Linear Actuator FEA & Design Optimization

Context

A capstone subsystem required a compact linear actuator capable of delivering 12 N·m of torque within a constrained envelope. Off-the-shelf solutions either exceeded the mass budget or lacked the stroke length required for the application. A custom design was pursued to meet all functional, manufacturing, and assembly requirements.

Requirements & Constraints

• Torque output: ≥ 12 N·m continuous at operating speed
• Stroke length: 85 mm minimum
• Mass budget: ≤ 420 g for the actuator assembly
• Environment: Indoor lab conditions, no IP rating required
• Manufacturing: Must be producible using a combination of FDM 3D printing and off-the-shelf fasteners
• Assembly: DFA principles — minimize unique parts, ensure tool-accessible fasteners

Approach

The design followed a structured workflow:

1. Concept generation — Four concepts evaluated using a Pugh matrix against torque, mass, manufacturability, and assembly criteria
2. Preliminary sizing — Hand calculations for lead screw geometry and motor torque requirements
3. Detailed CAD — SolidWorks model with full GD&T callouts per ASME Y14.5
4. FEA validation — ANSYS Mechanical static structural analysis on critical load paths
5. Iterative optimization — Topology-informed material removal guided by von Mises stress distribution

Design & Analysis

The critical housing bracket was analyzed under worst-case loading: 150% of rated torque applied as a distributed load on the mounting face, with fixed supports at the four bolt locations.

Mesh details:

• Element type: SOLID187 (10-node tetrahedral)
• Element size: 0.8 mm on critical fillets, 2.0 mm global
• Convergence study confirmed < 2% change in peak stress with further refinement

Results:

• Peak von Mises stress: 42 MPa (occurring at fillet radius on mounting boss)
• Material yield strength (6061-T6 Al): 276 MPa
• Factor of Safety: 6.57 at peak stress (well above 2.0 minimum)
• Maximum displacement: 0.012 mm (negligible for application)

After topology optimization, the bracket mass was reduced from 52 g to 42.6 g (18% reduction) while maintaining FoS > 2.3 everywhere.

Build & Integration

• Housing printed in PETG on a Prusa MK4 with 0.2 mm layer height, 40% gyroid infill
• Lead screw and nut sourced as COTS components (Misumi catalog)
• Motor mount uses four M3 socket head cap screws with thread-locking compound
• Assembly requires only two tools: M2.5 hex key and M3 hex key

Testing & Validation

• Torque output measured using a calibrated spring scale at 50 mm moment arm
• Results: 12.4 N·m ± 0.3 N·m across 10 test cycles (exceeds requirement)
• Stroke linearity verified with dial indicator: < 0.05 mm deviation over full stroke
• No visible deflection or creep after 1-hour sustained load test

Results

Metric	Target	Achieved
Torque	≥ 12 N·m	12.4 N·m
Mass	≤ 420 g	387 g
FoS	≥ 2.0	2.3 (minimum)
Stroke	≥ 85 mm	90 mm

Next Improvements

• Replace PETG housing with nylon PA12 for improved fatigue life
• Add integrated strain gauge mounting points for closed-loop force feedback
• Investigate harmonic drive option for higher gear ratio in same envelope