Designing for Weight, Stiffness, and Manufacturability

The advanced structural mindset#

A 115 lb robot weight limit (excluding battery and bumpers, per recent FRC rules) forces constant tradeoffs. Strong teams design for stiffness-to-weight and manufacturability, not just 'it holds'. The goal: the lightest structure that keeps deflection small along the real load path.

Think in load paths#

For every mechanism, trace the force from where it's applied to where it's reacted into the frame. Put material on that path and remove it elsewhere. A cantilevered shaft or single-supported gearbox plate deflects under load and ruins gear mesh, support both ends (dual-shear) wherever a real load exists.

Dead axle vs live axle#

• Dead axle: the shaft is fixed; bearings live in the rotating part. This lets you clamp the axle in dual shear, hugely stiffer, and is the classic choice for high-load pivots. The FRCDesign 6328 A-frame pivot case study highlights dead-axle clamping, custom gearbox packaging, and serviceable joints exactly for this reason.
• Live axle: the shaft rotates; simpler to drive but harder to support in dual shear. Use for lighter, lower-load rotating elements.

Lightening without losing stiffness#

• Pocketing: remove material from the center of plates where stress is low; keep material at edges and around bolt holes where it carries load. A pocketed plate can be far lighter at nearly the same stiffness.
• Box/tube structure: 2x1 aluminum tube resists bending far better per gram than flat plate; build frames and arms from tube, not slabs.
• Don't over-lighten near fasteners or load points, that's where cracks start.

Standardize on COTS dimensions#

Design around common standards so parts interchange and spares are trivial:

• 1/2 in hex shaft with matching heavy-duty hex bearings (e.g., AndyMark am-2986, 1.125 in OD) so torque goes through the flats, not set screws.
• COTS gearboxes (REV MAXPlanetary, system kit REV-21-2100, with 3:1/4:1/5:1/9:1 cartridges; VEXpro VersaPlanetary) for proven, serviceable reductions.
• Standard tube sizes and bolt patterns so brackets and bearings drop in.

Design for service#

A mechanism that can't be repaired in a short queue is a liability. Make joints serviceable: accessible bolts, modular subassemblies you can swap whole, and routed/strain-relieved wiring with service loops. The 2910 dead-axle pivot case study explicitly calls out chain tensioning, load paths, and serviceable structure as co-equal design goals.

Putting it together#

For a high-load arm pivot: dead axle clamped in dual shear, pocketed side plates, a COTS planetary you can swap as a unit, hex output, and bolt-accessible joints. That's a mechanism that's light, stiff, repairable, and built from parts you can carry spares of.