Backlash Gear system

[RM12]

I am trying to understand where a backlash is measured in a gear system.
I have a shaft with sun gear on it and it meshes with another shaft having a pinion gear.

Sun gear shat has a coupling to on end which is being driven by a motor. (But this system is uncoupled)
Pinion gear shaft has a fan blade to one end driven by the Sun gear.

Do I use an indicator on the fan blade and rock it back and forth to check for backlash or on the sun gear? (Back-forth as in slightly letting it engage and stop)

 

[mfgenggear]

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.

 

[RM12]

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

 

[mfgenggear]

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

 

[geesaman.d]

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

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.