Introduction: How to Draw a Sprocket Gear
When it comes to power transmission in DYI project sprocket gears, sometimes called chain gears, offer several advantages over other types of methods of power transmission such as belts/pulleys and meshing gears.
1. Unlike belts they can transmits large amounts of torque
2. Unlike meshing gears they don't need to be constantly lubricated
3. Unlike belts and mesh-gears the spacing between centers can be easily adjusted by shortening/lengthening the chain
Despite these advantages, few people know how to draw sprocket gears, which are designed based on pitch rather than mesh.
Step 1: Get DraftSight or Other 2D CAD
The first thing you will need is a drafting program to draw the sprocket gear with. I recommend DraftSight, which is the free CAD program I used for this demo.
For those who learned how to use, and are hopelessly addicted to AutoCAD's GUI, I might note that DraftSight has the look and feel of classic AutoCAD without the $4000+ price tag.
Step 2: Determine Your Key Dimensions
Next step is to determine the Pitch and Roller Diameter for your chain size. Also you need to determine how many teeth the sprocket needs to have.
I will be demoing 40 tooth #40 chain, which has a Pitch of .5in and a roller diameter of .313in.
Step 3: Draw the First Sprocket Tooth
This step is the hardest. You need to draw the first tooth of the sprocket gear, which is accomplished as follows.
1. Pick a start point and draw upwards 1/2 of the pitch, and out to the right some distance (longer than expected gear radius).
2. Rotate the line extending to the right up an angle of [360deg / (# Teeth * 2)].
3. Draw a line from the upwards extending line to the intersection of the angled line.
4. At that intersection draw a line downwards the length of the pitch, and then at each end of that line draw circles whose diameters are the roller diameter plus .005in.
5. Clear away the reference geometry cluttering the drawing and then draw a line on the backsides of the two circles (will be the length of the pitch).
6. From the center of that reference line draw a line out that is 20 percent the length of the Pitch and another that is 60 percent the length of the Pitch. At the end of the 20 percent line draw a vertical line that intersects both circles.
7. At the end of the 60 percent line draw a vertical line that is 10 percent the length of the Pitch and extends evenly in both directions.
8. Use the 20 percent line and the line on the backside of the circles as trim boundaries to trim down the circles.
9. Draw a concave arc that acts as an extension from the end of the trimmed circles to the one end of the 10percent line.
10. Mirror that arc about the start point.
11. Fillet the two arcs together with a radius that is 8percent of the pitch.
Step 4: Use Circular Pattern to Finish the Sprocket
Once the first tooth is drawn and the reference geometry is removed (1), finishing the gear is a simple two step process of first using the circular pattern tool to pattern the tooth around the start point per the number of teeth. Then simply draw the needed center whole size about the start point and delete the start point plus any leftover reference geometry.
Note: for previous AutoCAD users you will likely find the "Fill Angle and Total Number of Elements" setting in DraftSight to be the best way to create the circular pattern.
Step 5: Downloadable Sprockets
As I create different sprockets I will attack them to this page so feel free to download them and use them for your projects. Also if you are no CAD expert and really need a custom sprocket drawn, let me know and if I have time I will draw it up and attach it to this page...
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