## 1 Introduction  
Push-fit joints are one of the most frequently used assembly methods [1]. This is owing to the advantages of that type of joint, which include: precise alignment, the capacity to transfer high variable and impact loads, and the possibility of repeated disassembly. Depending on the assembly method, push-fit joints may be divided into two groups. The first is shrink joints, and the other is the push-fit ones. In the case of a shrink joint, the assembly process requires a change in the temperature of one or two elements joined together in order to obtain sufficient assembly clearance. When the temperatures have become equal, suitable pressure is obtained, which makes it possible to obtain the joint. Making a push-fit joint requires the use of an appropriate force, which is determined on the basis of relevant formulae and standards. In this case, joint life depends on the value of the friction force exerted by the mutual pressure of the elements joined together. In turn, that force generates suitable surface pressures preventing joint disconnection. Among the push-fit joint connection methods mentioned above, forced-in joints are used most often, which is undoubtedly owing to their advantages. Among these, the ease of joint making, which does not require the use of special workplaces or complicated instrumentation, and the high reliability of joint operation may be mentioned. Despite many advantages of forced-in joints, they also have weak points, which may significantly influence the operation of that type of joint. Scratches, grooves, and surface seizures [2; 3; 4], as well as plastic deformations resulting from the adhesive bond formation, may occur during both joint assembly and disassembly. The form and magnitude of damage to and wear of the elements depend on many factors and directly influence the cost of reworking repairs [5; 6]. In turn, joint life is strongly influenced by the technical condition of the top layer of the elements joined together, which changes depending on the kind and magnitude of wear. Surface roughness is one of the factors which influence the rate and kind of wear of the mating elements. Extensive scientific research was conducted, which confirmed that the elements with smaller roughness are more susceptible to wear due to a larger contact area and possible adhesive wear [7; 8; 9]. Wear is the main phenomenon that deteriorates the element's life [10]. In view of the advantages mentioned above, forced-in joints are used, for example, in motor vehicle power transmission systems [11] and in some aircraft systems [12]. Forced-in joints are also used in rail vehicles. First of all, wheel systems and, within them, the brake disc/wheelset axle assembly and the wheel/wheelset axle assembly [13; 14] can be mentioned here. Among the examples of the use of forced-in joints, the joint between the wheel and wheelset axle of the rail vehicle deserves attention. This is the assembly which requires special attention due to its functions in the vehicle. The main task of the wheelset is to run the vehicle on the track, but the set also directly influences the safety [15; 16] and comfort of the journey. Any damage not found on time may develop into fatigue wear and cause a railway incident, with vehicle derailing as its natural consequence. Such a situation may also involve high financial losses and cause damage to the passengers' health. In relation to that, the wheelset is subject to strict manufacturing technology and control at both the manufacturing and operation stages. Due to the rail vehicle wheel set operation conditions, the set is exposed to wear [17], especially in the wheel/axle joint zone, which is quicker than the wear of other vehicle assemblies. Heavy operation conditions are due to external loads acting during the ride. These loads comprise static loads from the vehicle weight and dynamic loads from riding on the track. In these conditions, it is also appropriate to take forced-in joint assembly stresses into consideration [18].