# Study on Temperature Characteristics of Lubrication Film of Valve Plate Pair in Axial Piston Pumps
## 4 Results and Discussion
### Temperature Characteristics of the Valve Plate Oil Film  
The highest film temperature of the valve plate pair appears in the auxiliary support belt due to the minimum-film-thickness point located in the auxiliary support belt. To protect the auxiliary support belt film and utilize the valve plate's high thermal conductivity, eleven holes for thermocouples are placed in the back of the auxiliary support belt in the high-pressure side in Figure 14. The diameter of every thermocouple hole is 2 mm, and its depth is four-fifths of the valve plate thickness. The temperature measured with a thermocouple in one hole was considered to be the oil film temperature at the corresponding point of the valve plate auxiliary support belt. Figure 15 shows the theoretical temperature distribution in the highest-temperature area of the auxiliary support belt. The curve A is the temperature distribution along the inner side of the auxiliary support belt. The temperature first increases then decreases in the circumferential direction of the valve plate, and the highest temperature of about 60 \({}^{\circ}\)C is near the middle of the range [\(-\)60\({}^{\circ}\), 60\({}^{\circ}\)] of the valve plate. The temperature increases slightly in the radial direction from the inner side to the outer side of the auxiliary support belt. Figure 16 shows the theoretical calculation results of the temperature distribution along curve A in Figure 15 in the high-pressure side of the auxiliary support belt at working conditions 1-4. In Figure 16, the theoretical and experimental film temperature values agree with each other; however, the theoretical maximum film temperatures at working conditions 1-4 are always higher than the experimental values. It is probably because the thermocouples installed on the back of the valve plate cannot directly measure the gap in the film temperature between the cylinder block and the valve plate. In Figure 16a, under working conditions 1 and 2, the experimental maximum film temperatures are 39.91 \({}^{\circ}\)C and 51.73 \({}^{\circ}\)C, and the theoretical values are 41.83 \({}^{\circ}\)C and 53.72 \({}^{\circ}\)C, respectively. The theoretical and experimental maximum film temperature errors at working conditions 1 and 2 are 1.92 \({}^{\circ}\)C and 1.99 \({}^{\circ}\)C, respectively. In Figure 16b, under working conditions 3 and 4, the experimental maximum film temperatures are 41.06 \({}^{\circ}\)C and 59.12 \({}^{\circ}\)C, and the theoretical values are 44.25 \({}^{\circ}\)C and 62.26 \({}^{\circ}\)C, respectively. The theoretical and experimental maximum film temperature errors at working conditions 3 and 4 are 3.19 \({}^{\circ}\)C and 3.14 \({}^{\circ}\)C, respectively. In Figure 16c, under working conditions 1 and 3, the gradient of experimental temperature curves is smaller than the theoretical temperature gradient in the circumferential range [\(-60^{\circ}\), \(60^{\circ}\)] of the valve plate. In Figure 16d, under working conditions 2 and 4, the gradient of experimental temperature curves is smaller than the theoretical temperature gradient in the circumferential range [\(-60^{\circ}\), \(-20^{\circ}\)]. The gradients of  
Figure 14: Thermocouple installation diagram. Figure 15: Temperature distribution in the highest–temperature area of the auxiliary support belt at working pressure 7 MPa and swash plate tilt angle \(16^{\circ}\). both experimental and theoretical temperature curves are very close in the circumferential [20\({}^{\circ}\), 60\({}^{\circ}\)] range of the valve plate. It is seen in Figure 16a,b that the experimental and theoretical film temperatures in the high-pressure side of the auxiliary support belt increase with the increasing working pressure from 3 MPa to 7 MPa. This is because the pump pressure increase causes an increase in the film wedge angle, a decrease in the minimum film thickness, higher viscous friction loss, and more leakage loss in the valve plate pair.