Designing
Depending on the application the type (bending beam, column, shear beam, etc.) is decided. Load range and the output are important factors to consider while deciding the material to be used. While aluminum is used for lower load ranges steel is preferred for higher loads. For weigh scale applications bending beam load cells are used. Tensile loads cells are used in automatic packing machines to measure tensile forces. High capacity column or shear beam load cells are used for weigh bridges/truck scales.
A load cell designing software reduces work to a great extent. Most software provide dimensions for the most critical part of the load cell. For shear beam load cells web thickness is most critical. For column load cells column width and width is important. And for binocular beam load cells thickness of the thinnest part of the profile & distance between the holes are important.
Material Procurement
Material procurement involves purchase of metal (steel or aluminium), strain gauges (transducer class), bonding adhesive, terminals, PCBs, cables, bellows, fasteners and name plates.
Strain gauges are selected based on the application; linear or shear. Strain gauges are available in various sizes such as 3mm, 6mm, etc. Strain gauges can be procured from any of the reputed manufacturers like HBM, Micro-Measurements (MM), Shinkoh, BLH, etc. Bonding adhesive and matching accessories such as terminals, wires for internal wiring, cable glands, etc. are also procured. Teflon coated multi-core cable (4 core or 6 core) with right color code (red, black, white, green, yellow and blue) is procured from the right vendor. Cable should be tested for continuity and also quality of strands within the cores, strands should be silver-coated and flexible.
Alloy with right cross section (circular or square or rectangular) is selected so that material wastage is at its least. Most manufacturers prefer to use circular sections of EN24 for steel load cells. Next step is testing the metal's chemical composition and internal cracks (ultra sound testing) from a reputed testing services provider. Alloys not confirming to industry standards is rejected. Also material with internal cracks cannot be used for load cell manufacturing.
Machining and Heat Treatment
Machining raw material to required form is performed with lot of care. Commonly used machines are shaping machine, milling machine, lathe, column drilling machine and surface grinding machine. Machines should be in good working conditions and capable of producing accurate dimensions. Right coolant is used at all stages to avoid excessive heating during the process. Dimensions are checked at every stage using precision measuring instruments such as height gauge, digital vernier, depth gauge, micrometer, etc. at accuracy of 1 micron. Material in process (steel) is oiled to avoid oxidation. Surface grinding is the last stage of machining, its done after the process of hardening.
Only steel elements undergo the process of hardening at a heat-treatment plant. The elements are heated slowly to a high temperature and cooled rapidly in a oil bath followed by further cooling in a water bath. Harness is tested in a Rockwell Hardness Tester. Hardness value should be between HRC 40 to 45. If the value is lesser than 45 then elements need to be hardened again or if the value is higher than 45, elements will be softened. Some batches of steel fail to harden to the required value, elements have to be rejected in such cases.
Surface grinding achieves two objectives; accurate dimensions and smooth surface finish. Material removed in the process is usually few microns. The elements undergo one last round of deburring and ready for the next stage.
Electroplating
Zinc-plating was the used commonly during earlier years. However in the last 2 decades electroless nickel is the preferred protective coating since it offers good protection and also makes the elements aesthetically good. Elements undergo a process called buffing to improve the surface finish. Then it is cleaned and rinsed in chemicals to remove grease and other matter. Elements are kept dipped in a chemical bath for a specified period of time during which nickel adheres to the elements. The last step is polishing which is done to enhance aesthetics.
Bonding Strain Gauge and Internal Wiring
This is a crucial stage of load cell manufacturing. On the element, the surface where strain gauge is to be fixed is prepared by polishing it with water emery in circular movement. Using a height gauge and a surface plate the cross-hairs are drawn to mark the precise position of the strain gauge on opposite sides of the element. The strain gauge surface is cleaned thoroughly using chemical agents like Trichloroethylene (TCE) and Acetone. Alternate chemicals are used in place of TCE since its banned in many countries.
Once the element is free from grease and other impurities, adhesive is applied at the cross-hairs (matching the approximate area occupied by the strain gauge). Adhesive is also applied to the bottom-side of strain gauges and solder terminals and allowed to settle for few minutes. Its important to apply just the right amount.
Under a microscope, the strain gauge is positioned by aligning the marks with the cross-hairs and then taped to hold it in position. The adhesive tape used is of special quality capable of withstanding temperatures in the range of 250 degrees Centigrade for about 2 hours. With strain gauges in places, pressure pads and clamps are fixed. This is done to arrest movement and also maintain uniform thickness of adhesive between the strain gauge & element. With clamps in position, elements are placed in an electrical oven (with a air blower) and heated to about 180 degrees for about one hour. The process is usually known as curing. Temperature and duration of heat treatment depend on the adhesive used. The elements require about 12 hours to cool down to room temperature and should happen naturally. After curing, clamps and adhesive tapes are removed. The elements undergo another round of heat-treatment called post-curing. This done to destress starin gauges and adhesive.
The next step is to solder strain gauge terminals to solder tabs and fix wires to create a circuit so that the strain gauges are in Wheatstone Bridge configuration. High-end soldering stations (temperature controlled) with special solder tips are used for this job. The internal wiring terminates at a small PCB to which the multi-core cable is joined. At this stage, we have a working load cell. A basic test is done; 10V DC (or 12V DC) is applied is measured using a multimeter with least count 0.1V and no-load or zero output is noted. Load is applied in right direction to check if output is positive. Ideally no-load is adjusted to -0.25 mV.
Temperature Compensation
Load cells are required to behave consistently through a specified temperature range ~ 0 to 60 degrees Centigrade. To achieve that, load cells are studied at 0°C and 60°C for 6 to 12 hours. Based on the differences in output, a length of wire made of a special alloy is introduced into the circuit to counter the effect of temperature. A second round of temperature test is run to ensure the load cell behavior is constant through out the range i.e. between 0°C and 60°C. With recent developments in strain gauge technology, self-compensating strain gauges have eliminated one stage of load cell manufacturing. However, companies serious about quality do check load cell behavior at different temperatures.
Load Testing and Calibration
In this stage, load cells undergo a host of tests- full scale output, repeatability, linearity, creep, hysteresis, and many more. Load cell output is tuned to 20mV (or 10mV, 30mV depending on the specification) at rated load. Load cells also undergo overload tests to ensure they withstand 150% of the rated load.
Repeatability Test: Load cell undergoes full scale (and partial load also) test number of times and output noted at every instance of loading. The output should be within the claimed accuracy level.
Linearity Test: Load cell undergoes incremental & decremental loading and output noted at every instance. The Load versus Output graph should be a straight line.
Creep Test: Load cell is loaded at full scale for an extended period, say one hour, and the out output is observed. Ideally the output should neither increase nor decrease.
Hermetical Sealing
This is the final stage where the load cell is made dust-proof, moisture-proof and water-proof. Some lower-end load cells are not hermetically sealed for cost reasons. Larger load cells (higher capacities) are coated with expoxy paint to provide additional protection. The load cell is tested again to ensure that outer cover or bellow has not affected load behavior.
Every load cell is supplied with a data-sheet which has load cell serial number, date of manufacturing, cable color code, external dimensions and electrical parameters like excitation voltage, input & output impedance, no-load output, rated capacity, full scale output, sensitivity, etc.
For definitions of load cell terminology, please refer this webpage- http://www.transducertechniques.com/load-cell-terminology.cfm
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