September 8, 2011
Conveyor belt scales should be calibrated periodically to ensure the accuracy of your material weight total.
by Austin Amos
It is important to note that a conveyor belt scale is no different than any other type of scale (i.e., lab balance or truck scale) in regards to the need for recalibration. Because a belt scale provides a “weight-in-motion” (unlike the static weight of a lab balance or truck scale), if the speed, zero, or span reference numbers of your belt scale have drifted, you will face a compound effect of inaccuracy over a given period of time (hours, days, or months) with a running conveyor belt. This will amplify the difference, positive or negative, between the weight shown on your belt scale integrator and the actual weight of the material you conveyed.
Recalibration of the belt scale once per month is recommended; however, some applications, at a minimum, can be lengthened to once every three months, with each particular application being somewhat different. The calibration of the belt scale can drift slightly (either positive or negative) due to wear on the belt scale, changing conditions of the conveyor belt, or deviation through the continued operation of the conveyor. Monthly calibration of the scale is preferred to keep these changing factors in check with the conveyor belt scale.
It is also important to note that the calibration procedure presented in the reference manual provided by the conveyor belt scale manufacturer should be followed in order from start to finish. Most discrepancy issues regarding accuracy are related to not following the required steps of the calibration process correctly or totally ignoring some steps completely.
Typically, the conveyor belt scale integrator will have a Setup/Calibrate mode that must be selected by the user in order to change the instrument parameters or run calibration tests. When in this mode, integration stops. The calibration process involves the completion of speed, zero, and span tests.
Some integrators feature a menu-driven display that has multiple lines of full English text for display/entry of the setup parameters for the scale; some require the user to refer to their operator’s manual for a code decipher listing of each code that is given on a strictly numeric display.
The addition of a speed sensor to the conveyor belt scale allows the integrator to process and display a “weight-in-motion.” The exact measurement of the conveyor belt speed is critical to conveyor belt scale accuracy. The speed sensor provides a constant measurement of the rate (unit of length/unit of time) of the conveyor. Any change in speed from a loaded or unloaded belt based on material flow is directly transmitted to the belt scale integrator along with the signals from the load cells. The integrator handles the processing of the “integrated” speed and weight and provides the user a “weight-in-motion.”
The user would complete a speed test by first calculating or measuring actual belt speed using a handheld tachometer. For the speed test, the conveyor is run empty (without material) up to normal speed.
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.
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.
Speed sensors utilize electronic proximity switches and wiring in addition to mechanical wheels and greaseable bearing sets. Due to the mechanical nature of the speed sensor design, a catastrophic failure of a proximity switch may also require the replacement of a cog or counting wheel, in addition to a new proximity switch, to complete a repair of the speed sensor.
It is best to contact a manufacturer or local scale service company with your specific issues and further questions for their review and expert recommendation.
About the author: Austin W. Amos has 16 years of experience in the weighing industry and currently holds the position of general manager at ConveyWeigh, LLC., a West Virginia-based manufacturer of conveyor belt scales, and is an appointed member of the West Virginia District Export Council.