# Influence of Contamination of Gear Oils in Relation to Time of Operation on Their Lubricity
## 1 Introduction  
Ensuring that machines and devices are able to conduct high-quality work and maintain operational reliability is not only a very important issue in chemical applications [1; 2; 3], e.g., transport [4; 5], electronic systems [6], or scientific works, e.g., for chemical purposes [7], it is also of great importance for applications that are related to the production of high-quality food products [8]. Lubrication is essential for all sliding pairs in all tribosystems [9; 10; 11]. The physical essence of lubrication processes is the conversion of adverse external friction to friction that takes place inside of the tribofilm [12; 13; 14]. In order for good lubrication to be maintained, the grease must have high adhesion to the frictional surfaces in question, and the grease layer that is between these components must be maintained at a certain thickness. This should be maintained regardless of friction speed, pressure, and temperature [15], and this is usually difficult to achieve. The formation of a layer of greaseon the friction surface is associated with the phenomena of the physical absorption of polar particles and the chemical absorption of boundary films as well as a hydrodynamic effect [16]. However, load transfer occurs through a layer of grease that is generated by the hydrodynamic effect in the machine kinematic nodes that are under increasing motion [12]. High-pressure lubrication takes place in concentrated contacts, including in gear contacts [17]. High pressure in the contact area increases the viscosity of the lubricant as well as the elastic deformation of the surfaces that are in contact with each other indirectly through the lubricant film. This is the case for elastohydrodynamic lubrication. The term elasthydrodynamic film is used to refer to the intermediate lubricant film [15]. In gear transmissions, lubrication conditions are not favorable for the formation of a lubricating film. It seems that gullet pressure is one of the determinants. However, as stated in [18], high pressure in the contact area increases the viscosity of the lubricant and the elastic deformation of the contact surfaces in an indirect way through the lubricant film. Moreover, pressure leads to teeth bending, and when teeth pairing occurs, the tips of the teeth belong to the powered wheel rub the grease from the surface of the powering wheel. Modifying the teeth only partially eliminates this phenomenon [15]. Moreover, high circumferential speeds result in oil being removed from the contact area. It is known that the formation of a lubricating film in oil is intended to prevent metal surfaces from coming into direct contact with each other, but it also refers to a situation in which wear and high friction occur between sliding surfaces. In light of the above situation, this becomes difficult in the context of gear boxes. In addition, oil properties change during exploitation, and this represents another disadvantage. It can be assumed that the contamination of the used oil may impair the ability to form a permanent tribofilm that reduces the friction and wear of sliding pairs [19]. The lubricant that is used in closed-circuit mechanical systems is subject to aging [18]. It undergoes oxidation because it comes into contact with air. The particles, which are a product of tribological wear, also get into the oil. The contamination of gear oils with wear products may result in power losses, among other consequences, as well as decreases in the flow resistance of the lubricant [19]. Therefore, the quality and condition of the lubricant affects the friction resistance in gear boxes and may affect the efficiency of tribomechanical systems [20; 21]. The applications that oil is used for may lead to changes in its viscosity [19]. On the other hand, as shown in paper [21], oil viscosity results in the power losses in meshing when operating under a working load. It is for this reason that periodic oil changes are performed [15]. Synthetic oils are usually used in the gear boxes of modern vehicles.