HELP
Description
Instructions
I've just uploaded the OpenSCAD file on purpose.
I think I have the involute gears correct.
Whole gear chain works, but it is way too tight.
I started off with plastic on plastic bearings, tried in ABS, gears seemed to floppy and ABS seemed too abrasive, switched to PLA, much harder, seemed to be close, but too tight. Current file allows for 2mm ball bearings for every gear bearing, better but still too tight.
I'm only a software engineer, I am dearly hoping that some mechanical engineer with OpenSCAD experience could take a look and give me some advice.
You must be logged in to post a comment.
Hi Trevor,
Have you examined the Makerbot Clock design? You might get some ideas from that. http://www.thingiverse.com/thi...
Best of luck with it!
NumberSix
I know it is very hard to make a clock, any gear will work down ratio (fast to slow) but going upratio is very hard, gears have to be correct and bearings have to be correct.
This clock goes from 1 revolution per day (weight wheel) to 1 revolution per 15 seconds (pendulum wheel).
I have tried top assembly with gears 1 thru 7, that works freely under weight power.
I have tried gears 7 thru 14, that works freely pushing lightly with finger.
I have tried top assembly with gears 1 thru 11, that is just slightly too tight, works with weight plus finger help.
But put it all together and no go
The scad file does include pendulum, but left that off for now, whole gear chain needs to be free running first, then I add the pendulum to regulate down to accurate seconds.
I have not included stl files on purpose, I don't want anyone to go through pain of printing just to end up with something that doesn't work.
Don't feel so bad. Clocks are the pinnacle of mechanical engineering. In the old days, if you were an mechanical engineer apprentice, your final exam would be to design and make your own clock. It is the culmination of years of learning and experience. Keep working at it, you'll get it right some day. Be sure to look at some old grandfather clocks that are working and see how they work.
License
More from TrevM
More from Engineering
Very interesting design. You have obviously put a lot of time and effort into this. You even wrote your own involute gear module(s). I am not a clock expert, but I think it looks like a pretty solid design that could be made to work with some tweaks.
Here are some things that I that you are doing correctly:
* When increasing speed with large ratios, it is best to split the change down into several stages, each with a modest speed increase. You do this well. All of your steps seem reasonable. The biggest is 5:1. If this gives you too much trouble it could be split into 2:1 + 5:2
* Bigger gears are more forgiving of printing imperfections and allow looser tolerances. Your gears are nice and large.
* You have a raised area on the frame, not much bigger than the shaft, by each gear to keep the teeth from rubbing on the frame.
* You support your gear shafts on both ends. I noticed to make that printable, you have the "top" of the shaft male and the bottom female. That should work, but the larger diameters required are causing extra friction. See below.
* PLA is a better choice than ABS.
Here are some suggestions that I came up with:
* The friction of the gear's shaft goes down as you decrease the diameter. It looks like you gears have large diameter shafts. This will cause a lot of friction as the gears turn. Consider reducing the diameters of the shafts where sliding contact occurs. For a first order model, the force of the friction is the same regardless of shaft diameter, but the distance travelled is proportional to the diameter, so the energy lost and the torque are both proportional to the diameter. This is especially important on the gears closest to the weight.
* It is often hard to get low friction between to similar materials. That is to say it is hard to make PLA on PLA bearings. Especially printed ones, which will never be perfectly round or smooth. I think it would help your design a lot to use metal shafts (that would incorporate my reduce shaft diameter suggestion at the same time as this one). The frame and gears would still be printed, but you would purchase metal shaft such as 3/32" brass rod or tubing. Amazon sells 36" of 3/32 brass tubing for $2.80.
http://www.amazon.com/Brass-Se...
I have used similar hobby tubing in the past. It is easy to cut and slides well. For shafts with more force on them (weight pulley and first few gear stages) you can use 1/8" stainless steel rod.
http://www.amazon.com/dp/B000H...
This is more expensive and harder to cut, but it is probably worth it. It would not be too hard to modify the design to use the rods. Put a hole through your gears and the frame and insert the shafts. I recommend making the hole undersized and drilling it out to the correct diameter after printing. Be sure to keep a small raised area around the shaft so the teeth don't rub. If you use the metal rods, it can be much smaller in diameter than you are using now.
* You could consider more experimental 3D printing materials. Nylon would be a good fit here because it is strong, tough, and has a low coefficient of friction. I just saw thing 34801 found a nylon filament that prints at a similar temperature to ABS. I haven't tried it, but it looks promising.
Thank you very much indeed.
I had no choice but to create the involute gears, there does not seem to be that functionality anywhere else for OpenSCAD.
I really wanted fully printable with as few vitamins as possible, I started with pure friction bearings, then I altered so that I had a 1mm ring making contact within each female part. I finally went to using 2mm ball bearings on every shaft end. So I learned by trial & error that friction was the problem, I had not thought of using metal rod, thanks again.
I have not tried anything other than ABS and PLA so far, I've only been playing with this printer for 4 months.