1. How do load pin load cells work?
Load pin load cells, often referred to as shear pin load cells or force sensing clevis pins, are a class of load sensor used for measuring tensile and sometimes compressive loads. They are intended to replace the shear pin in lifting shackles, thereby enabling load to be measured without increasing the size of the system or requiring special integration. They consist of strain gauges, bonded to the inside of a metal load pin (hence the name) in a Wheatstone bridge arrangement.
A strain gauge is a thin laminated foil pattern used to measure mechanical strain. Applying a force to the load cell causes the load pin to deform. This in turn stretches (in the case of a tensile force) the strain gauge, causing its length to increase and its cross sectional area to decrease, thereby increasing its electrical resistance. By measuring the change in electrical resistance, it is possible to determine the amount by which the strain gauge has stretched. Load pin load cell are suitable for measuring forces perpendicular to the load cell. Any sideways forces and moments will cause the sensor to output a ‘nonsense reading’ and may also result in damage.
2. General characteristics of load pin load cells
Load pin load cells are medium cost sensor with reasonably high measurement accuracy. They are intended to replace the existing shear pins of lifting shackles, enabling load to be measured without increasing the size of the system. They can measure a wide range of tensile and compressive loads, available with full-scale measurement ranges from 100 kg to as large as 1,000 tonnes. Load pin load cells are very compact and provide excellent ingress protection. However, their accuracy is lower than that of many other types of load cells, including S type load cells. Most load pin load cells provide a mV/V unamplified analog output, though amplified outputs are also available. There are no moving parts within the load cell and so it has a long lifetime. If the sensor is unamplified then the the process controller must include a built-in amplifier or an external amplifier must be used.
3. Input and output signals
Load pin load cells require fixed 11-30 Vdc supply voltages. The strain gauge elements are arranged in a full bridge Wheatstone bridge formation, providing temperature compensation, common noise cancellation and a high signal to noise ratio. The sensor’s output signal takes the form of millivolts per volt (of supply voltage). Most sensors have outputs in the range of 1-2 mv/V. Many load cells include built-in amplifiers that amplify the mV signal and convert it to a standard output signal such as 0-10 V or 4-20 mA.
4. Applications of load pin load cells
Load pin load cells are intended for replacing the shear pins of lifting shackles, thereby providing load measurement without increasing the size of the system. Popular applications include crane scales, hanging scales, and mechanical overload protection for cables.
5. Typical specification
|Measurement range||0-0.1 tonnes to 0-1,000 tonnes|
|Zero error||1% F.S.|
|Ambient temperature||-55 to 90 °C (up to 120 °C possible)|
|Supply voltage||1-2 mV/V, 0-10 V, (serial available but less common)|
|Output voltage||11-30 Vdc|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Output sensitivity: Unamplified load pin load cells are typically available with 1-2 mV/V output signals. Larger output signals improve noise immunity and effective resolution.
- Tension only or tension + compression: All load pin load cells are capable of measuring tension. Many but not all are also capable of measuring tension.
- Overload capacity: Load cell data sheets specify an overload capacity, typically 150-300% of rated load. This is the maximum load that can be applied without damaging the load cell.
- Natural frequency: The load cell is sensitive to mechanical vibrations (as strain gauges are also used in vibration sensors). It is therefore important to choose a sensor whose natural frequency does not overlap with the vibration frequency of the system.
- Creep: Creep effects long term temporal output stability, particularly when operating near to the maximum rated load. The majority of load pin load cells have creep induced output variations of 0.02-0.1% F.S. after 30 minutes of loading at their maximum rated load.
- Built-in amplifier: Load pin load cells are available with built in amplifiers which amplify the mV output signal of the strain gauges. Such load cells normally provide either a 0-10 V or 4-20 mA output signal.
- Mounting: Many models include a threaded end to better secure the shackle, as well as a hole through which a locking pin may be inserted.
7. Advantages of load pin load cells
Load pin load cells:
- Are compact and easy to integrate into existing systems. They can replace the shear pin of a lifting shackle so that they do not add extra length to the system.
- Provide excellent environmental protection (IP67 or IP 68)
8. Disadvantages of load pin load cells
Load pin load cells:
- Are sensitive to sideways loading (i.e. non-perpendicular loading) . The sideways loads are detected by the load cell but are measured incorrectly, possibly leading to a ‘nonsense output’.
- Are more expensive than s type load cells, which, like load cell load pins, are also relatively simple to integrate.
- Are of lower accuracy than most other types of load cells (though their accuracy is still reasonably high).
9. Application tips
- Overload capacity: When placing the load cell in tension (or compression), the inertia of the system (e.g. the inertia of a mass placed on the load cell) will cause a sudden and temporary increase in load to above the steady state load. It is important that the load cell is gradually placed into compression to avoid the instantaneous load from surpassing the overload capacity of the load cell.
- Sideways loading: Load pin load cells are sensitive to sideways loading (i.e. non-perpendicular loads). Sideways loads will be detected but measured incorrectly, leading to a nonsense output. When the load cell is properly setup, it should be comparatively simple to avoid sideways loading.
- Creep: Due to the time dependence of mechanical strain, permanent deformation of the load cell can occur, which may increase readings by up to 0.1% within 30 minutes. The creep error can be accounted for by remeasuring the zero error of the load cell. The rate of creep can be reduced by utilizing only part of the full-scale measurement range of the sensor. This will improve long term stability but will not significantly improve accuracy as the reduction in creep will be offset by a reduction in accuracy caused by not using the full measurement range.
- Lifetime: Load cells have a high lifetime, typically millions of cycles. The lifetime can be improved further by not using the entire full-scale measurement range of the sensor as well as avoiding mechanical overload.
- Mechanical vibrations: Mechanical vibrations cause periodic mechanical deformation of the load cell and are therefore detected by the strain gauge, resulting in high frequency noise. The noise amplitude can be reduced by choosing a load cell with a natural frequency far from the vibration frequencies of the system. Furthermore, the signal noise caused by mechanical vibration can be filtered by analog or digital filtering.
- Orientation: Despite their cylindrical geometry, load pin load cells have a specific orientation about their cylindrical axis, which must be adhered to for correct operation. When properly orientated, the grove on the load cell will face upwards. As well as indicating the correct orientation, the groove also provides a means to lock the load cell in its correct orientation. This is performed by inserting a keeper plate into the grove and securing the plate to the opposing side of the shackle assembly.