Bulk Engineering, Technical articles, White Papers

Enhancing energy efficiency in belt conveyors through large diameter rollers

TUNRA Bulk Solids, in collaboration with Big Roller and ContiTech Australia, aims to evaluate the influence of larger diameter idler rollers on energy savings for overland belt conveying systems.

Belt conveying systems are effective solutions for transporting bulk materials over long distances. The indentation rolling resistance in belt conveying systems, caused by the viscoelastic contact between the conveyor belt and an idler roll, can account for up to 60 per cent of total drive power for overland conveying systems. Typically, the indentation rolling resistance is known to be a function of idler roll diameter, belt loading profile, belt cover thickness, cord diameter and pitch for steel cord belts and the viscoelastic material properties of the bottom belt cover, which are also dependent on the operating conditions of the system including temperature, belt speed and belt load.

This article explores the relationship between indentation rolling resistance and idler diameter. A larger idler diameter results in a larger contact area between the conveyor belt and idler. This in turn allows for a more evenly distributed pressure between the belt and the idler, reducing the rate of indentation, and the localised pressure, and hence reducing the indentation rolling resistance. 

There are several additional benefits to increasing idler diameter, in that the viscous resistance from the grease in the bearings and labyrinth seals reduces by lowering the angular velocity of the idler, and the corresponding drag force reduces due to the belt acting at a larger radius to oppose the viscous torque.

The advantages of larger-diameter idlers have been known for some time; however, practical implementation for roller sizes greater than 219mm has been constrained primarily by a lack of quantifiable information regarding their benefits, potential challenges associated with the manual handling and maintenance of larger components and supply chain manufacturing limitations. In existing systems, especially those prioritising efficiency improvements, it is common practice to substitute the central idler in a three idler set with a larger diameter, typically around 219 mm. The central idler bears approximately 50-70 per cent of the total idler load, therefore simple economic advantages can be gained by increasing the diameter of the central idler roll only. 

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Taking this concept to more substantial dimensions, this article presents a study investigating the impact of idler diameter on indentation rolling resistance for idlers with diameters up to 400mm. Big Roller engaged TUNRA Bulk Solids to conduct indentation rolling resistance measurements with idler roll diameters of 152.4mm, 219mm, 316mm, and 400mm, to quantify the impact of larger idler rolls.

Testing was conducted on the large indentation rolling resistance test facility operated by TUNRA Bulk Solids at The University of Newcastle, Australia using an ST1000 belt supplied by ContiTech Australia Pty Ltd, manufactured with Continental Eco Plus cover compounds. Pictures of the test facility set up with the 152.4 mm and 400 mm test idler rollers are shown in Figure 1. 

Figure 1: Test facility set up with the 152.4mm diameter test idler
Figure 1: Test facility set up with the 152.4mm diameter test idler.
Figure 1: Test facility set up with the 400mm diameter test idler.
Figure 1: Test facility set up with the 400mm diameter test idler.

In addition to the experimental data, the QC-N analytical model was utilised to predict the indentation rolling resistance of a generic low rolling resistance compound for a range of idler diameters.

The percentage decrease in indentation rolling resistance compared to an idler roller diameter of 152.4mm is shown in Figure 2 for belt loads of 5 kN/m and 8 kN/m, for the idler roll diameters tested. Included in the comparison is the relative decrease predicted from the QC-N model at 20 degrees, with a belt speed of 5 m/s. These results show an improvement in indentation rolling resistance performance of over 45 per cent when comparing a 400mm diameter roller to a 152.4mm roller for this system, representing a saving in energy. Extrapolating this data based on the QC-N model would yield a 55 per cent reduction for a 508mm idler roll.

Figure 2: Decrease in indentation rolling resistance (IRR) compared to 152.4 mm diameter idler roller [4].

To illustrate the influence of idler diameter on overland conveyor design, a hypothetical case study is presented with parameters detailed in Table 1. To allow a direct correlation, a flat system is considered to remove lift forces.

Belt Velocity 5 m/s
Belt Width 1.5 m
Belt Mass 81.0 kg/m
Material Copper
Throughput 6,000 t/h
Conveying Length 10,000 m
Carry side idler configuration 3-idler trough @ 2.0 m spacings
Return side idler spacing 2-idler vee @ 4.0 m spacings
Topography Flat

Table 1: Case Study Parameters

The results of this case study, depicted in Figure 2, benchmark the performance of larger rolls against a conventional 152.4mm idler. The case study presents two scenarios; the relative improvement attained from replacing both carry and return idlers with larger diameter rolls, and the case where only the carry side idlers are replaced while the return idler rolls remain at 152.4mm. Extrapolations predict the improvement based on a 508mm diameter idler roll, considering it as an approximation due to the absence of direct testing on this size idler. 

Figure 3 highlights that using a 400mm idler roll predicts a 30 per cent and 36 per cent improvement in total drive power when replacing carry side or all idlers, respectively, compared to a 152.4mm idler. 

Figure 3: Case study – Relative improvement in power consumption
Figure 3: Case study – Relative improvement in power consumption.

This aligns with Hager’s research, estimating that IRR accounts for about 60 per cent of the total drive power for a flat, one km conveying system. Extrapolating to a 508mm idler predicts a further 3 per cent improvement in drive power, with reductions of 33 per cent and 39 per cent, respectively.

Figure 4: Big Roller ground (low and mid) level module design dimensions.

The comparison assumes ‘all other factors’ are equal, maintaining constant rim drag, flexure, and secondary resistances between scenarios. Flexure resistance should be relatively constant for a given sag ratio of the belt, while secondary resistances follow ISO5048 guidelines, particularly for systems longer than 1.5km. Rim drag may vary with idler diameter due to bearing and seal design differences, however should reduce with bearing and seal designs consistent across larger diameters.

Larger idlers also allow for design changes, notably in idler spacing. Increased idler diameter reduces contact stresses on the belt’s bottom cover, enabling larger idler spacings and hence a reduction in the total number of idler sets needed, leading to further energy savings. However, the impact on the dynamic response of the belt due to the span increase requires further consideration.

Trade-offs arise between increasing roll diameters and spans to achieve optimal efficiency versus adhering to material strengths and weights for roller components. Considerations such as shell wall thickness, abrasion design allowances, lubrication characteristics and component design life expectations also play a crucial role in determining practical limits for roller diameters. The practicality and cost-effectiveness of employing larger-diameter idler rollers may be more suitable for specific applications and need to be optimised.

In summary, the use of larger diameter idler rollers can lead to significant energy savings, particularly in long overland conveyors. 

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