A spreader frame is used in lifting operations that require vertical connection points to the load, with height, or equipment limitations, which means a standard spreader beam system wouldn’t be suitable. Here’s how load distribution works with a spreader frame:
Basic Structure of a Spreader Frame
- A spreader frame typically consists of two or more spreader beams connected by corner joints or apex points, forming a rectangular or square structure.
- The load is attached to the spreader frame via slings, hooks, or shackles, and the frame itself is connected to the lifting crane or hoist via slings.
Principles of Load Distribution
- Force Distribution: The spreader frame transfers the vertical lifting force from the load to the crane, while distributing these forces across several connection points, reducing stress on the load and preventing deformation or damage.
- Load Distribution: When properly rigged, the spreader frame load distribution is spread across all lifting points, ensuring that none of the lifting points exceeds the maximum WLL given to each individual component. The load distribution on the frame may not be even as it depends on the CoG of the item being lifted.
- Tension in Slings: Each sling connected to the load experiences tension. The length and angle of these slings impact how the forces are distributed. Slings positioned at shallower angles will experience higher tension, which is why the angle of slings is critical to safe load distribution. Sling angles need to be calculated for each lift, depending on the CoG of the load to ensure a level lift.
- Compression in Spreader Beams: While the slings are in tension, the spreader beams themselves experience compressive forces.
Effect of Sling Angles
- Shallow Sling Angles: If the slings are positioned at shallow angles, they can generate higher tension forces, which need to be accounted for to prevent overloading.
- Optimal Sling Angles: An optimal sling angle helps in achieving balanced load distribution and minimizes excessive tension in the slings. A proper angle ensures both vertical and horizontal forces are managed efficiently. Optimal sling angles, depend on the lifting environment, load and CoG position. If the CoG position is offset, then the slings will need to be calculated to ensure a level lift.
Load Symmetry and Distribution
- Symmetrical Loads: If the load is symmetrical, the spreader frame will distribute the weight evenly across all lifting points, making it relatively straightforward to balance.
- Asymmetrical Loads: In the case of asymmetrical or irregularly shaped loads, care must be taken to adjust sling lengths and connection points. When lifting uneven loads, the slings need to be calculated correctly above the CoG position to ensure a level lift, otherwise the load may tilt when lifting.
Load Weight and Distribution Capacity
- Spreader frames are rated for specific load capacities. Exceeding the rated capacity can lead to failure of the frame and slings.
- The lifting device must have enough capacity to handle the combined forces, each lifting point not exceeding the maximum WLL of the individual component.
Load Movement and Dynamic Forces
- During the lifting process, movement of the load (swaying or shifting) can cause dynamic forces that affect spreader frames load distribution. Operators must ensure smooth, controlled lifting throughout.
Applications of Spreader Frame Load Distribution
- Heavy Lifts: Spreader frames are commonly used in heavy-lifting operations, such as moving large machinery, shipping containers, or structures – either temporary, prefabricated or permanent.
- Multiple Lift Points: For large or irregular loads with multiple lifting points, spreader frames help avoid damage by ensuring that the weight of the load is shared across several points, reducing pressure on any single point.
- Low Headroom: For loads that require a lift with low headroom a spreader frame will likely be more suitable than a cascading spreader beam arrangement, as it doesn’t require multiple sets of spreader beams and slings to carry out the same lift.
- Vertical Bottom Slings: For loads that require multiple lifting points with vertical bottom slings, spreader frames are beneficial as this reduces the amount of equipment needed.
Conclusion
The load distribution in a spreader frame ensures that forces are evenly spread across multiple points, minimizing stress on the load and ensuring safety during lifting operations. The correct use of sling angles, appropriate load balance, and accounting for dynamic forces are all key factors in achieving optimal load distribution. Modulift spreader frames can lift from 6t to 300t with DNV type approval. For more information regarding DNV’s international standards, please visit: www.dnv.co.uk
Need help choosing the right spreader beam or frame? Call 01202 621 511 or email [email protected] for a quote. Download the Modulift Brochure to view Load v Span Charts. For worldwide delivery, find your nearest Modulift Distributor.
