UIC 811-2: Axle Tolerances & Geometric Specifications – 2026 Engineering Guide

Precision is not a luxury in railway engineering; it is a safety requirement. UIC 811-2 establishes the rigorous dimensional and geometric tolerances for the supply of axles for tractive and trailing stock. It ensures that the axle fits perfectly into the bogie assembly and operates without dangerous eccentricity.

1. Dimensional Tolerances (The Fitment Zones)

An axle is divided into functional zones, each requiring different precision levels. UIC 811-2 specifies tolerances for length, diameter, and chamfers.

• Journal Zone (Bearing Seat): Requires the highest precision (often ISO Tolerance Grade IT6/IT7) to prevent bearing fretting or seizure.
• Wheel Seat: Critical for the Interference Fit (Press-fit). If the diameter is too small, the wheel slips; if too large, the hub cracks.
• Body (Middle Section): Tolerances are looser here, as this area does not mate with other components, but surface finish remains critical for fatigue.

2. Geometric Tolerances: Run-out and Concentricity

A railway axle rotates at high speeds. Any deviation from a perfect cylinder causes vibration. UIC 811-2 focuses heavily on “Run-out”.

3. Surface Roughness ($Ra$)

Surface texture directly impacts fatigue life. A rough surface acts as a stress raiser (crack initiation point). UIC 811-2 mandates specific roughness values ($Ra$):

• Journals & Wheel Seats: Must be ground or rolled to a mirror-like finish (e.g., $Ra \le 0.8 \mu m$) to ensure uniform stress distribution during press-fitting.
• Body: Even the painted middle section requires a controlled finish (e.g., $Ra \le 3.2 \mu m$) to prevent corrosion pits from becoming fatigue cracks.

4. Final Balancing

For high-speed applications ($>120 km/h$), the finished axle must be dynamically balanced. UIC 811-2 defines the maximum permissible residual unbalance (usually expressed in gram-millimeters or g-mm/kg), ensuring smooth running and reducing track damage.