Crane Main Beam Camber Subsidence & Deflection

Basic Definition & Judgment Standard

1.1 Camber Subsidence: After long-term operation of the crane, the factory pre-set upward camber of the main beam gradually decreases and disappears.

1.2 Main Beam Deflection: The middle span or cantilever end of the main beam sinks below the horizontal line, which belongs to irreversible permanent plastic deformation.

1.3 Repair & Scrap Critical Standard

Under rated load and trolley mid-span working condition, if the main beam deflection value reaches S/700 (S = beam span), repair is mandatory; scrap the beam if restoration fails. The allowable horizontal side bending limit is S₁/2000. Excessive deviation will easily cause rail gnawing and wheel derailment risks.

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2. Main Hazards of Beam Deflection

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2.1 Trolley Operation Failure: Deflection forms an inclined slope on the beam, increasing the trolley climbing resistance by up to 40%, which easily causes motor overload and burnout. It also leads to trolley sliding after braking and inaccurate positioning. Uneven subsidence of double beams causes trolley deviation and rail gnawing.

2.2 Trolley Structure Damage: Inconsistent deformation of two main beams makes the four trolley wheels fail to contact the track simultaneously, resulting in the “three-leg support” phenomenon. The trolley frame bears unbalanced load, causing structural deformation and welding cracks.

2.3 Accelerated Bridge Deformation: Vertical deflection is usually accompanied by inward horizontal side bending of the main beam, triggering rail clamping and derailment risks. The web plate changes from tension state to compression state, and the wave deformation is significantly aggravated.

2.4 Main Beam Structural Failure: Long-term overload operation causes web instability and fatigue cracks on the lower cover plate and web bottom. In severe cases, overall structural collapse accidents may occur.

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3. Core Causes of Camber Subsidence

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3.1 Manufacturing Defects: Straight web plates are forced to form upward camber by welding and baking, resulting in huge residual internal stress. The pre-set camber quickly disappears after operation. Unreasonable welding procedures also induce inherent beam deformation hidden dangers.

3.2 Release of Internal Residual Stress: Assembly and welding produce uneven tensile and compressive residual stress. During long-term crane operation, stress homogenization and release lead to permanent beam subsidence deformation.

3.3 Improper Transportation, Lifting and Installation: The crane bridge is a flexible long structural component. Unreasonable stacking, transportation, single-point lifting and forced installation will cause invisible plastic deformation, which gradually appears as deflection in later service.

3.4 Non-standard Operation: Long-term overload lifting, oblique hoisting, heavy dragging and violent operation exceed the design load and working level, accelerating structural fatigue and deformation.

3.5 Irregular Welding & Maintenance: Random gas cutting and electric welding on the main beam, end beam and walkway cause local high-temperature shrinkage and beam deformation. Removing track pressure plates by gas cutting will cause 3–10 mm main beam subsidence in a single operation.

3.6 High-temperature Working Environment: Thermal radiation in hot workshops makes the lower beam cover plate hotter than the upper one, resulting in uneven thermal expansion and bending deformation. High temperature reduces the yield strength of steel materials, making the beam more prone to plastic deformation.

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4. Solutions & Preventive Measures

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4.1 Main Repair Method: Flame Straightening

Principle: Heat the designated beam parts to use the plastic shrinkage property of steel, so as to restore the original upward camber.

Operation Key Points: Jack up the mid-span before construction to assist correction with self-weight stress. The heating temperature for low-carbon steel shall be controlled at 700–800℃ (dark cherry red).

Strict Prohibitions: Do not repeatedly heat the same position; do not cool with water to avoid steel brittleness; avoid heating dangerous sections to prevent material performance degradation and structural cracking.

4.2 Daily Preventive Measures

① Regularly detect and record beam camber and side bending data, rectify deviations in time;

② Strictly prohibit overload and oblique hoisting to match the design working level;

③ Install heat insulation plates for cranes working in high-temperature environments;

④ Use grinding machines instead of gas cutting when removing track pressure plates;
⑤ Adopt standard support and special lifting points during transportation and hoisting.

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