Reynir Hilmisson, diagnostic specialist from Brüel & Kjær Vibro’s Surveillance and Diagnostic Service Center in Denmark, will be presenting a poster on the “evaluation of bearing rotational looseness” at the AWEA Windpower Conference and Exhibition (New Orleans, May 23-26). Bearing rotational looseness, or bearing creep, refers to rolling-element bearing inner race looseness on the shaft or outer race looseness on the bearing housing. In this work, the former case is considered. It most often occurs on the intermediate speed or high speed shaft of the helical gear stages of the multi-stage wind turbine gearboxes. A loose inner race can result in wear on the shaft that could require replacing the entire gearbox stage. The rotational looseness can also result in improper gear meshing, which can lead to broken gear teeth. In the case of broken teeth, the entire gearbox may have to be replaced including the entire lubrication oil cooling system (because of metal pieces present in the system). This costs approximately US$ 216,000 for a 5 day shutdown or US$ 276,000 for a one-month shutdown (this includes equipment, downtime, labor and crane, with the actual costs depending on a number of factors). By comparison, if the inner bearing race fault was detected and the bearing replaced before a catastrophic failure occurs, this would cost only US$ 12,000 for a two-day shutdown! A loose inner race is typically detected 8 months before service has to be done in most cases. Once detected, this condition is immediately considered at the highest severity and action has to be taken quickly to avoid wear on the shaft and damage to the gear teeth. Reynir Hilmisson will give actual case study information, together with statistics on remaining useful life and a description of the measurements used for detecting the condition. The Brüel & Kjær Vibro diagnostic specialists give several papers every year on various topics in condition monitoring of wind turbines. Based on both research and the experience of monitoring over 8,000 wind turbines worldwide, these papers mostly deal with promoting refined measurement techniques that give earlier, more reliable fault detection for some difficult potential failure modes of the wind turbine drive train. The aim is to reduce maintenance costs for the owners and operators.