Repository Files

/Parts/Drive

Parts

Geared DC motor

3D printed mounting bracket

3D printed wheel

Adhesive foam gasket

Glue

Elastic Band

Knife

M3 Bolts

To move Billy around, a single geared DC motor was used. It makes for simple continuous motion and easy speed control when using PWM or a motor hat. Stealth and power are also concerns. The trike with Billy will have some heft to it, so the motor needs to be strong, but having a bigger motor makes it really obvious, which would take away from the scare factor.

A geared motor is a DC motor with a gearbox built onto it. These generally have a fair amount of power and keep the size down, but can be a little loud.

Mount

This was the first 3D printed piece of many. 3D printing little brackets is much easier than machining them and lets me make little changes and test in reality how they would work.

There are 2 bolt holes that go across the motor. These are the only reasonable mounting points for the motor. To make sure there was space for the drive wheel, the whole motor was tipped up about 5 degrees so the space between the axle and the bottom mounting point is larger than the space between the axle and contact point with the wheel.

On the underside of the front fender is a single screw holding it on. Not the strongest mounting point, but probably good enough for my application.

The mount was designed in Solidworks. Since it's a parametric modeling program, that made it easier to make quick modifications between tests.

Wheel

The wheel was pretty basic. A cylinder that had a slot out of it to mount onto the motor.

Since this is an old tricycle, nothing about it is square or rigid or consistent. This meant that something needed to compensate for the wheel wobbling or being out of round. The drive wheel fit that bill. It could be made to be compressible to handle non-roundness of the wheel, and wide enough to handle the front wheel tipping side-to-side. A consistent speed isn't an important trait of this vehicle.

Like the mount, the wheel was 3D printed. Mostly for ease, but partly because making that internal mounting hole manually would be very difficult. It wasn't round and needed sharp edges. Hard for a CNC and very hard by hand.

Once it was printed, 2 layers of rubber gasket were added around the outside of it. This made the wheel larger, but also made it compressible, which gave it better contact with the front wheel. I ended up adding an extra elastic band around the outside of this wheel to help keep the gasket from snagging on anything and peeling off.

The adhesive on the gasket was never meant to hold it onto a surface for a long time, just long enough for it to be pressed between two parts. Because of this, some decent glue was used to hold it all together.

*Note: This wheel was replaced with a toothed wheel, as after some use it seemed to work better.

Mounting

The fully glued wheel (with gasket and elastic) were press-fit onto the motor. The motor was attached to the mount using M3 bolts, and the mount was attached to the tricycle using the only screw there was. The wheel was a little too large and ended up binding with the under-side of the fender. Using a box-cutter and some patience, a slot was cut out of the plastic fender to make space for the wheel. *Note. Be very careful when doing this. One slip with a really sharp knife can have terrible consequences*