IMPROVEMENT OF THE TECHNICAL AND OPERATIONAL CHARACTERISTICS OF HYDRAULIC CYLINDER LINERS OF AUTOMOTIVE AND TRACTOR MACHINERY BY APPLYING THERMOMECHANICAL STRENGTHENING TO ENHANCE THEIR RESTORATION TECHNOLOGY

Abstract

Based on engineering methods, the technical and operational characteristics of hydraulic cylinders of automotive and tractor equipment have been improved by applying a combined technology of thermomechanical hardening of the working surface of the hydraulic cylinder sleeve. The analysis of traditional technologies has allowed us to identify their limitations and substantiate areas of improvement. The proposed process combines heating, deforming mechanical rolling and temperature tempering, for the implementation of which appropriate equipment and tools have been selected. Experimental studies have confirmed the effectiveness of the developed solutions: the deformation temperature (Tdef), the degree of deformation (λ) and the tempering temperature (Ttempering) of the surface, the distribution of hardness along the wall thickness, and after running-in – the amount of wear. The optimal thermomechanical hardening modes should be taken as: deformation temperature 850–880 ºC, deformation degree 14–18 % and tempering temperature 170–240 °C. The proposed technology, compared to industrial production, provides: – maintaining hardness at the level of 42–50 HRC with a more than twofold increase in plasticity; – increasing the depth of the hardened layer by 1,25 times, which allows for repairs to repair dimensions; – reducing the tension of the surface layer by 4–10 times; – increasing the stability of mechanical properties, confirmed by reducing the coefficients of variation of hardness by 1,6–1,7 times and plasticity by 1,8–2,3 times. A comparative analysis showed significant advantages of the new technology, in particular, increasing processing productivity by up to 4 times, reducing tool costs by three times, and reducing the overall cost of the process of restoring hydraulic cylinder liners. It was found that the use of the proposed technology provides hardening of the surface layer to a depth of up to 0,25 mm and achieving a roughness close to the operational one. The purpose of the research is to increase the durability and reliability of hydraulic cylinder sleeves through the use of thermomechanical hardening, which will contribute to the improvement of the restoration processes of hydraulic systems of automobile and tractor equipment. Object of research: hydraulic cylinder sleeve of automobile and tractor equipment, which is subject to wear in operating conditions and is subject to restoration using hardening technologies. Subject of research: processes of thermomechanical hardening of the working surface of the hydraulic cylinder sleeve, which affect their durability, reliability and maintainability. Research methods. The research was based on the use of methods of physical and mathematical modeling of a real friction unit, as well as methods of mathematical statistics in the processing and analysis of experimental data obtained during laboratory tests. To study changes in the structure and determine the depth of the hardened layer, the metallographic method and the hardness measurement method were used. The adequacy of the mathematical model and the reliability of the research results are confirmed by the satisfactory convergence of the data obtained by theoretical calculations with the results of experimental studies. Experimental studies were conducted in laboratory and production conditions using model samples, experimental planning methods and mathematical statistics. As a result of the research, the possibility of improving the repair technology of hydraulic cylinder sleeves of automobile and tractor equipment through the use of thermomechanical strengthening of their working surface, made of carbon steels, by thermomechanical strengthening of the working surface (compared to industrial samples) was scientifically substantiated and experimentally confirmed.