Overhead Crane Technical Guide: Structure & Safety
Bridge crane technology spans six core disciplines: structural classification, main girder design, travel mechanisms, electrical control, safety devices, and selection & installation. This article distills insights from 59 technical papers on overhead cranes, covering everything from fundamental concepts to engineering design, selection calculations, and maintenance management—building a comprehensive knowledge base for bridge crane professionals. It references key standards including ISO 4301 (Crane Design Standard), FEM 1.001 (General Purpose Bridge Cranes), and GB/T 28264 Safety Monitoring and Management System, helping practitioners quickly establish a full-spectrum understanding of bridge crane technology.
Bridge Crane Anatomy: A Visual Overview
A bridge crane—also known as an overhead crane, traveling crane, or gantry crane—is the most common lifting equipment in industrial factory buildings. Its bridge travels longitudinally along crane rails on both sides of the facility, while the hoisting trolley moves laterally across the main girder, enabling material handling in three-dimensional space. This article systematically covers bridge crane technology across six key dimensions: structural classification, main girder design, travel mechanisms, electrical control, safety standards, and selection & installation—providing a practical reference for equipment selection, design, operation, and maintenance.
Bridge Crane Types and Application Scenarios
| Type | Lifting Capacity(t) | Span(m) | Work Duty / Classification | Structural Features | Typical Applications |
|---|---|---|---|---|---|
| LD | 1~20 | 7.5~31.5 | A3~A5 | Single Girder+Electric Hoist | Light Industry Workshop, Warehouse |
| LX | 0.5~5 | 3~16 | A3~A4 | Underhung Single Girder | Assembly Line, Light-Duty Handling |
| QD | 5~320 | 10.5~34 | A5~A7 | double girder Box-Type+Trolley | General Purpose Workshop, Maintenance |
| QZ | 5~50 | 10.5~31.5 | A6~A8 | Grab (grab bucket)/electromagnetic lifting magnet | Bulk Material Handling, Scrap Steel |
| YZ | 5~200 | 10.5~34 | A7~A8 | Heat Insulation+Forced Cooling | Metallurgical, Casting |
| YH | 5~100 | 10.5~31.5 | A5~A7 | Insulated Protection Structure | Electrolysis, Electroplating |
LD Type Electric Single-Girder Crane (Lifting Capacity 1–20t, Span 7.5–31.5m): The main girder uses a combined structure of I-beam and structural steel, with the electric hoist running along the lower flange of the I-beam. Designed for light to medium intermittent material handling in applications such as machining workshops, maintenance bays, and warehouses, it offers the most cost-effective general-purpose solution.
QD Type General Purpose Bridge Crane (Lifting Capacity 5–320t, Span 10.5–34m): Featuring a box-type main girder with a trolley hoisting mechanism and a double-girder configuration, this is the most widely applicable crane type, commonly used as the primary lifting equipment in fabrication shops, assembly plants, and maintenance areas.
YZ Type Overhead Crane for Metallurgic Plants (Lifting Capacity 5–200t): Specifically engineered for the metallurgical industry, this crane incorporates heat insulation shielding, forced cooling, and high-temperature-resistant structures, making it suitable for harsh environments such as steelmaking, continuous casting, and rolling mills.
YH Type Insulated Bridge Crane (Lifting Capacity 5–100t): Equipped with additional insulation isolation measures within the electrical system, this crane is designed for facilities with specific electrical insulation requirements, such as aluminum smelting and electroplating plants.
Main Girder Design and Selection Guide
The main girder is the core load-bearing structure of an overhead crane, and its design directly impacts the crane's overall lifting capacity, stiffness, dead weight, and cost-effectiveness. The box girder is the most prevalent structural form for overhead crane main girders.
Box Girder: Constructed by welding top and bottom flange plates to two side web plates, forming a closed box section, this design offers high torsional stiffness, substantial load capacity, and mature manufacturing processes. It is the most widely adopted main girder type for overhead cranes. Box girders are suitable for general applications with lifting capacities from 5t to 320t and spans from 10.5m to 34m. The section height typically ranges from 1/14 to 1/18 of the span, web plate thickness is determined based on shear strength calculations, and transverse stiffeners are incorporated to prevent local buckling. Careful control of welding distortion and fatigue crack prevention are critical during fabrication.
Selecting the appropriate main girder requires a comprehensive evaluation of lifting capacity, span, work duty classification, manufacturing capabilities, and economic factors. For a detailed discussion, please refer to our dedicated article on Overhead Crane Main Girder Design and Deflection Calculation Methods.
Crane Travel and Hoisting Mechanisms
The movement of an overhead crane is achieved through the coordinated action of three independent mechanisms: the Crane Travel Mechanism drives the entire crane longitudinally along the runway rails; the Trolley Travel Mechanism moves the hoisting mechanism transversely across the main girder; and the Hoisting mechanism handles the vertical lifting and lowering of the load. Together, these mechanisms cover all material handling requirements within the workshop. The Crane Travel Mechanism consists of an electric motor, reducer, brake, coupling, and wheel blocks, and can be configured with either individual or centralized drive. Variable Frequency Speed Control ensures smooth start/stop and multi-speed operation.
The Hoisting mechanism is the most critical component of an overhead crane, comprising the hoisting motor, reducer, drum, wire rope, pulley block, and hook block. The braking system is configured according to the work duty classification, typically employing a dual brake or a single brake supplemented by a Safety Brake. The drum can be designed for single or multi-layer rope spooling. The wire rope Safety factor complies with ISO 4301, requiring a minimum of 5:1 (and 6:1 for metallurgical cranes). For heavy-lift cranes, the hoisting mechanism often incorporates a redundant dual drive system (Dual Motor + dual reducer + double drum) to enhance safety and reliability.
Electrical Control System Explained
The electrical control system of an overhead crane manages three primary functions: power distribution, motion control, and safety protection. Modern overhead cranes commonly utilize a PLC + VFD (Variable Frequency Drive) control architecture. The PLC handles logic control and interlock protection, while the VFDs provide Variable Frequency Speed Control for the travel and hoisting motors, enabling smooth speed regulation and precise positioning of the bridge, trolley, and hoist. The system integrates safety control circuits for the Overload Limiter, Travel Limit Switch, Hoisting Height Limiter, Door Limit Switch, and Emergency Stop, ensuring safe operation under all conditions.
For power supply, the crane bridge typically uses Enclosed Conductor Rail (or angle steel conductor rail), while the trolley is powered via Cable Trolley or Cable Reel. Control methods have evolved from traditional Cam Controller and relay systems to modern Wireless remote control and PLC Control, allowing operators to manage all functions via a handheld Remote Control or from the Operator Cabin. With the advent of Industry 4.0, intelligent electrical systems are increasingly incorporating Internet of Things (IoT) technology and Remote Monitoring capabilities. For more details on system configurations, please see our guide on Overhead Crane Electrical Control System Design.
Safety Devices and Compliance Standards
As special equipment, overhead cranes must strictly adhere to national mandatory standards regarding the configuration of safety devices and periodic inspections. Key safety devices include: Overload Limiter (mandatory for cranes with Lifting Capacity ≥ 10t, with an accuracy of ±5%), Lifting Height Limit Switch (providing dual protection via counterweight and worm gear mechanisms), Travel Limit Switch (installed at both ends of the bridge and trolley travel), Buffer (rubber/spring/Hydraulic), and anti-wind anti-slip device (Rail clamp/Anchor device, mandatory for outdoor cranes). Additionally, standard configurations include Anti-Collision Device on the end carriages, Audible & Visual Alarm, Emergency Stop Button, door interlock switches, and Hook Latch.
The design, manufacture, installation, retrofitting, and maintenance of overhead cranes must comply with a comprehensive set of standards, including ISO 4301 (Crane design standard), FEM 1.001 (General purpose bridge crane), GB/T 28264 Safety Monitoring and Management System, FEM 1.001 (Crane test procedures), and TSG Q7015-2016 (Rules for periodic inspection of lifting appliances). The Safety Monitoring and Management System (per GB/T 28264) mandates real-time monitoring and recording of parameters such as Lifting Capacity, load moment, Lifting Height, travel distance, and operating time, providing essential data to ensure safe equipment operation.
Overload Limiter
Accuracy ±5%, mandatory for cranes with Lifting Capacity ≥ 10t. Issues a pre-warning at 90% of rated load and automatically cuts off the hoisting power source above 105%.
Braking System
The hoisting mechanism must be equipped with a normally closed brake. Work Duty classifications of M5 and above require a Dual Brake or Safety Brake. Braking Safety factor ≥ 1.5 (hoisting) / ≥ 1.25 (travel).
Travel Limit Devices
Includes Hoisting Height Limiter (with counterweight and worm gear dual protection) and bridge/trolley Travel Limit Switch. Automatically cuts power and applies the brake when the mechanism reaches its extreme position.
Buffers
Rubber/spring/Hydraulic buffer installed at both ends of the end carriages to absorb impact energy during travel. Buffer stroke is calculated per ISO 4301 to ensure deceleration ≤ 4m/s².
Anti-Collision Device
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When multiple cranes operate on the same runway, laser, infrared, or ultrasonic anti-collision devices must be installed with two-stage alarm distances for deceleration and stop to prevent collisions.
Safety Monitoring System
Configured per GB/T 28264, this system provides real-time monitoring of load, torque, height, travel, and operating hours. The black-box recorder stores data for at least 30 days, providing a basis for incident analysis.
Sizing, Installation, and Maintenance
Sizing calculations are the first step in applying a bridge crane. Key parameters to determine include: Lifting Capacity (maximum load weight + spreader dead weight + safety factor), Span (building width minus end clearances), Lifting Height (building height minus hook upper limit space), and Work Duty (determined by annual duty cycles and load spectrum factor, typically A3~A8). Standard series products should be prioritized; non-standard designs require detailed structural analysis and finite element verification.
Installation and maintenance of a bridge crane involves seven stages: foundation acceptance, rail installation, main girder erection, electrical wiring, no-load commissioning, static load test, and dynamic load test. After installation, a special equipment inspection body must perform a supervisory inspection. Daily maintenance per FEM 1.001 focuses on checking wire rope wear and broken wires (replace immediately if scrapping criteria are met), brake clearance and wear, rail fastening and straightness (≤2mm/m), insulation and contact of current conductors, and the lubrication system. We recommend establishing a full life cycle management ledger; planned preventive maintenance effectively extends equipment service life.
References: ISO 4301 Crane Design Standard, FEM 1.001 General Purpose Bridge Cranes, GB/T 28264-2017 Safety Monitoring and Management System for Lifting Appliances, FEM 1.001 Cranes – Test Code and Procedures, TSG Q7015-2016 Rules for Periodic Inspection of Lifting Appliances.
Frequently Asked Questions
Q: What is the difference between a bridge crane and a gantry crane?
A: A bridge crane's bridge ends are supported by end carriages that travel on rails mounted on the building's corbels or runway beams. A gantry crane, in contrast, is supported by outriggers that travel on ground-level rails, eliminating the need for building support structures. For applications within an existing building, a bridge crane is typically preferred; for outdoor use or facilities without overhead support, a gantry crane is the better choice. Kelude offers a full range of both bridge and gantry cranes and can recommend the optimal solution based on your specific operating conditions.
Q: How is the work duty of a bridge crane selected?
A: The work duty (A1~A8) is determined by combining the utilization class (total number of duty cycles, U0–U9) and the load spectrum (light Q1 to heavy Q4). General machine shops typically use A4~A6, while foundries and steel mills typically use A7~A8. We recommend selecting the classification based on your actual lifting frequency. An excessively high classification increases cost, while an insufficient one shortens the crane's service life.
Q: What are the key structural features of a box girder main girder?
A: A box girder is a closed section formed by welding the top and bottom flange plates to two web plates. This design offers high torsional stiffness, excellent load-bearing capacity, and a mature manufacturing process. Its section height is typically 1/14 to 1/18 of the span. Transverse stiffeners are used to prevent local web plate buckling. This is the most common main girder type for bridge cranes, suitable for general applications with lifting capacities from 5t to 320t and spans from 10.5m to 34m.
Q: Which safety device items require periodic inspection on a bridge crane?
A: According to FEM 1.001 and TSG Q7015, periodic inspections include: overload limiter accuracy (annual calibration), brake torque (monthly spot checks), wire rope broken wire count (monthly checks), travel limit switch functionality (tested before operation), crane rail wear (every six months to one year), and main girder deflection (annually). All inspection results must be documented for review, serving as the management basis for safe equipment operation.
