The Gripper That Dropped the Part
We mounted a parallel pneumatic gripper on a robot. The gripper was rated for 40 N per finger. The part was a smooth aluminum cylinder, 200 g. On the bench, it held. On the robot, when the arm accelerated at 5 m/s², the part slipped out. The holding force was fine for static weight (2 N), but the acceleration force (m × a = 0.2 kg × 5 m/s² = 1 N) combined with the low friction (smooth aluminum on smooth steel fingers, μ = 0.1) meant the grip force needed was F = (m(g+a))/μ = 0.2 × 14.81 / 0.1 = 29.6 N. The gripper delivered 40 N — close, but the smooth fingers had low friction. We added serrated aluminum pads (μ = 0.4). Required force dropped to 7.4 N. The gripper held. The mistake was calculating grip force for static weight, not for acceleration with the actual friction coefficient.
Robot gripper selection isn’t picking a gripper that “looks strong.” The required grip force depends on the part weight, the robot acceleration, and the friction between the fingers and the part. This article runs the numbers.
Pneumatic vs. Electric Grippers
Pneumatic Gripper
Air actuates the fingers. Simple, cheap, high force (for the size), but the force is fixed (by air pressure). Open/close position is fixed (two positions). No force control (it’s either open or closed).
Best for: Simple pick-and-place, consistent part sizes, high speed, low cost.
Electric Gripper (Servo)
A small servo motor drives the fingers. The grip force and position are programmable. Can grip soft parts (low force) or hard parts (high force). Can stop at any opening position.
Best for: Mixed part sizes, delicate parts (force control), intelligent gripping (detect part presence by force).
| Type | Force Control | Cost | Best For |
|---|---|---|---|
| Pneumatic parallel | Fixed (air pressure) | Low | Consistent parts, simple cycles |
| Pneumatic angular | Fixed | Low | Large parts, wide opening |
| Electric servo parallel | Programmable | High | Mixed sizes, delicate parts |
| Vacuum (suction cup) | N/A (vacuum force) | Low | Flat parts, sheets, boxes |
| Magnetic (permanent/electro) | Fixed (part-dependent) | Medium | Ferromagnetic flat parts |
Step 1: Calculate Required Grip Force
The grip force must hold the part against gravity and acceleration.
F_grip = m × (g + a) / μ × safety_factor
Where m is the part mass (kg), g is 9.81, a is the robot acceleration (m/s²), μ is the friction coefficient between finger and part, and the safety factor is 2–3.
For our example: m = 0.2 kg, a = 5 m/s², μ = 0.1 (smooth aluminum on smooth steel), safety factor = 2. F_grip = 0.2 × (9.81 + 5) / 0.1 × 2 = 0.2 × 14.81 / 0.1 × 2 = 2.96 × 2 = 59.2 N. Per finger? No — total grip force. If two fingers each deliver 40 N, total is 80 N. But the friction force is μ × F_normal. The normal force per finger is F_grip_per_finger. The friction holding the part is 2 × μ × F_finger.
Let me redo this. The two fingers squeeze the part. Each finger exerts F_normal on the part. The friction force holding the part is 2 × μ × F_normal (two contact surfaces). This must exceed the force trying to drop the part: m(g+a).
2 × μ × F_normal ≥ m(g+a) × safety_factor
So F_normal per finger = m(g+a) × SF / (2 × μ). For our example: F_normal = 0.2 × 14.81 × 2 / (2 × 0.1) = 5.92 / 0.2 = 29.6 N per finger. The gripper delivers 40 N per finger — it should hold. But with smooth fingers (μ = 0.1), it was marginal. With serrated pads (μ = 0.4): F_normal = 0.2 × 14.81 × 2 / (2 × 0.4) = 5.92 / 0.8 = 7.4 N per finger. Easy.
Step 2: Finger Stroke (Opening)
The gripper’s stroke (how far the fingers open) must clear the part plus some margin.
- Part width: The part’s dimension at the grip point.
- Clearance: Add 5–10 mm so the gripper can approach without hitting the part.
For a 50 mm wide part: gripper opening needs 60–70 mm. A gripper with 25 mm stroke per finger (50 mm total opening) is too small. Pick a gripper with 40 mm per finger (80 mm opening).
Step 3: Gripper Type: Parallel vs. Angular
Parallel Gripper
The fingers move straight in and out (parallel). They stay parallel to the part. Best for gripping flat or rectangular parts (boxes, machined parts). The contact surface is consistent.
Angular (2-Jaw or 3-Jaw)
The fingers pivot open/closed (like a chuck). They open wider for the same gripper size. Best for round parts (shafts, cylinders) or large parts that need a wide opening. But the contact angle changes as the fingers close.
For rectangular parts: parallel. For round parts: angular (or a 3-jaw centering gripper).
The grip force formula: F_per_finger = m(g+a) × SF / (2 × μ). Use the actual μ (smooth = 0.1, rubber = 0.5, serrated = 0.4). Add a safety factor of 2–3. The part that dropped was gripped with smooth fingers (μ = 0.1) at high acceleration. Rubber or serrated pads reduced the required force by 4×.
Finger Material and Surface
The finger surface affects friction and part damage.
- Steel (hardened): Durable, but low friction (μ = 0.1–0.2). Can scratch soft parts.
- Aluminum: Light, but still low friction. Soft — can deform.
- Urethane/rubber pad: High friction (μ = 0.5–0.8), non-marring. Best for holding smooth parts without damage.
- Serrated/knurled steel: Very high friction (μ = 0.6–0.8). For rough or heavy parts. But it marks the part.
For a smooth aluminum part, use urethane pads. For a rough casting, use serrated steel. Match the pad to the part surface.
Part Orientation: Grip at the CG
Grip the part at (or near) its center of gravity. If you grip off-CG, the part creates a moment that the fingers must resist (it wants to rotate in the gripper).
For an elongated part, grip at the middle, not the end. If you must grip off-CG, add a support (or use a gripper with longer fingers that hold both ends).
Sensors on the Gripper
Add sensors to know the gripper state.
- Part present (jaw position): A proximity sensor on the finger that triggers when the jaw closes on a part (vs. closing on nothing — the jaw closes further).
- Grip force (electric): The servo gripper reports the grip force. If it’s lower than expected, no part was picked.
Don’t assume the part was picked. Verify with a sensor. A “missed pick” causes the next station to fail.
A Gripper Selection Checklist
- What is the part mass? (kg)
- What is the robot acceleration? (m/s²)
- What is the friction coefficient? (Finger material vs. part.)
- F_per_finger = m(g+a) × SF / (2 × μ).
- Does the gripper deliver this force? (With margin.)
- What is the finger stroke? (Part width + clearance.)
- Gripper type: parallel or angular? (Part shape.)
- Pneumatic or electric? (Fixed or programmable force?)
- Finger pads: urethane, steel, serrated? (Non-marring?)
- Is the grip at the CG? (No off-CG moment?)
- Is there a part-present sensor?
- Does the gripper weight fit the robot wrist moment? (See article 59.)
The Bottom Line
Robot gripper selection is the friction-limited force calculation. The part that dropped wasn’t gripped too lightly by weight — it was gripped on smooth fingers at high acceleration. Calculate F_per_finger = m(g+a) × SF / (2 × μ), pick the right finger material (urethane for smooth parts, serrated for rough), and use a part-present sensor. The gripper that holds every part through every acceleration wasn’t the biggest one — it had the right friction pads for the part.
