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Worm gear discussion
04-19-2014, 09:24 AM (This post was last modified: 04-19-2014 09:28 AM by steamboat.)
Post: #1
Worm gear discussion
I have not printed a Micropede gear but I have some fear about it's general function and durability.

From the general mechanical point of view a worm gear has the advantage of turning the movement by 90° by using only a limited space for this.

The main disadvantage is the bad efficiency of such a gear. This results because of the cinematic of how the teeth are rolling and gliding with each other. A fine grinded metal worm gear running in oil has an efficiency of 70-80%. To archive this, a fine alignment of the wheels is necessary too. The 20-30% lost energy will create heat and axial forces. This axial forces again will be transferred into heat in the axial bearing - in our case at the touch points of the worm gear faces with the Shellmo walls.

I know this from my steam boat pumps which I designed lately. Here I am using out of the shelf grinded wheels with an efficiency of 78% running in oil and with ball bearings for radial and axial forces:
[Image: 2014-03-22.214001.jpg]

[Image: 2014-03-23.171800.jpg]


Because the Shellmo gear is made from rough plastics and even with a linear structure produced by the printing layers we will have even more loss in the gear. Additional we have no lubricant and a bad alignment of all parts. My expectation about efficiency would be less than 60%.

What does this mean? 40% and more of the battery capacity would be directly used for global warming - or on other hands. With a better gear you could make 20-30% longer trips with Shellmo - not talking about the reliability and the problems of printing this parts at home...

I would love to get some touch to such a Shellmo gear. Maybe Sho can tell us from his experiences. What wattage are the motors taking?

I already mentioned the use of ordinary cog wheels combined with a belt. Don't know if this is a realistic scenario but I will give it a try when I find some time capacity. Would love to modify the modular chasis for that...
[Image: 1240387_1382671532015857_3850299958859524185_n.jpg]

[Image: 1010218_1382671915349152_7501036329230473917_n.jpg]
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04-21-2014, 03:21 AM
Post: #2
RE: Worm gear discussion
Hello James,

my goal is still to print Micropede. My RapWalker efforts are only to get something moving into my hands...

You told us about your worm gear print out. Please show me some detailed pictures of your result and tell me how you printed it.

Rainer
http://www.steamboating.de
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04-21-2014, 06:20 AM
Post: #3
RE: Worm gear discussion
(04-21-2014 03:21 AM)steamboat Wrote:  You told us about your worm gear print out. Please show me some detailed pictures of your result and tell me how you printed it.
Yeah last week I split the worm gear in to two equal pieces since you need to print this part at the highest quality you can possibly print. I wanted to working surface to be printed vertically without support.

I have not done anything else on this since the first test because I started working on the chassis once I knew I could print the drive train parts.

I am attaching a couple screenshots from makerware. The part itself is at work so I can get better pictures tomorrow.


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04-21-2014, 07:11 AM
Post: #4
RE: Worm gear discussion
Great - I never would print it like this - but why not. You must have a strong binding between table and part when printing such a height!

Is the nozzle resting in the air to cool down the part or do you have such a strong fan to keep everything cool while printing? I am not familiar with this Makerbots. I only know the home brewed printers.

We have one extra day of Easter Holidays at Monday. But you have 6 hours more left at Sunday - it's 23:11 here in Germany already. And 90 minutes till end of printing some RepWalker parts ;-)

Rainer
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04-21-2014, 11:44 AM
Post: #5
RE: Worm gear discussion
(04-21-2014 07:11 AM)steamboat Wrote:  Great - I never would print it like this - but why not. You must have a strong binding between table and part when printing such a height!

Yeah, haha. I can't imagine printing any gear any other way. Honestly, since I learned to print gears when I started building a reprap huxley I have never seen gears printed any other way and was doing just what I heard others doing. Its impossible to get a good print horizontally IMO, though something this tall does seem out of the norm.

With the replicator 2 there is a air nozzle blowing and I never have a problem keeping a part to stick on painters tape. I do think this is about the limit for this diameter in a single print, the part would get too hot any smaller, you would have to print several items at once to give one a chance to cool between passes.

If you find that interesting check out this image of how I printed a knife handle with support and raft. (link has more pics)
http://www.thingiverse.com/make:61989


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04-21-2014, 04:55 PM
Post: #6
RE: Worm gear discussion
(04-21-2014 11:44 AM)jmccartney Wrote:  Yeah, haha. I can't imagine printing any gear any other way.
Yea - but with this height...
(04-21-2014 11:44 AM)jmccartney Wrote:  how I printed a knife handle with support and raft.
crazy!

Rainer
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04-21-2014, 06:12 PM (This post was last modified: 04-21-2014 06:20 PM by sho.)
Post: #7
RE: Worm gear discussion
About efficiency
I can turn shaft by hand by using "hand crank", and I can feel it's heavy to turn without no load to leg. Exactly, the efficiency is low.
You are right, Rainer. It's contribute to global warming.

I couldn't find the other best methods to move these 16 legs yet.
But, only 1 method I'm thinking yet is the original mechanism of Gakken "Centipede". I'm calling this, "link rod" method.

When I started to make Micropede, I made a simillar 3D model to Centipede.
Please see .skp file. (I'm sorry about the 3D data is not beautiful this version yet)

At first, I printed like this model at Shapeways, and it's not work.
Because, link rod was thin and everything was fragile.
Also, I felt that's hard to install links to rotary axis (bearing ?).

So, I started to find another mechanics and I found "helical gear"(worm gear) my old Japanese mechanism book. And I designed current one.


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04-21-2014, 06:31 PM
Post: #8
RE: Worm gear discussion
First of all thank you for sharing this file. I did not know that you can go so far with Sketchup. Maybe sometime I should learn to use it.

Also the link rod method has a lot of friction because of the material pairing and the absence of a lubricant.

The helical gear is still a common method to reduce speed and or redirect motions. But as mentioned already because of the geometry it has a lot of friction. Combined with the material and the surface produced by the fabrication method everything become worth. Don't take me wrong - it will work - but my fabrication method (3D printing) is not able to reproduce it. I could mill the worm gear on my home made CNC mill with a 4th axis from brass or bronce - but that is not the goal...

If I find some time during the next weeks I will try out my belt gear drive.

Rainer
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04-24-2014, 01:37 AM
Post: #9
RE: Worm gear discussion
I think i about have a decent print of one of the worm gears.


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04-24-2014, 01:57 AM
Post: #10
RE: Worm gear discussion
I am happy to see this!

Rainer
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