# Experimental Investigation on Contaminated Friction of Hydraulic Spool Valve
## 2 The Real-Time Experimental System of Contaminated Friction Measurement
### The Oil of Suspended Solid Particulates  
The oil of the suspended solid particulates is composed of hydraulic fluid and solid particulates at certain size. No. 46 anti-wear hydraulic oil is selected as the working medium, and its specifications are shown in Table 2.  
To reach the measuring universality in solid particulates and fit clearance, the solid particulates were selected as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 times the size of clearance between spool with standard morphology and valve body. Ten oil samples were made with a volume fraction of 2% under different particulates size.  
Recent evidence ([PERSON] et al., 2012; [PERSON] et al., 2011; [PERSON] et al., 2009) [23; 24; 25] suggests that metal particulate pollutants account for \(\sim\)75% of total pollutants in the working medium of the hydraulic system. Therefore, the solid particulates used in the test should be metallic, light in mass, and have good following performance. By comparing the physical and chemical properties of different commonly used metals, we found that pure aluminum can meet the testing requirement with good following, chemical stability, and fire resistance in air, and, with the help of the metal powder separation sieve (as shown in Figure 3), the particulates are separated into 10 samples, including 60 \(\upmu\)m, 120 \(\upmu\)m, 180 \(\upmu\)m, 240 \(\upmu\)m, 300 \(\upmu\)m, 360 \(\upmu\)m, 420 \(\upmu\)m, 480 \(\upmu\)m, 540 \(\upmu\)m, and 600 \(\upmu\)m, and images of the particulates are shown in Figure 4, taken by optical microscope (as shown in Figure 5).  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Experiment Model**} & \multicolumn{8}{c}{**Structural Parameters**} \\ \cline{2-10}  & **D/mm** & **d1/mm** & **d2/mm** & **d3/mm** & **L1/mm** & **Ra1** & **Ra2** & **Ra3** & **m/Kg** \\ \hline Standard morphology spool & / & 28.8 & 28.8 & 30 & 35 & / & 0.8 & 0.8 & 0.2983 \\ Rough spool & / & 28.8 & 28.8 & 30 & 35 & / & 12.5 & 0.8 & 0.3027 \\ Cis-conical spool & / & 28.7 & 28.9 & 30 & 35 & / & 0.8 & 0.8 & 0.3117 \\ Inverted cone spool & / & 28.9 & 28.7 & 30 & 35 & / & 0.8 & 0.8 & 0.3117 \\ Valve body & 30.01 & / & / & / & / & 0.8 & / & / & / \\ \hline \hline \end{tabular}
\end{table}
Table 1: Structural parameter.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Oil density/(g/cm\({}^{3}\)) & 0.879 \\ Oil dynamic viscosity/(Pa-s) & 0.0364 \\ \hline \hline \end{tabular}
\end{table}
Table 2: No. 46 anti-wear hydraulic oil trait parameters.  
Figure 2: The spools have been processed: (**a**) standard morphology spool, (**b**) conical spool, and (**c**) rough spool.  
The suspension time of aluminum particulates in oil was further measured. One-hundred milliliter samples of the oil of aluminum particulates were suspended with a volume fraction of 2% at each size. Particulate suspension time was measured, and the variation curve of the time under different particulates size was drawn, as shown in Figure 6. It is apparent from Figure 6 that the suspension time of aluminum particulates at each size is greater than 45 s. The aluminum particulates are in a uniform distribution in the oil during experiments at the single experimental time of 45 s.  
Figure 4: The microscopic view of particulates (figure below) and partial enlarges.  
Figure 5: The optical microscope.  
Figure 3: The metal powder separation sieve.