Weighing
are present and working correctly to ensure the belt is clean and running efficiently. Again, poor roller alignment will cause the belt to move to one side presenting off set weight fixings that can reduce the effect of downward forces being exerted on the weigh scale structure, especially if only one load cell used. One of the most important factors in the influence zone is the angle of the belt, creating downward forces as the weight passes over the weigh scale to maximise the weight measurement. Insufficient trough angle design of the belt can influence the accuracy of the product being weighed, typically for best results the belt trough angle is recommended to be 20 to 30 degree creating the most effective down force. Adjustable belt frames provide the most advantageous results to provide flexibility to maximise accuracy and performance.
Detection of belt speed
The detection of the belt speed is important to the calculation, as we know that the speed of the belt influences the weighing span calculation. It is therefore important to provide a reliable and quality signal and not an ambiguous signal as a result of poor positioning or machine detection. Belt ‘wheel detectors’ operate at their best when used on dry products but are prone to fail when used with wet and sticky products, which can cause the wheel to slip and clog up and stick, providing inaccurate real time measurement of belt speed.
Roller measurement is good providing the rollers are away from the material spillage, are in good condition and they are operational and effective i.e. in contact with the belt.
By far the best method of detecting belt speed is to measure the belt speed at the tail end drum. A pulse generator and sensor attached to the drum will accurately provide the pulse signal of the number of revolutions of the drum indicating the optimum belt speed for calculation.
In summary
The accuracy of a belt weigher is dependent on the care you put into the design mechanically and to the environment to which it will operate in, as well as the way it is installed at the erection stage, will all influence the accuracy you will achieve. • Aligning the weighing plane is necessary for +/- 1 % and better • Replacing a roller without re-alignment may cause an error up to 5 %.
• Due to the gutter shape, the trough creates a belt constraint increasing with the angle value. At 45°, reachable accuracy is +/- 2 % • To reach +/- 0.5 %, up to 30° required (ideally 20° for Legal Trade)
• The Influence Zone Z2 must be respected and positioning of stringers to maximise performance. • The zero calibration tests must be done daily, or at least weekly, to ensure fidelity.
• Drop tests must be done once a year and after each modification of the mechanical part of the scale as speed, idler or belt replacement.
For more information contact Precia-Molen on tel: 01527 590320 or visit: www.preciamolen.com
www.solidsandbulk.co.uk
March 2010 • Solids & Bulk Handling 21
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