The Vacuum That Kept Losing Hold
We picked a small vacuum ejector (venturi) for a suction cup picking cardboard boxes. The ejector was rated for -80 kPa. On the bench, it held. On the line, the boxes had small leaks (corrugated cardboard leaks). The vacuum dropped, and the box slipped. The ejector’s flow rate was too small to compensate for the leak. We upsized to a larger ejector (higher flow) and added a vacuum reservoir tank. The vacuum held. The mistake was sizing the ejector for the holding force but not for the leak rate through the porous part. The box holds with a small force, but the leak requires flow to maintain vacuum.
Vacuum system selection has two questions: how much holding force (from the suction cup area) and how much flow (to compensate for leaks). This article covers ejectors vs. pumps.
Two Sources: Ejector vs. Pump
Vacuum Ejector (Venturi)
Uses compressed air to generate vacuum (the venturi effect). No moving parts. Small, light, mounted on the robot wrist. But it consumes compressed air (noisy, energy-wasting). Best for short cycles (pick and release quickly).
Best for: Small-to-medium cups, robot-mounted, short cycles, clean dry parts.
Vacuum Pump (Electric)
An electric pump (diaphragm, rotary vane, or claw) generates vacuum. Placed in a fixed location (not on the robot). Continuous vacuum (doesn’t consume air). Quieter. But heavier, needs a power connection, and hoses run to the robot.
Best for: Large cups, long holding times, porous parts (high leak rate), continuous cycles.
| Type | Vacuum Level | Flow | Cost | Best For |
|---|---|---|---|---|
| Ejector (small) | -80 to -90 kPa | 5–20 NL/min | Low | Small cups, short cycles |
| Ejector (large) | -80 kPa | 30–100 NL/min | Medium | Medium cups, porous parts |
| Diaphragm pump | -80 to -90 kPa | 10–50 NL/min | Medium | Fixed location, continuous |
| Rotary vane pump | -90 to -95 kPa | 50–200 NL/min | High | Large cups, high flow |
| Claw pump (oil-free) | -85 kPa | 100–500 NL/min | High | Multiple cups, high leak |
Step 1: Holding Force (Suction Cup Size)
The holding force depends on the cup area and the vacuum level.
F = ΔP × A
Where ΔP is the pressure difference (atmospheric minus vacuum, in N/m²), and A is the cup area (m²).
For a -60 kPa vacuum (typical for cardboard) and a Ø50 mm cup (A = π × 0.025² = 0.00196 m²): F = 60,000 × 0.00196 = 118 N. With a safety factor of 4 (for vertical holding, acceleration), the effective holding is 118/4 = 29 N. That holds about 3 kg vertically.
For a heavier part, use multiple cups or a larger cup. Two Ø50 mm cups hold ~6 kg. A Ø100 mm cup (4× the area) holds ~12 kg.
Step 2: Leak Rate (Flow Demand)
The holding force is one question. The other is the flow needed to maintain the vacuum when the part leaks.
Porous Parts (Cardboard, Wood, Foam)
Porous materials leak. Air seeps through the material. The vacuum source must supply flow to compensate. A tight (non-porous) part needs no flow (just initial evacuation). A cardboard box needs continuous flow.
The required flow depends on the material:
- Non-porous (glass, metal, smooth plastic): Negligible leak. Small ejector works.
- Semi-porous (cardboard, wood): Moderate leak. Need 20–50 NL/min flow.
- Porous (foam, particle board): High leak. Need 100+ NL/min. Use a pump, not an ejector.
Leaks at the Seal
If the cup doesn’t seal perfectly (rough surface, dust, gaps), air leaks around the edge. The vacuum source must compensate. A soft (foam) cup seals better on rough surfaces than a rubber cup.
The vacuum selection rule: Size the cup for the holding force (F = ΔP × A, with 4× safety). Size the pump/ejector for the leak rate (non-porous = small flow, cardboard = medium, foam = high flow). The ejector that lost hold on cardboard had enough holding force but not enough flow to compensate the leak. Add flow (larger ejector or a pump) and a reservoir tank.
Step 3: Vacuum Reservoir Tank
A small tank (0.5–2 liters) between the vacuum source and the cups acts as a buffer. The tank holds a volume of vacuum. When the cup picks a leaking part, the tank supplies the initial flow (so the vacuum doesn’t drop instantly). The vacuum source then re-evacuates the tank between cycles.
For a short pick (1–2 seconds), the tank provides enough vacuum. The source only needs to recover between cycles. This reduces the required flow size (and cost).
Step 4: Vacuum Switch and Sensor
Install a vacuum switch (or pressure sensor) in the line. It signals the PLC when the vacuum reaches the setpoint (part picked) or when it drops below (part dropped or leak).
Don’t assume the part is held. The vacuum sensor confirms it. If the vacuum doesn’t reach -50 kPa within 0.5 seconds, no part was picked. The robot knows.
Cup Selection
The suction cup material and shape depend on the part.
- NBR (nitrile rubber): General purpose. Oil-resistant. For metal, plastic, cardboard.
- Silicone: Heat-resistant. For hot parts (ovens, baking). Also food-grade.
- Polyurethane (PU): Abrasion-resistant. For rough surfaces (sheet metal, textured plastic).
- Foam (sponge): Seals on rough or uneven surfaces (corrugated, textured). Best for cardboard.
Shape: flat cups for flat parts. Bellows (accordion) cups for uneven or angled surfaces (they compensate for height variation).
Energy: Ejector Air Consumption
An ejector consumes compressed air continuously while active. A typical ejector uses 10–30 NL/min at 6 bar. If it runs 50% of the time, that’s 5–15 NL/min of air. Over a year, this adds up in compressed air costs.
For long holding times (the robot holds the part for 10 seconds while another operation happens), an electric pump is cheaper (it doesn’t consume air). For short picks (grab and go in 1 second), an ejector is fine (the air consumption is brief).
A Vacuum System Checklist
- What is the part weight? (kg)
- What is the part surface? (Smooth, rough, porous?)
- Cup size and number? (F = ΔP × A, with 4× safety.)
- What vacuum level? (-60 kPa for cardboard, -85 kPa for smooth.)
- Is the part porous? (Leak rate: low, medium, high?)
- Ejector or pump? (Short cycle = ejector. High leak = pump.)
- Is a reservoir tank needed? (For leaks.)
- Is there a vacuum switch? (Part present detection?)
- Cup material? (NBR, silicone, PU, foam?)
- Cup shape? (Flat or bellows?)
- Is the ejector mounted near the cups? (Minimize hose length.)
- Is there a filter? (Protect the ejector from dust.)
The Bottom Line
Vacuum system selection is two calculations: holding force (cup area × vacuum) and flow (leak compensation). The ejector that lost hold on cardboard had enough holding force but not enough flow for the porous leak. Use a larger ejector (or a pump) for porous parts, add a reservoir tank, and put a vacuum switch in the line. The vacuum that holds every part, every cycle, wasn’t the strongest vacuum — it had enough flow to compensate for the leak.
