How to Choose a Magnetic Powder Brake

How to Choose a Magnetic Powder Brake

How to choose a magnetic powder brake comes down to matching four variables — the torque your unwind roll demands, the physical form that fits your machine, the heat the unit must dissipate in continuous slip, and the control loop that keeps tension flat from full roll to core. A magnetic powder brake resists the pull of the web on an unwinder so the roll pays off at a controlled tension instead of free-spinning; it is the mirror image of the clutch that drives the rewind side. If you are still deciding what this component is for, start with our explainer on what a magnetic powder brake is used for before you size one.

Start With the Job: Brake on the Unwind Side

The brake sits on the unwind shaft and always works against the web. Unlike a clutch that transmits drive torque to a rewinding roll, the brake's only task is to hold a stable drag torque. That distinction drives every selection decision that follows: you size for the largest roll the unwinder sees, because the brake's peak torque demand occurs at the start of the run when the roll is biggest.

Step 1 — Define the Torque Requirement

Every brake selection begins with the same mechanical backbone:

  • T = F × r
  • T = required braking torque (newton-metres, N·m)
  • F = peak web tension (newtons, N)
  • r = effective roll radius (metres, m) at the point where tension acts

The detail that trips up most first-time specifiers is that the radius is never constant. As the roll unwinds it shrinks, so the brake's highest torque load is at the full roll at the beginning of the run. Always size for that worst-case (largest) radius, then apply a safety margin of roughly 1.2 to 1.5 times the calculated value. Our dedicated guide on how to calculate magnetic powder brake torque walks through the formula with two worked unwinding examples.

Step 2 — Match the Brake Type to Your Mounting

Once the torque band is known, pick the physical form that fits the shaft and envelope of your machine. XW Machinery supplies several proven configurations:

The rated torque and torque–current curve for each model are printed on the nameplate and characteristic chart — use those exact figures, not a generic slope, when you confirm the frame size.

Step 3 — Check Thermal Rating and Duty Cycle

A magnetic powder brake in tension control runs in continuous slip, and the heat it must dissipate equals torque multiplied by slip speed. On an unwinder the rotor may turn slowly while the web moves fast, so slip heat can be large even at modest torque. A brake that is torque-adequate but thermally undersized will overheat, lose torque, and damage the powder. This is why duty cycle and cooling matter as much as the peak number. Read our brake overheating causes and solutions article before finalizing a high-slip application, and confirm the thermal rating with the manufacturer for your run profile.

Step 4 — Decide Open-Loop vs Closed-Loop Control

How you drive the brake changes how stable your tension is:

  • An open-loop tension control system sets a fixed excitation current. It is simple and cheap, but tension rises as the roll shrinks because the same torque acts on a smaller radius.
  • A closed-loop tension control system reads actual tension from a load cell or sensor and trims current as the radius falls, holding tension flat from full roll to core.

For precision slitting, printing, and laminating, closed-loop is the safer choice. The miniature tension controller for magnetic powder brake is one compact option for small frames and narrow lines.

Step 5 — Confirm Voltage, Shaft, and Interface

Before you place the order, lock down the interface details that are hard to change later:

  • Supply voltage and excitation current range, matched to your controller.
  • Shaft diameter, keyway, and hollow-bore size for direct-mounted units.
  • Heat-sink or forced-air provision if the duty is continuous.
  • Signal and feedback compatibility with your existing tension controller.

These are mechanical and electrical fit items, not performance items, but a mismatch here is the most common reason a correctly-rated brake still cannot be installed.

Brake vs Clutch — Don't Confuse the Two

A clutch and a brake share the T = F × r formula but sit on opposite sides of the web. The clutch drives on the rewind side and peaks at the end of the run when the roll is largest; the brake resists on the unwind side and peaks at the start. Size each for its own worst-case radius, and pair them through a shared tension system. If you are matching a clutch to the rewind side, our clutch-to-rewinding-machine guide covers that half, while the clutch-side math is detailed in the magnetic powder clutch torque calculation.

Common Selection Mistakes

  • Sizing for average tension instead of peak tension on the unwind side.
  • Forgetting that radius changes through the run; always use the largest radius for an unwinder.
  • Picking a torque-adequate brake that is thermally undersized for continuous slip.
  • Choosing open-loop control for a process that needs constant tension.
  • Treating one model's torque–current curve as universal across the range.

Frequently Asked Questions

Should I choose a brake or a clutch for my unwinder?

An unwinder needs a brake, because its job is to resist the web, not drive it. A rewinder needs a clutch. The two are not interchangeable even though they share the same powder technology.

How do I know which frame size fits?

Calculate the peak torque from your maximum tension and largest roll radius, apply the safety margin, then read the nameplate-rated torque of each candidate frame. The DIN-standard brake product page lists the rated torque ranges for each model.

Can one brake handle both narrow and wide web lines?

Only if the torque and thermal ratings cover the wider line's peak. A brake sized for a narrow web will saturate and overheat on a wide, high-tension line, so confirm both numbers against the larger duty.

Conclusion

Choosing a magnetic powder brake is a four-step discipline: define the peak torque from T = F × r at the full roll, pick the mounting form that fits the machine, verify the thermal rating for your slip duty, and select the control loop that holds tension flat. Cross-check the brake against the clutch on the rewind side so the whole line stays matched. For help matching a brake to your exact web, material, and run profile, contact XW Machinery and our engineers will recommend a rated frame and model from the magnetic powder clutches and brakes range.

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