In this project, the working process of an excavator will be simulated by multi-body dynamic software Adams. The mechanical system consists of the rotation platform, bucket arm, bucket arm cylinder, boom, boom cylinder, bucket, rocker and rod et al. It can complete various mining tracks through controlling the movement of cylinder. The working process of excavator includes the processes of excavate, bucket rise, rotation platform swing, unload and back swing, et al. The force condition and motion trail of excavator can be analyzed by multi-body dynamic simulation. Therefore, its performance can be determined rapidly. The analysis and comparison between different designs can be conducted during the concept development stage. The critical parameters can be determined by simulation, which can support the entity design and the FEM analysis of strength and apparatus.
The aim of this project is to recreate and study in Adams View environment the behavior of an excavator. The analyses which are carried out aim at:
– evaluating the dimension of the working scope
– evaluating the forces at critical points during the working cycle.
The modelling problem
The model of excavator consists of five parts: platform, cab, boom, arm and bucket. The geometry model of platform is neglected and represented by a coordinate system with specified quality. The constraint between the platform and ground is fixed joint. The revolute joint is created between platform and cab, cab and boom, boom and arm, arm and bucket. The cab, boom, arm and bucket are connected by hydraulic rod and hydraulic cylinder with each other. The joint between hydraulic rod and cylinder is translational.
The working cycle of the excavator commonly consists of four parts: preparation, excavation, transfer and unload. Different components experience different movement during the corresponding stage. In this work the time of a cycle is 47s . The following table describes the movement of different component during one cycle:
Table 1. The movement of different component during the working cycle
platform rotates 100°
platform reversely rotates 100°
Note: E: extend; R: retract
The following figure shows the excavator status at different time during the working.
Different quality attributes assigned to different components. The mass of boom, arm and bucket are 800kg, 500kg and 200kg, respectively. The mass of the earth excavated for one cycle is 5000N. In terms of the working process, the force on the center of the bucket increases linearly from 0 to 5000N during 20s-30s, remains unchanged during 30s-43s, and decreases to 0 during 43s-47s
Simulations and analysis of results
The kinetics and dynamics of the excavator during the working cycle was mainly analyzed.
In order to measure the dimension of the working scope, we need to obtain the trace of movement. The following figure shows the displacement trace of the tip of bucket. According to the definition of the parameters of the excavator, we define A is maximum excavating radius, B is maximum excavating depth, C is maximum excavating height, D is maximum unloading height. The main size of parameters of excavator can be shown in Fig. 3.
The following figure shows the displacement-time curves of bucket tip along y and z axis direction. In terms of the results, the simulated main size of parameters of excavator can be acquired:
maximum excavating radius A：6004.69mm
maximum excavating depth B：4525.91mm
maximum excavating height C：1485.50mm
maximum unloading height D：1722.11mm
Fig. 5 shows the variation of acceleration of the bucket. The maximum value is 3453.82mm/s2.
In order to excavating, the power of the excavator transmits to the bucket mainly through the hydraulic cylinder force of different components. Therefore, the force conditions of the joint points and the thrust of the hydraulic cylinder are the main property indication.
The following figure shows the variance of the force at the joint points and hydraulic cylinders between cab and boom, boom and arm, arm and bucket. At the beginning of the working cycle, the boom, the arm and the bucket moves simultaneously. The forces on the joint points and hydraulic cylinders mainly produced by the gravity of the components. The movement of the boom and the arm finished at 4s and 5s. Thus, the force remained unchanged after that time. During 8-15s, the platform rotated and the forces are basically the same. During 15-20s, the boom dropped and the force on the joint between cab and boom increased due to the increasing of the moment. The forces increased during the excavation period (20-30s) because of the increase of the gravity of the material in the bucket. The forces decreased when the boom lifted (30-35s). During the reversely rotation (35-40s), the forces is comparatively stable. During the unloading process (43-47s), the forces decreased due to the decrease of the gravity of the material.
The force on the joint between the cab and the boom is higher than the rest joints. The maximum forces on the joint point and hydraulic cylinder are 1.35E+5N and 1.30E+5N. The maximum force occurs at the end of the excavation and the beginning of the boom lift.
An excavator has been modeled and simulated to analyze the kinetics and dynamics. The main size of parameters of excavator can be obtained. The forces variation at the critical points have been studied. The maximum value of the force is 1.35E+5N and locates at the joint between cab and boom. The maximum force occurs at the end of the excavation and the beginning of the boom lift. According to the simulation results, the performance evaluation of the excavator can be acquired and the design of the machine can be improved.
The following video shows the whole process of the movement.
 Wang G. X., Yang Y. L., “The process of work Elements of Excavators”, Journal of mechanical engineering, 2009.