This gear is a little experiment I did to see how well I could make these kinds of gears, and so far it works really well. Eventually, I plan to 3D print a 4-speed gearbox for a LEGO car or something. In which case, this gear is massive and I should redesign it for LEGO-sized things.
I designed this gear for free using Google Sketchup and some plugins for it to make the gear shape and export as a .stl file. You can find the .stl exporter on the "Extensions Workshop" within GS, and here is a link to the gear plugin page. I did not create this plugin, Cadalog Inc. did. Installation instructions are included in the download in the readme.txt file.
I used purple PLA plastic in my high school's Makerbot 2 5th generation printer, and it took about 7 hours to print. I don't really know what resolution it is done in, since the librarians handle all of the gcode stuff. The layer height seems really small, though and that accounts for the long print time. The things are really light, so I'm guessing the infill was at about 30%, honeycombs.
If you want to print this yourself, I provided the stl file for you to do as you please. The gear is 30mm tall and has a pitch radius of 32mm with 16 teeth.
If you want to know how to create your own gears, then follow the rest of this instructable!
Step 1: Pre-Design Steps
If you are going to make a gear, you will also want to make gears of other sizes to work with it. In this case, there are some variables you need to standardize between the gears. I have not printed another gear of a different size yet, and I will do so in the future. I think it is safe to assume that the standard variables for normal gears (key involute gear) will apply to these gears as well. For this purpose, I created a text file that contains these standard values so I don't need to remember them. It looks something like this:
My Constants for Herringbone Gears:
20° Pressure Angle
Pitch Radius = #Teeth * 2
Shaft specs in component Each half is 15mm tall, total 30mm
30° Twist Angle
Let's go over what all these mean:
The pressure angle dictates the shape of the teeth. The GS plugin for gears gives you 3 options: 14.5º, 20º, and 25º. I put pictures of each example in this step, in the listed order. At a low pressure angle, the teeth have more contact with each other but that means more noise and backlash (space between meshed teeth, not good). The higher pressure angles provide less contact, so they are smoother and quiet.
Pitch radius is the radius of the circle that represents the point of contact between 2 gears. In the images, this is represented by the thin red circle that goes around through the teeth. It is not the radius of the outermost teeth, so keep that in mind while designing. It needs to be kept at the same ratio for different numbers of teeth between gears for the gears to be compatible with each other.
A herringbone gear is just 2 helical gears put together to counteract each other's push through the axle that happens because of their shape. The helical angle of the gear is how many degrees of incline is in the teeth of the helical gear. I'm using the term "twist angle" because that is how many degrees I rotated the top face of the gear in Google Sketchup relative to the bottom face.
Step 2: Work in Google Sketchup
I'm going to assume that you know some Google Sketchup basics, because I don't want this to be a Google Sketchup tutorial.
So to make a herringbone gear, start by going under the draw menu and clicking the "key involute gear" option. A window will pop up for the gear design inputs. Set the first 3 options according to what you now know from the previous step. Set the shaft radius, keyway radius, and keyway depth to 0 if you are making a helical or herringbone gear. Those settings only help you for untwisted gears.
Explode the gear face that was created. Now use the push/pull tool to extend it up by 1/2 of the total width of the gear size that you want. Then select the top face and use the rotate tool to rotate it around the center however many degrees you want your twist angle to be. Use a value that doesn't look like it won't work, if you know what I mean (i.e. 180º).
Then use the push/pull tool to extend the gear face by the other 1/2 of the desired gear width, and rotate it again in the opposite direction using the same twist angle, but negative.
To make the shaft, figure out what kind of profile you want for it using 2D lines. Before using the push/pull tool to extend that profile into a shaft, make a copy of that profile using the offset tool to create a hole for the gears that will have 0.1mm clearance. This profile will be used as the center hole for the gears. We are making 2 separate profiles because if we used the same profile for both the shaft and the hole, they wouldn't fit together when printed unless you did some sanding. I recommend saving these profiles as components for later use.
When you extend the shaft hole profile through the center of the gear using the push/pull, Sketchup should make a hole in the gear rather than overlapping solid.
When you are done with both the gear and shaft, you can copy/paste them however you want to get them ready to be exported for 3D printing.
Step 3: Finishing Thoughts and Design Flaws
These gears a pretty big for the project I'm working on, but they are more of an experiment to see what is possible. When you make your gear, it is important to know the pros/cons to having an odd vs. even number of teeth. This is because when you use an even number of teeth, the same tooth on one gear will always meet the same tooth on the other gear. This can cause isolated damage to a set of teeth. If you use an odd number of teeth, then the teeth on each gear will always meet a new partner on the next revolution. This will make any damage more spread out across the teeth rather than focused in one place, which is good or bad depending on what the gears are being used for. If lubrication is involved like in real gearboxes, then you want an odd number of teeth to distribute it more evenly.
For the other flaw, I'm not sure if it is serious or not. Real herringbone gears have the middles of their teeth slants curved outward to make the gear edge profile straight. As illustrated in the video in this step, these gears are not so. This is because when you rotate the face in Sketchup, it just follows the curve with straight lines. It is sort of hard to explain, but this effect becomes pretty serious when using low numbers of teeth (2nd image). If anyone out there knows a solution to fix this issue, I would be grateful.
I hope that this instructable inspires some of you to try this out for yourself and get into 3D printing mechanical parts. That's my favorite thing to print!