There's a need for speed,
The OG stock is lacking,
Brushes need to go
This post serves as a summary, as of the beginning of 2023, of the various drive gearmotor modifications that (to my limited knowledge) have been tried/tested for beetleweight (3lb) combat robot builds.
(I hope that this will also get me to finish my beetle, but let's not hope for miracles just yet)
Why Modify Drive Gearmotors
This can vary a great deal depending on the type of drive and wheel size you select, but a typical beetleweight build with direct-drive (wheels mounted directly to the gearmotor shaft) would utilize a 20-25mm diameter gearmotor with a standard 130 to 150 size DC motor, spec'd to run at 12-16V, and geared to a 700-1000 no-load rpm speed. There are plenty of generic DC motors (,) that fit these parameters, but those standard COTS options are (presumably) designed for elongated life and conservative operating conditions, not the aggressive bang-bang conditions of a (optimistically) 3-min fight. By mixing and matching gearheads and motors not normally paired together, we could get:
- equivalent speed/torque in a smaller size or lighter weight
- gains in max speed/torque at the same size/weight
- more robust gear transmission that's more resistant to impacts/hits
- mounting bolt pattern that's more secure
- better geometry for packaging
Selecting/swapping gearheads to better match the desired operating performance is a fairly common practice in mechatronics, but finding compatible combinations at an affordable price (for a hobby) wasn't all that easy until the past few years. While you could almost certainly get whatever gearhead you wanted via a comprehensive vendor like Maxon, it's (in my opinion) the combination of lucky finds on eBay/Aliexpress and the willingness of some builders to directly negotiate with and buy from Chinese vendors directly that has made these sort of higher-performance gearmotors all that more obtainable.
This also isn't some magical upgrade for success, especially for those of us just starting out, but there's significant room for optimization and improvement just through component selection. Learning and reading about different mods reminded me a lot of the Tamiya Mini 4WD kits I played with as a kid, where it was all about picking the right motor and gearing for the particular racetrack.
Gearmotor Modification Process Summary
The overall process is pretty straightforward and similar for most gearbox/motor combos:
- Find compatible pair of motor and gearhead/gearbox that you want to combine.
- Mechanical compatibility is largely determined by if the motor can attach to the appropriate pinion gear for the gearbox.
- The pinion gear could be sourced separately, but it typically either comes with the gearbox or can be extracted from the motor that originally came with the gearbox
- As the pinion gear is typically press-fit onto the motor shaft, it's sometimes recommended that you source a fresh pinion gear (of the same tooth count, tooth modulus, and ID) instead of pulling it off an existing motor
- Make necessary mechanical modifications (if possible) so that the two can be fastened together.
- Sometimes the motor shaft needs to be cut down so that the pinion mates properly with the gearset
- There often is an interface/adapter plate (usually as part of the gearbox) that has a set of threaded holes for the rest of the gearbox and then a set of through-holes allowing you to fasten the motor. This may need to be re-drilled/re-tapped or replaced entirely for some combinations. Sometimes, it's a good opportunity to integrate a more robust mounting strategy
- Many interface/adapter plates by default come with multiple sets of through-holes for compatible motors, so it's often possible to find combinations where you don't need to make mechanical modifications at all
- With brushless outrunners, sometimes the shaft needs to be replaced entirely or pressed through to work
- When possible, some builders replace gears (planetary/sun/pinion) with equivalent-sized ones of a stronger material. This is particularly prevalent w/ gearboxes that use nylon gears in the earlier stages.
- The electronics may need to be upgraded/replaced to support the new motor
- If switching from brushed to brushless, you'll need a brushless ESC modified to run on either SimonK or BLHeli firmware
- If moving to a more power hungry brushed motor, you may need to move to a brushed ESC with higher current rating or consider modifying a brushless ESC to drive a brushed motor
- Ironically, even though hobby brushless motors are newer, the proliferation of quadrotors have (seemingly) given us more brushless ESC options in more compact form factors than brushed
Here's a list of other (way better documented) guides on the process:
- The 5 minute, $20 brushless gearmotor (Russ Barrow)
- "5 Minute" Brushless Gearmotor for Beetleweight Combat Robots (StephenH102)
- How to make brushless 22mm planetary gear motors (Alex Mordue)
- Beetleweight Brushless Drive Tutorial (Robert Cowan)
For the purposes of this discussion, by alternative I mean alternative to the typical 22mm spur gearset you'd find from a vendor like Servocity, as that's the barebones, simplest option you could start with. You can see from the typical antweight (,) or beetleweight kit (,) that it's easiest to just directly mount the wheel onto the gearmotor shaft (as opposed to an indirect drive with dead shaft and belt/gear transmission) and fasten the motor to the side walls with the default mounting threads. By default, your standard 20-24mm diameter gearmotors will probably not reliably withstand the impacts or stall conditions of robot combat.
A typical spur gearhead transmits force along a single line of contact through the various gear stages. At each stage, there's generally a 1-to-1 tooth contact between independent gears (unless you're dealing w/ helical gears, but you shouldn't be). This makes construction simpler/cheaper, but reduces the load capacity. Apparently, you can get 25mm diameter gearboxes with a secondary, redundant set of gears for increased robustness. Absolute Chaos Robotics sells a variation of these with extended shafts in their beetleweight drive kits. A downside of these is that at 25mm diameter, they end up being quite heavy, and curiously enough, I haven't found such a double-gear setup in gearboxes of other sizes. The exact model number seems to be JGA25-370DG, but a search for "double-gear box" on Aliexpress should return a list of the different variations.
Flat/Box Spur Gearboxes
While a typical cylindrical spur gearbox has gears stacked in two primary columns, the gears could be spread out to minimize the axial length, like you're more likely to see in standard RC servos. For DC motors, they seem more prevalent for RC tank builds (,,), and I think they're interesting because we could leverage the larger housing as a structural member in the bot's design, and the default shafts tend to be larger and include additional threaded features.
That said, these are pretty heavy, and I haven't come across any bots that have used these COTS gearboxes directly, but there have been a few custom solutions (,) proposed/tested, built around gear train components salvaged from COTS boxes or bought directly.
If the design can package it, most drive modules will probably use a planetary gearbox nowadays. They have higher load capacity and distribute wear/tear/lubrication better than the simpler spur alternatives. You can find a multitude of options if you just search by gearbox diameter (22-24mm) on Aliexpress or look for gearmotors that come with a 370 class DC motor. Despite the increased weight, a typical early gearbox upgrade used to be switching over to the Servocity planetaries from the cheaper/simpler spur gearboxes.
Though a new builder may be better off nowadays going with one of the prebuilt/pre-tested, upgraded gearmotors listed further below, a crawl of the interwebs uncovered some pretty interesting alternative options in the past few years:
- 370 Micro Metal Planetary Gear Motor - Super lightweight (<25g) and with all steel gears (instead of brass/nylon), but the gear modulus is smaller and should be used only in indirect-drive setups, according to those I met who had used it. (Note: the link as of this post's writing seemed to work, but it also seemed to point to a listing for a gearmotor that's not quite the same as the one I remember when I first came across this motor)
- Rotalink Planetary Gearbox - The pictures show a rusty gear motor. The actual product has way more rust. I don't know what overstocked toy these came from, but they're as light as the micro metal option above (I think the ring gear housing is aluminum), but the initial stage has nylon gears, so it's common to buy a few excess so you can swap out those out for metal gears.
- PZ22GR9120R - Cheap (<$4 each) and available directly off of Amazon, but with a much higher reduction (1:84) than you'd typically want for drive (usually 1:10 to 1:40 depending on your wheel size). Also comes with a 6mm shaft (atypicaly for gearboxes of this size), though with a slot instead of a flat.
- Diameter 22mm Planetary Gearbox - Discontinued, but these look very similar to the Dartbox gearheads offered by Just 'Cuz.
- M22GXR - This looks like what's being sold as the MercuryBox by Robot Matter. The larger 6mm shaft offering makes this a lot more appealing for direct-mount drives.
In the beetleweight class, the 1806 size brushless motor seems to be the de facto standard. The 2mm shaft is compatible with most pinion gears from the various gearbox options, and the default mounting holes match up well to many of the stock adapter plates. Robert Cowan's video mentions the Aerodrive SK3-2118 as another option, but I've yet to see that motor in stock, while a quick search for "1806 brushless" on Amazon, Hobbyking, or Aliexpress brings up a multitude of options (though quality/longevity may vary). Assuming you battle-harden the motors properly, variation in performance during robot combat may be minimal. (Note: again, keep in mind that this is written by someone who hasn't finished building his beetle)
Nerf DC Motors
Brushless (sensorless) motors don't work great at stall or low rpm's, and apparently you can get DC motors that can spin at speeds as high as brushless ones. Out of Darts (and probably other vendors) offer higher quality DC motors in the 370 form factor that use stronger neodymium magnets and subsequently draw considerably more current. I honestly still cannot comprehend that there can be small hobby grade motors of this size that can draw as much as 50A or more at stall. Unfortunately (and ironically), there aren't readily available brushed ESCs that let builders leverage the full power and stall capabilities of these motors. However, maybe that's not too big a deal: in drive applications, we should lose traction way before we even get close to hitting the stall current for some of these higher-end motors.
Stock Upgraded Gearmotors
It's interesting: when I first read about gearmotor mods (probably at least three years ago at this point), you had to Frankenstein your gearmotors from sketchy Aliexpress/eBay purchases and do some custom work on both the adapter plate and motors themselves, but as of early 2023, there are several vendors/builders selling these sort of upgraded gearmotors completely assembled and ready to go:
- Rectified Robotics Brushless Planetary Gearmotor (4mm shaft, 22mm diameter, 1806 2300kV motor, $45)
- OwObotics MKIV Brushless Drive (6mm shaft, 24mm diameter, 1806 2300kV motor, 63g, $55)
- Repeat Drive Max (6mm shaft, 24mm diameter, 2004 2100kV motor, 66g, $55)
- Megaspark Brushless Drive (6mm shaft, 24mm diameter, 1806 2300kV, 71.5g, $55)
- DartBox V2 (4mm/6mm shaft, 22mm diameter, Nerf brushed motor, 57g, $35.5)
It would seem that everyone's converging towards larger diameter shafts and planetary gearboxes, with the primary differentiating factor being how/where weight is reduced. Some options have portions of the gearbox housing milled away or replaced with aluminum (in place of steel) to minimize weight. Seems to make sense: planetary gearsets transfer torque and support the output shaft better, and the larger diameter shaft allows for direct-mounting of wheels without worrying about impacts as much as you would with the (historically standard) 4mm diameter shafts. The larger gearhead sizing increases weight but (probably) allows for a more secure mounting to the side walls and frame of the bot.
Despite all these options, I'd like to note that we've so far only been mixing/matching existing gearheads and motors from disparate (affordable) catalogs. Again, there's nothing really out of the ordinary in what builders are trying in terms of speed reducers, and the means of locomotion remain wheel-based or wheel-ish. There's been some attempts at friction/tangential drive, single-stage reduction via printed or metal gears independent of a standalone gearbox, direct brushless drive (pretty much only in meltybrains), and even "drive-less" setups (,,). We may be now entering the stage where improving drive means improving the locomotion strategy rather than just the quality of components.