Measuring ring gear backlash--at the heel, or the pitch diameter - JeepForum.com
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post #1 of 5 Old 10-23-2019, 12:22 AM Thread Starter
Guy_Van_Horn
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Measuring ring gear backlash--at the heel, or the pitch diameter

Here's something I ran into recently. While checking the backlash on my differential ring gear, I found a difference between measuring it on the heel of the tooth with a dial indicator, vs. at the center of the tooth profile (approximate pitch diameter) using a test indicator. In searching online, most of the how-to's seem to show people checking it at the heel of the ring gear with a dial indicator. I get why people do it this way, since it's easier to check at the heel, and nearly impossible to check with a dial indicator at the pitch diameter. However this seems to me to be wrong since that is not the theoretical diameter where the gear meshes with the pinion, i.e., the pitch diameter. The diameter at the heel is larger than the pitch diameter and checking it there would give a larger backlash reading. I admit, it isn't a huge difference (about .001"), but when you are shooting for a backlash from only .006" to .010", that small amount is significant.
Where do you check the backlash on you differential ring gear?

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post #2 of 5 Old 10-23-2019, 04:46 AM
Jonny Jeep
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FSM shows the heel being used to check backlash and that's what I did. As you state, it's near impossible to do it anywhere else.
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post #3 of 5 Old 10-23-2019, 10:09 AM Thread Starter
Guy_Van_Horn
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It's near impossible to do anywhere else with a dial indicator. It's quite easy to do on the pitch diameter with a test indicator.

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post #4 of 5 Old 10-23-2019, 12:12 PM
Jerry Bransford
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It doesn't matter, the ring gear moves all parts of the gear teeth equally so it doesn't matter where it's measured. But most, probably 99.999%, would use a dial indicator so they'd use it on the heel.
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post #5 of 5 Old 10-25-2019, 10:36 PM
CSP
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The distance that the ID of the gear moves at xx degrees of rotation is smaller than at the OD of the gear.
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