Spoke railway wheels: return to the future

Railway wheels

Wheels are a key element of any rail vehicle. They make up a significant part of the cost, eg, the carriage during its service life. The main contribution to the cost of a wheel comes from its mass., which should accelerate at the beginning of the movement and decelerate when braking. Since the wheel mass is not spring-loaded ("Not sprung"), then forces, which act between wheel and rail especially depend on the mass of the wheel.

Picture 1 - Typical steel rail (carriage) wheel [1]

Steel wheels

Wear between wheel and rail is critical, which in addition to the dynamic behavior of the vehicle, highly material dependent, from which the wheel is made and the quality of the rolling surface of the wheel. Wheels are also a major contributor to noise., which the railroad cars emit.

Currently, the railway wheels are almost 100 % have steel rims. These rims are or are part of a compound wheel and fit over the wheel center, or are part of a one-piece wheel (picture 1).


Picture 2 - Various types of railway wheels:
and – composite (bandage); b – whole [2]

All steel, which are used for the manufacture of railway wheels, have basically the same chemical composition - these are carbon steels with various types of heat treatment. Steel as a wheel material is dominated by its high hardness and high wear resistance. Besides, steel has high fatigue strength and high toughness. The properties of steel at the point of contact between the wheel and the rail are enhanced by high-quality heat treatment of the wheel rim.

For the manufacture of wheels, special technologies of molding and casting are used.. Forging technology uses special forging and rolling tools, which are optimized for wheel production. Foundry technology adopts high quality multiple models and optimized injection molding technology.

but, the use of steel as a wheel material and modern technologies for their manufacture limit the freedom to create new designs of wheel rims. Typically, wheel rims are axisymmetric and subject to complex machining..

Austenitic-bainitic nodular cast iron wheels

Austenitic-bainitic nodular cast iron (ABCHSHG) is already widely used in mechanical engineering and power engineering. In English, this cast iron is called austempered ductile iron. (NAME) Until recently, it was not used as a material for railway wheels.. The main reason for this is the high requirements, which are presented to the wheels at the point of their contact with the rails.

The wheel material for a one-piece wheel must simultaneously have high strength and high wear resistance at the point of contact between the wheel and the rail., as well as high toughness and fatigue strength in the wheel disc. Austenitic-bainitic nodular cast iron has twice the strength, than conventional nodular cast iron, and with the elongation at break. It has a combination of high fatigue strength, high wear resistance, high viscosity, as well as more possibilities for the development of new wheel designs and good machinability.


Picture 3 – Wheelset with spoke wheels made of austenitic-bainitic cast iron with nodular bainite [2].

Strength properties of railway wheels

Characteristics and properties of austenitic-bainitic nodular cast iron sets EN 1564. This cast iron has a tensile strength of 800 to 1400 N / mm2, yield point from 500 to 1100 N / mm2, elongation at break from 1 to 10% and Brinell hardness 250-380 NV.

For comparison, according to EN 13262 European ER8 wheel steel has a tensile strength in the rim of 860 to 980 N / mm2, yield point not less 540 N / mm2, elongation at break, not less 13% and Brinell hardness not less 245 NV. The mechanical properties of austenitic-bainitic nodular cast iron are quite comparable to those of the used ER8 wheel steel..

The use of this cast iron gives wider possibilities for designing a wheel disc.. Instead of an axisymmetric disc, the rim and hub can be connected to each other using a disc, which has numerous holes to form spokes. These spokes can be placed in this way, to optimally transfer the load from the rim to the hub.

An additional advantage of the spoke wheel is the reduced noise emission. This is because, that the disc of a conventional wheel emits sound like a membrane, what does not happen with a spoke wheel.

While this new wheel material is not regulated by European standards. At the same time, IN 13262 provides an extensive test program, with which you can carefully study the properties of railway wheels from any material.

Sources:

  1. Design Technologies for Railway Wheels and Future Prospects Yoshinori OKAGATA*NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 105 DECEMBER 2013
  2. Application of Austempered Ductile Irons to structural components of railway vehicles / Gianluca Megna – PhD Thesis, University of Florence, 2019
  3. Back to the future for spoked wheels / Franz-Josef Weber - 2019 – railjournal.com