Load cells are sensitive instruments engineered to deliver accurate measurements yet are also often expected to perform reliably under harsh conditions. A load cell designed to withstand tons of applied load can still be susceptible to failure when misused or installed in unsuitable environments.
Load cell failure is characterized by the inability to produce a reliable signal output that accurately represents the forces being measured, provided that this inability is not due to external factors within the system and generally caused by either damage or defect. Essentially, a load cell fails when its electronic or mechanical components are compromised. This could be due to damage or defects in its electrical circuitry or any of its mechanical components. Understanding the causes of load cell failure is crucial for maintaining the accuracy and reliability of weighing and other force measurement systems.
A load cell is most fundamentally an electrical circuit housed within a load receiving element that deforms predicably when force is applied. The electrical circuit outputs a measurable signal that changes proportionally to the force applied to the metal element. Therefore, damage to either the mechanical element or the electrical circuit of a load cell can lead to a failure, leading to unreliable or no signal output.
Mechanical: The mechanical components of the load cell, primarily the load-receiving metal element of the load cell can be physically damaged or defective. The internal circuitry then outputs incorrect signal outputs based on changes to the mechanical properties that have exceeded the normal specification range that the circuitry was built to measure.
Electrical Failure: Load cells circuitry are highly sensitive and delicate, designed to detect miniscule changes in the mechanical properties of the deflecting element. This circuitry is generally protected by potting, epoxy sealing, and metal caps that are often welded sealed to ensure lasting durability in harsh environments. Compromised circuitry, either caused by a defect during manufacturing or damage will lead to irregular or no signal output.
Most load cell failures are caused by damage due to accidental or avoidable causes. Understanding these causes can help load cell users to avoid or reduce the likelihood of load cell failures in their applications, limiting the amount of costly service calls, production line downtime, product giveaways, and other inefficiencies.
DEFINITION: Overloading is caused when the load applied exceeds the rated capacity of the load cell. This can lead to irreversible damage to the load cell, permanently compromising weighing performance
CAUSES:
EXPLANATION:
AVOIDING OVERLOAD:
DEFINITION: A shock load is a sudden and drastic increase of load that can damage the load cell through overloading.
MECHANICS:
A shock load typically occurs when material is dropped uncontrollably onto the weighing platform or when there is a sudden, forceful impact. This sudden increase in force, even if it lasts only for a fraction of a second, can cause irreversible damage to the load cell. The mechanical structure of the load cell may deform permanently, and components such as strain gauges can exceed their elastic limits, leading to failure.
EXPLANATION:
Load cells operate using an analogue signal generated by a constant electrical circuit. During a shock load, the abrupt and extreme force causes a significant, instantaneous change in the resistance of the strain gauges within the Wheatstone bridge circuit. This can lead to a permanent imbalance in the circuit, compromising the load cell’s accuracy and functionality. Even a brief instance of shock loading can result in lasting electrical and mechanical damage to the load cell.
AVOIDING SHOCK LOAD:
DEFINITION: Surge damage refers to the harm caused to load cells due to sudden, excessive electrical voltage or current that exceeds the load cell’s designed limits, leading to malfunction.
COMMON CAUSES:
EXPLANATION: Surges of electricity, whether from lightning or welding, can pass through metal structures. When these surges travel through the load cell, they can cause significant electrical damage to the internal components. Surges that travel through the cables connecting the load cell can introduce high voltage directly into the strain gauges and the Wheatstone bridge circuit, leading to immediate damage.
AVOIDING SURGE DAMAGE:
DEFINITION: Moisture damage in a load cell refers to the adverse effects caused by the ingress of water or high humidity levels, which can compromise the electrical, mechanical, and structural integrity of the load cell, leading to inaccurate measurements and potential failure.
COMMON CAUSES:
EXPLANATION: Electrical circuits are vulnerable to moisture damage that can corrode sensitive components and cause shorts. Strain gauges are especially susceptible to moisture damage, as the thin films can easily have their resistive properties and adhesion compromised. ANYLOAD utilizes pressurized water chamber and humidity chamber testing to ensure that our load cells can endure in harsh environments by simulating extreme moisture exposure for extended periods.
AVOIDING SURGE DAMAGE:
DEFINITION: Cable damage in a load cell refers to any physical or chemical harm to the electrical cables that receives excitation and outputs signal.
COMMON CAUSES:
EXPLANATION: Cable damage in load cells occurs when the cables transmitting signals between the load cell and instrumentation are compromised, leading to inaccurate or lost data. This damage can result from mishandling, rodent activity, exposure to corrosive chemicals, environmental degradation, or physical impacts like pinching, crushing, cuts, and abrasion. Such damage disrupts the electrical pathways, causing signal interference, increased resistance, and potential short circuits.
AVOIDING SURGE DAMAGE:
DEFINITION: Fatigue damage in a load cell refers to the progressive and irreversible deterioration of the load cell’s structural and functional components due to repeated cyclic loading over time, leading to a loss of accuracy and eventual failure.
COMMON CAUSES:
EXPLANATION: Fatigue damage occurs when a load cell is subjected to repeated cycles of loading and unloading. This cyclic stress can lead to the gradual loss of mechanical elasticity or deformational linearity of the load cell element, compromising its accuracy over time. Most modern load cells from reputable manufacturers are designed to withstand over 5 million cycles at full capacity without significant performance degradation.
Vibrations induce high-frequency cyclic loading on the load cell. Each cycle of vibration imposes stress reversals on the load cell material. Even if these stresses are below the material’s yield strength, repeated application can initiate dislocation movements at the atomic level, leading to the initiation and growth of macrostructural changes over time. These result in the progressive degradation of the material’s mechanical properties, leading to signs of fatigue failure, including increased signal drift, hysteresis, and nonlinearity in measurements.
AVOIDING SURGE DAMAGE:
DEFINITION: Thermal damage in a load cell refers to the adverse effects caused by exposure to extreme high or low temperatures, typically below -40°C (-40°F) or above 70°C (158°F), which can compromise the load cell’s structural integrity, electrical performance, and overall accuracy.
COMMON CAUSES:
Operation in environments with extreme ambient temperatures can lead to thermal damage, especially to the sensitive electronic components inside a load cell.
EXPLANATION: Thermal damage occurs when a load cell is exposed to temperatures outside its specified range, affecting both mechanical and electrical components. High temperatures can cause thermal expansion, reducing material elasticity and leading to deformation, while low temperatures can make materials brittle. Strain gauges within the load cell are particularly sensitive to temperature changes, which can alter their resistance and cause inaccurate readings.
AVOIDING SURGE DAMAGE:
Load cells from reputable manufacturers are very unlikely to have defect failures as a result of rigorous production and quality assurance procedures. Failure rates ‘out-of-box’ can be as high as 1-2% for some suppliers, while ANYLOAD tries to adhere to a <0.01% failure rate.
DEFINITION: Component failures due to manufacturing defects refer to imperfections or faults in the various parts of a load cell, arising from errors during the manufacturing process, which can lead to compromised performance and accuracy.
COMMON CAUSES:
EXPLANATION: Component failures in load cells due to manufacturing defects can significantly impact their performance and accuracy. Material impurities can introduce inconsistencies in electrical properties, leading to erratic or inaccurate readings. These defects can lead to issues such as signal drift, increased hysteresis, nonlinearity, and overall unreliability in load cell outputs, compromising the accuracy of the measurements.
AVOIDING SURGE DAMAGE:
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