Railway wheels: technical requirements for design

The railway wheel is a critical element of the wagon

The railway wheel is one of the critical elements, which ensure the safe operation of railway rolling stock. The wheels carry the entire weight of the wagon, however they cannot be designed as

the system, which, in the event of a breakdown of one of the elements, has reserve funds, which prevent catastrophic destruction. Therefore, the strength of the wheel requires absolutely high reliability.. Therefore, the most important and fundamental characteristic in wheel design is strength.. but, although "being indestructible" is the true main characteristic of the wheel, there are other important characteristics, which determine the efficiency of the wheel, such as:

  • Wear resistance
  • Resistant to thermal cracking
  • Noise and vibration characteristics

In particular, since the wheels are wearable (expendable) elements, their service life plays an important role in saving the cost of rolling stock maintenance.

There are two main approaches to improving wheel quality.:

  • wheel material development (wheel steel) and
  • development of an optimal wheel design.

There are two types of railway wheels - one-piece and composite, which consist of a wheel center and a tire. Solid wheels are most widely used. Only they will be considered below..

Wheel design

A typical construction of a one-piece railroad wheel consists of three main elements, as it shown on the picture 1. The main elements of the wheel are:

  • trap, into which the axle of the wheelset is pressed
  • rim, which contacts the rail and
  • disk, which connects the hub and rim.

Wheel surface, which directly contacts the rail is the rolling surface and flange (crest).

Picture 1 - One-piece railroad wheel design

Railway wheel design parameters

Weight

Wheels are unsprung elements of the car. Therefore, it is preferable, to keep them light, to have as little influence as possible on the smooth movement of the wheeled cart. This is especially important for high speed trains..

Disc fatigue strength

The disc must have sufficient fatigue strength, to withstand cyclic mechanical stresses from the weight of the car.

Fatigue resistance of the rolling surface

Sufficient fatigue strength, to withstand contact stresses (Hertz voltage) between rolling surface and rail.

Resistance to thermal stress braking

Shoe braking brings a lot of heat into the rim. As a result, significant temperature stresses arise in the rim and disc.. Sometimes excessive heat flux into the rim can change the normal distribution of residual stresses. (compressive hoop stresses) for unfavorable stress distribution (tensile hoop stresses) (picture 2).

Picture 2 – The emergence of tensile residual stresses during braking heating of the wheel [1]

Thermal cracks and brittle fracture toughness

During shoe braking, as a result of friction between the shoe and the rolling surface of the rim, thermal cracks appear in the rim., which can extend deeper into the rim. In the worst case, wheel destruction occurs. (cm. picture 2).

Wear resistance

Abrasion (export) occurs on the rolling surface, there, where does she contact the rail. In the case of shoe braking, wear also occurs between the brake pads and the rolling surface.. The service life of the wheel directly depends on this characteristic.. In some cases, more problematic, than wear by itself, is uneven wear.

Rail Traffic Characteristics

The stability of movement on straight sections and when cornering is strongly influenced by the design of the bogie.. However, the rolling surface profile also plays a role..

Acoustic performance

Reducing noise from a moving railway wheel is required in terms of environmental protection requirements. There are several approaches to this problem., such as: improvement of bogie design or rail lubrication. One of the solutions is to install noise dampers and absorbers on the wheels..

Vibration

Vibration, produced by the wheel, comes from damage to the rolling surface and from its imbalance. The first factor depends on the fatigue strength of the rolling surface from rolling contact loads and the resistance of the rolling surface to wear.. The second factor depends on the accuracy of machining during manufacture and subsequent maintenance.. This characteristic is especially important for high-speed trains..

Axle connection force

This characteristic ensures a strong connection between the wheel and the axle.. Usually there are no problems with this., if the insertion force is properly controlled according to the technical instructions.

Types of mechanical loads on the wheel

There are two types of mechanical stress, which act on the railway wheel: vertical load and horizontal load. As it shown on the picture 2, vertical load is the load from the weight of the car in the vertical direction. With regard to the horizontal load, then they are of two types: side load and rear load. Side load acts on the flange (flange) in curved sections of the track. The rear load acts in the opposite direction to the side load on the rear face of the flange when passing the guide rails, eg, on the arrows.

Since the lateral load occurs constantly when passing curved sections of the path, then it is the most important in the design of the disk. On the other hand, rear load only occurs when the rear side of the wheel is in contact with the guide rail, mainly when passing arrows. It means, that rear loading occurs much less frequently, than side load. Therefore, the rear load is usually not considered when designing a disc..

Picture 3 – Main mechanical loads on the wheel [1]

Wheel design

Disc design

Among all the characteristics, the above, characteristics are of particular importance, which refer to the disc and rim. There are almost no possible changes to the hub design. Disc and rim design can improve or degrade these characteristics.. Therefore, it is the design optimization, as well as the material of the disc and rim, special attention is usually paid.

Consider the following issues when designing a disc:

  • the influence of the design of the disc on the stresses in it from mechanical loads
  • fatigue strength of disc material
  • relationship between disk configuration and thermal stress distribution, which occurs during braking.

(cm. disk design article - in progress)

Rim design

When designing a rim, consider the following characteristics:

  • tread wear parameters
  • thermal cracks and brittle fracture resistance
  • Fatigue strength of the rolling surface under rolling contact loads.

(cm. rim design article - in progress)

Source:

  1. Design Technologies for Railway Wheels and Future Prospects / NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 105 DECEMBER 2013