Powder Brake and Powder Clutch for Web Tension Control

Powder brakes and clutches are selected by torque rating and shaft size for the unwind or rewind they will control. RK sizes the unit against the machine, so please share the following for a quotation:

• Machine make and model, and whether the unit is for the unwind (brake) or the rewind (clutch)
• Maximum roll diameter and roll weight
• Web width and the materials you run
• Shaft diameter and mounting arrangement
• Control preferred: manual with amp meter, or automatic with load cell feedback

For reference, the RK full servo flexo press carries an auto electronic tension controller with a powder brake and load cell feedback on the unwind and a powder clutch on the rewind; the RK offline roll to sheet cutting machines use a powder brake with an amp meter on the unwind.

Both are torque devices that use magnetic powder. Inside the housing, a fine iron powder sits between an inner rotor and an outer drum. When current passes through the coil, the powder links up under the magnetic field and transmits torque between the two parts. More current means a stronger link and more torque; less current means less. The torque is smooth and steady, with no sudden grab, which is what a moving web needs.

A powder brake has one side fixed to the machine frame and the other on the unwind shaft. It resists rotation and so holds back the roll to keep the web tight. A powder clutch has both sides free to rotate: one side is driven by the machine motor and the other turns the rewind shaft. It lets the rewind slip as the roll grows, so the winding torque stays under control.

Smooth, Stepless Torque

Because torque follows coil current, it can be adjusted continuously from very light to full rating while the machine runs. There are no friction pads to wear unevenly and no jerk when the setting changes.

Steady Tension as the Roll Diameter Changes

On the unwind, the brake current is reduced as the roll gets smaller; on the rewind, the clutch current is raised as the roll grows. Tension at the web stays constant from a full roll to the core.

Works With Manual, Amp Meter or Load Cell Control

A simple installation uses a potentiometer and an amp meter, as on the RK offline roll to sheet cutting machine. A closed-loop installation adds a load cell that measures actual web tension and corrects the brake automatically, as on the RK full servo press unwind.

Suits Thin Films and Heavy Papers Alike

The wide torque range lets the same brake hold thin PVC or PET film without stretching it, and heavier label paper or board without letting it run loose.

Fitted Across the RK Machine Range

Powder brakes and clutches are standard on RK flexo presses, roll to sheet cutting machines, the PVC shrink sleeve glueing machine and the wrist band machine, so the accessory is a proven part of the RK line, not an add-on.

Retrofit for Existing Machines

A powder brake with its controller can be fitted to an older unwind that runs on a mechanical or friction brake, giving a real improvement in registration and roll quality without changing the machine.

On the unwind, the roll shaft is coupled to the powder brake. The controller sends a set current to the brake coil, the magnetic powder stiffens, and the brake resists the pull of the web by the set amount. With an amp meter, the operator reads the current and lowers it as the roll runs down. With a load cell, a roller in the web path measures tension directly and the controller trims the current on its own, so the operator sets a tension value once and the machine holds it.

On the rewind, the motor drives the input side of the powder clutch and the rewind shaft is fixed to the output side. The clutch transmits only as much torque as its current allows, slipping the difference. As the rewound roll grows and needs more torque for the same web tension, the controller raises the current. The result is a roll wound evenly from core to full diameter, without tight starts and loose ends.

• Flexo label printing presses – unwind brake and rewind clutch for stable registration across the print units.
• Slitting and rewinding machines – even tension for straight slits and firm rewound rolls.
• Roll to sheet cutting machines – unwind tension held steady so sheet length stays accurate.
• Shrink sleeve and film converting – controlled tension on thin PVC and PET film that stretches easily.
• Inspection and rewinding machines – smooth stop and start without web breaks.
• Retrofit on older machines – replacing friction or mechanical brakes on an existing unwind.

The powder brake or clutch does not touch the material; it controls the shaft that carries it. That makes it suitable for any web an RK machine runs: self-adhesive label stock, paper and board, BOPP, PE, PET and PVC film, shrink film, foils, foam and tapes. The torque rating is chosen for the roll weight and the tension the material needs, which is why RK asks for material and roll details before quoting.

• Rubber Rollnip and pressure rolls that carry the web through printing and pasting machines.
• Printing Cylinderaluminium plate cylinders for RK flexo printing units.
• Magnetic Cylindercarries flexible dies in rotary die cutting stations.
• Flexible Diethin steel dies made to your label shape.
• Solid Dieengraved rotary dies for long production runs.

RK Label Printing Machinery Pvt. Ltd. has designed and built label printing and converting machinery in Ahmedabad, Gujarat since 1995, operates three manufacturing units and supplies customers in 23+ countries. Tension control is part of every roll-fed machine RK makes, so the company selects powder brakes and clutches from working experience on its own presses, slitters and cutters rather than from a catalogue. Buying the accessory from the machine builder also means the controller, load cell and brake are matched to each other and to the machine.

• Position: unwind needs a brake, rewind needs a clutch. Some machines need one of each.
• Torque rating: decided by the maximum roll diameter and weight, and the tension your material needs. Oversizing makes fine adjustment harder; undersizing lets heavy rolls run loose.
• Control type: an amp meter and potentiometer are simple and reliable for slower machines; load cell feedback is worth it where registration or thin film matters.
• Shaft and mounting: confirm the shaft diameter and the space available on the machine.
• Materials: stretch-sensitive films need a wider low-torque range than heavy paper.

The price depends on the torque rating, whether it is a brake or a clutch, the controller supplied with it and whether load cell feedback is included. Prices are quoted ex-works Ahmedabad with GST at 18% applicable. If an RK engineer fits and sets the unit at your plant, the engineer's transport and stay are borne by the customer. Send RK your machine details, roll size and material, and the team will quote the right unit.