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Thread: Weigh scale accuracy is required within +/- 1%

  1. Weigh scale accuracy is required within +/- 1%

    Dear experts,

    We were trying to check the accuracy of a weigh scale newly installed in one of our conveyors, which is having horizontal take up trolley.

    During taring process, belt is made to run empty for 3 revolutions, without any material. Then the weigh scale final reading is made to zero.
    This is known as zero calibration.

    If the take up trolley remains at the same position in the subsequent exercises, the drop test value becomes with in range of 1%.
    If the take-up trolley shifts from the taring process, the value is changed. If the trolley goes towards the conveyor, ( i.e., the belt becomes slightly loose ) , the readings are less than the known weight of material passed. If the trolley moves away from the conveyor ( i.e., the belt becomes tight ), the readings are higher than the actual quantity of material passed.

    Even the trolley moves for 100 mm +/- from the position of taring process, the readings becomes erratic in +/- 3% of actual value. We need accuracy with in +/-1%.

    As a mechanical engineer, I ensured all the rollers, take up trolley wheels and sheave pulleys used in the take-up arrangement are smooth. Drive pulley is not slipping.
    Belt was recently replaced, only to get the accuracy of weigh scale, though old belt was pretty good. Weigh scale is new. We are taking drop test at 180 TPH, 30% of weigh scale capacity ( 600 TPH ).

    Though I told operation engineers that due to belt dynamics, we cannot expect the trolley will attain the same position, as it was at the taring process.
    But they & weigh scale commissioning engineer are not convinced.

    Requesting help from the forum for advice to solve this problem.

    Thanking in anticipation with lots of regards,

    S.Ganesh

  2. #2
    Belt tension is a major factor in belt scale accuracy. The best control is of course vertical take-up as it applies a constant tension in the system. But when you have a horizontal take-up tension will fluctuate depending on how the tension to the trolley cable is applied and maintained. In other words you need a very good and reliable trolley cable tension monitoring system with feedback to the belt tension control system to maintain an accurate tension in the system. This should be maintained to the design sag parameter of maximum 2% for belt scales.

    Also installing the belt scale in the lowest tension area of the belt (typically near the tail end) helps to reduce fluctuations in belt tension percentage wise. Too many times I see belt scales installed near the head end which is the worst place to install them due to the largest fluctuation in belt tension.
    Gary Blenkhorn
    President - Bulk Handling Technology Inc.
    gblenkhorn@bulkht.ca
    http://www.bulkht.ca

    Offering Conveyor Design Services, DEM Flow Simulation Services, Conveyor Transfer Design Services, SolidWorks Design Services for equipment layouts and Software for the Bulk Material Industry.

  3. #3

    Question Commissoning Commissioning

    What does your calibration curve say?
    Have you checked the commissioning chain?
    John Gateley
    johngateley@hotmail.com
    www.the-credible-bulk.com

  4. #4
    davidhyer Guest

    Check Idler Alignment

    S. Ganesh,

    As Gary correctly pointed out, ensuring consistent belt tension is critical to maintaining accuracy of the belt scale over a wide range of rates. In addition, you should check the alignment of the idlers within close proximity of the scale. (At least 2 idlers directly before and after the scale). These should be adjusted (shimmed) to within 1/32" of the scale idlers. This will improve immunity to tension variation.

    Good luck,

    David

    David G. Hyer
    President

    Thayer Scale / Hyer Industries, Inc.
    www.thayerscale.com


    Quote Originally Posted by sganesh View Post
    Dear experts,

    We were trying to check the accuracy of a weigh scale newly installed in one of our conveyors, which is having horizontal take up trolley.

    During taring process, belt is made to run empty for 3 revolutions, without any material. Then the weigh scale final reading is made to zero.
    This is known as zero calibration.

    If the take up trolley remains at the same position in the subsequent exercises, the drop test value becomes with in range of 1%.
    If the take-up trolley shifts from the taring process, the value is changed. If the trolley goes towards the conveyor, ( i.e., the belt becomes slightly loose ) , the readings are less than the known weight of material passed. If the trolley moves away from the conveyor ( i.e., the belt becomes tight ), the readings are higher than the actual quantity of material passed.

    Even the trolley moves for 100 mm +/- from the position of taring process, the readings becomes erratic in +/- 3% of actual value. We need accuracy with in +/-1%.

    As a mechanical engineer, I ensured all the rollers, take up trolley wheels and sheave pulleys used in the take-up arrangement are smooth. Drive pulley is not slipping.
    Belt was recently replaced, only to get the accuracy of weigh scale, though old belt was pretty good. Weigh scale is new. We are taking drop test at 180 TPH, 30% of weigh scale capacity ( 600 TPH ).

    Though I told operation engineers that due to belt dynamics, we cannot expect the trolley will attain the same position, as it was at the taring process.
    But they & weigh scale commissioning engineer are not convinced.

    Requesting help from the forum for advice to solve this problem.

    Thanking in anticipation with lots of regards,

    S.Ganesh

  5. Thank you all.
    I never thought even a very few jammed return rollers at head end can make huge impact on weigh scale which is installed approx. 250 mtr away at tail end. Now problem is solved.

    Regards,

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