“Dynamic Characteristics of an Overhung P/A Fan in Rolling Element and Fluid Film Bearings,”
This paper presents some of the dynamical characteristics of a class of overhung primary air utility fans mounted in rolling element and ﬂuid ﬁlm bearings. This particular class of fan has experienced high vibrations and has had reduced bearing life. Experimental studies of the fans mounted in rolling element bearings indicate that the fans are operating at the 1st critical speed. The overhung fans supported by rolling element bearings are extremely difficult to balance because of operation near the 1st critical speed, higher order harmonics and lack of damping in the rotor-bearing system. Because of the low damping level of the rolling element bearings, high rotor amplification factors are encountered.
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.
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