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.

http://www.3ds.com/products/draftsight/overview

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...

Comments

author
AlexJ56 (author)2016-03-01

I have never attempted drawing a solid before. I followed your instructions carefully and 4 hours later I had my completed sprocket. I want to machine a 150 tooth sprocket for number 25 chain. Now i only need to turn this into g code. Thanks!

author
shelly142 (author)2015-09-19

Using Draftsight for this Drawing but I do have one question. On step #9 the instruction is to draw a concave arc that acts as an extension....................
How do you calculate the value for the arc. Draftsight provides many different options for arc.

author
AR88 (author)2014-10-19

Hello EV Builder. I've been trying to make a 13t sprocket with a 0.5 pitch and a 0.313 roller diameter for a 1/2 x 1/8 single speed chain but it's been years since I've used AutoCAD and would like to know if it's not too much for you to draw one as well as a 58t sprocket with the same pitch and roller diameter. Also, don't worry about the bore diameter. I can't seem to find sprockets that will fit this chain size as they're too wide. Thank you and I really appreciate it!

author
Mechtechnal (author)AR882014-10-20

Check out the last page to download them and verify they were what you wanted.

author
AR88 (author)Mechtechnal2014-10-20

Your awesome! That's just what I needed. Thank you EV Builder.

author
nathanaloysiusbash (author)2014-10-11

At least the free version of draftsight I have seems to have issues with printing multi-page drawings. As in it only prints one page of the drawing. EV builder, I know you have used the program succesfully. Did you print directly from the draftsight program or import the drawing into another program to print it?

author

Sorry been out of town, but AutoCAD & Draftsight only print one page at a time. What you need to do is draw an 8x10.5in (8.5x11in minuses 1/4in border) rectangle around what you want to print and then use the Specify Window printing option to select that box by snapping to it.

Then just make an 8x10.5in rectable around everything else you want to print at 1:1 scale and repeat the process with new boxes.

author

Ahh. That makes sense. Will try that tomorrow. Thanks!

author
nathanaloysiusbash (author)2014-09-15

This instructable is awesome but it need's a video. Someone unfamiliar with CAD and Draftsight is going to have no idea how to follow these steps. A screen recording of the process would help people know which icons to click on/commands to enter without having to spend hours learning how to use the draftsight program. (Not that learning DraftSight isn't worthwhile.)

author

Thanks for the input, when I get a chance I will have see if I can figure out one of those screen capture programs. If you can tell me what size sprocket do you need.I will make the video showing how to draw that one exactly :)

author

Cool. The one I want to make is 105 teeth, 17 inches in diameter (not sure if that's actual outer diameter or circular pitch diameter), then it has a hole in the middle 3.4 cm in diameter. It's for a 1 speed crank. It has the same pitch and roller diameter of #40 chain. That would be awesome if you'd find time to do that. I don't know what operating system you have on your computer but I do know that for the last few version of Windows there are screen recorder utilities made by microsoft. What operating system do you have?

author

Hey its going to be a while before I can post an update video so in the mean time I added a new page with your sprocket already drawn on it, feel free to request other sizes.

author
gibbert (author)2014-04-01

Thanks for putting this up. I've been looking for this for a while. I might be able to make my own chainring now!

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