Executive Summary / Key Takeaways
- Primary Function: The crane beam is the main load-bearing component that supports both the trolley and the suspended load.
- Critical Design Considerations: Proper beam selection, load calculations (vertical, longitudinal, and lateral), and the incorporation of brake structures and auxiliary supports are essential.
- Track System Importance: Reliable installation and maintenance of crane tracks, track connections, and buffer stops ensure smooth operation and safety.
- Maintenance and Safety: Regular inspection and adherence to strict design and construction standards extend service life and reduce operational risks.
Introduction
An overhead crane is a lifting machine used to transport materials above workshops, warehouses, or stockyards.
Typically, the crane is suspended between tall concrete columns or metal supports, giving it a bridge-like appearance. The crane’s bridge travels longitudinally along rails mounted on elevated supports, thereby maximizing the use of space below and avoiding interference with ground-level equipment.
For these reasons, overhead cranes are among the most widely used and numerous types of lifting equipment.
1.Structural Components
The structural system of an overhead crane comprises several key components, each critical to safe operation and longevity.
1.1 Crane Beam
The crane beam is the primary load-bearing component. Its design is directly related to the safety and service life of the crane.
1.1.1 Beam Selection
- Solid Web Beams:
- Typically, welded I-beams or hot-rolled wide-flange H-beams are preferred.
- Asymmetric Section Beams:
- Suitable for larger spans and heavier loads.
- Options include beams with an increased or widened top flange, or beams with a tapered web (with reduced mid-span thickness or thickened upper web).
- Box Girders:
- Employed when there is sufficient technical and economic justification.
- Crane Trusses:
- An acceptable option for medium-span cranes where the working load is relatively light or does not exceed 30 tonnes.
1.1.2 Crane Loads
The design of the crane beam must account for several types of loads:
- Vertical Loads:
- Based primarily on the maximum wheel load of the crane.
- For fatigue analysis, the minimum wheel load is also considered to compute the stress range.
- Longitudinal Horizontal Loads:
- Generated mainly by the braking force of the crane’s bridge (trolley drive).
- Typically, 10% of the sum of the maximum wheel loads on the braking wheels is applied.
- As shown by the equation (in below pictures):
- Lateral Horizontal Loads:
- Result from the braking force of the trolley and the inertial force of the rated load.
- Load is usually distributed evenly among the wheels.
- With the following calculation:
Note: For heavier loads, reduced sway and lower operating speeds may justify a decrease in the applied percentage and corresponding horizontal load.
1.1.3 Beam Calculations
Due to the dynamic movement of the trolley, the load distribution along the crane beam varies. The following calculations are performed using the influence line method:
- Local Compression Analysis: Evaluates the top flange.
- Flexural Strength Assessment: Determines the capacity of both upper and lower flanges.
- Shear Capacity Verification: Checks the web’s ability to resist shear forces.
- Overall Stability Analysis: Assesses the beam’s stability under combined loads.
- Stiffening Ribs Design: Ensures the cross section is adequately reinforced.
1.2 Brake Structure, Auxiliary Trusses, and Supports
To ensure satisfactory braking performance and structural stability, the crane beam system incorporates brake structures, auxiliary trusses, and supports.
1.2.1 Brake Structure
Applicability:
- For medium- to heavy-duty cranes with a beam span of 12 m or greater, a horizontal brake truss or brake plate is required on the top flange.
- For side-mounted crane beams (or trusses) with spans of 12 m or more, or when only one side of the central column is equipped with a crane beam, auxiliary trusses should be provided.
Calculation Requirements:
- When the beam or truss is subjected to both vertical and horizontal loads, it is acceptable to assume that the top flange (or top chord) and the brake plate (or truss) together bear the horizontal load.
- These components should be designed to resist bi-directional bending stresses.
- If a brake structure is provided on the compressed top flange, an overall stability analysis may be omitted; otherwise, a critical moment check based on overall stability capacity is required.
1.2.2 Auxiliary Trusses and Supports
- Horizontal Supports:
- Provided if the crane truss span is 12 m or greater, or if the bottom flange is required to function as a wind-resistant element.
- Vertical Supports:
- Generally not provided between beams on central columns to avoid additional load transfer.
- If necessary, should be placed within one-third of the beam span from the end.
- Maintenance Walkways:
- For heavy-duty cranes operating on a shift system, a safe maintenance walkway should be provided on one side of the brake structure.
- The clear width should be no less than 400 mm.
- A stairway connecting the ground to the crane cabin and the walkway is recommended.
1.3 Crane Tracks, Track Connections, and Buffer Stops
Crane tracks, their connections, and buffer stops are essential for ensuring smooth operation and safe control of the overhead crane.
1.3.1 Crane Tracks
Crane tracks guide the movement of both the bridge and the trolley, transmitting wheel loads directly to the crane beam.
Track Types:
- Railway Steel Rails (e.g., P-type):
- Suitable for small to medium-sized cranes due to their high strength and wear resistance.
- Crane-Specific Steel Rails (e.g., QU-type):
- Feature larger cross-sectional dimensions and high load-bearing capacity; used in heavy-duty cranes.
- Flat Bars:
- Typically employed in light-duty cranes or applications where high precision is not critical.
Installation:
- Tracks are generally fixed to the top flange of the crane beam using pressure plates or bolts.
- It is essential to install the tracks level and maintain the correct gauge to prevent rail wear or derailment.
- Expansion joints should be included at track joints to accommodate thermal expansion and contraction.
Maintenance:
- Conduct regular inspections to assess track wear, and perform repairs or replacements as necessary.
- Keep track surfaces free of debris to ensure smooth operation.
1.3.2 Track Connections
Track connections secure the tracks to the crane beam and ensure effective load transfer.
Connection Methods(Rail Clips/Clamps):
- Clamping Plate Connection:
- Secures tracks to the beam’s top flange using pressure plates and bolts; a simple and widely used method.
- Bolted Connection:
- Involves pre-embedded bolts in the track and beam, used with nuts to fasten securely; ideal for heavy-duty cranes.
- Welded Connection:
- Directly welds tracks to the beam’s top flange, providing high connection strength; however, disassembly is difficult when the track’s position must remain fixed.
Design Considerations:
- Connection devices must have sufficient strength and rigidity to withstand dynamic loads from crane wheels.
- Consider thermal expansion and contraction to prevent deformation or connection failure.
1.3.3 Buffer Stops
Buffer stops are safety devices installed at the ends of crane tracks to prevent the bridge or trolley from overrunning the track limits, thereby avoiding derailment or collisions.
Types of Buffer Stops:
- Fixed Buffer Stops:
- Secured to track ends via bolts or welding; structurally simple and suitable for most cranes.
- Shock-Absorbing Buffer Stops:
- Equipped with internal damping mechanisms (e.g., springs or rubber pads) to absorb collision energy; ideal for high-speed or heavy-duty cranes.
- Hydraulic Buffer Stops:
- Utilize hydraulic systems for advanced energy absorption; designed for applications requiring high-performance buffering.
Key Design Considerations:
- Must possess sufficient strength and rigidity to withstand impact loads without deformation.
- Should be positioned based on the crane’s operating speed and braking distance to ensure complete stoppage before contact.
- The height must align with the crane’s wheel elevation to prevent overrunning.
Maintenance Guidelines:
- Regularly inspect fasteners (bolts/welds) for integrity.
- Monitor the condition of damping components (springs, rubber pads) and replace them as necessary.
1.3.4 Integrated Design of Tracks and Buffer Stops
- Holistic Approach:
- Design must consider the crane’s operating speed, load capacity, and environmental conditions (e.g., temperature variations, corrosion).
- Robust Connection:
- Ensure buffer stops are rigidly anchored to the tracks to prevent displacement or deformation during impacts.
- Safety Optimization:
- For heavy-duty or high-speed cranes, prioritize shock-absorbing or hydraulic buffer stops to enhance overall safety.
Conclusion
The structural system of the crane beam for overhead cranes encompasses several key components:
- Crane Beam: Designed to handle various load types (vertical, longitudinal, lateral) using methods like influence line analysis to ensure adequate bending, shear resistance, and overall stability.
- Brake Structures and Auxiliary Supports: Enhance safety under complex loading conditions.
- Crane Tracks, Connections, and Buffer Stops: Critical for smooth operation and accident prevention.
By following rigorous scientific design principles, employing strict construction practices, and maintaining regular inspections, the service life of overhead cranes can be substantially extended, reducing operational risks while ensuring both production safety and economic efficiency.
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