The effect of friction modifiers on wheel/rail isolation at low axle loads

Signalling block occupancy is triggered by the wheelset of a rail vehicle 'shunting' the track circuit. The main cause of loss of shunting is the many materials that can be present on the rail head. These include iron oxides, leaves, ballast dust, oil, as well as products deliberately applied to the rail/wheel such as grease, friction modifiers, lubricants and sand. These may lead to an indication that a signal block is unoccupied when a train is actually present. Friction modifiers (FMs) are being increasingly used for reduction of noise, lateral forces, wear, rolling contact fatigue, etc. The Kelsan (R) high positive friction (HPF) solid stick FM is applied directly to the wheel tread creating a thin film. Although no effects have been observed during field operation, the intention in this work was to build on previous studies and evaluate conductance across a wider range of contact conditions as studies have shown that loss of shunt is more common with light axle loads where films could have a greater effect. The testing was performed at a contact pressure of 470 MPa (equating to a 4.9 tonne axle load). An HPF stick was spring loaded against the rotating wheel disc to generate a FM film at the contact. Tests were run to measure the impedance across the discs using a modified TI21 track circuit. Static testing was also performed using discs with a pre-generated HPF film. Analysis of the results showed that the application of HPF friction modifier had no significant effect on the measured level of impedance. The highest impedance levels were recorded under pure rolling (0% slip) where FMs were not being applied. In cases where the application of FM increased impedance, the recorded impedance values were still lower than the highest values measured in the absence of a FM. This is of key relevance as several light rail operators coast their trains.