Warehouse managers and facility owners with limited vertical space or structural constraints often face challenges when installing overhead cranes. An underhung bridge crane (under-running bridge cranes) offers a smart solution for these scenarios. Underhung cranes maximize space by suspending the crane from the building's roof support, eliminating the need for floor-mounted runway columns.

This article explains what underhung cranes are, describes their key components and operation, compares their advantages and limitations to top-running cranes, discusses single vs. double girder variations, and provides practical considerations for selecting an underhung crane. We also highlight how Hoj Innovations, with 60+ years of overhead crane experience, can assist in implementing underhung crane systems.

 

What Is an Underhung Crane?

An underhung crane is an overhead bridge crane where the bridge (the horizontal beam carrying the hoist) is supported from the bottom flange of elevated runway beams rather than sitting on top. In an underhung (or under-running) configuration, the crane’s wheels ride on the bottom of the runway beam’s flange, allowing the entire crane to hang below the runways.

The runways are usually attached to the facility’s roof structure or ceiling trusses, so the crane is under-hung from above. This design is ideal for buildings that lack the headroom to accommodate a traditional top-running crane system. By suspending the crane, underhung systems free up floor space since no large support columns are needed on the ground (assuming the roof can bear the load).

Underhung cranes are sometimes called under-running overhead cranes for this reason.

Underhung cranes are commonly used for lighter loads and are frequently built as single-girder cranes (one bridge beam) for low—to medium-capacity applications.

Double-girder underhung cranes(two bridge beams) are possible, and they can support higher capacities and offer different hoist mounting options. However, double-girder underhung designs become complex and are less common beyond certain size limits. In general, underhung bridge cranes excel in situations where space is at a premium or the crane needs to integrate into an existing building without heavy structural modifications.

 

Key Components and Operation of Underhung Cranes

Like all bridge cranes, an underhung crane consists of several main components that work together to lift and move loads:

• Runway Beams – These are the horizontal structural beams that the crane runs along. In underhung systems, runways are typically attached to the roof trusses or ceiling structure and have a flat bottom flange that acts as the travel surface for the crane’s wheels. The runways carry the weight of the crane and the load, transferring it to the building structure (or to support columns if a freestanding runway is used).

• Bridge Girder – The bridge is the beam (or girders, in a double-girder design) that spans between the two runways. It carries the hoist and trolley and moves perpendicular to the runways. In an underhung crane, the bridge is suspended via end trucks from the bottom of the runway. The bridge girder travels along the runways to position the hoist over the load.

• End Trucks – These are wheeled assemblies at each end of the bridge girder that enable the bridge to roll along the runway beams. In underhung cranes, the end truck wheels ride on the underside of the runway flange. The end trucks connect the bridge to the runways and house the drive mechanism (for motorized cranes) that moves the bridge.

• Hoist and Trolley – The hoist is the lifting mechanism (with rope or chain and a hook) that raises and lowers the load. The hoist is mounted on a trolley, which allows it to move back and forth along the length of the bridge girder. In an underhung single-girder crane, the trolley wheels typically run on the bottom flange of the bridge (an under-running trolley). This means the hoist hangs below the bridge. In a double-girder underhung crane, the hoist can either be underslung (under the two girders) or placed between/above the two girders for extra lift height.

When an underhung crane is in operation, the bridge travels along the runways (covering the length of the building or work area) and the trolley moves across the bridge (covering the width). This provides two axes of horizontal motion, allowing the hoist to be positioned directly over any point in the coverage area.

Because the entire crane hangs from the runways, underhung cranes occupy minimal vertical space – the bridge and hoist are below the runways, enabling operation in buildings with low ceilings or obstructions. However, it’s important that the building’s roof structure is strong enough to support the crane’s weight and the loads to be lifted; engineers must verify the roof capacity or add supports if needed.

 

Underhung Crane Solutions by Hoj Innovations

If you believe that anunderhung crane is the right fit for your warehouse, partnering with an experienced team is crucial. Hoj Innovations, with over 60 years of expertise, designs and installs custom crane solutions for warehouses with limited space or unique structural needs.

Our engineers and project managers provide complete support, from initial assessments and design through installation and ongoing service. We’ll ensure your underhung crane integrates seamlessly into your existing operations, maximizing efficiency and safety.

Contact us today for a personalized consultation and detailed project quote.

 

Advantages of Underhung Cranes

Underhung cranes offer several advantages that make them attractive for certain applications, especially compared to top-running overhead cranes:

• Maximized Floor Space: Because underhung cranes are supported from above, they do not require floor-mounted runway columns in many cases. An underhung crane can free up valuable floor area for operations and storage by hanging from existing ceiling or roof supports. This is a major benefit in crowded warehouses or factories where floor obstructions (like support columns) would interfere with workflow.

• Ideal for Low Headroom: Underhung systems are well-suited for facilities with limited vertical clearance. The crane operates below the runway beams, allowing installation in buildings with lower ceilings unsuitable for a top-running crane. Experts commonly recommend underhung cranes for facilities that “lack vertical clearance to install a top-running overhead crane system. (Do note that because the hoist hangs under the bridge, the hook height – the highest point the hook can reach – will be somewhat less than in a top-running crane. We discuss this trade-off later.)

• Excellent Side Approach: Underhung cranes can typically allow the hoist hook to get closer to the walls or the end of the runway than top-running cranes. Since the bridge and hoist are underneath the runway, the hook isn’t constrained by large end truck assemblies sitting atop a beam. This means improved end approaches – the crane can cover more of the building’s width and reach near the building edges. In practical terms, an underhung crane minimizes “dead zones” at the extremes of travel, maximizing the use of the facility’s layout.

• Flexible Layouts and Multi-Crane Systems: Underhung cranes offer greater flexibility in certain multi-crane or complex layouts. Installing multiple underhung cranes on parallel runways in the same bay without interfering with each other is possible. In wide areas, you could have two or more underhung bridge cranes operating side-by-side on separate runways, even passing each other if the runways are independent. Moreover, underhung systems can incorporate interlock mechanisms and switches to transfer a hoist from one crane or monorail track to another. This means you can design an integrated material handling system with branch tracks, curves, and transfer points – a level of flexibility that top-running cranes (which are confined to their fixed rails) cannot easily match. Such adaptability is particularly useful in production processes where loads need to move through multiple workstations or along complex paths.

• Lower Installation Cost (If Structure Allows): If a building’s existing roof structure is strong enough to support an underhung crane, you can avoid the cost of installing new support columns or a gantry structure. Utilizing the existing ceiling support can reduce material and construction costs for the crane runway. Additionally, underhung cranes tend to be used for lighter capacities, meaning the runways and bridges are smaller and weigh less than a heavy top-running crane – this can translate to cost savings in equipment and installation. (For instance, eliminating runway columns saves steel and avoids adding obstacles on the floor.)

"We often recommend underhung cranes, this frees up floor space and eliminates the need for floor mounted columns but is also dependent on the ceiling structure being engineered to allow the additional crane loads to the ceiling. The approach of the hoists to runway centerlines also maximizes the available floor space.” - Jerry LeBaron – HOJ Innovations Engineer

In summary, underhung cranes shine in applications up to moderate capacities where maximizing floor space, maintaining flexibility in the workflow, or coping with low overhead height are critical.

You commonly find them in assembly shops, maintenance facilities, manufacturing lines, and warehouses handling lighter loads and where the building structure can support the crane.

 

Limitations of Underhung Cranes

Despite their benefits, underhung cranes have some limitations and trade-offs compared to top-running cranes. Key considerations include:

• Lower Load Capacity: Underhung bridge cranes are generally limited to lighter lifting capacities. Because the crane is suspended (with wheel loads hanging from the bottom flange), there are practical limits to how much weight can be handled. Typical underhung crane capacities range from a few hundred pounds up to about 10 or 20 tons maximum. In contrast, top-running bridge cranes can be built for extremely heavy loads (21, 50, 100+ tons) because they put the load directly onto the rails above the beams. An underhung crane may be unsuitable if your application involves very large, heavy lifts. Manufacturers often suggest that for loads above 20 tons, a top-running crane is the better choice.

• Reduced Hook Height: In an underhung system, the hoist and bridge are mounted below the runway, which inherently eats up some vertical space in the lifting geometry. The top position of the hook will be lower than it would be on a top-running crane where the hoist may sit on top of the bridge girders. This reduced lift height means you must ensure that an underhung crane can still lift your loads high enough for your needs. In some cases of very low headroom, a top-running single-girder (or especially a top-running double-girder) crane might achieve an extra 3-6 feet of hook height over an underhung crane. If maximum vertical lift is a priority and the building can accommodate it, a top-running design could be preferable.

• Dependency on Building Structure: Since, Underhung cranes rely on the building’s roof or ceiling structure to support the runways. Not all buildings are built to handle these loads. If the roof trusses or support beams are not strong enough, you might have to reinforce the structure or add a freestanding support system – which can reduce the cost advantage. If the existing support structure can't support the crane, you can mount underhung cranes on dedicated runway columns—but you'll lose the floor-space benefit. Always have a structural engineer evaluate the building for the additional loads.

• Span and Coverage Limits: Underhung cranes typically have shorter maximum spans (distance between runway beams) than comparably sized top-running cranes. This is partly because the runways and bridge in underhung designs need to be stiff enough to carry the load while hanging; beyond a certain span, the deflection and stresses become impractical without beefing up the structure (which adds cost). In general, facilities use underhung cranes for moderate bay widths, typically under about 60–65 feet spans. Very large buildings that need wide crane span coverage might necessitate a top-running crane or additional runways.

• More Complex Maintenance: Since an underhung crane’s components (trolley, end trucks, etc.) are literally hanging beneath the runways, accessing them for inspection or maintenance can be trickier. Maintenance personnel may need to use lifts or temporarily remove components to service wheels or motors. By contrast, a top-running crane sits atop the runway where there may be catwalks or easier access for technicians. Underhung crane installation and alignment can also be more time-consuming because everything must be suspended and precisely leveled. While underhung systems generally have fewer tracking issues once aligned (since they ride in a fixed flange of the beam), the initial setup and any future adjustments will require careful work at elevation.

• Higher Capacity Underhung = Higher Cost: As mentioned, it is possible to build double-girder underhung cranes for higher capacities, but these designs can become impractical and expensive to engineer beyond a certain point. Suppose you push an underhung crane to handle heavier loads or long spans. In that case, the system might need custom engineering, very robust (and costly) patented track or support systems, and extensive building modifications. At some point, it may be more sensible to switch to a top-running crane for heavy-duty needs.

In summary, underhung cranes are best suited for light to medium-duty operations. They trade off brute strength and maximum lift height for flexibility and space efficiency. Always weigh the capacity and performance needs of your application against these limitations when deciding between underhung and top-running configurations.

 

Single vs. Double Girder Underhung Cranes

We've already touched upon many key differences between single and double girder underhung cranes, but here’s a brief recap.

• Single-girder cranes are simpler, lighter, and more economical, ideal for most warehouse and industrial uses with moderate lifting requirements, though their hook height is limited by the girder’s position.

• Double-girder cranes use two parallel bridge beams, allowing the hoist to sit higher and handle heavier loads or greater spans; however, they’re typically costlier and more complex to install.

Always consult a crane specialist to choose the best type for your facility’s specific needs.

 

Practical Considerations for Selecting an Underhung Crane

We've covered many of these points already, but here’s a simplified recap, provided as a quick-reference list for easy decision-making:

• Building Support: Confirm your existing roof or structure can handle the crane or if new supports are needed.

• Load Capacity: Underhung cranes typically handle lighter loads (up to 10–15 tons); heavier loads might require a top-running crane.

• Headroom: Check if your available ceiling height suits the lower profile of underhung cranes.

• Lift Height: Ensure the underhung crane provides sufficient vertical lift for your tallest loads.

• Coverage Area: Consider if you need a better hook approach near walls—underhung cranes excel here.

• Floor Space: Decide if maximizing unobstructed floor space is critical—underhung cranes eliminate columns.

• Maintenance: Plan for slightly more complex servicing, as underhung components hang beneath the runway beams.

• New vs. Retrofit: Determine if you're integrating the crane into a new build or retrofitting an existing structure.

Always consult a crane specialist to match your crane selection closely with your facility's specific needs.

Conclusion

Underhung cranes are a powerful option for facilities facing space constraints or looking for flexible overhead lifting solutions. By hanging the crane from above, they maximize floor space and can adapt to complex layouts, all while efficiently handling light to medium-duty loads.

We’ve discussed underhung cranes, how they differ from top-running cranes, and what to consider when selecting one. Ultimately, the decision should hinge on your capacity needs, building structure, and workflow demands. With careful evaluation and the guidance of experienced professionals, an underhung crane system can significantly improve material handling in your warehouse or plant.

For businesses needing expert assistance, Hoj Innovations is ready to design, install, and support a custom underhung crane solution that meets your needs today and for decades to come. Reach out to our team to start optimizing your overhead lifting system, and take advantage of our 60+ years of overhead crane experience to ensure your project’s success.