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.

Key takeaways

• Design for stiffness-to-weight: trace the load path, support loads in dual shear, and pocket only low-stress regions.
• Prefer dead axles clamped in dual shear for high-load pivots (per FRC 6328’s A-frame), and build from tube, not flat plate.
• Standardize on COTS dimensions (1/2 in hex, MAXPlanetary/VersaPlanetary) and design serviceable, swappable subassemblies for fast field repair.

This lesson was adapted from learnfrc.com.