# Experimental Investigation on Contaminated Friction of Hydraulic Spool Valve
## 1 Introduction  
Hydraulic spool is one of the basic structural forms of hydraulic control valve. There is a circular clearance in micron level between spool and body, which plays a part in guiding and lubricating the spool movement. Solid particulates whose size nearly fit clearance probably enter into the clearance with fluid in hydraulic system and have impacts on spools movement, and furthermore cause stagnation of the spool. The size of solid particulates and morphology of fit clearance are the main factors of stagnation in spool valve. Therefore, research on contaminated friction in different size of particulates and morphology of fitting clearance will have important theoretical and practical values regarding enhancement of spool contamination resistibility.  
Domestic and international researches have been carrying out wide and deep research into the mechanism of spool contaminated stagnation. Factors thought to influence stagnation have been explored, and the optimal size and profile error of fit clearance that provides a reference for stagnation of hydraulic valve caused by machining quality has been obtained ([PERSON] 2005) [1]. Surveys such as that conducted by [PERSON] et al. [2] have shown a robust designing method to solve stagnation of spool caused by viscous heating. Several attempts have been made to research the influence ofmicrometer and relative roughness on the clearance, finding that roughness can cause greater flow resistance, which is a basis for improving the morphology of kinematic pair ([PERSON] et al., 2005; [PERSON] et al., 2017; [PERSON] et al., 2007; [PERSON] et al., 2013; [PERSON] et al., 2000) [3; 4; 5; 6; 7]. Numerous studies have been attempted to design the valve contaminated friction test device and have obtained the friction from static to motion under the different conditions, like the size of clearance, surface roughness, eccentricity, and static time, but up to now these researches have been at the initiating stage of the spool and do not measure during the whole movement in real-time ([PERSON] et al., 2014; [PERSON] et al., 2014; [PERSON] et al., 2001; [PERSON] et al., 2016; [PERSON] et al., 1980; [PERSON] et al., 2011; [PERSON] et al., 2009) [8; 9; 10; 11; 12; 13; 14]. Much of the literature on particulates pays particular attention to the movement characteristics in the clearance ([PERSON] et al., 2005; [PERSON] et al., 2012, [PERSON] et al., 1994, [PERSON] et al., 1979; [PERSON] et al., 1993; [PERSON] et al., 2000; [PERSON] et al., 2000; [PERSON] et al., 2018) [15; 16; 17; 18; 19; 20; 21; 22]; model simulation calculation and visualization experiments were carried out, which found that rotation phenomenon of particulates during the movement increased contacts with the clearance surface, which plays an important role in stagnation of spool  
It is the contact of solid particulates with the spool valve and body that is the main resistance of movement on spool. The aim of paper is to analyze the influencing factors of spool valve contaminated stagnation in a novel way based on the point of view of friction, and an experimental test bench for real-time measuring contaminated friction of spool valve was designed and built. According to equilibrium condition of forces, the contaminated friction is measured in real-time with the movement of spool. Analysis of the mechanism of morphological factors that affects spool movement, such as cis-conical, inverted cone, and roughness of fit clearance, and the stagnation-sensitive size of solid particulates were explored quantitatively.