What a mess!!!
Good catch, @mac7595. To make the annular gear, I used a standard gear to make the inside and added the outer circle and then deleted the inside gear. I’ve modified the plugin to reverse the sense of the tip and root allowances and came up with something that’s a lot closer to what it should be. I think I will add another option to the plugin to specify whether the gear faces inward or outward.
planetary_gears_2.skp (608.6 KB)
Gear design is well documented. If you have access to machinist hand book they will have about 100 pages on design with tables that give the info you need plus picture that define all the nomenclature used. If not that just search gear design on the net and you will find more than needed.
My farther was a tool and die maker and I still have his old book, dated 1927 with about all the info needed. Interesting at that time there were about 12 types of different gears and now think it is up into 20s or more.
Thank you for the additional info on how you plan to drive the edge gear. That answers one more question about function;
More questions; Are the 18 planetary gears installed on a carrier and do they have any output on their shaft or their only function is to turn to angle you have shown; What about the work done on the other plate with drawing holes. Do the dimensions there have any relation to this question?
FYI YouTube on defs https://www.youtube.com/watch?v=qQCh3jYm0Sc although he does not show the base circle which is used for involute design to create the profile.
Sorry about confusing you on root:
Some additional fro you;
- Any gears that mesh must have the same circular pitch;
- Gare sisze is ref to the pich dia, not the the top of the teeth nor bottom of them. Think the pluin uses the bottom of tooth;
- From the pitch dia to top of the teeth is generally used 2/N where n is number of teeth and the same for pitch dia to bottom of tooth;
- Then from bottom of tooth a clearance factor is used which is .157/P where P is the diametral pitch and is measured from top of the tooth to bottom of mating tooth
I still see no allowance for backlash. If you do not have it the assembly of gears may be a problem. I have to look up the typical allowance for that. That is defined as the difference between the tooth width and width space. Measured along the pitch circle. - In a normal planetary gear set the carrier rotates so you have the case where planetary gears rotate at the same time the carries rotates. I under stand from your function spec you want only the planetary gears to rotate and not the carrier and by implication the is no sun ( aka ring, annulus shaft) . But you need to have a carrier planetary gear mounting and supporting the carrier in the middle;
- The alignment spec has me confused. It can be set at build and then it is just a matter of how much you want to rotate the outer sun? .
To clarify: the plugin uses the pitch radius, not the bottom of the tooth. It also uses +/- 1/(diametral pitch) for the tip and base with a clearance allowance for the tip and root.
[added – there is no tip clearance in the published plugin - just the one I used for this example]
In my experience, backlash is something to be avoided, although it is pretty much impossible to eliminate. However, the gears produced by the plugin can be altered with the Offset Tool to shave a thousandth or so off the gear edges if backlash is needed or desired.
BTW, I don’t think you ever specified how these are going to be made. You mention 3D Hubs, so I assume that you are planning to have this 3D printed. Be aware that the resolution of the printer and the medium will determine the accuracy of the result. I had the gear train shown below printed in laser-sintered nylon. They work really well, all things considered.
The gears have pitch radii of 0.5", 0.375", and 0.25", respectively.
Going to be machined from polycarbonate. And the outer gear from an acrylic pipe section. Too big for 3D printing.
My impression was that drawing all gear teeth is not generally necessary for fabrication. Most fabricators would prefer to use ready made gears (tho probably not available for the annular), as setting up for gear milling is a pain unless you do a lot of it. Even if they do, the hobs come ready to mill standard teeth so drawing the exact profile doesn’t really matter. FWIW, hobs are shaped according to a range of pitch diameters, with annular gears having a negative PD (in other words, you wouldn’t use the same hob for a 3" gear as a 12" one). A reference such as Machinery’s Handbook would take you through standard practices.
When you find a fabricator, ask them what they prefer— unless you really know what you are doing, it’s better to leave the details to someone with more expertise than you’ll have without going through a long learning curve.
Speaking of annular, if that’s how you drew it, isn’t your tolerance backwards? IIRC, gears are milled with a bit of clearance That would make what you’ve drawn a bit big for an annular, rather than a bit small for a normal spur gear. I’m not sure if the clearance is at the sides, I think it’s just at the tips. The thing is, with gears there is no room for error, it has to be spot on.
Just to be an old stick in the mud. How do you know what you are using are correct dias??
What is the gear ratio of a planetary gear system ( not germane for your design since at minimum a fixed carrier used ) Your original post I think wanted at least 2PI() radians rotation for the planetary gears for x rotation of the ring. You have 18 planet gears on what I think is a fixed carrier( 18 arms) or installed some way but that does not establish the gear ratio. I have seen: 36 and 288 and x18 teeth discussed and the 1 and 10 dias.
I agree that spending a lots off time on gear design is not required but how about some time on system engineering to get the design you need. I have not seen any thing on carrier but, technically that is not required based on the functional spec but, those gears have to be installed and aligned some way? With out carrier that may be fun?
End of discussion. I already purchased components of the assembly which were carefully researched beforehand for feasibility of their intended use. All else must be designed to fit them. As for the pipe section to create the outer gear from (only sold in 1’ sections at approx. $143/ft machining not included). If I cannot find the precise diameter I need - it will be water cut from an appropriate thickness of polycarbonate for probably near the same price. As to the carriers for all the gears. … All gear mountings are in the housing section not shown… I’m starting with known component dimensions and designing from the inside to the outside if that makes any sense.
I haven’t stated what I’m designing for reason. The original concept of 2003 contained over 181 moving parts. I’ve improved upon it and reduced that number to 20.
SketchUp for Dummies - Pg 1 is my next move.
Thank you for your interest and responses. You’ve been a great help overall