The Wheel’s Shield: EN 14535-2 Wheel-Mounted Discs

Introduction to EN 14535-2

In standard passenger trains, brake discs are usually mounted on the axle (Part 1). However, in powered bogies (where the motor takes up all the space) or low-floor trams (where small wheels leave no room), there is no space on the axle shaft. The solution is to bolt the friction surfaces directly onto the sides of the wheel itself.

EN 14535-2, titled “Railway applications – Brake discs for railway rolling stock – Part 2: Brake discs mounted on the wheel,” is the safety manual for this compact design. It addresses the unique danger of this configuration: transferring the immense heat of braking directly into the structural wheel, which carries the train’s weight.

Snippet Definition: What is EN 14535-2?

EN 14535-2 is a European standard specifying the design, testing, and performance requirements for brake discs mounted on the wheel web (often called “Cheek Discs”). It covers the friction ring’s material (cast iron, steel, or composite), the fastening system (bolts/sleeves), and the rigorous dynamometer testing required to prove that the heat generated during braking will not cause the wheel to crack or the disc to warp.

The Engineering Challenge: Heat Transfer

Unlike axle-mounted discs which are isolated, a wheel-mounted disc is intimate with the wheel. EN 14535-2 focuses heavily on managing this thermal relationship:

1. Thermal Isolation

If the friction ring gets red hot (600°C) and touches the cold wheel web, the wheel might warp or lose its interference fit with the axle.

• Ventilation: The standard encourages designs with internal cooling vanes that act as a fan, pumping air between the disc and the wheel web.
• Contact Points: The disc usually touches the wheel only at specific mounting bosses, minimizing conductive heat transfer.

2. The Fastening System (Expansion)

A hot disc expands (grows larger) while the cold wheel stays the same size. If they were rigidly bolted, the bolts would shear off.

• Sliding Sleeves: EN 14535-2 validates the use of special “sliding” fasteners or radial slots. These allow the friction ring to expand radially outwards as it heats up, without stressing the bolts or the wheel.

Testing Regime

Before approval, the disc assembly undergoes “Bench Testing” on a full-scale inertia dynamometer.

• Energy Absorption Test: Simulating a fully loaded train stopping from maximum speed (e.g., 200 km/h) multiple times.
• Drag Braking Test: Simulating a long mountain descent (e.g., 30 minutes at constant power) to saturate the assembly with heat.
• Acceptance Criteria: No thermal cracks penetrating to the core, no loosening of bolts, and the wheel web temperature must stay below critical limits (typically defined by the wheel standard EN 13262).

Comparison: Axle-Mounted (Part 1) vs. Wheel-Mounted (Part 2)

Two solutions for the same problem.

Operational Relevance

Noise Reduction: Wheel-mounted discs act as a “shield” or damper on the wheel web. EN 14535-2 compliant discs often have the secondary benefit of reducing the “squeal” of wheels in curves by damping the wheel’s natural vibration modes.