Introduction
Crane girders are manufactured with a precise upward curvature, known as camber, to counteract the deflection caused by the weight of the trolley and the rated load. Over an extended period of service, this camber gradually diminishes. When the central portion of the girder sags below the horizontal line, it results in a permanent downward deformation known as "deflection" or "downward sag." A common benchmark for intervention is when the deflection at mid-span under rated load reaches S/700 (where S is the span in mm). At this point, repairs are recommended; if correction is not possible, the structure should be considered for retirement. Continuing operation beyond this limit can lead to progressive deterioration of the bridge structure and potential safety hazards. This article outlines the primary impacts of girder sagging, its root causes, and the standard method of correction.
Impacts of Girder Sagging
The deformation of the main girder adversely affects several key components and operational characteristics of the crane:
1. Impact on Trolley Operation:A sagging girder creates an upward slope that the trolley must climb when traveling from the center to the ends of the bridge. This introduces significant additional resistance. Calculations indicate that when the mid-span deflection reaches S/500, the running resistance can increase by as much as 40%. This added strain often leads to the overheating and frequent burnout of trolley drive motors. Furthermore, the slope can cause the trolley to drift or "creep" after the brake is applied, hindering precise load positioning. If the two main girders sag unevenly, the trolley may tend to shift sideways towards the lower rail, leading to flange wear and the characteristic "gnawing" or rail clipping.
2. Impact on the Trolley Frame: Differential sagging between the two girders results in the four wheels of the trolley no longer being in the same plane. This often leads to a "three-point" support condition, where one wheel loses contact with the rail. This uneven contact creates torsional stresses and uneven loading on the trolley frame, potentially causing fatigue and structural issues.
3. Impact on Horizontal Bending: Vertical sagging of the main girder is frequently accompanied by an inward horizontal bowing of the girders. For a box-section girder, this horizontal bend should not typically exceed S1/2000. Excessive horizontal bending misaligns the trolley rails, further contributing to trolley binding, rail clipping, and, in severe cases, poses a risk of derailment.
4. Impact on Web Plates:The redistribution of stresses caused by the sagging girder can increase the severity of existing waveform buckling in the web plates. The deformation often shifts from the tension zone to the compression zone of the web. Under continued operation, this can compromise the stability of the web or lead to the formation of fatigue cracks in the highly stressed areas of the lower flange and the lower portion of the web plates.
Causes of Girder Sagging
The loss of camber is typically a result of cumulative factors related to manufacturing, handling, and operation:
1. Manufacturing and Welding Deficiencies:Proper fabrication requires the web plates to be cut with a shape that corresponds to the desired camber. Attempting to create camber solely through welding or flame straightening of straight-cut plates introduces high residual stresses without providing a stable geometry. Similarly, an incorrect welding sequence can cause the progressive loss of camber during the manufacturing process itself.
2. Relaxation of Residual Stresses: The welding process inherently introduces significant residual tensile and compressive stresses within the girder structure due to localized heating and subsequent cooling and contraction of the weld metal. Over time, and under cyclic service loading, these internal stresses gradually redistribute and relax. This relaxation process is a primary cause of the progressive and permanent loss of initial camber.
3. Improper Handling and Installation:The crane bridge is a long, flexible structure. Incorrect procedures during transportation (e.g., inadequate support), storage, lifting, or installation can introduce unintended bending moments and permanent deformations before the crane even enters service.
4. Unfavorable Service Conditions: Cranes are designed for specific load ratings and duty cycles. Operation beyond these design parameters is a major cause of premature sagging. This includes frequent or chronic overloading, exceeding the designed work cycle class, and misuse such as using the crane to drag loads or pull on embedded objects, which subjects the structure to unplanned lateral and longitudinal forces.
Remedial Method: Flame Straightening
The most common and flexible method for correcting crane girder deformation is flame straightening (also known as thermal stress relieving or heat矫正). This technique relies on the plastic contraction that occurs when a localized area of steel is heated and then cools. By strategically heating specific zones, the resulting shrinkage forces can be used to reshape the girder and restore its camber.
A typical procedure involves raising the girder at its mid-span with a jack until the wheels at one end just lift off the rail. This uses the structure's own weight to assist the compression of the heated zones on the top flange. The process requires careful planning regarding the location, size, and number of heating spots or lines. Key practical considerations for successful and safe application include:
Avoid Repeated Heating:The same area should not be heated multiple times, as this can introduce new internal stresses, alter the metallurgical structure of the steel, and potentially reduce its yield strength.
No Artificial Cooling:The heated areas must be allowed to cool naturally in the air. Forced cooling with water or air jets can cause the steel to become hard and brittle.
Strategic Location:Heating zones should be carefully chosen to achieve the desired correction. Critical cross-sections, such as the very center of the span, are typically avoided to prevent weakening the main load-bearing area.
Temperature Control:The heating temperature must be carefully controlled. For mild steel, the optimal temperature range is typically 700–800°C (corresponding to a dark cherry red color). At this temperature, the steel retains sufficient plasticity for effective forming without undergoing significant phase transformations that could degrade its mechanical properties.
Conclusion
The gradual loss of camber in an overhead crane is a serious indicator of structural fatigue and stress redistribution. Its impacts range from increased operational costs due to motor burnout to significant safety risks like trolley derailment. Understanding the root causes—from manufacturing stresses to operational misuse—is key to prevention. When correction is necessary, flame straightening remains a proven and adaptable technique, provided it is executed with strict adherence to proper procedures to avoid introducing new flaws. Regular inspection and adherence to the crane's rated capacity are the most effective strategies for prolonging the life of the structure and ensuring safe operation.
HENAN ZEHUA HEAVY INDUSTRY EQUIPMENT CO., LTD
Email: sale@zehuacranes.com
Website: [https://www.zehuacranes.com/]
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