Cause analysis and measures of gear pump shell burst
Cause analysis and measures of gear pump shell burst
During the delivery inspection of an equipment vehicle, it was found that the indicated value of the hydraulic gauge could not meet the specified requirements. The commissioning personnel immediately increased the opening pressure of the main overflow valve of the vehicle slightly. It was found that the indicated value of the Hydraulic Gauge still could not rise, and then further increased the opening pressure of the overflow valve. The problem was still not solved. The commissioning personnel immediately retracted the outrigger and waited for the technicians to check. At this time, the gear pump burst suddenly.
2. Failure cause analysis
After shutdown, the gear pump was inspected, and it was found that the fastening screw hole of the end cover of the aluminum alloy shell was cracked along the axial direction, with a through crack about 100mm long and 2mm wide, which was located at one side of the high-pressure oil chamber of the gear pump. In view of the phenomenon of the problem, the causes of the problem are analyzed from the following aspects.
(1) Material and casting quality analysis. The shell is made of ZL111 aluminum alloy, which is manufactured by casting and processed by boring, grinding and other processes. If the shell material is unqualified, or there are slag inclusions, bubbles during casting or cracks during processing, local stress concentration will occur under high-pressure load, resulting in burst. In order to verify whether the pump shell meets the requirements, the burst pump shell is subjected to chemical composition analysis and mechanical property test.
The results show that the measured tensile strength of the material is 315mpa, which meets the standard requirements; The chemical composition of the shell material shall meet the requirements of GB / t1173-1995; According to GB / T 9438-1988, no bubble or slag inclusion with diameter greater than 0.5mm is found through macroscopical inspection, and the casting quality meets the requirements; The fracture surface of the pump casing is a ductile tear fracture without old cracks, which indicates that there are no cracks during processing or fatigue cracks during use. This shows that the material and casting quality of the gear pump shell meet the requirements.
(2) Finite element analysis of gear pump housing. Using finite element analysis, we must first build a solid model (this process is also called solid modeling), which generally has two methods: bottom-up and white top-down. From bottom to top, define key points, and then use these key points to define higher-level entities. This modeling method from point to line, from line to face, from face to body, and from low-level to high-level is called bottom-up method. The other is the top-down method, which starts modeling from high-level primitives and generates low-level primitives directly. According to the characteristics of complex shell structure, the bottom-up modeling method is adopted.
After the shell solid model is established, it is meshed. Figure m shows the finite element model of the shell solid after meshing in the finite element software ANSYS. When imposing boundary constraints on the shell, it is considered that the end cover forms a low-pressure oil cavity and a high-pressure oil cavity together with the shell in function; In terms of constraint relationship, the end cover restricts the degrees of freedom of the shell through the 8 connecting bolt holes of the shell, so all DOF is applied to the interior of the installation and fixing bolt holes, and UX, uy and UZ are applied to the 8 connecting bolt holes of the shell. Considering that the center line of the meshing gear outside the inner cavity of the gear pump is the boundary, it is divided into high-pressure oil cavity area and low-pressure oil cavity area. The pressure in the low-pressure oil cavity area is negative pressure (to absorb oil). Therefore, during loading, only 31.5Mpa pressure load (the maximum adjustment value of overflow valve is 31.5Mpa) is applied to the inner wall of high-pressure oil chamber and high-pressure oil outlet channel to simulate the working condition of gear pump under 31.5Mpa pressure load. The simulation results are shown in Figure n.
It can be seen from figure n that the maximum stress point of the shell is located at the fastening bolt hole of the end cover of the pump shell (MX in the figure), which is consistent with the actual position of burst, which is the weakest place of the shell. The maximum stress value here is 77MPa, which is less than the tensile strength value of shell material (315mpa). The calculation results show that the design strength of the pump shell can fully meet the service requirements.
(3) Pressure regulation system analysis. Fig. o is a partial schematic diagram of the hydraulic system of the ammunition transport vehicle. Among them, the fine-tuning valve 5 of the upper pressure controls the pressure of the oil control port K of the hydraulic control 4, that is, it determines the opening pressure value of the overflow valve 4.
The function of multi-way directional valve 2 is to select whether to supply oil to the upper system or to the lower outrigger oil circuit. Only one of the two can be selected.
When the oil supply to the upper loading system is selected, the hydraulic oil is controlled by the overflow valve 1. It enters the upper loading system through the multi-way directional valve and performs a certain action after being controlled by the overflow valve 4. At this time, the working pressure value of the whole system can be known from the hydraulic indicator table 3.
When selecting to supply oil to the upper loading system, the overflow valves 1 and 4 are in parallel in the whole hydraulic system, and only the one with lower opening pressure plays a role. If the upper (or lower) overflow valve fails, only the opening pressure of the lower (or upper) overflow valve will be increased, and the value of the pressure indicator is still the opening value of the overflow valve 4 (or 1). When commissioning the opening pressure of two relief valves, the general method is; Fine tune one of the relief valves first, and then adjust the other if the indication of the pressure gauge still fails to reach the specified value.
Due to the failure of the upper overflow valve 4, although the operator has raised the opening pressure of the lower overflow valve for many times and has exceeded the maximum pressure of the gear pump, the value shown in the hydraulic indicator 3 is still the opening value of the overflow valve 4. At this time, choose to supply oil to the off vehicle system and retract the outrigger. Because the system hydraulic pressure rises when the outrigger is in position, but the overflow valve cannot be opened in time, the system hydraulic pressure rises rapidly, exceeding the maximum pressure value of the gear pump, resulting in shell burst.
From the above analysis, it can be seen that the burst of gear pump is mainly caused by imperfect design of hydraulic system and improper adjustment of system hydraulic pressure. At the same time, there is no clear pressure adjustment method in the operation specification to guide the operators.
Through analysis, the causes are found out. In order to ensure the normal operation of hydraulic equipment and avoid similar accidents in the future, the following measures are taken.
(1) Improve the hydraulic system. One method is to install a hydraulic indicator in the lower vehicle hydraulic system, but the adjustment times of the lower vehicle overflow valve are not many. Installing a hydraulic indicator increases the cost and is not necessary. Another method is to improve the valve body structure of the off vehicle overflow valve and limit its opening pressure adjustment value to 3 ~ 23mpa. Even if the pressure adjustment bolt is screwed to the maximum, the overflow valve can still be opened and the pressure is 23mpa, which can not only meet the use requirements, but also not exceed the rated pressure of the gear pump (25MPa). After expert review, the second method is finally selected.
(2) Improve the final assembly and commissioning documents. Add the above commissioning methods into the General Assembly commissioning process and operation instructions, and train relevant personnel to conduct commissioning and operation according to the specified procedures.
After implementing the above measures, the follow-up products and products delivered to the army were improved, tested and tracked, and no similar accidents occurred. This shows that the cause analysis is comprehensive, the measures are in place and the effect is good.