UDC 532 © Darko Knezevi, Zdravko Milovanovi, Aleksandar Milasinovi, Mitar Jocanovi DETERMINATION OF THE FLOW RATE THROUGH LONG RADIAL CLEARANCES INSIDE HYDRAULIC COMPONENTS In hydraulic control components (directional control valves, servo valves, LS regulators, etc.) there is a problem of determining of flow rate through the radial clearances of different lengths overlap between the control piston and body of component. <...> As lengths of overlap are relatively small, it is necessary to analyze the development of a full profile of fluid velocity through radial clearance (in sleep mode servo valve has length of overlap equal zero; directional control valves, depending on the diameter, have lengths of overlaps of 0.6 to 0.8 mm; while in work of the load sensing (LS) regulator, piston is continuously located in areas of small opening, zero overlap and small length of overlap). <...> Common values of radial clearance is approximate in the range from 1 m (minimum values for servo valves) to 50 ?m (maximum value for axial piston pumps). <...> In this paper is analyzed development of full-length profile of velocity inside radial clearance (i.e. development of boundary layer) and derived an expression for the flow rate through the so-called long radial clearances, taking into account the influence the changes of thermodynamic state of hydraulic oil on the value of viscosity. <...> Under the long radial clearances implies such clearances where they can be ignored the impact of the development of full-length profile of velocity on flow rate through clearance and change of thermodynamic state of the oil considered isothermal. <...> Keywords: hydraulic components, radial clearance, length of overlap, boundary layer, flow rate, long radial clearance. 1. <...> INTRODUCTION Complexity ofmathematical description of the flow of hydraulic fluid through the radial clearances stems fromthe fact that the mathematical description cannot be the same for f low of hydraulic f luid through the clearances at zero overlap between the piston and cylinder, the small lengths of overlap and the long lengths of overlap. <...> Flow of the hydraulic fluid through orifice at zero overlap (cylindrical ring), depending on pressure, temperature, fluid type and size of theclearance, can be laminar or turbulent. <...> Due to small size of clearances in hydraulic components, flow of hydraulic fluid <...>