Influence of impact load form on dynamic response of chock-shield support

▪ Mechanical-Hydraulic co-simulation model is developed for chock-shield support. ▪ The influence of different loading forms on the reliability of the support is analyzed. ▪ The joint that may reduce the reliability of the support is obtained. ▪ The load at each joint of the support is typically non-uniformity. Chock-shield support is usually used in undergroud coal mining to protect the roof. However, as the mining depth gets deeper, impact load that came from the roof becomes stronger and more frequent. This causes the support to bear a large number of dynamic loads, and reducing its reliability. To improve the support performance of the chock-shield support, the mixed-kinetic model was established using the mechanical-hydraulic co-simulation method. The load distribution law of the support joint under impact load form different stability forces, impact load amplitude, and impact frequency is discussed. The mechanical-hydraulic cooperative response of the chock-shield support are obtained. The results show that different joints show typical non-uniformity characteristic during the loading process. The proposed mechanical-hydraulic co-simulation method can more accurately obtain the dangerous points of hydraulic support reliability. The results of this study will help to improve the reliability of the chock-shield support.


Introduction
With the rapid and stable growth of China's economic transformation, the mining and utilization of the coal resource are accelerated. As the key supporting equipment for coal mining, chock-shield support has been more and more widely used to protect the roof [3,9,16,18]. During the mining process, the top beam of the chock-shield support is in contact with the roof plate and the base plate is in contact with the bottom plate.
The roof plate, bottom plate and the coal seam together form the "strata system". The function of the support is to help prevent the subsidence of roof strata and ensures the mining face working normally. As the working face advances, the roof plate behind the support canopy will cave randomly or periodically, forming an impact load on the support. The released impact energy caused by the roof plate caving reaches up to 10 6 -10 8 J [14,15]. When the impact load appears, key structural components such as the hinge points, top beam plates will undergo significant deformation. This reduces the support stability of the support and poses a threat to the safety of workers. Therefore, the reliable operation of the supports is an important prerequisite for ensuring the safe production of the working face. In recent years, the shallow buried coal resources are gradually exhausted, and people have had to mine deeper coal. Meanwhile the mining height of the working face is also getting higher. These all cause the support to bear a large number of dynamic loads and reducing its reliability [8,12,22,27].
Since the support equipment has an important influence on safe mining. Many scholars have done lots works to improve the performance of the support. By establishing a mechanical model of two-column shield support, Wang et al. [19] introduced the impact coefficient to describe the adaptability of the joint to impact load. Wo et al. [21] analyzed the rule of support strength change on roof settlement characteristics. He points out that stronger support strength will be beneficial to roof settlement control. By using the Finite Element Method analysis method and applying horizontal and bending load to the support, He et al. [6] studied the stress distribution law of the top beam and determined the dangerous area that may lead to the support failure. By modelling a hydraulic support simulation model and applying a uniform load to the top beam position, Li et al. [10] obtained the stress nephogram of the support under different loads and proposed a structural optimization plan for the support. To analyze the load distribution rule of the two-column shield support, Zeng et al. [25,26] built the mechanical model of the support and applied the impact load to different position on the support. According to the D'Alembert principle, Guan et al [5] established a multidegree-of-freedom numerical analysis model of the hydraulic support and modified the front linkage structure. Then he compared the operating characteristics of the hydraulic cylinder and found that the improved structural components could further ensure the safe operation of the hydraulic support. Liang et al. [11] proposed an impact dynamics model considering the The existing research has focused on the impact of changes in impact load position on the dynamic response of two-column support, and less on the impact of different impact load forms on the dynamic response of four-column chock-shield support.
The existing research generally only involves mechanical system or hydraulic system, without considering the synergy of these two factors. To obtain the performance of chock-shield support under different impact loads more comprehensively, it is necessary to establish a mechanical-hydraulic collaborative simulation (MHC) model. The method of co-simulation analysis used in this paper is more applied in the research fields of automobile and robotics [4,13,23,28], which can cut down the production process, reduce the cost and improve the accuracy of simulation analysis in the practical application process. However, this method is less applied in the direction of hydraulic support.

The mechanical system model
In this study, the ZZ18000/33/72D type chock-shield support is chosen, the specific parameters of the support are shown in Table 1. The "ZZ18000/33/72D" is the product code of the selected support. The "ZZ" represents a four-column chockshield support, the "18000" represents the rated working resistance of the support (18000 kN), the "33/72" indicates that the minimum and maximum working height of the support is 3300 mm and 7200 mm, respectively. The "D" represents that the support is electro-hydraulic controlled. The ADAMS software is selected to establish the mechanical system model.
The connecting mode of the columns are defined as moving pair constraints and that of the others parts are rotating pair constraints. The selected support is equipped with double telescopic columns. To establish the force, speed and displacement transmission interface between the mechanical system and hydraulic system, Marker points are set at the center of the piston cylinder and piston rod, respectively. The force variables are defined as Input variables (input from hydraulic system to mechanical system), the speed and displacement variables are defined as Output variables (output from mechanical system to hydraulic system). Fig. 1 shows the established mechanical model of the ZZ18000/33/72D support.

The hydraulic system model
The hydraulic support studied in this paper contains four columns. For the convenience of narration, it is named the front raw column (FRC) and the back raw column (BRC) according to the spatial position of the column. Figure 2 shows the defined hydraulic system model.  Table 2.

(1) Dynamic response of the column system
The reactions of the column system with the same impact load and different setting forces is shown in Fig.6  (

2) Dynamic response of the joints
The existing studies show that the joints, as the key connection structure, is the most vulnerable part of the support.
Therefore, studying and improving the load distribution law of the joints is helpful to improve the reliability of the support [18,24].

Influence of the impact load amplitude
To further study the effect of impact loading variation on the dynamic response of support under the same setting force, this section fixes the setting force at 11000 kN. By changing the amplitude of impact load (5000 kN ~9000 kN, increments 1000 kN), the load transfer performance of support with different impact forces is compared [17,20].
(1) Dynamic response difference of the column system (2) Dynamic response of the joints Figure 9 displays the dynamic response of the joints under differing impact loading. When the impact load appears, the joints load increases and reaches the peak value rapidly. When the impact loading is 9000 kN, the load of joint 1, 2, and 3 is 1385 kN, 6534 kN, and 5815 kN, respectively. Compared with the impact load of 5000 kN, the maximum LIR of the joints is 1.28 and it appears at joint 2. Therefore, the joint 2 has higher probability reducing the reliability of the support.

Influence of the impact frequency
When the support reaches the rated operation height, it forms an elastic coupling with the roof. Therefore, the hydraulic support (1) Dynamic response difference of the column system

Influence of impact time gaps
The relative movement of the hydraulic support and the roof is (1) Dynamic response difference of the column system

Conclusions
To research the effect of impact loading on the operational reliability of the chock-shield support working reliability, this paper takes ZZ18000/33/72D type support as the research object. After building the MHC model, the dynamic change differences of the support under different forms of impact loading is analyzed. The main results are shown below: (1) The MHC analysis model of the support is established, and the rationality is tested by applying setting force to the model. In the stability test stage, the supporting force of the FRC increases gradually with the stability force (about 1.14 times of the BRC). When the impact loading is fixed, the joint load shows typical non-uniformity with the increase of the stability force.
(2) As the impact loading increases, The FRC shows a larger LIR (about 1.34) than that of the BRC (only about 1.13).
Compared to others joints, the joint 2 shows a continuous high additional load characteristic. During the mining process, the cylinder diameter ratio of the front and BRC can be appropriately increased to enhance the reliability of the support. (4) With the increase of impact interval, the peak load of the support is significantly reduced. Therefore, additional energyabsorbing structures can be considered to reduce the impact frequency of the hydraulic support, so as to improve the reliability of the support.
To study the effect of varying forms of loading on the impact reliability of the support, an ideal MHC model is developed in this paper. Due to the calculation time and efficiency, the influence of the joint clearance is not considered. To improve the accuracy of the results, further consideration will be given to the effect of clearance position and size on the operational reliability of the chock-shield support.