Mechanical Design Overview
Introduction
Mechanical design is the physical foundation of robot engineering, determining a robot's form factor, motion capabilities, load capacity, and durability. Excellent mechanical design requires balancing functional requirements, manufacturing feasibility, cost, and aesthetics.
Design Process
graph TD
A[Requirements Analysis] --> B[Concept Design]
B --> C[Solution Evaluation and Selection]
C --> D[Detailed CAD Modeling]
D --> E[Structural Analysis / Simulation]
E --> F{Meets requirements?}
F -->|No| D
F -->|Yes| G[Prototype Manufacturing]
G --> H[Assembly and Testing]
H --> I{Passes testing?}
I -->|No| J[Design Iteration]
J --> D
I -->|Yes| K[Production / Final Version]
style A fill:#e3f2fd
style K fill:#c8e6c9
Requirements Analysis Phase
Define the following key requirements:
| Requirement Category | Specifics | Example |
|---|---|---|
| Functional requirements | Motion type, speed, payload | Wheeled mobile, 0.5 m/s, 5 kg payload |
| Environmental requirements | Work environment, temperature, protection | Indoor flat ground, IP54 |
| Size constraints | Maximum dimensions, passability | Width <50 cm (pass through doorways) |
| Weight constraints | Maximum allowable weight | <15 kg (single-person carry) |
| Runtime requirements | Working time | >2 h continuous operation |
| Cost constraints | Material and machining budget | Mechanical parts <$500 |
Concept Design Phase
- Sketching and ideation: Hand-drawn or simple 3D draft models
- Locomotion mechanism selection: Wheeled / tracked / legged / hybrid
- Module division: Chassis, robotic arm, sensor mounts, shell
- Reference existing designs: Open-source robots, competitor analysis
Structural Analysis Fundamentals
Basic Mechanics Concepts
Stress and Strain:
\[\sigma = \frac{F}{A} \quad \text{(Stress, Pa)}\]
\[\epsilon = \frac{\Delta L}{L_0} \quad \text{(Strain, dimensionless)}\]
Hooke's Law (within elastic range):
\[\sigma = E \times \epsilon\]
Where \(E\) is Young's Modulus.
Safety Factor:
\[n = \frac{\sigma_{yield}}{\sigma_{actual}}\]
Typically \(n \geq 2\) for static loads and \(n \geq 3\) for dynamic loads.
Common Load Types
| Load Type | Description | Source in Robots |
|---|---|---|
| Tension / compression | Axial force | Support columns, links |
| Bending | Transverse force causing bending moment | Cantilever arm, sensor mounts |
| Shear | Force parallel to cross-section | Bolt connections, pins |
| Torsion | Torque along axis | Motor output shaft, drive shaft |
| Impact | Sudden large force | Collision, drop |
Beam Bending
Cantilever beam with end load \(F\), length \(L\):
Maximum bending moment: \(M_{max} = F \times L\)
Maximum deflection:
\[\delta_{max} = \frac{F L^3}{3 E I}\]
Where \(I\) is the second moment of area.
Rectangular cross-section (\(b \times h\)): \(I = \frac{b h^3}{12}\)
Circular cross-section (diameter \(d\)): \(I = \frac{\pi d^4}{64}\)
Degrees of Freedom (DOF)
Degrees of Freedom describe the number of independent parameters for a robot's motion.
Rigid Body DOF
A free rigid body in space has 6 degrees of freedom:
- 3 translational: \(x, y, z\)
- 3 rotational: \(\text{roll}, \text{pitch}, \text{yaw}\)
Robot DOF
| Robot Type | Typical DOF | Description |
|---|---|---|
| Differential wheeled | 2 (v, omega) | Non-holonomic constraint |
| Omnidirectional wheeled | 3 (vx, vy, omega) | Holonomic constraint |
| 6-DOF robotic arm | 6 | Can reach any pose in workspace |
| 7-DOF robotic arm | 7 | Redundant DOF, can avoid obstacles |
| Quadruped robot | 12 (3/leg) | Each leg: hip-thigh-knee |
| Humanoid robot | 20–40+ | Depends on design |
Gruebler's Formula
Planar mechanism DOF calculation:
\[F = 3(n-1) - 2j_1 - j_2\]
Where \(n\) is the number of links, \(j_1\) is the number of lower pairs, and \(j_2\) is the number of higher pairs.
Connections and Fasteners
Bolt Connections
| Size | Pitch | Wrench Size | Grade 8.8 Tensile Load |
|---|---|---|---|
| M2 | 0.4 mm | 4 mm | 1.9 kN |
| M3 | 0.5 mm | 5.5 mm | 4.4 kN |
| M4 | 0.7 mm | 7 mm | 7.8 kN |
| M5 | 0.8 mm | 8 mm | 12.3 kN |
| M6 | 1.0 mm | 10 mm | 17.6 kN |
| M8 | 1.25 mm | 13 mm | 31.4 kN |
Anti-Loosening Measures:
- Spring washers, lock nuts (nylon insert)
- Thread locker (Loctite 243 blue = removable, 271 red = permanent)
- Anti-loosening measures are mandatory in robot vibration environments
Other Connection Methods
| Method | Suitable Scenario | Pros / Cons |
|---|---|---|
| Bolts | Detachable connections | Universal, repeatable disassembly |
| Rivets | Thin sheet connections | Lightweight but non-removable |
| Welding | Permanent metal connections | High strength but non-removable |
| Adhesive | Dissimilar material connections | Uniform stress distribution |
| Snap fits | Shells, covers | Quick assembly/disassembly |
| Pins | Precise positioning | High repeatability |
Sealing and Waterproofing
IP Protection Ratings
IP (Ingress Protection) ratings are expressed as two digits:
| IP Rating | Dust Protection | Water Protection | Typical Application |
|---|---|---|---|
| IP20 | Protection from >12 mm objects | No water protection | Indoor electronics |
| IP44 | Protection from >1 mm wires | Splash-proof | Indoor robots |
| IP54 | Protection from harmful dust | Splash-proof | Outdoor robots |
| IP65 | Completely dust-tight | Protected against water jets | Industrial robots |
| IP67 | Completely dust-tight | Brief immersion (1 m/30 min) | Harsh environments |
| IP68 | Completely dust-tight | Continuous immersion | Underwater robots |
Sealing Methods
- O-rings: Most common rotary/static seal
- Gaskets: Planar seal, silicone rubber/EPDM
- Sealant: RTV silicone, for irregular surfaces
- Waterproof connectors: Aviation plugs (GX12, M12)
- Potting: Epoxy resin fully encapsulating PCB
Design Tools and Methods
Finite Element Analysis (FEA)
- Fusion 360 Simulation: Basic static analysis
- SolidWorks Simulation: Professional FEA
- ANSYS: Advanced multi-physics simulation
- FreeCAD FEM: Open-source FEA
Design Verification Checklist
- [ ] Safety factor >= 2 for all load-bearing parts
- [ ] Tolerances annotated on critical dimensions
- [ ] Assembly sequence feasibility verified
- [ ] Maintenance convenience (accessibility of removable parts)
- [ ] Reasonable center of gravity position (not prone to tipping)
- [ ] Cable routing space reserved
- [ ] Heat dissipation paths unobstructed
- [ ] Sensor mounting positions unobstructed
Chapter Structure
This chapter covers the various aspects of robot mechanical design:
- Chassis and Locomotion Mechanisms: Wheeled, omnidirectional, steering mechanisms
- Robotic Arm Structure: Serial/parallel mechanisms, joint design
- Thermal Management and Protection: Heat management, IP protection
- 3D Printing and Machining: Rapid prototyping manufacturing
- Material Selection: Metals, plastics, composites
- CAD Tools: Comparison of mainstream CAD software
References
- Shigley's Mechanical Engineering Design (11th Ed.)
- GrabCAD: Open-source 3D model library
- McMaster-Carr: Standard parts reference