An aging crane typically refers to equipment in service for more than 10 years, with accumulated structural fatigue, outdated electrical systems, and safety standards that have since been upgraded. Common issues include: degraded camber of the main girder (restoration required when measured values fall below L/1500), fatigue micro-cracks in critical weld seams, motor insulation class no longer meeting current standards, and oxidized or aged wiring in the control cabinet. A systematic structural inspection and strength verification must precede any retrofit work — proceeding blindly based on experience alone is never acceptable.

Step 1: Structural Inspection

Structural inspection forms the foundation of any retrofit decision and must cover the following items: Main girder camber measurement — measure mid-span camber with a level instrument and compare against factory records and standard requirements (factory camber L/1000–L/800; minimum service limit L/1500). Main girder side bow measurement — use the wire-stretching method to measure horizontal side bow; allowable tolerance is ≤3 mm (for spans up to 22 m) or ≤5 mm (for spans of 22–30 m). Weld inspection — 100% ultrasonic testing (UT) on butt welds of the main girder, and no less than 25% magnetic particle inspection (MPI) on fillet welds, with particular attention to end carriage connections and stiffener ends. Wall thickness measurement — use an ultrasonic thickness gauge to check remaining steel plate thickness; sections with corrosion exceeding 15% of the original plate thickness must be replaced.

Electrical system inspection covers: motor insulation resistance testing (using a 500V megohmmeter, winding-to-ground resistance ≥1 MΩ), cable insulation testing (aged cables must undergo a dielectric test), and control cabinet component aging checks (contactor contacts, relay coils, terminal blocks). Inspection results are classified into three grades — good / fair / poor — and serve as the basis for the subsequent retrofit solution design. The inspection report must include test data, assessment conclusions, and preliminary recommendations, with two copies filed for record.

Step 2: Strength Verification

Based on inspection results, a finite element analysis (FEA) strength verification is performed per ISO 4301 under three loading conditions (detailed in FEA of Large-Tonnage Non-Standard Structures): Condition A (full load at mid-span + dead weight + wind load) verifies maximum stress and camber of the main girder; Condition B (full load at the end + horizontal inertia force) verifies stress in the end carriage and wheel block connection zones; Condition C (maximum wind load + earthquake) verifies overall stability. Eigenvalue buckling analysis determines whether the flange width-to-thickness ratio and stiffener arrangement meet requirements. Fatigue life verification uses S-N curve analysis on welded joints, requiring a service life of no less than 2×10⁶ cycles.

FEA results are presented as stress ratios: zones with a stress ratio above 0.9 require reinforcement; zones between 0.6 and 0.9 may be retained with continued monitoring; zones below 0.6 have excessive margin. For main girders with insufficient camber, the FEA must simulate the camber recovery effect after plate reinforcement. For capacity-increase retrofits, the FEA must recalculate all conditions based on the new rated load. The verification conclusion must clearly specify reinforcement recommendations (reinforcement locations, material specifications, and welding process requirements), which serve as the basis for shop drawings.

Step 3: Upgrade Solution Implementation

The upgrade solution is developed based on inspection and verification conclusions and generally includes: Structural reinforcement — plate welding on the bottom flange of the main girder, densified web stiffeners, and reinforcement plates on the end carriage. Welding is performed in accordance with GB/T 50661 procedure qualification, using E5015 low-hydrogen electrodes. 100% weld inspection is conducted within 24 hours after welding. Electrical upgrade — replacement with variable-frequency motors + VFDs + PLC control, with the safety circuit redesigned per IEC 60204-32 (see Non-Standard Crane Control Cabinet Solutions). Safety device installation — installation of a safety monitoring system per GB/T 28264, including a lifting capacity limiter, travel limit switches, an anemometer, and an anti-collision device.

After construction, the project enters the inspection and acceptance phase: 125% static load test (residual deformation of the main girder ≤ L/2000), 110% dynamic load test (3 full-stroke cycles), item-by-item functional verification of safety devices, and TSG supervision inspection filing. Upon passing acceptance, a retrofit Conformity Certificate is issued and the equipment file is updated. For detailed load test procedures after retrofit, refer to Load Testing and TSG Acceptance for Non-Standard Crane Installation. A complete old crane retrofit — from inspection to acceptance — takes approximately 14–35 days and extends equipment service life by 5–10 years.

Comparison of Three Retrofit Solutions