How Load Moment Limiters Enforce Overload Protection for Cranes
📋 Key Summary
Crane overload is a serious hazard—it can deform structural components, snap wire ropes, or even tip the entire machine over. But a crane has no eyes, so how does it know when the load is too heavy? The answer lies in load sensors paired with limiters: overhead and gantry cranes use a Lifting Capacity Limiter to measure the load, while jib and boom cranes rely on a load moment limiter (LML) that calculates load multiplied by working radius. This article explains how these overload protection systems work and how they enforce the safety red line.
📌 Quick Distinction
Lifting Capacity Limiter: Used on overhead and gantry cranes. It only measures how heavy the load is—if it exceeds the rated capacity, an alarm sounds and hoisting is cut off.
Load Moment Limiter (LML): Used on jib and boom cranes. It monitors load × working radius to prevent the overturning moment from exceeding safe limits when the radius increases.
The most non-negotiable safety red line for any crane is "no overloading." But a machine can't look at a load and sense that it's too heavy—so how does it actually know?
The answer is an overload protection system built on three steps: weighing, decision-making, and cut-off. Load sensors weigh the load, the limiter decides whether it's within limits, and if it's over, an alarm triggers and power is cut. This system is the most fundamental—and most critical—safety defense on any crane.
Here's how it works, broken down.
How Overload Protection Works: Weigh, Decide, Cut Off
Overload protection revolves around three core functions.
Weighing: Load sensors measure the actual load. Mounted on the wire rope dead end, drum support, or hook block, they convert the weight of the suspended load into an electrical signal—this is the foundation for knowing how heavy the load really is.
Decision-making: The limiter compares the measured load against the rated capacity. If the load exceeds the rated value, an alarm is triggered. The benchmark is the Rated Lifting Capacity, a red line set during the design phase.
Cut-off: Once the overload alarm sounds, the limiter interrupts the hoisting action—either allowing only lowering (no lifting), or shutting the crane down entirely to prevent the overloaded condition from continuing. These three steps—weighing, deciding, cutting off—work in sequence, and ISO 10245-5, "Limiting and indicating devices for overhead and gantry cranes," sets clear requirements for this system.
Lifting Capacity Limiters vs. Load Moment Limiters: Overhead & Gantry vs. Boom Cranes
Overload protection falls into two main categories, each matched to a specific crane type.
Lifting Capacity Limiters are used on overhead and gantry cranes. They measure only the lifting capacity, because the primary risk for these cranes is lifting too heavy a load—exceed the Rated Lifting Capacity and the limiter triggers an alarm and cuts power. The logic is simple and direct.
Load Moment Limiters (LMLs) are used on jib and boom cranes—truck cranes, tower cranes, and portal-base cranes. These devices measure the moment, which is load multiplied by working radius. The danger for boom cranes isn't just about weight; it's about radius. The greater the radius, the larger the overturning moment generated by the same load. So the LML monitors both load and radius simultaneously, calculates the moment, and alarms if the moment exceeds safe limits.
The fundamental difference lies in what each system protects against: one prevents "lifting too heavy," the other prevents "overturning from excessive moment." Kelude selects the appropriate system by crane type—overhead and gantry cranes are fitted with Lifting Capacity Limiters, while boom cranes are equipped with Load Moment Limiters.
The Safety Red Line: Rated Lifting Capacity and Safety Factor
Where does the overload protection "red line" come from? It's determined by the Rated Lifting Capacity and the safety factor.
Rated Lifting Capacity is the maximum load a crane is designed to lift, dictated by the strength of the structure and wire ropes. GB/T 28264-2017, "Safety Monitoring and Management System for Lifting Appliances," requires that overload conditions be recorded and traceable. Exceed the rated capacity and structural stress goes beyond design limits, eating into the safety margin.
The safety factor is the built-in reserve of structural strength relative to the rated load. Wire ropes and structural components are all designed with a specific safety factor. Overloading consumes that reserve—and when it's gone, you have an accident.
That's why the overload protection logic is uncompromising: exceed the rated value, and the system cuts off power, keeping the load within the safety factor. This red line isn't arbitrary—it's calculated from the actual strength of the structure and wire ropes. Kelude treats the Rated Lifting Capacity as an absolute, non-negotiable red line in its overload protection systems.
Common Overload Protection Mistakes in the Field
The first mistake is running a crane with a faulty overload protection system. When sensors drift or limiters fail, the load reading becomes inaccurate and an overload goes undetected. Overload protection devices require periodic calibration and verification.
The second mistake is bypassing the overload alarm to save time. Some operators short-circuit the overload signal to "squeeze out a bit more capacity"—effectively dismantling the safety red line. When an accident happens, it's a major one.
The third mistake is poor calibration. The accuracy of overload protection depends entirely on calibration. If it's off, the system either fails to alarm when it should, or triggers false alarms when it shouldn't. Kelude mandates periodic calibration of overload protection as a hard maintenance requirement—equipment with inaccurate calibration is not permitted to operate.
Lifting Capacity Limiter vs. Load Moment Limiter: A Comparison
| Dimension | Lifting Capacity Limiter / Load Limiter | Load moment limiter (LML) | Differentiation Point | Applicable Crane Type |
|---|---|---|---|---|
| Measured Parameter | Lifting Capacity | LoadMultiplyWorking radiusTorque | MonitoringMeasurement Discrepancy | — |
| Protection Target | Overload | overturning momentExceeding Limit | Protected Component Variation | — |
| Applicability | overhead typegantry type | Jib Crane / Boom Crane | Crane Type Variation | Type-Specific Selection |
| Complexity Level | Simple | Requires DualParameter | Complexity Variation | — |
Quick Reference of Standard Clauses for Overload Protection
| Standard | Clause Key Points | andOverload protectionRelationship with |
|---|---|---|
| ISO 10245-5 | overhead typegantry typelimiterindicator | overloadDevicerequirements |
| GB/T 28264 Safety Monitoring and Management System | safety monitoringRecord Retentionrequirements | overloadStatus Logging |
| FEM 1.001 Crane Design Standard | crane design specification | rated loadSafety factor |
Overload Protection FAQ: Load Limiters, Torque Limiters & Safety Devices
Q: What is the fundamental difference between a Lifting Capacity Limiter and a Load Moment Limiter (LML)?
A: The core difference lies in what they measure and what they prevent. A Lifting Capacity Limiter (Load Limiter) measures only the lifted load and is used on overhead-type and gantry-type cranes to prevent "lifting too heavy a load." A Load Moment Limiter (LML) calculates load multiplied by working radius and is used on jib cranes and boom cranes to prevent overturning caused by excessive torque at a given radius. In short, one is a single-parameter device guarding against overload; the other is a dual-parameter device guarding against tipping. They protect against different failure modes.
Q: How do I select the right overload protection device for my crane?
A: Selection depends on the crane type. For overhead-type and gantry-type cranes, choose a Lifting Capacity Limiter — it measures the load directly and is straightforward. For jib-type cranes (including truck cranes, tower cranes, and portal base cranes), you need a Load Moment Limiter (LML) that monitors both load and working radius. Also consider the Rated Lifting Capacity, accuracy requirements, and whether digital display and data logging are needed. The rule of thumb: overhead and gantry cranes use a Load Limiter; boom-type cranes use an LML.
Q: How can I tell if my overload protection device has failed?
A: Check three key indicators. First, load reading accuracy — verify with Standard Weights or a known load; a significant deviation means the Sensor has drifted. Second, overload alarm function — deliberately apply a slight overload to confirm the alarm triggers and the control circuit cuts off; if it doesn't, the limiter is faulty. Third, calibration validity — if the device is past its calibration date, it cannot be trusted. If any of these three signals is abnormal, stop using the crane and perform verification immediately.
Q: How is the overload protection safety red line determined?
A: It is derived from the Rated Lifting Capacity and the Safety factor. The Rated Lifting Capacity is the maximum load the crane is designed to handle, determined by the strength of the structure and Wire Rope. The Safety factor represents the reserve strength beyond the rated load. Overloading consumes this reserve, and when it's exhausted, an accident occurs. Overload protection therefore keeps the load within the rated capacity — the red line is calculated from structural and wire rope strength, not arbitrarily set.
Overload protection and Limit Switches are part of the same safety device system. For verification procedures, refer to Crane Limit Switches & Safety Device Commissioning: Hoisting Limit Switch, Travel Limit Switch & Overload Limiter Calibration Procedure.
Overload protection is the most critical safety red line on any crane. Kelude selects the right device for each crane type — Load Limiters for overhead and gantry cranes, Load Moment Limiters for boom cranes — with a three-stage approach of weighing, decision-making, and power cut-off to keep loads within the safety factor, plus periodic calibration to maintain accuracy.
