Hello! I’m just wondering how I can fix this weird glitch in blender eevee. It’s only noticable when motion blur is on and it happens for only certain frames.
This might be wrong, because I don’t use eevee much, but for cycles I would say turn up the samples. I am assuming it’s similar for eevee.
Well, why is it happening? What’s causing it?
AFAIK here’s how motion blur works. When rendering frame 5 it looks backwards to frame 4.5 (half a frame backwards) and forwards to frame 5.5 (half a frame forwards). This way it knows how fast the object is moving and how much to blur it. Settings for this are in the motion blur panel for the render engine.
I have glitches with this when using a child-of constraint for a character picking up an object. Child-of constraint is off (0) on frame 4, on (1) on frame 5. When rendering, at frame 4.5, the constraint is half on/half off (0.5) so the object has moved off to someplace crazy and causes this glitch.
For me, when this happens, I just re-render the 2 bad frames with motion blur off, because it’s not needed. It works for me because I don’t need to blur an object being picked up. Won’t work for you because of the speed of the action.
You need to figure out what is going on with that gear. How are you rotating it? Are you using a constraint to rotate the gear? Step thru the animation at those frames and watch the gear. Maybe it’s rotating 180 or 360 degrees between one frame.
I ask about constraints because in blender for gear rotations, were one gear rotates another, you should stay away from using them. Use drivers on the gear rotations instead.
Feel free to ask questions,
Randy
I agree with @revolt_randy , it is most likely caused by some very fast movement in the scene. Either the camera or the objects. Rotations bigger than half a circle can cause problems if you are using quaternions instead of euler rotation. In Blender, quaternion rotation difference between two frames can be max 180 degrees. And even then it could rotate the wrong way around, I guess it has something to do with the way quaternions are calculated, so that the software doesn’t know which way to turn when rotating 180 degrees. The only way to rotate things in Blender over 180 degrees when using quaternions is to keyframe them at in between values so that all the rotation differences between keyframes are under 180 degrees. I wonder if there could be some constraint mismatch, like the target using euler but the constrained object using quaternions. Could that cause the constrained object to “reset” its rotation at certain points and thus cause an extremely fast rotation? 
idk, I don’t understand the math so well…
You could ofc try to adjust the motion blur settings in render settings. By default it should be center on frame, but it can be changed to start of frame or end, if I remember right.