Bridge Crane Overhaul Cycle: Standards & 5 Key Inspection Areas
The overhaul cycle for an overhead crane typically ranges from 6 to 10 years, depending on its work duty classification (A3 to A8), daily operating hours, and environmental conditions. A full overhaul covers five main areas: structural inspection, hoisting mechanism disassembly and repair, comprehensive crane travel mechanism maintenance, electrical system insulation testing, and calibration of safety protection devices—encompassing no fewer than 20 inspection items. After the overhaul, the crane must pass both static and dynamic load tests at rated load and receive approval from a special equipment inspection body before returning to service.
As the most critical material handling equipment in factory buildings and warehousing logistics, bridge cranes operate under heavy loads, high frequency, and demanding environmental conditions. Over years of service, the metal structure develops fatigue damage, transmission components suffer wear and failure, and the electrical system becomes a safety hazard due to aging and insulation degradation. A well-timed overhaul not only restores rated performance but also extends the crane's overall service life and minimizes the risk of costly downtime. When is the right time to schedule an overhaul? What does the scope of work include? This article breaks down everything you need to know.
ISO 4301 (Crane Design Standard) and ISO 4306 (General Purpose Bridge Crane) set clear technical requirements for design life, structural safety factors, and maintenance classifications. Overhaul planning should be based on these standards.
Overhaul Cycle vs. Work Duty Classification
The overhaul cycle for a bridge crane is not a fixed number—it's a dynamic indicator closely tied to work duty classification, daily utilization, environmental conditions, and maintenance practices. Work duty is the primary variable determining overhaul frequency: it reflects both the severity of the load spectrum and the cumulative effect of total work cycles. Across the six classifications from A3 to A8, overhaul intervals decrease from over 10 years down to 4–6 years—a difference of more than twofold.
The chart below illustrates the relationship between overhaul cycle and work duty classification for bridge cranes:
Bridge Crane Overhaul Cycle vs. Work DutyA1–A3 Light DutyOverhaul: 6–8 yearsAnnual use: under 1,000 hExample: maintenance cranesA4–A5 Medium DutyOverhaul: 4–6 yearsAnnual use: 1,000–3,000 hExample: machining workshopsA6–A7 Heavy DutyOverhaul: 2–4 yearsAnnual use: 3,000–6,000 hExample: metallurgy / foundriesA8 Extra Heavy DutyOverhaul: 1–2 yearsAnnual use: over 6,000 hExample: steel mill continuous casting<r
Bridge Crane Overhaul Checklist: Five Key Areas
Overhauling an overhead crane is a systematic process aimed at fully restoring the equipment's technical performance. It goes far beyond the localized tightening and lubrication of routine inspections, and it is not merely a functional confirmation like an annual examination. An overhaul requires complete disassembly, inspection, cleaning, measurement, repair or replacement of parts, and reassembly with re-commissioning of all mechanisms and systems. The standard overhaul checklist is organized into the five key areas outlined below. The diagram below illustrates the five key overhaul areas and their core checklist items for bridge cranes:Overhead Crane Overhaul: Five Key Work Areas1. Steel StructureMain girder deflection measurementUT/MT weld seam flaw detectionEnd carriage bolt tighteningCrane rail joint/clamp inspectionGuardrail/walkway/ladder replacementDuration: 3–5 days2. Hoisting MechanismFull wire rope replacementDrum groove wear detectionGearbox overhaul and oil changeBrake friction lining replacementHook block UT/MT flaw detectionDuration: 5–7 days3. Travel MechanismWheel block tread surface inspectionGearbox lubricating oil replacementCoupling shaft alignmentBuffer/rail clamp serviceMotor bearing replacementDuration: 4–6 days4. Electrical SystemCable/conductor rail replacementControl cabinet cleaning and tighteningLimit switch/overload limiter calibrationMotor insulation/resistance testingLighting/horn/warning light checkDuration: 3–5 days5. Safety DevicesOverload limiter calibrationHeight/travel limit switch replacementEmergency stop button testingAnemometer/lightning protectionSafety monitoring system verificationDuration: 2–3 days<text x="480" y="258"Basis: GB/T 5972 "Specification for maintenance, inspection and discard of steel wire ropes for cranes" | TSG Q7015 "Rules for Periodic Inspection of Lifting Appliances" After overhaul, the crane must be inspected and accepted by a special equipment inspection body before being put back into service.1. Metal Structure Inspection & Repair — Measure the mid-span camber of the main girder and the static deflection under rated load. Camber should be maintained between 1/1000 and 1.4/1000 of the span, while static deflection must not exceed 1/700 of the span (for A4~A5 duty) or 1/800 (for A6~A7 duty). Perform a 100% visual inspection and no less than 20% Ultrasonic Testing (UT) on the weld seams connecting the main girder web plate to the lower flange plate, as well as the welds between the end carriage and the main girder. Check the cover plate and web plate for wave deformation — the amplitude must not exceed 5mm/m². The reduction in steel plate cross-section due to corrosion must not exceed 10% of the original thickness. Spot-check the tightening torque of high-strength bolts at all connections; the torque value must not be less than 90% of the design value.
2. Hoisting Mechanism Overhaul — Disassemble the drum assembly and inspect rope groove wear — the wall thickness reduction at the groove bottom must not exceed 20% of the original wall thickness. Replace all wire ropes (standard overhaul item); new ropes must comply with GB/T 8918 and have a safety factor of no less than 5–6. Open the gearbox and inspect the gear tooth surfaces — the pitting area must not exceed 30% of the working tooth surface area, and tooth thickness wear must not exceed 15% (hoisting mechanism) or 20% (travel mechanism) of the original tooth thickness. Replace all gearbox oil seals and fill with fresh lubricating oil. Disassemble the brake, inspect brake wheel wear (wear must not exceed 30% of the original thickness) and the remaining thickness of the brake shoe lining (must not be less than 50% of the original thickness), and readjust the brake clearance to 0.5–1.0mm.
3. Comprehensive Maintenance of Trolley & Bridge Travel Mechanisms — Disassemble the wheel blocks and inspect wheel tread wear and flange thickness — flange wear must not exceed 50% of the original thickness, and tread wear must not exceed 3%–4% of the original wheel diameter. Clean and inspect bearings; replace any with excessive clearance and repack with fresh grease. Open the gearboxes for inspection, replace the input/output shaft oil seals and all lubricating oil. Replace all coupling elastic elements (standard overhaul item). Disassemble and inspect brakes, and adjust clearances to ensure the contact area between the brake lining and the brake wheel is no less than 70%.
4. Electrical System Insulation Test & Wiring Inspection — Measure the insulation resistance of the main circuit to ground using a 500V megohmmeter; it must not be lower than 1MΩ. The control circuit must not be lower than 0.5MΩ. Measure grounding resistance with a ground resistance tester; it must not exceed 4Ω. Inspect all contactor and relay contacts, cleaning or replacing them (replace if contact wear exceeds 50% of the original thickness). Test the stator winding insulation resistance of motors — the insulation resistance to ground must not be lower than 0.5MΩ, and the absorption ratio must not be less than 1.3. Tighten all terminal blocks and replace any aged wiring. Inspect the internal capacitor banks of the frequency inverter (overhaul item) — replace any capacitor bank with a capacitance decay exceeding 20%.
5. Comprehensive Calibration of Safety Protection Devices — Calibrate the load limiter: perform load tests at 90%, 100%, and 110% of the rated load, with an action error of no more than ±5%.
Hoisting Height Limit Switch — Test the trigger positions of both the upper and lower limits. After triggering, the power-off margin must not be less than 100mm.
Travel Limit Switch — Test the triggering reliability of the bumpers in both directions for the bridge and trolley travel.
Overspeed Switch — Simulate an overspeed condition of 115% to verify the activation and protection functions.
Anti-Collision Device (e.g., laser distance measurement or radar) — Calibrate the detection distance and alarm thresholds.
Audible & Visual Alarm — Verify the power-on self-test and overload triggering functions.
Emergency Stop Button — Test each button to ensure the power-off protection is activated correctly.
Overhaul Acceptance Standards and Test Requirements
After the overhaul is complete, the crane must not be put into service directly; it must pass a strict acceptance procedure. The acceptance process includes five stages: review of technical documentation, visual and assembly quality inspection, no-load test run, static load test, and dynamic load test. The static and dynamic load tests are the most critical acceptance checkpoints, as they directly determine whether the overhaul is deemed qualified.
Static Load Test — Use 125% of the rated load as the test load. Lift the load 100–200mm off the ground and hold it suspended for no less than 10 minutes. During and after the test, verify that the main girder shows no residual deformation, the weld seams show no cracking, and no connections are loose or damaged.
Dynamic Load Test — Use 110% of the rated load to perform hoisting, lowering, bridge travel, trolley travel, and combined motions. Repeat all actions for at least 3 cycles. After the test, verify that all mechanisms operate normally, braking is reliable, and limit switches are responsive.
Once both tests are passed, the special equipment inspection body will issue the "Overhaul Supervision Inspection Report" and record it in the equipment's technical file.
Acceptance Item Checklist
The following table outlines the technical requirements and inspection methods for each acceptance item after the overhaul:
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Bridge Crane Overhaul vs. Annual Inspection: Key Differences
Q: What is the difference between a bridge crane overhaul and an annual inspection?
A: An annual inspection is a statutory safety examination conducted by a special equipment inspection body in accordance with TSG Q7015 – Rules for Periodic Inspection of Lifting Appliances. It focuses on verifying the functionality of safety protection devices and performing a visual inspection of the metal structure, without disassembling components. An overhaul, by contrast, is a comprehensive engineering operation organized by the equipment owner to restore the crane's performance. It involves full disassembly, wear measurement, and replacement or repair of the hoisting mechanism, travel mechanism, and electrical system—a scope of work far beyond that of an annual inspection.
Q: How can I tell if my bridge crane needs an early overhaul?
A: Any of the following signs indicates that an early overhaul should be considered: main girder deflection reaches or exceeds 1/700 of the span under rated load; persistent abnormal noise or oil temperature above 80°C in the hoisting gearbox; repeated braking force deficiency in the brake requiring frequent adjustments; the number of broken wires in the wire rope within one lay length approaching or exceeding 10% of the total wires per ISO 4309; or electrical system insulation resistance dropping below 0.5 MΩ and failing to recover after drying.
Q: What is the inspection procedure after a crane overhaul?
A: An overhaul is classified as major repair work. In accordance with TSG Q0002 – Rules for Supervision Inspection of Installation, Retrofit, and Major Repair of Lifting Appliances, the owner must file a construction notice with the local market supervision authority before work begins, and a certified inspection body must carry out full-process supervision inspection. The inspection is divided into Category A (hold points, e.g., static load test and dynamic load test), Category B (witness points, e.g., limit switch calibration and insulation testing), and Category C (documentation review). Only after all categories are passed will the Overhaul Supervision Inspection Report be issued.
Q: Which wear parts should be replaced during an overhaul?
A: Standard replacement items during an overhaul include: the wire rope (regardless of its external condition), all gearbox oil seals and lubricating oil, all coupling elastic elements, all brake linings, brake return springs, and limit switch contacts and contactor contacts in all mechanisms (where wear exceeds 50%). In addition, bearings with excessive clearance, aged seals, and corroded fasteners that cannot be safely removed should also be replaced at the same time to avoid unplanned downtime shortly after the overhaul due to these components.
Overhauling a bridge crane is a highly technical, multi-disciplinary engineering undertaking. A well-planned overhaul cycle should be based on the equipment's work duty classification, adjusted dynamically using daily inspection records and operating hours. Full coverage of overhaul items and strict adherence to acceptance standards are essential to ensuring the crane operates safely and reliably after the overhaul.
