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Many LEGO wheels include a cross-axle center hole to attach to a LEGO motor or axle. A third-party motor can be connected to the cross axle hole by using a special coupler.
Sadly, the plastic LEGO cross-axle is too thin and slippery to hold onto a large LEGO wheel when actually put into service. The wheel either slides off or the cross-axle becomes bent under the weight.
Fortunately, the LEGO engineers include peg holes around the center of some hubs to permit multi-point attachment of wheels.
Left: A 1.25-inch UHMW rod with holes for a motor shaft and screws. Right: Rod being inserted onto a motor shaft with setscrew holes visible.
A 1 1/4-inch diameter rod of UHMW (ultra-high molecular weight) polyethylene is cut to a length of 1 3/4-inches. The diameter and length are not critical, so long as it fits inside the LEGO wheel.
Using a lathe, an 8 mm center hole is drilled for the Faulhaber motor shaft. A pair of setscrews will press against the flat portion of the motor shaft after the coupler rod has been slid into place. Interestingly, plastic tends to compact during drilling and expand afterwards, leaving a slightly smaller hole than the drill size. Thus, the plastic rod grips the motor shaft fairly tightly even without the setscrews.
Screw holes in the front of the rod correspond to existing holes in the LEGO hub.
Three screw holes are drilled in the front of the coupler rod to match the hole pattern on the LEGO hub. For accuracy, the holes should be measured and laid out on a template using a computer drawing program. Or, since this LEGO Technic hub features a 7.5 mm tube in the center which almost matches the 8 mm motor shaft hole, the hub can be pressed against the coupler rod and the screw holes can be marked by hand.
After the holes are drilled and tapped, the wheel is screwed onto the coupler. I am amazed at how solid the connection is. In fact, when stress tested, the rubber wheel tends to flatten and slide off the hub before the setscrews loosen and slide off the motor shaft. Normally, the setscrews are the weakest link.
For the remainder of the page, we'll be taking a slight detour from the main topic of mounting motors. The particular model of Faulhaber motor that is pictured above is double shafted. This means that the motor shaft extends out of the front of the motor to connect with a gearhead, like usual. And, the motor shaft extends out of the rear of the motor to connect with an encoder.
A plastic disc with a short length of aluminum rod is painted half black and half white to detect motor rotation.
In my book, Intermediate Robot Building, I show how to make aluminum or colorful acrylic disc wheels for a robot using a rotary table on a milling machine. That same approach can be used to make plastic discs for encoders.
In this case, a white ABS plastic sheet is rounded into a disc. A short length of aluminum rod with a center hole is glued to the disc. The aluminum rod includes a set screw to attach the disc to the rear motor shaft.
A motor with a rear shaft to attach an encoder. The encoder disc can be flipped to make it nearer or farther from the sensors as needed.
The disc is painted half white and half black to vary the reflectivity of the surface as it rotates. A reflective pair of light sensors detects this change in light as the motor rotates the disc. An attached microcontroller reads the sensors and can determine the direction and speed of the motor.
Since the encoder disc is attached to the motor shaft, the resolution of the encoder will be significantly better than it would be if the encoder disc were attached to the wheel. Not only is the rear of the motor exposed to less ambient light than the wheel, but the wheel rotates more slowly than the actual motor speed due to gear reduction. For example, with a 66:1 ratio gearhead, an encoder disc mounted on the motor shaft gets 66 counts for every 1 rotation of the wheel.
To learn more, you can see the final motor setup and sensor circuitry on the optical encoder page.
A motor mounting bracket is made from 90 degree L-shaped aluminum angle stock.
The large (38 mm diameter) Faulhaber motor is mounted using the standard mounting holes at the front of the gearhead. A custom bracket is made from L-shaped aluminum angle stock. Let’s see how that is made...