UIC 812-2: Solid Wheel Tolerances & Geometric Specifications – 2026 Engineering Guide

At 300 km/h, a wheel isn’t just a rolling part; it’s a centrifuge. UIC 812-2 defines the rigorous tolerances for the supply of solid wheels for tractive and trailing stock. It governs the precise geometry required to prevent dangerous vibrations and track damage.

1. Run-out: The Battle Against Vibration

The most critical aspect of UIC 812-2 is limiting “Run-out” (deviation from a perfect circle or plane). Excessive run-out destroys bearings and rails.

• Radial Run-out (Ovality): The difference in radius as the wheel rotates.
  
  Risk: Creates vertical pounding forces on the rail (Flat wheel effect).
• Axial Run-out (Wobble): The side-to-side movement of the rim.
  
  Risk: Causes flange wear and lateral instability (Hunting oscillation).

2. Web Geometry and Brake Discs

For wheels with Web-Mounted Brake Discs, the tolerances on the web (the plate connecting hub and rim) are even stricter.

• Web Thickness: Must be uniform to prevent thermal warping during braking.
• Flatness: The mounting surface for brake discs must be perfectly flat to ensure 100% contact area and heat transfer.

3. Residual Unbalance (Balancing)

A wheel might look round but be heavy on one side due to internal density variations. UIC 812-2 requires Dynamic Balancing for high-speed stock.

• Measurement: Expressed in gram-meters (g.m).
• Limit: For a typical passenger coach wheel running at 200 km/h, the maximum residual unbalance is often capped at 75 g.m.
• Correction: Excess mass is removed by machining (grinding) specific non-stress zones on the wheel web.

4. Surface Finish ($Ra$)

The roughness of the machined surfaces impacts fatigue life and fitment.

• Bore (Hub Hole): Requires a fine finish ($Ra \approx 1.6 – 3.2 \mu m$) to ensure a secure interference fit with the axle.
• Tread Profile: Must be smooth to reduce rolling noise and initial rail wear.