Doctor blades and variations

By Clarence Klassen, P. Eng.

Doctor blades are often used to scrape web from a roller or prevent the buildup of contaminants on a roller. Generally these doctor blades are oscillated in the transverse direction across the face of the roller just after the outgoing web leaves the roller.

We often see that the doctor-blade pressure creates a considerable drag on the drive for the roller. In many cases on paper-machine dryer cans, the doctor blade can use up 100 HP on a dryer with a 500-HP motor. This is easy to prove by monitoring the motor load while relieving the doctor-blade pressure or removing it entirely.

Secondly, the load created by the doctor blade can often vary with the period of the oscillation.

A steady load created by the doctor blade merely wastes energy every hour the doctor blade is applied. A load variation caused by the doctor will also result in small speed variations, which may result in large tension variations.

Moral – used the lightest doctor pressure and change blades when wear becomes uneven.

Image courtesy of Yellow Submarine Original Artwork. Copyright 2014