planetary gear geometry query!

[simplystupid]

Hi, memory loss is my excuse for having lost the answer Miles gave to this last year.
Using Bosch starter planetary ring of 47 teeth, carrier planets have 16 teeth.
I think Miles said 15 or 14 teeth on the sun.
Computer blew up with his answer saved on it.

The OD of the planets is 13.2mm.
OD = (N+2) x MOD
So MOD is 0.733333
Possibly MOD 0.75 with some adjustment having been made to the profiles actually being used?

Maximum space between tooth roots of two opposite planets is 11.2mm. Not enough room for 14 or 15 teeth?
Even if a carrier has unevenly spaced planets, they always seem to be opposed in pairs across the centre sun axis. Cutaway views of this starter show this arrangement.
Based on the OD space available, I originally made a brass 13 tooth sun, which didn't work. And was nigh on impossible to get off even with a blowtorch, after using threadlock.
It should really be hardened steel, aka silver steel, which is so difficult and tedious to broach that I wouldn't want to make another tooth count error.

Mark

 

[amberwolf]

To find any previous discussions you've had, just click on your own profile link, you can then click the "search users posts" link, which finds this thread as your previous one
https://endless-sphere.com/forums/viewtopic.php?f=28&t=73492

Or you can click the link at the top right of the forum "view your posts" and it will also show it.



If you ask a moderator they can merge this new thread with that thread for you, so all the info is in one place.

 

[simplystupid]

Ok Thanks, Amberwolf.
I found the previous post as you describe.
15 teeth for the sun.

The problem now is that I did a sherlock manoeuvre , checking from the result backwards. What is OD of a MOD 0.75 15 teeth spur gear on HPCGears website. And it is too big by far. With the planets meshed fully with the ring there is possibly a tight fit for a 14 teeth gear.

Miles would have used the right formula, so what I have here must be some odd-ball MOD. Which just sounds so unlikely for a low-tech starter motor.

 

[Punx0r]

Just followed a hunch on Google regarding the oddball Mod number you came up with from your measurements: http://khkgears.net/gear-knowledge/abcs-gears-b/basic-gear-terminology-calculation/

It seems Mod is based on the Reference Diameter of the gear, which is not the O.D. of the gear (i.e. measured across opposite tooth tips), but a smaller circle passing approx. 1/3 between tooth tip and tooth root. I'll leave it up to determine how to measure your gears, but it might be that yours are 0.7 Mod

 

[simplystupid]

Just checked HPC Gears again, see what else is possible.
They list a 15 teeth 0.7 MOD as OD 11.9mm.

But the 16 teeth planets should therefore be OD 12.6mm for MOD 0.7.
Which they are not. They are OD 13.2mm.
HPC Gears list an 0.75 MOD 16 teeth as having an OD of 13.5mm. That may be a perfect volute calculated value and my gears are a slightly modified version. Or a different 'pressure angle' or any number of subtle unknowns.

My only option seems to be to cut another brass gear with 15 teeth and just keep shifting the broach centre line, matching a planet until they look like a reasonable mesh? And hope there is actually some brass left.

Thanks, Just seen your repy Punx0r.
Since I just have the finished gear set and no specs, all there is to go on is the OD. HPC Gears for example make their spurs to all mesh with any other of the same MOD, but that doesn't mean they'll mesh well with any other manufactured gear of the same MOD. The volute profiles get tweaked depending on the size/number of teeth.

 

[simplystupid]

Condition No.1
zc = za + 2 zb (17.1)
This is the condition necessary for the center distances of the gears to match. Since the equation is true only for the standard gear system, it is possible to vary the numbers of teeth by using profile shifted gear designs.
To use profile shifted gears, it is necessary to match the center distance between the sun A and planet B gears, a1, and the center distance between the planet B and internal C gears, α2.
α1 = α2 (17.2)

This is from the link to HRK website knowledge base. In their technical references section on planetary gears.
I await further enlightenment as to what 'profile shifted gears' are and how they are designed or made.

 

[simplystupid]

http://khkgears.net/gear-knowledge/introduction-to-gears/profile-shifted-gears/

So, my existing 16 tooth planets are already 'shifted profiles'? And possibly the ring tooth profiles as well!
Therefore it is not surprising the measurements I have made do not match the 'normal' calculated or catalogue dimensions for particular mods.

Heck, what a bad decision to use an incomplete set of gears, albeit perfectly hardened and ground ring and planets for use in the millions of Bosch starter/demolition drill motors.

Now I want to make sure I can make a matching pinion sun which has as wide a tooth base as possible, as it will have 4 times the wear of the already very hard Bosch planet gears. Unlike my 1st. 13 tooth which was very 'peg-like'.

 

[simplystupid]

That is one strange pinion profile. So now I know what I'm aiming for.

this image is from vax in 2011
https://endless-sphere.com/forums/viewtopic.php?f=39&t=34532