Investigating the Lubricity and Electrical Insulation Caused by Sanding in Dry Wheel–Rail Contacts

The adhesion (or available friction) in the wheel–rail contact is the most important parameter for the braking and traction operation of rail vehicles. Since the beginning of railway transportation, sanding from the locomotive has been a common practice to enhance the wheel–rail adhesion. In recent years, sanding from electrical multiple units (EMUs) and sand-based friction modifiers (FMs) have been adopted in some railway networks to overcome low adhesion incidents caused by leaf contamination in autumn. Although sanding has been proven to improve the adhesion under most of the typical contamination conditions, laboratory and field investigations have shown that sand may act as a solid lubricant in dry wheel–rail contacts. Nevertheless, the influence of the current sanding parameters on the solid lubrication effect has not been entirely investigated. Depending on the resulting adhesion coefficient, the traction and braking operations of rail vehicles could be affected. Furthermore, the influence of those parameters on the electrical insulation is also of special importance because it may affect the train detection. This article presents a laboratory investigation of the influence of three sanding parameters (i.e., feed rate, particle size, and slip) on the adhesion and electrical insulation in dry wheel–rail contacts. The tests have been carried out with a twin-disk roller rig in rolling-sliding motion under closely controlled conditions. Three different slips representative of the actual traction and braking operations have been considered. Sands of four different sizes and up to five feed rates have been used. The results show that using smaller particle sizes and higher feed rates promotes the lubrication and causes more electrical insulation in the wheel–rail contact. Furthermore, the increase in slip is found to reduce the lubrication, leading to a higher adhesion coefficient.