Material Selection
Introduction
Material selection is a critical decision in robot mechanical design, directly affecting weight, strength, durability, and cost. This section covers the properties and selection methods for metals, plastics, and composite materials commonly used in robotics.
Material Performance Metrics
Key Parameters
| Parameter | Symbol | Unit | Significance |
|---|---|---|---|
| Density | \(\rho\) | kg/m³ or g/cm³ | Weight |
| Yield strength | \(\sigma_y\) | MPa | Stress at onset of plastic deformation |
| Tensile strength | \(\sigma_{UTS}\) | MPa | Maximum stress before fracture |
| Young's modulus | \(E\) | GPa | Stiffness (resistance to deformation) |
| Elongation | \(\epsilon_f\) | % | Toughness / ductility |
| Hardness | HB/HRC | — | Surface wear resistance |
| Fatigue limit | \(\sigma_f\) | MPa | Life under cyclic loading |
Specific Strength and Specific Stiffness
Specific strength measures load-bearing capability per unit weight:
\[\frac{\sigma_y}{\rho}\]
Specific stiffness measures stiffness per unit weight:
\[\frac{E}{\rho}\]
These two metrics are particularly important for lightweight design.
Aluminum Alloys
Aluminum alloys are the most commonly used metal for robot structural parts, balancing strength, weight, and machinability.
Common Aluminum Alloys
| Grade | Series | Density (g/cm³) | Yield Strength (MPa) | Tensile Strength (MPa) | Young's Modulus (GPa) | Features |
|---|---|---|---|---|---|---|
| 6061-T6 | 6xxx | 2.70 | 276 | 310 | 69 | Most versatile, easy to weld and machine |
| 6063-T5 | 6xxx | 2.70 | 185 | 230 | 69 | Extrusion profiles, good surface finish |
| 7075-T6 | 7xxx | 2.81 | 503 | 572 | 72 | High strength, aerospace grade |
| 5052-H32 | 5xxx | 2.68 | 193 | 228 | 70 | Corrosion resistant, sheet metal |
| 2024-T3 | 2xxx | 2.78 | 345 | 483 | 73 | High strength, good fatigue |
Aluminum Alloy Selection Guide
| Application | Recommended Grade | Reason |
|---|---|---|
| Chassis plate / brackets | 6061-T6 | Versatile, low cost, easy to machine |
| Extrusion profile frames | 6063-T5 | Rich standard profile selection |
| High-strength structural parts | 7075-T6 | High specific strength |
| Shell sheet metal | 5052-H32 | Good bendability, corrosion resistant |
| Shafts / pins | 7075-T6 or 2024 | Strength + fatigue performance |
Aluminum Extrusion Profiles (V-slot / 2020 / 2040)
Industrial aluminum extrusion profiles are a rapid solution for building robot frames:
| Specification | Cross-Section | Weight per Meter | Suitable For |
|---|---|---|---|
| 2020 | 20x20 mm | ~0.5 kg | Small robot frames |
| 2040 | 20x40 mm | ~1.0 kg | Medium chassis |
| 3030 | 30x30 mm | ~1.0 kg | Medium frames |
| 4040 | 40x40 mm | ~1.7 kg | Large structures |
| 4080 | 40x80 mm | ~3.2 kg | Large frames |
Connected with T-nuts and corner brackets, no welding needed.
Carbon Fiber
Carbon fiber reinforced polymer (CFRP) offers extremely high specific strength and stiffness, making it the top choice for lightweight design.
Carbon Fiber Products
| Product Form | Specification | Features | Suitable For |
|---|---|---|---|
| Carbon fiber tubes | 6–50 mm dia | Round, good bending/torsion stiffness | Arm links |
| Carbon fiber plates | 0.5–5 mm thick | Flat, CNC-cut | Chassis plates, brackets |
| Carbon fiber square tubes | 10x10–30x30 mm | Rectangular cross-section | Frame structures |
| Carbon fiber wound tubes | Custom | Specific fiber angles | High-performance drive shafts |
Carbon Fiber Performance
| Parameter | Value | Comparison to Al 6061 |
|---|---|---|
| Density | 1.5–1.6 g/cm³ | ~57% of aluminum |
| Tensile strength | 600–3000 MPa | 2–10x |
| Young's modulus | 70–200 GPa | 1–3x |
| Specific strength | Very high | 3–6x aluminum |
| Specific stiffness | Very high | 2–4x aluminum |
Carbon Fiber Considerations
- Anisotropic: Strong along fiber direction, weak perpendicular
- Brittle: Cannot deform plastically; no visible warning before failure
- Machining dust: Cutting produces carbon fiber dust; protection needed
- Electrically conductive: Carbon fiber conducts electricity; may cause short circuits
- Expensive: Approximately 3–10x the cost of aluminum
- Difficult to join: Cannot be welded; requires adhesive or bolted connections
Engineering Plastics
Engineering plastics are widely used in robots for non-critical structural parts, shells, gears, etc.
Common Engineering Plastics
| Material | Density (g/cm³) | Tensile Strength (MPa) | Young's Modulus (GPa) | Features | Typical Application |
|---|---|---|---|---|---|
| POM (Polyoxymethylene) | 1.41 | 60–70 | 2.8–3.5 | Wear-resistant, self-lubricating, dimensionally stable | Gears, bearings |
| PA6 (Nylon 6) | 1.13 | 70–85 | 2.7–3.3 | Good toughness, wear-resistant, hygroscopic | Gears, structural parts |
| PA66 | 1.14 | 80–100 | 3.0–3.5 | Stronger than PA6 | High-load gears |
| PC (Polycarbonate) | 1.20 | 55–70 | 2.3–2.4 | Transparent, impact-resistant | Shells, windows |
| ABS | 1.04 | 40–50 | 2.0–2.6 | Easy to process, low cost | Shells, non-structural parts |
| PEEK | 1.30 | 100–110 | 3.5–4.5 | Heat-resistant, chemical-resistant | High-end bearings/seals |
| PPS | 1.35 | 75–85 | 3.5–4.0 | Heat-resistant, flame-retardant | Electronic enclosures |
Plastic Gear Material Selection
| Material | Wear Resistance | Noise | Strength | Cost | Recommendation |
|---|---|---|---|---|---|
| POM | Excellent | Low | Medium | Low | First choice |
| PA66 | Good | Low | High | Low | High loads |
| POM+PA66 pair | Excellent | Very low | — | — | Best pairing |
| PEEK | Excellent | Low | Very high | High | Extreme conditions |
Steel
Steel is used in robots primarily for shafts, gears, springs, and other high-strength components.
Common Steels
| Grade | Density (g/cm³) | Yield Strength (MPa) | Hardness | Application |
|---|---|---|---|---|
| 1045 | 7.85 | 355 | HB 197–241 | Shafts, pins |
| 4140 | 7.85 | 785 (quenched) | HRC 28–33 | High-strength shafts |
| 52100 | 7.81 | — | HRC 61–65 | Bearings |
| 304 Stainless | 7.93 | 205 | HB 187 | Corrosion-resistant structures |
| 65Mn Spring Steel | 7.85 | 784 | HRC 42–50 | Springs, snap rings |
Rubber and Elastomers
| Material | Hardness (Shore A) | Features | Robot Application |
|---|---|---|---|
| Natural rubber (NR) | 30–90 | Good elasticity, tear resistant | Tires |
| Silicone rubber | 20–80 | Temperature resistant (-60 to 200°C) | Seals, soft grippers |
| NBR (Nitrile rubber) | 30–90 | Oil resistant | O-rings |
| EPDM | 30–90 | Weather resistant | Outdoor seals |
| TPU | 60–95 | 3D printable, wear-resistant | Tires, bumpers |
| Sponge rubber | — | Shock absorbing | Bumper strips |
Comprehensive Material Comparison
| Material | Density (g/cm³) | Yield Strength (MPa) | Young's Modulus (GPa) | Specific Strength | Relative Cost |
|---|---|---|---|---|---|
| Al 6061-T6 | 2.70 | 276 | 69 | 102 | 1x |
| Al 7075-T6 | 2.81 | 503 | 72 | 179 | 2x |
| Carbon fiber plate | 1.55 | 600+ | 70–200 | 387+ | 5–10x |
| 1045 Steel | 7.85 | 355 | 210 | 45 | 0.5x |
| 304 Stainless | 7.93 | 205 | 200 | 26 | 1.5x |
| POM | 1.41 | 65 | 3.0 | 46 | 0.8x |
| PA66 | 1.14 | 85 | 3.2 | 75 | 0.8x |
| PLA (3D printed) | 1.24 | 60 | 3.5 | 48 | 0.3x |
| PETG (3D printed) | 1.27 | 50 | 2.1 | 39 | 0.3x |
Material Selection Flow
graph TD
A[Determine part function and loads] --> B{Metal needed?}
B -->|Yes| C{Lightweight important?}
C -->|Yes| D{Budget sufficient?}
D -->|Yes| E[Carbon Fiber CFRP]
D -->|No| F[Aluminum Alloy 6061/7075]
C -->|No| G{High strength needed?}
G -->|Yes| H[Steel 4140/1045]
G -->|No| F
B -->|No| I{Wear resistance needed?}
I -->|Yes| J[POM / PA66]
I -->|No| K{Toughness needed?}
K -->|Yes| L[PC / PA6]
K -->|No| M{Rapid prototype?}
M -->|Yes| N[3D Print PLA/PETG]
M -->|No| O[ABS / PC Injection Molded]
Surface Treatments
| Treatment | Compatible Materials | Effect | Cost |
|---|---|---|---|
| Anodizing | Aluminum | Corrosion protection, coloring, wear resistance | Low |
| Hard anodizing | Aluminum | High hardness (HV 300+), wear resistance | Medium |
| Sandblasting | Metals | Uniform matte finish | Low |
| Spray painting / baking | Metals/plastics | Aesthetic, corrosion protection | Medium |
| Nickel/chrome plating | Steel | Corrosion protection, wear resistance | Medium |
| Blackening/phosphating | Steel | Basic corrosion protection | Low |
References
- MatWeb: matweb.com — Material database
- CES EduPack (Granta Design) — Material selection software
- ASM Handbook — Metals reference
- McMaster-Carr material catalog