Conveyor Belt Design
CDI developed extensive testing data, project experience and analysis capability over 30 years. Our areas of expertise in conveyor belt analysis and testing are listed below. These technologies are used in conveyor design to achieve optimal conveyor engineering. They are also in conveyor field services to find root cause of belt failure or conveyor failure, and to assist engineering for conveyor upgrade.
Belt Splice Testing and Analysis
CDI does splice design, analysis, testing and inspection. During splice inspection, the rubber cover is removed carefully to expose the underlying steel cords. The straightness and layout of steel cords are checked against the splice specification. We study if there are voids in the rubber between steel cords, if there is rubber losing adhesion on steel cords and try to determine the locations of the starting point of the failure and its path of propagation.
We can test splice samples to measure the static strength and dynamic fatigue strength on a tensile test machine. Splice of both fabric belt and steel cord belt can be tested.
|Splice Strength Test on a Tensile Test Machine|
Dynamic splice fatigue test can verify the splice design and splice material of high strength conveyor belt above ST-4000. A belt loop is made and spliced in the factory for the test. The test machine has two pulleys placed on a big steel frame. One pulley is driven by an electric motor to rotate the belt; the other pulley is connected to two parallel hydraulic cylinders to move back and forth to create tension load cycles in the belt loop. In 1997, CDI designed and built a dynamic splice fatigue machine that can test ST-10000 splice. In 2017, CDI designed a new dynamic splice fatigue machine that can test ST-15000 splice, maximum 12m distance between the center of two pulleys. The project is a collaboration with Double Arrow Rubber Co., Ltd., where the test machine is also housed. The test methodology conforms to DIN 22110-3 standard.
|New Conveyor Belt Splice Dynamic Fatigue Testing Machine commissioned in 2017|
CDI developed a finite element analysis program to analyze the rubber shear stress in the splice. In the figure below, the red and blue areas (positive and negative values) show the rubber shear stress in two different shearing directions between steel cords (white columns), in a 3-step splice. This program is used to optimize splice designs by changing rubber gap, step length and splice pattern so that the shear stress is minimized. It is also used in belt splice failure analysis to investigate the shear stress level.
|Splice FEA Analysis|
Rubber Rheology Testing
Where does the energy loss in running conveyors come from? Research and field measurement have shown that the Indentation Rolling Resistance (IRR) between belt and idler rolls can account for ~60% of the total energy loss (excluding gravity effect). Reducing the IRR is an effective way to reduce conveyor power consumption and the belt tension. The IRR is due to the hysteresis energy loss of the viscoelastic deformation in the belt bottom cover, which is closely related to belt bottom cover properties. The Low Rolling Resistance belt has a modified bottom cover rubber with less hysteresis energy loss, which can reduce conveyor power demand by 10%~40% compared to the conventional belt. The LRR conveyor belt is also named as Energy Optimized Belt or Energy Saving Belt by different belt manufacturers.
CDI is a pioneer in utilizing the LRR belt to optimize the conveyor design with reduced capital and operating cost. We first applied this technology in 1989 on the 20km Channar overland conveyor in Australia, and our latest applications are 27km Impumelelo overland trough conveyor in S.Africa and 15km Yubei pipe conveyor in China.
A key question is the measurement of a particular rubber’s IRR property. CDI uses the Rheometrics’s RSA3 Dynamic Mechanical Analyzer (DMA) to measure the viscoelastic properties E’ (elastic modulus) and E” (loss modulus) of a rubber sample over wide range of temperature, strain and frequency. The data acquired from DMA testing is processed by a special program to generate master curves and then incorporated into Beltstat software for conveyor calculations. This is our proprietary program. Conveyor Dynamics, Inc. (CDI) has been developing this technology, applying it to conveyor design, and accumulating verification data since the 1989.
|Conveyor Belt Rubber Rheology Test for Indentation Rolling Resistance Measurement|
Beltstat software provides a lot detailed analysis on trough belt for conveyor engineering. It includes analysis on vertical curves, horizontal curves, flat turnover, belt transitions, belt flap (vibration), and idler junction pressure index. The idler junction pressure index calculation is also a proprietary technology. In addition, we developed special finite element models to analyze unique cases and gain deeper insight of the trough belt behavior.
The figure below shows the FEA of a trough belt. The idler junction pressure is referring to the extent of belt ending between center roll and wing roll. If the bending is too severe, the bottom cover rubber can fail.
|Strain in Trough Conveyor from Finite Element Analysis|
|Stress in Trough Conveyor Cross Section from Finite Element Analysis|
The FEA can also simulate the trough belt behavior at horizontal curve, especially when there is special idler arrangement (5 roll or deep trough), small horizontal curve radius or specialty belt.
|Conveyor Belt in Horizontal Curves from Finite Element Analysis|
Pipe conveyor belt has a much greater influence on the conveyor system performance than trough belt does. The main factor is the pipe belt stiffness. As the belt is rolled into the pipe shape from the flat shape, the intrinsic bending stiffness of the belt turns into the contact pressure on surrounding idler rolls. Higher belt stiffness will result in higher contact pressure. This gives the pipe belt more stability and better resistance to twist and collapse during horizontal and vertical curves. But the penalty is increased power consumption, increased belt tension and the necessity for stronger belt and larger drive size. From the design point of view, there is a balance in the belt stiffness to achieve for each individual project. If the conveyor routing is straight, there is no need to design a pipe belt with high belt stiffness. Lower power consumption and capital cost can be achieved. If the conveyor routing has a lot of curves with small radii, the belt stiffness should be increased accordingly to accommodate the bending effect by curves, so that the belt does not have excessive rotation, twist and collapse.
CDI uses Three Point Bending Test to measure the pipe belt stiffness, and calibrate the material properties of the pipe belt finite element model. The three point bending is a well defined bending test. Compared to the six point stiffness test, it is much easier to make test samples and generate design database. Once good numerical and experimental correlations between the three point bending stiffness and the six point pipe belt stiffness are established, there is no need to make full width belt samples and go through a trial-and-error process for each design before the actual belt production.
|Three Point Bending Test and FEA Modeling of the Three Point Bending Test|
Unlike other published FEA of pipe belts, where simplified shell or solid elements are used to represent both rubber, steel cord and fabric materials, CDI uses a full 3-D pipe belt model where the rubber, steel cords and fabric layers are modeled individually with their own material properties. The overall bending behavior of the pipe belt in FEA is compared and calibrated with three point bending and six point stiffness testing results. From the finite element mode, contact pressure on 6 idler rolls can be obtained. Combining rubber rheology test results and contact pressure, the pipe belt’s IRR can be calculated for each belt and conveyor. This is a more fundamental approach than empirical methods based on predefined DIN friction factor. It allows pipe conveyor optimization based on LRR and specialty belts like the heat resistant EPDM pipe belts.
|Full 3D Finite Element Pipe Belt Model|
CDI designed a full scale pipe belt testing machine. The test machine is 11.4m long, can accommodate pipe diameter from 200mm to 800mm. It has hydraulic cylinder at one end to apply tension to the test belt. The idler panels can adjust position to simulate horizontal and vertical curves. There is a panel where load cells are attached to 6 idler rolls to measure the contact forces. The project is a collaboration with Double Arrow Rubber Co., Ltd., where the test machine is housed.
|Full Scale Pipe Belt Testing Machine|
Broken Steel Cords
The problem of broken steel cords in conveyor belt is complicated. There are many variables to consider, for example, the number of broken cords, the location of broken cords, different trough angles, belt transition and trough, etc. The conventional rule-of-thumb about broken steel cords in conveyor belt, is that if there are less than 5% cords broken in the belt edge and 10% cords broken in the belt center, the belt can be repaired without re-splicing. If the number of broken cords exceeds this limit, the belt needs to be re-spliced. Typical repair fills the belt puncture region with rubber. Sections of steel cords may also be inserted into the puncture region. However, the repair doesn’t restore the loss of tension carrying capability from broken steel cords in the belt. It just prevents material spillage through the punctured region. CDI developed a program to study the broken cords in conveyor belts. It is a tool to advise conveyor operators whether to repair, re-splice, or run the conveyor at lower capacity to maintain the belt safety factor. It is also used to do failure analysis where broken cords in conveyor belt lead to sudden belt breakage.
|Analysis of Broken Steel Cords in Conveyor Belt|