Overhead travelling cranes are essential tools used in factories and workshops for lifting and transporting heavy materials, equipment, and products from one location to another. These cranes play a vital role in ensuring smooth operations in industries such as manufacturing, warehousing, and logistics. They come in two main types: hand-operated cranes and electrically operated cranes. Both types feature a similar basic structure but differ in how they are powered and controlled.
Components of an Overhead Travelling Crane
An overhead travelling crane consists of several key components that work together to lift and move heavy loads. The primary components of the crane include:
- Bridge: The bridge spans the bay of the shop, providing a horizontal support structure for the crane. It serves as the main framework for the crane’s movement.
- Trolley or Crab: This component is mounted on the bridge and is responsible for moving the load along the bridge. The trolley can move in either direction along the length of the bridge to position the load where it is needed.
- Gantry Girders: These girders rest on rails at both ends of the bridge and support the weight of the crane, the trolley, and the loads being lifted. They ensure the crane operates efficiently and safely by distributing the weight evenly across the structure.
The crane’s movement occurs along the rails that are positioned at either end of the bridge, which allows it to move longitudinally across the bay. The gantry girders support the weight transmitted through the travelling wheels of the crane, allowing it to perform its lifting tasks effectively.
Design Loads Considerations
When designing overhead travelling cranes, it is crucial to account for various loads and forces that affect the crane and its supporting structure. These include vertical and horizontal forces, impact factors, and thrusts that arise during crane operation. Below are some of the key design load considerations:
A. Factors to Consider
- Vertical Loads: These are the primary loads that the crane must bear, coming from the weight of the crane itself, the trolley, and the materials or equipment being lifted.
- Eccentricity Effects: When vertical loads are applied, they can create eccentricities (offsets) that affect the balance and stability of the crane. These effects need to be considered during the design phase to ensure the crane operates smoothly.
- Impact Factors: Impact loads arise from the dynamic nature of crane operations, such as when the crane starts or stops. These forces must be factored in during design to prevent structural failure.
- Internal (Surge) Thrust: Surge thrust occurs when there are sudden changes in direction or speed, which cause internal forces along the crane rails.
- Longitudinal Horizontal Thrust: This refers to forces that act along the length of the crane rails, which must be accounted for to prevent excessive stress on the structure.
B. Load Combinations
The design of crane components involves considering different load combinations to ensure safety and efficiency. For example:
- Vertical Loads with Full Impact: This combination includes the vertical load from a single crane with full impact, or the load from two cranes operating in tandem, along with the effect of any additional cranes that may be positioned for maximum load.
- Multi-Bay Gantries: For multi-bay crane systems, the loads must be calculated for the maximum effect across adjacent bays of the building. This ensures that the crane system is designed to handle the combined loads from multiple cranes in operation.
- Longitudinal Thrust: The longitudinal thrust from two fully loaded cranes on the same track should be considered to ensure the gantry girders and other supporting structures can handle the stresses.
C. Earthquake Considerations
When designing crane systems, earthquake forces must also be considered, especially for structures located in seismic zones. The effect of dead loads from cranes parked in each bay should be evaluated to ensure the structure can withstand seismic activity.
D. Bumper Impact Loads
Cranes are often equipped with bumpers to reduce the impact when they reach the end of their travel. The gantry girders supporting these bumpers must be checked for the potential impact loads to ensure they are strong enough to withstand these forces.
E. Permissible Stress Increases
To accommodate the combined effects of vertical and horizontal surge loads, the permissible stresses in the crane’s design can be increased by up to 10%. However, this increase does not apply in cases where wind load is the primary force acting on the structure.
Design Basis of Gantry Girder
The gantry girder is a critical component of the overhead crane system, as it bears the weight of the crane and its loads. When designing the gantry girder, several factors must be considered to ensure its strength and stability:
A. Design Assumptions
The gantry girder is designed based on the assumption that both horizontal and vertical forces will act simultaneously on the structure. These forces include the weight of the crane, the load being lifted, and any dynamic forces such as impact loads. The forces primarily act at the level of the rails, which means that the gantry girder must be able to resist bending and twisting due to these loads.
B. Design Simplifications
To simplify the design calculations, the channel section at the top flange of the girder is used. This section helps resist bending in the horizontal plane and increases the moment of inertia about the y-axis. While transverse loads (those acting perpendicular to the rails) are relatively small, the use of a channel section helps in resisting the horizontal bending stresses without introducing significant error into the design calculations.
C. Bending Stress Computations
The gantry girder experiences bending both in the vertical and horizontal planes. The design calculations must account for these bending stresses to ensure the girder is strong enough to withstand the forces. The combined bending stresses are the sum of the stresses in both planes, and the total stress must be kept below the allowable bending compressive stress to avoid structural failure.
Types of Loads and Additional Loads
In addition to the primary loads described above, additional loads must be considered in the design of overhead travelling cranes. These additional loads are typically based on the type of crane (electric or hand-operated) and are given as a percentage of the maximum static wheel load. The static wheel load is the reaction on the crane wheels due to the total load, which includes the weight of the crane, the trolley, and the lifted load.
A. Vertical Loads
- Electric Overhead Cranes: 25% of the maximum static wheel load
- Hand-Operated Cranes: 10% of the maximum static wheel load
B. Horizontal Forces
- Transverse to Rails:
- Electric cranes: 10% of the total weight of the trolley and lifted load
- Hand-operated cranes: 5% of the total weight of the trolley and lifted load
- Along the Rails: 5% of the static wheel load
Conclusion
Overhead travelling cranes are integral to the efficient movement of heavy materials in industrial settings. The design of these cranes requires careful consideration of various loads, forces, and stress factors to ensure the crane system operates safely and effectively. From vertical and horizontal loads to dynamic forces such as impacts and surges, every aspect of crane operation must be taken into account during the design process. By following the guidelines set forth in standards such as IS:875-1964 and IS:800-1984, engineers can ensure that overhead cranes are built to handle the stresses they will face, providing reliable and safe performance in a variety of industrial applications.