I’m building a simple helicopter scene and had the main and tail rotors parented and key framed just fine and both showed up in a test animation.
After adding some more assets to the scene and doing an other render I noticed the main rotor stopped spinning in the animation but still spins when played in the 3D view port.
I tried all can think of, any help will be appreciated
Please post the blend file, otherwise it’s just guesswork from everyones side 
On main menu, window->toggle system console to open up a console with warnings. Notice the warnings about dependency cycles. That means that you have relationships that operate in a loop, leading to unpredictable behavior (and quite possibly, different behavior in render from preview.)
Fix the loops.
I’ll check that out and see if that fixes the problem 
looked at the console and I get these warnings.
Blockquote
Dependency cycle detected:
OBMrotor/Transform Component/TRANSFORM_CONSTRAINTS() depends on
OBEmpty/Transform Component/TRANSFORM_FINAL() via ‘Copy Rotation’
OBEmpty/Transform Component/TRANSFORM_SIMULATION_INIT() via ‘Simulation -> Final Transform’
OBEmpty/Transform Component/TRANSFORM_EVAL() via ‘Transform Eval -> Simulation Init’
OBEmpty/Transform Component/TRANSFORM_PARENT() via ‘Eval’
OBEmpty/Transform Component/TRANSFORM_LOCAL() via ‘ObLocal -> ObParent’
OBEmpty/Transform Component/TRANSFORM_INIT() via ‘Transform Init’
OBMrotor/Transform Component/TRANSFORM_FINAL() via ‘Parent’
OBMrotor/Transform Component/TRANSFORM_CONSTRAINTS() via ‘ObConstraints ->
Done’
Detected 2 dependency cycles
Not sure how the 2 are conflicting with each other
I think the simulation stuff refers to the particle system. The obvious loop you have here is that your empty is parented to Mrotor, while Mrotor has a copy rotation constraint targeting the empty. The rotation of Empty depends on Mrotor, which depends on Empty, which depends on Mrotor…
Thanks very much. Cleared the parent on the empty now its rendering like before.
