The Vacuum That Lost Grip After Lunch: Degradation Over a Shift
A vacuum gripper running a 3-shift cell. Morning shift: perfect. Afternoon shift: dropping parts. Night shift: nothing holds. The customer thought the cups were worn. We measured the vacuum at 9 AM, 1 PM, and 9 PM. The vacuum level dropped steadily through the day. The cups were fine. The air supply wasn’t.
9 AM: baseline
The ejector draws -85 kPa at the cups. The vacuum switch (set at -60 kPa) confirms grip every cycle. The parts (plastic trays, 0.5 kg) are picked and placed without drops. The compressor starts at 5 AM, by 9 AM the system is at 6.5 bar. The dryer is cold and working.
1 PM: the first drops
We measure again. The vacuum is -65 kPa. The vacuum switch is right at its threshold. Some picks (especially with slightly dirty cups) dip to -55 kPa — below the switch threshold. The robot retries. About 2% of cycles have a retry. The customer doesn’t notice yet.
What changed? The plant’s air demand increased. Two other machines started up at 11 AM. The pressure dropped from 6.5 bar to 5.2 bar at the FRL. The ejector’s vacuum is proportional to supply pressure. At 6.5 bar, the ejector draws -85 kPa. At 5.2 bar, it draws -68 kPa. The vacuum margin shrank.
9 PM: the failure
Night shift. The plant’s other machines are off, but the compressor is now hot. The dryer’s refrigeration capacity drops as the ambient temperature rises (the compressor room is 35°C). The air is wet. The water in the line reaches the ejector. The ejector’s nozzle (a 0.5 mm orifice) partially clogs with moisture. The vacuum drops to -45 kPa. The vacuum switch doesn’t confirm. Parts drop.
What we found
Three problems, layered:
Supply pressure fluctuates. The ejector needs 5.5 bar minimum. The plant’s air system drops to 4.8 bar at peak demand. We added a dedicated reservoir (20 L) and a regulator set to 6 bar, dedicated to the vacuum gripper. The gripper doesn’t share air with the rest of the plant.
Water in the line. The dryer can’t keep up in a hot compressor room. We installed a separate refrigerated dryer for the vacuum system, plus an auto-drain filter. The water stops reaching the ejector.
The ejector was undersized. A small ejector (10 Nl/min air consumption) draws -85 kPa at 6 bar, but drops off sharply at lower supply pressure. We upsized to a larger ejector (20 Nl/min) that maintains -75 kPa even at 4.5 bar supply. The larger ejector uses more air, but it’s stable across the pressure range.
The vacuum stability table
| Supply pressure | Small ejector (-kPa) | Large ejector (-kPa) |
|---|---|---|
| 6.5 bar | 85 | 90 |
| 5.5 bar | 70 | 82 |
| 4.5 bar | 45 | 72 |
| 3.5 bar | 0 (no vacuum) | 55 |
The monitoring that prevents this
We added a vacuum transducer (analog 0–10 V) on the gripper. The PLC logs the vacuum level every cycle. If the vacuum drops below -65 kPa (even if the switch still confirms), the PLC alarms. The customer sees the trend before the drops start. They can service the dryer or adjust the regulator before night shift.
The bottom line
A vacuum gripper that works in the morning and fails at night isn’t a cup problem. It’s an air supply problem. The ejector’s vacuum depends on supply pressure, and the supply pressure fluctuates through the day. Dedicated air, dried, regulated, and monitored. The cups lasted 3 years. The air system needed work after 3 months.
The vacuum rule: The ejector is a precision nozzle. It needs clean, dry, regulated air. If the gripper drops parts late in the shift, measure the supply pressure at the time of failure. Don’t replace cups that aren’t worn.
