All-electric knife holder for slitting applications

Knife holders are an essential element in slitter rewinder construction. UK-based converting and packaging equipment design consultant Sigmala has replaced, for the first time, conventional pneumatic controls with electrically powered embedded servo motors in its new SL50 knife holder.

The result is a system that requires no manual set-up or intervention. Servo controls throughout have enabled Sigmala to introduce another new concept – adaptive slippage compensation (ASC) – to monitor knife wear and adjust the side-load automatically to prevent slippage between upper and lower knife blades. This development is not possible with pneumatic knife holders since they do not incorporate a sensor to detect the rotation of the upper knife.

Each of the three major components of the SL50 – traverse unit, upper knife holder and lower knife holder – are controlled by servo motors. The first two are embedded and incorporate individual microprocessor controls. The lower knife unit uses an AC pancake servo motor for the main drive and a Gates Polychain belt to connect the drive to the knife hub.

Built-in to the traverse unit that positions and guides the knife holder along the backplate on the slitting machine is an encoder that can pinpoint knife position to 1,000th/inch (0.025mm) at two-inches/sec (50mm/sec).

The upper knife holder with three embedded servo motors, servo drives and microprocessors controls the up/down, side-load and cant or slitting angle. Up/down is accurate to 0.025mm; side-load to one Newton and the cant angle to 0.1 degree.

The ASC has been designed to run in ‘threshold’ mode whereby the point at which slippage occurs then increase the side-load automatically to a slightly higher value. This ensures the machine can run with the smallest possible side-load which in turn reduces wear on both knife blades.

The 50mm width SL50 is the first product to be designed and manufactured by Sigmala under its own name. Previously, the company has operated behind-the-scenes and been responsible for many design engineering innovations for packaging and converting machinery manufacturers.