The Function of the Coil Core
The coil core, also called the spindle or mandrel, is the central cylindrical support of a steel coil. During production it ensures stable uncoiling and rewinding on rolling mills, galvanizing lines and annealing furnaces. During storage and transport it acts as the main support and stress point for hoisting, stacking and transportation, bearing the weight of the coil itself.
Forces Acting on the Core
Two force patterns dominate. When coils are stacked flat, the core of the bottom coil is compressed by the weight of the coils above, producing compression and radial pressure. When coils are stood upright and lifted with C-hooks through the bore, the core behaves like a beam and carries bending stress generated by the full weight of the coil.
Key Factors Determining Load-Bearing Capacity
| Factor | Influence |
|---|---|
| Core material | Steel cores are strongest and reusable; fiber cores are light and low cost but have very limited capacity |
| Wall thickness | The most critical factor; thicker walls resist compression and bending better |
| Inner diameter | Standard bores of 508 mm (20 in) and 610 mm (24 in); larger bores allow stronger cores |
| Coil weight and length | Capacity must be matched to the heaviest coil in the pallet or stack |
Industry Practice and Safe Stacking Rules
Core manufacturers publish maximum safe load capacities for each specification (inner diameter x wall thickness x length) based on the principles of material mechanics, and this is the most direct source of load values. In warehouses and logistics, stacking layers are strictly limited: coils over about 10 tonnes are stacked in one layer; medium-weight coils in a maximum of two layers; light coils in three layers, provided the bottom core and foundation have sufficient capacity. More than three layers is absolutely prohibited because the pressure on the bottom core increases sharply with each added layer. A design safety factor of 3 to 5 times is normally applied to cover dynamic impacts such as sway during hoisting.
Safe Operating Guidelines
Label each coil clearly with its weight; where possible mark the core with its rated load.
Use vertical C-hooks for lifting, the most favorable loading mode for the core.
When using wire ropes through the bore, install corner guards to prevent cutting the inner wall.
Use dedicated wooden blocks or supports between stacked layers to keep cores aligned and avoid direct compression of the steel strip.
Keep the stacking foundation firm and level.
Inspect the core before lifting for flattening, cracking or severe deformation; stop using it immediately if damaged.
FAQ
Is there an international standard for coil core capacity?
There is no single universal standard number for core capacity; the authoritative values come from the core manufacturer load tables, while coil handling practices follow the logistics and safety codes applicable at the site.
What are the standard core inner diameters?
The common industry-standard bores are 508 mm (20 inches) and 610 mm (24 inches).
Why can a coil stack have only three layers maximum?
Because the compression on the bottom core increases with each layer, and the risk of core collapse and coil damage grows sharply beyond three layers.
Is a steel core always better than a fiber core?
Steel cores are stronger and reusable, but fiber cores are cheaper and lighter for light coils; the choice depends on coil weight and handling frequency.
What is the recommended safety factor for core design?
A safety factor of 3 to 5 times the maximum static load is common practice to cover dynamic loads and impact.
How should a damaged core be handled?
Any core with visible flattening, cracks or severe deformation should be taken out of service immediately.