912 Gearbox Installation - How to check radial gear backlash at the propellor flange

In General.

gearbacklash by mounting a dial indicator so it is similar to a pinion tooth profile, block the gear to prevent its rotation and rock the pinion through the backlash. To measure shaft endplay, mount a dial indicator at the end of a shaft and move the shaft in the axial direction. 

I tend to agree with you that this makes no sense ... Until ... You blow up the drawing in the manual until it is the size of a Movie Poster do you see what it is they want you to measure.

At First glance, it looks like you would be measuring bearing runout if you were to put the dial on the edge of the flange and measure the radial motion.

But what you are supposed to measure is the tangental motion of the Outer Mounting Bolt Boss in the Prop Flange as you ROTATE the Prop Flange.

In essense, How much play you would see at a Prop Tip, except there is no propeller to measure.

I agree! Clear as Mud!  But at least this makes sense.

Something was Lost in translation I am sure.

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Converting this to something practical...

The manual specifies a 100% wear limit of 0.2mm (0.0079") at the 101.6mm (4.00") Diameter Bolt Circle Mounting Boss.

This extrapolates to a backlash of 0.1975" at 50" ... or ...  3.94mm at 1 meter Radius.

. . .

For those of you that do not run a 100" Diameter Prop (50" Radius Blades) ...

You would expect to see no more than 70/100 x 0.1975"= 0.138" of backlash at the tip of a 70" Prop.

I hope this helps.

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Note: This Post has been Edited/Corrected as noted in a post below.

Thanks Bill - that does make sense (more than can be said for the manual) !

So, I went out and checked the prop tip runout on my 912 and found that on and engine with less than an hour on it it was already at the 100% limit.  What the Heck!

Something is NOT right here.

Going back over everything, I found my mistake.

The Bolt Boss is on a 4" DIAMETER Bolt Circle, Not a 4" RADIUS.

All the numbers were off by a factor of 2.

I have corrected the original post to reflect the 2" Radius and normalized the prop to 100" to make the conversion to your actual prop simpler.

Example: If you have a 68" Diameter Prop Just multiply the 0.1975" for the 100" prop by 0.68 to get YOUR Backlash Number (0.134") at the Prop Tip.

. . .

Now my Backlash is well within specs!

Topics such as this, are why iitoi created rotax certified mechanics (doctors) to begin with!