MECHANICAL ANALYSIS AND RESEARCH OF THE CONVEYOR BELT OF PLANE TURNING BELT CONVEYOR BASED ON DISCRETE ELEMENT METHOD

In view of the size and the change of the load force of the conveyor belt at the turning point of the plane turning belt conveyor, the influencing factors of the stress of the conveyor belt at the turning point of the plane turning belt conveyor under full load condition are analyzed. A three dimensional model of the turning point of the plane turning belt conveyor is established. Combined with previous research experience, the formula for calculating the load is put forward. Based on discrete element method, multiple sets of internal curve elevation angle and the belt speed are used for dynamic simulation analysis. The results showed that the middle of conveyor belt is the most stressed, the lateral force second, the force of the inner conveyor belt is the least. Outside force increases with the increase of speed; there is no change in the middle band; the inner band force decreases with the increase of the velocity. Outside force decreases with the decrease of the inclination angle. With the change of the inclination angle, the force is basically unchanged. With the decreasing of the inclination angle, the force increases gradually. By optimizing the design parameters of the plane turning belt conveyor, the force of belt is reduced, and the service life of belt is improved.


Introduction
Belt conveyor generally use straight line layout, however, there are some occasions due to the external factors such as terrain constraints, the belt conveyor cannot be used for linear transportation (Rocha et al., 2012) Therefore, the curve conveyor belt conveyor emerges (Pang & Lodewijks, 2007).The curve layout of the existing belt conveyor generally adopts series connection, forced to change direction and natural change direction, which means to change the natural setting conveyor roller group forward, elevation curve and other measures to make the realization of nature oriented conveyor, in place of a plurality of overlapping conveyor transfer modes.This can not only reduce the overlap of belt conveyor, reduce the investment of the driving device, but also reduce dust pollution (Wang, Yang & Song, 2014;Tao, 2013).The analysis of the force of the conveyor belt in the turning section is directly related to the normal operation of the belt conveyor, therefore, the stress analysis of the conveyor belt in the plane turn has attracted the attention of many researchers both at home and abroad.German scholar Holger Lieberwirth (1994) analyze the stress of the adhesive tape and the material in the horizontal turning section has only considered the slot angle and the elevation angle, and has not considered the influence of the rake angle of the roller.Austria scholars Grimmerk and Kessler (1992) carried out the design and calculation of the possible two kinds of limit position, analyzed the large plane turning belt conveyor by dynamic design method, which can improve the design level, avoid the design defects of the traditional design method and the occurrence of the accident.In recent years, Domestic scholars have carried out some researches on the plane turning belt conveyor.Professor Sun Ke-wen (1991) analyzed two dimensional stresses of the conveyor belt and the material in the horizontal turning section, and derived the belt tension and turning radius in the horizontal curve section.Professor Song (Song, 1996

Mechanical model of plane turning conveyor belt 2.1. General assumption
Under the actual conditions, the turning point of the belt conveyor adopts the three roll form of the bearing segment groove type roller group.There are two kinds of running state of groove type bearing support roller group, which is load operation and noload operation.The difference between the load and no-load operation is that the load operation of the material needs to be part of the force calculation by gravity.Therefore, the three section roller type roller group is selected as the research object.

Force calculation of conveyor belt in plane turning
The necessary condition for the plane turning of the belt conveyor is the force balance at the turning point.Set FT as the conveyor belt centripetal force of plane turning belt conveyor in one roller spacing, N; R as the radius of curvature at the corner of the plane, m; l0 as the distance between the support roller group at the turn of the plane, m; v as the belt speed of belt conveyor, m/s.The analysis (Liu, 2013) indicates that the FT parallel the supporting roller group surface elevation, and divided the force into the following three components according to the conveyor belt in each roller width, then The force along roller axial which let the conveyor belt shift to the inside as following: FT along the normal force of roller which make conveyor belt have a positive pressure on the roller and increase the roller friction.The component force is helpful for conveyor belt turn.Take all the forces as In the formula, B is the width of conveyor belt, m; lb1 is the width of conveyor belt on the inner roller, m; lb2 is the width of the conveyor belt on the middle roller, m; lb3 is the width of the conveyor belt on the outer roller, m; λ1 is roller group for groove angle, º; λ2 is the outer roller group groove angle, º; γ is internal curve elevation angle, that is, the theory of slope angle,º.

Force calculation of material in plane turning
Because of the characteristics of the bulk material, it is decided that the conveyor belt is loose and has certain fluidity.Material is different from the conveying belt at the time of operation and material on conveyor belt in addition to the gravity.And the friction force between the conveyor belt and material.So the analysis of the structure of the plane turn needs to be separated from the conveyor belt and the material.When the material is turning along with the conveyor belt, due to the relative motion the horizontal relative position of the material and the conveyor belt has been changed.And because the material is continuous, the cross-sectional area of the material is constant, and the cross section of the material is still the same.And the curve radius of the arc, the maximum height of the material and the width of the material are all the same.When running the material on the groove type supporting roller group, the cross section of the material is approximated as a combination of a trapezoidal cross section and a section of the chord.The roller is divided into three parts, and its bearing capacity is shown in Figure 2. Set FGB1、FGB2 and FGB3 in turn for the 3 regions of the conveyor belt gravity, set FGG1、FGG2 and FGG3 are 3 regions for the gravity of the material, set FG1、FG2 and FG3 in turn for the total gravity of 3 regions.
According to the literature (Song, Xu & Wang, 2014) available, the conveyor belt gravity calculation is as follows: The type (6) according to the axial and radial roller conveyor belt is decomposed along the gravity roller axial force as follows: Similarly, the conveyor belt along the gravity roller radial component as follows: In the formula, lB1, lB2 and lB3 are the contact width of the 3 rollers and conveyor belt respectively, m.The vertical component of the material gravity effect on the three rollers was measured by the (Piao, 2010) in the literature.The section material is divided into three parts, which are corresponding to the three rollers.Measure the weight of the material on each roller: k1qGgl0, k2qGgl0, k3qGgl0, thereinto, k1+ k2+ k3=1.The gravity of the material is decomposed into two forces along the radial and axial direction of the roller, then the calculation formula for the axial component of roller.
Similarly, the calculation formula for the radial force of roller as follow, Set FN1、FN2 and FN3 are in turns as the force of three rollers acting on the belt, then Set FR1、FR2 and FR3 as the three roller friction effect on the conveyor belt, μ as the coefficient of friction, then

Analysis
From formula (11) ~ ( 16), ① When the belt conveyor in the plane turn, the conveyor belt is supported by the support force (FN1、FN2 and FN3) and friction (FR1、FR2 and FR3) of the roller group, that is, the total force of the conveyor belt is the vector of the support force and the friction force.② The total force of the conveyor belt is nonlinear with the speed of the belt, which increases with the increase of the v, and the increment gradually increased.The total force of the conveyor belt is also nonlinear relation with the elevation angle of the inner curve γ, which decreases with the increase of the γ, and the reduction is a trend of fluctuations.

EDEM modeling 3.1. Plane turning belt conveyor model
By making use of Solidworks, the author establishes a 3D model of the conveyor belt in transportation, maps the complete machine model in the modern discrete element model software EDEM (Kruggel-Eden et al., 2007), as shown in Fig. 3. Belt conveyor turning length is approximately 5.3m, the bandwidth is 0.8m, the average transport inclination is 0.74 degrees, the spacing of the roller group is 0.6m, the belt speed is 1m/s, the horizontal turning radius is 2.1m, the elevation angle of the internal curve is 5 degrees.

Establishment of contact model
In order to accurately describe the contact and crash process between coal material and conveyor belt, as well as those between coal material and coal material, the granular distinct element method generally adopted the elastic-damping-frictional contact mechanical model, as shown in Fig. 4.

Simulation Analysis 4.1. Simulation parameter settings
The uneven granularity of raw coal and mixed gangue after mining makes the separation more difficult.For this reason, lumps of raw coal and mixed gangue should undergo primary breakup before being transported by belt conveyor.In general terms, a jaw breaker can reduce the lump coal to the size ranging from 18mm to 48mm (Li, Du & Xu, 2011), so the size of the simulated coal material is 40mm.Conveyor belt model using Moving-Plane model in order to simulate linear motion of the conveyor belt.The recovery coefficient of each material is set to: between coal and coal is 0.5, between coal and belt is 0.45; the static friction coefficient of each material is set to: between coal and coal is 0.6, between coal and belt is 0.5; the coefficient of rolling friction of each material is set to: between coal and coal is 0.05, between coal and belt is 0.05; The basic material flow parameters: the production rate of the particle factory is 1050 particles/s, simulation time is 10s.increase of the belt speed and with the increase of internal curve elevation angle, the total force of the conveyor belt is gradually increased, which is consistent with the theoretical calculation results.It can be seen from the fig.7, when the belt speed is increased from 1m/s to 3m/s, the lateral force of the conveyor belt is gradually increasing, the middle of the conveyor belt is basically unchanged, the inner side of the conveyor belt is gradually reduced; And it can be seen from figure 8, when the internal curve elevation angle is increased from 5 degrees to 10 degrees, the lateral force of the conveyor belt is gradually decreased, the middle of the conveyor belt is basically unchanged, the inner side of the conveyor belt is gradually increasing.Table 2 is simulation data of the total force of conveyor belt for coal transportation stable stage, when the belt speed is 3m/s and internal curve elevation angle is 15 degrees, the total force of the conveyor belt is 206N.While the belt speed is 1m/s and internal curve elevation angle is 5 degrees, the total force of the conveyor belt is 135N, which reduced 34.4%.This is consistent with the theoretical analysis.

Conclusions
(1)In view of the size and the change of the load force of the conveyor belt at the turning point of the plane turning belt conveyor, the influencing factors of the stress of the conveyor belt at the turning point of the plane turning belt conveyor under full load condition are analyzed.The total force of the conveyor belt at the turning point is nonlinear with the speed of the belt, which increases with the increase of belt speed, and increment gradually increase.The total force of the conveyor belt is also nonlinear relation with the internal curve elevation angle, which decreases with the increase of belt speed, and the reduction is a trend of fluctuations.
(2) Using the discrete element simulation, the pressure curve of each supporting roller of the roller group at the turning point is analyzed.The force of the conveyor belt is in a non-linear relationship with the speed of the belt and the internal curve elevation angle.The force of middle area of the plane turning conveyor belt is the largest, the outside area is the second, and the inside area is the smallest.This is consistent with the actual situation of the conveyor belt when horizontal turning.The total force increases with the increase of the inner curve elevation angle, and increases with increase of the belt velocity, which is consistent with the theoretical calculation results.
(3) In the actual design of the plane turning belt conveyor, optimizing the belt speed and the elevation angle of the internal curve, which can reduce the pressure of the conveyor belt, improve the reliability and service life of the conveyor belt.

Fig. 2 .
Fig. 2. Schematic diagram of the gravity and friction force of the material and the conveyor belt

Fig. 6 .Fig. 7 .Fig. 8 .
Fig. 6.Simulation process (1.Measurement area of the inner conveyor belt; 2. Measurement area of the middle conveyor belt; 3. Measurement area of the outside conveyor belt) ; Song et al., 2005; Wang & Song, 2005) analyzed the dynamic characteristics of the large belt conveyor starting from the plane turning and compared with the linear transportation belt conveyor, which obtained the De-yong Li, Shuang Wang, Kun Hu Mechanical analysis and research of the conveyor belt of plane turning belt conveyor based on DEM 56 influence of the turning conveyor on the belt conveyor.Piao Xiang-lan et al., (2007) researched the tape and the material in the horizontal curve section of the stress, although considered the roller forward influence on the force, its mechanical model is not general.At present, the study of the dynamics of the plane turning belt conveyor is still relatively few.

Table . 2
Simulation data of the total force of the conveyor belt in the stable transport stage