The Gripper That Dropped the Part on Acceleration
We used a pneumatic gripper (article 91) to hold a metal part. The gripper force was set to 200 N (enough to hold the static part). On the bench, it held. On the floor, during a fast robot move (high acceleration), the part slipped out. The problem: the gripper force was sized for the static load, not the dynamic (acceleration) load. During acceleration, the part’s inertia pulls it out (F = m × a). For a 1 kg part accelerating at 10 m/s², the dynamic force is 10 N (plus the friction factor). The gripper’s 200 N was enough for static, but the acceleration (and the safety factor) needed more. We either upsized the gripper (more force) or slowed the acceleration. We upsized the gripper (400 N). The part held. The mistake was sizing the gripper for the static weight, not the dynamic acceleration.
Robot gripper force and stroke sizes the gripper for the dynamic load. This article covers the calculation.
The Gripper Force
A gripper’s force (at the finger) holds the part. The force must overcome:
- Static weight: The part’s weight (gravity). For a vertical hold, the gripper supports the weight.
- Dynamic (acceleration): During a fast move, the part’s inertia pulls it out (F = m × a).
- Friction: The gripper’s fingers (and the part’s surface) determine how much the force translates to holding. A smooth part (low friction) needs more force.
The Force Calculation
The required gripper force (per finger) is:
F_grip = (m × (g + a)) / (2 × μ × SF)
Where m is the part mass (kg), g is 9.81, a is the acceleration (m/s²), μ is the friction coefficient (finger on part), and SF is the safety factor (2–3).
For m = 1 kg, a = 10 m/s², μ = 0.2 (smooth metal), SF = 2: F_grip = (1 × (9.81 + 10)) / (2 × 0.2 × 2) = 19.81 / 0.8 = 24.8 N (per finger). Two fingers: 49.6 N total. The gripper’s 200 N is enough (with margin). But for a low-friction part (μ = 0.1, e.g., oily plastic): F_grip = 19.81 / (2 × 0.1 × 2) = 49.5 N per finger (99 N total). Still under 200 N. For a heavier part (5 kg, a = 20): F_grip = (5 × 29.81) / 0.4 = 373 N per finger. The 200 N gripper is too small.
The gripper force rule: Size the gripper for the dynamic load (m × (g + a)), not just the weight. The gripper that dropped the part had 200 N on a 1 kg part accelerating at 10 m/s². Calculate F_grip = m(g+a)/(2μ·SF). Use a safety factor of 2–3. For low-friction parts (oily, smooth), use more force (or a textured finger).
Step 1: The Stroke (Finger Opening)
The gripper’s stroke (how far the fingers open) must clear the part (and the placement). A gripper with 10 mm stroke can’t pick a 30 mm part (the fingers don’t open wide enough). Size the stroke for the part (plus clearance). For a range of part sizes, use a gripper with adjustable fingers (or a longer stroke).
Step 2: Friction (Finger Material)
The finger material (the gripping surface) affects μ. A smooth steel finger has μ = 0.1–0.2. A rubber (or polyurethane) pad has μ = 0.4–0.6. For a low-friction part (oily, smooth), add a rubber pad (higher μ, less force needed). The pad also protects the part (no marring).
Step 3: Acceleration (Robot Speed)
The robot’s acceleration (article 136) affects the dynamic load. A fast robot (high a) needs more gripper force. For a slow robot (low a), the static weight dominates. If the gripper is marginal, slow the robot’s acceleration (the dynamic load drops).
Step 4: Gripper Type (Parallel vs Angular)
A parallel gripper (fingers move straight) holds the part centered (consistent force). An angular gripper (fingers pivot) is simpler but the force varies with the opening. For a precise, centered hold, use a parallel gripper. For a simple pick (no precision), an angular gripper is fine.
| Parameter | Typical |
|---|---|
| Part mass (m) | 0.5–5 kg |
| Acceleration (a) | 5–20 m/s² |
| Friction (μ, steel) | 0.1–0.2 |
| Friction (μ, rubber pad) | 0.4–0.6 |
| Safety factor (SF) | 2–3 |
A Gripper Sizing Checklist
- What is the part mass? (kg?)
- What is the robot acceleration? (m/s²?)
- What is the friction (μ)? (Finger material?)
- Calculate F_grip = m(g+a)/(2μ·SF)?
- Is the gripper force ≥ F_grip? (With margin?)
- Is the stroke enough? (Part size + clearance?)
- Is the finger material right? (Rubber pad?)
- Does the part slip on acceleration? (Test?)
- Is the gripper parallel? (Centered?)
- Can you slow the acceleration? (If marginal?)
The Bottom Line
Robot gripper force and stroke size for the dynamic load. The gripper that dropped the part had static-only force. Calculate F_grip = m(g+a)/(2μ·SF). Use a safety factor of 2–3. Add a rubber pad (higher μ). Size the stroke for the part. The gripper that held the part wasn’t the strongest one — it was sized for acceleration.
