Hardening Motors

The shocks and forces that combat robots endure during matches are far higher than in almost any other application. Just as you can pad the internals of your robot with foam, or have well-planned strain relief in your wiring, you can harden motors and make them more robust.

Most hobby motors are designed for drones, planes and RC cars, where they never get hit. In a robot, the danger isn't usually a direct hit on the motor – your frame and armor deal with that. It's the huge acceleration when the robot is slammed by a spinner, or when a weapon spinning at thousands of RPM stops dead on impact. Those forces can reach hundreds of g, and they're what hardening protects against.[1] This page covers the common failures and the usual fixes. For choosing motors, see Motors and Insect Class Motors.

How motors fail in combat

• Magnets come loose or crack. Outrunner magnets are only glued to the inside of the can (the "bell"). A hard stop can shear the glue or crack a magnet, and a loose magnet jams the motor.[2]
• The can or bell comes off the shaft. Many hobby outrunners hold the shaft with a small C-clip or set screw, which can break and let the can fly off.[3]
• Bent or broken shafts. Side loads from a wheel, pulley or weapon put a bending load on a thin shaft that it wasn't designed for.[4]
• Stripped gearbox teeth. The fine gear teeth in small gearmotors can strip under shock or stall loads. Coarser (larger-pitch) gears are less likely to strip.[5]
• Wires and tabs torn off. A brushed motor's solder tabs, or the thin enamelled wires coming out of a brushless stator, break when the leads get pulled or flexed.[6] Some combat motors now come with silicone-jacketed leads instead of bare enamelled stator wires for this reason.[7]
• Worn or broken bearings. The small bearings inside an outrunner are meant for a light propeller, not a weapon hitting things.[8]
• Overheating. Stalling or overloading a motor cooks the windings and can weaken magnets.[9]

Brushed gearmotors

These tips apply to N10 and N20 micro gearmotors in 150g robots, 16mm motors in antweights and 22mm planetary motors in beetleweights.

• Support the output shaft. Don't let the wheel hang off the end of the shaft. Add a bearing on the far side of the wheel. For example, FingerTech's Silver Spark has an extra-long shaft so you can fit a bearing block on the outside of the wheel.[10][11] See Drive Systems.
• Clamp the gearbox, not the end cap. The plastic end cap holds the brushes, so don't let it carry any load. N20 motors have an open gearbox and need more support than a face-mount plate gives, so use a clamp-style bracket.[12][13] Mount motors with proper clamps or screws, never just glue, tape or zip ties.[14]
• Threadlock the gearbox. Screws and joints shaken loose by hits are a common failure. Rectified Robotics ships its 22mm and 24mm drives with threadlocker already on the gearbox screws and the motor joint. They advise letting it cure overnight before an event, not applying it in the pits.[15]
• Limit the current. Stalling a small brushed gearmotor with no current limit can destroy it. Rectified Robotics recommends ESCs with current limiting for its brushed drive.[16] See ESCs.
• Protect the brush end and tabs. Solder carefully, and anchor the wires to the motor or frame so a tug can't reach the tabs. Some builders add a dab of hot glue or epoxy over the tabs. Keep glue away from the brushes and vents.

Brushless motors

Brushless outrunners are used for weapons and, more and more, for drive (in gearmotors or as friction/tangent drives). Weapon motors take the worst of it.

Reinforcing motor magnets

With brushless motors, a common failure is the cracking of the motor's magnets on impact. Robert Cowan has a great video on how to use glass microspheres and epoxy to strengthen the magnets.

Links are provided by Robert.

• Glass Microspheres
  
• Loctite Heavy Duty Epoxy

Other approaches:

• Use slow-cure epoxy, not 5-minute epoxy. To re-glue loose magnets, Ask Aaron suggests cleaning off the old adhesive, putting each magnet back exactly where it was, and coating its whole back face with slow-set epoxy before clamping it gently until it cures. Magnets are brittle, so don't clamp hard.[17]
• More epoxy isn't better. Ask Aaron disagrees with building a thick epoxy-and-microsphere shell, because it holds heat in. Instead, they warm thin, slow-cure epoxy so it soaks into the gaps. Soaked-in epoxy bonds the parts and actually helps carry heat away. The same thin-wicking approach works for rotor magnets.[18]
• Thin CA (superglue) wicked into the gaps between magnets is sometimes used as a quick fix in the pits. It's quicker but more brittle than slow-cure epoxy, so treat it as temporary.

Stator windings

Hits can shift the windings on the stator and rub through the enamel, causing a short. Coating the windings in epoxy stops them shifting and can improve cooling.[19] As with magnets, a thin coat that soaks into the windings is better than a thick shell.

Replacing motor shafts

It's not uncommon to reinforce shafts with an additional bearing or pillow block to resist side impacts on the shaft (radial impact). However, many builders have found that the design of the motor itself is a weakness to be overcome. Here's an example:

• Replacing a stock shaft with a longer one is fairly easy if you have the tools. You need a shaft of the right length with the correct clip groove and flat. A larger shaft is a much bigger job: new bearings, machined bearing seats and precise boring.[20]
• Don't trust the C-clip. Combat-specific motors retain the shaft with a screw or a flanged shaft instead of a C-clip, and key the shaft into the can so it can't spin.[21][22]
• File a flat for set screws. If a pulley is held by a set screw, file a flat on the shaft for it to bite on and threadlock the screw.[23]

Supporting the motor

• Support both ends of the shaft where possible. If you can't, mount the pulley as close to the motor base as you can, so the motor's internal bearing tube takes less leverage.[24]
• Pulleys on the can make a compact package and are common in beetleweight horizontal spinners. However, they put more load on the motor's internal bearing support, and a taller motor makes it worse.[25]
• Don't make the motor the weapon bearing. Direct-driving a weapon from an unmodified outrunner is hard on the motor, because it isn't built for the side loads of weapon hits. Pack spares if you do it.[26] A belt drive with a little slip protects the motor and gives better bite.[27] The stronger option is a dead shaft: the weapon spins on its own bearings on a fixed shaft or shoulder bolt held by the frame, and the motor only drives it. See Weapon Shafts, Bearings and Mounting and Spinner Design.
• Hubmotors (the weapon bolts straight onto a combat-built motor) work because they're designed for it, with large bearings and a through shoulder bolt. Mount both sides of the motor to the frame if the seller says to.[28][29]
• Friction (tangent) drives press the motor shaft into the tire, which side-loads the shaft. Too much pressure overloads the motor.[30][31]
• Use short enough screws. Mounting screws that go in too deep can hit the windings and destroy the motor. Check the seller's maximum screw depth.[32][33]

Shock mounting

• Mounting a motor, or the module it sits in, on rubber or TPU spreads out the peak force of a hit. Properly sized rubber mounts are quite firm and barely flex when you push on them by hand.[34] See Armor and Materials and 3D Printing.
• A printed TPU motor clamp or wheel hub gives a little, which can save a gearbox from a sharp shock. Don't make it so soft that the motor moves enough to misalign belts or gears.
• Keep a weapon motor and its weapon on the same rigid structure, so the belt or gears stay aligned when the frame flexes.

Wires

• Use flexible silicone-insulated wire, and secure it close to where it leaves the motor so the motor's own leads or tabs never take the strain.[35] See Wiring and Soldering.
• Leave a little slack for the motor to move on its mounts, but not enough for a weapon or wheel to catch.
• Cover solder joints with heatshrink and keep them away from sharp frame edges.

Heat

Hardening shouldn't stop the motor cooling. Stalled drive motors and overworked weapon motors fail from heat as often as from impacts.[36]

• Don't bury a motor in glue or foam. Leave airflow to the can and keep the vents clear.
• A combat hubmotor's outside can get very hot, so plan its surroundings for that.[37]
• Gear for the arena. Correct gearing keeps drive motors from overheating. See Drive Systems and Speed Reduction.

Buying pre-hardened motors

Combat robot shops now sell motors that come hardened from the factory. These often cost little more than hardening a hobby motor yourself (prices as of October 2026):

• Repeat Robotics – weapon outrunners with screw-retained, keyed shafts and double-supported magnets (2822 Mk3, about $20), and "battle ready" hubmotors with hardened magnet rings.[38][39] Their gearmotors have hardened steel shafts on two bearings.[40]
• Just 'Cuz Robotics – hubmotors with epoxied magnets and windings, no C-clips or set screws (RDY-5022, about $62), and the JCR-4935, which ships with the bigger bearings, stronger shaft and epoxied magnets that builders used to add themselves (about $85).[41][42]
• Rectified Robotics – 22mm and 24mm drive gearmotors shipped with threadlocker applied.[43]
• FingerTech – Silver Spark gearmotors with coarser gears than its Gold Sparks, plus matching bearing blocks.[44]

See Insect Class Motors and Suppliers for more.

Between matches

Check motors after every fight: spin each by hand for grinding or rubbing, look for bent shafts and loose magnets, and re-tighten set screws. Carry spare motors, as they're one of the most commonly replaced parts. See Repairs Between Matches and Pit Kit and Event Checklist.

See also

• Motors
• Insect Class Motors
• Drive Systems
• Spinner Design
• Weapon Shafts, Bearings and Mounting
• Repairs Between Matches
• Wiring and Soldering
• Armor and Materials