When handling steel or aluminum coils in roll forming or metal processing lines, the coil’s inside diameter (ID) and the uncoiler’s mandrel (arbor) size are critical for safe and efficient operation. Mismatched sizes can lead to machine damage, downtime, and safety hazards — often evidenced by a shear pin failure.

What Is the Coil ID and Mandrel?

• Coil ID (Inside Diameter): The hollow center of a coil of metal that slides onto the uncoiler.

• Mandrel / Arbor: The shaft inside the uncoiler that expands to grip the coil ID.

  • May include expanding fingers, jaws, or wedges.

  • Often protected by pads or sleeves.

  • Has an end stop or coil keeper to prevent the coil from sliding off.

Think of the mandrel as the “hand” that holds the coil steady while the uncoiler rotates. The coil ID is the “hole” the hand must fit.

Common Coil ID Sizes

Most coils come in standardized inside diameters:

Uncoiler design range examples:

• 16″–20″ (406–508 mm)

• 20″–24″ (508–610 mm)

Coils outside this range require an adapter sleeve or a properly sized uncoiler.

What Happens if the Coil ID Is Too Large?

When a coil has an ID larger than the mandrel’s expansion range:

1. Coil sits loose on the arbor – it cannot be gripped properly.

2. Slippage occurs – uneven feed, vibration, or “walking” of the coil.

3. Shock loads – sudden torque spikes transfer to the expansion system.

4. Shear pin failure – the sacrificial safety pin breaks to protect the gearbox and drive.

5. Potential damage to bearings, mandrel fingers, and gearbox if repeated.

6. Safety hazards – risk of coil dropping or clock-springing.

The shear pin is not failing due to a machine defect; it is doing its job to prevent far more expensive damage.

Why Shear Pins Break on Expansion

Shear pins are designed to fail under overload conditions:

• Overload from oversized coil ID – the mandrel cannot grip, and the expansion system is stressed.

• Operator forcing mandrel – pushing beyond its designed stroke causes sudden torque spikes.

• Worn or misaligned expansion components – binding jaws or wedges increase force on the pin.

• Hydraulic issues – over-pressurized cylinders can overload the pin.

Key point: Replacing the pin with a stronger one does not solve the problem — it can damage the gearbox or mandrel.

Corrective Actions & Best Practices

1. Confirm coil ID before loading – never force a coil beyond mandrel limits.

2. Use coil ID adapters / sleeves – safely increase the effective mandrel size.

3. Replace shear pins with OEM-rated parts – maintain safety design.

4. Inspect expansion system regularly – check jaws, wedges, bearings, and mandrel shaft.

5. Train operators – highlight risks of running oversized coils and how to identify proper coil sizing.

Visual Reference (Optional Diagram)

• Small coil ID – fits comfortably, jaws engage without stress.

• Correct coil ID – mandrel grips fully, smooth rotation.

• Oversized coil ID – loose fit, risk of shear pin failure, vibration, and safety hazards.

A simple diagram helps operators quickly see which coils are safe to run.

Summary

• The shear pin broke because the coil ID was too large for the mandrel.

• The coil could not be secured, overloading the expansion system.

• The pin’s failure prevented more serious damage to the mandrel and gearbox.

• Long-term solutions:

  • Use coils within standard IDs (16″, 20″, 24″)

  • Use adapters for larger IDs

  • Never force mandrels beyond their expansion range

By following these best practices, operators can protect the uncoiler, maintain coil quality, and avoid costly downtime.