Magnetic Powder Brake Torque Selection Guide

Magnetic powder brake torque selection guide is the short list of steps you follow to size a powder brake so it holds web tension on an unwinding roll from full diameter down to the core without overheating or running out of torque. The whole job comes down to two numbers — the peak resisting torque the brake must supply and the slip power it must absorb — plus choosing a shaft style that fits your unwind. This guide walks through each step with the relationships that matter, and points to the deeper references on our site when you need the math or a specific model.

Why Torque Rating Is the Heart of Brake Selection

A magnetic powder brake does one thing on a converting line: it resists rotation of the unwind shaft with a torque that a controller sets by coil current. There are no pads or air cylinders, so the rating that decides whether a given unit is right for your machine is its torque range. Undersize it and the web goes slack at the start of the roll; oversize it and you pay for capacity you never use and may struggle to hold low tension smoothly.

Because the transmitted torque rises almost linearly with current, a correctly sized brake lets the controller hold any setpoint inside its range. The selection question is therefore not "which brake is strongest" but "which brake covers my peak demand with margin and still reaches my lowest tension cleanly."

Start From the Worst-Case Unwind Torque

The torque a brake must supply equals the target web tension multiplied by the current roll radius. At the beginning of an unwind the parent roll is at its largest diameter, so the radius — and therefore the torque demand — is at its peak. That peak, not the average, is the number you size from. Our article on how to calculate magnetic powder brake torque lays out the exact relationship and the units, but the intuition is simple: bigger roll, same line speed, more resisting torque needed.

A sound torque selection always begins with the full-roll case. Take your highest production tension and your largest roll radius, multiply them, and you have the peak torque the brake must reach. Everything else — slip power, cooling, shaft style — builds on that starting point.

Match the Continuous Slip Power, Not Just Peak Torque

Sizing only on peak torque is the most common mistake. A brake on an unwinder is slipping the entire run: the rotor turns while the housing is held, and the difference is converted to heat inside the sealed gap. The slip power equals the resisting torque multiplied by the slip speed, and that heat has to be carried away by the unit's thermal rating. If the thermal rating is too low, the powder warms, its coupling weakens, and the brake loses torque mid-run — the classic symptom covered in our guide to magnetic powder brake overheating causes and solutions.

So a proper selection guide checks both curves on the rating chart: the torque curve across the full current range and the continuous slip-power limit. Pick the model whose thermal rating clears your worst-case slip power with margin, not just the one whose peak torque looks big enough.

Sizing for the Roll Radius Swing

The roll does not stay at full diameter. As it pays out, the radius falls toward the core, so torque demand drops to a fraction of the peak. That swing is good news and bad news. Good, because the brake only needs peak torque briefly; bad, because the controller must trim current down smoothly as the radius shrinks or the tension will overshoot and stretch the web.

This is where the way web tension changes during rewinding matters: the radius swing is the largest single cause of tension drift, and a brake sized only for the average will be far too large to hold the low end. Choose a brake whose minimum stable torque sits below your lowest required tension times the smallest roll radius, so the controller has room to back off.

Single-Shaft vs Hollow-Shaft Brakes

Once torque and slip power point to a model family, you choose how it mounts. A single-shaft magnetic powder brake carries the load on its own shaft and couples to the unwind through a coupling or safety chuck, which suits retrofit and compact stations. A hollow-shaft magnetic powder brake slides directly over the unwind shaft, removing a coupling and simplifying the build on new machines.

The torque-sizing logic is identical for both; the choice is mechanical, not electrical. If you are unsure which fits your frame, our overview of the four types of magnetic powder brakes breaks down flanged, shafted, and hollow builds so you can match the body to your station.

Brake vs Clutch: Sizing the Unwind Side

A brake and a clutch share the same powder-coupling physics, which is why the sizing mindset transfers directly. The difference is duty: the brake resists an unwinding roll while the clutch drives a rewinding one. If you already sized a clutch for the rewind, you can read the mirror logic in our magnetic powder clutch torque selection guide and flip the direction of the torque swing.

For the unwind specifically, the brake must hold against the web pulling the roll, so its peak torque matches the full-roll case described above. Understanding how a magnetic powder clutch works helps you see why both ends of the line are really the same actuator doing opposite jobs, and why a matched brake-and-clutch set keeps tension bookended through the knives.

Reading the Rating Chart and Nameplate

Every XW brake ships with a rating chart and nameplate that state its rated torque, rated slip power, and allowable continuous current. Selection in practice is just: plot your peak torque and peak slip power on that chart, find the model whose envelope contains both points, then confirm the lowest stable torque covers your core-diameter tension.

Do not read the chart as "pick the biggest." A brake running at the very top of its torque range runs hot and leaves no headroom for a thicker web or a bigger future roll. Aim for peak demand to land around the middle of the rated band so the unit stays cool and the controller has authority above and below the setpoint.

Common Sizing Mistakes

The recurring errors are predictable. First, averaging the roll radius instead of using the full diameter — this silently undersizes torque by a wide margin. Second, ignoring slip power and choosing on peak torque alone, which leads to the overheating and torque loss covered in our magnetic powder brake overheating guide. Third, forgetting the core-diameter case, so the brake cannot hold low tension at the end of the roll.

A fourth, subtler error is mismatching the brake to the rest of the loop. The brake is only the actuator; it needs a controller and a sensor that can close the loop fast enough. Our write-up on open loop vs closed loop tension control explains why a closed loop matters most exactly when the radius is swinging and the torque demand is moving.

Pairing With the Rest of the Tension Loop

Torque selection does not stop at the brake. The unwind brake must work with a web tension sensor that measures actual tension and a controller that trims coil current in real time, all inside a web tension control system built for your line. If you are still choosing the controller itself, our guide to choosing a web tension control system walks the options.

For lines that also rewind, pair the unwind brake with a differential shaft for slitting and rewinding machines so each lane can be tensioned independently, and review what a differential shaft does in web handling if you need the background. The brake sets the unwind tension; the shaft balances the rewind.

Conclusion

Magnetic powder brake torque selection guide in one line: size peak torque from your highest web tension times the largest roll radius, confirm the model's slip-power rating clears the heat of a full run, make sure its lowest stable torque still holds tension at core diameter, then pick a single- or hollow-shaft build that fits your station. Read the rating chart so peak demand lands mid-band, avoid the usual mistakes of averaging the radius or ignoring slip power, and close the loop with a proper sensor and controller. When you need model specifics, see how to choose a magnetic powder brake and how to install a magnetic powder brake, or contact XW Machinery with your line speed, web tension, and roll dimensions and we will specify the exact unit for your unwinder.