A vacuum gripper that dropped parts on a pick-and-place machine. The gripper had two Ø50 mm suction cups. The part was a flat steel plate, 2 kg. The ejector was a small Venturi type, rated for 80 L/min vacuum flow. At 6 bar supply, it pulled 60% vacuum. The cycle was: pick (0.5 s), lift (0.5 s), move (1 s), place (0.3 s). The part dropped during the move cycle. The customer thought the cups were worn. They were fine. The ejector was undersized for the volume — and there was no vacuum reservoir. This is about vacuum gripper sizing and ejector selection.
The vacuum holding force
The holding force of a suction cup depends on the vacuum level and the cup area:
F = P_vac × A_cup × n
Where P_vac is the vacuum pressure (bar absolute difference from atmosphere) and A_cup is the effective area. For a Ø50 mm cup at 60% vacuum (0.6 bar): A = π × 25² = 1963 mm² = 0.00196 m². F = 60,000 N/m² × 0.00196 = 118 N per cup. Two cups: 236 N. The 2 kg plate weighs 20 N. The safety factor is 12x. The holding force is fine. The problem isn’t the holding force — it’s the response time.
The response time
The ejector must pull the system to vacuum fast enough to grip the part before the robot moves. The volume to evacuate is: cup volume + hose volume + ejector internal volume. For two Ø50 cups (10 mm deep each: V_cup = 2 × 1963 × 10 = 39,260 mm³ = 39 mL) plus 2 m of Ø6 hose (V_hose = π × 3² × 2000 = 56,549 mm³ = 57 mL). Total volume: 96 mL. The ejector flows 80 L/min = 1333 mL/s. Time to pull 60% vacuum: t = V × ln(P_atm / P_target) / Q = 96 × ln(1/0.4) / 1333 = 96 × 0.916 / 1333 = 0.066 s. That’s fast. So why did the part drop?
Because the supply pressure dropped. The machine has 6 bar at the compressor, but the ejector is 5 m from the compressor. The tubing is Ø6 mm. When the ejector fires, it draws 200 L/min of compressed air. The pressure at the ejector drops to 3.5 bar. At 3.5 bar supply, the vacuum level drops to 40%. The holding force drops to 157 N (still 8x the 20 N weight). But the cups leak slightly (the steel plate has a rough surface). At 40% vacuum with a leak, the vacuum decays during the 1-second move. By the time the robot reaches the place position, the vacuum is 20%. The part drops.
What was changed
1. Up-sized the supply tubing to Ø10 mm. The pressure at the ejector stayed at 5.5 bar during operation. The vacuum level recovered to 55%. The part held through the move.
2. Added a vacuum reservoir. A 0.5 L reservoir mounted near the ejector provides surge flow. When the ejector fires, the reservoir supplies the initial vacuum volume. The vacuum reaches 60% in 0.1 seconds instead of 0.3. The part is gripped before the robot moves.
3. Added a vacuum switch. A pressure switch set at 40% vacuum confirms the part is gripped before the robot moves. If the vacuum doesn’t reach 40% within 0.5 seconds, the alarm triggers and the robot doesn’t move. The dropped parts stopped. The robot only moves when the part is confirmed held.
The ejector type choice
| Ejector type | Vacuum flow | Air consumption | Use for |
|---|---|---|---|
| Single Venturi (small) | 20-80 L/min | 10-20 Nl/min | Small cups, light parts |
| Multi-stage Venturi | 100-300 L/min | 30-50 Nl/min | Large cups, porous parts |
| Vacuum pump (electric) | 100-1000 L/min | No compressed air | High flow, multiple grippers |
The ejector sizing I use: calculate the volume to evacuate, then size the ejector to pull 60% vacuum in under 0.3 seconds. The dropped part wasn’t worn cups — it was supply pressure drop at the ejector. Use larger supply tubing, add a reservoir, and confirm vacuum with a switch before moving. For multi-gripper systems, an electric vacuum pump beats multiple Venturis.
