The axle box assembly is a component installed on railway locomotives and rolling stock, fitted onto the axle journal to connect the wheelset and the bogie frame or the body of a two-axle vehicle. It is commonly referred to as an axle box. On railway vehicles, the axle box is installed on the outer side of the wheels, while on steam locomotives, the axle boxes on the driving wheels and leading wheels are installed on the inner side of the wheels. Its function is to transfer the weight of the vehicle body and the load to the wheelset, lubricate the axle journal, reduce friction, and lower running resistance.
Axle box assemblies can be divided into two types based on the type of bearing used: sliding bearing axle box assemblies and rolling bearing axle box assemblies. Early locomotives and rolling stock all used sliding bearing axle box assemblies. Rolling bearing axle box assemblies were first tested in the early 20th century and have now replaced sliding bearing axle box assemblies.
Compared to sliding bearings, rolling bearings have a lower coefficient of friction, reducing the starting and running resistance of locomotives and rolling stock; rolling bearings have less wear and generate less heat, which is easily dissipated, reducing the incidence of "hot axle" and "hot bearing" accidents; rolling bearings have high rotational speed and high load capacity; they shorten maintenance time and improve efficiency; they reduce operating costs and eliminate the need for regular axle inspections; and they offer advantages such as ease of use and interchangeability.
Currently, all railway locomotives and passenger cars in my country are equipped with rolling bearing axle box assemblies, while railway freight cars use axleless rolling bearing assemblies.
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