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I have some fans that I need to rotate using an armature:

enter image description here

the problem here is blender's bone posing system makes it so if you key a rotation, then rotate around multiple times, blender just goes to the closest rotation. I cant have this - I need to keep spinning (like a fan).

how can I do this?

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In the 3D View properties panel (activated with N), or in the Properties panel > Bone > Transform, switch the rotation mode from Quaternion to XYZ Euler, otherwise you will have the kind of problem you describe.

If you want your rotation to be linear (and not accelerate and decelerate, which will happen by default), go in the Graph Editor, select all and click T > Linear.

If you want the rotation to be infinite, select all and press shiftE > Linear Extrapolation.

enter image description here

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Hit "N" and set the rotation manualy under the tranform tab.

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A cycle and the NLA

Simplest way would be to change to Euler rotation and use the method in @moonboot's answer

To remain with quaternions Another option is to make a cycle and use the (oft forgotten) NLA to scale and repeat the action.

Here I've made an action to rotate using quaternions, about Y axis one revolution in 24 frames. Rotated in two parts from 0 to 180 and from 180 to 360 (0). Converted to linear interpolation.

At 24 fps is 1 rev per second or 60 rpm. Switched the frame range to 1 to 23 to test.

enter image description here One rev in Y in quaternion. Have removed the non required X and Z and the middle keyframe of W

When happy with cycle, "push down" the action to the NLA.

enter image description here

Switch to NLA editor. In Action Clip panel set the end frame to 23 (one less so next frame is back to 0 or 360 degrees)

enter image description here

Use repeat for revolutions and scale for speed, reversed for switching direction. These properties can be animated.

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