Vacuum ejector selection based only on a catalogue’s maximum vacuum can fail when a robot tries to lift a real workpiece. The suction confirmation takes too long, compressed air runs continuously, or the part will not release consistently. For a Thai factory, begin with the actual workpiece and suction cup: measure leakage, define the time available to evacuate the cup and tubing, and test holding, release and fault behavior. This guide turns those measurements into a comparison, an RFP and FAT/SAT acceptance evidence.
Define the handling duty before choosing an ejector
An ejector uses compressed air to create vacuum. Electric pumps and blowers are other generator options. Schmalz lists workpiece permeability, available energy, size and weight, transport distance and cycle time among the selection factors. A compact ejector near the robot tool may suit a short cycle, but that does not make every ejector fast in every circuit. Leakage, long tubing and multiple cups change the operating point.
Document the workpiece material, minimum and maximum mass, shape, surface roughness, holes, warpage, temperature, oil, water, dust and variation between lots. Record orientation, robot acceleration and rotation, travel path and required holding time during a stop. Then specify cup number, diameter, material, layout, allowed contact positions and any marking limit. Tubing length and bore, fittings, filters, manifolds and check valves affect evacuated volume and pressure loss. Measure supply pressure at the ejector inlet while other pneumatic devices run; a compressor-room gauge is not enough.
Split the cycle into time from cup contact to confirmed grip, time available for verification, transfer and release. State what the robot must do if the signal is late or lost. Record installation space, exhaust direction, noise constraints, spare-part availability in Thailand and PLC interfaces. Our robot hand and gripper selection guide covers the wider choice of gripping method. For tools that change automatically, pair this design with our robot tool changer selection guide and check the pneumatic connection after each exchange.
Measure leakage on the real workpiece
Do not apply an airtight-workpiece rule to porous material
Smooth metal, glass or coated plastic may seal well. Carton, fabric, foam, perforated trays and rough castings admit air through the material or around the cup lip. Schmalz’s suction-capacity examples by cup diameter are guidance for smooth, airtight surfaces; the company explicitly recommends a suction trial with the original porous workpiece. Multiplying a catalogue maximum flow by cup count does not reveal the actual pressure-flow operating point.
Leakage can also come from ribs, printed areas, warpage in motion, aged fittings and damaged tubing. A single ideal sample hides variation. Include acceptable samples near the limits of flatness and permeability, multiple lots and the dust or moisture encountered in production. Where product contact is controlled, confirm cup material and cleaning requirements separately.
Find the operating point, not merely the maximum rating
Use the proposed cups, supply pressure and tubing. Measure vacuum close to a cup, ejector inlet pressure, time to grip confirmation, pressure during transfer and time to release. SMC describes an adsorption test that reads the vacuum achieved with the workpiece and uses the ejector’s flow-characteristic graph to estimate leakage. It is a way to assess the real cup-and-workpiece pair. Assume zero leakage only after tests justify it.
Compare candidate flow at the required vacuum pressure. Maximum suction flow can be measured at a different point from the vacuum-switch threshold. Two ejectors with similar maximum vacuum can hold different pressure under continuing leakage. For each candidate, request flow curves, recommended supply pressure, air consumption, valve and filter configuration. Convert units consistently when datasheets mix negative kPa, mbar and percentage vacuum.
Size for tubing volume and the available grip time
An ejector evacuates the cup and the tubing, fittings and branches between cup and generator. A long hose and a tool-mounted ejector therefore cannot be judged by the same nameplate figure alone. SMC’s selection guidance combines circuit volume, target response time and workpiece leakage to estimate required average suction flow. The calculation narrows candidates; a production test must still account for valve switching, flow resistance and part deformation.
Start the timer when the cup first touches the part. Define how long it may take to reach the verified grip threshold. Measure inlet pressure during operation, determine the circuit volume, and add measured leakage. If cups touch the part at different times, test that transient as well. Do not enable robot acceleration from a one-sample crossing of the pressure threshold; specify stabilization or an appropriate confirmation logic.
Schmalz publishes cup-diameter flow examples for smooth airtight surfaces. Do not directly apply them to carton or bags. Its −600 mbar figure is an assumption in one airtight-workpiece calculation, not a universal target. Required holding force depends on load, acceleration, cup area and layout, and excessive suction may deform a thin part. Test both typical running and peak simultaneous pneumatic demand at the factory: pressure may fall at the robot even when the main air header appears adequate.
Select the cup and generator as a system
A larger cup offers contact area but may bridge a curved or narrow surface and leak at the lip. Thin sheet can deform or double-pick. More cups distribute load but add branches and evacuated volume. Prove the cup shape, material and placement with the real workpiece first; then choose a generator for that configuration. Otherwise an oversized ejector may conceal a poor seal rather than solve the underlying problem.
Test at least a well-sealed part and the most permeable acceptable part. On the first, measure evacuation time and whether air can be turned off during the hold. On the second, measure the stable vacuum and continuous air demand. Also test a missed cup, angled contact and a dirty filter; the controls must detect these states. Compare the suction valve, blow-off valve, check valve, vacuum switch, filter and silencer as well as the ejector body. Integrated functions change wiring and maintenance. Check that exhaust does not disturb operators or light parts and that blow-off does not throw the product.
Compare generators using compressed-air duty
Ejector, electric pump or blower
An ejector can be compact near the robot but consumes compressed air. An electric pump needs electrical power and space and can be evaluated for longer holding or a central vacuum system. A blower may be relevant to larger permeable surfaces. Schmalz notes that high suction rate combined with high vacuum brings higher energy use and operating costs. Purchase price or motor nameplate power alone is therefore an incomplete comparison.
Use the same cost boundary: generator, valves, sensors, fittings, pipework, compressor capacity, installation and maintenance. For an ejector, combine measured compressed-air flow with the actual on-time, cycles per hour and annual running hours. Convert air use to electricity cost with the factory compressor’s efficiency, pressure, load, leakage and tariff. Do not paste a generic percentage saving into an investment case.
Can the jet turn off while holding?
With a tight part, the control may stop the air supply after reaching a threshold and restart it as needed. Verify vacuum decay, check-valve behavior, restart delay and the state during power loss. A leaking part may rapidly lose holding force when supply stops, so the same control cannot be assumed suitable. Piab describes a product that adjusts ejector feed pressure for leaking materials; its benefit still requires testing under the particular circuit and workpiece conditions.
Measure air used per good part moved safely, not only instantaneous flow. Retries and scrap can erase a nominal efficiency gain. Oversizing may shorten evacuation but raise compressor demand and exhaust noise. Track throughput, accepted parts and retries under the same test conditions.
Design vacuum monitoring beyond one “grip OK” bit
Set the vacuum-switch threshold using the minimum verified holding condition, the pressure normally reached, measurement variation and hysteresis. Too high a threshold delays a sound pick; too low can approve an insecure one. SMC’s troubleshooting guidance cites supply-pressure variation, leakage and changes after cup replacement as causes of unstable adsorption. Setpoints should follow measured distributions across acceptable parts, not one convenient bench reading.
Keep separate PLC states for suction command, confirmed grip, transfer permission, vacuum loss during movement, release command and release confirmation. If grip is not established before the deadline, do not lift. If vacuum is lost in travel, apply the machine’s assessed stop or recovery behavior, considering the path and possible drop zone. At placement, commanding blow-off is not by itself proof that the part has left the tool. The exact logic comes from machine-level risk assessment; a single vacuum switch must not be treated as a complete machine-safety function.
A pressure trace helps diagnosis. Slow buildup suggests inlet pressure, nozzle contamination, excessive circuit volume, filter restriction or poor contact. A gradual drop after confirmation points to cup wear, fitting leaks or surface changes. One cup missing a part may be hard to see in a shared pressure reading; zones or flow sensing may be useful. Store part type, lot, cup replacement date, filter service date and supply pressure with the trend. Define normal ranges, warning triggers and who can change settings. Test sensor disconnection, power loss and communication failure so they cannot be read as grip confirmation.
Specify release, power loss and restart behavior
Stopping the jet does not always release a smooth part promptly: residual vacuum, a check valve or narrow tubing can delay it. If blow-off is used, tune pressure and duration without ejecting the part or moving a neighboring one. Different products may need controlled recipes with change history. Stable release can affect total cycle time as much as fast gripping.
Whether a part must remain held during a power or air failure depends on the hazard and where people can stand. A check valve or reservoir may provide a temporary hold, but leakage may prevent a guaranteed duration. Verify valve position on loss of power, behavior at restart and robot stop posture. Where falling remains possible, consider physical containment, access control and path changes in the machine risk assessment. During recovery, show the holding state, give workers a method to remove the part, and prevent an unexpected grip or release. Limit automatic retries so a defective part is not repeatedly dropped.
Put comparable evidence in the vacuum-ejector RFP
Attach the acceptable range of material, mass, geometry, surface, temperature, dust, moisture and lot variation. State required parts per minute, time for pick and place, acceleration, orientation, simultaneous air demand and behavior during a stop. Supply test samples and define the hardest acceptable good part.
Ask bidders for the cup and ejector model, allowable supply-pressure range, flow curve around the required vacuum, air consumption, suction and release valve scheme, pneumatic circuit, filters, silencers, switches and electrical interface. Ask for Thailand spare-part lead times and a reasoned maintenance interval. If suppliers quote ratings at different test conditions, request a demonstration at common supply pressure, tubing and part conditions.
Score cycle achievement, grip on leaky but acceptable parts, air use, fault detection, maintainability and local support separately from price. Give every criterion a pass threshold, measurement point and record format. A proposal containing only maximum suction flow has not answered the question of flow at the required pressure. Record differences in tubing and cup designs rather than hiding them behind an “equivalent” model designation.
Define vacuum-gripping FAT and SAT acceptance
FAT: verify circuit and control behavior
At FAT, use representative real parts on the bench or assembled machine. Record grip confirmation time, release time, holding pressure, air consumption, noise and continuous-run results. Include the leakiest acceptable part, changes in orientation, the specified supply-pressure range and simultaneous pneumatic demand. Simulate a missed cup, tubing leak, dirty filter, failed blow-off, disconnected sensor and stuck input. Confirm the robot does not lift before grip is valid; document alarm, stop position and recovery.
Keep raw traces or time-series graphs, sensor models, sample IDs, tube bore and length, cup condition and supply-pressure measurement location in the FAT report. A cycle-time value without test conditions cannot be repeated. Carry RFP pass criteria into FAT. Any change requires a documented reason, product-quality and safety impact, and approval.
SAT: prove performance in the Thai factory
At SAT connect to the actual compressed-air network and run the production robot path with neighboring equipment. Measure dynamic pressure at the ejector inlet, vacuum near the cup, cycle-time distribution, air use and fault rate during production-representative operation. Include shifts with materially different air demand. Let local maintenance staff replace a cup, clean a filter and verify settings after the intervention.
If FAT passes and SAT fails, compare supply pressure, as-installed tube length, workpiece lots, humidity, dust, acceleration and PLC timing. A longer installed tube changes evacuation time; stronger blow-off can move an adjacent part. Align measurement points and timestamps, isolate the cause and record the retest. SAT acceptance is the evidence that a catalogue selection works in this installation.
Make a decision table and feed operating lessons back
Compare candidate systems with the same part, cup, tubing and supply pressure where possible. If cups differ, label the offers as different system designs and expose the differences. Include grip-time distribution by workpiece, lowest vacuum in transfer, release time, failures, air per good part, exhaust noise, filter service, spare parts, total cost and fault response. Use repeated results and the hardest acceptable part instead of a single best cycle.
Weight the criteria for the process. Fast pick-and-place emphasizes reproducible grip and release and correct stopping. Long holds emphasize air duty and power-failure state. Porous parts emphasize flow at the operating pressure and cup sealing. Frequent product changes require recipe control and protection against incorrect selection. Build the budget from the whole system, including possible compressor upgrades, robot payload, tubing, maintenance and downtime. Request a calculation sheet with input assumptions so FAT and SAT readings can update the business case.
After startup, diagnose symptoms before replacing the ejector. For slower gripping, compare the same part, cup and inlet pressure with earlier traces; investigate common-air demand, filters, nozzles, pipe changes and cup contact in turn. If hold pressure declines only on certain lots, inspect material and surface changes. If all lots deteriorate, inspect cup wear, fittings and tubing. After replacing a cup, check height, compression and switch setting; SMC documents instability after maintenance and fitting leaks among its failure examples.
For inconsistent release, check residual vacuum, check-valve location, tubing, cup adhesion and the part before increasing blow-off. For multi-product lines, bind cup position, threshold, wait time and release duration to the product recipe, confirm the loaded recipe against the actual part and record changes. These observations should become standard input to the next machine’s selection, not remain a one-off repair note.
Keep test records usable for the next machine
Do not retain only a verdict such as “model A worked.” Store the part number, lot and photo, contact surface, cup model and layout, tube bore and length, generator location, filter state and inlet-pressure measurement point as one test configuration. A slow sensor sampling interval can miss a brief pressure dip. Align the pressure plot with PLC events so contact, suction command, threshold, robot motion, release command and release confirmation can be reconstructed. When a later machine performs differently, these records help separate workpiece changes from pipework and control changes.
Record why candidates were rejected: a fast unit may consume too much air, need more noise treatment space, or lack an acceptable Thailand spare-part lead time. Fix the evaluation-sheet revision before ordering. When a new product is added, update the acceptable part range and decide whether tests must be repeated. For rollout across Thai plants, compressor-air quality, supply pressure, installed tubing and maintenance support can differ. A first site’s SAT is a baseline, not acceptance at the next site; repeat SAT there and record changes against the common FAT file.
FAQ: vacuum ejector selection and robotic suction
Is the ejector with the highest maximum vacuum the best choice?
No single maximum decides the result. Test whether the actual leaky part reaches a safe holding condition within the available time. Compare flow near the required pressure, sealing, supply pressure and circuit volume. Excessive suction can deform a thin part.
How should I start selecting an ejector for carton or fabric?
Test actual samples with the proposed cups and measure pressure and leakage during holding. Do not transplant an airtight-surface guide. Include acceptable variations in lot, moisture and warpage, and reconsider cup shape or contact area first if the seal is poor.
How should compressed-air cost enter a generator comparison?
Measure air flow and jet on-time under the actual duty. Apply cycle count and annual hours, then the factory compressor and tariff conditions. Include retries, rejected parts and downtime in the cost comparison.
Is one pressure switch enough for vacuum monitoring?
It depends on the machine. Test buildup time, pressure loss in travel, disconnected sensors, parts left on the tool and partial leakage across multiple cups. Separate process monitoring from machine-safety design and follow the equipment risk assessment.
What belongs in the ejector RFP and vacuum FAT/SAT?
Specify the real workpiece range, circuit, supply pressure, cycle, hold and release requirements, fault behavior, measurement methods and pass criteria. FAT checks circuit and PLC logic on actual samples; SAT repeats performance checks on the Thai factory air network and robot path. Keep conditions and time-series evidence for both.
Conclusion
Select a vacuum ejector by establishing leakage, cup contact, circuit volume, inlet pressure and available grip time on the real workpiece. Compare candidates under common conditions, including air consumption and stable hold and release. Put fault handling into the RFP, verify control behavior at FAT and prove installed performance at SAT.
TOMAS TECH can help a Thai factory define sample tests, comparison criteria, an RFP and FAT/SAT measurements for robotic vacuum handling while a project is still being scoped. Contact us to discuss the conditions of your line.
Primary references
- Schmalz, Vacuum Generator Selection
- Schmalz, Selection Aid for Vacuum Generators
- SMC, Vacuum Equipment Model Selection
- SMC, Vacuum Equipment Best Pneumatics Guide
- SMC, Vacuum Adsorption Transfer System Model Selection Software
- Piab, Vacuum Technology
- Piab, piSAVE ESL
*Sources checked on 4 October 2026. Set final design values, costs and acceptance limits from measurements and the individual machine risk assessment.*
