Transient Discharge of Entrained Air From a Wound Roll

As magnetic tape or other web-like material is wound onto a roll, air moving with the incoming stream and the roll becomes forced into the converging wedge at the stream’s point of tangency. The spiral-shaped air bearing so formed can extend many wraps into the roll’s interior. When the roll is subsequently brought to rest, the entrained air discharges from it, and the roll’s radius gradually decreases until all adjacent tape layers have come into direct contact. In the present paper, a model is developed for this transient discharge process, and for the rate at which the roll stabilizes following steady-state winding. Predictions of the model are compared with results from laboratory experiments in which the roll’s radius is measured through laser interferometry during the steady entrainment, and transient discharge, stages of winding. Parametric effects of the tape’s tension, speed, width, and surface roughness are specifically addressed with a view towards reducing the time required for the roll to stabilize.