“Dynamic Characteristics of an Overhung P/A Fan in Rolling Element and Fluid Film Bearings,”
E. J. Gunter, A. Hobbs & C. Phelps, 1992.
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This paper concerns the experimental and theoretical investigation of a class of overhung primary air utility fans mounted in rolling element and fluid film bearings. This particular class of fan has led to considerable difficulties in balancing and bearing life. An experimental investigation of the fans mounted in rolling element bearings indicated that the fans are operating at the 1st critical speed.

The overhung rotor in rolling element bearings was extremely difficult to balance because of its operation in the vicinity of a critical speed, higher order harmonics and lack of damping in the rotor-bearing system.

A detailed theoretical analysis of the fan critical speed in rolling element bearings indicated that the critical speed should have been 15 to 20 % above the operating speed range. The reduction in fan critical speed to below the operating speed range was caused in part by the flexibility of the overhung disk, and also by flexibility or elasticity of the fan end bearing support structure.

One of the fans was replaced with fluid film bearings. It is shown that the fluid film bearings have effectively higher stiffness and damping than the rolling element configuration. A satisfactory balance was possible with the fan in fluid film bearings, whereas in the case of the rolling element bearings, accurate balancing could not be achieved. The bearing forces transmitted with fluid film bearings may be reduced in half by reducing the shaft diameter of the overhang section.

An elastomeric isolation support was designed for the fan end rolling element bearing to reduce the critical speed to below the operating speed range. It is demonstrated that the isolation support reduces the forces transmitted to the fan rolling element bearing by a factor of 20 at running speed. Extended bearing life and ease of field balancing was achieved by using the isolation support system with the rolling element bearings.