This paper summarizes a theoretical study of a method of controlling the clearances of hydrodynamic foil bearings. The air-bearing clearance for certain large-wrap cylindrical foil bearings (that is, for perfectly flexible webs) is determined exclusively by the tension, the viscosity of air, the velocity of the web, and the radius of the cylinder. This clearance cannot usually be modified without changing one of the above parameters. However, a pressure pad placed above the web where the web enters the air bearing enables this uniform clearance to be reduced considerably. The pressure pad can thus be used to control the position of the web relative to the cylinder. This paper presents the relationships of the pressures and size of the pad to the reduction in uniform clearance. Clearance profiles for many different pads are also presented, and the practical limitations on reductions in clearance are discussed.

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