Shafts, bearings & mounting

The weapon rotor stores more energy than anything else on the robot and dumps shock loads into its support every hit. This is the most common structural failure point.

Dead shaft is the default

Almost every modern beetleweight weapon runs a dead shaft — the shaft is fixed to the frame and the rotor spins on bearings pressed into it — supported at both ends in double shear. Rotor loads go straight into a static shaft, and the motor is isolated by the belt or gear. Live shafts (shaft spins with the rotor) are now rare.¹

Shaft

Size for bending and shock, not torque — torque is modest, impact bending is severe.²
Off-the-shelf options, roughly in order of preference:¹
- End-tapped linear shaft (e.g. Misumi) — hardened, precise, secured by a screw and washer at each end, nothing exposed. Hollow versions save weight.
- Hardened rod + clamping shaft collars — use clamping collars, never set-screw; the collars are exposed and can loosen.
- Shoulder bolt — cheapest; use an alloy-steel bolt, not stainless, and a nylock nut.
Sizes: ≥ 5/16” (8 mm) for small verts, 3/8” (10 mm) for larger or wider verts (drums, beaters), 3/8”–1/2” for horizontals. 1/2” (12 mm) hollow is common on big verts. Titanium shafts must go a size up — and not with needle bearings (below).¹
Avoid stress risers at the impact zone: no sharp shoulders, threads, or cross-holes where bending is highest. Use generous fillets.²

Bearings

Needle-roller bearings are the usual choice for vertical spinners: the rollers run directly on the shaft (no inner race), so they are thin — easy to fit a small rotor — and far more impact-tolerant than ball bearings. They need a polished hardened-steel shaft (they will indent titanium or soft steel), two bearings spaced well apart to stop wobble, and thrust washers to take axial load.¹
Ball bearings (often angular-contact) are used on large-diameter weapons where low slop matters. The catch: only half the race carries axial load, so a hard off-plane hit can burst them. Size the bore to the shaft and check the dynamic rating against shock with a large safety factor.
Cheap bearings are fine — the precision of an expensive bearing is gone the first time the weapon hits something.¹
Shielded or sealed to keep grinding debris out; treat them as wear items.
Provide axial location (shoulders, snap rings, thrust washers, or clamping collars) so the rotor can’t walk off the shaft in a hit.

Blade thickness

The blade itself is part of the stack. Rough beetleweight guidance for 2D AR500:¹

3/16” (~5 mm) — minimum; will still bend on hard off-plane hits. Common on large-diameter bars, which rarely get hit off-plane.
1/4” or 6 mm — the common vertical-spinner thickness and a chunky horizontal option.
3/8”–1/2” — heavy verts; essentially won’t bend.

Mounting and load path

Bolt bearing blocks / weapon mounts (commonly 3/8”–1/2” UHMW uprights, or stiffened TPU) into the frame so impact load goes into the thickest, best-supported structure, spread across multiple fasteners in shear.
Keep the rotor’s plane of rotation close to the bearing supports — long overhangs multiply bending stress.
On a two-tooth “drisk” (two disks on one hub), hitting with one disk twists the whole load through the hub — connect the disks with large (M4+) standoffs or a keyed aluminium hub, not just a printed part.¹
Isolate delicate electronics from the bearing-block structure with shock mounting where practical.

References

Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies (Fall 2025 revision) — builder-maintained beginner guide distributed via the NHRL Discord. Sections: Spinner Weapon Systems (Bearings, Shafts, Weapon Stacks); Designing for Durability. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷
Meggiolaro, M. A. RioBotz Combat Robot Tutorial, v2.0, PUC-Rio, 2009 (CC BY-NC-ND) — weapon shaft and bearing design for impact loads, live- vs dead-shaft trade-offs, and stress-riser avoidance. ↩ ↩²