Friction mechanisms of brake pads made of cast iron and composite material on the surface of railway wheels

Abstract

The friction mechanisms of brake pads made of cast iron and composite material (rubber-asbestos or rubber-free mixtures, including those with cast-iron inserts) on the surface of wheels of railway rolling stock were analysed. Values of the friction coefficient are given. The main role in the mechanism of friction of cast iron pads on the surface of railway wheels belongs to the microstructure of cast iron, namely, the amount, shape, distribution in volume of graphite inclusions, dispersion of pearlite and cementite inclusions, and the construction of phosphide eutectics. The friction mechanism of pads made of composite material on the wheel surface is determined by the content and properties of the components from which the composite rubber mixture is formed, their chemical and granulometric composition, as well as the degree of burnout of the composite rubber mixture during the operation of the brake pads. It is shown that due to the lack of regulation in the standards and technical conditions for composite blocks of the percentage content of ingredients and their chemical composition in the composite material, its properties and the value of the coefficient of friction are uncontrolled. Cyclic temperature changes on the contact surface of the brake pads with the wheels during train braking and high peak temperatures (up to 800 °C) in individual periods of braking lead to changes in the structure and properties of the surface layer of the metal of the wheels and the material of the brake pads. This is reflected in the frictional characteristics of the friction process of brake pads with wheels. The disadvantage of brake pads made of composite material is their low thermal conductivity compared to cast iron pads. As a result, the temperature in the contact zone of composite pads with wheels increases more than when using pads made of cast iron. The hydrodynamic effects of moisture (water) in the contact zone of brake pads with the surface of vehicle wheels are considered from the standpoint of classical tribology approaches. The microrelief of the working surface of the brake pads was studied. The influence of various factors on the coefficient of friction of the brake pads on the surface of the wheels is shown. Recommendations are provided for improving standards and technical conditions for brake pads made of cast iron and other materials. Keywords: brake pads, wheels of railway transport, friction, cast iron, composite materials, surface microrelief.