9. Investigation of the isolation and frictional properties of hydrophobic
9.2. Test methodology
9.4.2. Pendulum Testing
It was found that both products had an effect on the adhesion levels between the rubber pad and the rail head under all conditions tested. From Figure 10 it can be seen that both products at both concentrations when applied to the rail head would reduce friction levels below that of pure water to coefficient of frictions levels around 0.10. This level is low, but as it is a rubber on steel contact it can only be said that this product would reduce adhesion, not that the product would reduce adhesion to this exact level in the actual wheel-rail contact. Having said this, the matchup between the friction values found in the twin disc testing and the pendulum testing is rather good, with the same
trends being found; water alone giving highest level of friction, product A giving higher levels of friction than product B, and product B at the highest concentration giving the lowest levels of friction for the comparative tests. The hydrophobic products used in the tests have been specifically designed for use on windscreens, with a solvent to cut through and remove any
contamination that can be found on these, such as bug residue. From purely visual inspection, there was little noticeable difference in the hydrophobicity of the steel before and after treatment. Product A when sprayed onto the rail
sprayed with just water would have water droplets forming. It is therefore difficult to attribute the change in levels of adhesion to the change in hydrophobicity at the rail head due to the treatment with either of these products, rather than just the products themselves being friction modifiers. With a moist layer applied to a dry film of product, an increase was seen in friction levels, but there was a high level of scatter.
9.5. Conclusions
This paper summarises an assessment of the suitability of two commercially available hydrophobic products for use in addressing low adhesion levels which occur in water-contaminated wheel-rail contacts. The test program also provided some insights into the mechanics of this low adhesion phenomenon. • Traction levels obtained with product A seem to be in a safe operating
range under all dilutions.
• At the manufacturer’s recommended dilution product B showed a low traction coefficient sometimes below the safe limit for braking and traction, which is considered to be 0.1.
• A black liquid was witnessed during the second part of the pure water test after the water supply was stopped. This liquid kept the traction at approximately 0.2 (see Figure 3). This liquid seemed to disappear from the disc surface very rapidly and as this happened the friction almost instantaneously started to rise toward dry levels. It is thought that the liquid may be mixed with an oxide which acts as a solid lubricant as the contact initially dries, before being removed.
• Impedance levels seen with the hydrophobic products were below the Network Rail threshold.
• Pendulum friction values from testing on an actual rail head were in agreement with the twin disc values. With a layer of moisture (that perhaps best represents the dew point condition) a dried film of product did increase friction over a moist clean rail head. This, however, needs further investigation as there was considerable scatter in the data, likely due to temperature sensitivities.
• In some cases the hydrophobic solutions degraded the adhesion conditions compared to a purely wet or untreated surface. At higher concentrations in the SUROS tests of product B, the adhesion is almost half that of the equivalent went test in the initial phased (0.1 from 0.2). Although this value is not in the ultra-low adhesion range it should still be considered that this product may actual caused low adhesion when applied – exactly the problem it is intended to treat. • At present there is not a convincing case for applying hydrophobic
products to help reduce low adhesion incidents, but the moist layer effects need further investigation. Another aspect that needs
consideration is the role the hydrophobic liquids may play in
suppressing oxide formation. Water alone is probably not enough to cause low adhesion problems. As noted by Beagley (1976), solid material is required, which at the dew point could be the oxide that the water film on the rail head creates.
9.6. References
Beagley, T.M. and McEwen, I.J. and Pritchard, C., 1975a, “Wheel/rail adhesion-boundary lubrication by oily fluids”, Wear, 31, pp. 77-88.
Beagley, T.M., Pritchard, C., 1975b, “Wheel/rail adhesion – The overriding influence of water”, Wear, 35, pp. 299 -313.
Beagley, T.M., 1976, “The rheological properties of solid rail contaminants and their effect on wheel/rail adhesion”, Proccedings of the Institute of Mechanical Engineers, 190, pp. 419-428
BS 7976-1:2002, Pendulum Testers: Part 1 – Specification.
Fletcher, D.I. and Beynon, J.H., 2000, “Development of a machine for closely controlled rolling contact fatigue and wear testing”, Journal of Testing and Evaluation, 28, pp. 267-275.
Gallardo-Hernandez, E.A. and Lewis, R., 2008, “Twin Disc Assessment of Wheel/Rail Adhesion”, Wear, 265, pp. 1309-1316.
Lewis, R., Dwyer-Joyce, R.S. and Lewis, J., 2003, “Disc machine study of contact isolation during railway track sanding”, Journal of Rail and Rapid Transit Proceedings of the IMechE Part F, 217, pp. 11-24.
Lewis, R. and Masing, J., 2006, “Static Wheel/Rail Contact Isolation due to Track Contamination.” Journal of Rail and Rapid Transit Proceedings of the IMechE Part F, 220, pp. 43-53.
Lewis, S.R., Lewis, R. and Olofsson, U., 2011a, “An alternative method for the assessment of rail head traction, Wear 271, pp. 62-70.
Lewis, R., Gallardo, E.A., Cotter, J. and Eadie, D.T., 2011b, “The effect of friction modifiers on wheel/rail isolation”, Wear, 271, pp. 71-77.
Lewis, R., Lewis, S.R., Zhu, Y. and Olofsson, U., 2013, “The modification of a slip resistance meter for measurement of rail head adhesion”, Journal of Rail and Rapid Transit, Proceedings of the IMechE Part F. 227, pp. 196-200. Rail Safety and Standards Board (RSSB), 2013, “Hydrophobic additives for rail sand (S136)” Project report.