Belt Scales BW10 and BW20

The accuracy of a gravimetric (weight-loaded) belt weigher depends on many factors. The system errors for a belt weigher are nearly always caused by disturbance from the belt itself. The high level of accuracy of the electronics and the weighing frame can be disregarded in the calculation of the measurement error. In principle, they work with more precision, the greater the weight of the conveyed goods on the scale. Constant absolute error interferences thus have less effect on the weight. Crucial sources of error are the lateral „wandering“ / misalignment of the belt and the changing belt tension, e.g. as a consequence of exposure to the sun or weathering of the belt. Therefore, for belt weighers approved for certification, it is compulsory to have a tensioning station, but this can also ensure more precise measurement in process applications. Furthermore, the following factors can produce greater accuracy: The belt should be as new as possible without carriers or reinforcers, with gradient up to 20% (10% for Class 1 approved scales), max. 30° troughing (20° for Class 1 approved scales), tracking of the belt to avoid misalignment, a sufficient distance to inlet and outlet, no side scrapers / guides near the scales. The product weight on the scale can also be increased by reducing the transport speed. However, this is possible only up to a certain loading of the belt.

In technical terms approved belt weighers are not especially different from „normal“ scales. However, components approved for certification are used and the scale has to undergo a test at a notified body (for instance the German national metrology institute, the PTB or Physikalisch Technische Bundesanstalt). In addition, once installed, the belt weigher must undergo a variety of test measurements at different transport speeds within the specified boundaries for the respective accuracy class.

The requirements for approved weighing systems are described in the European directive MID 2004/22/EG. According to this directive belt weighers are available in various accuracy classes. Belt weighers approved for certification are classified into different accuracy classes 0.5, I, and II. At the acceptance test the scale must not deviate by more than +/- 0.25% (accuracy class 0.5), +/- 0.5% (accuracy class I) or +/- 1.0% (accuracy class II) from the actual value at various transport speeds. The later operating margins of error lie within double error in the acceptance test, i.e. max. +/- 1% in accuracy class I.

Belt weighers can be used for mass flows from 1 t/h. There is nearly no upper limit. For large belts the weigh frame must be reinforced accordingly, in order to bear the loads of the belt and the conveyed goods.

Typical applications are balancing material flows, truck loading, process control, dosing and continuous precision mixing processes.

The force is transmitted into the weighing cell by maintenance-free spring steel strips. Only the metering wheel with the non-contact proximity switch turns and therefore is subject to slight wear over the long term. For moist, clumping products, (brick clay, clay, etc.) we nevertheless recommend a periodic visual inspection, in order to remove heavy deposits, where necessary.

A belt weigher comprises the weigh frame, which by means of the integral load cell(s) determines the weight (in kg) of the conveyed goods being transported on the belt. The belt speed (in m/s) is also determined by means of a speed measuring wheel or a rotary encoder on the tail drum. From the two measurement values (the weight and the speed) the electronic encoder calculates the flow stream (in kg/h or t/h). In addition, the electronics system transmits the absolute quantity transported across the scale (in kg or t). All measurement values can be displayed on the electronics system or made available as an interface.

All state of the art interfaces, such as Profibus, Profinet, analogue outputs, impulse signals, Ethernet, serial RS232/485, etc. are available. Depending on the interface, the flow (kg/h or t/h), transported quantity (kg or t), meter readings, time and date, reference values, state and status of the measurement system are transmitted.

In the case of very small mass flows, significantly below 1 t/h, precise measurement of the flow stream becomes difficult, by virtue of the small weight on the scale in proportion to the disturbances on the belt.

The measuring principle is suitable for an extremely wide variety of products and therefore for nearly all sectors.

Built-in belt weighers can be easily integrated into new or existing conveyors. To do this one or more idlers are removed from the support frame of the conveyor and mounted or welded onto the weigh frame of the belt weigher. The scale is then positioned on the existing support frame and secured. The speed measuring wheel is secured to the scale and normally runs on the bottom chord. Alternatively, a rotary encoder is installed on the tail drum to measure the speed. The electronics system can be installed close to the belt weigher or at a greater distance in a control room.

Yes, a belt weigher should always be calibrated with a known quantity of the conveyed goods. If it is not possible to calibrate with conveyed material, calibration with chains can be carried out. To do this, chains are laid over a relatively long distance on the belt and secured. The weight per metre of chain is used as a reference. In this way, both the belt speed and the current flow can be calculated and recorded in the electronics system. In practice a „genuine“ calibration is preferable to a chain calibration. The calibration with test weights on the weighing frame is only suitable as a function test but not as calibration of the whole system.

Many manufacturers offer scales with automatic calibration or a test device. In this system, a known weight is stored, manually or automatically, on the weigh frame of the belt weigher. However, this type of calibration is insufficient, since the major error interferences caused by the conveyor belt remain undetected. Therefore, this system is merely a device for testing whether the measuring chain on the belt weigher has stopped working. However, this can be detected simply and manually by applying a known or unknown weight.