Mounting Variations for Small DC Motors

To have the most flexibility in robot designs, I’m constantly experimenting with various methods of attaching motors and wheels to robot platforms. This article shows several different motor mounts and wheel couplers built over the years. I don’t expect that you'll want to recreate these exact arrangements. However, I hope these setups will spark some creative solutions for unusual problems that you may encounter.

LEGO Brick Motor Mount

Most robot builders are fans of LEGO parts for prototyping or creating entire robots. However, LEGO has a limited selection of electric motors. Since the torque (strength), speed, and size of a motor determines the boundaries of a robots capabilities, it is often desirable to have access to the wide selection of non-LEGO motors. Thus, some parts need to be created to adapt a non-LEGO motor to a LEGO base and wheel.

For example, precision Maxon motors are often available online at eBay and surplus stores. I bought a bunch of 13 mm diameter Maxon motors that I wanted to use on a LEGO body for a robot.

LEGO bricks glued together and drilled to accept a Maxon motor. Drill high in the pair to avoid weakening the tubes.

LEGO bricks glued together and drilled to accept a Maxon motor. Drill high in the pair to avoid weakening the tubes.

A straightforward approach is to glue two 2x4 LEGO bricks together with super glue and drill a 13 mm hole in the center for the motor. But be careful, drilling through the vertical center of the pair leaves too little bracing material to maintain the integrity of the bottom tubes. Instead, the LEGO bricks should be drilled such that the top of the hole is just beneath the roof of the top brick.

To hold the motor within the LEGO bricks, initially I thought I could drill a hole through a wall of the LEGO bricks and tap the hole for a set screw. But, the plastic seems too thin and fragile to securely hold a screw -- particularly with a motor pushing and twisting against it. Too strengthen the brick, a portion could be filled in with epoxy, but epoxy is too malleable to hold threads.

A square of aluminum with a motor hole drilled in the middle and a setscrew in the corner.

A square of aluminum with a motor hole drilled in the middle and a setscrew in the corner.

Aluminum is an inexpensive, lightweight, comparatively-strong material. A piece of aluminum is machined square with a 13 mm hole slightly offset from the center. A smaller hole is drilled in the thickest corner. The hole is tapped for a setscrew to press against the motor and hold it in place.

Maxon motor between two LEGO bricks with aluminum bracket to attach to motor to the LEGO bricks.

Maxon motor between two LEGO bricks with aluminum bracket to attach to motor to the LEGO bricks.

With the aluminum square glued to the LEGO bricks, the Maxon motor can be inserted and the setscrew tightened. Now the Maxon motor and LEGO bracket can be snapped above or below a LEGO base.

Since this motor is particularly long, additional motor holders can be added if desired. Or another pair of bricks can be glued to the other side of the aluminum square. If you’re going to do that, make sure the width of the aluminum is a multiple of 8 mm so that all of the LEGO studs will continue to line up properly.

Don’t complain about the difficulty of machining the aluminum square motor holder until you see how the LEGO wheel is attached (pictured below). Then you'll have something to complain about.

Single piece Maxon-motor-to-LEGO-wheel coupler made of solid aluminum on a lathe and milling machine.

Single piece Maxon-motor-to-LEGO-wheel coupler made of solid aluminum on a lathe and milling machine.

My previous attempts at LEGO wheel couplers were made with multiple pieces that fit together. But, since the acquisition of a lathe, I decided to try my hand at this slightly complex single-piece aluminum coupler.

Left: Original center of LEGO wheel hub. Middle: Drilled and tap two screw holes with another hole vertically. Right: Aluminum coupler screwed into the center.

Left: Original center of LEGO wheel hub. Middle: Drilled and tap two screw holes with another hole vertically. Right: Aluminum coupler screwed into the center.

The dimensions of the single-piece coupler match the inside of a large LEGO hub. Two holes are drilled into the thick band in the LEGO wheel hub. The aluminum coupler press fits into the LEGO hub and is then fastened in place with a pair of screws.

Left: Hex key tightens setscrew against the motor shaft through a vertical hole in the hub. Right: Completed assembly.

Left: Hex key tightens a setscrew against the motor shaft through a vertical hole in the hub. Right: Completed assembly.

The motor shaft is inserted into the aluminum coupler in the center of the LEGO hub before the rubber tire is squeezed over the hub. This permits a set screw to be inserted through the vertical hole in the hub and into the aluminum coupler. A hex key tightens the screw against the Maxon motor shaft.

With the tire in place, the completed motor and wheel assembly is ready to attach to a robot.

Results

Although the aluminum coupler inside the LEGO wheel hub is attractive and functional, it is difficult to make. And, since the LEGO bricks need a chunk of aluminum to secure them against the motor, it would be more practical to simply mill a block of aluminum and glue (or screw) LEGO plates above and below it.

Actually, the torque of the long motor and large-diameter wheel prove too much for the minor friction that attaches LEGO bricks together. This fatal flaw prevents this setup from making it into a finished robot.

The next design is a bit more practical...