I hate writing and thinking about gimbal lock because it is given such connotations of fear. It’s just a thing—do your rig with some forethought and it isn’t that big of a deal. It is frustrating as an animator, though. So, if you don’t look out for this in your more advanced rigs, you are liable to get that good old phone call, email, text, IM, smoke signal, fuming animator at your desk. You can pick up all sorts of explanations on the subject from any manner of book, help documentation, or on YouTube. They all explain the same issue.
Gimbal lock is when your axis rotates in such a way that you lose one axis of freedom. This happens because two axis align and, when rotated, both cause the object to move in the same direction. ...