# Degradation of Hydraulic System due to Wear Particles or Medium Test Dust
## 2 Materials and Methods  
### Test Rig  
Two identical hydraulic test rigs were set up in our laboratory. The effect of oil cleanliness in the hydraulic system on the durability of the system itself was tested, and the effect of wear particles or test dust (MTD) was compared. Figure 1 shows both test rigs, with wear particles added to the first test rig (right) and test dust added to the second test rig (left). Initially, 30 L of hydraulic oil ISO VG 46 was added to both units. Flow through the 2 SP A06 D 10-G gear pump was measured at a displacement of 6 cm\({}^{3}\)/rev and a pressure of up to 270 bar. Internal leakage was measured on a 4/3 directional control valve with a solenoid model DS3-S1/11-N-D24K1. In the zero position of the directional control valve, all ports were closed (P, T, A, B). The oil temperature in the tank was about 65 \({}^{\circ}\)C. The thermostat, which switched the oil cooling on or off, was set to 60 \({}^{\circ}\)C. The directional control valve was switched with a frequency of 5 Hz. The test rigs developed in the Laboratory for Hydraulic Power and Controls consisted of all the key hydraulic components that are also commonly used in the field. Table 1 shows the bill of material (BOM) of the test rigs with the corresponding system positions. The pump (pos. 1, Figure 2) sucked the oil from the tank (pos. 19, Figure 2), which was inclined at an angle of 15\({}^{\circ}\) (to prevent particles from settling, the suction pipe was located at the lowest point (corner) of the tank), and pushed the oil through the check valve (pos. 12, Figure 2) to directional 4/3 valve with solenoid (pos. 4, Figure 2). During the parallel or diagonal positions of the valve, the oil flowed through working lines A or B, the orifices (pos. 15, Figure 2), hydraulic motor (pos. 6, Figure 2), manually operated 3/2 valve (pos. 17, Figure 2), cooler (pos. 9, Figure 2), priority valve (pos. 7, Figure 2), filter (pos. 8, Figure 2), and back to the reservoir (pos. 19, Figure 2). The priority valve can be set to an adjustable pressure differential so that it can divert the oil flow around the filter into reservoir. In this way, the oil can be filtered and the cleanliness level is consequently higher, or the oil bypasses the filter and the cleanliness level decreases. Factory values of the maximum permissible pressure of all tested hydraulic components were inspected, and the pressure was set to somewhat lower at 200 bar. The hydraulic oil in both test systems was initially filtered to a cleanliness level of 16/15/13 or less. The temperature was monitored and ranged between 60 \({}^{\circ}\)C and 70 \({}^{\circ}\)C during the test. Cleanliness was monitored by manually controlling the 3/2 valve to position 17 and then taking oil samples for cleanliness measurements through the test port on the pressure relief valve. Control spool inside a directional 4/3 valve had the function of redirecting fluid flow (fluid energy). It made a connection from port P (pressure port of the pump) to port A (working port) and at the same time connection from port B (another working port) to port T (port of the tank). Described position is called parallel position. The diagonal position connected port P to port B and port A to port T; in this case, the fluid flow was reversed from the directional valve onwards. This is necessary when actuating hydraulic cylinders for extending and retracting piston and rod in and out of the housing of the cylinder or rotating hydraulic motor in desired direction (clockwise, counter clockwise). The duration of one cycle was 0.2 s, i.e., solenoid a was activated so that the spool of the directional 4/3 valve reached a parallel position, and then solenoid b was activated so that the spool returned to its initial position, i.e., the diagonal position. The pressure conditions in the system using pressure measurement system (Service Master Plus K-SCM-500-01-01, [PERSON], Bielefeld, Germany) and pressure sensor (model A-10, Wika, Klingenberg, Germany) 0-400 bar with 0-10 V output were measured.