Detailed Introduction to Truck Crane Load Moment Indicator (LMI)

The Moment Limiter/Load Moment Indicator (LMI) is the most critical safety protection device on truck-mounted cranes, hailed as the crane's “brain” and “safety guardian.” Its core function is to monitor lifting operations in real time, preventing overturn accidents caused by unsafe practices such as overloading or exceeding the crane's reach.

I. Core Functions

The primary functions of the moment limiter can be summarized as: monitoring, calculating, comparing, warning, and protecting.

1. Overload Protection (Core Function)

Principle: The fundamental cause of crane overturn is when the “overturning moment” exceeds the crane's “stability moment.” Overturning Moment = Lifted Load × Working Radius (horizontal distance from boom center of gravity to outrigger centerline). The Moment Limiter continuously calculates the actual moment under current operating conditions and compares it with the pre-programmed rated moment (safety limit) stored in the system.

Action: When the actual moment reaches 90%-100% of the rated moment, the system issues an audible and visual warning. Upon reaching or exceeding 100%, the system immediately outputs control signals to automatically halt movements in the hazardous direction (e.g., hook lifting, boom lowering) while permitting operations in the safe direction (e.g., boom raising, hook lowering). This forces cessation of dangerous operations to prevent accidents.

2. Working Radius Monitoring and Limitation

Using angle and length sensors, the system precisely calculates the working radius from the hook's current position to the turntable center. For any given boom length and angle, the system corresponds to a maximum safe lifting capacity.

3. Human-Machine Interaction and Display

The in-cab instrument panel provides operators with real-time critical information, typically including:

Actual Lifting Capacity / Rated Lifting Capacity / Actual Working Radius / Actual Moment / Rated Moment (usually displayed as a percentage, e.g., 85%) / Boom Length / Boom Angle / Operating Condition Information / Warning and Alarm Information

4. Data Logging and Black Box Functionality

The moment limiter continuously records operational data, including time, weight, radius, moment percentage, and operating actions. Following accidents or violations, this data can be retrieved and analyzed for root cause investigation and liability determination.

5. System Self-Diagnostic Function

During startup or operation, the system performs self-checks on key sensors and components. Upon detecting faults (e.g., sensor damage, circuit breakage), it immediately alerts the operator with a fault warning, prompting maintenance to prevent system operation in a failed state.

II. Primary Components

The moment limiter is a mechatronic system primarily composed of three major parts: hardware sensors, a central processing unit, and an actuation output mechanism.

1. Main Processor (Host/Black Box)

Function: Serves as the system's computational core. It receives signals from all sensors, performs high-speed calculations based on built-in load charts and algorithms to determine the current torque percentage, and makes logical judgments. Internally stores load charts for various boom lengths, radii, and operating conditions, acting as the system's “knowledge base.”

Installation Location: In some load moment limiters, the monitor integrates the main processor's functions and is installed in the crane operator's cab.

2. Length Sensor

Function: Continuously detects the extended length of the crane boom.

Installation Location: Mounted on the crane boom.

3. Angle Sensor

Function: Continuously detects the angle between the boom and the horizontal plane (elevation angle).

Installation Location: Typically mounted on the boom, often integrated with the length sensor.

4. Pressure Sensor

Function: Indirectly calculates the actual weight on the hook by monitoring hydraulic pressure in the hoist winch system. Pressure is linearly proportional to load weight.

Installation Location: Mounted on the hoist winch motor's inlet or return oil line.

5. Anti Two-Block Switch

Function: Prevents the hook from over-hoisting and colliding with the boom tip. Automatically cuts off the hoisting action when the hook reaches its maximum height.

Installation Location: The very front end of the boom.

6. Display Unit and Control Panel

Function: The system's human-machine interface. Typically a liquid crystal display (LCD) screen, it shows all real-time data, warnings, and alarm information. Operators can also use it to select operating modes, set parameters (password required), and perform other operations.

Installation Location: Mounted in the most visible position within the operator's cabin for easy monitoring.

7. Control Output Unit (Relays/Solenoid Valves)

Function: The system's “execution mechanism.” When the host detects a hazardous condition, it outputs electrical signals to control relay activation/deactivation. This in turn de-energizes hydraulic solenoid valves governing dangerous operations (e.g., hoisting, boom extension), halting the action or restricting movement to safe directions only.

Installation Location: Typically integrated within the main processor.

8. Alarm Device

Function: Provides audible (buzzer) and visual (LED warning lights) warnings and alarms to promptly alert the operator.

Installation Location: Inside the operator's cab or in front of it, in a location where warning signals are easily visible.

Summary

The truck crane's load moment indicator (lmi) collects physical signals such as boom length, angle, and pressure through sensor arrays (eyes and ears). These signals are transmitted to the main processor (brain) for high-speed computation and logical judgment. Results and information are then communicated to the operator via the display unit (mouth). During hazardous situations, the control unit (hands and feet) directly intervenes in operations, forming a complete closed-loop monitoring-alert-protection system.

It is an indispensable critical device for ensuring safe crane operations and protecting equipment, personnel, and property. All operators must fully understand its importance and must never arbitrarily disable or tamper with its functions.