Compact Wheel Coupler

On the prior page, we looked at some unusual motors and the brackets to mount them. The motor power is transmitted to the wheel to move the robot. A piece, called a coupler, is needed to connect them. I selected commercial miniature motors but LEGO wheels, rather than motors and wheels manufactured to connect together. Therefore, I had to make my own couplers.

I generally followed my usual process for making motor couplers on a lathe. However, for this robot, I wanted to make the coupler much more compact than usual.

Compact wheel coupler

Compact wheel coupler

The major change is to press fit the Lego cross axle all the way into the coupler, and to do this before drilling the hole for the motor shaft.

Drilling motor shaft hole with cross axle in place

Drilling motor shaft hole with cross axle in place

Then, drill the set screw hole, again with the cross axle in place.

Drilling set screw hole with cross axle in place

Drilling set screw hole with cross axle already inserted

I wasn’t thinking ahead during that last step. In hindsight, I should have been more careful to align the thickest point of the cross axle with the setscrew hole. That way, when the motor shaft is inserted and the set screw tightened, there will be material on the opposite end of the shaft to hold it centered.

In the photograph below, the left cross axle is almost aligned with the setscrew. Preferably, the setscrew would be nearer to the top. On the right side, the cross axle is in the worst possible position, as it provides no support when the setscrew pushes down. Worst case, the cross axle may deform and tear at that weak point; average case, the coupler will be offset or slightly angled, causing wheel tilt.

Motor shaft view of motor couplers with setscrew in cross axle

Motor shaft view of motor couplers with setscrew in cross axle

Above, notice the drill holes in the center of the cross axle. That’s where the motor axle fits. You’re looking at the back of the coupler.

Compact coupler in Lego wheel

Compact coupler in Lego wheel

Despite the errors in the positioning of the cross axle, the couplers turned out really well. They are compact yet strong. Aluminum was a good choice over brass for this robot -- for color consistency and reduced weight.

We’ve got motors and wheels attached, but how does the battery power reach the motors?