Online Equipment Focus: Maintaining Conveyor Belt Scales
A material test of the conveyor belt scale can also be completed where the weight produced by the conveyor belt scale is factored to another known weight. This known weight can be either a pre-weight, such as bagged material, or the unloading of a rail car or truck load of material across the conveyor scale.
Material that has been conveyed can also be collected in a truck or rail car and then weighed out on either a truck or rail scale. The conveyor belt scale can be factored to match the known weight of either the pre-weight or collected material to achieve the best accuracy.
Once the conveyor belt scale is properly installed and calibrated, proper routine maintenance will ensure proper operation. Since the conveyor belt scale will likely be located in a rather hostile environment, it is recommended that the user perform periodic checks whenever inspecting the conveyor or servicing other components in the area of the belt scale.
The conveyor belt scale should be kept clean of debris or spilled material. Heavy buildup of static material will register as weight on the scale and be added to the total on the integrator. This will produce a compound effect and can produce a high error in the accuracy of the belt scale over a long period of time. The scale idler(s) attached to the weigh bridge(s) and speed sensor should also be kept properly lubricated.
Wear and repair parts
The following parts of a conveyor belt scale are subject to a need for service due to wear and tear and, possibly, repair or replacement: integrator, weigh bridge, load cells, and speed sensor proximity switch.
The integrator is prone to potential failure due to issues with seasonal lightning if not properly grounded during electrical installation of the belt scale. Additionally, with a long cable run (i.e., between a conveyor and a control room) there is the opportunity for noise from wire interference (from anything that uses or produces power) to be introduced into the signal wires and affect the accuracy/operation of the scale.
The weigh bridge has additional electronics including junction cards, load cells, and failure potential for frayed wiring due to neglect. Additionally, the belt scale installation can suffer from problems as a result of a need for shimming/alignment of the scale weigh bridge and/or the idlers both before and after the scale. Some conveyor belt scale manufacturers require the idlers before and after the scale idlers to be shimmed.
Load cells are the devices which receive the force transferred by the conveyor idler supported by the weigh bridge. The force is then converted to an electrical signal that can be received by the integrator, which processes the signal as a load on the conveyor belt scale. Typical load cell designs for conveyor belt scales include both “shear-beam” and “S-type” load cells similar to beam and platform scales. The capacity of the conveyor in tons per hour will determine the load rating of the load cells commonly in 100-pound, 250-pound, 750-pound, and 1-kilogram capacities. A weigh bridge will use either one or two load cells. A multiple idler scale system consisting of multiple weigh bridges can use up to a total of eight load cells.
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