The Cup That Leaked on a Round Can
We used a flat suction cup (article 113) to pick up cylindrical cans. The flat cup sat on the curved surface. The seal was bad — only the edges touched. The vacuum leaked. The can slipped. The problem: a flat cup on a curved surface doesn’t seal (the surface curves away from the flat cup face). We switched to a bellows (accordion) cup. The bellows flexes to conform to the curve. The seal held. The mistake was using a flat cup on a curved surface.
Vacuum cup on curved surfaces needs a conformable cup (bellows), not a flat one. This article covers the choices.
The Cup Shapes
Flat Cup
A flat (disc) cup seals on a flat, smooth surface. Rigid, good for flat parts (sheets, boxes, flat panels). Doesn’t conform to curves.
Bellows (Accordion) Cup
A bellows cup has folds (like an accordion). It compresses and conforms to curved surfaces (cylinders, spheres). It also cushions (the bellows absorbs the Z move). Good for curved or uneven parts.
Oval Cup
An oval (elliptical) cup seals on cylindrical parts along the axis. It wraps around the cylinder (better contact than a round flat cup). For long cylinders (tubes, pipes).
| Cup Shape | Surface | Best For |
|---|---|---|
| Flat (disc) | Flat, smooth | Sheets, boxes, flat panels |
| Bellows (accordion) | Curved, uneven | Cylinders, spheres, uneven parts |
| Oval (elliptical) | Cylindrical (along axis) | Tubes, pipes, long cylinders |
Step 1: Match the Cup to the Curvature
For a cylindrical part (a can, a tube), the cup must wrap around the curve. A flat cup contacts only at the edges (leak). A bellows cup (1.5 or 2.5 folds) flexes to match the radius. An oval cup (along the cylinder axis) gives a long contact line.
For a sphere (a ball), a small bellows cup (or a custom cup) conforms. A flat cup won’t seal on a sphere.
The curved-surface rule: Don’t use a flat cup on a curve. The cup that leaked on the can was flat. Use a bellows cup (flexes to the radius) or an oval cup (along the cylinder axis). The cup must conform to the surface geometry.
Step 2: Cup Diameter and Radius
The cup’s diameter must match the part’s radius. A large cup on a small-radius cylinder won’t seal (the cup is too big — it doesn’t wrap). A small cup on a large radius is fine (it nearly seals). For a cylinder of radius R, use a cup with a diameter under 2R (so the cup fits the curve). For a 50 mm diameter cylinder (R = 25 mm), use a cup under 50 mm diameter.
Step 3: Bellows Cushion (Z Travel)
A bellows cup compresses (it has Z travel). This is useful for:
- Cushioning: The bellows absorbs the Z move (the cup touches the part gently, no slam).
- Height variation: Parts of varying height (stacked sheets) — the bellows compresses to take up the difference.
But the bellows adds spring-back (the cup pushes off the part). For a heavy part, the bellows pushes it off (the cup doesn’t hold). Use a flat cup (rigid) for heavy, flat parts. Use a bellows for light, curved parts.
Step 4: Material (Article 113)
For curved surfaces (especially in food/pharma), the cup material matters. NBR (standard) for general. Silicone for food/high temp. The material must grip (not slip) on the curved surface.
A Curved-Surface Cup Checklist
- Is the surface flat or curved?
- If curved: bellows or oval cup?
- Is the cup diameter under 2× the part radius?
- Does the bellows compress enough? (Z travel?)
- Is the part heavy? (Bellows spring-back?)
- Does the cup seal? (Block the cup, test the vacuum?)
- Is the material right? (Food? Oil?)
- Does the cup slip on the curve? (Test?)
- Is there enough vacuum? (Article 128?)
- Is the vacuum switch set? (Article 115?)
The Bottom Line
Vacuum cup on curved surfaces uses a conformable cup. The cup that leaked on the can was flat (didn’t wrap). Use a bellows cup (flexes to the radius) or an oval cup (along the cylinder axis). Keep the cup diameter under 2× the part radius. For heavy flat parts, stay with a rigid flat cup. The cup that held the round can wasn’t the biggest one — it was bellows.
