link
The Basics
A link is a rigid structural segment of a robot—the solid "bones" that form the robot's skeleton. Links are connected by joints and transmit forces between them without bending or deforming.
Key Characteristics
Links are:
- Rigid - don't bend or flex during normal operation
- Solid structures - aluminum bars, steel frames, composite tubes
- Connected by joints - linked together at connection points
- Load-bearing - transmit weight, forces, and torques
- Passive components - provide structure (joints provide movement)
Link 3
│
◯─────◯ (Joint)
│ Link 2
◯─────◯ (Joint)
│ Link 1
◯─────◯ (Joint/Base)
══════════
Base
Anatomy of a Link
Each link typically has:
Component | Purpose |
Main body | Structural core (bar, tube, or frame) |
Attachment points | Where joints connect |
Internal channels | Route cables, hydraulics, wires |
Mounting surfaces | Secure motors, sensors, grippers |
Real-World Examples
Typical robot arm links:
Link 1 (Shoulder):
- Heavy-duty aluminum tube
- Connects base to elbow joint
- Bears all downstream weight
Link 2 (Forearm):
- Lighter composite material
- Connects elbow to wrist
- Transmits forces to end-effector
Link 3 (Wrist):
- Compact, minimal mass
- Connects to gripper/tool
- Enables fine positioning
Link vs. Joint: The Difference
Link | Joint |
Rigid body | Connection point |
Doesn't move by itself | Causes movement |
Transmits forces | Enables rotation/translation |
"Bone" | "Hinge" |
Example: arm segment | Example: elbow |
Link 1 ←Joint→ Link 2 ←Joint→ Link 3
(moves together) (rotates here) (moves with Link 2)
Number of Links and DOF
Degrees of Freedom (DOF) = Number of joints (roughly)
3-link robot arm:
Link 1 ─ Joint 1 ─ Link 2 ─ Joint 2 ─ Link 3 ─ Joint 3 ─ Gripper
(3 DOF)
More links = more complex motion possible, but harder to control.
Link Design Considerations
Engineers choose link design based on:
Material selection:
- Aluminum - lightweight, good strength-to-weight ratio
- Steel - strong but heavier
- Carbon fiber - ultra-light, expensive
- Composite - optimized strength and weight
Length:
- Longer links = greater reach
- Shorter links = faster movement, less inertia
- Balance between functionality and performance
Cross-section shape:
- Hollow tube - lightweight, good rigidity
- Solid bar - simpler but heavier
- I-beam - strong in specific directions
Structural Requirements
Links must withstand:
Force Type | Impact |
Tension | Pulling forces from joints |
Compression | Weight and pushing loads |
Bending | Moments from offset loads |
Torsion | Twisting forces from rotation |
Fatigue | Repeated stress cycles |
Real Robot Example: Industrial Arm
Base (Link 0)
↓
Shoulder Link (Link 1) - Heavy, 2kg aluminum
↓ Shoulder Joint
Upper Arm Link (Link 2) - Medium, 1.5kg aluminum
↓ Elbow Joint
Forearm Link (Link 3) - Light, 0.8kg composite
↓ Wrist Joint
Tool Plate (Link 4) - Tiny, 0.3kg steel
↓
Gripper/Tool
Each link gets progressively lighter because less mass hangs below it.
Link Rigidity Trade-offs
Rigid links:
- ✓ Predictable motion
- ✓ Accurate positioning
- ✓ Precise control
- ✗ Heavier materials needed
- ✗ Higher cost
More flexible links:
- ✓ Lighter weight
- ✓ Lower cost
- ✗ Unpredictable deflection
- ✗ Difficult to control accurately
Most robots prioritize rigidity for precision.
Link Deformation (In Practice)
Though links are "rigid," in reality:
- Long links may sag slightly under weight
- High-speed movement may cause vibration
- Extreme forces can cause temporary deflection
- Engineers account for this in design
Solution: Over-design links with safety factor (1.5×–3×) to ensure true rigidity under all operating conditions.
Key Takeaway
Links are the structural skeleton of a robot—rigid segments that form the physical framework. Linked together by joints, they enable the robot to move and manipulate while maintaining the structural integrity needed for precise, repeatable control. Link design directly impacts robot speed, strength, precision, and cost.