I do enjoy fatuous off topic discussions. I try to stay out of them, but I have succumbed again. The word axle originally always referred to the part of an axle-tree on which the wheel revolves, so whether the wheel revolved on or with the part, it was an axle. A spindle started out as a rod on which thread was wound. When more sophisticated spinning machines evolved, the part was still called a spindle. It was natural for engineers to extend these usages into other fields and so they tended to call the part on or with which a large rotating member turned an axle, and that on which a smaller turned a spindle. There was plenty of overlap and no rule to determine where there might be a dividing line. But that was the beauty of the English language before the bean-counters got to it; words were used to convey meaning and not used as weapons in a court of law. Stuart Tallack in William Blake's Felpham
('The gilded car of day His glowing axle doth allay In the steep Atlantic stream.' Milton) Which suggests that the Gods were not in the habit of checking that there was grease in their hubs.