pic: Butterfly Drive V2

[GeeTwo]

Originally Posted by GeeTwo

One of these days I'm going to have to have a good session at a dry-erase board to figure out what the proper angles and proportions are for one of these modules.

Today was the day (though I used a sheet of paper rather than a board). I'll follow up later (probably over the weekend) with a diagram, but the bottom line is that if the module has a short pendulum arm (vertical distance between the drive axle and the CoG), and ignoring bearing friction, wheel slippage can be avoided by ensuring that

• cos(θ-φ) > r cos(φ)/μR

where

• θ is the angle between the drive axle and wheel axle then the wheel contacts the carpet, measured down relative to horizontal
• φ is the pressure angle of the gears
• r is the pitch radius of the wheel gear
• R is the radius of the wheel
• μ is the coefficient of friction

The optimal angle for floor engagement is therefore the pressure angle of the gears, though small variations from this should not cause too much of a problem.

I also see that doing this with belt or chain would not work; the key point is that the drive gear pushes down on the wheel gear to provide the basis for the normal force. In order to apply enough torque to the module, there would have to be a significant gearing up within the module.