Profile shifting on small shaft - planetary mechanism
Profile shifting on small shaft - planetary mechanism
(OP)
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 :)
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 :)





RE: Profile shifting on small shaft - planetary mechanism
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.
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JHG
RE: Profile shifting on small shaft - planetary mechanism
nr. of teeth are counted ok , the weird profile is what i am trying to determine :)
RE: Profile shifting on small shaft - planetary mechanism
Lots of things to consider with this gear drive besides just the sun gear tooth mods.
RE: Profile shifting on small shaft - planetary mechanism
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
RE: Profile shifting on small shaft - planetary mechanism
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/planet...
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.
RE: Profile shifting on small shaft - planetary mechanism
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.
RE: Profile shifting on small shaft - planetary mechanism