# A Study on Failure Analysis and High Performance of Hydraulic Servo Actuator
## 2 Failure Mode Analysis of Hydraulic Servo Actuator  
In order to analyze the cause of the failure of the hydraulic servo actuator, the characteristics of use in the test equipment are summarized as follows: first, very fast operating frequency; second, micro displacement; third, high load; fourth, durability; and fifth, increasing the frequency of use.  
The results of failure of the hydraulic servo actuators are external leakage and power reduction, reduction in control precision, discoloration of the rod and tube parts, as shown in Figure 3, and noise or overheating. Fast frequency (0.1 to 100 Hz) produces rapid iterative friction on the seal assembly and on the rod and cylinder tube surface. Micro displacement, which is smaller than the width of the seal, inhibits sufficient lubrication and increases friction. In particular, it is not possible to release the residual air and gas inside the hydraulic servo actuator, resulting in high temperature due to the insulation compression of the gas. Additionally, if the operating pressure is increased for the high load test, the pressure of the seal increases and the friction increases.  
Hydraulic servo actuators are used in the approximate (0.1-80%) range of maximum stroke.  
As the displacement of the actuator fails to operate in the 100% range, the hydraulic oil introduced inside the actuators is not fully drained, the initial pressurization state is maintained continuously, and the hydraulic oil of the micro displacement control level is supplied. At this time, the incoming gas remains inside the actuator, reducing the control precision with compression. The gas in the cylinder and piston chamber is heated by insulation compression, as shown in Equation (1), accelerating the lubrication degradation and resulting in mechanical efficiency degradation and wear failure due to excessive friction.  
The oil temperature of the hydraulic system, \(T_{1}\), is managed at 50 \({}^{\circ}\)C and the maximum working pressure, \(P_{2}\), is used at 21 MPa. Thus, the internal gas rises high to about 500 \({}^{\circ}\)C as shown in formula (1), which damages the seal by burning it, which increases friction. Adiabatic coefficient for air \(k\) is 1.4 [25; 26].  
\[T_{2}=T_{1}\times\frac{P_{2}\cdot V_{2}}{P_{1}\cdot V_{1}}=T_{1}\left(\frac{P_ {2}}{P_{1}}\right)^{\frac{k-1}{k}}=\left(273+50\right)\times\left(\frac{21}{1 }\right)^{\frac{1.4-1}{k}}\approx 771(^{\circ}\mathrm{K})\approx 498(^{\circ} \mathrm{C}) \tag{1}\]  
Figure 3: Fault picture of hydraulic servo actuator (**a**) Tube burning (**b**) Rod burning.  
Therefore, the main failure mode was analyzed and found to be the over burning of the seal contact area due to friction, which was analyzed and found to be caused by overloading the seals, incorrect misalignment of the precision device and compression of gas. To solve this problem, low friction hydrostatic bearing should be employed to prevent the overload of the seal part, and misalignment requires the recruitment of bush-use design technology to secure concentricity and the insulation compression of gas requires the use of air-dissipating design technology [27; 28].