The railway wheelset is one of the important parts of the trains. It withstands various forces transmitted from the car body and the rails, and guides the wheels to roll along the rails to complete the operation of the vehicle. The performance of the wheel set directly affects driving safety. Therefore, the wheel set must be strong and durable, and the dimensions of each part must comply with technical regulations to ensure driving safety.

The wheel set consists of an axle and two identical wheels. During assembly, an interference fit is used, and the two wheels are press-fitted on both ends of the axle on an axle press (hydraulic press).

The rail axle is made of high-quality carbon steel that is heated and forged, and is made by heat treatment and mechanical processing. The axle is the main accessory of the wheel set. In addition to forming the wheel set with the wheels, both ends must also cooperate with the axle box oiling device to ensure the safe operation of the vehicle.

Axles are divided into powered axles and non-powered axles. The two axles on the power bogie are both powered axles, and the two axles on the non-powered bogie are both non-powered axles.

The rail wheel is the final stress-bearing accessory of the vehicle. It transfers the load of the vehicle to the rail and rotates on the rail to complete the operation of the vehicle. Its performance directly affects driving safety.

Names of various parts of rail wheels.

• Tread Surface: The outer circumferential surface of the wheel that comes into contact with the rail. The tread surface and the rail surface achieve rolling motion under a certain level of friction.

• Flange: The radially protruding part on the inner side of the wheel, serving to prevent the wheel from derailing and maintaining normal operation on the track.

• Rim: The radial thickness section of the wheel with a complete tread surface, ensuring sufficient strength and facilitating tread surface maintenance.

• Spokes: The part that connects the rim to the hub, providing support.

• Hub: The part of the wheel that interacts with and is fixed to the axle wheel seat. It serves as the backbone and support for the entire wheel structure.

• Hub Hole: The hole used for mounting the axle, with an interference fit with the wheel seat.

The train wheelset runs on the track. When the line is in normal condition, the distance between the inner sides of the wheelset is an important factor affecting driving safety. For this reason, the inner distance of urban rail transit vehicle wheelsets is stipulated to be (1353 ± 2) mm. The reasons are as follows:

① Reduce the wear of wheel rim and rail

In order to reduce the wear of the wheel flange and the rail head, appropriate play must be left between them. According to my country's "Metro Technical Management Regulations", the minimum gauge of a standard gauge line in the straight section is 1433mm, while the maximum wheel back inner distance of a standard wheel set is 1355mm.

When the maximum thickness of the wheel rim is 32mm, the minimum play δ between the rim and the rail can be obtained by the following formula:

δ=1433-(1355×32×2)=14mm

It can be seen from the calculation that the average minimum play between the wheel flange and the rail on each side is 7mm. This play can completely ensure that the wheel flange and the rail will not suffer serious wear under normal conditions. On the other hand, from the perspective of vehicle running quality, excessive play will increase the amplitude of the snaking motion. Therefore, from the aspects of reducing wheel-rail wear and improving vehicle running quality, the travel clearance cannot be too large or too small.

②Safe passage curve

In order to facilitate vehicle steering, the track gauge in the curve section must be appropriately widened. The "Metro Technical Management Regulations" stipulates that the maximum track gauge at the minimum curve radius is 1456mm. When the wheel set runs to the curve section, due to the centrifugal phenomenon, the wheel rim on one side is close to the outer rail, and the wheel tread on the other side should ensure sufficient width on the inner rail to prevent excessive contact stress per unit of the wheel set tread. Cracks or deformation may occur, which may cause wheel derailment in severe cases. The necessary width of the inner wheel tread on the inner rail is called the safe loading capacity.

When the inner distance of the wheel set is at least 1351 mm, the wheel rim thickness is at the thinnest 23 mm, and the maximum track gauge of the minimum curve radius section is 1456 mm, the safe carrying capacity of the wheel set tread (i.e., the theoretical carrying capacity) can be obtained by the following formula:

λ=1351+23+135-1456=53mm.