This week at The Tinker's Workshop I came across an interesting video on Youtube.
This video animation is of a planetary gear set in motion. This kind of mechanism is used in automatic transmissions and is a key assembly in making it all work. I was so intrigued by the animation that I immediately had to see if I could make one for real. So I set about spending some major design time creating the assembly and its parts in Inventor 2013 cad software. Even with the cad software it was a real trick to get everything lined up and working properly. This is how it turned out.
The mid-section of the gear set I laid out in Inventor with a 26 toothed ring. This would be the track for the four inner gears to rotate around on.
The large inner ring was then flanked by two outer rings which allow the blue gears to rotate and not slide out of the assembly. This is where the animation is incorrect. I could not build the actual working model the same way the animation was depicting the assembly. It simply would not hold together. It needed an outer lip on both the front and back of the assembly to hold the inner gears in place while they were being rotated.
The four outer sprockets were the next pieces that needed to be created. The first attempt at these pieces did not work out quite as planned. The spacing or thickness of the center white ring was half an inch and the blue sprockets were three eighths. Just was to much slop in the assembly so I redesigned the blue gears to be just under a half inch thick. This would allow the gears to rotate without side to side friction (or at least very little friction) and little or no slop.
The center sprocket was another problem in the assembly. The first go around in putting it together looked good in the computer but not in real life. I assembled the center gear and tried to spin the assembly like the animation. This worked but not as smoothly as I had hoped. I removed the center gear and started taking measurements and seeing what could be done to free up the assembly to make it run smoother. I reduced the number of teeth from ten to eight thinking that this might be the cause. Not so. I printed an eight tooth center gear and it would not even go into the assembly. So I went back to a ten tooth sprocket and decreased the overall diameter of the part along with the diameter of the inner valleys of the part. This did the trick. Now I had a working assembly.
To hold everything together I mounted a white outer plate to the front and back of the center assembly as shown in the photo above. This held all the blue sprockets in place with an inner bushing so that the gears would rotate freely and all the hardware could be tightened down so as to not interfere with the rotation of the gears. The center gear did not need a center bolt or shaft as the four blue outer gears hold this gear in place.
The final assembly is a nice size standing five inches tall and wide with stabilizing feet at the front and rear. All the bolts in the assembly are 1/4 inch and 1 1/2 inches long.
This was an interesting project that stretched my designing skills just a bit. Which is always fun to do once in a while. No matter what I think my efforts paid off once again with something challenging to create and a nice conversation piece for my computer desk when company comes by to see what new project I have been working on in the shop.