How does vacuum pick-and-place work?
A gripper presses suction cups or a foam pad onto a part and removes air from the space inside the seal. Atmospheric pressure outside then pushes the part against the cup. The holding force is roughly the pressure difference multiplied by the effective sealed area, reduced by safety factors for acceleration, orientation and friction. When the cycle ends, vacuum is released, often with a short blow-off pulse, so the part drops cleanly.
Vacuum can be made in two ways:
- Compressed-air ejectors (venturi generators): compressed air flows through a nozzle and entrains air from the vacuum port. They are compact, fast and fitted close to the cups, but they consume compressed air whenever they are on.
- Electric vacuum pumps or blowers: rotary vane, claw, side-channel or liquid-ring machines that serve a central or machine-mounted vacuum line. They use electrical power directly and suit continuous or high-flow duties such as porous parts.
The choice matters to the compressor room. A press line or packaging hall with dozens of ejectors can become one of the largest compressed-air users in the plant.
Vacuum terms that cause confusion
Vacuum is quoted in many ways: negative gauge pressure (for example −0.6 bar), percentage vacuum, kPa absolute, or inches or millimetres of mercury. Make sure everyone on the project uses the same reference. Ejector catalogues also quote suction flow and air consumption at a stated feed pressure, often around 5–6 bar (72.5–87 psig); flows are usually in normal litres per minute.
| Term | Meaning | Why it matters |
|---|---|---|
| Vacuum level | Pressure difference below atmosphere | Sets holding force for a given cup area |
| Suction flow | Air the generator can remove | Sets evacuation time and leakage tolerance |
| Air consumption | Compressed air the ejector uses | Sets the load on the compressor |
| Feed pressure | Ejector inlet pressure while running | Ejector performance falls sharply if it sags |
| Evacuation time | Time to reach working vacuum | Sets cycle time |
Higher vacuum is not always better. Deep vacuum on a flexible part can distort it, mark it or slow release, and many ejectors trade suction flow for depth. The right level is the one that holds the part with margin.
Sealed parts versus porous parts
The part surface decides the design more than anything else. Sealed parts such as sheet metal, glass, plastic panels and coated boxes let the cup reach vacuum quickly and hold it with little flow. Porous or leaky parts such as cardboard, sacks, wood, textiles, perforated sheets or textured castings let air through continuously, so the generator must keep moving air just to maintain vacuum.
- For sealed parts, an ejector with an air-saving function can shut off once vacuum is reached and restart only if vacuum falls, which cuts compressed-air use sharply.
- For porous parts, a high-flow ejector or an electric blower is usually needed, and the cups or foam must be matched to the leakage.
- Rough, curved or oily surfaces call for bellows cups, softer compounds or foam seals; worn cup lips are a major leak source.
Holding force and dynamic loads
The theoretical force from vacuum times area is only the starting point. A robot accelerating a part sideways, rotating it or carrying it vertically with cups on its face loads the seal in shear and peel, which is far weaker than a straight pull. Gripper makers publish selection methods with safety factors for these cases; use them, and confirm with tests on the real part at full speed and at worst orientation.
- Weigh the part and identify the worst orientation and acceleration in the cycle.
- Choose cup number, size and layout so the load is shared and moments are small.
- Apply the gripper maker’s safety factor for the load case.
- Test with representative parts, including the worst surface condition, at full robot speed.
- Decide what happens if vacuum is lost: part drop zone, vacuum switch and safe stop.
Robot payload limits, guarding and functional safety are for the machine builder and the plant’s safety engineer to approve.
What an ejector costs in compressed air
An ejector’s compressed-air demand is its consumption at the feed pressure multiplied by the time it is on in each cycle, summed over all ejectors that can run together. Add the blow-off pulses. The table shows how to build the duty.
| Item | Value to collect | Source |
|---|---|---|
| Ejector consumption | Normal litres per minute at feed pressure | Ejector datasheet, confirmed by measurement |
| On-time per cycle | Seconds from pick to release | Machine cycle chart or signal trace |
| Air-saving behaviour | Off-time while holding sealed parts | Controller setting and trace |
| Blow-off | Pulse length and pressure | Valve setting |
| Concurrency | How many ejectors run at once | Line sequence |
Average demand sizes the compressor; peak concurrent demand sizes the pipework, valves and local receiver. Ejectors left running between cycles, or on machines that are idle but energised, are common hidden losses. Because compressed air is an expensive form of energy, a high-duty porous-part application often justifies an electric vacuum pump instead.
Feed pressure at the ejector
Ejector suction flow and vacuum depend directly on the feed pressure at the ejector inlet while it is running. If the supply to a manifold sags when several ejectors and cylinders fire together, vacuum falls and parts slip. Check:
- Filter-regulator flow class and droop at the combined peak flow
- Valve manifold and tubing bore; long, narrow tubing to ejectors is a frequent restriction
- A small local receiver close to the machine to cover simultaneous demand
- Clean, dry air: ejector nozzles are small and block with rust, water and oil; a 5 micron filter or finer is typical, and lubricated air should not be used
Reducing compressed-air use on vacuum lines
Once the duty is measured, savings usually come from control rather than new hardware. The steps below are typical and do not change the grip design.
- Use ejectors with air-saving (vacuum hold) control on sealed parts, so they switch off once vacuum is reached.
- Switch ejectors off when the machine is idle, in manual mode or waiting for parts.
- Keep blow-off pulses as short as reliable release allows.
- Set the ejector feed pressure to its optimum value from the datasheet; more pressure often adds air use without more vacuum.
- Replace worn cups and damaged tubing, which force ejectors to run continuously.
- Move high-duty porous-part stations to electric vacuum pumps or blowers.
Commissioning and fault-finding
Measure, don’t guess. Fit a vacuum gauge or transducer at the cup side and a pressure gauge at the ejector feed, and record them together with the cycle signals. A trace shows evacuation time, holding vacuum, leakage recovery, release and whether feed pressure dips.
| Symptom | Likely cause | Action |
|---|---|---|
| Slow pick-up | Low suction flow, large volume, long tubing | Move ejector closer, shorten tubing |
| Part slips during move | Feed pressure dips, worn cups, leakage | Log feed pressure, replace cups |
| Vacuum never reaches setpoint | Porous part, damaged hose, blocked silencer | Leak test, clean or replace |
| Part does not release | Weak or missing blow-off | Check release valve and pulse |
| Plant pressure falls when line runs | Ejector demand underestimated | Demand logging; air-saving or electric pump |
How Reddy Compressors helps
We look after the compressed-air side of vacuum handling. Our engineers log plant pressure and flow while packaging, press-shop or robot lines run, measure feed pressure at ejector manifolds, and identify whether the problem is distribution, treatment or compressor capacity. We supply new and used/refurbished compressors, dryers and filtration, and we sell, rent and service vacuum pumps and blowers (Roots, screw, centrifugal and side-channel) where a vacuum or low-pressure duty suits them better. All brands, including ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic; we are an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands.
From Tarnaka, Secunderabad, our 20+ service engineers and technicians cover plants across Telangana and Andhra Pradesh, and we supply equipment and spares across India. Cup and gripper selection stays with the machine builder; we make sure the air behind it is right.
What to send us
- Ejector make and model, number per machine and the datasheet consumption
- Cycle time, on-time per pick and how many machines run together
- Part type: sealed or porous, weight and surface
- Plant pressure and any trace of feed pressure at the manifold
- Compressor and dryer nameplates and the complaint (slipping, slow, pressure loss)
Frequently asked questions
How much compressed air does a vacuum ejector use?
It depends on the ejector size and feed pressure; datasheets quote consumption in normal litres per minute at a stated pressure, often around 5–6 bar. The real load on your compressor is that rate multiplied by the on-time in each cycle and the number of ejectors running together. Air-saving ejectors that switch off once vacuum is reached on sealed parts use much less than continuously running ones.
Should I use a vacuum ejector or an electric vacuum pump?
Ejectors suit compact grippers, fast cycles and sealed parts where they can switch off between picks. Electric vacuum pumps or blowers usually suit porous parts, continuous high-flow duty and central vacuum systems, because generating vacuum with compressed air is energy-intensive. Compare the real duty cycle and leakage of the part; on high-use lines the electric option often has the lower running cost.
Why does my vacuum gripper drop parts at high speed?
Common causes are feed pressure dipping at the ejector when several valves fire together, worn or wrong cups, porous or oily surfaces leaking, and acceleration loads in shear or peel exceeding the cup rating. Log vacuum at the cup and feed pressure at the ejector during the cycle. If feed pressure dips, fix the regulator, tubing or local receiver before changing grippers.
What air quality do vacuum ejectors need?
Ejectors have small nozzles that block with rust, water and oil, so they need clean, dry air, typically filtered to around 5 micron or finer and supplied from a dried system. Lubricated air should be avoided because oil collects in the nozzle and silencer. Check that the filter-regulator feeding the ejector manifold is sized for the combined peak flow, not just the port size.
Why does the plant air pressure drop when the packaging line runs?
Ejectors, blow-off and cylinders on a packaging or robot line can draw large peak flows together. If the compressor’s free air delivery, the pipework or the local supply cannot meet that peak, pressure sags. Demand logging shows whether it is a capacity, distribution or leak problem. Remedies include air-saving ejectors, a local receiver, larger supply lines, or switching high-duty stations to electric vacuum pumps.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Vacuum ejector compressed-air consumption
Average and peak compressed air used by vacuum ejectors from their rated motive-air flow, the on-time per cycle and blow-off pulses, for one gripper and for the whole line.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Ejector motive-air consumption at its feed pressure (Nl/min): 60
- Cycle time (s): 10
- Ejector on-time per cycle (s): 4
- Blow-off flow (Nl/min; 0 = none): 100
- Blow-off pulse per cycle (s): 0.2
- Number of ejectors on the line: 20
- Number of ejectors that can be on at the same moment: 20
- Compressor specific power (kW per m³/min; 0 = skip energy): 0
- Running hours per year of the line: 6000
Result
- Ejector duty: 40 % of the cycle (6 cycles a minute)
- Average motive air per gripper: 24 Nl/min
- Blow-off per gripper: 2 Nl/min (0.333 litres per cycle)
- Total per gripper: 26 Nl/min = 0.918 cfm
- Average for all 20 ejectors: 520 Nl/min = 18.36 cfm = 8.67 L/s = 0.52 m³/min
- Peak motive flow with 20 on together: 1,200 Nl/min = 42.4 cfm (2.3 × the average)
Method. Average motive air = rated consumption (Nl/min at the stated feed pressure) × on-time ÷ cycle time. Blow-off per cycle = blow-off flow × pulse ÷ 60 (litres); per minute = that × 60 ÷ cycle time. Total per gripper = motive + blow-off; line total = × number of ejectors. Peak = rated consumption × the number that can be on at once: it sizes the supply pipework, regulator and local receiver. 1 cfm = 28.317 l/min; normal litres are at reference conditions. Energy: line flow (m³/min) × specific power × hours. Air-saving (vacuum-hold) circuits reduce the on-time on sealed parts; use the measured on-time. Leaks are not included.
What to take from it. Average demand sizes the compressor, peak concurrency sizes the pipework and local receiver; on porous, high-duty stations an electric vacuum pump or blower usually costs less to run.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Vacuum holding force
Holding force of suction cups or a vacuum table from the vacuum at the part and the sealed area, reduced by a safety factor, and checked against the part weight and acceleration.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Vacuum at the part: pressure difference below atmosphere (bar): 0.6
- Sealed area entered as: Suction cups: diameter and number
- Cup effective diameter (mm): 50
- Number of cups sharing the load: 1
- Table or pad area under vacuum (m²): 1
- Share of that area covered and sealed by the workpiece (%): 100
- Load case: Horizontal cup, vertical lift (safety factor 2)
- Own safety factor, if chosen (times): 2
- Part mass (kg; 0 = skip the check): 0
- Highest acceleration incl. emergency stop (m/s²): 0
Result
- Pressure difference: 600 mbar = 60 kPa (−0.6 bar g)
- Effective sealed area: 19.6 cm² (1 × 19.63 cm²)
- Theoretical holding force: 118 N = 0.118 kN = 12 kgf
- Force per square metre of sealed area: 6.118 tonnes-force per m²
- Usable force after the safety factor: 58.9 N (safety factor 2)
Method. Force F = Δp × A, with Δp the pressure difference below atmosphere in Pa (1 bar = 100,000 Pa; 1 mbar = 100 Pa) and A the effective sealed area in m² (cup: π/4 × d² × number of cups; table: area × share covered). 0.2 bar acts as about 2 tonnes-force per m². Usable force = F ÷ safety factor: at least 2 is common for a straight vertical lift, 4 or more for a cup loaded in shear on a vertical face (friction carries the load). Required force = m × (g + a), with a the highest acceleration including emergency stops. Use the vacuum measured at the part, not the pump rating; the maximum possible Δp is the local atmospheric pressure (about 1.013 bar at sea level, about 0.95 bar at Hyderabad). Use the cup maker’s data for oily, dusty, textured or curved surfaces.
What to take from it. More vacuum gives little extra force and more cup wear: add cup area or cups first, and measure the vacuum at the part on the worst workpiece.
Compressed air demand estimate
Add up the air your tools and processes use, apply duty and simultaneity, then add leakage and growth allowances to reach a capacity to enquire about.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Unit for the air each tool uses: L/s
- Group A: number of tools: 6
- Group A: free air each (in the unit above): 4
- Group A: duty (%): 40
- Group B: number of tools: 2
- Group B: free air each (in the unit above): 15
- Group B: duty (%): 60
- Group C: number of tools: 1
- Group C: free air each (in the unit above): 40
- Group C: duty (%): 80
- Simultaneity of all tools (%): 65
- Leakage allowance (%): 15
- Growth allowance (%): 10
Result
- Connected demand: 94 L/s = 199 cfm
- Average demand (duty-weighted): 59.6 L/s = 126 cfm
- Simultaneous peak: 61.1 L/s = 129 cfm
- Design basis (higher of average and peak): 61.1 L/s = 129 cfm
- Design capacity with allowances: 77.29 L/s = 4.64 m³/min = 163.8 cfm
Method. Average demand = Σ(number × air per tool × duty). Peak = Σ(number × air per tool) × simultaneity. The peak can never be lower than the average, so the design basis is the higher of the two. Design capacity = design basis × (1 + leakage) × (1 + growth).
What to take from it. Leakage and growth allowances move the answer as much as any single tool. Where you can, replace catalogue figures with measured ones.