Europe Sets Standard: High-Speed Rail Safety Confirmed

Understanding EN 14865-2: Mechanical Stability Testing for Axlebox Greases

EN 14865-2 is a European standard that specifies the test method for evaluating the mechanical stability of lubricating greases used in railway axleboxes for vehicle speeds up to 200 km/h. This test is critical for ensuring the safety, reliability, and long-term performance of rolling stock wheelsets.

The standard provides a rigorous, repeatable procedure to simulate the harsh operating conditions an axlebox bearing experiences, including high rotational speeds and sustained mechanical stress. By assessing how a grease withstands these forces, manufacturers and operators can verify its suitability for demanding rail applications.

The Critical Role of Mechanical Stability in Axlebox Grease

The axlebox is a safety-critical component that houses the bearings supporting the axle of a railway vehicle. The lubricating grease within this assembly performs several vital functions:

• Reduces Friction: It creates a lubricating film between the rolling elements and raceways of the bearing, minimizing wear and heat generation.
• Protects Against Corrosion: It forms a barrier against moisture and contaminants, preventing rust and corrosion.
• Dissipates Heat: It helps to carry heat away from the contact zones within the bearing.
• Provides Sealing: It helps to seal the bearing housing, preventing the ingress of dirt and water.

Mechanical stability is the ability of a grease to maintain its consistency and structure when subjected to mechanical work or shear forces. In a high-speed railway application, grease is constantly churned and sheared by the rolling elements of the bearing. A grease with poor mechanical stability will break down, leading to several potential failure modes:

• Softening and Leakage: The grease can become too fluid, causing it to leak out of the axlebox housing. This results in insufficient lubrication, leading to overheating and catastrophic bearing failure.
• Oil Separation (Bleeding): The base oil may separate from the thickener, leaving a hard, soap-like residue that provides no lubrication.
• Reduced Performance: A change in consistency compromises the grease’s ability to form a stable lubricating film, increasing friction and wear.

EN 14865-2 directly addresses this challenge by providing a standardized method to stress the grease and measure its response.

The EN 14865-2 Test Method Explained

The test is performed using a specialized test rig, most commonly the FAG ROF test rig, which is designed to simulate the dynamic conditions inside an axlebox. The procedure involves several distinct phases.

Test Apparatus and Setup

The core of the test rig consists of a housing containing two standard axlebox roller bearings (e.g., cylindrical or spherical roller bearings). A defined quantity of the test grease is applied to the bearings and the housing. The assembly is driven by an electric motor, and temperature sensors are placed on the outer ring of the bearings to monitor thermal behaviour throughout the test.

Test Procedure Steps

1. Preparation: The test rig, including bearings and housing, is meticulously cleaned to remove any residual lubricant or contaminants.
2. Grease Application: A precise amount of the test grease is carefully distributed within the two test bearings and the free space in the housing, as specified by the standard.
3. Running-in Phase: The rig is operated through a series of incremental speed steps at a relatively low load. This phase allows the grease to distribute evenly within the bearing assembly and reach a stable operating temperature.
4. Main Test Phase: Following the run-in, the rig is operated at a constant high speed (corresponding to a vehicle speed of 200 km/h) for a prolonged duration, typically 500 hours or more. Throughout this phase, the bearing temperature is continuously monitored to detect any signs of lubricant failure, such as a thermal runaway.
5. Evaluation: After the test duration is complete and the rig has cooled down, the assembly is dismantled for a thorough evaluation of the grease and the components.

Evaluation Criteria and Acceptance Limits

A grease’s performance under EN 14865-2 is judged based on several key quantitative and qualitative criteria. The goal is to determine if the grease has maintained its structural integrity and protective properties.

Post-Test Analysis

• Grease Leakage: The amount of grease that has leaked from the bearing housing is collected and weighed. Excessive leakage is a primary failure criterion, as it indicates the grease has softened unacceptably under shear.
• Change in Consistency: The consistency of the grease is measured before and after the test using a cone penetrometer (according to ISO 2137). A significant change in penetration value (either softening or hardening) indicates poor mechanical stability.
• Visual Inspection: The remaining grease is visually inspected for changes in colour, texture, and evidence of excessive oil bleeding. The bearings and housing are also inspected for signs of wear, corrosion, or heavy grease deposits.

Key Test Parameters Summary

The following table outlines the critical parameters assessed during and after the EN 14865-2 test and their importance.

Conclusion: Ensuring Reliability at High Speeds

EN 14865-2 is more than just a laboratory test; it is a crucial benchmark for ensuring the operational safety and reliability of modern railway vehicles. By subjecting axlebox greases to realistic and severe conditions, the standard allows rolling stock manufacturers and operators to select lubricants that can withstand the demands of high-speed service. Passing this test demonstrates that a grease possesses the mechanical stability required to protect critical axlebox bearings over long service intervals, ultimately reducing maintenance costs, preventing costly downtime, and safeguarding the integrity of the entire rail system.