Bearings & Wear Parts

ENHANCING SAFETY MEASURES IN RAIL FREIGHT BOGIES

Recent events have shown the importance of using approved materials in centre pivot liner and side barer liner applications.

Issues with abnormal wear of safety critical components in bogies and resulting derailments have caused further investigations of the non-metallic parts. The shocking discovery was that cheap plastic components, neither specified on the drawings nor approved, were used in these cases. Not complying with specifications and using non-original materials was the leading cause of many unfortunate accidents. These cheap plastic materials, also called thermoplastics are polymers which become pliable and soft in use.

While this allows for low cost manufacturing of the parts, it also causes the finished product to be dimensionally unstable. Plastic materials are not reinforced, and are severely affected by changes in atmospheric moisture, heat and high loads. Obviously the wrong material for use in the arduous and high-load environments of any railway application. The only fully tested material with approvals from Deutsche Bahn (Germany), SNCF (France), Trenitalia (Italy) and other major railway operators are thermoset composites, such as RAILKO NF, resin-based materials that include reinforced fibres and friction modifiers. Once a thermoset composite is formed, it remains stable even when exposed to harsh environments.

Thermoset composites have higher wear resistance, strength and dimensional stability which is crucial for a freight wagon’s long term operation and safety. They are therefore used within the rail industry to provide a sliding surface between two mating metal components. Thermosets are specified and approved for other safety critical applications such as brake bushes, friction damper bushes, Anti roll bar bushes and hook guide plates.

Centre Pivot Liners & Side Bearer Liners (CPL and SBL's)

CPL and SBL are installed in all standard Y series freight bogies running throughout Europe carrying products such as timber, coal and dangerous goods such as gasoline and volatile gases (LPG). These sliding parts provide a smooth bearing surface to allow the wagon to safely negotiate around curves in the track. Recently, several official reports written by the main operators point out failures with thermoplastic components. The problems appear to have been caused by the low wear resistance of the material, high thermal expansion and water swell. These pose a significant risk to safety, and have been the possible causes of train derailments.

The bent thermoplastic SBL (shown above) indicates high wear. In dismounted condition the sliding plates showed a bend of approximately 13 mm upwards. The high wear, which is above the allowable limit, is reported to have cause the incident.

Derailment Pau, France – Propane Gas Wagon

In November 2009 a potentially near fatal accident was investigated by the BEA-TT, Bureau d’enquêtes sur les Accidents de transport terrestre. Many contributory factors were identified including the wear on the SBL's. The wear rate on the SBL's was reported to be “out of specification after only 13 months against expected life of at least six years”. “Over the 13 month period the thermoplastic SBL had worn by 3.5mm, SNCF maintenance rules state a wear limit of 2mm between maintenances”. The abnormal SBL wear can have detrimental affects on the stability of the wagon. Wagon owners have found that some plastic wear parts could also induce accentuated track wear increasing the risk of derailment.

The wear on the SBL's was found to be irregular and shows “signs of seizure”. A seizure on the SBL can cause the wagon's freight car to be unable to manoeuvre itself around curves in the track. The 27 wagons involved were carrying a total of 160 tons of volatile gas that posed a public health risk, environmental issues and potential loss of life.

Severe Damage: United Kingdom - Freight Wagon

A major UK freight company had used thermoplastic CPLs and SBLs on their freight wagon bogies. The bogies had been in service for less than two years and had become damaged and were removed for overhaul. The thermoplastic CPLs showed high wear and degradation. The worn plastic CPLs had caused damage to the bogies making it necessary for these wagons to be removed from service. On the contrary the other bogies fitted with approved composite CPLs and SBLs continued to operate as required. It was stated that the composite SBLs were lasting about 5 times longer than the plastic SBLs. During the overhaul all of the bogies were fitted with Thermoplastic were replaced by Thermoset composite CPLs and SBLs and returned to service.

Oberhausen, Germany Major Derailment

The German railway reported major safety critical issues on their bogies after one train with a loaded vehicle (type Sggmrs), derailed at the railway station of Oberhausen. On investigation of the vehicle involved it was realized the following defects were seen on the SBL of the car body. The thermoplastic SBL which sits onto the spring mounted part has been worn beyond its wear limit.

Summary

All specifications and guidelines enforce the usage of tested and approved components for safety critical applications in bogies, such as CPL's and SBL. Large penalties can be levied if original specified wear part materials are not used.

Furthermore thermoplastic, un-reinforced materials show cold flow, creep and abnormal wear rates that will lead to unscheduled maintenance, costly repairs or worse. Thermoset composites have reinforcement that gives them more robust mechanical properties, durability and better performance than thermoplastics. The Thermoset composites have been reported to last 5 times longer than thermoplastics due to their high wear resistance.

by Adam Fletcher