2024-07-24
How is the compression load cell created?
From machining the steel workpiece to packaging and shipping, through gluing the strain gauges, wiring, calibration and testing...here are all the steps in the construction process that turns a simple steel cylinder into the most important component of a weighing system.
The construction of the load cell starts in the mechanical workshop where the 17-4 PH stainless steel bars arrive to be transformed.
17-4 PH stainless steel, due to its elasticity and hardness, is a particularly suitable material for load cells.
When combined with the proper strain gauges, it has a very small error on the hysteresis cycle.
This characteristic allows it to remain flexible even for very high forces: it deforms under a load and then returns to its initial condition when the load is no longer acting.
All bars are certified with an ultrasonic machine to rule out any imperfections in the steel. Then each bar is marked with its diameter and casting number so that the batch is easily traceable.
The bar is cut into cylinders, which are then checked one by one to make sure that there are no "cracks".
Cracks are often hairline cracks invisible to the human eye and may cause future problems when manufacturing the load cell or operating it.
Once the checks are passed, the cylinder is machined in 3 stages: two on the lathe and one on the milling machine.
The load cell mechanics are ready for moving on to the next stage of manufacture.
Once the blank body of the load cell has been received by the workshop, the first mechanical test is carried out.
The operator, using a periodically certified gauge, checks that the dimensions are within the tolerances specified on the design drawing.
To remove all traces of impurities, the part is then washed a first time with a degreasing emulsion at a temperature of 50° for 40 minutes.
This is only the first wash the load cell will undergo, as a matter of fact it will be cleaned at least another 4 times.
Finally, all the clean parts are dried by hand one by one.
The heat treatment of aging serves to eliminate and ease the tensions in the material and to bring the load cell to thedesired hardness, improving its elasticity.
Aging leads to the core of load cell production.
The load cells are put in batches into the furnace while the management control panel oversees the operation and produces a graph and a certificate on the heat treatment carried out.
There are two types of industrial furnaces used for this treatment: air furnaces and muffle furnaces.
The air furnace acts by ventilation, while the muffle furnace uses spiral heating elements to produce heat.
For special load cells (such as those certified OIML R 60) the heat treatment involves a further step before aging: resolubilization.
During resolubilization, the load cell is treated at 1050°C in a high vacuum industrial furnace and then cooled to -80°C in a cryogenic oven.
The high vacuum furnace ensures that the load cell does not deform, while the thermal shock in the cryogenic oven eliminates tensions in the 17-4 PH stainless stell so that the load cell will be even more accurate and linear.
This stage of pre-aging is carried out on the blank piece before machining and eliminates all the tensions in the load cell.
After heat treatment, the load cell is polished and sandblasted.
The surface is brushed with an abrasive paste and made shiny to lower the roughness of the load cell and prevent material build-up, making it also easy to clean.
With the aid of a sandblasting machine, the operator sprays glass microspheres onto the surface of the load cell making it porous.
This sensor are polished on the outside and sandblasted only on the inside, so the strain gauges will adhere better to the body of the load cell due the porosity given to its surface by the sandblasting process.
The load cell is lastly washed again at 50°C and completely cleaned of sandy remains of the microspheres and greasy remains of the abrasive paste.
When the washing is completed, the load cells are stacked, being careful to insert a silicone disc between them. This will prevent them from getting damaged or scratched later on in the production cycle.
The load cell is transported to the laboratory, where it is subjected to two checks: the dimensional and flatness checks.
The dimensional check is performed with gauges, which in their turn are calibrated with Johansson blocks.
What are Johansson blocks?
Also referred to as "flat parallel gauge blocks", they are magnetic steel parallelepipeds of varying thickness (0.01 to 10 mm) machined to obtain 2 perfectly parallel faces.
By rubbing and attaching together multiple blocks of different nominal value, the gauges used to measure the thickness of the load cell diaphragms can be verified.
Johansson blocks have an expiration time. They are therefore to be periodically reviewed to obtain a new certification, or replaced with new blocks.
To check the flatness, the load cell is placed on a perfectly flat surface and a force is exerted on it with your fingers.
If the load cell moves, the support crown is not perfectly flat and the load cell will be discarded. It is essential for it to remain still, otherwise it will not be able to work properly.
After polishing and the mechanical checks, the hardness of the load cell is checked.
Approximately 10% of the pieces are checked at random, which is sufficient to ensure that the whole batch meets the required parameters.
For this test, the operator uses a durometer, that is a machine with a penetrator that measures the degree of hardness taken on by the metal after the aging heat treatment, and confirms that the measured parameter is correct.
Different methods and scales can be used to perform the check. The most appropriate one is chosen depending on the shape and material of the penetrator and the preload and load forces applied.
In this case the chosen scale is HRC (Hardness Rockwell Cone), which is the one used for hardened steel parts.
The load cell is now ready for axis tracing and marking.
Both tracing and marking are done with a laser marker that can engrave any material, for an extremely clean and precise result.
The tracing phase is critical. It is precisely here that, following the instructions on the drawing provided by the mechanical designer, the axes are engraved inside the load cell slot which during the gluing phase will show the operator where to position the strain gauges precisely.
The marker moves from one load cell to another and on each one engraves the specific label with the nameplate data to ensure full traceability.
Once all the mechanical checks have been completed, the load cell is washed again.
This time an industrial ultrasonic washing machine is used to make sure it gets completely degreased.
The operator places approximately 10 kg of load cells into a basket and lowers it into the water at a temperature of 70°C. Here the load cells are subjected to a series of ultrasounds at 70Hz for 2-5 minutes and then rinsed.
After washing, the differce between polishing and sandblasting is clear and noticeable.
The surface of the polished load cell is smooth and causes the water to run off immediately; the surface of the sandblasted load cell is porous and retains the water, which takes longer to evaporate.
The strain gauge is a small grid consisting of constantan wires (a binary alloy of copper and nickel) and is the sensitive measuring component of the load cell.
Glued inside the load cell slot, it follows the deformation of its surface.
When deformed, it causes a change in resistance, and therefore in electrical signal, proportional to the force applied to the load cell.
These variations are measured by making a Wheatstone bridge circuit.
The bridge connects multiple strain gauges to each other with electrical wires and a printed circuit board (PCB), which together detect the magnitude of deformation as a change in electrical signal expressed in mV/V (millivolts per volt).
The load cell now reaches the most delicate department. It is here where the strain gauges are selected, positioned and glued expertly by skilled and specialized technicians.
Everything in this department is kept under the utmost control: the temperature is always between 20°C and 24°C, the level of humidity is kept constant and there is no dust whatsoever.
Everything is carefully monitored, nothing must affect the strain gauge and its isolation.
However, the load cell must be cleaned once again before gluing.
The technician cleans it by hand with two products: the conditioner and the neutralizer.
The conditioner is a compound of phosphoric acid which slightly corrodes the surface of the load cell and allows it to be completely degreased.
The technician pours a drop of conditioner into the slot to clean it thoroughly , then dries the residues with gauze.
The neutralizer is used to block the chemical reaction initiated by the conditioner and to create the optimum conditions of cleanliness for gluing the strain gauges.
It is applied after the conditioner and helps to ensure that the load cell is perfectly clean: if the conditioner has been effective, the drop of neutralizer will immediately expand on the surface of the slot.
The technician can now proceed with gluing.
