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Characterizing railway lubricants using twin disc tribometer

June, 2021

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.

How can railway lubricants reduce noise?

Noise comes from the wheel vibration due to stick-slip at the wheel-rail interface. The vibration is higher at the curves, that is widely known as wheel squealing. Friction modifiers in ToR lubes and wheel flange grease can reduce noise by suppressing these vibrations and maintain a positive or neutral traction coefficient. Recommended friction coefficient on top rail is between 0.25 to 0.40 for optimum braking and acceleration.

Stick-slip is a result of difference in lateral and longitudinal motion of the wheels. Minimum difference between lateral and longitudinal motion enables energy efficiency and reduction in fuel consumption.

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. 

Twin disc and grease retentivity

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”.

What type of tribometers are used for screening rail lubricants?

Tribometers must enable test methods to simulate the key rail/wheel contact conditions. Five essential control parameters can be used in selection of a tribometer as described below.

  1. Generate contact pressure (0.5 to 2.5 GPa) over a larger contact area
  2. Controll creepage (0.1 to 20 %) at a surface speed of 1 m/s.
  3. Holders compatible with top disc carved out of railway wheel and bottom disc carved out of rails
  4. Sensors that enable traction coefficient vs. creepage curves for rail materials in dry or lubricated conditions such as greased/oiled or solid lubricant stick.
  5. Safety controls to configure lubricated wear (oxidative, rolling fatigue, etc) on the rollers in the range of 0.1 to 20 µg/m.

Twin disc tribometer comply with all the five essential parameters required for railway lubrication research. Mini-Traction Machine or MTM is a ball-on-disc that can only comply with creepages and sensors enabling traction coefficient. However, the contact pressure is on a small point contact compared to the same contact pressure at larger area contact in twin disc tribometer. 

Solid lubricants in twin disc

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.

How can railway lubricants reduce wear?

Wear is a material loss on the wheel flange and rail gauge due to starved lubrication at extreme pressure between flange-gauge interface in the range of 0.5 GPa to 2.5 GPa. Extreme pressure additives such as MoS2 and graphite in grease is used to reduce wear. Furthermore, the grease components like high viscosity base oil and thickeners contribute towards preventing wear due to rolling contact fatigue and gauge corner cracking. In general grease must prevent catastrophic wear in the range of 2 to 16 µg/m.

Reduction in wear can reduce maintenance cost as the number of wheel/rails grinding intervals can be reduced.


Grease and top of rail lubricants traction curves

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.

How to minimize the railway lubricants effect on environment?

Greases and oils that are not consumed at the rail-wheel interface are bound to pollute the surrounding environment. Environmentally acceptable lubricants (EALs), for example the biodegradable ester base oils can be a suitable replacement to mineral oil-based lubricants used in wayside or onboard lubrication system (Ex. SKF and Lincoln Lubrication Systems). Another widely adopted solution is solid lubricant stick, a matrix embedded with graphite or MoS2 solid lubricants, used in onboard lubrication of wheel/wheel flange (Ex. Schunk). Solid lubricant stick is known to reduce lubricant wastage and reduce fatigue cracks compared to grease.

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.

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