## 3 Experimental Tests  
Experimental tests were carried out, where the main stage of a 4/3 4 WEH 10 J46/6 EG24 NETK4/B10 directional control valve manufactured by Bosch-Rexroth was subjected to external mechanical oscillations in a frequency range of 10 to 100 Hz. During the tests, the control chambers of the main stage of the directional control valve were pressure-relieved, i.e., the pressure in the chambers was equal to the atmospheric pressure. The acceleration of the oscillations of the directional control valve spool and the body were recorded. The tests were carried out on a directional control valve immersed in HL68 or Azolla ZS22 oil at a temperature of 20 \({}^{\circ}\)C (with a dynamic viscosity of 612 \(\times\) 10\({}^{-4}\) or 198 \(\times\) 10\({}^{-4}\) N\(\cdot\)s/m, respectively). The centring spring pair was replaced after each series of tests. The spring stiffness and the oil type are given in Table 1. Mechanical oscillations were generated using a Hydropax ZY-25 linear hydrostatic drive simulator. The direction of excitation oscillation was aligned with the direction of movement of the spool in the sleeve. The directional control valve was mounted in a special holder on the simulating table. Figure 3 shows the directional control valve during testing. Figures 4 and 5 show a diagram of the directional control valve and the measurement path. \begin{table}
\begin{tabular}{c c c} \hline
**No.** & **Spring Stiffness–c (N/m)** & **Oil Type** \\ \hline
1. & 816 & Azolla ZS22 \\ \hline
2. & 2923 & HL68 \\ \hline
3. & 2923 & Azolla ZS22 \\ \hline
4. & 816 & HL68 \\ \hline \end{tabular}
\end{table} 
Table 1: Selected parameters of the directional control valve model. As shown in Figure 5, two accelerometers were used for the acceleration measurement. AC1 was mounted to the spool while AC2 was mounted to the valve body. The sensor uncertainty \(\delta\mathrm{a}_{\mathrm{acc}}\) was equal for both sensors. The signal from the accelerometers a\({}_{\mathrm{acc}}\) was  
Figure 4: Diagram of the directional control valve: 1—directional control valve body; 2a and 2b—centring springs; 3—spool acceleration measuring pin; 4a and 4b—spool control chambers; 5—mounting plate; 6—spool; 7—lubricant reservoir. Figure 5: Block diagram of the mechanical oscillation acceleration measurement path: BO—tested item—main stage of the 4 WEH J46/6 EG24 NETK4/B10 directional control valve; AC1—accelerometer measuring the oscillation acceleration of the directional control valve body; AC2—accelerometer measuring the oscillation acceleration of the directional control valve body; KS—VibAmp PA3000 signal conditioner; OC—Tektronix TDS224 digital oscilloscope; PC—personal computer. Figure 3: Directional control valve in the simulating table holder: 1—tested directional control valve; 2—simulating table holder. obtained by a signal conditioner with gain factor K\({}_{1}\) and error of gain \(\delta\)K\({}_{1}\). The signal was then passed to the oscilloscope which further amplified it. The oscilloscope had a gain factor of \(K_{2}\) with gain error \(\delta\)K\({}_{2}\). From the above, the measured acceleration can be described by the following equation:  
\[\text{a}=\text{a}_{\text{acc}}K_{1}K_{2} \tag{30}\]  
As the measurement uncertainty for each device is different, the acceleration is a function of three variables. The total measurement uncertainty can be presented as follows:  
\[\Delta\text{a}=\frac{\partial\text{a}}{\partial\text{a}_{\text{acc}}}\Delta \text{a}_{\text{acc}}+\frac{\partial\text{a}}{\partial K_{1}}\Delta K_{1}+ \frac{\partial\text{a}}{\partial K_{2}}\Delta K_{2} \tag{31}\]  
The relative uncertainty \(\delta\)a is described by the equation:  
\[\delta\text{a}=\delta\text{a}_{\text{acc}}+\delta K_{1}+\delta K_{2} \tag{32}\]  
For the measuring apparatus used for the experiment, the relative uncertainty of the acceleration was equal to 5%. The results of each test are presented individually below (Figures 6-9) and then compared (Figures 10 and 11).