Weapon Shafts, Bearings and Mounting

A spinning weapon is only as good as the shaft, bearings and mounts that hold it. Every hit sends the full impact back through the weapon hub into the shaft, the bearings and the frame, and a weak link there can end your match even when the blade itself survives. This page covers how insect-class builders support, retain and drive spinning weapons. For the physics of spinners see Spinner Design; for each spinner type see Vertical Spinners, Horizontal Spinners and Drum Spinners; for screws, threadlocker and general hardware see Fasteners and Hardware.

Safety: only spin a weapon up in an arena or a test box, and always fit a weapon lock outside the arena. A shaft or bearing that fails at speed can throw parts. See Safety and Failsafes.

Live vs. dead shafts

A live shaft spins with the weapon. The bearings sit in the frame, and the weapon and pulley must be locked to the shaft so they can't turn on it.
A dead shaft is fixed to the frame. The bearings sit in the weapon (and pulley), and the weapon spins around the shaft.

Team Run Amok's view is that there's essentially no difference between the two in bearing protection, speed or stored energy. The main advantage of a dead shaft is that it can be clamped tight and become a structural part of the chassis, adding strength and stiffness.[1]

Dead shafts are the norm in the insect classes. A live shaft needs a reliable way to lock the weapon to it, and keyways aren't practical on small shafts: keys are rarely used below about 1/2 in diameter because a keyway weakens a small shaft too much. That lack of small keyed shafts is why Ask Aaron recommends dead shafts for insect robots.[2] Kit weapons such as the Repeat Robotics Peter Bar beetleweight kit, the Shock! antweight undercutter and many hub-motor weapons all run on fixed shafts or bolts.[3][4]

Shaft materials and sizing

Materials

Hardened alloy steel is the usual choice. Ask Aaron recommends a solid rod of tough steel made for a similar job, such as carbon steel drive shaft, and warns that standoffs are only spacers and aren't strong enough to be shafts.[5]
Shoulder bolts (shoulder screws) have a precision ground, smooth shoulder that bearings can ride on, and a short thread at the end. Good ones are hardened alloy steel and much stronger than an ordinary hardware-store bolt.[6] For example, Just 'Cuz Robotics sells 4 mm × 35 mm class 12.9 shoulder bolts with a ground shoulder hardened to 39–44 HRC, made for 4 mm bearings and bushings.[7]
Titanium (Grade 5) saves weight. Repeat Robotics sells 5/16 in Grade 5 titanium weapon shafts, tapped at the ends so screws through the frame clamp them in place.[8][9] Titanium is less stiff than steel, so a titanium shaft bends more under the same load.
Hardened dowel pins are precision ground and very hard, and some builders use short ones as dead shafts. Because they're hard, they're less forgiving of shock than a tougher shaft, and you can't easily drill or tap them, so plan how they'll be held in place.

Bolts as shafts, and why that's a compromise

A bolt is convenient: the head and a nut clamp it through the frame. But:

The threads are the weak point. The core of a thread is much smaller than the bolt's nominal size. Just 'Cuz notes that an M5 thread has a core diameter of about 4.1 mm, so a 6 mm shoulder has about 2.1 times the cross-section.[10] Threads also concentrate stress, so cracks start there.
Bearings shouldn't sit on threads. The threads chew up the bearing bore and give a sloppy fit. Use a bolt whose smooth (unthreaded) part runs through the bearings, or a shoulder bolt.
Cheap bolts are weak. Use high-grade fasteners (Grade 8 or class 12.9). The Repeat Peter Bar kit, for example, uses a 5/16-18 × 5 in Grade 8 bolt, well supported on both ends.[11]

A simple sanity check

Nobody can tell you the exact force your shaft will see: combat hits come from any direction with unknown force, so Ask Aaron's advice is to see what successful robots with similar weapons use, and "if it breaks, make it stronger."[12][13] Two simple beam rules still help you compare designs:

Bending moment. For a load midway between two supports a distance L apart, the maximum bending moment is M = F × L ÷ 4.[14] For a weapon hung off one support (cantilevered) a distance L away, it is F × L, four times worse for the same distance. Support both sides and keep supports close to the weapon.
Diameter. Bending stress is σ = y × M ÷ I;[15] for a solid round shaft this works out as σ = 32 M ÷ (π d³). Strength grows with the cube of diameter: going from 6 mm to 8 mm makes a shaft about 2.4 times stronger in bending, and going from 1/4 in to 5/16 in about 2 times.

Use these to compare options, not to predict a safe load. If you want a full calculation, the Engineering ToolBox beam stress page has the formulas, and many CAD packages can run a simple stress study.

Bearing types

Type	Good for	Watch out for
Deep-groove ball bearings (radial)	The default for weapons. Cheap, low friction. Ask Aaron says ordinary ABEC 3 steel bearings are fine, since precision isn't critical here.[16]	Small ones can crack or brinell (dent) under big hits. Choose shielded or sealed ones (suffix ZZ or 2RS) to keep grit out.[17]
Flanged ball bearings	The flange locates the bearing in the weapon or pulley, and the frame holding the dead shaft keeps it in. Washers or spacer tubes take up any extra side clearance.[18]	The flange takes side loads, so make sure it bears on something solid.
Needle roller bearings (drawn cup)	High load capacity in a thin section; popular in beetle weapons. For example, Repeat's Peter Bar spare bearing is an SCE59 (5/16 in bore, 1/2 in OD).[19]	Usually a press fit, which needs a precisely sized hole. They only take radial load, so pair them with thrust bearings and spacers.[20]
Bronze (oilite) bushings	Cheap, light, and survive enormous shock loads; flanged ones stay in place.[21][22]	A bit more friction (negligible when well oiled). You can't clamp a dead shaft tight through them the way you can through ball bearings' inner races.[23]
Thrust bearings (needle thrust with washers)	Take the side (axial) load of a weapon pushed sideways in a hit, for example between a needle-bearing weapon and the frame. Repeat sells a 5/16 in TC512 set.[24]	Adds width and weight.

At heavier weights, Ask Aaron suggests a pair of tapered roller bearings as a stronger and cheaper alternative to angular contact bearings.[25]

Common sizes in insect weapons

As of October 2026. Shaft sizes in kits and vendor parts give a feel for what works:

Antweights (1 lb): about 4–6 mm or 1/4 in. The Shock! kit used a 1/4 in shoulder bolt;[26] small flanged ball bearings from 3 mm up to 3/8 in bore are sold by FingerTech (for example MF63ZZ, FR168ZZ and FR188ZZ sizes).[27]
Beetleweights (3 lb): commonly 5/16 in (8 mm in metric designs) for the weapon shaft, with needle or ball bearings to match, such as the Repeat Peter Bar's 5/16 in Grade 8 bolt and SCE59 needle bearings.[28][29]
Fairyweights (150 g): most weapons are direct drive on the motor itself, so the motor's own shaft and bearings carry the load.[30]

Prices are low: as of October 2026 a pair of SCE59 needle bearings is US$6 at Repeat,[31] FingerTech flanged ball bearings are about US$3.34 each,[32] and a 5/16 in titanium shaft is US$10–15.[33] McMaster-Carr stocks bearings, bushings, shoulder screws, dowel pins and belts in every size. Carry spares: bearings are a common between-match replacement.

Installing bearings

Press fits vs. retained bearings

Press fit (interference fit): the hole is slightly smaller than the bearing, so it's pressed in and stays put. It needs a precisely drilled or reamed hole – much tighter tolerance than a typical waterjet-cut hole. With the right hole, a needle bearing can be tapped or pressed in with a vise.[34]
Retaining compound: some builders fix bearings in slip-fit holes with a retaining compound such as Loctite 640 (not the same as blue threadlocker). Don't get it inside the bearing.[35]
Retained by the structure: flanged bearings, or bearings trapped between the frame plates and the weapon, need no press fit and are easy to swap at events.[36]
Avoid epoxy as the only thing holding a bearing in. If an adhesive failure would cause a weapon failure, Ask Aaron suggests changing the design.[37]
Never use a set screw on a bearing's outer race – the point load damages the bearing.[38]

Support both sides

A weapon shaft should be supported on both sides of the blade. Hanging a weapon off the end of a motor or gearbox shaft destroys that shaft's bearing within a few good hits.[39] Place the weapon hub as close to a bearing as possible; on a live shaft, putting the pulley between the bearings helps.[40] Wider-spaced chassis plates also make a stronger base for an undercutter.[41]

Clamping and spacers

On a dead shaft with ball bearings, put a spacer tube between the inner races so you can tighten the shaft down and make it part of the frame.[42] Then:

Add a strong spacer block between the two chassis plates around the shaft to take the clamping force.[43]
Use metal washers between plastic frame parts and the bolt head or bearing races so the plastic isn't crushed.[44]
Don't put both bearings in one 3D printed hub. In the Silent Spring and Shock! undercutters, one bearing sits in the pulley and the other in the weapon disk.[45]

Keeping bearings clean

Use shielded or sealed bearings (ZZ/2RS).[46]
Arena dust, metal chips and plastic shavings work their way in. Between matches, spin the weapon by hand: grinding, roughness or new play means a bearing is on its way out.
Keep oilite bushings oiled.[47]
Keep retaining compound and threadlocker out of bearings.

Weapon retention

The weapon must not walk along or off the shaft:

Shoulder bolt and nylock nut – the Silent Spring undercutter's approach.[48] Tapped shaft ends with screws through the frame do the same job.[49]
Shaft collars – clamping collars hold far better than set-screw ones.
Retaining (snap) rings – light and don't come loose; FingerTech's Snap Hubs use them for those reasons.[50]
Threadlocker on metal-to-metal threads, but not on plastics (see Fasteners and Hardware).
Set screws on small motor shafts are often unavoidable. Grind a flat for the screw, use threadlocker, and check them before every match.[51]

Driving the weapon

Direct vs. belt drive

Most outrunner motors are built for model aircraft and aren't strong enough to take spinner impacts directly. Ask Aaron reports that direct-drive weapons with unmodified motors can need several motor replacements in a single tournament, while a well-designed belt drive isolates the motor and rarely throws its belt.[52] Direct drive is still normal in 150 g robots and common in hub-motor designs, where the weapon spins on a strong dead shaft through the motor, as in Repeat's battle-ready hub motors with replaceable titanium shafts.[53][54] See Spinner Design and Hardening Motors.

Belt types

Belt	Notes
Timing belts (S3M/HTD 3 mm, GT2, XL, MXL)	Toothed, so they don't slip under load. Even small MXL belts are rated to about 20,000 RPM and 400 W.[55] FingerTech sells S3M pulleys and belts,[56] and Repeat's HTD3M belts are compatible with S3M pulleys.[57] Don't mix profiles you haven't checked: GT2, HTD and S3M teeth differ.
Round urethane belts	"Big rubber bands": stretchy, cut short and welded into a loop. The stretch and slip protect the weapon motor and smooth out spin-up current, but they lose elasticity over time, so check the tension. Pulleys are usually home-made.[58]
O-rings	Similar to round belts. Choose EPDM rather than nitrile (Buna-N), which degrades in air.[59]
V-belts	Grip well with some slip. Repeat sells 6 mm cogged V-belts and pulleys for beater bars.[60]

For a little slip with a timing belt, some builders run the belt over a smooth or shallow-toothed pulley, but a smooth pulley makes the drive more sensitive to tension.[61]

Pulleys and belt tension

Size the belt with a calculator. The FingerTech Pulley & Belt Calculator gives the center distance for a belt and pulley pair, or the belt for a given distance.[62]
Timing belts need only enough tension to keep the teeth from jumping. They tolerate a fair range, so most insect robots with two toothed pulleys need no tensioner.[63] FingerTech recommends keeping slack on their S3M belts under 1 mm.[64]
Smooth or stretchy belts need adjustment: idlers or an adjustable motor mount.[65]
Skip spring-loaded tensioners; they don't survive combat shock. Ask Aaron would rather spend the weight on a stiffer chassis, and carry a belt one size smaller as a spare.[66]
Pulleys on outrunner cans are often a press fit, a few thousandths of an inch smaller than the can.[67] Ask Aaron prefers a good press fit to a set-screw pulley, but carry complete spare motor and pulley assemblies, since a press-fit pulley is hard to replace in the pits.[68]

Protecting the belt

Keep the belt inside the frame or behind a guard. On undercutters, a blade that bends upward in a hit can cut its own belt.[69]
A stiff frame keeps the center distance fixed, so the belt doesn't go slack when the robot is hit.[70]
Carry spare belts; they are cheap (as of October 2026, about US$3–8 for one or two).[71][72]

Mounting the weapon motor

Outrunners don't like side loads. Belt tension and hits pull sideways on the can, so set the tension correctly. If the motor shaft isn't supported at its far end, put the pulley as close to the motor base as you can to limit stress on the motor's bearings.[73]
Belts wrapped on the can. Many beetleweights run a timing belt inside-out directly around the outrunner can as a slip clutch. Because timing belts barely stretch, the gap between "too loose" and "crushing the can" is small; Ask Aaron suggests mocking the parts up on the bench, measuring the snug center distance, and putting that into your CAD.[74]
Mount the motor firmly on a motor plate or bracket. Don't leave the motor's base hanging unsupported.[75]
Slotted mounting holes let you slide the motor to set belt tension, which you'll need with stretchy belts or a smooth pulley.[76] Tighten firmly and use threadlocker so the motor can't creep.
Support a direct-drive shaft with an extra bearing beyond the weapon where you can. It helps a great deal, though motors still break; the best fix is to replace the motor shaft with a dead shaft.[77] See Hardening Motors.
Cushion the mount if it suits your design: some hub-motor blades are mounted on shoulder bolts through TPU sleeves so the blade can shift slightly and pass less shock into the motor.[78]

Common failure modes

Failure	Usual cause	Prevention
Bent or snapped shaft	Too thin, cantilevered, or a weak bolt or standoff	Support both sides; larger diameter; hardened shaft or shoulder bolt
Shaft breaks at the threads	Threads in the loaded zone	Shoulder bolt, or keep threads outside the bearings
Bearing seizes, grinds or gets loose	Shock damage, grit, poor fit	Shielded bearings, correct fit, inspect and swap between matches
Bearing walks out	Loose hole, epoxy failure	Flanged bearings, press fit or retaining compound, frame trapping
Weapon slides on the shaft	No axial retention	Spacers, thrust washers, collars, snap rings, nylock nut
Belt thrown or cut	Slack, misalignment, exposed belt, frame flex	Correct length, stiff frame, guard, spare belts
Motor shaft bent, magnets cracked	Direct-drive impacts, side load	Belt drive, extra support bearing, hardened motor
Set screw or pulley slips	No flat, no threadlocker	Flat, threadlocker, check before every match, or press fit
Frame plates crack around the shaft	No spacer block or washers	Spacer block between plates, washers on plastic