“Joints” on rails: the technical code for wheelset devices

Behind the speeding train, there is a set of precision devices that silently bear tens of millions of tons of weight and endure high-frequency vibrations – the wheelset axle box device. It is like the knee joint of the human body, which not only supports the walking of the vehicle body but also flexibly turns. It is the core guarantee for the safe and efficient operation of railway vehicles.

1.Wheelset: “Running Shoes” on Rails

The wheelset consists of an axle and two wheels and is the only part of the train that contacts the track. When the train is speeding at a speed of 350 kilometers per hour, the contact area between each wheel and the rail is less than the size of a coin, but it has to bear a vertical load of more than 10 tons. To meet this challenge, the wheels are made of high-strength alloy steel, and a high-hardness tread is formed through a heat treatment process while retaining the toughness of the wheel rim to ensure an accurate fit to the track when cornering.
The wheelset directly transfers the weight of the vehicle to the rail, generates traction or braking force through the adhesion between the wheel and the rail, and realizes the operation of the vehicle on the rail through the rotation of the wheelset. When braking, the vehicle can stop within the specified distance; in addition, it also bears static and dynamic forces from the car body and rails, and the forces are very complex. The performance of the wheelset directly affects the running quality of the vehicle.

What is even more amazing is that when manufacturing the wheelset, the wheels and axles must be pressed together to a “seamless” degree through interference fit technology – the assembly error between the two must be controlled within 0.01 mm, which is equivalent to 1/6 of the diameter of a hair. This high-precision process enables the wheelset to maintain a stable state under the condition of extreme torque, effectively avoiding the potential derailment caused by loosening.

2.Composition of wheelsets

1) Axle
The axle is one of the basic components of the bogie, it is like a “big tie”, connecting the wheels and the bogie frame, and can also support the bogie and the body, passing the traction force and braking force, and the weight of the body is carried by it.
2) Axle journal
The axle journal is the part with the smallest outer diameter on the axle. Its functions are: 1. to install rolling bearings; 2. to bear the weight of the vehicle; 3. to transmit static and dynamic loads in all directions
3) Wheel seat
The wheel seat is the part where the axle and the wheel match, and it is also the part with the largest diameter on the axle. At the same time, the wheel seat is the part where the wheel is subjected to the greatest force. In order to ensure that there is sufficient clamping force between the wheel and the axle, the wheel seat diameter is often 0.10~0.35mm larger than the wheel hole diameter.
In order to facilitate the press-fitting of the wheel axle and reduce stress concentration, the outer diameter of the wheel seat often gradually decreases outward to form a cone. The cone length is 12~16mm, and the small end diameter is 1mm smaller than the large end diameter. This makes it easier to park the wheel when the wheelset is pressed, and it is also easier to press-fit and prevent the wheel hole from scratching the axle.
4) Dust plate
The dust plate seat is the part where the axle and the dust plate fit together. Its diameter is larger than the journal diameter and smaller than the wheel seat diameter. It is the intermediate transition part between the axle diameter and the wheel seat to reduce stress concentration.