Backlash Gear system

RM12
Generally using a mag base indicator holder, a measurement should be taken perpendicular to the tooth flank. There plenty of videos on line showing this procedure.
Generally a sun gear is on a fixed axis with planetary, and a ring gear. The back lash is in part designed according to the mod or Diametral pitch and pressure angle. The manufacture should specify the back lash and where to inspect.
To low or zero back lash is a bad. In order for the gears to roll correctly, and prevent damage the correct back lash is required.

Got it. What if I measure it on the fan instead of directly going to the gear itself?

Rm12
Then it would be verifiing the accumulated backlash which would give a false result.
Follow the manufactures instructions.

RM12 said:

Got it. What if I measure it on the fan instead of directly going to the gear itself?

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If you can equate the backlash measured on the fan side to a backlash as per the gearing instructions, then it's fine. But that takes some work up front.

Note about backlash: think of it as room for the teeth to flex under load. Zero backlash is only reasonable (in theory) for gears that take zero load. You want enough to support maximum load but not so much that impact loads and NVH get too high.