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Characteristics and applications of the most common types of load cells
Strain gauge load cells, depending on their designed use and application, differ in many features, including shape, method of loading, deformation measurement technology, construction material, capacity, ingress protection rating and many others.
1. Single-point load cells
Also known as platform load cells, single-point load cells are ideal when it is not possible to ensure centering the load to be weighed.
They have a cutout in the middle of the mechanical body, which often has a binocular shape.
The cutout is obtained by machine tooling and during commissioning it is fine-tuned so as to give the sensor the characteristics of a symmetric articulated quadrilateral.
This feature allows single-point load cells to ignore the torsions caused by the decentralization of the load and to ensure a correct reading of the weight within its work area.
Therefore, set in the middle of a surface of known dimensions, single-point load cells are capable at any point of correctly weighing the weight force applied, within a certain maximum area specified on the data sheet.
The load area varies depending on the model and capacity of the load cell.
Single-point load cells are very accurate, also reaching very high accuracy classes such as C6 in accordance with OIML R 60.
These are very versatile load cells that are used in many applications: platforms, bench scales, small roller conveyors, small tanks and hoppers, filling machines and packing machines, bagging machines, conveyor belts and rollers in tensioning systems.
They are also used in dynamic weighing.
Single-point load cell applied to the middle of a roller conveyor.
Application on a roller in a tensioning adjustment and measuring system.
Single-point load cells usually work individually.
For reasons of construction, in fact, they have very different rated outputs from each other that, in group work, could create such interference as to compromise accuracy.
However, it should not be ruled out that in certain applications they have to be connected in parallel.
In this case, at the stage of purchase, it is important to inform the vendor about this need since it will be necessary to try and couple load cells with a similar rated output and equalize their output signals.
single-point load cells are chiefly made of aluminum alloy, an advantageous material besides being ideal for many of their most common applications, in which stainless steel is not a necessary requirement.
However, they are also available in stainless steel, both AISI 420 and 17-4 PH.
They have average capacities, from 250 g up to 2000 kg, and reach the C5 accuracy class in accordance with OIML R 60.
The protection rating they can provide (that is their degree of resistance to the ingress of dust and water) is high both for cleaning with more or less strong pressure jets (IP65, IP66 and IP69K) and for immersion with different duration and depth (IP67 and IP68).
2. Compression load cells
Compression load cells contain a deformable circular diaphragm, whose thickness varies depending on the capacity of the load cell. The amplitude of the diaphragm ensures that the load cells of the same model all have the same rated output and are all the same as each other.
For this reason they are perfectly interchangeable and replaceable even after some time and suitable for working in groups, connected in parallel in the same weighing system.
The design of the compression load cells, in particular the top load cylinder with its convex surface, allows them to have a single load point.
As a result, any slight angling of the load will not affect the accuracy of its measurement, since it is always brought back to the middle.
Compression load cells are generally used in weighing silos, tanks, hoppers, mixers and reactors, even of large dimensions and capacities.
Weighing system under a tank with 4 legs.
They have capacities from 200 kg up to 1 000 000 kg and are available with high protection ratings, according to need (IP67 and IP68 or IP69K).
A feature of the low-profile compression load cells is their ease of installation.
In fact, they accompany the specifically designed mounting kits to make their installation easier and safer.
Column load cells are compression load cells whose deformable part takes on the form of a cylindrical column that shortens under load: this allows the load cell to support high loads, while keeping a limited diameter.
They are generally installed on weighbridges or on high-capacity silos, in groups of at least 4 load cells connected in parallel.
.column load cells are made of 17 4-PH stainless steel and AISI 4340 nickel-plated steel. They have high capacities, from 10 000 kg to 50 000 kg, but can have even higher capacities to custom design.
They have an IP68 protection rating.
Bending beam load cells are the simplest load cells and they work thanks to the principle of “shelf deformation”: part of the load cell has a reduced section (by means of circular or square holes) that deforms and bends when a load is applied.
The weakened part is always protected with welded covers that sometimes have a bellows shape; in this case the load cell is also called a “bellows load cell”.
Bending beam load cells are generally used for making weighing systems of small capacity such as dosing units and filling systems, bagging machines, filling machines, packing machines, small hoppers.
Bending beam load cells as a rule work in groups of 2, 3 or 4 load cells connected in parallel. They can also be used in single load cell applications, for example installed on the tie-beam of a mechanical scale to convert it into an electronic scale.
Single use is however less frequent and requires the force to always be applied perfectly perpendicular to the load point.
Shear beam load cells offer very high precision and accuracy.
Their geometry is similar to that of bending beam load cells, but they differ in the “shear” technology with which the deformation is measured.
They have a plate in the middle of the hole, with an increasing thickness as the capacity increases, on which the strain gauges are glued at 45°, to maximize the rated output and minimize errors due to any rotation of the load cell.
Shear beam load cells always work under compression and depending on the type of application they can be mounted either on the frame of the structure, with the loading zone supported on the ground, or vice versa: on the ground, with the loading zone supported on the frame.
Shear beam load cells are usually used within multi load cell platforms.
They are ideal for weighing silos of small and medium dimensions, hoppers, mixers, big bag fillers and dischargers and for transport systems with belts, roller conveyors, etc.
As a rule, shear beam load cells are especially cost-effective.
When the loads to be weighed have to be suspended tension load cells are used.
They are special shear or bending beam load cells, depending on the capacity.
Thanks to their various options, they can be fitted on all tension force measuring systems, through threaded fasteners or with the classic lifting eyebolts/shackles.
S tension load cell, applied on the tie-beam of a mechanical scale to digitize it.
Tension load cells are used both individually and in groups connected in parallel.
They are used in suspended weighing systems such as bridge cranes, cranes, lifting machines, overhead platforms, crane scales, but also in hoppers, weighing belts and concrete mixing systems, or to transform old mechanical scales into electronic scales (revamping).
Compression and tension load cells are special versions of compression load cells that can also measure forces of tension thanks to the fixing holes and threaded loading zones.
There are, in fact, models with a central thread (male) and others with a hole in the middle (female).
A force is applied onto the threaded pin or hole, both under tension and under compression, while the holes on the outer crown permit fixing the load cell to the structure.
There are two types of compression and tension load cells:
They are used both individually and in groups, connected in parallel.
The most common applications are: material test machine, tests in the field, quality control machines and presses.
Double shear beam load cells are a special and particularly precise and accurate version of shear beam load cells, characterized by symmetry in relation to the load point. They are, in fact, designed to be secured at both ends and loaded in the middle.
This symmetry improves load cell performance and ensures a certain insensitivity to any lateral forces, making them suited also to special uses.
double shear beam load cells, both made of AISI 4340 nickel-plated steel and with an IP68 protection rating.
The high capacity ranges from 9000 kg to 27 000 kg.
Double shear beam load cells besides being especially precise and accurate are also sturdy, suitable for weighing high loads.
They are typically used in weighbridges, on-board weighing for vehicles and high-capacity silos, even installed outdoors.
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