Research and application of ultra-high pressure control system in strength tests

1. Introduction

The working pressure of the hydraulic system in engineering applications is usually about 20 MPa, which can meet most of the requirements generally and is easy to use and maintain [1]. In some engineering fields, the working pressure of equipment exceeds 32 MPa, which is called ultra-high pressure.

In order to achieve the goal of weight reduction, the hydraulic system of the aircraft adopts high pressure and ultra-high-pressure technology always. If the rated working pressure of the hydraulic system is increased from 21 MPa to 28 MPa, the weight of the actuator is reduced by about 8 %, the weight of the working fluid is reduced by about 21 %, and the total weight of the hydraulic system is reduced by 5 % [2]. In a landing gear static strength test of an aircraft, it is demanded to apply oil pressure to the actuating cylinder. Pressure in the test reaches 43 MPa, considering the design safety factor1.5 [3-4].

In previous pressure tests, a hand pump is often used manually for loading, which may lead to problems of slow pressurization speed, low control accuracy and prominent potential safety hazards, and cannot be applied to fatigue tests. The ultra-high pressure hydraulic system is widely used in hydraulic tools and forging industry, which do not require precise control of pressure, nor can they set a control protection limit. It does not match the requirements of an aircraft static test [5-9]. Yang Hu realized high pressure (52 MPa) loading by carrying out pressurization transformation on a hydraulic system, but the automation degree of the system is limited, and the operation is relatively cumbersome [10]. Zhipeng Zhang et al. designed an ultra-high pressure automatic control system, which can achieve high pressure not greater than 120 MPa, but the interference problem limits its application, either the low acquisition frequency and low control accuracy [11].

Aiming at improving the control ability of ultra-high pressure, an ultra-high pressure device was designed, which composed of a low-pressure servo pump, servo valves and a booster cylinder. The precise loading control of ultra-high pressure became reality by combining the device with the existing control system and hydraulic system. The system has the following advantages:

1) Low cost, good system compatibility and stable high pressure is achieved by adopting low pressure hydraulic components;

2) By giving full play to the role of low-pressure servo components in controlling ultra-high-pressure accurately, the problem that the high-pressure with low flow cannot be accurately controlled is solved;

3) The pressurization, pressure maintaining, and pressure reduction of ultrahigh pressure equipment are realized.