PiWars Robot Traction System

Vex Rorbotic's tank tread parts. SKU# 276-2168, SKU# 276-6558 is also available.
Vex Rorbotic’s tank tread parts. SKU# 276-2168. SKU# 276-6558 are also available.

I still need B to give me more design hints beyond “Tank Tracks” for the PiWars robot design. However, I have been making progress with the design of the Traction System.

A company called Vex Robotics sells a kit of parts to make up tank treads. The kit includes about 1.5 meters of tank tread links, four drive sprockets, four double bogey road wheels, two tensioner idlers and fixing hardware. The drive sprockets can also be used as idler wheels. The mounting hole for the sprockets is a square 0.125″ (on a side). Vex Robotics sell 1/8″ axle rod in various lengths.

One of the comments on my Drill motor post was from Max Allan, who commented: “My guess is that the driveshaft will get shredded/snap. I’d suggest making a square hole down the middle and put a length of bar down it.”

Since that post I have re-designed the gearbox, going from 17:37 teeth to 31:61which approximately doubles the number of teeth and keeps the ratio about the same. Both 90° bevel gears have a prime number of teeth.

Also included in the redesign is a simplification of the drive shaft design, from four printed parts to two plus the 1/8″ rod. I imported the drive sprocket step file from Vex Robotics into Autodesk Fusion 360 to allow me to model this setup.

PiWars 3D printed traction system gearbox testrig.
PiWars 3D printed traction system gearbox test rig – I am going to run this for as long as I can to confirm that there is no wear or damage due to the design.

This redesign simplifies the assembly and reduces failure points in the Traction System, while the insertion of the axle will allow for more force to be transmitted to the drive sprocket.

Test rig for 3D printed gearbox, using current limit to show motor stall point and re-start point.
Test rig for 3D printed gearbox, using voltage limit to show motor stall point and motor re-start point.

The Traction Control system still uses a feedback ring, this ring will pass between an IR LED and IR photodiode, and give the ESC (Electronic Speed Controller) feedback on the number of rotations of the output shaft, this system is still being designed, I will post about it in more detail.

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