Vacuum Generators

What is a Vacuum Generator

A vacuum generator, also called a vacuum ejector or venturi pump, is a static pneumatic component that produces a partial vacuum from a supply of compressed air. Unlike a mechanical vacuum pump, it contains no pistons, rotors, vanes, or motor. The only thing that moves is the air itself. Compressed air passes through a precisely machined motive nozzle, accelerates, and creates a region of low pressure that pulls air out of the connected vacuum line. This is the same venturi effect that lets a carburettor draw fuel or an aspirator pull liquid, applied to industrial automation.

The device sits at the boundary between two worlds: the pneumatic supply side, where it behaves like any air-consuming actuator, and the vacuum side, where it behaves like a small pump. A complete vacuum-handling system usually pairs the generator with suction cups, a non-return valve, a vacuum switch or sensor, tubing or a manifold, and often a blow-off (release) valve that injects a short pulse of pressure to break the vacuum and drop the part cleanly. The generator is the heart, but the cups and the switching logic determine whether the system works.

Vacuum itself is defined relative to local atmospheric pressure. One standard atmosphere is about 101.3 kPa absolute, equal to 1.013 bar or 1013 mbar. A reading of 0 percent vacuum corresponds to atmospheric pressure, 50 percent vacuum to roughly 50.7 kPa absolute, and a hypothetical 100 percent vacuum to absolute zero. In practice, most industrial suction-cup applications operate between 60 and 85 percent vacuum, which is comfortably inside what a venturi generator can deliver. Engineers must watch units carefully: the same operating point may appear on different datasheets as -80 kPa, -800 mbar, or 80 percent vacuum, and confusion between gauge negative pressure and absolute pressure is a frequent source of error.

The appeal of the generator is operational. With no wear parts it offers a long service life and near-zero maintenance, it responds in milliseconds so cycle times are short, it is small and light enough to mount on a moving robot wrist without adding meaningful inertia, and it works anywhere compressed air reaches without running electrical cable into the hazardous or wet zone. These properties make it the default choice for distributed, intermittent gripping tasks across electronics assembly, food and beverage packaging, printing and paper handling, glass and sheet-metal loading, and general robotic palletizing.

The countervailing fact is energy. Compressed air is one of the most expensive utilities in a factory, and a generator consumes it whenever it runs. For continuous, high-flow, or deep-vacuum duty, a centralized mechanical pump is usually far cheaper to operate, even though it costs more to buy and responds more slowly. The engineering question is therefore never "generator or pump" in the abstract; it is which device minimizes total cost of ownership for a specific part, cycle, duty factor, and number of pick points. Chapter 6 turns that question into a checklist.

Types and Classification

Pneumatic vacuum generators are classified first by the number of venturi stages and then by their construction and integrated functions. The single most consequential split is single-stage versus multi-stage, because it sets the relationship between vacuum depth, suction flow, and air consumption. The table below summarizes the main families and where each fits.

Single-stage generators use one nozzle and one diffuser. They are the simplest and cheapest, and a high-vacuum nozzle geometry can reach about -85 to -88 kPa. The drawback is that all suction flow must pass through a single diffuser, which caps the suction flow obtainable for a given amount of compressed air. A separate high-flow variant uses a larger nozzle to move more air at a shallower vacuum, suited to porous loads. Single-stage units remain popular for their low cost, tiny footprint, and predictable behavior on small, tight parts.

Multi-stage ejectors stack two or three diffusers in series. The air leaving the first diffuser still carries kinetic energy, so it is used to entrain additional free air through a second and sometimes a third stage. For the same compressed-air consumption this raises suction flow substantially, with some multi-stage series delivering on the order of one and a half times the suction flow per unit of air compared with a single stage. Three stages is the practical ceiling, since the residual jet energy after the third stage is too low to do useful work. Multi-stage units are also noticeably quieter and can reach slightly deeper vacuum, around -90 to -93 kPa, which is why they dominate higher-throughput handling.

Construction format is a second axis. Body-ported (manifold) generators bolt to a base and route vacuum and exhaust through machined channels, suited to multi-circuit valve islands. Inline cartridge generators are lightweight plastic units with push-in fittings that clip directly onto the tubing next to the cup, minimizing dead volume and the mass carried by the robot. Larger box-type generators integrate the generator, valves, switch, and silencer into one enclosure for heavy material handling. Piab's patented COAX cartridge approach packages a multi-stage element so compactly that several can be combined to scale flow, while SMC, Festo, and Schmalz offer parallel single-stage, multi-stage, and integrated ranges.

Integrated functions increasingly distinguish products at the same vacuum and flow rating. Built-in vacuum switches or analog sensors, blow-off (purge) valves, supply valves, non-return valves, IO-Link communication, and the air-saving control described in Chapter 3 turn a bare ejector into a self-contained handling module. For automation projects the integrated unit usually wins on wiring, footprint, and commissioning time even if a bare ejector is cheaper on paper.

Working Principle and Control

The core of every pneumatic vacuum generator is the venturi, named after the Italian physicist Giovanni Battista Venturi. Compressed air enters a convergent motive nozzle, where the contracting cross-section forces the flow to accelerate. At the design supply pressure the jet reaches sonic or supersonic velocity at the nozzle throat. By Bernoulli's principle and the conservation of energy, this high velocity corresponds to a sharp drop in static pressure immediately downstream of the nozzle. That low-pressure zone is connected to the vacuum port, so air is continuously drawn out of the load.

The entrained load air and the motive jet then enter a diffuser, a gradually expanding passage where the mixed stream decelerates and its pressure recovers toward atmospheric, allowing the combined flow to discharge through a silencer. In a single-stage device this happens once. In a multi-stage device the partially spent flow from the first diffuser is led into a second nozzle and diffuser, and possibly a third, each adding suction flow. A bank of small non-return flaps between stages prevents back-flow so that each stage only contributes when its pressure conditions are favorable, which is why a multi-stage curve shows a knee where successive stages cut in.

Two performance curves define a generator and should be read together. The first plots vacuum level against time, the evacuation curve, which rises fast at first and then asymptotes toward the maximum vacuum. The second plots suction flow against the achieved vacuum level: flow is highest at zero vacuum (atmospheric) and falls to zero as the maximum vacuum is approached. Engineers use the evacuation curve to estimate cycle time for a sealed load and the flow curve to confirm the generator can overcome leakage on a porous load while still holding the working vacuum.

Supply pressure is the dominant control variable. Most venturi generators are tuned for an optimum around 0.4 to 0.5 MPa (4 to 5 bar), with many datasheets citing peak efficiency near 0.45 MPa. Raising pressure above the optimum does not deepen the vacuum once the nozzle chokes; it simply burns more air and lowers efficiency. Dropping below the optimum cuts both vacuum and flow. Best practice is to feed each generator through a dedicated regulator set to the datasheet optimum rather than the full network pressure of 0.6 to 0.7 MPa, paired with clean, dry air from an upstream FRL unit, because moisture and particulate quickly foul the fine nozzle.

The decisive control feature for sealed parts is the air-saving (energy-saving) function. On a gas-tight part the system reaches target vacuum and then leaks only slightly. The air-saving architecture combines three elements: a vacuum switch or sensor that monitors the level, a small 2/2 supply valve that gates the compressed air, and a non-return (check) valve that traps the vacuum on the load. When vacuum reaches the upper set point the supply valve closes and the check valve holds the load; if vacuum decays to the lower set point the supply valve reopens for a brief top-up. The generator thus runs in short pulses instead of continuously, and on tight workpieces this cuts air consumption dramatically, with some manufacturers citing reductions exceeding 90 percent. The function delivers nothing on porous or leaky loads, where vacuum cannot be held and the ejector must run continuously.

Sizing for Suction-Cup Handling

Most vacuum generators are bought to lift parts with suction cups, so sizing starts at the cup, not the generator. The holding force of an ideal cup equals the pressure difference across it multiplied by its effective area: F = (P-atmospheric minus P-vacuum) times A-cup, often written as F = vacuum level times effective area. A cup of 40 mm diameter has an effective area near 12.5 square centimeters; at -70 kPa (700 mbar) of vacuum it generates roughly 88 N of theoretical pull. That theoretical figure must then be divided by a safety factor to give the usable lifting force.

Safety factors are not optional. For smooth, dense, dry workpieces lifted horizontally a factor of 2 is the accepted minimum, meaning the cups must be able to pull at least twice the part weight. For vertical lifts, accelerated motion on fast robots, or rough, porous, or oily surfaces a factor of 4 or more is required, because dynamic loading and partial seal loss erode the real holding force. Underestimating the safety factor is the most common cause of dropped parts on a new line. Once the number and size of cups are fixed by force and safety factor, the generator must supply the vacuum level and the suction flow the cup array demands.

The two dimensions to balance are vacuum level and suction flow, and they pull in opposite directions on a porous part. A tight, non-porous part such as glass leaks almost nothing, so once the generator pulls it down to working vacuum almost no flow is needed to hold it; a deep-vacuum, low-flow single-stage unit with the air-saving function is ideal. A porous part such as corrugated cardboard leaks continuously, so the generator must supply enough suction flow to outpace the leakage while still holding a useful vacuum; here a high-flow or multi-stage unit running continuously is the right answer, and the air-saving function is useless because the vacuum can never be trapped.

Evacuation time, the time to pull the system from atmospheric to working vacuum, depends on the total volume to be evacuated (cups plus tubing plus any vacuum reservoir), the leakage of the gripping surface, and the suction flow of the generator. Minimizing dead volume by mounting an inline generator close to the cups, keeping tubing short and of the correct bore, and avoiding oversized reservoirs all shorten the cycle. Where a fast initial grab is needed on an otherwise tight part, a deliberately oversized suction flow or a multi-stage unit shortens the evacuation phase even though the steady-state flow demand is low.

A worked sequence ties it together. Suppose a 5 kg steel plate is lifted vertically by four cups. Required holding force is 5 kg times 9.81 times a safety factor of 4, about 196 N total, or roughly 49 N per cup. Solving F = vacuum times area at a target -70 kPa gives an effective area near 7 square centimeters per cup, satisfied by a 32 to 40 mm cup with margin. Because steel is non-porous, flow demand is low and a deep-vacuum single-stage generator with the air-saving function will both seat the part quickly and then idle, minimizing air cost. The same plate handled by a fast robot might push the safety factor higher and demand a multi-stage unit purely to cut evacuation time.

Key Specification Parameters

A vacuum generator datasheet typically lists a dozen parameters, but only a handful drive the selection. Below are the ones that matter, with the units to watch and typical ranges drawn from mainstream single-stage and multi-stage venturi products. Always read each value at your actual supply pressure, because every figure is quoted at a stated reference pressure.

Maximum vacuum is the deepest level the generator can reach against a sealed port at the optimum supply pressure. Standard high-vacuum venturi units sit near -85 to -88 kPa; deep multi-stage units approach -90 to -93 kPa. A venturi physically cannot reach the near-absolute vacuum of a mechanical pump because exhaust back-pressure and the entrainment limit cap it around 90 percent vacuum. Match this to the cup demand from Chapter 4, with margin, and confirm it is the value at your line pressure rather than the headline best case.

Suction flow and air consumption are the cost-and-capability pair. Suction flow is how much air the generator can pull from the load; it must exceed the leakage of a porous part and governs evacuation speed. Air consumption is what the generator draws from the network, and it scales directly with nozzle size and supply pressure. Both are commonly given in litres per minute at ANR (atmospheric normal reference, 20 deg C, 1 atmosphere, 65 percent humidity per ISO 8778) in Europe and Asia, or in standard cubic feet per minute (SCFM) in North America. Mixing the two unit systems is a frequent comparison error; convert before judging. A representative single-stage body-ported unit might consume 15 to 50 L/min for 20 to 90 L/min of suction flow, while a larger multi-stage series can deliver several hundred L/min of suction flow at correspondingly higher air consumption, but with a markedly better flow-to-air ratio.

Nozzle diameter is the lever behind the previous two parameters. A larger nozzle passes more air, producing higher suction flow and higher air consumption but generally a shallower maximum vacuum; a smaller nozzle does the reverse. Datasheets commonly offer a family of nozzle sizes (for example 0.5, 0.7, 1.0, 1.3, 1.5, and 1.8 mm in one popular series) so the same body can be tuned across applications. Selecting the nozzle is effectively selecting the operating point on the flow-versus-vacuum curve.

Optimum supply pressure defines where the published vacuum and flow figures apply, typically 0.4 to 0.5 MPa. Operating temperature and ambient conditions matter for ovens, freezers, and washdown zones, where standard plastic-bodied units may be out of range. Noise level is a genuine workplace concern: a bare venturi exhaust is loud, so an integrated silencer is standard and multi-stage units are inherently quieter. Finally, the integrated function set (vacuum switch, blow-off valve, supply valve, non-return valve, IO-Link, air-saving control) is itself a specification, because it determines wiring, commissioning, and running cost as decisively as the raw vacuum and flow numbers.

Selection Decision Factors

Selecting a vacuum generator is a sequence, not a single spec lookup. Most mistakes come from deciding the device before the part and the cycle are characterized. Work through the following steps in order; they double as an RFQ template.

1. Characterize the part and surface: weight, dimensions, orientation of the lift (horizontal or vertical), and whether the surface is gas-tight (glass, metal, smooth plastic) or porous and leaky (cardboard, wood, perforated sheet). This single distinction drives almost every later choice.
2. Size the cups and set the safety factor: compute holding force as vacuum level times effective area, divide part weight by usable force, and apply a safety factor of 2 for smooth horizontal lifts or 4 or more for vertical, accelerated, or rough or oily duty. Fix the number and size of cups before choosing the generator.
3. Decide single-stage versus multi-stage: tight parts favor a deep-vacuum single-stage unit with the air-saving function; porous or high-throughput parts favor a high-flow or multi-stage unit running continuously. Multi-stage also wins where evacuation time must be short.
4. Set vacuum level and suction flow targets: read them off the flow-versus-vacuum curve at your actual supply pressure, not the headline best case, and ensure suction flow exceeds the leakage of the worst-case surface while holding the working vacuum.
5. Choose construction and connections: inline cartridge for lowest mass on a robot wrist, body-ported for valve-island integration, box-type for heavy handling. Specify process and exhaust ports (G or NPT threads, push-in tube sizes) and the electrical or IO-Link interface.
6. Specify integrated functions: vacuum switch or analog sensor for grip confirmation, blow-off valve for clean release, supply valve and non-return valve and the air-saving control for sealed parts. On a multi-pick line these features cut wiring and commissioning far more than they cost.
7. Provide clean, dry air at the optimum pressure: feed each generator through a dedicated regulator at the datasheet optimum (around 0.45 MPa) and an upstream FRL unit, because the fine nozzle clogs quickly with moisture or particulate, and over-pressuring wastes air without deepening vacuum.
8. Compute total cost of ownership: purchase price plus the cost of compressed air over the duty cycle plus filtration and maintenance. For continuous high-flow duty, run the numbers against a centralized mechanical pump, which often wins on running cost despite a higher purchase price and slower response.

One dimension that buyers routinely undervalue is serviceability and ecosystem fit: availability of spare nozzles and silencers, compatibility with the plant's valve island and fieldbus, local technical support, and whether the device exposes diagnostics over IO-Link for predictive maintenance. SMC, Festo, Piab, and Schmalz all maintain broad distribution and configurator tooling, which shortens lead time and simplifies replacement years into a line's life. A generator that is two dollars cheaper but orphaned on the factory floor costs far more in downtime than it ever saved at purchase.