Hydraulic systems are very common in industry and other technical fields. The most frequently used valves for control are sliding directional control valves, which do not have seals on the control pistons. This causes internal leakage. The stability of the leakage is highly dependent on the purity of the hydraulic fluid. The thesis deals with the influence of the purity of the hydraulic oil on the durability and volumetric efficiency of the sliding directional control valve under consideration. As part of the thesis, a hydraulic test rig was designed and assembled, on which three durability tests of a hydraulic 4/3 directional control valve were performed. For each test, a different amount of MTD test powder recommended for accelerated testing was added to the hydraulic oil. The valve was switched at a frequency of 4 Hz until 2 million cycles were reached. At the valve, the internal leakage of the A and B lines was measured using two measuring cylinders and a stopwatch. The leakage increased visibly with an increase in the amount of test dust. After the tests were carried out, the casings were cut open and the sliding surfaces of the casings and pistons of the tested valves were inspected for wear.
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