Spur differential. Functional sample of spur differential for students or any one. You can see and show how it is made and works.You can help me to do more things like this. Buy my designs at pinshape:http://pinshape.com/users/27920-jsirgado The OpenSCAD file is there, you can change all.Recommended allen screws: 6x 3mm diameter 35mm tall for 12 teeth gears. 2x 4mm diameter and minimum of 40mm tall for axis. 1x 4mm diameter and minimum of 35mm tall for handle. 1x 4mm diameter and minimum of 20mm tall for grip. Lots of nuts and washers. Use more washers to align the gears and reduce the friction. Use 2 nuts together to hold the frame and gears in position. And remember: You have 2 aligned but separated central axis. One screw for each 24 teeth gear.