Maintenance-free belt scales

Depending on the application and boundary conditions, accuracies of 99% and better are possible in practice, especially with calibratable belt scales. Typical error limits are +/- 1 to 2%. 

When planning a belt scale, the boundary conditions should be coordinated with us in order to achieve the intended purpose of the belt scale. 

Accuracy is generally positively influenced by the following factors:

• a high belt load through the clever selection of the conveyor speed,
• the choice of a conveyor belt that is as new and thin as possible without stiffeners,
• the use of gravimetric tensioning stations or other devices for uniform belt tension,
• avoiding external influences such as fluctuating belt tension or misalignment,
• low troughing of the conveyor belt

Read here how you can improve the accuracy of belt scales in operation:

https://hense-waegetechnik.de/blog/chancen-bessere-genauigkeit-bandwaage/#more-1635

From a technical point of view, calibrated belt scales hardly differ from "normal" belt scales. However, calibratable components are used and the scale must undergo a conformity assessment by a notified body. Furthermore, after installation, the belt scale must undergo several test/reference measurements at different conveyor capacities. The deviation must be within the specified limits of the respective accuracy class.

Belt scales are available in different accuracy classes. The more valuable the product being transported on the scale, the higher the calibration class and thus the measurement accuracy that must be maintained. For calibratable belt scales, a distinction is made between accuracy classes 0.5, 1, and 2. During calibration, the scale may deviate by a maximum of +/- 0.25% (accuracy class 0.5), +/- 0.5% (accuracy class I) or +/- 1.0% (accuracy class 2) from the actual value at different conveyor speeds. The subsequent traffic error limit is twice the error of the initial acceptance, for example, max. +/- 1% in accuracy class 1.

In order to obtain a calibratable measuring system, it is not only the belt scale that must meet certain criteria. Specific requirements are also placed on the conveyor belt (e.g., a gravimetric belt tensioning station, etc.), which must be met for the respective accuracy class.

Belt scales can be used for mass flows starting at 1 t/h. There are practically no upper limits. For large belts, the weighing frame must be reinforced accordingly to bear the load of the belt, the roller stations, and the conveyed material. This allows mass flows greater than 10,000 t/h to be measured.

Typical applications include balancing material flows, loading trucks, railcars, or ships, performance control (output or input), process control, dosing, precise continuous mixing processes, or simply visualization for plant drivers and operators.

After installing and initially calibrating the scale, the zero point of the scale should be adjusted regularly. This can be done automatically or by pressing a button on the electronics. During zero point adjustment, the conveyor belt runs without product. Since the belt ages and changes during operation or due to sunlight, adjusting the zero point is an important factor in maintaining measurement accuracy. 

Calibration by means of comparative measurement is recommended at reasonable intervals for critical applications or for quality purposes. After changes to the conveyor belt system, e.g., replacement of the belt, a new zero point calibration and reference measurement should be performed.

If a reference measurement with the actual conveyed material is not possible, calibration with chains is a possible alternative for adjusting the belt scale.

The actual weighing frame is designed as a maintenance-free system with no moving parts.

Only the measuring wheel with the non-contact proximity sensor rotates and is therefore subject to very little wear over the long term. 

An occasional visual inspection is recommended for sticky or clumping materials to ensure that the weight measurement is not distorted by caking or foreign objects.

The rollers in the roller chairs in front of and behind the scale should not be stiff and should rotate freely.

A belt scale usually consists of four components:

• The weighing frame, which uses the built-in load cell(s) to determine the weight (in kg) of the material being transported on the belt. The weighing frame is integrated into the conveyor belt construction.
• the speed measurement, i.e., a speed measuring wheel or a rotary encoder on the deflection drum 
• a terminal box/connection box to which the speed measurement and the load cell are connected. The two signals are transmitted from the terminal box to the belt scale electronics via a cable.
• The evaluation electronics, which calculate the current mass flow (in kg/h or t/h) from the two measured variables, weight and speed. In addition, the electronics determine the absolute quantity transported over the scale (in kg or t). All measured variables can be displayed on the electronics and transmitted to a control technology or EDP system via various standard interfaces.

From Profinet, OPC UA, and Profibus via Ethernet to analog outputs and RS232/485—all common standards are supported. The following data is transmitted: flow rate (kg/h or t/h), conveyed quantity (kg or t), meter readings, setpoints, condition, and status of the measuring system. 

With very small mass flows, significantly below 10 t/h, precise measurement of the mass flow becomes difficult due to the low weight on the scale in relation to the disruptive influences of the belt. In such cases, it must be checked on a case-by-case basis whether a belt scale can deliver plausible measurement results.

The measuring principle is suitable for a wide variety of products. That is why our belt scales are used in almost all industries. 

Built-in belt scales can be easily integrated into existing or new conveyor belts without having to make major structural changes. One or more roller carriages are removed from the conveyor's support structure and mounted or welded onto the weighing frame of the belt scale. The scale, or more precisely the weighing frame, is inserted into the existing conveyor belt structure and secured in place. The speed measuring wheel is attached to the scale and usually runs along the bottom belt. The electronics can be installed either directly near the belt scale or at another location up to 200 m away via a cable.

Yes, a belt scale should always be calibrated using a reference measurement, i.e., with a known quantity of the material being conveyed. If calibration with conveyor material is not possible, a so-called chain calibration can be performed. For this purpose, chains are placed on the belt and secured over a longer distance. The weight per meter of chain is used as a reference. Together with the belt speed, the current flow rate can thus be calculated and stored in the electronics. In practice, a "real" calibration is preferable to chain calibration. We are happy to assist you in carrying out both methods. 

Some manufacturers offer scales with automatic calibration or testing devices. In this case, a known weight is placed or attached (manually or automatically) on the weighing frame of the belt scale. However, this calibration is insufficient, as the significant error influences caused by the conveyor belt are not taken into account. Therefore, this is at best a device for testing whether the load cell and mechanics of the belt scale are functioning properly and respond to loads. However, this can be achieved simply by manually placing a known or unknown weight on the scale.