The Vacuum That Took Too Long to Grip
We used a vacuum ejector (article 73) with a suction cup (article 113) to lift a metal panel. The vacuum switch (article 115) confirmed the grip. But the cycle was slow — the ejector took 1.5 seconds to pull the vacuum. The robot waited (for the vacuum switch) before moving. The takt time (article 114, 120) was too long. The problem: the ejector was too small for the volume (the cup + the hose + the reservoir). The ejector’s flow was low, so it took long to pull the vacuum. We upsized the ejector (higher flow) and shortened the hose. The evacuation time dropped to 0.3 s. The mistake was choosing the ejector by the holding force, not by the evacuation time.
Vacuum evacuation time is how long the ejector takes to pull the vacuum. This article covers the calculation.
Why Evacuation Time Matters
The ejector pulls air out of the cup (and the hose, and the reservoir). The vacuum builds over time. The robot waits for the vacuum switch (article 115) to confirm the grip. If the evacuation is slow, the takt time (article 120) suffers. In a fast cycle (0.5 s move), a 1.5 s evacuation is the bottleneck.
The Evacuation Time Formula
The time to pull the vacuum (from atmospheric to the setpoint) is roughly:
t = V_eff / Q_ejector
Where t is the time (s), V_eff is the effective volume (L) being evacuated (the cup + hose + reservoir), and Q_ejector is the ejector’s effective flow (L/min, at the working vacuum).
For V_eff = 0.5 L (a 50 mm cup + 1 m hose), Q_ejector = 30 L/min: t = 0.5 / 30 × 60 = 1.0 s. That’s slow. For Q_ejector = 100 L/min: t = 0.5 / 100 × 60 = 0.3 s. Fast.
The evacuation rule: Size the ejector by the evacuation time, not just the holding force. The vacuum that took 1.5 s had a small ejector on a 0.5 L volume. t = V/Q. Upsize the ejector (higher Q) or shorten the hose (smaller V). For a fast cycle, aim for t under 0.3–0.5 s.
Step 1: The Effective Volume
The volume being evacuated includes:
- The cup: A 50 mm cup holds about 0.02 L (small).
- The hose: A 6 mm ID hose, 1 m long: V = π × 0.3² × 1000 = 0.28 L. The hose is the big volume.
- The reservoir (article 128): A reservoir adds volume (it cushions leaks but slows evacuation).
Shorten the hose (less volume). Use a smaller ID hose (less volume). Mount the ejector close to the cup (article 73) — the hose is short. The ejector at the cup (not at the manifold) cuts the volume.
Step 2: The Ejector’s Effective Flow
An ejector’s spec lists the “standard flow” (at atmospheric pressure). But at the working vacuum (e.g., -60 kPa), the flow is lower (the ejector’s flow drops as the vacuum builds). Use the ejector’s curve (flow vs vacuum). At -60 kPa, the effective flow is maybe 60% of the standard flow. Size for the working point.
Step 3: The Vacuum Switch Setpoint
The vacuum switch (article 115) triggers at a setpoint (e.g., -50 kPa). The time to reach -50 kPa is what matters (not the time to reach max vacuum). Set the switch at a level the ejector reaches quickly (not at the max vacuum). If the switch triggers at -50 kPa (reached in 0.3 s), the robot moves in 0.3 s. If it triggers at -80 kPa (reached in 1.5 s), the robot waits. Don’t over-set the switch.
Step 4: Leaks (Article 128)
A leaky cup (or fitting) slows the evacuation (the ejector has to pull air faster than it leaks). A clean, well-sealed system evacuates fast. A leaky one never reaches the setpoint (or takes long). Fix leaks (article 128).
| Volume | Typical Size |
|---|---|
| Cup (Ø50) | 0.02 L |
| Hose (6 mm ID, 1 m) | 0.28 L |
| Reservoir | 0.5–2 L |
| Effective total (typical) | 0.3–0.5 L (without reservoir) |
An Evacuation Time Checklist
- What is the effective volume? (Cup + hose + reservoir?)
- What is the ejector’s flow at the working vacuum?
- t = V / Q? (Under 0.5 s?)
- Is the hose short? (Mount ejector close?)
- Is the hose ID small? (Less volume?)
- Is the vacuum switch set right? (Not at max vacuum?)
- Are there leaks? (Article 128?)
- Is the reservoir needed? (Adds volume but cushions?)
- Does the cycle time include the evacuation? (Article 120?)
- Is the ejector upsized? (Test?)
The Bottom Line
Vacuum evacuation time is V/Q. The vacuum that took 1.5 s had a small ejector on a big volume. Shorten the hose (less V), upsize the ejector (more Q), and set the vacuum switch at a reachable level. Fix leaks. For a fast cycle, aim for under 0.5 s. The gripper that gripped fast wasn’t the biggest ejector — it had a small volume and a matched flow.
