related typification of lubricants is certainly helpful but in no way mandatory and cannot under any circumstances replace an expert consultation.
Further limiting factors are the conditions under which the lubricant must work. Especially the operating and ambient temperatures as well as the temperature peaks possible are of great importance in the selection. Details of the drip point, ignition point, and the pour point of the lubricant apply as characteristic quantities.