# Railway Axle and Wheel Assembly Press-Fitting Force Characteristics and Holding Torque Capacity
## 1 Introduction  
A railway wheelset is an assembly consisting of an axle and two wheels fitted with interference and, where necessary, applicable associated components, e.g., gear-shaft, bearing bushing, etc. Wheelsets need to be safe because failures could lead to derailment and potentially to major safety issues, including loss of life and heavy damage to railway vehicles. Therefore, it is important to assemble the wheelset correctly to reduce accidents and derailments. Wheelsets are assembled to remain attached to all components while in operation. Wheelsets may be press-fitted or shrink-fitted to the axles. Press-fit, or interference fit, tightens two parts together by relying on friction and joining the parts that can take a different form. This is commonly used in the industry because the process is reliable, simple, and does not require heating, cooling, or soldering. One problem with railway wheelsets is their failure due to excessive interference in press-fitting, and this can result in a slip between the wheel and axle. Fretting wear takes place at the interface of the axle wheel seat and the wheel hub surface, and it can cause surface fatigue, adhesion, oxidation, exfoliation, and scratching [1, 2]. If a press-fit is used for assembling the wheels onto the axle, the wheel mounting peak press force limits and the press-fitting curve must be considered. Furthermore, many standards have been defined for railway wheelset assembly; for example, JIS E 4504 [3], EN 13260 [4], ISO 100-5 [5], and AAR [6]. Railways tend to follow national standards, so the authors followed the State Railway of Thailand's practice and used the European standard EN 13260 for the assembly of wheels and axles [4]. Following the standard improves the quality of the wheelset, and this would also help to standardise production. Very few publications were found in the literature that discussed the characteristics of the press-fitting curve for railway wheelsets. [PERSON] and [PERSON] analysed the contact pressure and speed related to lubrication and predicted a press-fitting curve using an FEM model and a simplified theoretical model [7]. [PERSON] et al. constructed and tested a mathematical model for the press-fit curve based on the AAR standard [8]. [PERSON] et al. predicted the residual stress and plastic strain for forging, machining, and assembling [9]. [PERSON] et al. addressed the effect of the strength of the wheelset press-fit joint and recommended a frictional coefficient value that resisted sliding between the interface for specific conditions by comparing experiment and theory [10]. [PERSON] and [PERSON] investigated fretting wear in a wheelset press-fit joint and conducted contact surface and fretting wear tests; they found that the adhesion phenomenon played a critical part in the initiation of fretting wear in a press-fit joint [11]. [PERSON] and [PERSON] reviewed the factors affecting the interference fit joints' strength and discussed the modelling and limitations related to the friction coefficient, boundary condition, geometry, and material [12]. [PERSON] et al. have also discussed the relationship of radial interference and holding torque capacity, the friction coefficient of camshaft based on the finite element analysis and experiment [13]. [PERSON]'s equation can also be used to determine the contact pressures at the interface of the assembly parts and to estimate the maximum holding torque capacity of a press-fit. A major challenge of press-fit assembly is to choose reasonable interference values that would ensure the safety of the assembled components. Too large an interference may damage the part being assembled, and less resistance may result in plastic deformation. To prevent slipping, the wheelset maximum holding torque capacity must be known. The magnitude of the press fit-force depends on the geometry, friction coefficient, material properties, and operating conditions [14].