Determining Tension and Torque for Unwinding & Rewinding Systems with Magnetic Particle Brakes

By Sam Hoffman | August 26, 2025

When it comes to unwinding and rewinding systems, precision is everything. From films and foils to wires and web materials, the ability to maintain consistent tension directly impacts product quality, machine efficiency, and overall process reliability.

That’s where magnetic particle brakes and clutches come in. These components offer smooth, adjustable tension control that makes it possible to design simple yet highly effective systems for a wide variety of materials. By pairing them with a DC power supply and a roll diameter sensor (such as an ultrasonic sensor or follower arm potentiometer), manufacturers can achieve precise and repeatable control—without overcomplicating the setup.

In this guide, we’ll walk through the key steps in determining tension and torque for your application, while also addressing speed, heat dissipation, and power supply considerations.

Why Tension Control Matters in Unwinding & Rewinding

Whether you’re unwinding supply rolls or rewinding finished material, maintaining the correct tension is critical:

  • Too much tension can stretch or deform materials.

  • Too little tension can cause wrinkles, slack, or misalignment.

  • Inconsistent tension leads to poor winding quality and potential machine downtime.

Magnetic particle brakes and clutches excel because they provide smooth, adjustable braking and torque across a wide speed range, making them ideal for roll-to-roll applications.

Step 1: Determining Tension Requirements

The first step in designing a system is to determine the proper tension for your material. This is often expressed in pounds, ounces, or grams.

  • Material manufacturer specs: Many suppliers provide recommended minimum and maximum tensions.

  • Experimental measurement: A force gauge or even a simple fish scale can be used to measure actual tension.

Getting this step right ensures that downstream calculations—like torque and thermal load—are accurate.

Step 2: Choosing Roll Diameters

Tension interacts directly with roll diameter. Two key diameters must be defined:

  • Full roll diameter – larger rolls rotate slower but require higher torque.

  • Core (empty) diameter – smaller cores rotate faster but require lower torque.

⚠️ Avoid pairing an extremely large full roll with a very small core. This combination demands a wide torque and speed range, complicating control.

Step 3: Determining Torque

Once you know your material tension and roll diameters, you can calculate torque requirements.

  • Maximum Torque = Max. Web Tension × (Max. Roll Radius)

  • Minimum Torque = Min. Web Tension × (Min. Core Radius)

Example:

  • Full roll: 18″ diameter, 5 lbs. tension → 18 ÷ 2 × 5 = 45 lb-in torque

  • Core: 3″ diameter, 2.5 lbs. tension → 3 ÷ 2 × 2.5 = 3.75 lb-in torque

If multiple rewind rolls are involved (e.g., interleaf film separation), repeat calculations for each roll.

This step is critical—determining torque correctly ensures your magnetic particle brake or clutch is sized appropriately for the job.

Step 4: Calculating Speed (RPM)

Next, calculate the roll speed requirements.

  • Maximum RPM occurs at the smallest roll diameter and the highest line speed:

    MaxRPM=3.82×(LinearSpeedinft/min)÷(CoreDiameterininches)

Example: At 200 ft/min with a 3″ core:

MaxRPM=3.82×200÷3=255RPM

⚠️ Keep in mind: At very low slip speeds (below 25 RPM), elastic webs may experience stick-slip issues.

Step 5: Managing Heat Dissipation

Every brake and clutch generates heat due to slip. Correctly calculating thermal load prevents overheating.

  • For brakes:

    SlipHeat(watts)=Tension×WebSpeed÷44.2

  • For brakes & clutches:

    SlipHeat(watts)=Torque×SlipRPM×0.0118

If heat dissipation is too high, choose a larger brake/clutch or use a gearbox to reduce slip RPM while increasing torque.

Step 6: Power Supplies for Constant Tension

To maintain constant tension control, torque must automatically adjust as roll diameter changes. This is achieved with:

  • Ultrasonic sensors or follower arm potentiometers that measure roll diameter.

  • A DC power supply (with scaling capability or through a signal conditioning board) that adjusts output in proportion to roll size.

This closed-loop control keeps winding tension consistent from full roll to empty core.

Why Magnetic Particle Brakes Are Ideal for Tension Control

  • Smooth performance: Unlike mechanical brakes, particle brakes avoid stick-slip at higher speeds.

  • Adjustable torque: Easy integration with DC power supplies.

  • Scalable: Suitable for both small web operations and high-speed industrial systems.

By carefully determining tension, torque, speed, and thermal requirements, you can build efficient, reliable unwinding and rewinding systems with minimal complexity.

Build Your Tensioning System with Confidence

At Placid Industries, we specialize in helping manufacturers design web handling systems that run smoothly and efficiently. Whether you’re determining torque for a new unwinding line or troubleshooting rewinding inconsistencies, our magnetic particle brakes and clutches provide the reliability and control you need.

👉 Contact us today to discuss your application and get expert guidance on sizing the right tension control solution.

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