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Planetary Gear Design - Profile Shifted

TXY

Petroleum
Joined
Jul 15, 2025
Messages
6
Hi guys,

I'm trying to reverse engineer a planetary gear design because of urgent needs for getting the spares part (the planet gears had worn out) and supplier is quoting a 10 weeks lead time.

The existing design is as such - 11T Sun Gear, 12T Planet Gear and 37T Ring Gear.
Based on measurement of the 12T planet gear (18.03mm OD), I reckon it is a DP20 gear which might be profile shifted (which it should be, otherwise there would be issue with these gears meshing based on these numbers of teeth). The PCD of the planet gears is approximately at 31mm (I can't get an accurate measurement because of the 3 planet design).

My issue is with the gear dimensional calculation. I'm referring to the SDP/SI document where there are isolated calculations for an external-external gear pairing and for an external-internal gear pairing. I can't get the numbers (tip, working pitch, root diameters, center distance, etc.) to match between these two set of calculation for the same planet gear. And I realize that the numbers only match when I change the Ring Gear to 35T. However, as it is an existing design, I have to keep the teeth numbers unchanged.

Have anyone encountered such situation before and how should I tackle this? Or would it be acceptable to try fabricating a DP20 12T profile shift x=0.1 and specify a tip diameter of 18.03mm and hope it fits. Thanks!
 
Measuring only the gear diameters is unfortunately insufficient to accurately identify and reproduce the gear parameters, especially when a profile shift may be involved. Precise determination of the gear’s specifications requires measurement with a dedicated CNC gear measuring machine capable of capturing the tooth profile. Therefore, I recommend sending the original planet gear to a specialized metrology lab or company equipped with such machinery. Only with this detailed measurement data will it be possible to accurately reverse-engineer the gear and manufacture a replacement that properly meshes and functions within the planetary set.
 
@spigor Thanks for your advice. I'm trying to use this opportunity to learn about profile shifted planetary gear design. Have searched up the web for information but they are pretty much limited to profile shifted external-external gear pairing or external-internal gear pairing instead of designing the entire planetary gear set altogether. Do you know of any good references to recommend.
 
Look at why profile shift is used on single gear pairs and what the circumstances are that make such a shift useful.
 
Try
Planetary gear sets are one of the most difficult to manufacture.
Here's why
Ringgears are difficult to maintain roundness. Depending on the heat treat, hardness, if it is case harden.

Unknown
Material
Heat treat requirements
Agma quality requirements
Involute profile.

As stated it is essential to obtain correct gear geometry from a cnc gear checker.
Measurement over/under wires.

Profile shift is used to reduce pinion wear and under cut.

No wonder the planets wore out wit 12 teeth.
Not a conducive design. It is what it is.
11t and 12t will have under cut in the root diameter. Terrible design.

If you have no gear manufacturing experience you will scrap these parts.
If the involute is not correct the gears will self destruction at high loads and velocity.

Pay your supplier for expedite and shorten the delivery. Case harden gears will make the ring gear even more difficult to manufacture.
 

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