Characterizing railway lubricants using twin disc tribometer

Railway track or rails take the beating while transporting millions like us every day. We can even feel the vibration and noise when the rails cry for help. This is due to severe wear of top of rails (ToR) and railway wheel flange. Therefore, lubricants are used to reduce wear, noise, and vibration without affecting the safety factors like braking distance and acceleration. Often the top of rail lubricants is water-based polymer friction modifiers, and the wheel flange lubricants is a grease. Rail networks have preferences to certain lubricants/grease suppliers, however there is no preferences for ToR friction modifiers. For example, UK rail network standard NR/L3/TRK/3530/A01 recommends the use of RS Clare Supreme and Whitmore BioRail EP 1.5 grease for wheel flange lubrication in UK rail networks.

Although there are no laboratory test standards for lubricant selection, the preference to any brand/supplier is based on the field performance metrics like grease pick-up, retentivity and carry down. An imporant drawback of field testing is that it is difficult to control parameters that can help lubricant formulation. In the lab such controlled testing of lubricants can be easily done. In this regard there are several working groups in ELGI, NLGI and ASTM like standards setting organizations working on development of lab test standards and protocols for railway lubricants. 

Figure 1. Real time changes in traction coefficient at a fixed creepage and varying lubricant contact conditions in Ducom twin disc or roller on roller tribometer. 

Twin disc or roller on roller tribometer has been widely used for railway lubricants characterization by rail network research labs, industrial and academic tribology labs. The advantage of twin disc is that it allows larger contact area testing compared to a point contact in ball-on-disc (Ex. MTM Rolling-Sliding). Moreover, the rollers can be carved out of the rails and wheel, therefore the tests are closer to the field contact condition when compared to steel ball and glass disc/steel disc in a MTM type tribometer. Therefore, it will not come as a surprise when we read that twin disc is the preferred screening test method for R&D labs in EVRAZ (Russian Railways), ALSTOM, UK railways and Indian Railways. In this report, we will briefly describe a case study on twin disc test method and relevant metrics used to characterize “railway solid lubricants”.

Figure 2. Traction coefficient and rail wear in twin disc tribometer for graphite or MoS₂ based solid lubricant stick. (A)Evolution of traction coefficient over increase in creepage, (B) Changes in wear rate over increase in creepage. 

Differentiate lubricant products based on retentivity, wear and traction curves using Ducom Twin Disc or Roller on Roller.

Institute of Railway Technology in Monash University (Australia) has designed a unique test method to characterize solid lubricants on Ducom Twin Disc or Roller on Roller Tribometer. A specialized solid lubricant stick holder is designed for controlled application of lubricant in contact with the top roller (carved from railway wheel). Creepages in the range of 0.1 to 20 % is achieved using two direct drive servomotors for top and bottom rollers. Pneumatic loading system is used to apply force in the range of 0.1 to 8 kN on the top roller that translates to a contact pressure range of 0.5 to 2.5 GPa. Retentivity, wear and traction curves were used in characterizing the solid lubricants. Retentivity is direct measure of longer product response time after lubricants ceased (see Fig. 1). Wear is measured in terms of mass loss of roller after the test. Traction curves were derived from in situ friction measurements at different creepages.

Figure 3. Evolution of traction coefficient over time for various lubricant contact conditions. Graphite based lubricants showed higher retentivity and lower traction coefficient compared to MoS₂ based lubricant. 

Results showed that the graphite based solid lubricant stick showed lower stable traction curve compared to dry condition or MoS₂ based solid lubricant (see Fig. 2 A). And the rail wear was lower for graphite-based lubricant compared to MoS₂ based solid lubricant (see Fig. 2B). Furthermore, retentivity of graphite-based lubricant was better than MoS₂ (see Fig. 3). Longer retentivity enabled low stable traction and wear of rails

For an in-depth description of these results please refer to the article on Tribology International.

Twin disc can enable development of railway lubricants for energy efficiency, safety and low maintenance cost. However, its general acceptance in the laboratory will depend on the precision in repeatability and reproducibility. Precision can be developed by collecting the traction coefficient and wear data from multiple twin disc tribometers following an established test protocol. We are working on this task and currently, we are looking for collaborators. Please contact us if you are interested to be a part of this work.