Abstract
Extra-large hydraulic excavators for mining are indispensable for improving the efficiency and quality of open-pit mining. Research on the load spectrum of excavator operations is fundamental for enhancing their intelligence, efficiency, and reliability. This study employs the Enhanced Discrete Element Method (EDEM) simulation software to analyze the loads on the bucket teeth of excavators. It investigates the spatial distribution of the working device from two aspects: the included angle and amplitude distribution of loads between bucket teeth. The simulation analysis reveals that the bucket teeth loads under typical working conditions exhibit similar time histories, corresponding peak loads, and spatial distribution patterns. This provides theoretical and data foundations for subsequent research on performance degradation and life prediction of excavator working devices.
1. Introduction
Extra-large mining excavators serve as crucial mining machinery for open-pit mining and shovel-loading operations. The random load variation during excavation operations is a significant basis for studying the fatigue life, reliability, and operational performance evaluation of excavators. It plays a vital role in improving product life, reliability, and operational performance, as well as shortening the research and development cycle. Current research on the operational loads of mining face shovel excavator working devices can be broadly classified into three areas: direct data generation from theoretical empirical formulas [1], actual measured load data [2-3], and data generated through discrete element simulation [4-9]. Discrete element simulation data closely aligns with actual measured load data, while avoiding the need for extensive field measurements, thereby saving considerable time and economic costs [10-11]. The Enhanced Discrete Element Method (EDEM) simulation is an effective method for analyzing the excavation process of excavator working devices.
2. Simulation Setup
2.1 Simulation Model
The simulation model in this study focuses on the bucket teeth of a mining excavator. The bucket teeth are modeled as rigid bodies, with a total of six bucket teeth mounted on the bucket. The bucket teeth and the bucket are combined into a single entity, which rotates unidirectionally and uniformly around the hinge point between the bucket arm and the boom. In EDEM, this unidirectional rotation is represented by “Rotation,” with motion parameters detailed in Table 1.
Table 1: Motion Constraints and Parameters of the Working Device
| Component | Speed (°/s) | Duration (s) |
|---|---|---|
| Bucket arm (arccos) | -0.001t² – 0.0864t – 0.1736 | 8 |
| Bucket (arccos) | -0.001t² – 0.0864t – 0.1736 | 8 |
2.2 Particle Model Creation and Coal Particle Factory Generation
Based on the actual physical density range of coal in Table 2, the material properties of several common coal particle types are set in EDEM, as shown in Table 3.
Table 2: Densities of Different Types of Coal
| Coal Type | Density (kg/m³) |
|---|---|
| Lignite | 1.1 – 1.3 |
| Anthracite | 1.4 – 1.7 |
| Bituminous coal | 1.1 – 1.4 |
| General coal | 1.2 – 1.7 |
Table 3: Physical Properties of Common Coal Types
| Rock Name | Density (kg/m³) | Elastic Modulus (GPa) | Shear Modulus (GPa) | Poisson’s Ratio | Cohesion (MPa) | Friction Angle (°) |
|---|---|---|---|---|---|---|
| Coal 1 | 1380 | 5.3 | 2.0 | 0.32 | 1.25 | 32 |
| Coal 2 | 1400 | 0.99 | 0.38 | 0.31 | 1.28 | 30 |
| Coal 3 | 1430 | 1.2 | 0.46 | 0.31 | 1.22 | 28 |
| Coal 4 | 1420 | 0.5 | 0.19 | 0.32 | 0.8 | 20 |
| Hard coal | 1850 | 12.2 | 0.956 | 0.15 | 1.88 | 42 |
| Soft coal | 1300 | 0.4 | 0.145 | 0.39 | 0.34 | 30 |
The density of the bucket is 7500 kg/m³, with a Poisson’s ratio of 0.28 and a shear modulus of 10 GPa. The physical parameters of Coal 2 are selected as the input parameters for generating coal particle simulations. The friction coefficients between the working device and the particles in EDEM are shown in Table 4.
Table 4: Friction Coefficients Between the Working Device and Particles
| Attribute | Between Coal Particles | Between Coal and Bucket |
|---|---|---|
| Static Friction Coefficient | 0.6 | 0.4 |
| Rolling Friction Coefficient | 0.15 | 0.04 |
| Restitution Coefficient | 0.5 | 0.5 |
The physical properties of coal particles in EDEM are shown in Figure 6.
Figure 6: Physical Properties of Coal Particles in EDEM
Using a single particle type to simulate rock can lead to significant errors. To better match the actual material shape, spheres of the same or different sizes are arranged to form particles of various shapes, representing different forms of coal particles found in coal piles, such as flat, columnar, pyramid-shaped, and approximately spherical. The simplified particle models are shown in Figure 7, with shape coefficients of 0.433, 0.389, 0.279, and 0.753, respectively.
Figure 7: Actual Shapes of Coal and Their Simplified Discrete Element Particle Shapes
In the simulation, these four particle shapes are used to establish the distribution model of the pile. The number and proportion of each type of coal particle are shown in Table 5.
Table 5: Quantity and Proportion of Four Types of Particles
| Particle Type | Diameter (mm) | Quantity | Proportion (%) | Generation Location |
|---|---|---|---|---|
| Spherical | 200 | 6000 | 21.42 | Random |
| 400 | 1000 | 3.57 | Random | |
| Columnar | 200 | 6000 | 21.42 | Random |
| 400 | 1000 | 3.57 | Random | |
| Pyramid-shaped | 200 | 6000 | 21.42 | Random |
| 400 | 1000 | 3.57 | Random | |
| Flat | 200 | 6000 | 21.42 | Random |
| 400 | 1000 | 3.57 | Random |
The particle factory is used to configure the time, position, and method of particle generation during the EDEM simulation. All types are set to dynamic, with the settings detailed in Table 6.
Table 6: Particle Factory Parameter Settings
| Name | Particle Count | Generation Rate (particles/s) | Initial Velocity (mm/s) | Particle Type | Particle Diameter (mm) |
|---|---|---|---|---|---|
| F1 | 6000 | 2000 | 100 | Spherical | 200 |
| F2 | – | – | – | Columnar | – |
| F3 | – | – | – | Pyramid-shaped | – |
| F4 | – | – | – | Flat | – |
| F5 | 1000 | 400 | 200 | Spherical | 400 |
| F6 | – | – | – | Columnar | – |
| F7 | – | – | – | Pyramid-shaped | – |
| F8 | – | – | – | Flat | – |
2.3 Simulation Parameters
2.3.1 Simulation Region
To minimize computational load and improve simulation efficiency, the simulation region is set using the EDEM automatic method, resulting in a cuboid region of 43.64 m × 30.60 m × 21 m. This includes a circular truncated cone region for generating coal particles, with a height of 21 m, an upper base radius of 1 m, and a lower base radius of 21 m, simulating a coal pile in a mine.
2.3.2 Simulation Time Step
The EDEM simulation time step represents the time interval between two consecutive simulation steps. The ideal time step is the Rayleigh time step (TR), which represents the time required for a shear wave to propagate between coal particles.