Load cells, as the core measurement components of weighing systems, have their measurement accuracy not only dependent on the performance parameters of the sensor itself but also closely related to the details of the installation process. Among these, the selection of screw strength is a crucial factor that is often overlooked. The strength of the screws directly affects the stability of the connection between the sensor and the mounting base, the uniformity of force transmission, and consequently has a chain effect on the accuracy, repeatability, and service life of the measurement results. This article will analyze the specific impact of screw strength on measurement, the pros and cons of different strength selections, and the standardized selection criteria from three aspects.
1. Core Impact Mechanism of Installation Screw Strength on Load Cell Measurement
The measurement principle of load cells is based on the strain effect of elastic bodies. When external forces act on the sensor, the elastic body undergoes slight deformation, and the strain gauge converts this deformation into an electrical signal, which is ultimately output as weight data by the instrument. The installation screws serve as the “connecting bridge” between the sensor and the base, and their strength determines whether the connection point can stably bear the load and whether the external force can be uniformly transmitted to the elastic body. The specific impacts are reflected in three dimensions:
(1) Uniformity of Force Transmission
In an ideal state, the load should act vertically and uniformly on the force center of the sensor’s elastic body, at which point the deformation signal collected by the strain gauge accurately reflects the load magnitude. If the screw strength is insufficient, during the application of rated or impact loads, the screws may undergo slight elastic deformation or even plastic deformation, leading to “loose gaps” in the connection between the sensor and the base. This gap disrupts the force balance, causing the load transmission path to shift, and part of the load may be converted into lateral forces or torques acting on the elastic body, resulting in distorted strain gauge signals, ultimately manifesting as measurement results that are “too light” or “too heavy,” with errors increasing as the load increases.
(2) Connection Stability and Repeatability
The “repeatability” of load measurement (the consistency of multiple measurements of the same load) is a key indicator of weighing performance, and screw strength is central to ensuring repeatability. When the screw strength is below the operational requirements, during each loading and unloading process, the screws will undergo slight deformation due to the applied force, leading to subtle changes in the installation position and force angle of the sensor. Even with the same load, the deformation state of the elastic body may differ during different measurements, resulting in decreased measurement repeatability. For example, in industrial batching systems, if the screw strength is insufficient, cumulative errors after multiple batchings can significantly increase, severely affecting product quality.
(3) Long-term Reliability and Safety
From a long-term usage perspective, insufficient screw strength can lead to the risk of “fatigue failure.” Weighing equipment frequently endures load cycles (loading – unloading) during daily operation. If the screws remain in a “super-stress” state for an extended period, fatigue cracks may gradually develop, ultimately leading to screw breakage. On one hand, screw breakage can directly cause the sensor to fall off, resulting in equipment damage or safety accidents; on the other hand, in the “sub-healthy” state before breakage, the deformation of the screws will continue to exacerbate measurement errors, which are unpredictable and cannot be corrected through calibration.
2. Pros and Cons Analysis of Different Strength Screws
The strength of installation screws is typically categorized by “tensile strength,” “yield strength,” and corresponding material grades (such as ISO standard grades 8.8, 10.9). The advantages and disadvantages of screws of different strength grades in weighing scenarios are significant and must be selected based on specific operational conditions.
(1) Low Strength Screws (≤ Grade 6.8, such as Grade 4.8, 5.8)
Low strength screws are often made from low carbon steel (such as Q235), with tensile strengths typically ranging from 400-600 MPa and yield strengths from 240-480 MPa. Their advantages and limitations are quite pronounced:
- Advantages Low cost, easy to process, suitable for temporary installations under light loads and static conditions. For example, a small laboratory scale (maximum capacity ≤ 50 kg) used only for light loads and low-frequency measurements can meet basic connection needs with low strength screws, and during installation, it is less likely to cause deformation of the sensor base due to over-tightening.
- Disadvantages Firstly, poor resistance to deformation, easily leading to plastic deformation when subjected to loads above 50% of the rated load, resulting in loose connections with measurement errors of up to 2%-5%; secondly, short fatigue life, with fatigue cracks typically appearing within 3-6 months in frequently loaded scenarios (such as conveyor belt scales), requiring frequent replacements; finally, insufficient safety, as screws are prone to breakage under unexpected impact loads exceeding 1.2 times the rated load (such as material drops), leading to sensor damage.
(2) Medium Strength Screws (Grade 8.8, Mainstream General Grade)
Grade 8.8 screws are made from medium carbon steel (such as 45 steel) and are treated through quenching and tempering, with tensile strengths ≥ 800 MPa and yield strengths ≥ 640 MPa. This grade is currently the most widely used in weighing equipment, with clear advantages in performance balance:
- Advantages Firstly, precise deformation control, producing only elastic deformation within the rated load range, fully recovering after unloading, ensuring stability in force transmission, with measurement errors controllable within 0.1%-0.3%; secondly, long fatigue life, with a service life of 3-5 years under load cycles ≤ 10^6 (such as in electronic scales in shopping malls and industrial platform scales), requiring no frequent maintenance; thirdly, strong adaptability, meeting the needs of most weighing scenarios (capacity 100 kg – 50 t), with costs between low and high strength, offering excellent cost-performance ratio.
- Disadvantages There are limitations under extreme conditions. For example, in high-temperature environments (≥ 150℃), the yield strength of screws decreases with increasing temperature, potentially leading to “stress relaxation”; in ultra-large load scenarios (≥ 100 t) such as truck scales and platform scales, there remains a risk of fatigue failure if subjected to full loads for extended periods.
(3) High Strength Screws (≥ Grade 10.9, such as Grade 10.9, 12.9)
High strength screws are made from alloy structural steel (such as 40Cr, 35CrMo), with tensile strengths ≥ 1000 MPa (Grade 12.9 can reach 1200 MPa) and yield strengths ≥ 900 MPa (Grade 12.9 ≥ 1080 MPa), suitable for extreme loads or special conditions:
- Advantages Firstly, strong overload resistance, capable of withstanding short-term impact loads of 1.5 times the rated load without easily undergoing plastic deformation, suitable for ultra-large load scenarios (≥ 100 t) such as truck scales and railway scales; secondly, good high-temperature stability, with Grade 10.9 screws having a strength degradation rate of ≤ 10% in environments below 200℃, suitable for high-temperature weighing scenarios in metallurgy and chemical industries; thirdly, high long-term reliability, with fatigue life reaching 5-8 years under full load continuous operation, significantly reducing maintenance costs.
- Disadvantages Firstly, higher cost, approximately 2-3 times that of Grade 8.8 screws, increasing overall equipment costs; secondly, strict installation requirements, as high strength screws have high hardness and brittleness, improper torque control during installation (over-tightening) can easily lead to screw breakage or damage to the sensor mounting holes; finally, they require high-strength bases, as insufficient strength of the base material (such as ordinary cast iron) can lead to damage of the base threads due to the high strength of the screws, adversely affecting connection stability.
3. Strength Selection Standards for Load Cell Installation Screws
The selection of screw strength must adhere to the “matching principle”—the screw strength must match the sensor capacity, operational load, installation environment, and base material, while also referencing international standards and industry norms to avoid “over-strength” or “under-strength”. The following are specific selection standards:
(1) Selection Based on Sensor Capacity and Load Characteristics
The sensor capacity directly determines the basic load that the installation screws must bear, and the actual force on the screws must be calculated in conjunction with the “safety factor” (usually taken as 1.2-1.5) to select the appropriate strength grade:
- Small Capacity Sensors (≤ 1t) For static, light load scenarios (such as laboratory balances or small platform scales), Grade 8.8 screws can meet the requirements; if there are slight impacts (such as weighing on a packaging line), upgrading to Grade 10.9 screws is advisable to avoid deformation of the screws due to impact loads.
- Medium Capacity Sensors (1t-50t) The mainstream choice is Grade 8.8 screws; if the operational condition involves continuous full load (such as weighing modules for storage tanks), the safety factor should be increased to 1.5, at which point Grade 10.9 screws are recommended to ensure long-term stability.
- Large Capacity Sensors (≥ 50t) Grade 10.9 screws or higher must be used, and high-strength installation bases (such as Q345 steel) are required to avoid damage to the base due to mismatched screw and base strengths. For example, installation screws for truck scale sensors commonly use Grade 10.9, while some ultra-large platform scales (≥ 200t) may use Grade 12.9 screws.
(2) Selection Based on Operational Environment
Environmental factors can affect the actual load-bearing capacity of screws, which must be additionally considered in strength selection:
- High-Temperature Environments (≥ 150℃) Ordinary Grade 8.8 screws show significant strength degradation at high temperatures, requiring the use of high-temperature resistant Grade 10.9 screws (such as those made from 35CrMo material), or the installation of thermal insulation pads between the screws and the base to reduce the impact of temperature on strength.
- Moist / Corrosive Environments (such as food processing, chemicals) In addition to strength, “corrosion resistance” must be considered, for example, selecting Grade 10.9 stainless steel screws (such as A4-80, A4-100), which have tensile strengths comparable to Grade 10.9 carbon steel and excellent corrosion resistance, preventing strength degradation due to rust.
- Vibration / Impact Environments (such as mining, logistics sorting) Grade 10.9 screws or higher should be selected, along with anti-loosening measures (such as spring washers, thread adhesives), utilizing the deformation resistance of high-strength screws to counteract the loosening risks posed by vibrations.
(3) Selection Based on International and Industry Standards
The selection of screw strength must comply with relevant standards to ensure measurement compliance and safety:
- International Standards Follow ISO 898-1 “Mechanical Properties of Fasteners Part 1: Bolts, Screws, and Studs,” which specifies the tensile strength, yield strength, and hardness requirements for Grades 8.8, 10.9, and 12.9 screws. When selecting, ensure that the screws meet the testing requirements of this standard.
- Industry Norms For example, the OIML R60 “Non-Automatic Weighing Instruments” standard requires that the installation components of weighing equipment possess “sufficient mechanical strength to ensure no permanent deformation under rated loads,” thus the screw strength must meet the “load cycle testing” requirements in this specification (usually 1.2 times the rated load cycled 1000 times without deformation).
- Manufacturer Requirements Most sensor manufacturers clearly recommend screw strength grades in their product manuals. For instance, brands like Mettler-Toledo and HBM generally recommend Grade 8.8 screws for medium capacity sensors, while Grade 10.9 screws are recommended for large capacity sensors. When selecting, prioritize manufacturer recommendations to avoid warranty voiding due to mismatched screws.
(4) Control of Installation Torque
The screw strength must match the installation torque; even when using high-strength screws, improper torque can still affect measurement accuracy:
- Insufficient Torque If screws are not fully tightened, gaps exist, leading to uneven force transmission and increased errors;
- Excessive Torque If the torque exceeds the yield strength of the screws, it can lead to plastic deformation or breakage of the screws, and may also compress the sensor base, causing the elastic body to undergo pre-deformation, resulting in “zero drift.”
Typically, manufacturers provide torque reference tables. For example, for M12 Grade 8.8 screws, the recommended torque is 55-65 N·m; for M12 Grade 10.9 screws, the recommended torque is 80-90 N·m. During installation, a torque wrench should be used to strictly adhere to the recommended values.
4. Conclusion
The strength of installation screws for load cells is not a matter of “the higher, the better,” but rather must be based on “precise matching”—low strength screws are suitable for light loads and static simple scenarios, but their stability and safety are insufficient; medium strength Grade 8.8 screws, with their balanced performance, become the first choice for most weighing scenarios; high strength screws are suitable for ultra-large loads and extreme environments, but they come with higher costs and installation requirements. In actual selection, it is essential to comprehensively consider the sensor capacity, operational load, environmental conditions, and manufacturer standards, while also ensuring precise torque control to ensure that the screws provide stable connection without interfering with the measurement accuracy of the sensor, ultimately ensuring the long-term reliable operation of the weighing system.