"5 Minute" Brushless Gearmotor for Beetleweight Combat Robots

The idea of the “5 minute brushless gearmotor” seems to have been floating around the online forums/Facebook groups for a while as a drive option in beetleweight bots. As brushless motors pack a lot of power for their size/weight, this is an attractive option for the weight conscious builder. The two main issues with using brushless motors for drive has been gearing them down (as they don't come with gearboxes attached, unlike a lot of brushed motor drive options in the insect classes), and ESCs that allow forwards/reverse – which is a solvable problem if you use an ESC which can be flashed/reflashed with SimonK (or BLHeli) firmware.

The concept of the “5 minute brushless gearmotor” is that you can take an 18mm brushless outrunner and a 25mm brushed gearmotor (a common drive solution in itself) and swap the gearbox onto the outrunner. But is it that simple? Being lazy I looked for a step-by-step guide to the swap but failed to find one. Undeterred, I vowed to not only recreate this superior drive system (as I have beetleweights that a smaller/lighter drive system would be helpful for) but to also document the process for future lazy bot builders following in my footsteps.

Components:

18mm brushless outrunner with a 2mm shaft – I used the DYS BE1806 2300KV motor, sourced from Banggood

25mm / 25GA gearmotor – available all over the place, I initially used a 1000rpm version from Banggood as they are my go-to brushed drive motor for beetleweights. However subsequent builds have used a lower RPM / higher ratio version (210rpm or 35:1) which worked better.

SimonK or Blheli_32 programmable brushless ESC – I used the 12A Afro ESC as it already had the SimonK bootloader installed, making it easier to reflash for forwards/reverse. Unfortunately these (or other SimonK ESCs) are no longer readily available and in future I will need to look at alternatives running Blheli_32.

Optional: spare pinion gears - these gears are small, easy to lose in a cluttered work environment (is there any other kind?), and are sometimes difficult to remove in one piece from an unwilling donor brushed motor. However, all the flavours of 25mm gearmotor I have opened up so far have used 0.4 module 12 tooth gears, which I have found on AliExpress

Tools:

Calipers (or a small ruler, if you want to measure things like some sort of peasant)

Smallish philips head screwdriver

Pliers / circlip removal tool

Soldering iron

Hammer (the small kind, for precision adjustments)

Threadlocker (the good stuff, "super stud lock" or similar)

2-3 screwdrivers with progressively larger shafts (or a pinion removal tool, if you're feeling fancy)

A nail 2mm in diameter with the sharp end cut off

A vice

First, remove the two screws holding the gearbox onto the motor block, and put these and the gearbox to one side. Then remove the two screws holding the motor block to the motor – feel free to immediately lose these, as the brushless motor uses different size screws.

Next, measure the distance from the front plate of the motor to the end of the pinion, and take note of it. This distance will need to be replicated on the brushless motor, and have ranged from 4mm to 7mm for the ones I have so far converted. For the sake of example, the 1000rpm gearmotors featured in this Instructable were 7mm.

First, use your soldering iron to heat up the pinion gear – this will degrade the loctite or similar holding it to the shaft. Once it has cooled remove the pinion gear – I have heard tell of mythical devices designed purely for this purpose, but I just prized it off with a couple of progressively larger screwdrivers (try not to let this ping off into the depths of your shed/workspace, never to be found).

Remember that 7mm (or whatever you got) measurement? That's how far the shaft will need to stick out of the brushless motor (you may actually get away with 1-2mm less, as some of the donor motors I used didn't have the pinion gears quite on the shafts). Rest the brushless motor on the vice (with the jaws just wider than the diameter of the circlip on the base of the shaft), and very gently whack the top on the shaft with your hammer to drive it through the motor until it's flush with the top of the motor can. You can then remove the circlip – I just used pliers to lever it off.

At this point the shaft won't be sticking out anywhere near 7mm. For this sort of length it will need to be pushed even further through the motor can, but first I recommend attaching the pinion gear.

Put some threadlocker on the shaft, then press the pinion gear on. It will probably need persuading a few gentle taps with the hammer to go on completely – just make sure the top on the motor can is resting on a hard surface so the shaft doesn't get pushed back through.

Now we need to push the shaft through until the pinion gear sits out the magic 7mm. I found a small nail worked well, with the pointy end cut off and filed flat. Put the motor back on the vice but now open just beyond the diameter of the gear, put the nail over the shaft and give it gentle taps with the hammer to drive it further through, until the requisite length is achieved.

Now you can attach the motor block to the brushless motor. I've found that the ones that come with all the cheap gearmotors I've tried actually had a few different holes that could be used, so find the ones that line up with the mount holes on the brushless motor, and use two of the 2mm screws that came with the motor. You can then attach the gearbox back to the motor block, and then you're done!

The newly brushlessified gearmotors can generally be attached to your bot the same way as their humdrum brushed counterparts. The first bot I tried them in (pictured above) used clamping mounts, but I've also face mounted them using the M3 holes in the gearbox. The main difference is making sure nothing can contact the spinning motor cans on the outrunners, so think about wire routing and suchlike (rather than the usual method of shoving it all in however it vaguely fits) - this bot ended up getting a 3D printed cover over them.

For all my brushless-drive bots so far I've used Afro ESCs as they have the SimonK bootloader already installed and can be flashed to be reversible through the signal wire, a boon for lazy bot builders like myself. They are no longer being made (and I believe are unavailable now outside of Australia), and in general SimonK has fallen out of favor in the quadcopter world for Blheli. Fortunately Blheli_32 ESCs can now more easily be used for drive - going through part selection and flashing of these is beyond the scope of this Instructable (at least in part as it's something I haven't yet had the time or need to try), but this video covers it well.

The first bot I used for brushless drive was my beetleweight full body spinner (or shell spinner, if you're a pedant) Zim. I used 1000rpm gearmotors with (I think) 5:1 gearboxes in this and the resultant drive was ridiculously too fast - I had to drop the rate down to 30% to make it drivable. For my more recent 4WD beetleweight Bunyip I used 210rpm gearmotors with 35:1 gearboxes and while fast it was a lot more controllable (pity the wheels keep falling off in combat).