SkyMoBot
Newbie
Posts: 33
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« on: October 21, 2011, 03:57:58 PM » |
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I can make a basic planetary gear train with a single planet, but I haven't figured out how to make one with multiple planets. I thought I read somewhere along the line a thread describin g how to do it, but I cannot seem to find it..
I watched the tutorials that I thought might pertain, but I didn't see it there either. Is there one?
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ArtF
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« Reply #1 on: October 21, 2011, 05:26:34 PM » |
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Hi:
I made one yesterday in fact. Place a planetary gear, then place a pinion on the gear and rotate it to 90 degrees, drop it.
Place a pinion on the one you just placed and put it at -90 degrees .. Repeat this for each angle you wish to have planets at..
Trick: The trick to this is the number of teeth of the three gears across must equal the number of teeth in the ring. I was doing a
104 tooth as I recall, so I placed a 40 tooth on top, a 24 tooth in the center by rolling it off the 40 tooth, then places 2 more 40 tooth gears off the
center 24 tooth.
24 + 40 + 40 = 104 which matched the planetary gears count, so all the gears meshed fine..
Art
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Thanks, have fun,
Art
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SkyMoBot
Newbie
Posts: 33
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« Reply #2 on: October 22, 2011, 12:11:22 PM » |
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Is there certain ratio's that work? Or a formula I need to follow? Using a 80T Ring, 20T planet, 40T sun seems to work fine, but 75/18/39 and 76/19/38 do not. This is what I am doing. - I have 12x12 pieces of "gear wood"
- On the Spur Gear screen, press the "??" button and put in 80T, diameter of 8, and ratio 4:1
- Calculate and send to design
- Checkbox on wheel internal to get my planetary gear
- Place wheel, and 4 planet gears at 0,90,180....
- Back on Spur Gear screen, uncheck wheel internal, change teeth of wheel to desired number
- Place wheel on one of the planet gears, choose to use same shaft as sun gear
Is that right? Attached is one that doesn't seem to fit quite right. (I changed the gprj extension to a .doc so it would upload) |
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ArtF
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« Reply #3 on: October 22, 2011, 02:20:56 PM » |
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I think the rule is the gears must have even numbers. An odd numbered tooth gear doesnt seem to work out because it leaves 1/2 tooth which cannot be meshed.
Im not sure what hard and fast rules apply to it, Ive only leanred to use the tooth count of the pinions and the count of the main to make them. But I do think odd numbers are the issue..
Art
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Thanks, have fun,
Art
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bosr
Newbie
Posts: 39
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« Reply #4 on: October 22, 2011, 03:05:43 PM » |
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It might pay to consider setting up a reference section on this site with docs that describe all these little "rules" that are scattered about this forum.
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Dan
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« Reply #5 on: October 22, 2011, 03:07:27 PM » |
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From SDP/SI Gears Handbook:
In order to determine the relations hip among the numbers of teeth of the sun gear A (Za), the planet gears B (Zb) and the internal gear C (Zc) and the number of planet gears (N) in the system, the parameter s must satisfy the following three condition s:
1) Zc = Za + 2Zb
Necessary for the centre distances of the gears to match.
2) (Za + Zc)/N = integer
Necessary for placing the planet gears evenly spaced around the sun gear.
If an uneven placement is desired then:
[(Za + Zc) x theta]/180 = integer
where, theta = half the angle between adjacent planet gears.
3) Zb+2 < (Za + Zb)sin(180/N)
Necessary to ensure that adjacent planet gears can operate without interferi ng with each other. This is for a design with equal planet gears placement .
Dan
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ArtF
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« Reply #6 on: October 22, 2011, 03:18:42 PM » |
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Dan:
Thank you. As I suspected, its about uneven numbers. Nice to have the real rule though.
I will look at a ref section, I imagine Bob can prune out any such "rules" and combine them into a
good section of such things.
Art
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Thanks, have fun,
Art
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SkyMoBot
Newbie
Posts: 33
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« Reply #7 on: October 31, 2011, 03:41:55 PM » |
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In the meantime, what's been working for me to cut planetary gears is to stick to even numbers. I did run into a issue where the the gears bind very tightly, Since the gears mesh with multiple gears and gearotica doesn't seem to account for that, I cannot use the backlash compensat ion. It might not be the "right" thing to do, but what I did was make the SUN gear slightly smaller. Since you cannot just resize the entire gear, I trick sheetcam into making it smaller by having the cutter size for the end mill set smaller than it is for just the teeth. For example, I am using a end mill .124 inches in diameter. For the teeth, the endmill size is set to .120. This works for me, especiall y for wooden gears. Photo of my latest... http://www.smolkowski.com/planetarygears4.jpg |
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