A heavy component accidentally falls onto a weighing platform. Afterwards, the scale is unloaded and the indication no longer returns exactly to zero, but instead shows, for example, +0.6 kg. After taring, everything initially appears to function normally again. However, with a known test load of 80 kg, the scale indicates only 78.9 kg. Has only the zero point shifted, or has the load cell been permanently affected by the shock load?
This distinction is crucial after a shock overload. A load cell may appear completely undamaged externally and still provide a stable electrical signal even though the elastic measuring body, strain gauges or mechanical force transmission have changed. Simply resetting the display to zero is therefore not sufficient to confirm proper measurement performance. Zero point, characteristic curve, repeatability and behavior during loading and unloading must be checked under defined conditions.
The surrounding mechanics must also be included in the inspection. An existing overload stop may have protected the sensor during the impact. However, it may also have shifted or deformed as a result of the event. If the stop subsequently contacts the platform within the normal measuring range, a force shunt is created: part of the load bypasses the sensor and the scale indicates too little.
After a shock overload, a load cell should therefore never be assessed solely on the basis of its zero point. Only the combination of zero return, characteristic-curve testing, repeatability and inspection of the mechanical force path shows whether the measuring point can still be used reliably.
What happens during a shock overload?
A load cell is designed for a defined force range. Within this range, the measuring body deforms elastically and largely returns to its original state after the load is removed. With a shock load, however, it is not only the static mass that matters. If, for example, a workpiece is dropped onto a platform, the briefly acting peak force can be significantly higher than the gravitational force of the workpiece when stationary.
The actual magnitude of this dynamic load depends, among other things, on drop height, mass, platform stiffness, load-cell deflection and damping of the mechanical system. A load cell can therefore be subjected to a much higher load by an apparently harmless impact than by the same mass being placed slowly onto the platform. If the load exceeds the permissible limits of the sensor, a permanent change may occur.
| Observation after the event | Possible cause | Assessment |
|---|---|---|
| Zero point permanently shifted | Permanent deformation, mechanical stress or changed force transmission | Further characteristic-curve testing required |
| Zero point correct, full-scale value incorrect | Sensitivity or rated output changed | Perform multi-point test |
| Medium loads correct, high loads read too low | Overload stop engages too early | Check overload protection and force shunt |
| Values differ during loading and unloading | Hysteresis, friction or mechanical stress | Investigate mechanics and sensor together |
| Measured values vary between repeated loads | Mechanical damage, loose connection or unstable force application | Check repeatability |
Why is the zero point the first important check?
After complete unloading, the first step should be to check whether the load cell or weighing system reproducibly returns to its original zero signal. It is important not to use the tare or zero function of the weighing electronics immediately. Doing so would hide exactly the change that may provide an indication of overload.
A changed zero point can originate directly from the sensor, but it can also be caused by the mechanics. A deformed platform, shifted support, stressed screw connection or a stop that remains in contact after the impact can apply a residual force to the load cell. It should therefore be observed whether the offset remains stable, changes slowly over time or changes when the mechanics are moved or relieved.
Even a zero point that initially appears normal does not prove that the load cell is undamaged. A change in sensitivity can mean that the cell still looks correct at zero but increasingly deviates under load. The zero point is therefore only the beginning, not the end, of the inspection.
Why must the entire characteristic curve then be tested?
After checking the zero point, the load cell should be tested with several known loads over as much of the intended working range as possible. Testing only at zero and at nominal load may overlook certain fault patterns. A changed characteristic curve may, for example, still appear plausible in the lower range and only deviate noticeably at higher loads.
For a platform scale with a working range of 100 kg, for example, a test could be carried out at 0, 20, 40, 60, 80 and 100 kg. What matters is not only the absolute deviation at each point, but also whether the deviation develops proportionally with load or suddenly becomes greater above a certain range.
An increasingly low indication in the upper measuring range is particularly interesting. It can result from a changed load-cell characteristic curve, but also from a mechanical stop that starts making contact above a certain platform deflection and carries part of the additional load.
Assessing hysteresis and repeatability after overload
For a complete assessment, the same characteristic curve should then be run in the decreasing direction. The load is therefore increased in steps and then removed again in the same steps. If the measured values at the same load differ depending on whether the point is approached from below or from above, a hysteresis effect is present.
After a shock overload, increased hysteresis can originate both in the sensor and in the mechanics. Rubbing stops, shifted joints, stressed mounting points or a deformed platform can behave differently during loading and unloading. Several complete load cycles should therefore also be carried out. If, for example, the value at 60 kg changes noticeably with each cycle, repeatability is impaired and a simple calibration factor is not an adequate solution.
| Test load | Increasing load | Decreasing load | Interpretation |
|---|---|---|---|
0 kg |
+0.0 kg |
+0.2 kg |
Observe zero return |
20 kg |
20.0 kg |
20.1 kg |
Unremarkable |
40 kg |
39.9 kg |
40.1 kg |
Small hysteresis |
60 kg |
59.8 kg |
60.0 kg |
Continue monitoring |
80 kg |
78.9 kg |
79.2 kg |
Noticeable non-linearity; check mechanical stop |
100 kg |
96.7 kg |
97.1 kg |
Do not simply readjust the measuring point |
Checking the mechanical overload stop
A mechanical overload stop is intended to prevent the load cell from deflecting beyond its permissible travel when the load becomes too high. Within the normal measuring range, there is a defined clearance between the stop and the moving weighing structure. Only in the event of excessive movement should the stop make contact and create an additional force path.
After a shock load, it must therefore be checked whether this clearance is still present. The stop itself may have settled, shifted or bent. The base plate or weighing platform may also have been permanently deformed. This can reduce the originally correct gap or eliminate it completely.
The correct clearance cannot be specified universally for every load cell. It must match the deflection of the specific sensor and the deformation of the complete mechanical structure. If the gap is too large, the load cell may not be protected in time; if it is too small, the normal measuring range may already be affected.
How an incorrectly adjusted stop causes measurement errors
If the stop touches the platform, a parallel force path is created. The applied load is then no longer transmitted entirely through the load cell into the base structure. Part of the load is transferred directly through the stop. As a result, the load cell sees only part of the actual force and the indication becomes too low.
A typical symptom is that the measurement initially appears correct and then increasingly deviates downward above a certain load. This fault pattern can easily be interpreted as damaged sensor linearity. The mechanics should therefore be observed at the same time as the electrical measurement. It is particularly important to determine whether the stop makes contact at exactly the point where the characteristic curve begins to change.
Contamination can cause a similar effect. Metal chips, product residues or other foreign objects between the platform and stop can bridge the intended clearance. After an impact, not only the adjustment but also the entire stop area should therefore be cleaned and inspected.
Test the load cell installed or removed?
Testing in the installed condition evaluates the complete weighing system and is therefore particularly important for practical operation. If the system shows an error in this condition, however, it is not yet clear whether the load cell itself is the cause. Mechanical stress, frame deformation, stops, cables or bearings can produce the same deviation.
If possible, an additional test of the load cell outside the machine can help. If the sensor operates correctly in a defined test setup while the installed scale deviates, this strongly indicates a mechanical influence from the system. If zero-point shift or characteristic-curve errors remain under controlled force application, a change in the sensor itself becomes more likely.
This distinction is especially important before deciding on readjustment. An electronic correction may compensate for a constant rated-output error under certain circumstances, but it cannot eliminate a contacting stop, poor repeatability or mechanical hysteresis.
Systematic test procedure after shock loading
- Completely unload the weighing system: First document the zero point without taring again.
- Carry out a visual inspection: Check the load cell, platform, screws, force application and cable for visible damage.
- Check the overload stop: Inspect free clearance, mounting, deformation and possible contamination.
- Observe zero return: Check whether the value remains stable or changes over time after unloading.
- Perform a multi-point test: Apply several known loads across the entire relevant working range.
- Reduce the load again: Record the same points in decreasing direction and check for hysteresis.
- Repeat the test several times: Compare repeatability of zero point and characteristic curve.
- If abnormalities remain, separate sensor and mechanics: Where possible, test the load cell separately under defined conditions.
- Only then decide on adjustment: Mechanical or sensor damage must not simply be hidden by calibration parameters.
Practical example: 100 kg platform scale after shock loading
An industrial platform scale has a working range of 0 ... 100 kg. During production, a heavy tool falls onto the platform. After the event, the unloaded scale initially indicates +0.6 kg. The operator tares the display and a 20 kg test load is then indicated correctly. At 80 kg, however, the indication is already more than one kilogram too low.
A multi-point test shows that the deviation remains small up to approximately 50 kg and then becomes increasingly larger. The load cell is not recalibrated immediately. Instead, the mechanics are observed. It becomes apparent that the adjustable overload stop shifted slightly due to the impact and starts touching the underside of the platform at around 55 kg.
Part of the additional load is therefore transferred directly into the base frame. After correctly readjusting the stop, the complete characteristic curve is tested again. If zero point, linearity and repeatability are then correct again, the main cause was mechanical. If the deviation remains despite a free force path, the load cell itself must be investigated further or replaced.
This example shows why a shock overload does not automatically mean that the sensor is defective – but also why simply taring the system after the event is not an adequate functional test.
When is recalibration no longer sufficient?
A calibration determines the actual behavior of the measuring point. A subsequent adjustment can only meaningfully correct a changed zero point or rated output if the system continues to operate stably, linearly and reproducibly. If the load cell shows pronounced non-linearity, increased hysteresis or strongly varying values between repeated load cycles, the cause should first be investigated technically.
A permanent change immediately after a known overload is particularly critical. If only a new calibration factor is stored, the scale may once again indicate correctly at one individual test point while still showing significant errors at other points. Progressive mechanical damage also cannot be reliably corrected electronically.
For quality- or safety-relevant applications, the permissible tolerances and test specifications should therefore be used after a significant shock load to decide whether the load cell may remain in service, should be recalibrated or must be replaced.
Common diagnostic mistakes
- Taring immediately after the impact: Important information about a possible zero-point shift is lost.
- Using only one test load: Non-linearity or an overload stop engaging too early may remain undetected.
- Suspecting only the load cell: Platform, mounting and overload stop can produce the same fault patterns.
- Allowing the stop to contact within the normal measuring range: This creates a force shunt and causes load-dependent measurement errors.
- Testing only with increasing load: Hysteresis and mechanical friction may remain hidden.
- Trying to correct poor repeatability by calibration: Scattered measured values cannot be eliminated with a fixed correction factor.
- Treating dynamic shock load as equivalent to static weight: The briefly acting peak force may be significantly higher than the stationary gravitational force.
Load cells and force transducers for industrial applications
Different load-cell and force-transducer designs are available for platform scales, machines, test benches and industrial force measurements. In addition to the rated measuring range, important selection criteria include the permissible load direction, mechanical installation conditions, available deflection, possible lateral forces and required overload protection.
For platform applications, the WIKA model F4801 platform load cell is one suitable option. It can be installed directly beneath a weighing platform and is available for different weighing ranges. In applications where shock loads may occur, the mechanical design should also ensure that impermissible overload of the load cell is prevented as far as possible.
Suitable load cells, force transducers and other sensors can be found under force, weighing and displacement measurement technology at ICS Schneider. Information about the load cell used as an example here can be found under WIKA model F4801 platform load cell.
Conclusion
A shock overload can permanently alter a load cell without any visible external damage. The zero point is therefore an important first indication, but it is not sufficient for a reliable assessment. Even a cell with a correct zero signal can have altered sensitivity or a changed characteristic curve under load.
The test should therefore include several known load points across the relevant measuring range and then be repeated in the decreasing direction. This allows linearity deviations, hysteresis and poor repeatability to be identified much more reliably than with a single test weight.
The mechanics are equally important. A deformed or incorrectly adjusted overload stop can create a force shunt above a certain load and therefore produce exactly the same fault pattern as a non-linear load cell. The sensor and the mechanical force path must therefore be assessed together.
For inspection after a shock overload, the following therefore applies: document the zero point before taring, check the complete characteristic curve during loading and unloading, compare several cycles and inspect the mechanical overload stop for sufficient free travel and unintended contact. Only then should a decision be made about recalibration or replacement of the load cell.
FAQ: Checking a load cell after shock overload
Can a load cell be damaged by a brief impact?
Yes. During a shock load, the brief peak force can be significantly higher than the static gravitational force. The permissible overload of the load cell can therefore be exceeded even though the applied mass itself is within the normal weighing range.
How can you recognize an overloaded load cell?
Possible indications include a permanently shifted zero point, changed sensitivity, non-linearity, increased hysteresis or reduced repeatability. However, these symptoms can also be caused by the surrounding mechanical structure.
Is it sufficient to tare the scale again after an overload?
No. Taring only hides a zero-point offset. Whether the load cell still measures correctly under load can only be determined using known test loads.
How do you check the linearity of a load cell?
Several known loads are applied across the relevant measuring range and the indicated values are compared with the reference values. A test in several steps from zero to the maximum intended test load is useful.
Why should the load cell also be tested with decreasing load after an overload?
This makes it possible to identify whether different values occur at the same test load depending on the loading direction. Such hysteresis can originate from the sensor or from friction and mechanical stress in the installation.
What is a mechanical overload stop on a load cell?
An overload stop limits the movement of the weighing platform or force-introduction component. A free gap exists during the normal measuring range. Only if deflection becomes excessive does the stop take over part of the load and protect the load cell.
Why must the overload stop not make contact within the normal measuring range?
If contact occurs, an additional force path bypasses the sensor. The load cell then no longer measures the complete load and the indication typically becomes too low or non-linear.
Can an overload stop be shifted by a shock load?
Yes. The stop, platform, base plate or mounting can change as a result of a strong impact. The clearance should therefore be checked after a known overload event.
When should a load cell be replaced after overload?
If zero point, characteristic curve, hysteresis or repeatability lie outside the permissible requirements and mechanical causes have been ruled out, the load cell should not simply be electronically readjusted. Continued use must be assessed according to the requirements of the specific application.
Which load cell is suitable for an industrial platform scale?
For platform applications, the WIKA model F4801 platform load cell is one suitable option. The specific measuring range and mechanical overload protection must be matched to the platform and the maximum loads that may occur.