Flexible Couplings Reduce Fan Maintenance Costs

Issue 2 and Volume 104.

A MAJOR GENERATING plant in the eastern United States with 1,400 MW of baseload and peaking capacity was experiencing excessive maintenance and downtime due to vibration on its forced draft (FD) and induced draft (ID) fans.

Up to five years ago, the utility used lubricated gear couplings between the FD and ID fans and the electric drive motors. These couplings, however, tended to wear and develop backlash that sometimes caused vibration. They also had to be heated and shrink-fitted to the shaft for installation, which took several hours.

Because of thermal growth during normal operation, adjustments are often required to keep the fan and its drive motor aligned. The original couplings, with their interference-fit design, had to be heated before they could be moved. After each adjustment, the maintenance staff then had to wait for the couplings to cool to see if they were properly aligned. This procedure often had to be repeated several times before achieving proper alignment.

To simplify alignment and reduce maintenance costs, plant management decided to switch to Rexnord’s Rex Omega couplings, which are split, flexible polyurethance couplings that consist of only two hubs and two half-elements. The non-lubricated, material-flexing design incorporates torsionally soft flex elements that protect connected equipment by cushioning shock loads, reducing torsional vibration and absorbing unavoidable misalignment.

The new couplings are joined to the shaft using a keyless shrink disk that is installed over the hub projections. By eliminating the keyway, maintenance staff can now slide or rotate the hub for adjustment without having to worry about the length of the key or rotating the shaft. The bore capacity and torque transmission have also been increased.