Belt cleaners, Belts, Bulk Engineering, Technical articles

Optimising scrapers for improved belt performance

Dr Jayne O’Shea, from TUNRA Bulk Solids, discusses conveyor carry-back and testing techniques that can be utilised to evaluate belt scraper configurations and pressures to optimise scraper performance.

Dr Jayne O’Shea, from TUNRA Bulk Solids, discusses conveyor carry-back and testing techniques that can be utilised to evaluate belt scraper configurations and pressures to optimise scraper performance.

Conveyor belts face significant challenges in industries such as mining due to carryback issues. Carryback occurs when material adheres to the belt and is transported back along the return side, leading to problems like premature wear of the belt, idlers and pulleys, decreased efficiency, spillage, and reduced product quality. Additionally, frequent belt cleaning and maintenance are necessary, resulting in unplanned downtime and additional costs. To address these concerns, many companies use belt scraper systems to remove carryback. However, the effectiveness of these systems can vary, making it crucial to find the optimal scraper pressure for installation, striking a balance between carryback removal and minimising belt wear. 

Belt scraper systems typically include primary scraper blades mounted on the discharge pulley and secondary scraper blades positioned under the return side of the conveyor belt. The main purpose of these scrapers is to remove carryback, improving conveyor performance and reducing maintenance requirements. However, the effectiveness of scraper systems depends on various factors such as design, blade material, installation position, operating conditions, and the angle of attack of the scraper blades.

Research has highlighted that finer particles tend to move toward the belt surface, and moisture content affects the adhesion and cohesion characteristics of bulk material [1, 2, 3].  The surface roughness of the belt also influences the extent of carryback. 

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To optimise scraper performance and evaluate their effectiveness, testing protocols can be employed where different scraper configurations are assessed under simulated operating conditions. These tests enable engineers to compare the performance of various scraper designs by measuring their efficacy in removing carryback. The test facility located at TUNRA Bulk Solids, The University of Newcastle, can be used for belt scraper performance testing.  

This test facility includes a recirculating system with a discharge hopper that evenly distributes bulk solids across the width of the conveyor belt’s top cover. The primary and secondary cleaners scrape the bulk material off the belt, which is collected in a lower storage hopper. The carryback adhered to the belt surface after passing through the cleaners is measured using the TUNRA carryback measurement system. The amount of carryback is quantified and presented as a value per area of the belt or per unit of time [6, 5].

Testing was conducted as part of a recent project to compare the effectiveness of four different belt scrapers in removing iron ore carryback, utilising the test facility. Figure 1 presents a summary of results averaged over three tests, with the scraper tensions set at eight kg and 15 kg. The results are expressed in kg per hour.

Figure 1 Particle and surface interaction: a) interlocking due to overlap, b) no overlap [4]
Figure 1 Particle and surface interaction: a) interlocking due to overlap, b) no overlap [4]

When installing scraper blades, two important factors to consider are the angle of attack and pressure. The angle of attack refers to the angle at which the scraper blades make contact with the belt surface, and two common approaches to scraper blade configuration are peeling and scraping [1]. In the peeling configuration, the scraper blade initiates contact with the belt at a shallow angle, gradually peeling off the carryback material. This approach is effective for removing loosely adhered material and minimising belt wear. Conversely, in the scraping configuration, the scraper blade directly contacts the belt at a steeper angle, exerting more force to scrape off stubborn or tightly adhered material. However, this approach may increase belt wear due to higher friction and scraping forces. To determine the optimal angle of attack for scraper blades, it is essential to consider the material characteristics, belt properties and operating conditions. Peeling angles are often preferred when dealing with loosely adhered carryback and minimising belt wear is a priority, while scraping angles are suitable for applications involving stubborn or tightly adhered carryback, but they may result in increased belt wear.

Scraper pressure plays a critical role in effective carryback removal. Insufficient pressure can result in incomplete removal, reduced conveyor performance, and increased material spillage. Conversely, excessive pressure can lead to excessive belt wear, causing premature failure and downtime.

Achieving the optimal scraper pressure involves striking a balance between carryback removal and belt wear. While a universal solution does not exist due to variations in operating conditions, belt characteristics, and material properties, testing offers valuable insights for determining an optimal pressure range. By systematically adjusting scraper pressures in controlled test environments and evaluating the results, engineers can identify the pressure that maximises carryback removal while minimising belt wear. 

To illustrate this, the results from carryback performance testing performed at the University of Newcastle, with conditions described in [6], are shown in Figure 2. The results showed that the optimal pressure range for minimising carryback for the tested arrangement was between 60 and 80 kPa. At lower pressures, carryback levels are high due to a larger effective cross-sectional area between the belt and scraper blade profiles caused by minimal indentation of the blade into the belt’s top cover. As pressure increases, belt deformation reduces the mean surface roughness, resulting in a decrease in the effective cross-sectional area and a reduction in carryback. When the pressure applied to the blade exceeds approximately 80 kPa, the amount of carryback gradually increases. The higher force applied to the belt scraper blade causes the leading edge of the contact surface to lift off the belt as the blade pivots, thereby enlarging the effective cross-sectional area between the belt and the blade. These findings emphasise the importance of selecting the appropriate pressure range to effectively manage carryback and optimise conveyor belt performance.

Conclusion

Carryback presents persistent challenges for industries relying on conveyor systems, impacting operational efficiency, maintenance costs, and the longevity of belts and belt components. Fortunately, scraper systems offer a viable solution to mitigate carryback-related problems. However, the effectiveness of scrapers depends on careful evaluation of scraper configurations and pressures. By striking the right balance between carryback removal and belt wear, companies can optimise their conveyor systems, improve operational efficiency, and achieve sustainable reductions in maintenance costs.

Findings from carryback performance testing can provide crucial insights into the factors influencing belt cleaning efficiency. Scraper selection, tension adjustment, and maintenance all play pivotal roles in effectively mitigating carryback. Through thorough evaluation of scraper configurations, optimisation of pressure, and implementation of proper maintenance practices, companies can successfully address carryback issues, enhance operational efficiency, and extend the lifespan of their conveyor belt components. 

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