Profile shifting on small shaft - planetary mechanism 1

[mane08]

Hello Everyone, first-time poster here .

Need some help with a planetary mechanism design . This mech. is found inside a B&D EPC-12CA Cordless drill . I am trying to 3d model + manufacture a spare gear later ( a project of mine) . The outer ring gear (annulus) has 42 teeth + there are 3 planet gears with 16 teeth each. The middle shaft has been profile shifted + modified i am not sure , as it only has 9 teeth ( standard planetary mechanism law is outer ring teeth = 2 x planets + sun --> 42 = 2 x 16 + 10 - and not 9 - my problem here ). From approx measurements it appears the gears have a module m= 0.7 .
The midle sun /shaft gear has only 9 teeth instead of 10 , so i guess it has a profile shift of x=0.5mm ?
I am not sure about the reducing of teeth. So my problems is the design / profile of the middle shaft - i did found some info about profile shifting , but the method of creating a compatible 9-teeth sun gear with profile shift is the mystery here.


Best Regards

 

[drawoh]

mane08,

I would start by counting all the teeth again.

Gear teeth are efficient when they work on their line of action. Consider the possibility that the sun gear has a weird profile, that actually works in this configuration.

--
JHG

 

[mane08]

Hi there,
nr. of teeth are counted ok , the weird profile is what i am trying to determine

 

[tbuelna]

Your gear tooth numbers agree with what AGMA 6123-B05 table 5 describes for correct meshing in a simple epicyclic with 3 evenly spaced planets. As for the geometry mods used, every part of the drive likely has some amount of modification applied. The 9T sun shown in the photo appears to have a fair amount of profile shift, since the teeth don't show much undercutting. I'd imagine the designers did what they could to improve the bending strength of the planet gear teeth. These planet gear teeth are subject full reverse bending each rotation, and they are made from plastic. So using an enlarged PD for these planets would be helpful. The ring gear teeth may also be modified to minimize contact sliding with the plastic planet gear teeth, and the friction heat the contact sliding would produce. The ring gear teeth probably have a fair amount of tip relief to permit smooth meshing with the modified planet gears.

Lots of things to consider with this gear drive besides just the sun gear tooth mods.

 

[mane08]

Thanks for the reply. The requirement for a evenly spaced planetary system from AGMA 6123-B06 table 5 is that ( Zr + Zs ) / Ncp = integer so i have (42+9)/3 --> 51/3 checks out ok. But another "law" of a planetary epicyclic mech. is Ring teeth = 2x planet teeth + sun teeth --> 42= 2 x 16 + 9 which does not work here, so i am a bit confused. Is it possible that the Ring and Planet gears have not been altered, just the middle sun/shaft geo suffered a geo modification + profile shifting? .
Also, input RPM from shaft is ~21.600rpm , with 2 stage reduction . The carrier has to transmit the momentum to the other set of 3 planets ( same set only made from metal ). This carrier has the same geometry as the shaft, 9 teeth meshing with 3 equally distanced planets. Output RPM with no load is ~750rpm , so total reduction ratio is ~ 29:1 . see attached pic

 

[3DDave]

From

https://en.wikipedia.org/wiki/Epicyclic_gearing

it looks like the 'law' is used to simplify a more general equation.

According to the Wiki link to a simplified analysis the overall should be 32:1; pretty close to your estimate.

http://www.me.unm.edu/~starr/teaching/me314/planetary.pdf

I expect that all the gear teeth have been altered; as long as they are involute, one should be able to use measuring pins to determine the factors needed to reproduce it, though harvesting the shafts from drills may be cheaper.

 

[tbuelna]

mane08-

If you measure the sun/planet center distance I'm sure you'll find it is enlarged. This is because the sun and planet gears both have positive profile shift.

The positive profile shift on the 9T sun gear helps with undercutting. The narrow top lands shown in the photo linked in your OP would seem to indicate the max practical amount of positive profile shift was applied.

The positive profile shift on the 16T planets helps increase bending strength of the plastic teeth.

The 42T ring gear may or may not use a standard PD. Sometimes the tooth profile geometry of an internal ring gear is adjusted to produce a recess action type contact, which gives better efficiency. The tooth tips are likely shortened to prevent interference with the planet gear roots.

Annex A of AGMA 901-A92 gives a good description of how to approach profile shift and all the considerations involved.

Hope that helps.

 

[mane08]

Thanks for the support, i will continue to study this mechanism, i will get back with a dxf/dwg file if i can upload it to the site.