What is a load cell and how does it work?
A load cell (sometimes called a load cell transducer or load sensor) is a force gauge that consists of a transducer. This transducer converts a mechanical force, such as tension, compression or pressure into a measurable electrical output. While there are many types of force sensor, the strain gauge load cell is the most common; this is because strain gauge load cells can offer accuracies from 0.03% to 0.25% and can be used for almost any industrial application.
Load cells are usually differentiated in one of the following ways:
The method by which they detect load: e.g. compression and tension load cells
The type of output signal generated: e.g. hydraulic load cells and piezoelectric load cells.
The term load cell has come to be used for a range of force sensing devices, though strictly the phrase refers only to force sensors which are designed to work in the direction of gravity.
Load cells provide non-intrusive, highly accurate measurements making them perfect for use across a wide range of sectors, including construction, industrial manufacturing, commercial shipping and the oil and gas industry. The technology can be used to improve safety by alerting operators if an exerted force exceeds an asset’s safe working load limits. This is particularly important for lifting operations.
A load cell can also increase efficiency and performance of your operations. Advanced load cell technology delivers critical data insights which can be used to optimise lifting and weighing procedures, while offering asset life extension opportunities.
History of the load cell
Mechanical scales were the preferred method for industrial weighing applications before strain gauge based load cell technology overtook them. Load sensors designed pre-strain gauges where typically hydraulic or pneumatic in design. Whilst mechanical lever scales and pneumatic or hydraulic load sensors can be accurate and reliable as long as they are properly calibrated, they cannot match the same level of accuracy and cost effectiveness as the strain gauge load cell.
English Physicist Sir Charles Wheatstone devised a ‘bridge circuit’ that could measure electrical resistances in 1843. The Wheatstone bridge circuit is perfect for measuring resistance changes that occur in strain gauges, however, despite the first bonded resistance wire strain gauge being developed in the 1940’s, it would require further developments in electronics for the technology to become both economically and technically viable.
The foil strain gauge was invented as an alternative to the wire strain gauge by James Fisher Strainstall’s (JF Strainstall), founders whilst at Saunders-Roe in 1952. A high proportion of the standard wire type strain gauge, used in the dynamic testing of helicopter components, notably rotor blades, were failing. Peter Scott Jackson of the Electronics Division was therefore tasked to devise an improved gauge and, in collaboration with Messrs. Technograph Printed Circuits Ltd., produced the foil strain gauge
Jackson had observed the copper laminating process, used for printed circuit boards in the aircraft electronic systems, which involved a photo-chemical etching technique. He realised that a bonded strain gauge could be designed in exactly the same way, using copper/ nickel foil laminated to a thin epoxy carrier.
Saunders-Roe used these first foil strain gauges on the airframe of the Princess Flying Boat, with great success. The flat foil strain gauges bonded to a flat surface provided the maximum transfer of thermal and strain sensing properties, whereas the wire-wound strain gauges made minimal contact with the surface that they were bonded to, giving poor heat sink properties in response to the applied excitation currents.
This invention of the foil strain gauge led to the devising of the load cell and ultimately, the founding of JF Strainstall in 1965 as the engineering and commercial platform for what was a revolutionary product of the time.
How do load cells work?
Spring elements, typically made from materials such as steel or aluminium, are attached to a strain gauge material. The spring elements are designed to be very strong but slightly elastic which means they will deform slightly under sufficient pressure. Although the human eye cannot detect this sufficient pressure in the form of compression or tension causes micro deformation before the force sensor is allowed to return to its original shape.
When this change takes place, the strain gauge is able to measure the exact deformation in the spring element to an extremely high level of accuracy. This micro deformation measurement is then converted electronically into a precise weight readout. To simplify the process, directional pressure on the load cell creates an electrical signal which indicates the force being measured.
When used in real world applications there are a multitude of outside influences that must be controlled to get the best results from your load cell. Temperature, weather fluctuations, moisture and dust can all affect the electrical resistance of the load cell. It is therefore important, when purchasing a load cell, to consider the working environment it will be used in and the material from which the load cell is made from. Commonly load cells are housed in:
As discussed above, there are many types of load cell. Some of the most common are:
Strain gauge load cells
The strain gauge load cell is the most popular style of load cell for industrial applications. Using four strain gauges, two in tension and two in compression, bonded to a structural element which deforms slightly when weight is applied causing changes to the electrical resistance of the gauges. This change in resistance can be output as a highly accurate weight measurement. The electrical resistance from the strain gauge is linear and so can be easily converted into a force or weight.
Strain gauge load cells have become increasingly accurate and cost effective making them the most popular choice.
Hydraulic load cells
Hydraulic load cell’s work by utilising pressurised liquid. They typically consist of:
An elastic diaphragm
A piston with a loading platform on top of the diaphragm
Oil or water that will be inside the piston
A bourdon tube pressure gauge
When a load is placed onto the loading platform, the piston applies pressure to the liquid contained inside. This change in pressure is proportional to the applied force or weight and therefore measurable, either via an analogue gauge or by conversion into an electrical signal. Due to the fact hydraulic load cells have no electrical components, they can provide a good option for hazardous area load monitoring applications.
Pneumatic load cells
Similarly to hydraulic load cells, a pneumatic load cell consists of an elastic diaphragm and a loading platform but rather than using pressurised liquid, it deals with air pressure. When an object is placed on top of the pneumatic load cell, pressurised air or gas is used to balance out the weight, escaping from one end of the load cell with a pressure gauge. The weight of the object can then be determined by knowing how much air is required to balance out the object. The pressure gauge can then convert the air pressure reading into an electrical signal for output.
The use of inert gases rather than fluid, which may cause contamination if something was to go wrong, mean that pneumatic load cells are a good choice for environments where cleanliness is paramount, however they can be slow to respond and do require a regulated flow of air or gas.
Piezoresistive load cell
Piezoresistive load cells generate a high level output signal, meaning they can be directly connected to a readout meter. The main drawback of piezoresistive load cells however is that their output is nonlinear and that they do not produce a useful readout under static load as standard. Combined with the ready availability of linear amplifiers and there is very little benefit over a strain gauge load cell.
Magnetorestrictive load cells
Magnetorestrictive load cells work based on the stresses caused by a load being applied causing distortions in the magnetic flux pattern which then generates a proportional output signal.
Types of load cell
Load pins are the simplest and most reliable method of measuring loads and they can be used in a wide number of applications, including the most difficult of environments such as offshore and subsea. Their design makes them a straightforward replacement for existing dumb pins without modifications to the system.
Applications of a load pin
Explore our case studies below for real world applications of JF Strainstall load pins:
Also known as ‘column load cells’, ‘pancake load cells’ or ‘donut load cells’, a compression load cell is designed to measure downward or ‘pushing’ forces. Their compact design makes them ideal for weighing applications where space is limited such as silo weighing.
Also known as tension links, load links or tensile load cells, a tension load cell measures ‘pulling’ forces, similar to a hanging digital scale. You can purchase compression/tension load cells for applications where the load may vary between compression and tension.
Applications of tension and compression load cells
Designed specifically with harsh environments in mind, load shackles are used in heavy lift applications such as under hook crane weighing or water weight bag testing where there is limited head room.
Applications of load shackles
Explore our case studies below for real world applications of JF Strainstall load shackles:
Micro load cells are extremely small load cells which typically use a semiconductor to measure deformation rather than a strain gauge. They are mostly used in applications where space is at a premium. Most micro load cells are designed to only measure force from a single direction so when they are in operation it is important to be aware of any forces which may be being exerted from another direction and therefore affecting the accuracy of the reading.
S-Beam load cells
Also referred to as a ‘z beam load cell’, an S-Beam load cell is designed in an ‘S’ shape which is how it derives its name. S-Beam load cells can provide an output if under tension or compression and are typically used for measuring suspended loads.
A popular choice for a wide variety of load cell applications, bending beam load cells feature low profile construction, are compact and rugged.
Platform and single point load cells
Platform and single point load cells can accept an off-centre load, enabling them to provide accurate readings regardless of where the load is positioned on the platform. Typically they would be used in a low capacity, compact weighing system.
Shear beam load cell
A shear beam load cell typically has two restraining bolts at one end for support and then the load cell at the other end, though there are double-ended variations where the load is placed in the centre.
Direct vs indirect load monitoring
Direct vs indirect, when discussing load cells, refers to where the load monitoring device sits within the load path. If the monitoring device is fitted within the primary load path and taking direct measurements from the load exerted on the equipment, this is direct load monitoring.
Conversely, indirect measuring refers to measuring which avoids the primary load path, determining load by measuring its results from outside that path. Due to avoiding the primary load path, indirect monitoring may be more cost effective at higher loads.
What is sensitivity in load cells?
The smallest force which a pair of load cells can detect reliably is referred to as ‘load cell sensitivity’. A greater sensitivity indicates a load cell is better at detecting smaller changes in tension – it is the minimum amount of force needed to cause a change in a load cells output. Resolution on the other hand refers to the load cells ability to produce an output relative to the detected change.
How to calculate load cell sensitivity
Find your load cells excitation voltage (V or VDC) and output (mV)
Divide your output by your excitation voltage to work out a ratio (eg. 250 mV/10 V = 25:1)
The greater the ration the greater the load cells sensitivity.
If you are purchasing a load cell you should look for information on the excitation voltage (usually given as V or VDC) and the output (mV).
From this information you can work out the ratio which will give you the load cell’s signal resolution. For example, if your load cell has a 10V excitation voltage and a 0-250mV output, the ratio is 250/10 or 25:1. The greater the ratio the greater the load cell’s signal resolution. A greater signal resolution means a higher signal to noise ratio and so a greater sensitivity.
‘Noise’ in this instance refers to any non-grounded electrical signals which may interfere with the signal produced by the load cell. When a load cells signal is amplified, any noise is also amplified and the ‘signal to noise’ ratio determines the accuracy of the load cell. The lower the ratio, the less accurate your load cells measurement is. This is because a low signal to noise ratio indicates that more noise is being amplified and measured compared to the load cell’s electrical signal.
Why is sensitivity important?
A higher sensitivity means that load cells are able to detect smaller changes faster and with less noise interference. This means that measurements are faster, more accurate and more reliable.
What are the applications of load cells?
There are many industrial uses for load cells, including:
Overhead crane scales
Weighing agricultural produce
Rail rolling stock
JF Strainstall load cell products
Did you know?
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