Building the Drivetrain

The drivetrain is everything that makes the robot move: motors, shafts, wheels and the parts that hold them. A drivetrain that is built carefully feels smooth when you push the robot by hand. One that is built in a hurry squeaks, drifts to one side and drains the battery fighting its own friction. This lesson shows how to build the smooth kind.

Shafts, axles and bearings

A wheel turns on a shaft (sometimes called an axle). The shaft passes through the frame, and wherever it passes through, it needs a bearing. A bearing lets the shaft spin with very little friction and stops it rubbing metal on metal, which would wear the hole and slow the robot. A hub connects the wheel to the shaft so they turn together, and shaft collars clamp onto the shaft to stop it sliding sideways.

Support the shaft on both sides

A wheel mounted only on the end of a motor shaft is called direct drive or cantilevered: all the robot's weight pushes sideways on the motor's gearbox. It works for light robots and many kits are designed for it, but it can bend shafts and wear gearboxes over time. A stronger design supports the wheel shaft with bearings on both sides of the wheel, often by placing the wheel inside a U-channel or between two plates. Follow your kit's recommended design; if it offers a supported option, it is usually the more reliable choice.

Mounting a wheel step by step

1. 1
   Fit the bearings

   Press a bearing into each hole the shaft will pass through. It should sit flat, not tilted.
2. 2
   Slide in the shaft

   Push the shaft through the bearings, adding spacers and the wheel with its hub in the correct order as you go.
3. 3
   Lock the wheel

   Tighten the hub screws so the wheel and shaft turn as one piece.
4. 4
   Add shaft collars

   Clamp collars on both ends so the shaft cannot slide, leaving a tiny gap so nothing is squeezed.
5. 5
   Spin test

   With the motor disconnected, spin the wheel by hand. It should turn freely for a moment and stop slowly, without grinding or tight spots.

Alignment

Alignment means every part of the drivetrain lines up as designed. If a motor is slightly tilted, or a shaft is pinched between two bearings that are not in line, the drivetrain binds. Binding causes heat, noise, battery drain and a robot that pulls to one side. Check alignment before you tighten everything fully, then check again afterwards.

• All four wheels touch the floor at the same time; the robot does not rock on a flat surface.
• Left and right wheels are the same distance from the centre of the robot.
• Front and back wheels on each side sit in a straight line; check with a ruler along the side.
• Each shaft is at 90 degrees to the frame, and each motor is mounted flat, not twisted.
• No wheel, hub or screw head rubs on the frame when it spins.

Mecanum roller orientation

Mecanum wheels have small rollers set at an angle, usually 45 degrees. Each wheel pushes the robot both forward and sideways, and by spinning the four wheels in different combinations the robot can drive forward, strafe sideways and turn. Mecanum wheels come in two mirror-image types, often sold as a left and right pair. Only the correct arrangement lets the sideways forces cancel or add up as intended.