The Cup That Leaked on a Round Can: Vacuum Cup Geometry, Not Size
A flat vacuum cup (Ø50 mm) trying to lift a round aluminum can. The cup flattened on the top of the can but leaked around the edges — the curved surface meant only the center of the cup sealed. The vacuum dropped to -30 kPa. The can tipped. We tried a larger cup (Ø75 mm). It leaked worse. The problem was the cup shape, not the size.
Flat cup vs curved surface
A flat vacuum cup seals against a flat surface. The cup’s lip lies flat. The entire cup opening contacts the surface. The vacuum holds.
A round can has a curved surface. A flat cup pressed against a curve contacts only at the center. The edges lift off. Air leaks in. The vacuum can’t build. A larger flat cup doesn’t help — the edges lift off more, not less.
The cup types for curved surfaces
| Cup type | Shape | Best for |
|---|---|---|
| Flat | Flat lip | Flat sheets, machined plates, glass |
| Bellows (1.5 folds) | 1–2 folds, conforms to curves | Curved surfaces, uneven parts |
| Bellows (3.5 folds) | Deep folds, high compliance | Highly curved, irregular shapes |
| Oval (long side) | Elongated footprint | Cans, tubes, round parts |
| Special (custom profile) | Machined to the part | Precision parts, high-value items |
What we used
We switched to an oval cup, 2 × Ø40 mm (an elongated footprint). The long axis matched the curvature of the can. The cup contacted the can along a line — not a point. The seal held. The vacuum built to -75 kPa. The can lifted without leaking.
For a 330 mm diameter can, the oval cup (40 × 80 mm) wrapped across the curvature. The bellows version (1.5 folds) would also work — the folds compress to match the curve. We chose the oval because it’s more rigid (better for robot positioning) and cheaper than a bellows.
The holding force check
For a 0.5 kg aluminum can, the holding force needed is:
F = m · g / μ = 0.5 × 9.8 / 0.4 = 12.3 N
The oval cup at -75 kPa has an effective area of about 2000 mm². F = 0.075 × 2000 × 0.4 = 60 N. Five times the required. The seal was good; the cup was more than enough.
The other factor: cup height
For a round can, the robot must position the cup so it contacts the can squarely. If the cup is off-center, it contacts the curve at an angle and leaks. We added a compliance compensator (a floating mount between the cup and the tool) that lets the cup self-align to the surface. The compensator has 5 mm of float in all directions. The cup lands squarely even if the robot is slightly off.
The spec sheet rule
When selecting a vacuum cup, match the cup shape to the part geometry, not just the part weight. A flat cup on a curved part leaks no matter how big it is. A bellows or oval cup on a flat part works but wastes vacuum (the folds don’t fully compress). Match the shape. The size is secondary — get the shape right, then size for the weight.
The cup rule: Flat surface = flat cup. Curved surface = bellows or oval. If the cup leaks on a round part, don’t go bigger — go shaped. A custom-machined cup profile is the last resort, but an off-the-shelf bellows or oval solves 90% of curved-surface picks.
