Le crane load moment indicator (also knows as crane safe load indicator,referred to as “grue LMI“) is a mandatory safety device required by GB 6067.1-2010, and the quality of its installation directly affects the monitoring accuracy and safety control response. In accordance with “Overload Protection Devices for Lifting Machinery” (GB/T 12602-2022), the following core points must be grasped during installation:
I. Preliminary Technical and Environmental Preparation
- Parameter Matching Verification
Check the nameplate of the crane LMI against the technical documents of the crane to ensure that core parameters such as rated moment, maximum radius, and boom length classification are consistent. For luffing cranes, it is necessary to verify the adaptability between the measuring range of the radius sensor (0°-80° elevation angle) and the boom angle, with an error of ≤±0.5°.
- Electromagnetic Compatibility Handling
Detect the electromagnetic environment of the installation area to ensure that the power frequency magnetic field is ≤50Hz/100μT. The sensor cable should use shielded twisted pair (shielding layer ≥0.5mm²) with single-end grounding (grounding resistance ≤4Ω) to avoid electromagnetic coupling with the electrical control system.
II. Precision Control for Installation of Core Components
- Sensor Positioning
Weight sensor: It needs to be close to the load-bearing point of the hook group to ensure vertical force and ensure that the axial force transmission efficiency is ≥98%.
Length&Angle sensor: Positioned with a laser line projector. The coaxiality error between the rotation axis and the hinge axis of the boom is ≤1mm, and the angle measurement delay is ≤50ms.
- Mechanical Fixing Requirements
The connection between the sensor and the machine body should use fitted bolts (GB/T 27) with H7/k6 transition fit. The pre-tightening force of 8.8-grade bolts should be 60%-70% of the yield strength. Mobile cranes need to be equipped with vibration-damping pads (hardness 50±5 Shore A), and the resonance frequency should avoid the 10-50Hz vibration frequency band of construction machinery.
III. Electrical System Integration Specifications
- Wiring Design
The signal lines adopt a star topology to avoid signal attenuation. Cable laying must meet the following requirements: the distance from power cables is ≥300mm, and they should cross at 90°; the bending radius is ≥10 times the cable diameter, and the fixing spacing is ≤500mm; the plug uses an IP67-grade aviation plug with pin plating thickness ≥5μm.
- Safety Circuit Access
The relay output of the crane LMI is connected to the normally closed point of the safety control circuit to ensure that when the monitored value reaches 110% of the rated moment, the power supply for lifting and boom extension directions is cut off within ≤0.2s. The cross-sectional area of the circuit conductor is ≥1.5mm², using GB/T 5023.5 flame-retardant copper core cable.
IV. Calibration and Acceptance Requirements
- Multi-condition Calibration
In accordance with Appendix C of GB/T 12602-2022, calibrate at the minimum, medium, and maximum radius positions under 10%, 50%, and 100% rated loads. Use a 0.1-class standard dynamometer and an angle encoder with 0.01° resolution to collect data, ensuring that the deviation between the displayed value and the standard value is ≤±2%.
- Acceptance and Maintenance
After installation, the following documents must be provided: sensor 3D coordinate detection report, electrical circuit insulation resistance record (cold state ≥100MΩ, hot state ≥1MΩ), calibration curve, and error chart. In daily maintenance, use standard weights to recheck the 100% rated load point quarterly and conduct full-range calibration annually; after major repairs, it is necessary to re-verify the installation accuracy and calibration parameters.
Adhering to the above specifications can ensure that the crane LMI operates stably under working conditions of -20℃~60℃ and relative humidity ≤95% (no condensation), meeting the safety requirements of GB 50490-2009.
