How to Calculate Magnetic Powder Clutch Torque

How to calculate magnetic powder clutch torque comes down to one relationship that every engineer in web handling eventually commits to memory: the torque a clutch must transmit equals the web tension acting on the roll multiplied by the radius at which that tension acts. Get that number right, and you can size the clutch, set the excitation current, and keep constant tension from core to full roll. This guide walks through the formula, a four-step method, two worked examples, and the slip-heat caveat that catches out first-time specifiers. If you are still deciding between models, pair this article with our magnetic powder clutch torque selection guide and our explainer on how a magnetic powder clutch works.

What "Torque" Means for a Magnetic Powder Clutch

In a magnetic powder clutch, torque is not produced by friction plates you clamp together — it is produced by a magnetic field acting on a fine powder suspended between the input and output members. The transmitted torque is set by the excitation current, and within the usable range it rises roughly in proportion to that current. For the purpose of sizing, however, you do not start from current; you start from the mechanical job the clutch must do in your machine, which is to hold or transfer a specific torque at the web.

The Core Formula — Torque Equals Tension Times Radius

The foundation of every clutch torque calculation is:

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

This is the same principle used on unwinders, rewinders, and pull rolls. If your tension is measured in kgf and your radius in mm or cm, convert first: 1 N·m ≈ 10.2 kgf·cm, and 1 kgf ≈ 9.81 N. Mixing units is the single most common source of a wrong clutch size.

Because a roll grows during rewinding and shrinks during unwinding, the radius is never constant. You must calculate torque at the worst-case radius, which is the largest radius your machine will see.

Step 1 — Determine Your Maximum Web Tension

Start with the tension your process needs. A tension controller or load cell in the line will tell you the actual running value; if you are designing from scratch, take the material width, the allowable tension per unit width for your film, paper, foil, or textile, and multiply. Write down the maximum tension F_max in newtons — not the average, the peak, because the clutch must survive the peak without saturating.

Step 2 — Find the Operating Radius (and Why It Changes)

Measure the core radius and the full-roll radius of the finished roll. For a 3-inch (76 mm) core, r_core ≈ 0.038 m; for a full roll of 500 mm diameter, r_full ≈ 0.25 m. The clutch sees the highest torque at the largest radius in its duty. On a rewinder, that is the full roll; on an unwinder, the highest torque is also at the largest radius, because the roll is largest at the start of the run.

Step 3 — Calculate the Required Clutch Torque

Multiply your peak tension by the worst-case radius:

• T_required = F_max × r_max

If your line runs constant tension, this single number defines the clutch's job. If tension varies by section, calculate T for each section and use the highest value. Position-control and registration lines can also link clutch torque to a closed-loop tension system that adjusts excitation current as the radius changes.

Step 4 — Add a Safety Margin and Match the Rated Torque

Never select a clutch whose rated torque equals your calculated requirement exactly. Apply a common safety margin of 1.2 to 1.5 times the calculated torque, then choose the nearest standard rated clutch from the manufacturer's range. XW Machinery offers the 6 Nm to 400 Nm magnetic powder clutch family, the hollow shaft magnetic powder clutch, and the external rotation magnetic powder clutch for high-speed printing and packaging lines. The exact rated torque and torque–current curve for each model are printed on the nameplate and characteristic chart — use those, not a generic slope.

Worked Example A — Film Rewinding

A BOPP film line rewinds at a constant tension of 50 N. The core radius is 0.04 m and the full-roll radius is 0.25 m.

• T_required = 50 N × 0.25 m = 12.5 N·m
• With a 1.3× margin: 12.5 × 1.3 ≈ 16.3 N·m
• Select the next standard clutch rated at or above ~16 N·m

Here a hollow shaft magnetic particle clutch in the low-to-mid range is a good fit, and a miniature tension controller can trim excitation current as the roll builds.

Worked Example B — Paper Unwinding

A paper unwinder runs at 220 N tension. The largest roll radius at the start of the run is 0.40 m.

• T_required = 220 N × 0.40 m = 88 N·m
• With a 1.3× margin: 88 × 1.3 ≈ 114 N·m
• Select a clutch rated at or above ~115 N·m

For this duty a higher-rated clutch such as the magnetic powder clutches and brakes range sized toward the top of the band is appropriate, paired with a closed-loop tension controller.

From Torque to Excitation Current (the Linear Slip Region)

Once you know the required torque, you set it with excitation current. Above the residual (slack) magnetization current, transmitted torque climbs roughly linearly with current until it reaches the rated value, after which it plateaus. The precise mapping is on the model's characteristic curve — do not assume one clutch's curve applies to another. If the clutch cannot reach the needed torque at rated current, you have undersized it and must move up a frame size. See our guide to clutch torque loss if you see drift in service.

Don't Forget Slip Heat — Torque Is Not the Whole Story

A magnetic powder clutch in tension control runs in continuous slip. The heat it must dissipate equals torque times slip speed:

• P_slip = T × ω_slip (watts = N·m × rad/s)

A clutch that is torque-adequate but thermally undersized will overheat and lose torque. This is why duty cycle and cooling matter as much as the peak number. Read our overheating causes and solutions article before finalizing a high-slip application, and confirm the thermal rating with the manufacturer.

Common Calculation Mistakes to Avoid

• Using average tension instead of peak tension when sizing.
• Forgetting that radius changes through the run; always use the worst-case (largest) radius.
• Mixing units — keep everything in N and m, or convert cleanly.
• Sizing only for torque and ignoring slip heat in continuous-duty lines.
• Treating one model's torque–current curve as universal.

Frequently Asked Questions

Can I size a clutch from tension alone without the radius?

No. Torque is tension multiplied by radius, so without the roll radius the number is undefined. Even a fixed core adapter changes the effective radius, and you should confirm it against the clutch selection method for your setup.

What if my tension varies during the run?

Calculate torque at the highest tension and largest radius combination your process reaches, then apply the safety margin. A closed-loop controller handles the variation between those limits by adjusting current.

How do I know the rated torque of a specific clutch?

Read the nameplate or characteristic chart supplied with the unit. XW Machinery lists rated torque ranges on every magnetic powder clutch product page.

Conclusion

Calculating magnetic powder clutch torque is straightforward once you anchor on T = F × r, use the worst-case radius, apply a safety margin, and respect slip heat. Pair the math with the right clutch and controller and you get stable tension from core to full roll. For help sizing a clutch to your exact web, material, and duty cycle, contact XW Machinery and our engineers will recommend a rated torque and model from the magnetic powder clutches and brakes range.