Those dusty fans
Be sure to put a drop or two where the bearings are. The bigger fans usually have an oil port, like a little half round cup with 'oil' over it. If the fan oscillates left and right, the rear bearing most likely gets it's lubrication from the gear box. Virtually every fan I've pulled from dumpsters and trash heaps needed nothing more than a little oil to bring them back to life,
You can buy a zoom spout oiler,has a retractable tube of 7" for easier reach and the oil is right for what you're doing. Try not to spill any oil on the windings, just wipe them off if you do. This will keep your fans going for a long time. Just a drop or two will do. If you remove the fan blades be sure to wash them in sudsy water to remove even the oil from your hands. Dust will build up faster on oily,dirty blades. I service mine about twice a year, only takes 15 or 20 minutes.
If you can't find the bearings, just unplug the fan and spin the blades by hand and watch inside for the moving shaft, it will go through or stop at a bearing. Post a pic if you need help . Billy