Revision as of 00:38, 25 December 2020

How to Design a Sprocket for Bulldozer Drive

The sprocket generator in OpenSCAD creates a proportionally correct sprocket.
How do we know how to scale it to match our specific case?
We first design the track. We settled on pin spacing of 3", and XXH 1.5" pipe (see Pipe Chart) for the roller, with 1" pins. Thus: roller = 1.9" OD (1.5

XXH pipe), pitch = 3" (based on geometry of pins:

1 - Number of teeth
2 - Roller (5 units)
3 - Pitch - 1.58x the roller size as determined by our track geometry (looks sensible, and 3" pitch was selected as a convenient distance, and 1.9" OD roller was selected as a readily-available stock material
4 - Thickness - that is the thickness of the plate material
5 - Tolerance - play in the distance between rollers (pitch)?

To generate a correct sprocket, we can scale the resulting design in CAD - by superposing the track piece over the sprocket to make sure the track pieces fit in the valley-to-valley distance in the sprocket (which corresponds to the distance between rollers, or pitch)

See the File:Sprocketgeo.dxf for how the track pieces fit into the sprocket (example shows a larger sprocket than an 8 tooth sprocket)

Hint: The bulldozer from 2015 used an 8 tooth sprocket, generated with sprocket(8,5,7.5,2,0.2); in the last line of the Editor

Note: stock Export DXF does not work for 3D files - they need to be projected first.

To project - projection(cut=false) import("sprocket.stl"); STL file must exist in same directory

Sample file for exercise in OpenSCAD - the sprocket STL and the .scad file that loads the STL and does a 2D projection. Note that the directory must be correct. File:Sprocketexample.zip
Note: original srocket was generated using Parametric Sprocket Generator using sprocket(12,5,7.5,8,0.2);

Sprocket geometry for a correct sprocket pitch and roller diameter - example - File:Sprocketgeo.dxf

@@ Line 3: / Line 3: @@
 *The sprocket generator in OpenSCAD creates a proportionally correct sprocket.
 *How do we know how to scale it to match our specific case?
-*We first design the track. We settled on pin spacing of 3", and XXH 1.5" pipe (see [[Pipe Chart]]) for the roller, with 1" pins. Thus: roller = 1.9", pitch = 3". Ratio of 3/1.9=1.58. Ratio of pitch to the roller is constant.
+*We first design the track. We settled on pin spacing of 3", and XXH 1.5" pipe (see [[Pipe Chart]]) for the roller, with 1" pins. Thus: roller = 1.9" OD (1.5
+ XXH pipe), pitch = 3" (based on geometry of pins:
+[[File:trackunit.png|200px]]
+*Ratio of 3/1.9=1.58. Ratio of pitch to the roller is constant.
 *Then we generate the sprocket using 5 parameters
 :*1 - Number of teeth