Development of Long Distance Belt Conveyor Design Ideas

The development of long-distance belt conveyor design points of view In the past 15 years, foreign research on the theory of belt conveyors has made great progress, and the technical performance of main components of belt conveyors has also been significantly improved. The foundation for the development of distance and large-scale development has been laid.

 

With the continuous improvement of the reliability and economic requirements for long-distance belt conveyors, its design point of view is also gradually developing. The advanced design point of view is based on the international standard ISO 5048 and the German industrial standard DIN 22101, trying to reduce the running resistance, reasonably determine the safety factor of the conveyor belt, using the controllable start, the braking device to start up smoothly, braking, using the conveyor Dynamic analysis with visco-elasticity theory to predict and optimize the condition of the conveyor.

 

1 Use of high-precision roller and high-performance conveyor belt to reduce the running resistance The main resistance of the belt conveyor is composed of the roller rotation resistance and the forward resistance of the conveyor belt. Foreign experimental research shows that the roller rotation resistance and the sag resistance of the conveyor belt account for 50% to 85% of the main resistance, and the average value is 70%. Therefore, improving the accuracy of the roller and the performance of the conveyor belt can effectively reduce the running resistance. In the past 10 years, the structural form of the idler has been innovated, especially the use of high-performance special bearings and high-precision seal rings, which effectively reduce the rotation resistance of the idler. At the same time, the surface adhesive and core material of the conveyor belt are also constantly updated, so that the conveyor belt not only has a certain degree of groove, but also has a certain degree of surface hardness and wear resistance, effectively reducing the sag resistance of the conveyor belt. According to the current standards, the main resistance is estimated using simulated friction coefficient and factory value.

 

The DIN standard and the ISO standard recommend that, under normal conditions, f fetches 0.017 to 0.020; according to domestic design experience, f usually takes 0.020 to 0.025. Studies have shown that the main drag, which is calculated according to the current standard, for the simulated friction coefficient f-value is, in most cases, too large and affects the economics of the conveyor considerably.

 

The revised DIN 22101-1998 (Draft) proposes a more accurate method of calculating the primary resistance. That is: FHo=(FRo+Fgo)/q. In the formula FHo - upper branch main resistance FRo - upper branch idler rotation resistance FEo - upper branch conveyor indentation resistance qo - coefficient, take 0.5 ≤ qo ≤ 0.85, average value is q0 = 0.7 Fhu = (FRu + FEu ./qu. In the formula FHu—The main resistance of the lower branch FRU—The rotation resistance FEu of the lower branch idler—The indentation resistance of the lower branch conveyor belt qu— Coefficient, taking qu=0.9 The calculation of the main resistance in the new standard is the calculation of the upper and lower branch idlers. It is based on the rotation resistance and the sag resistance of the conveyor belt. For long-distance belt conveyors, the main drag has a great influence on the whole machine. It is necessary to determine in advance the rotation resistance of the idler used and the indentation resistance of the conveyor belt in order to accurately calculate the main resistance of the conveyor. In the case of unknown roller rotation resistance and belt collapse resistance, the new standard gives the reference friction coefficient, f. Under normal conditions, f = 0.010 ~ 0.020; under severe conditions, f = 0.020 ~ 0.040. It should be noted that the f-value recommended in the standard is applicable to the case where the upper idler pitch is 1.0 to 1.5 m and the lower idler pitch is 2.5 to 3.5 m. By reducing the idler spacing, the f-number can be reduced, but the total resistance generally increases, which is generally not desirable. For long-distance belt conveyors, foreign countries usually adopt methods to increase the distance between the rollers to reduce the total resistance. The distance between the upper support rollers can be increased from 2.5 to 5.0m, and the spacing between the lower support rollers can be increased from 5 to 10m. However, this design should have sufficient dynamic analysis as a basis to ensure that the conveyor operates reliably.

 

2 Reasonably determine the safety coefficient of the conveyor belt The safety factor of the conveyor belt has a great influence on the economical efficiency and reliability of the belt conveyor and it is also the focus of many scholars. The current standards are based on the rated breaking strength of conveyor belts, taking into account factors such as the significant decrease in fatigue strength, the decrease in strength due to bending and elongation, the loss of joint strength, and the increase in dynamic tension under braking conditions. The safety factor. For example, the DIN 22101-1982 standard suggests that the steel cord conveyor belt has a dynamic safety factor of 4.8 to 6.0 and a steady-state safety factor of 6.7 to 9.5. In fact, this safety factor representation based on the rated breaking strength of conveyor belts is not intuitive and is conceptually misleading. The actual project requires the fatigue strength of the conveyor belt to have an appropriate safety factor based on the maximum tension of the working conditions. A study 20 years ago believed that the fatigue strength of a steel cord conveyor belt after pulsation cycles of 10,000 times was 36% of its rated breaking strength. Based on this, the standard gave the above safety factor values.

 

In the past ten years, foreign studies on the fatigue strength of conveyor belts have shown that by improving the manufacturing process and jointing process of steel cord conveyor belts, the fatigue strength of steel cord conveyor belts below St 6000 is increased by 45% to 55%. . In this way, the dynamic safety factor recommended in the DIN standard can be reduced to 3.8 to 4.8 and the steady state safety factor to 5.4 to 7.6. The DIN 22101-1998 (Draft) standard introduces the concept of conveyor belt fatigue strength. Based on this, it proposes a conveyor belt safety factor So related to joints and a belt safety factor S1 related to the service life and operating conditions. Conveyor belt fatigue strength safety factor: S = S0Sl then KN, min = Kt/Kt, rel = KK, maxS/Kt, rel where KN, min - minimum rated breaking strength of conveyor belt Kt - - conveyor with safety factor With fatigue strength Kt,rel - the ratio of the conveyor belt fatigue strength and rated breaking strength, generally take 0.45 ~ 0.55 Kk, max - the maximum edge tension of the conveyor belt minimum safety factor: Smin = (S0Sl) min = 1.0 × 1.5 = 1.5 Maximum safety factor: Smax = (S0S1) min = 1.2 × 1.9 = 2.28 When Kt, rel = 0.45, KN, min = KN, min × (3.33 - 5.1) When Kt, rel = 0.55, KN, min =KN,min×(2.72～4.15) The maximum tension of the conveyor belt usually occurs under braking conditions. The soft-start braking device can effectively relieve the effect of dynamic tension. The dynamic tension can be calculated more accurately by dynamic analysis, or it can be roughly estimated by multiplying the steady-state maximum tension by the activation coefficient Ka. When the soft-start braking device is used, the starting factor Ka may be 1.1 to 1.3.

 

3 Use reasonable controlled braking or soft braking device to reduce the power effect According to the current standard, the braking acceleration of the belt conveyor should be 0.1~0.3m/s2. The actual project shows that this value is not suitable for long-distance, complicated belt conveyors. Through dynamic analysis, it can be known that the long-distance, complex-lined belt conveyors preferably use a drive device with a controllable braking function to control the conveyor to start and brake in accordance with the desired starting and braking speed curves in order to reduce Conveyor belts and load-bearing parts dynamic load; For ordinary long-distance belt conveyors, soft-start brake drive can be used.

 

3.1 Ideal controlled start-up speed curve The ideal start-up speed curve should enable the belt conveyor to start smoothly, and the maximum acceleration during the entire start-up process is small, there is no abrupt change in acceleration to minimize the starting inertia force and Start impact. There are two kinds of ideal controllable starting speed curves for practical engineering applications.

 

(1) The start-up speed curve proposed by Australia expert Harrison (see Figure 1): v/(t)=v/(1—cosπt/2) 0≤t≤T Where: v—design belt speed T—start At the beginning of time start, the acceleration is 0 and the speed increases steadily; at T/2, the acceleration reaches the maximum and the speed reaches v/2; then, the acceleration gradually decreases symmetrically and the speed continues to increase; when the design speed is reached, the acceleration decreases. To 0, complete the start process. With the exception of the starting point and the ending point, the first derivative of the acceleration curve is continuous.

 

(2) The starting speed curve proposed by American expert Nordell (see Figure 2): At the start of the start, the acceleration is 0, the speed increases steadily; by T/2, the acceleration linearly increases to the maximum value, which is higher than the acceleration in Figure 1. The value is 27% larger and the speed reaches v/2; then, the acceleration gradually decreases symmetrically and the speed continues to increase; when the design belt speed is reached, the acceleration is reduced to 0 and the starting process is completed. The first derivative of the acceleration is discontinuous at times 0, T/2, T, but the peak value of the acceleration derivative is only 81% of that of Fig. 1. The above two kinds of starting control methods can achieve ideal starting effect. Since the conveyor belt is in a relaxed state before the conveyor is started, in order to avoid the impact of the conveyor belt, the conveyor belt is started after the belt is stretched, and the peak tension of the starting can be further improved. Therefore, it is necessary to add a time delay section at the start of the start. As shown in FIG. 3, the speed of the delay section is generally taken as 10% of the design belt speed. Starting time T is a very important design parameter. According to the design experience, the starting time can be preliminarily determined by controlling the maximum starting acceleration or average acceleration, and then optimized according to the dynamic analysis results. Under normal circumstances, the starting acceleration of the ultra-long-distance belt conveyor is not more than 0.05m/s2, and the starting acceleration of the medium and long-distance belt conveyor is not more than 0.1 m/s2. In order to avoid resonance and other dynamic phenomena during the start-up process of the conveyor, the starting time should meet the following conditions: T≥5L/Vw The starting time is greater than the time required for the longitudinal stress wave of the lower branch conveyor to travel from the nose to the tail 5 times. In the formula: L - the total length of the conveyor, m Vw - the longitudinal stress wave transmission speed of the conveyor belt, m/s E - the elastic modulus of the conveyor belt, N/mm B - bandwidth, mm qB - conveyor belt per unit length Quality,kg/m qRu——Roller mass of the idler of the lower branch unit, kg/m At present, the drive which has many applications in engineering and has controllable braking function includes AC variable frequency drive and CST. Controllable brake drive.

 

3.2 AC frequency conversion speed-regulating drive device The AC motor frequency conversion speed regulation has the characteristics of wide speed range, high precision, easy realization of the automatic tracking of the braking speed curve, and can provide ideal controllable braking performance. The starting coefficient can be controlled from 1.05 to 1.1, and the starting acceleration can be controlled from 0 to 0.05m/s2. It is suitable for long-distance, complicated belt conveyors and can control the conveyor to start with the set "S"-shaped speed curve. And braking to meet the dynamic stability and reliability requirements of the whole machine. The variable frequency speed control drive can also provide low speed tape speed. Since the frequency conversion speed regulation needs to solve a series of problems in the electrical industry, the cost is relatively high and the application is limited to a certain extent.

 

3.3 CST controllable brake drive device CST controllable brake device is a special controllable brake device developed by Dodge Corporation. From the structural point of view, CST is a fixed-axis plus planetary geared reducer with a liquid-viscous clutch at the output stage. The liquid-viscous clutch is connected to the inner ring gear of the planetary transmission, enabling the CST to have a differential output torque and Output speed function. CST controllable braking device is an ideal drive device for long-distance, large-capacity, and complicated-line belt conveyors. It has an automatic tracking control function for setting the braking speed curve, overload protection function, multi-machine balancing function and low speed. Test belt function. Starting coefficient can be controlled at 1.05 ~ 1.1, starting acceleration can be controlled at 0 ~ 0.05m/s2, control accuracy is 2%. The disadvantage of the CST controllable braking device is that it increases the maintenance work of the hydraulic system; for the inclined belt conveyor, a relatively large low-speed shaft brake and backstop must be provided.

 

3.4 Soft-starting drive for squirrel-cage motors plus speed-regulating fluid couplings The oil-filling capacity of variable-speed hydraulic couplings is adjustable. After the motor is started with no load, the coupling can increase the oil filling amount stably, and output the constant torque acceleration characteristics, so that the belt conveyor can be started smoothly under the set starting torque, and the starting coefficient can reach 1.1-1.3. Squirrel-cage motor plus speed-regulating fluid coupling drive method is an ideal soft-start device, often used for open-loop control, etc. to accelerate the brake, multi-machine drive is easy to adjust the power balance, suitable for large and medium-sized and line Simple long distance belt conveyor. The disadvantage is the large volume, additional oil cooling device is needed, and the area is large.

 

3.5 Soft-starting drive device for the series resistance of the rotor circuit of the winding motor The winding-type motor can soften the output characteristics of the motor by connecting the resistor in series with the rotor circuit. In the starting process, by switching the resistance, both the set starting torque and the starting current can be limited. The drive method of series resistance of the rotor circuit of the winding motor usually adopts open-loop control. Through the method of “binary” switching resistance, more starting acceleration stages can be obtained under the limited resistance series, so that belt conveyors, etc. Accelerated, more stable start. By adopting the driving method of series resistance of the rotor circuit of the winding motor, the no-load, full-load starting characteristics and full-load braking characteristics of the belt conveyor can be conveniently set respectively, and an ideal braking effect can be obtained. This type of drive is suitable for belt conveyors for large, multi-motor drive systems. The disadvantage is that it is difficult to perform explosion-proof treatment on the winding motor and resistor, and it is not suitable for use in underground coal mines.

 

4 Optimizing the design of large belt conveyors using dynamic analysis methods The current standards for the calculation of the dynamics during start-up and braking of belt conveyors are based on a rigid body dynamics method using the conveyor belt as a rigid body. Recent decades of research and engineering practice have shown that the results of rigid body dynamics analysis can only meet the requirements for engineering design accuracy of short-distance, small-capacity belt conveyors. For long-distance, large-capacity, and complicatedly arranged belt conveyors, the dynamic characteristics of the belt conveyor are more complex and important. The rigid body dynamics method is used for analysis, and its accuracy can no longer meet the needs of practical projects. Therefore, for large belt conveyors, more accurate dynamic analysis methods must be used. At present, the viscoelastic dynamic method of conveyor belts is widely used in the world to analyze the dynamic state of large belt conveyors. The dynamic analysis of the so-called belt conveyor is based on the mechanical properties of the conveyor belt according to the viscoelastic body, comprehensively calculating the braking characteristics of the human-driven device, the mass distribution of each moving body, the gradient of each section of the line, and various movement resistances. , the initial tension of the conveyor belt, the deflection of the conveyor belt, the form and position of the tensioning device, and the tensioning force, establish the mathematical model of the conveyor dynamics, and obtain the conveyor belt during the starting and braking process. The changes in speed, acceleration and tension that occur at different points over time. The dynamic hazards and unsafeness of conveyors that are designed according to the traditional static design method are forecasted. Improvements and adjustment measures are proposed for the design, and the optimized design and control parameters are determined. Using dynamic analysis, it is possible to find out the dynamics that may occur during start-up and braking of large belt conveyors.