Designing the Perfect Circle: EN 13979-1 Wheel Approval

Introduction to EN 13979-1

Designing a railway wheel is a balancing act. It must be strong enough to carry 25 tons of axle load without bending, yet flexible enough to expand when heated by brake blocks without cracking. EN 13979-1, titled “Railway applications – Wheelsets and bogies – Monobloc wheels – Technical approval procedure – Part 1: Forged and rolled wheels,” is the standard that engineers use to prove their design works before mass production begins.

While EN 13262 governs the manufacturing quality of the steel, EN 13979-1 governs the geometry and physics of the shape. It answers the question: “Will this wheel shape survive 30 years of braking and rolling?”

Snippet Definition: What is EN 13979-1?

EN 13979-1 is a European standard that defines the technical approval procedure for the design of forged and rolled monobloc railway wheels. It specifies the mandatory assessment stages, including geometric assessment, thermomechanical finite element analysis (FEA) to simulate braking heat, and physical bench testing to validate the wheel’s resistance to fatigue and thermal stress.

The Two Main Threats

EN 13979-1 requires the design to be validated against two distinct failure modes:

1. Mechanical Fatigue (Rolling)

Every time the wheel turns, the load from the axle compresses the web (the part between the hub and the rim). After millions of revolutions, this cyclic stress can cause cracks.

• Calculation: The standard requires a calculation of the stress in the web using the maximum vertical and lateral forces (including dynamic factors).
• Limit: The calculated stress must remain below the “Fatigue Limit” of the steel grade (e.g., ER7 or ER8).

2. Thermomechanical Stress (Braking)

This is critical for freight trains using tread brakes (blocks pressing on the wheel). The friction generates massive heat, causing the rim to expand. Since the hub remains cool, the wheel tries to tear itself apart or buckle.

• Drag Braking Simulation: The design is tested electronically (and later physically) against a scenario of “Drag Braking” (e.g., holding the brakes for 45 minutes while descending a mountain).
• Residual Stress: After cooling, the wheel must not have developed dangerous “tensile” residual stresses in the rim, which would promote crack growth. The rim must remain in “compression.”

The Approval Process: Two Stages

You cannot just build a wheel and put it on a train. EN 13979-1 mandates a phased approach.

Stage 1: Geometric & Theoretical Assessment

Engineers use computer models (FEM) to simulate the worst-case scenarios. If the computer says the web is too thin or the shape will warp under heat, the design is rejected on paper.

Stage 2: Bench Testing (Physical)

If the design passes Stage 1, prototypes are manufactured and put on a Brake Test Rig.

• The wheel is spun and subjected to repeated braking cycles until it glows red hot.
• Pass Criteria: After the test, the lateral displacement of the rim (warping) must be within limits (e.g., < +2mm / -0.5mm), and no cracks can be found using ultrasound.

Comparison: EN 13979-1 vs. EN 13262

These two standards are often confused but serve different phases of the lifecycle.

Operational Relevance

Why do curved webs exist? If you look at a modern freight wheel, the web is S-shaped, not straight. This shape is a direct result of EN 13979-1 optimization. The S-shape acts like a spring, allowing the rim to expand radially when hot without creating excessive stress at the hub connection.