Simple Clock Gears

thingiverse

I am working on creating a 3D printed clock and I wanted to share what I've learned about clock gears so far. Typically, gear teeth are involute, but clocks are one of the only places where you'll find cycloidal gear teeth. These teeth are designed to run without lubrication and can have fewer teeth than an involute gear. Cycloidal teeth are actually quite simple. The sides of each tooth are radial lines from the hub of the gear, while the tip of the tooth is just a semi-circle, rather than a cycloid. This design allows for pinion gears to be narrower than usual. The gears I'm working with have a 2:1 ratio and I used 123D Design to create them. I started by drawing circles that represent the gears as though they were just rollers. For my 2:1 gears, I used a 25mm and 50mm diameter, with the gears running on axes that are 37.5mm apart. To determine the angle of each tooth on the circumference of the circle, I calculated 360 degrees / 20 = 18 degrees. However, for the smaller (pinion) gear, I reduced this angle by 20% to create clearance and allow for backlash. I fit a semi-circle for the tip of this tooth and drew radial lines from the semi-circle to the hub of the gear. Creating a circular pattern of this tooth for as many teeth as planned, I terminated the radial lines to create tooth and space outlines. For pinions, it's best to have at least eight teeth, but ten teeth work even better. This is a simple gear train with three sets of 2:1 ratios, resulting in an overall ratio of 8:1. As the gear ratio increases, so does friction and interference between parts. My gear train was designed to evaluate the gears, which ran very well after some filing of the hubs. The 20 tooth gears are 2mm thick, while the ten tooth pinion gears are 4mm thick, with two 1mm thick washers used for assembly. To assemble the clock, I placed the large diameter washer on one shaft, followed by the 20 tooth gear. Then, I placed one of the other large gears, pinion down, on the other shaft and confirmed that these two gears ran well together. Next, I placed the other large gear above the single 20 tooth gear that rested on the washer. The three gears ran smoothly after confirming their alignment. Finally, I placed the ten tooth gear on the other shaft and tested the gears again. To complete the assembly, I placed the small diameter washer on the shaft with the large washer at the bottom of the stack, then pressed the plugs into the hollow shafts to hold everything together. The plugs may need filing before they'll press in. When it came to part finishing, I found that the teeth printed very well but the gear hubs needed a little filing to achieve the design dimension.