To solve rolling stock reliability issues and prevent the reoccurrence of premature bearing failures, operators need to track down their root causes, says STEVE WAKELAM, senior applications engineer at SKF

Modern rolling stock achieves extremely high levels of reliability, with assets operating for many hundreds of thousands of kilometres between major overhauls. The failures that do occur, however, remain a significant concern, affecting costs, service levels and safety. While detecting problems early and repairing them quickly remains critical, operators are placing increasing emphasis on actions that help them understand and control the underlying causes of those problems.

SKF has been involved in the supply of bearings and other equipment to the rail industry for decades. Increasingly, our customers in the sector are also making use of specialist root cause analysis to help them diagnose and fix difficult equipment reliability issues.

A structured methodology
Root cause analysis (RCA) is based on the theory that every failure stems from three causes:

  1. Physical or technical causes
  2. Human causes such as errors of omission or commission, or
  3. Latent or organisational causes that stem from the organisation’s systems, operating procedures and decision-making processes.

To identify those causes, our RCA procedure includes five basic elements:

  1. Identifying the problem
  2. Defining the root cause
  3. Developing the solution
  4. Implementing and delivering the solution
  5. Verifying and monitoring its effectiveness.

Identifying the problem
A typical root cause analysis project for the rail sector will begin when an operator experiences premature bearing failures on a single vehicle or category of vehicles; and our engineers are requested to commence an investigation into the reasons why.

The damaged bearing is returned to our engineering centre in Luton for detailed analysis. At the centre, the bearing will be weighed and lubricant samples will be taken for analysis before it is washed and degreased. Weighing the bearing a second time after cleaning allows the quantity of lubricant present at the time of failure to be calculated.

The cleaned bearing then undergoes a detailed inspection. Different causes of failure leave clues that can reveal a lot to the experienced investigator. Misalignment during service, for example, can produce excessive stresses in a bearing that ultimately lead to failure. Those stresses can show up as uneven wear on the bearing rings or races. Thermal damage caused by lubrication failure can produce discolouration, or even cause parts to become welded together in extreme cases.

Damage caused by stray electrical currents is a particular issue in some rail applications. Its presence can be revealed by distinctive forms of surface damage. For example, micro cratering of their surface may result in balls losing their original mirrored finish and taking on a dull, grey appearance, while raceways may exhibit a ‘fluted’ or ‘washboard’ effect.

Analysis of lubricant samples can provide additional information on possible causes of failure. Grease will be discoloured by high operating temperatures, for example, while the ingress of water can cause the lubricant to emulsify. Where a bearing has been damaged by solid contaminants like dust or dirt particles, these will be present in the lubricant.

Defining the root cause
Inspection and analysis can show how a bearing ultimately failed. Understanding why that failure occurred requires a full understanding of operating and maintenance conditions. That requires close collaboration between our engineers and those of our clients. Damage to axle bearings caused by water ingress, for example, could have been caused by a one-off event, like a flood incident, or it may suggest that bearings are inadequately sealed for operating environment.

Electrical damage may be the result of inadequate bonding arrangements on a particular vehicle, or could be caused by issues with network infrastructure. Where issues are recurring in nature, analysis of a sample of components over a period of time can help to build a more comprehensive picture of the underlying issues, aiding root caused identification.

Developing and implementing the solution
The value of root cause analysis comes from the introduction of measures designed to prevent the recurrence of issues. These measures will typically comprise a combination of technical, process and management changes. There may be several different ways to address a given issue, each with its own costs and benefits.

Bearing problems caused by misalignment during service, for example, can be addressed through improved training and enforcement of appropriate installation procedures. Alternatively, some operators have adopted non-clamped bearing designs that are more tolerant to misalignment. Where electrical damage is the issue, and problems with bonding cannot be identified or resolved, insulated bearing designs, like the INSOCOAT range from SKF, can be a cost-effective solution.

Similarly, the impact of problems caused by contamination or water ingress can be reduced by more frequent re-lubrication. Alternatively, better seals – and the installation of rear cover plates on axle bearings – can provide protection and also extend the operating life of the bearing.

Verifying and monitoring
The final phase in the RCA process is monitoring the chosen solution to ensure it has been properly implemented and that it has addressed the underlying problem. Where the solution required process changes, like improved installation or maintenance procedures, this must be incorporated into the operator’s ongoing management activities. The performance of new technical solutions may be evaluated by running a pilot program with a small number of assets before rolling out the solution to the whole fleet.

Bearing inspection and analysis can also play an important role in ongoing monitoring activities, for example. Our analysis teams are often asked to inspect axle bearings removed from rolling stock to identify any early signs of developing problems – like water ingress or uneven wear. Where problems are not present, the axle bearings can be overhauled and returned to service. This same approach is increasingly adopted by operators seeking to extend the operating life of their assets between maintenance intervals.