Understanding the deflection and standards of double girder Overhead Crane

Double girder Overhead Crane, as indispensable material handling equipment in modern industrial production, are widely used in steel, chemical, port, logistics, and other sectors. The deflection of their main girders serves as a core indicator for assessing the structural rigidity and safety of the crane, directly impacting equipment lifespan, operational stability, and work safety. This paper systematically analyzes deflection management standards for double girder Overhead Crane from four dimensions: definition of deflection, calculation methods, national standard requirements, and deflection repair techniques.

Definition and Mechanics of Deflection

Deflection refers to the vertical displacement of a beam under load and serves as a key parameter for measuring structural rigidity. In double-girder bridge cranes, main girder deflection manifests as the vertical sag at the midspan under full load. Based on the nature of deformation, deflection can be categorized into two types:

• Elastic deflection: Temporary deformation that recovers after load removal, conforming to Hooke's law.
• Residual deflection: Permanent deformation caused by repeated loading or material fatigue, requiring structural repair to eliminate.

From a mechanical perspective, deflection is inversely proportional to the beam's bending stiffness (EI) and directly proportional to the fourth power of both the load (q) and span (L). For instance, in a 28-meter span double-girder crane, a 10% reduction in the main girder's elastic modulus (E) would increase mid-span deflection by 23%, highlighting the decisive role of material properties in structural stability.

Engineering Impacts of Deflection

• Safety: Excessive deflection causes the trolley running track to tilt, leading to accidents such as brake failure and runaway incidents. Statistics show that when main girder deflection exceeds 1/500 of the span, the risk of motor overload burnout increases by 40%.
• Equipment Lifespan: Frequent large deflections accelerate metal fatigue, shortening the main girder's service life. For instance, a steel mill crane operating under prolonged overload saw its main girder deflection increase from an initial 10 mm to 80 mm, developing cracks after only five years of use.
• Operational Precision: Excessive deflection compromises hook positioning accuracy, potentially causing product quality issues in precision assembly applications.

Mandatory requirements for deflection in standards

(1) Permissible Deflection Limits

According to GB/T 3811-2008 “Design Specifications for Cranes,” the fully loaded vertical deflection of the main girder for double-girder bridge cranes shall meet the following requirements:

Class A1-A3 (Light Duty): Within 1/700 of the span

Class A4-A6 (Medium Duty): Within 1/800 of the span

Class A7 (Heavy Duty): Within 1/1000 of span length

Case Study: A 50-ton double-girder crane (Class A5) at an automobile manufacturing plant, with a span of 22.5 meters, has a permissible deflection of 32.1 mm. The measured full-load deflection of 28 mm complies with the standard requirement.

(2) Positive Camber Requirements

To counteract elastic deflection, the main girder of new cranes shall be pre-set with positive camber:

Standard Value: 0.9‰ to 1.4‰ of span length

Inspection Method: With the crane unloaded and trolley positioned at the outriggers, measure the mid-span height using a theodolite.

Engineering Practice: For a 28-meter span crane, the upward camber range should be 25.2–39.2 mm. If measured camber disappears or negative camber (downward deflection) occurs, immediate maintenance is required.

Inspection Cycle and Acceptance Criteria

Regular Inspections:

• First inspection after one year of new equipment use
• Subsequent inspections every two years
• Immediate inspection following overload or collision incidents

Inspection Method:

• Unloaded Measurement:With the trolley positioned on the outriggers, measure the original height at the midspan.
• Loaded Measurement:With the trolley carrying the rated load at the midspan, measure the residual height.

Acceptance Criteria:

• Deflection Value = Original Camber - Residual Height After Loading
• Deflection Value ≤ Allowable Deflection Limit
• No permanent deformation after unloading.

Repair Techniques for Main Beam Deflection

(1) Flame Correction Method

Restores camber by locally heating the metal to induce contraction. Key operational points:

Heating Zone: Middle third of the main girder's lower flange plate

Heating Temperature: 600-700°C (dark red glow)

Cooling Method: Natural air cooling; water quenching strictly prohibited

Repair Case: A 100-ton port crane with 120mm sag was repaired using the triangular heating method. The camber was restored to 28mm, meeting service requirements. No recurrence occurred after three years of operation post-repair.

(2) Prestressing Tensioning Method

Install prestressed steel cables or bars beneath the main girder to generate reverse bending moments through tensioning. Technical Parameters:

Tensioning Force: Typically 30%-50% of rated load

Anchorage Points: Mid-span and at 1/4 span locations

Durability: Requires periodic inspection for prestress loss, with retensioning as needed

Engineering Advantages: This method eliminates the need for open flames, making it suitable for crane repairs in flammable or explosive environments. After repairing an 80-ton explosion-proof crane at a chemical plant using this technique, the upward camber remained stable for five years.

(3) Structural Reinforcement Methods

Plate Laminating Reinforcement:

Adhere Q345 steel plates to the bottom flange plate

Steel plate thickness: 8-12 mm; width: 0.3-0.5 times the beam height

Secure using high-strength bolts or welding

Additional Support:

Install transverse tie beams at the midspan

Increase overall stiffness by 15%-20%

Structural Conversion:

Replace box girders with truss girders

Reduce self-weight while enhancing bending resistance

Economic Analysis: Plate reinforcement costs approximately 30% of main girder replacement, with a 60% shorter construction period, making it the most cost-effective repair solution.

Engineering Practices for Deflection Management

IV. Engineering Practices for Deflection Management

(1) Design Phase Optimization

Material Selection:

Main girder utilizes Q345B low-alloy high-strength steel

Yield strength increased by 50% compared to standard Q235 steel

Section Optimization:

Increased lower flange thickness

Enhanced web height

Implemented variable-section design

Effect Verification: Through optimized design, a heavy machinery manufacturer achieved a 25% increase in main girder stiffness and a 30% reduction in deflection for a 320-ton crane.

(2) Manufacturing Process Control

Welding Process:

Employed CO₂ gas shielded arc welding

Controlled welding distortion ≤ 2 mm/m

Assembly Precision:

Main beam span deviation ≤ ±5 mm

Diagonal difference ≤ 8 mm

Stress Relief:

Perform full annealing treatment

Residual stress ≤ 80 MPa

Quality Case Study: Through stringent manufacturing control, a crane manufacturer achieved 98% consistency in main girder camber, significantly exceeding industry standards.

(3) Usage and Maintenance Specifications

Load Management:

Strictly prohibit overload operation

Avoid uneven loading (load offset ≤ 1/10 span)

Operational Control:

Maintain uniform trolley speed

Avoid abrupt starts and stops

Scheduled Maintenance:

Quarterly track straightness inspection

Annual main beam deflection testing

Comprehensive non-destructive testing every 5 years

Maintenance Benefits: A logistics enterprise extended crane service life from 15 to 25 years through standardized maintenance, reducing repair costs by 40%.

Deflection management for Double girder Overhead Crane is a systematic engineering process spanning the entire lifecycle of design, manufacturing, and operation. By strictly adhering to national standards, implementing scientific repair techniques, and establishing standardized maintenance systems, equipment can be ensured to operate safely and efficiently. With the advancement of intelligent monitoring technologies, future crane deflection management will achieve real-time online detection, providing more reliable safety assurance for industrial production. Enterprises should incorporate deflection management into core equipment management metrics, continuously improving to enhance overall equipment performance.