AIR ENTRAINMENT DURING STEADY-STATE WEB WINDING

As a web is wound at speed onto a roll, a thin layer of air becomes en trapped between it and the incoming web stream. The resulting spiral-s haped air bearing separates adjacent web layers and can extend many wr aps into the roll. The air entrained during the winding process increa ses the propensity for lateral interlayer slippage and damage to the e dges of the web. In the present paper an in situ technique is develope d for measuring the thickness of the entrained air film during winding , and parameter studies quantify the effects of such winding variables as tension, width, transport speed, and surface roughness. With a vie w towards evaluating different transport designs and operating conditi ons, three measures of air entrainment are discussed: (i) the cumulati ve thickness of all air layers, (ii) the thickness of the outermost ai r layer at the nip, and (iii) the rate at which air bleeds from the ro ll once it comes to rest. Measured values of the first two metrics are compared with those predicted by a derived two-dimensional reduced-or der model for steady-state winding. The analysis treats the two bondin g configurations of symmetric and asymmetric stacking of web layers by specifying appropriate cross-web pressure profiles.