Analysis of Tribotechnical Properties and Comparative Evaluation of Polymeric Materials and Rubbers Used in Rolling Stock

Rubber materials are widely used in friction assemblies of railway rolling stock. Rubbers are used oil seals, other various seals, shock absorbers, corrugated hoses, sleeves, sealing rings, etc. During operation, rubbers are exposed to various mechanical influences that cause wear, cracks, abrasion, dents, burnthroughs, etc., which can lead to the failure of the entire unit and unforeseen unscheduled repairs of the rolling stock. Any failure on the route together with unplanned repairs incur heavy economic losses.Currently, the issue of wear of rubbers in rubber-steel tribopairs has not been sufficiently studied in case of supply of lubricant to the friction zone and of wear caused by a free and fixed abrasive. There is ongoing research on the possibility of replacing rubber products with other polymeric materials, which have shown significantly better results in tribological tests, both in terms of friction coefficients and in the wear mechanism. The data obtained will make it possible to choose the most optimal options for materials that can act as a replacement for standard rubber products in rolling stock friction assemblies.The paper presents the results of tribological tests of thermoplastic polyurethanes (TPU), ultra-high-molecular-weight polyethylene (UHMWPE) and polypropylene (PP2015) in comparison with rubber based on nitrile butadiene rubber (NBR). The tests were carried out according to two schemes: «plane (tested sample) – bushing» and «plane (tested sample) – generatrix of a rubber disk with supply of abrasive grain to the friction zone». The objective of the work is to determine the dependence of the change in friction coefficients on load and sliding speed, as well as the dependence of seizure pressures of tribo-pairs on speed, weight loss of samples after wear tests with a free and fixed abrasive, the morphology of wear surfaces, and wear mechanisms of polymer materials and rubbers.

Regularities of the influence of submicron ceramic powders TiO2, AlN, Cr2O3 on the tribological properties of a friction material

Friction units for automotive and special vehicles are designed to operate under boundary friction conditions. Modern vehicles contain friction assemblies that use friction materials. Currently, friction materials are actively used: based on thermosetting resins; pulp and paper-based materials; sintered powder materials; materials of carbon or carbon composition; materials with a ceramic matrix. The development of a unified understanding of the effect of the size and chemical nature of ceramic additives on the processes occurring in a friction material during friction is very important and can be obtained both on the basis of experimental and theoretical studies. The paper presents the results of a study of the effect of submicron TiO2, Cr2O3, AlN powders with a size of 0.2-0.5 microns on the tribotechnical properties of a frictional material based on copper intended for operation under boundary friction conditions. It was found that when using the addition of Cr2O3 powder, the greatest increase in the value of the friction coefficient is noted - from 0.042 to 0.082, a slightly smaller increase in the friction coefficient is shown by the use of AlN and TiO2 defects - 0.042-0.074 and 0.042-0.060, respectively. The least wear of the friction material was obtained when using 3.0 vol. % aluminum nitride additive - 2.1 microns / km. Increasing the addition of any of the submicron powders by more than 7 vol. % leads to a significant decrease in wear resistance. This is due to the formation on the surface of the friction material of a modified layer containing ceramic particles and the metallic phase of the friction material. For the friction material, an unstable value of the friction coefficient and increased wear were recorded

RESTORATION OF THE CONSTRUCTION AND ROAD MACHINES’ DETAILS FROM GREY CAST IRON: IMPROVEMENT OF THE REPAIR DIMENSIONS’ METHOD EFFICIENCY

Introduction. The total mass of some grey cast iron details in construction and road machines reaches 60%. Cylinder blocks, parts of braking systems, flywheels, etc. are made of cast-iron. One of the main causes of operational failures (up to 70%) of construction and road machines is failure of friction units. Considering that the contacting parts in these units take the load mainly by surface layers. The operability looses at relatively small values of dimensional wear. Although mass wear of the part usually does not exceed 1%.Materials and methods. The authors carried out a comprehensive technical and economic analysis of repair methods of friction assemblies. As a result, the best combination of criteria was the repair size method. However, the strengthened layer was removed to the repair dimension by conventional reconditioning technology. At the same time grey cast iron was strengthened only by very expensive and labor-intensive methods, which in conditions of real repair production were not applicable. The authors considered the possibility of using a new method of strengthening iron parts in repair works.Results. The developed method strengthened grey cast iron with layer thickness up to 3 mm. The distinctive feature was that hardness of the strengthened layer in thickness increased. At the same time the microhardness increased in comparison with the initial grey iron more than 2 times and wear resistance became comparable to high-strength hardened stun.Discussion and conclusions. The authors obtain the best results with the usage of reducing atmosphere furnaces. In order to expand the scope of the method, the researches proposed to use a special device creating the necessary conditions in any furnace. The application of the proposed technical solutions increases the efficiency of the construction and road machines’ repair.The authors have read and approved the final manuscript. Financial transparency: the authors have no financial interest in the presented materials or methods. There is no conflict of interest.

Composite metal-ceramic and metal-polymeric materials for friction assemblies of national civil aircrafts

Experience of using of metal-ceramic and a metal-polymeric material in friction assemblies of aeronautical engineering is generalized. Characteristics and conditions of operation of the sintered friction and antifriction materials are presented. Peculiarities of metal-fluoroplastic tapes of foreign production are considered. Efficiency of using of sintered and metal-polymeric materials in friction assemblies of modern civil aircrafts is shown.

CORRELATION OF BEARING FRICTION ASSEMBLIES WITH ABRASIVE WEAR OF THEIR TRIBO-COUPLING

To ensure wear resistance of all tribo-couplings of a bearing assembly at the expense of the abrasive wear decrease it is necessary the solution of problems connected with the impermeability of their movable joints. In this case by the impermeability we imply a process of sealed “wall” formation which is able to separate reliably volumes with different environments at considerable changes of external factors of environment: temperature, dustiness, aggressiveness and others. The contact of two hard surfaces of tribo-coupling of the “shaft-seal” type forms a system of cavities, capillaries of an arbitrary shape and contact patches. Under the action of pressure difference on separate capillaries arise leaks. The depressurization of bearing friction assemblies (leak moment) results in the decrease of wear rate of tribo-coupling of “shaf-tseal” type, but a wear rate of other tribocouplings such as “ringsolid of rolling motion”, for frictionless bearings or “shaft-bushing” for friction bearings increases because of abrasive penetration into a contact area. The penetration of quartz particles (pollutions) through a sealing into a bearing assembly results in the increase of a constant of friction and initiation of abrasive wear in tribocouplings of “ringsolid of rolling motion” and “shaftbushing” types. The wear rate of “ringsolid of rolling motion” and “shaftbushing” couplings depends on the abrasive concentration in lubricant and a leak at the outlet from the bearing assembly. For the wear resistance increase in tribo-couplings of bearing assemblies it is necessary not only to increase the hardness of contact surfaces, ensure optimum roughness, select materials in tribo-couplings, but also to ensure the impermeability and lubricant keeping in a friction area.