You spec a vacuum gripper by how much vacuum it pulls: 85 kPa, deep vacuum, good for sheet metal. On the first run, the cups leak, the pump cycles constantly, and the sheet slips. You sized the vacuum level right. You sized the flow wrong.
Vacuum level holds; flow fills
Vacuum level (the negative pressure in kPa or inHg) determines how hard the cup pulls against the part. Flow (the volume of air the pump moves) determines how quickly the cup seals and how well it handles leaks. These are two different parameters, and they’re specified by different pump technologies.
A venturi vacuum generator pulls high vacuum (80 to 90 kPa) but low flow. It’s fast to react and compact, but it can’t sustain a leak. If the part has a rough surface, a porous gasket, or a slightly damaged cup, the venturi can’t keep up and the vacuum drops.
A rotary vane or dry pump pulls lower vacuum (60 to 70 kPa typically) but high flow. It fills volume fast and handles leaks. It’s bigger, noisier, and costs more, but it won’t drop vacuum on a textured part.
When high vacuum is the wrong choice
Porous parts (cardboard, wood, molded composites) don’t need deep vacuum. They leak air continuously. A high-vacuum venturi on cardboard will cycle the pump constantly and still lose grip. You need flow: a pump that keeps replacing the air that leaks through the part.
Smooth non-porous parts (glass, sheet metal, plastic sheets) need deep vacuum to seal thin cups and hold flat surfaces. Here the venturi shines because there’s no leak to compensate for. Pick the technology based on the part surface, not the spec sheet.
Cup size is not just load rating
A cup’s load rating is based on vacuum level and effective area. But a larger cup also has more volume to fill. On a venturi system, a cup that’s too large takes longer to evacuate, which adds cycle time. On a pump system, the volume matters less because the pump has flow.
Match the cup to the part geometry, not just the weight. A flat sheet needs a flat suction cup. A curved part needs a bellows cup that conforms to the surface. A bellows cup adds compliance but also adds volume and reduces effective holding area.
The reservoir
A vacuum reservoir tank between the pump and the gripper smooths out pressure drops during pick and place. When the cup hits the part and draws down, the reservoir supplies volume instantly instead of waiting for the pump to catch up. This shortens the pick time and reduces pump cycling.
Size the reservoir for the total volume of the cups plus the plumbing. A reservoir that’s too small doesn’t help. One that’s too large takes too long to pull down at startup. One to two times the cup volume is a reasonable starting point.
Leaks are the design condition
Every vacuum system leaks. The question is how much you design for. A system on a clean, smooth part with new cups leaks almost nothing. A system on a machined casting with a rough surface and a worn cup leaks constantly. Size the pump for the worst-case leak, not the ideal condition.
If the pump cycles more than once per second on a steady pick, it’s undersized for the leak rate. Add flow, not more vacuum.
Bottom line
Specify a vacuum gripper by both vacuum level and flow rate, not just the number on the pump catalog. Match the pump technology to the part surface: venturi for smooth non-porous parts, pump for porous or leaky ones. Add a reservoir. Design for the leak, not the perfect seal.
