Over the past few decades, successive generations have painstakingly explored every aspect of belt scales. They have continuously refined component designs, enhanced the quality of load cells, weighing instruments, and speed sensors, improved the rigidity and structure of weigh bridges, optimized installation conditions, and fine-tuned components that influence performance.
In 2011, at the International Organization of Legal Metrology (OIML) conference, Nanjing San'ai and Jiangsu Saimo presented their high-accuracy belt scale products—capable of consistently achieving Class 0.2 accuracy [1]—prompting the international body to add this accuracy class to the R50 International Recommendation, "Continuous Totalizing Automatic Weighing Instruments (Belt Weighers)." Other manufacturers subsequently adopted similar structural features to develop and produce their own Class 0.2 belt scales.
A defining characteristic of these high-accuracy models is their "length": they utilize eight weighing units, extending the total weighing length to over 15 meters (assuming a 1-meter spacing between weighing idlers). This increases the sampling time as material passes over the scale—though this is only one factor—thereby improving measurement accuracy and stability, albeit at the cost of significantly higher manufacturing expenses.
Should this same approach be applied to the vast number of standard-accuracy belt scales? Is this the only way to enhance weighing stability? Here, we briefly analyze the forces acting on belt scale idlers to explore a method for relatively improving stability without requiring substantial investment. 1
Error Analysis
Belt scales are installed on belt conveyors. When material passes uniformly along the belt, the weight on the belt is converted into data via load cells and displacement sensors. If we examine the force state of the first weighing idler in the direction of travel (as shown in Figure 2)—assuming the belt material remains constant—we observe that greater material loads result in increased belt deformation. Consequently, the weighing idler is subjected not only to the gravitational force of the material but also to belt tension and lateral forces exerted by the material. Both belt tension and lateral material forces are closely linked to the belt itself; as material passes over the weighing idler, it generates an impact force. If material supply is non-uniform—resulting in discrete segments passing over the scale—the component of this impact force acting along the direction of belt travel increases approximately in proportion to the square of the belt speed; the greater the belt deflection, the more severe the impact. The vertical component of this impact force superimposes onto the material's weight, significantly affecting weighing accuracy, while the horizontal component compromises the stability of the weighing platform. Generally, reducing belt speed or shortening the spacing between idlers can substantially mitigate the impact of both vertical and horizontal force components, thereby reducing platform vibration and enhancing system stability. 2
Modification Plan
(1) For conveyors utilizing lightweight or standard belts (such as those with canvas cores and narrower widths) and featuring smaller idler trough angles, system stability can be improved by increasing idler density and reducing the spacing between carrying idlers and weighing idlers. The material on the belt between the weigh idler and the nearest conveyor idler is supported jointly by the weigh idler and that nearest conveyor idler; effectively, the material flow is supported such that each of these two idlers bears half of the material load on that section of the belt. The weighing length is defined as the distance between two imaginary lines located halfway between the axis of the weigh idler and the axis of the nearest conveyor idler at each end of the belt scale's weighbridge. If the idler spacing is reduced from L to L/2, the impact force exerted by the material on the scale structure in the direction of belt travel is significantly diminished. Although the longitudinal restraints of the weighbridge theoretically counteract impact forces in the direction of travel, practical factors regarding design and installation make it impossible to completely eliminate the influence of horizontal impact forces in that direction. However, the reduction in the material's impact force also decreases the resulting vertical force component, thereby mitigating a factor that affects weighing accuracy; consequently, the weighing length of the belt scale becomes 2.5L rather than 3L. By making a minor modification to the existing belt scale—specifically, shifting the conveyor idlers at both ends slightly inward to reduce the spacing relative to the weigh idler—we can at least diminish the vertical and horizontal impact of the material on the weighbridge. This reduces the magnitude of a key influencing factor and yields a modest improvement in the belt scale's stability. Example: Consider the modification of a three-idler electronic belt scale. The maximum flow rate is $Q_{max} = 1440\text{ t/h} = 400\text{ kg/s}$, the maximum belt speed is $V_{max} = 2\text{ m/s}$, and the initial spacing between weighing idlers is $1.2\text{ m}$. After modification, the spacing between the conveyor idlers and the weighing idlers becomes $0.6\text{ m}$, resulting in a weighing length of $WL = 3.0\text{ m}$. The formula for calculating the maximum weighing capacity is: $\text{Max} = WL \times Q_{max} / V_{max} = 3.0\text{ m} \times 400\text{ kg/s} \div 2\text{ m/s} = 600\text{ kg}$. When modifying an existing belt scale, if an additional set of weighing idlers is installed within the existing spacing, the number of idlers increases, thereby increasing the scale's tare weight; consequently, it is necessary to verify whether the load cell capacity needs to be increased. Since the weighing section length changes, the formulas for calculating calibration parameters must be adjusted accordingly, and the calculation method for the maximum weighing capacity must also be revised.
(2) Belt tension affects the forces acting on both the end weighing idlers and the intermediate weighing idlers. Depending on the stiffness of the belt, reducing the spacing between weighing idlers—thereby increasing the total number of idlers—can help minimize the impact of the vertical force component exerted by the material on the scale structure and reduce the shock caused by horizontal force components. Naturally, such modifications will lead to a slight increase in the belt scale's cost (as shown in Figure 4). However, for the user, the resulting improvement in the scale's stability justifies this additional cost.