Magnetic Powder Clutch Torque Selection Guide

Magnetic powder clutch torque selection comes down to three numbers: the web tension your material needs, the largest roll radius the clutch will ever see, and the slip power it has to dissipate while getting there. Get those three right and the clutch will hold stable tension for years. Get any one wrong and the unit cooks on long runs, drifts as it warms, or cannot build a tight core at the start of a roll.

This guide gives you the formula, a worked rewinder example, the service factor to apply, and the thermal check that most buyers skip.

Start From Web Tension and Roll Radius

The most common sizing error is starting from motor power. A 2.2 kW motor tells you nothing about the torque at the reel. Torque at the clutch output shaft is set by the web, not by the motor.

You need four inputs before you can size anything:

Input	Where to get it	Typical unit
Web tension required	Process spec, or the setting that currently runs well	N (total) or N/mm width
Web width	Machine spec	mm
Core diameter and maximum roll diameter	Machine spec	mm
Line speed and clutch input speed	Machine spec	m/min, rpm

If you have no written tension value, measure it. A hand-held meter, or a pillow block tension sensor on an idler roll, will give you the real number in an afternoon.

The Core Torque Formula

Torque is tension multiplied by radius:

T (N·m) = F (N) × R (m)

where F is the total web tension across the full roll width and R is the roll radius at the moment of maximum torque. If your tension is specified per unit of width, convert it first:

F = tension (N/mm) × web width (mm)

Two points catch people out:

• Maximum torque is not at the core. For a rewind clutch, peak torque occurs at maximum roll diameter. Size for the full roll, then check the core separately.
• The core is the control-resolution limit. There the radius is small, so the required torque is often 10–20% of the full-roll figure. If the clutch rating is too large, that core torque falls below the usable control band and rolls start loose or telescoped.

Worked Example: Sizing a Rewind Clutch

A 1,000 mm wide BOPP film rewinder runs at 150 m/min, winding from a 76 mm core up to a 400 mm finished roll, at 60 N total web tension, with a constant clutch input speed of 700 rpm.

1. Torque at full roll: R = 0.20 m, so T = 60 × 0.20 = 12 N·m.
2. Torque at the core: R = 0.038 m, so T = 60 × 0.038 = 2.3 N·m.
3. Apply a service factor of 1.5 for continuous converting duty: 12 × 1.5 = 18 N·m.
4. Select the next standard rating up — a 20 N·m or 25 N·m unit. Our magnetic powder clutch from 6 Nm to 400 Nm covers both.
5. Check control resolution: 2.3 N·m is roughly 11% of a 20 N·m rating, comfortably inside the usable band. On a 40 N·m unit it would fall under 6%, and core quality would suffer.
6. Check slip power, as shown below.

Choose the Service Factor by Duty

Duty	Typical service factor
Light intermittent, start/stop cycling under 30% duty	1.2 – 1.3
Standard converting: continuous slip, steady load	1.4 – 1.5
Frequent acceleration, shock loads, wide tension swings	1.6 – 2.0
Precision low-tension film, foil or battery electrode	Size for control resolution first, usually 1.2 – 1.4

Note the last row: oversizing is not automatically safer. On low-tension work a clutch rated far above the requirement sits at the bottom of the torque-current curve, where resolution is poor and thermal drift shows up as visible tension variation.

Check Slip Power Before You Buy

Torque rating is only half the specification. A magnetic powder clutch works in slip — input and output turn at different speeds — and slip turns mechanical energy into heat:

P (kW) = T (N·m) × Δn (rpm) ÷ 9550

where Δn is the slip speed, the difference between input and output rpm.

Continuing the example: roll speed at 400 mm diameter is 150 ÷ (π × 0.4) ≈ 119 rpm, so slip is 700 − 119 = 581 rpm and P = 12 × 581 ÷ 9550 ≈ 0.73 kW. At the core, roll speed is about 628 rpm, slip only 72 rpm, and P = 2.3 × 72 ÷ 9550 ≈ 0.02 kW. The worst case for heat is the full roll, not the start of the wind.

That 0.73 kW has to leave the housing continuously. Check the continuous slip rating, in watts or kW — not just the peak torque figure. If your calculated slip power is close to it, move up a frame size, add forced-air cooling, or switch to an external rotation magnetic powder clutch, which sheds heat far better at speed.

Speed, Residual Torque and Controller Resolution

Two final checks. Never exceed the rated input speed, or powder is thrown out of the working gap. And ask for the residual (drag) torque at zero current — on very light webs it can already exceed your minimum tension. Then match the coil: most industrial units run 0–24 V DC, compact machines 12 V, driven by a miniature tension controller or a full automatic web tension controller whose current resolution limits your torque resolution.

Typical Starting Points by Application

Broad ranges for first-pass sizing — confirm against your own process before ordering.

Application	Typical total web tension	Typical torque band
Narrow label and tape slitting	20 – 80 N	1 – 5 N·m
Film and flexible packaging rewinding	50 – 300 N	6 – 50 N·m
Paper and board converting	200 – 800 N	25 – 100 N·m
Foil and laminate handling	30 – 150 N	3 – 25 N·m
Textile and nonwoven winding	20 – 120 N	2 – 20 N·m
Lithium electrode and separator	10 – 60 N	1 – 12 N·m

For the smallest duties a 12 V micro magnetic powder clutch, 1–5 Nm is the right frame; for short-drivetrain retrofits, a hollow shaft magnetic powder clutch fits over the existing shaft.

Clutch or Brake: Same Math, Opposite Side

The torque calculation is identical — only the mechanical arrangement differs. A clutch drives the load; a brake holds it back against the frame. Most converting lines run a magnetic powder brake on the unwind for back-tension and a clutch on the rewind to pull the web. Size each side independently: unwind torque peaks at a full parent roll, rewind torque at a finished roll.

Five Mistakes That Undersize or Oversize a Clutch

1. Sizing from motor kW. Always start from tension and radius.
2. Ignoring the continuous slip rating. Peak torque alone sends you into overheating.
3. Oversizing for low-tension webs. You trade margin for control resolution and lose core quality.
4. Forgetting taper tension. Size for the maximum torque point in the taper profile, not the average.
5. Running open loop on a critical web. A controller with a cantilever tension load cell closes the loop and cancels thermal drift; open loop leaves the drift in your product.

FAQ

Q: What torque range does a magnetic powder clutch cover? A: Standard industrial units span roughly 6 Nm to 400 Nm; micro units cover 1–5 Nm. Choose a rating that keeps your minimum required torque above about 10% of rated.

Q: How much safety margin should I add? A: 1.4–1.5 for standard continuous converting duty, less for precision low-tension work where resolution matters more than margin.

Q: Why does my clutch lose torque when it heats up? A: Torque at a given current falls as the housing warms. Closed-loop control with a tension sensor removes the effect. If torque falls year on year rather than shift by shift, suspect powder degradation.

Q: Do I size a brake differently from a clutch? A: No — same formula, same service factor. What changes is the maximum-radius case: for an unwind brake that is the full parent roll, not the core.

Conclusion & Next Step

Magnetic powder clutch torque selection is four steps: convert web tension and maximum roll radius into torque, apply a duty-based service factor, verify slip power against the continuous thermal rating, and confirm that core torque stays inside the usable control band. Do all four and the clutch will hold tension for years with nothing more than periodic powder service.

Send XW Machinery your web width, tension, core and maximum roll diameter, line speed and duty cycle — we will run the torque and slip-power calculation and recommend a frame size, free of charge.