Choosing a gripper that is too weak means dropped parts. Choosing one that is too strong adds weight, cost and air consumption. This guide shows how to calculate the grip force a pneumatic parallel gripper needs, with a worked example.
Friction grip or form-fit grip?
First decide how the fingers will hold the part.
- Friction grip: flat fingers press on the part, and only friction stops it from slipping. This needs the most force.
- Form-fit grip: the fingers are shaped to the part (a V-groove for a shaft, a pocket for a flange), so the part sits in the fingers and the grip force only has to keep the fingers closed. This needs much less force and is more reliable.
Use form-fit fingers whenever the part shape allows. The calculation below is for friction grip, which is the worst case.
The formula
For a 2-finger parallel gripper holding a part by friction, with the part hanging below the gripper:
F = m × (g + a) × S ÷ (μ × n)
| Symbol | Meaning | Typical value |
|---|---|---|
| F | Required grip force (N) | — |
| m | Part mass, including any finger mass carried by friction (kg) | — |
| g | Gravity | 9.81 m/s² |
| a | Robot acceleration | 2 – 10 m/s² |
| S | Safety factor | 2 – 4 |
| μ | Friction coefficient between finger and part | 0.1 – 0.2 steel on steel |
| n | Number of fingers | 2 |
Use a higher safety factor when the robot moves fast, when the part is oily, or when an emergency stop could cause a hard deceleration.
Worked example
A steel part of 2 kg is gripped with flat steel fingers. The robot accelerates at 5 m/s².
- m = 2 kg
- g + a = 9.81 + 5 = 14.81 m/s²
- S = 2
- μ = 0.2
- n = 2
F = 2 × 14.81 × 2 ÷ (0.2 × 2) = 59.24 ÷ 0.4 ≈ 148 N
The gripper must deliver at least 148 N at the actual finger length and air pressure in your cell.
Check the force at your finger length
Catalog grip force is usually given at a reference finger length. Longer fingers reduce the effective force and increase the moment load on the jaw guides. Check the gripper's force-versus-finger-length chart and keep fingers as short as possible.
Check the air pressure
Grip force scales with air pressure. If your factory air drops to 0.4 MPa during peak demand, size the gripper at 0.4 MPa, not 0.6 MPa. A spring-assisted (self-locking) gripper adds holding force and keeps the part if pressure is lost.
Summary
- Prefer form-fit fingers.
- For friction grip, use F = m × (g + a) × S ÷ (μ × n).
- Check the force at your real finger length and minimum air pressure.
- Confirm the stroke per jaw covers the opening you need.
Not sure about your numbers? Send us the part drawing and robot model, and our engineers will size the gripper for you.