# Selecting proper tooth count for proper alignment

Back again… But this challenge is more complex than any previous… I have an outer cylinder with gear teeth on the inner face. I have 18 gears that are radially mounted about a center point. I have a fixed point within each of the 18 geared assemblies and I want all the identical fixed points of each assembly too be closest to the center point of the model at the same time or for a given angle of rotation of the outer cylindrical housing. If I place the 18 geared subassemblies first via radial copy of the involute gear I start with and then draw the outer involute cylinder in its proper position… the teeth only align properly on the gear on the red axis. What parameters/tools must I use for all teeth on all gears to align at the same time without repositioning any gears from their already designated center points?

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I half understand you. Could you upload a copy of your SKP file to clarify?

If my half understanding is right, you need to rotate each inner gear about its centre with the rotate tool so the midpoint of the end of one tooth aligns radially with the midpoint of the base of the tooth gap it should align with

When you radially copied (using the RotateTool,) the components (or groups) copies should have been rotated as they were copied.

This isn’t a SketchUp problem. This is just basic math. It would be useful to see a drawing of what it is you are trying to do.

It sounds like you’ll need to set up the tooth counts to be some multiple of 360/18.

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Something like this, perhaps?

As @DaveR mentioned, the key is to match the number of teeth on each gear and the pitch radii used. I went with 18 teeth on the smaller 1" radius gears and 180 teeth on the larger 10" radius annular gear.

planetary_gears.skp (1009.7 KB)

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[quote=“DaveR, post:4, topic:38026”]
It sounds like you’ll need to set up the tooth counts to be some multiple of 360/18
[/quote] They were rotated accordingly and you are correct… my best result so far is with 36 teeth on the inner assemblies and 288 teeth on the outer but the meshing is not proper because I must move the gear outward 1/16th inch but in doing so for the component to be positioned properly I must also use a slightly larger diameter gear which Sketchup will not permit due to overlapping of teeth on adjacent assemblies in the radial array. Making the outer gear ID slightly smaller causes the teeth to not mesh correctly and shows slight overlap of the teeth before fully engaging the opposing teeth of the inner assemblies. I was hoping there was some sort of tool or formula for this situation. The 1/1000" tolerance is out the window… The smallest increment I can adjust by is 1/16 inch diameter and I’m limited to pitches of 14.5, 20.0 and 25 degrees. Any suggestions

And they were… .SKP file too large to upload, sp screenshot it for Paint. This is only day three and a long way to go… Still searching for fabricator that doesn’t force forfeiting all intellectual property rights by accepting their terms and policies in order to open an account to have anything made through them… aka 3D HUB.

That sounds like you have length and angle snapping turned on. Go to the Units tab of model info and turn them off.

Yes… EXACTLY… But your gear teeth mesh beautifully. Mine have unnecessary gaps and overbite/clash depending on how I try to solve it.

I’m wondering if the problems might stem from the definition of “radius”? Where exactly in the tooth profile does the radius lie?

They mesh beautifully because the tooth counts and pitch radii are compatible. As @slbaumgartner pointed out, the term radius is typically applied to a circle. When the edge of the circle is deformed into teeth, the bottom of the gaps are smaller than the circle’s radius and the tips of the teeth are outside the circle’s radius. The deformation occurs about the pitch radius. Doing this by trial and error will quickly lead to insanity. In order to mesh properly, the teeth need to have the same spacing which means a gear with a pitch radius of 10 will need 10 times as many teeth as a gear with a pitch radius of 1. If the teeth count of the smaller gear is an even multiple of 18, this will simplify things quite a bit.

If you’re working in inches, it is a good idea to work in decimals:

Have you tried Shapeways? I’ve used them for several years now.

A cursoy ckeck looks like there is no allowance for back lash in the drawing and assembly may be a challenge;
It appears the there is the potential for the rack type teeth to have an interference on the adjacent tooth;
There is no clearance allowance at the tooth root for the outer gear

[quote=“jimhami42, post:11, topic:38026”]
Have you tried Shapeways? I’ve used them for several years now
[/quote]Never heard of them… thank you. will check into it.

Not me… It installs that way, but that was definitely the problem… I now also know it defaults back to snap turned on for every model and must be turned off for each (or am I missing a simpler method?
And again… Thank you. This is what I was struggling to do. It just never seemed correct until now. Thanks a bunch!

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You could create your own template with those snaps turned off.

[quote=“mac7595, post:12, topic:38026, full:true”]
A cursoy check looks like there is no allowance for back lash in the drawing and assembly may be a challenge;It appears the there is the potential for the rack type teeth to have an interference on the adjacent tooth;There is no clearance allowance at the tooth root for the outer gear
[/quote]Noted - There was interference one way and the other way was too mushy (loose).

This is my first attempt at gear placement and design. Now that the snap-to problem identified… What is your prognosis now kind sir?

Still learning the terminology… Is the tooth root the center tooth of all the teeth currently meshed with teeth of the adjacent gear? Or is it the crest of said tooth and the gap between the trough?

It is saved separately in the model info in each model; there is no global setting that overrides it as you load a model. So you start out with however the model was set when last saved. But for new models, you can change the setting in the template you are using (as well as other model info and also style) and save that as your default template. All new models you create will start out with those settings.

Thanks… just looked up gear terminology… more than I ever needed or wanted to know. and tooth root was nowhere in any of the listings… I think Mac7595 means the working gap in the dedendum of the opposing gear.

As you found, gear design is a complex and mathematical area of mechanical engineering. Be glad you aren’t trying to do something really messy like a hypoid gear!

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Not that messy (heck no clue what you’re even mentioning but curiosity demands I look it up) just planning on adding upper and lower brackets/blocks to the outer gear housing with a gap between them and place an axle mount therein with a threaded bore perpendicular to the axis of rotation and feed a threaded rod through it with handle to/through the outside housing (not shown) for “tuning” the rotation of the cylindrical housing and thereby also set the simultaneous rotation of the 18 assemblies inside. (fun stuff)