Online Equipment Focus: Maintaining Conveyor Belt Scales
The belt scale integrator receives pressure signals by voltages generated by load cells and mathematically converts them to the rate, commonly in short tons per hour. It also provides the total mass of the material which has passed over the scale for a certain period of time, also commonly in total short tons.
During the zero test, the conveyor should be run to speed without material being loaded onto the belt to achieve the zero of the belt scale. Specifically for the zero test, there are no calibration test weights used. Calibration test weights are further defined below.
The purpose of the span test is to set the upper limit of the weighing capacity of the conveyor belt scale to match the maximum capacity of the weighing application. The difference with the span test (versus the zero test) is the attachment of calibration test weight(s) to the weigh bridge(s) during the span test.
In the event that the design rate of your process changes, you can achieve the best accuracy by completing a new span test with more or less calibration test weights.
During the span test, the conveyor should be run to speed without material being loaded onto the belt with the calibration test weights attached.
Calibration test load methods
Typical calibration test weight designs include hand calibration test weights and calibration test chains, in addition to material tests. Hand calibration test weights are attached to the scale only during the brief completion of the span test and then stored until the next span test is performed. Hand test weights are universal, and one set of multiple hand test weights can be used across all belt scale(s) at the given site or regionally by all sites of a company.
Another option is to purchase an engineered calibration test chain for the belt scale. A calibration test chain looks like an oversized “bicycle chain.” Installation of a calibration test chain commonly includes a storage trough mounted above the conveyor and either a hand or electronic winch. The winch is used to lower the calibration test chain onto the conveyor belt to hold it stationary during the span test. The winch is then used to retrieve and store the calibration test chain in the trough until the next span test is performed.