## 1 Introduction
Such operation conditions are the cause of considerable damage to and wear of not only the wheel or axle rolling surface, as described in, for example [19; 20; 21], but also in the wheel/axle push fit joint zone. It is precisely in that zone that the wear is least recognised, and it also poses a real threat to the safety of the journey. Fretting is one of the possible kinds of wear in the wheel/axle forced-in joint zone. In view of the complex nature of physical, chemical, and mechanical phenomena and the influence of a large number of factors on the wear process, that kind of wear has not been fully recognised yet; therefore, the unambiguous description of the mechanism of the phenomenon is yet to be given. It follows from the review of the literature that refuting may consist of several mechanisms, among which fatigue and friction wear, as well as adhesion and corrosion, may be mentioned [22; 23]. The fretting damage phenomenon often appears in the interference fit assembly [24]. Fretting wear is tribological wear, whose development is influenced by many factors, for example, operation conditions, external loads, or the material's properties (hardness, roughness) [25]. The variety of factors influencing the initiation and development of fretting wear and the test methodology depending on the kind of tests mean that no unambiguous definition of fretting wear has been offered until today. However, contact between two elements and oscillatory tangential displacement between them are the necessary conditions for the development of wear [26; 27]. The typical image of fretting wear depicts corrosion traces on the surface of the elements, the increase of the surface roughness height, microcracks, micropits, as well as plastic deformations resulting from adhesive bonds. Adhesion is one of the most dangerous aspects of wear as it causes local overloading, which promotes the initiation and propagation of fretting cracks [28; 29]. In the case of the forced-in joint, the consequences of damage mentioned above include the reduction of surface pressures, which leads to joint weakening and then disconnection. Shafts onto which sleeves are inserted by pressing may have varying diameters. This may be related to the rise of fatigue wear, especially at the point of diameter change. That is why, in most forced-in joints, a periodic/diagnostic technical inspection plan is prepared to eliminate wear, if any. In most cases, process instructions permit the replacement of one of the joint elements. Most often, sleeves are replaced while the same shaft is left in place. That process may be repeated many times until the shaft nominal dimensions have been obtained. When these have been exceeded, hazardous situations may arise. It is economic considerations that weigh in favour of that state of affairs. Sleeve production costs are considerably lower than shaft production costs. Many users who want to reduce repair or inspection costs replace the sleeve with a new one without a detailed shaft inspection. Obviously, that situation does not concern rail vehicle wheel sets, where thorough inspections are carried out. Following such replacement, the next operation period is conducive to the development of wear, which may lead to joint destruction. The object of the research discussed in this article was the forced-in joints because, as it follows from the review of the literature, they are more susceptible to damage and wear than shrink joints. Whereas in the case of forced-in joints, the damage or wear may arise at the joint assembly and disassembly stage and during operation, in the case of shrink joints, the damage to the elements during the assembly or disassembly process is less likely. Considering all the information above, it was assumed that the aim of this article would be to assess the wear of the sleeve/shaft tribological kinematic pair, which operated in rotational bending moment conditions and was repeatedly disassembled from time to time.