Overhead cranes are a critical part of material handling systems across industries ranging from manufacturing and assembly to warehouses, steel plants, and shipyards. One of the key considerations when selecting the right crane system is its load capacity. The capacity of an overhead crane directly affects its suitability for specific applications, its cost, and its long-term performance. Among the many types of overhead cranes, two of the most common are top running cranes and underhung (or under-running) overhead cranes.
While both serve the same essential purpose – lifting and transporting loads within a facility – their designs differ significantly, leading to notable differences in their load capacities. Understanding these differences is crucial for choosing the right system that aligns with your operational needs.
What Are Top-Running Overhead Cranes?
Top-running overhead cranes feature a bridge that rides on rails mounted on the top of runway beams. This design allows the crane to carry heavier loads because the weight is distributed on top of the beams, which are usually integrated into a facility’s structural steel or supported by independent columns.
Top-running cranes are generally available in single girder and double girder configurations. Single girder overhead cranes are suitable for moderate lifting tasks, while double girder cranes can support much higher capacities, often exceeding hundreds of tons.
Key Characteristics:
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Operate on rails mounted above the runway beams.
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Support both light-duty and very heavy-duty lifting.
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Can be installed with long spans and higher lifting heights.
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Offer flexible customization for demanding industries.
What Are Underhung Overhead Cranes?
Underhung or under-running overhead cranes operate differently. Their end trucks are suspended from the bottom flange of runway beams, meaning they “hang” underneath the support structure. This design makes underhung cranes particularly suitable for facilities with limited headroom or for applications where space optimization is a priority.
Like top-running cranes, underhung models are also available in single girder and double girder designs. However, due to their suspended configuration, their capacities are typically lower than those of top-running cranes.
Key Characteristics:
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Travel on the underside of runway beams.
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Ideal for lightweight to medium-duty applications.
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Suitable for facilities with limited floor space or headroom.
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Commonly used in manufacturing lines, small workshops, and maintenance facilities.
Capacity Differences Between Top-Running and Underhung Cranes
The most significant distinction between these two crane types lies in their load capacities. This difference arises from their structural design, support mechanisms, and intended applications.
1. Structural Load Bearing
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Top-running cranes: Since the crane bridge runs on rails above the runway beams, the weight of the load is directly supported by the beams and transferred efficiently to the supporting columns or structure. This setup allows for much greater load-bearing capacity.
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Underhung cranes: The crane hangs from the lower flange of the runway beams, which limits the structural load they can safely carry. The bottom flange is less robust than the top of the beam, naturally restricting lifting capacity.
2. Typical Capacity Ranges
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Top-running cranes: Can handle very heavy loads, from 1 ton for small workshops up to hundreds of tons for heavy industries such as shipbuilding, steel plants, or power generation. Double girder top-running cranes are especially common in capacities ranging from 20 tons to 500 tons or more.
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Underhung cranes: Generally designed for lighter loads, usually between 1 ton and 15 tons, although some special designs can handle slightly more. Their capacities make them more suitable for assembly lines or facilities requiring repetitive handling of lighter components.
3. Girder Design and Its Influence
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Single girder top-running cranes: Often support loads from 1-20 tons, depending on the design.
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Double girder top-running cranes: Can accommodate 20 tons to over 500 tons.
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Single girder underhung cranes: Typically support loads up to 10 tons.
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Double girder underhung cranes: Rare, as the suspended design makes heavy-duty double girder crane systems less practical and more expensive to engineer.
4. Span and Height Capabilities
Capacity is also indirectly influenced by how much span and lifting height a crane can support.
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Top-running cranes can be built for long spans and high lifting heights without sacrificing capacity.
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Underhung cranes, in contrast, are limited in span and lifting height because of their reliance on the bottom flange of runway beams, further restricting their maximum load handling.
Choosing Between Top-Running and Underhung Cranes Based on Capacity
When selecting a crane, capacity is often the deciding factor, but it should be considered alongside your facility’s layout and operational needs.
When to Choose a Top-Running Crane:
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Your operations require lifting more than 15 tons.
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You need long spans or high lifting heights.
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Heavy industries such as steel mills, shipbuilding, power stations, or construction precast yards are involved.
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You require a crane system that can be expanded or upgraded in the future.
When to Choose an Underhung Crane:
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Your lifting requirements are less than 15 tons.
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You operate in a facility with limited headroom or confined space.
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You need cranes that can run on the same runway system for multiple workstations.
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Applications involve assembly lines, small machine shops, or maintenance workshops.
Safety and Capacity Considerations
Regardless of the crane type, it’s essential to remember that exceeding rated capacity can lead to catastrophic failures, including structural damage, accidents, or loss of life. Both top-running and underhung cranes must be equipped with load limiters, overload protection systems, and regular inspections to ensure safe operation.
Key Safety Practices:
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Always adhere to the manufacturer’s rated load capacity.
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Perform regular load testing as required by OSHA or other applicable standards.
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Train operators on the differences in handling loads with top-running vs. underhung cranes.
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Incorporate modern monitoring systems to track real-time loading conditions.
Conclusion
The choice between top-running and underhung overhead cranes largely depends on the load capacity your operations demand.
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Top-running overhead cranes are the go-to solution for medium to ultra-heavy lifting tasks, with capacities ranging from a few tons to over 500 tons. They are best suited for industries that require robust, large-scale material handling solutions.
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Underhung overhead cranes, on the other hand, are designed for lighter loads, typically under 15 tons, making them a cost-effective choice for smaller facilities or for operations where maximizing headroom and workspace is critical.
By carefully evaluating your lifting requirements, facility constraints, and long-term operational goals, you can choose the right crane system to enhance efficiency, safety, and productivity in your workplace.