Picking up a workpiece with vacuum pads means deciding the pad size and count, the vacuum level, and the ejector (or vacuum pump) size. The first two come from force, the last from time. This guide follows the formulas and selection example in SMC's «Vacuum Equipment Model Selection» (PDFs on SMC's official site).
Lifting force = vacuum × pad area ÷ safety factor
W (N) = P (kPa) × S (cm²) × 0.1 × 1/t P vacuum (magnitude: 60 for −60 kPa) S pad area = π/4 × (diameter in cm)² t safety factor — 4 or more horizontal, 8 or more vertical ø40 pad (12.57 cm²), −60 kPa: theoretical force 60 × 12.57 × 0.1 = 75.4 N horizontal (÷ 4) 18.8 N (about 1.9 kg) vertical (÷ 8) 9.4 N (about 0.96 kg)
The 0.1 converts kPa × cm² to newtons (1 kPa × 1 cm² = 0.1 N). SMC calls pressure × area without a safety factor the «theoretical lifting force»; it is a static value, so SMC says to divide by a safety factor in practice.
| Pad | Area cm² | −40 kPa | −60 kPa | −80 kPa |
|---|---|---|---|---|
| ø10 | 0.79 | 3.1 N | 4.7 N | 6.3 N |
| ø20 | 3.14 | 12.6 N | 18.8 N | 25.1 N |
| ø32 | 8.04 | 32.2 N | 48.3 N | 64.3 N |
| ø40 | 12.57 | 50.3 N | 75.4 N | 100.5 N |
| ø50 | 19.63 | 78.5 N | 117.8 N | 157.1 N |
| ø80 | 50.27 | 201.1 N | 301.6 N | 402.1 N |
The table is theoretical force with no safety factor. SMC's theoretical lifting force table truncates or rounds the last digit (e.g. ø40 at −60 kPa: 75 N), so the last digit differs slightly. Doubling the vacuum doubles the force; doubling the diameter quadruples it.
Safety factors 4 and 8 — why so large
- Theoretical force is static. Acceleration, air flow and impact while moving add to it.
- Pads resist forces parallel to the suction face (shear) and moments poorly. If the centre of gravity is off the pad, one edge lifts.
- Vertical lifting (gripping a side face) relies on friction alone; SMC says to avoid it where possible and use a safety factor of 8 or more if you must.
- Porous or rough workpieces leak and lower the vacuum. Calculate with the lowered vacuum.
Choosing a pad size — an example
A 5 kg plate (49.0 N), four pads, horizontal lifting, −60 kPa from an ejector (the value SMC gives as a guide for ejectors), safety factor 4:
Area per pad S = W × t ÷ (P × 0.1 × pads)
= 49.0 × 4 ÷ (60 × 0.1 × 4) = 8.17 cm²
Diameter = √(4 × 8.17 ÷ π) × 10 = 32.3 mm → ø32 is short, use ø40
With 4 × ø40: lifting force 75.4 N (after ÷ 4) — −39 kPa would be enough
Gripping the same plate by its side (t = 8): 45.6 mm → ø50SMC notes that unnecessarily high vacuum wears and cracks pads sooner, lengthens response time and costs more energy. If force is short, increase pad size or count before raising the vacuum.
SMC's selection example — a semiconductor chip
Chip 1 g = 0.0098 N, one ø4 pad (0.13 cm²), horizontal t = 4 0.0098 = P × 0.13 × 0.1 × 1/4 → P = 3.0 kPa → −3.0 kPa or more holds it
Light workpieces can be held at very low vacuum. In that case it is response time, not force, that sizes the ejector.
Adsorption response time — pipe volume ÷ suction flow
Pipe volume V (L) = π/4 × D (mm)² × L (m) ÷ 1000
Response time T1 = V × 60 ÷ Q (63 % of final vacuum)
T2 = 3 × T1 (95 %)
Flow needed Q = V × 60 ÷ T1 + QL (QL leakage)
Ejector max. suction flow = (2–3) × Q
Ejector average suction flow Q1 = max × (1/2–1/3), pipe max. flow Q2 = C × 55.5
Q = smaller of Q1 and Q2SMC's example holds the chip through a 2 mm ID, 1 m tube. V = π/4 × 2² × 1 ÷ 1000 = 0.0031 L; for 300 ms, Q = 0.0031 × 60 ÷ 0.3 = 0.62 L/min, so an ejector with 1.24–1.86 L/min or more max. suction flow will do. The 5 L/min ejector actually chosen gives Q1 ≈ 1.7, and the tube's conductance 0.22 gives Q2 = 12.2, so with the smaller 1.7, T1 = 0.109 s (109 ms) — within 300 ms.
| Tubing | Volume V | Ejector max. 20 L/min (Q1 6.7–10) | T2 (95 %) |
|---|---|---|---|
| 4 mm ID × 1 m | 0.0126 L | T1 75–113 ms | 226–339 ms |
| 4 mm ID × 2 m | 0.0251 L | T1 151–226 ms | 452–679 ms |
| 6 mm ID × 2 m | 0.0565 L | T1 339–509 ms | 1.02–1.53 s |
These assume the pipe conductance (Q2) is larger than Q1. With the same ejector, longer and larger tubing means more volume and a slower response — which is why ejectors are mounted close to the pads. If the workpiece leaks, add the leakage QL (= 55.5 × leak conductance) and size the ejector up.
In PortDock
- «Vacuum Pad Lifting Force Calculator» web tool: pad diameter, count, vacuum, safety factor and mass → lifting force and verdict, vacuum or pad size needed (SMC sizes), pipe volume, required flow and response times T1, T2.
- «Pneumatic Cylinder Force Calculator» web tool: the force of the cylinder raising and lowering the pads.
- Serial Plotter and Modbus Master: log a vacuum pressure sensor (analog → Modbus) to see the stable vacuum after adsorption and the response time on a graph.
Published 2026-10-24