How Crane Load Moment Indicator Protect Cranes and Prevent Accidents?

I. What Is a Crane Load Moment Indicator?

A crane load moment indicator is an integrated electronic safety protection system installed on cranes. Its core mission is to prevent accidents caused by crane overturning or structural damage due to overload or excessive working radius.

Its fundamental design is based on a core physics principle: Moment = Weight × Working Radius.

Weight: The mass of the load suspended from the hook.

Working Radius: Typically refers to the horizontal distance between the crane's pivot point and the vertical line of the hook (commonly known as the “working radius”).

Moment: Can be understood as the “tendency” or “effect” that causes crane overturn. The heavier the load and the greater the radius, the larger the overturning moment generated.

The moment limiter continuously monitors this “moment” value, ensuring it remains within the safe range for the crane's current operating conditions.

II. Core Components

A complete load moment indicator typically consists of three main parts:

1. Detection Device (the system's “senses”):

Load Cell: Usually employs a load cell or three-pulley weight sensor to monitor the load weight in real time.

Length/Angle Sensor: Mounted on the boom to detect its elevation angle and length, thereby calculating the actual working radius. For tower cranes, additional sensors may measure trolley travel.

Angle Sensor: Directly measures the angle between the boom and the horizontal plane.

Pressure Sensor: Indirectly calculates the load by detecting hydraulic pressure in the hoist or boom swing circuits (commonly used in truck cranes).

2. Computational Control Unit (the system's “brain”):

Receives data from all sensors.

Performs rapid calculations and logical judgments based on preset crane moment values (the control unit stores the crane's load chart).

Compares the current actual moment with the rated safe moment.

3. Actuation and Alarm Devices (the system's “hands, feet, and voice”):

Monitor: Displays critical information such as weight, boom angle, and percentage of rated load to the operator in real time.

Audible and Visual Alarm: Emits a buzzer and flashing light warning.

Output Control Unit: This is key to active protection. It can signal the crane's control system to automatically cut off operations in dangerous directions.

III. How to progressively protect cranes and prevent accidents?

The protection provided by the crane load moment indicator is layered and progressively escalating, acting like a strict “safety officer.”

Step 1: Real-time Monitoring and Display (Early Warning)

How it works: Operators in the cab can clearly view the current load weight, actual working radius, rated lifting capacity, and torque percentage (current actual torque / rated torque × 100%) on the display.

Protective function: Provides operators with a clear, quantitative understanding of the crane's load status, preventing “gut feeling” operation and proactively avoiding accidents.

Step 2: Early Warning Alarm (Level 1 Warning)

Trigger Conditions: When the moment percentage reaches 90% or 95% of the rated value (configurable).

Protective Action: The system triggers audible and visual alarms (e.g., flashing yellow warning lights and intermittent beeping) to alert the operator: “Current operation is approaching safety limits—proceed with caution!”

Purpose: To provide the operator with reaction time to halt potentially hazardous maneuvers.

Step 3: Load Limit Alarm (Level 2 Warning/Reduced Performance Operation)

Trigger Condition: When the torque percentage reaches 100% to 105% of the rated value.

Protective Action: More urgent and intense audible/visual alarms (e.g., red lights and continuous alarm sounds). In some systems, operating speed may be automatically restricted (e.g., reduced hoisting or slewing speed), but all operations will not be completely halted.

Purpose: To issue the highest level of warning, indicating the crane is operating at its limit.

Step 4: Automatic Termination of Hazardous Operations (Critical Protection)

Trigger Condition: When torque percentage exceeds 105% or 110% of rated value (configurable).

Protective Action: The system's central processing unit issues commands to the control system, automatically halting or prohibiting all operations that would further increase torque:

- Prohibit hook lifting (to prevent weight addition).

Prohibit boom lowering/reducing radius (prevents radius increase, as lowering the boom may actually increase radius and torque for certain cranes).

Prohibit boom rotation toward hazardous directions (centrifugal force also increases tipping risk).

At this point, only safe operations are permitted: e.g., lowering the hook to reduce load, raising the boom to decrease radius, or rotating toward safe directions.

Purpose: This represents the final and most critical mechanical defense. It shifts protection from “human intervention” to “technical intervention.” Even if the operator misjudges or acts recklessly, the system forcibly interrupts hazardous operations, fundamentally preventing tipping accidents.

IV. Specific Accident Types Prevented

1. Entire Crane Overturning: This represents the most severe accident. By controlling the moment within safe limits, the crane load moment indicator directly ensures the stability of the entire crane.

2. Boom Bending or Fracture: Overloading not only causes overturning but also subjects the boom structure to stresses exceeding its yield limit, leading to permanent deformation or fracture. The crane load moment indicator protects the metal structure itself.

3. Wire Rope Breakage: Overloading can directly snap the hoisting wire rope, causing the load to fall. The moment limiter indirectly safeguards the wire rope and hoisting mechanism by restricting the lifting capacity.

4. Foundation Damage: For tower cranes, excessive moments transfer to the foundation and anchor bolts, potentially causing foundation failure. The moment limiter also safeguards foundation integrity.

Summary

A crane crane load moment indicator is not merely an “alarm device”; it is an intelligent, proactive safety system integrating monitoring, calculation, early warning, and intervention. By integrating physical principles with electronic control technology, it elevates accident prevention from operator-dependent “human safeguards”—relying on individual experience and responsibility—to system-enforced “technical safeguards.” This significantly enhances the inherent safety of lifting operations, making it an indispensable “l(fā)ife-saving device” for modern cranes.