Vacuum components

Vacuum is pressure that is lower than atmospheric — 14.7 psia at sea level. In a vacuum system, the difference between atmospheric and vacuum pressure creates the ability to lift, hold, move and generally perform work.

There are two types of vacuum applications: closed, or non-porous; and open, or porous. In a closed system, removing air progressively decreases the air density within the sealed, confined space and creates a vacuum. In an open system, a vacuum unit must remove more gas molecules than are able to leak back into the system.

Vacuum is typically divided into three areas of application, depending on the level of vacuum required. Low-level vacuum applications are typically those requiring high flows and low force. These systems are primarily serviced by blowers. Screen printing on cloth is one application that falls into this range.

The majority of industrial vacuum falls within the range of 6 to 29.5 in.-Hg. Application examples include pick-and-place and thermoforming.

Scientific or process applications encompass the deepest levels — approaching a near-perfect 29.92 in.-Hg. Flow in this range is minimal. Examples of applications are ion implantation and space simulation.

The vacuum generators that evacuate air and create the required low pressure come in an extensive array of types, sizes, designs and efficiencies to suit widely ranging applications. Two basic types are electric-motor-driven vacuum pumps and vacuum ejectors.

Image courtesy of SMC

Vacuum pumps. Mechanical vacuum pumps generally fall into one of two different types: positive-displacement and dynamic/ kinetic. Displacement vacuum pumps essentially operate as compressors with the intake below atmospheric pressure and the output at atmospheric pressure. They draw in a fairly constant volume of air, which is mechanically shut off, expanded, and then ejected. The main feature of vacuum pumps of this type is that they can achieve a high vacuum with low flow rates. Types include reciprocating piston, rotary vane, diaphragm and rotary screw. They are often suited for precision industrial applications.

Kinetic vacuum pumps cause gas particles to flow in the delivery direction by applying additional force during evacuation. Rotary blowers, for example, operate according to the impulse principle: a rotating impeller transfers kinetic energy by impacting air molecules. In operation, air is drawn in and compressed on the suction side by the impeller blades.

These vacuum pumps generate a relatively low vacuum, but at high flow rates (high suction capacity). They are usually suited for handling extremely porous materials, such as clamping cardboard boxes.

Among the advantages, typical positivedisplacement industrial pumps generate up to about 98% vacuum — beyond the capability of ejectors. And blowers can offer high suction rates well beyond 1,000 m 3 /hr. However, electromechanical vacuum pumps tend to operate continuously with vacuum requirements regulated by valves. And compared to ejectors, they are larger, heavier, and usually cost more.

Vacuum ejectors. Vacuum ejectors basically generate vacuum using pneumatically driven nozzles without moving parts. They produce high vacuum at relatively low flow rates.

Image courtesy of Piab

A classic ejector consists of a jet nozzle (also called a Laval or venturi nozzle) and, depending on the design, at least one receiver nozzle. Compressed air enters the ejector and a narrowing of the jet nozzle accelerates the flowing air to up to five times the speed of sound. The ejector has a short gap between the jet-nozzle exit and the entry to the receiver nozzle. Here, expanded compressed air from the jet nozzle creates a suction effect at the gap which, in turn, creates a vacuum at the output vacuum port.

Vacuum ejectors come in two basic versions, single and multi-stage. A single-stage ejector includes a jet nozzle and one receiver nozzle; a multi-stage ejector has a jet nozzle and several nozzle stages, each of which has a larger diameter in proportion to the falling air pressure. Air drawn in from the first chamber, combined with compressed air from the jet nozzle, is thus used as a propulsion jet for the other chambers. In both versions, air exiting the receiver nozzle generally discharges via a silencer or directly to the atmosphere.

Among their benefits, vacuum ejectors are compact, lightweight, and relatively inexpensive and they respond quickly, with fast start and stop times. They resist wear, can mount in any position, experience no heat build-up in operation, and consume energy only as needed — as they switch off when no vacuum is needed. On the downside, vacuum ejectors only generate pressures to about 85% vacuum, and do not produce extremely high suction rates.

KEY USES OF VACUUM CUPS

Vacuum cups, or suction cups, are often used as grippers in manual or automated handling applications. They can secure and help move a wide range of products — everything from bottles and bags to bricks and wooden boards, and sheet metal, pipes and glass windows. In essence, they’re the interface between a vacuum system and the workpiece.

Typical vacuum handling systems are a mainstay in many industries, including packaging, food, beverage, woodworking, metalworking, automotive, semiconductor and electronics. Vacuum cups hold several advantages in such applications, including the fact that they are relatively simple, compact, light, inexpensive and require little maintenance. They are capable of firmly gripping parts in high-speed motion applications, as well as providing gentle handling of fragile parts. Here are some basics on how they work.

Technically, a suction cup does not attach itself and grip the surface of a product. Instead, when a suction cup contacts the workpiece surface, it activates a vacuum generator (such as a vacuum ejector, blower or pump) and draws out air from the cup interior and creates a vacuum. Given that air pressure inside is then lower than that outside of the cup, atmospheric pressure holds the workpiece against the cup. The greater the difference between ambient pressure and vacuum pressure inside the cup, or the larger the effective area of the cup acting on the workpiece, the greater the holding force pressing the cup onto the workpiece.

Ideally, a suction cup should mate against a smooth, nonporous surface. Then, when generating vacuum, the cup rim completely seals against atmospheric air and the interior air is quickly evacuated, resulting in a firm grip on the workpiece. However, non-ideal conditions are many times the norm because materials are often permeable, rough or uneven. In these cases, the cups cannot completely seal and outside air constantly enters the system. That’s termed a leaking system. Designers must compensate for leaking systems by using high-flow vacuum generators or using smaller cups to reduce the potential for leaks.

Types of suction cups range from simple, circular types to those designed for special applications like handling candy, greasy sheet-metal panels, or porous wood and cardboard. They come in two general shapes, flat and bellows.

Image courtesy of SMC

Flat suction cups are suited for handling workpieces with flat or slightly curved surfaces, such as metal and glass plates, plastic sheets and wooden boards. Flat cups have a small inner volume and, thus, evacuate quickly and can grip in a very short time. Properly designed, they have good stability to handle high shear forces and can withstand forces and accelerations from fast automated-handling movements.

Bellows suction cups, on the other hand, have one or more accordion-like convolutions. This lets them compensate for varying workpiece heights and handle parts with uneven surfaces. Evacuating the bellows also creates a lifting action which can be useful to lightly grip fragile parts, like electronic parts or even chocolate candy.

Bellows versions are typically used for handling curved parts like car body panels, pipes and tubes, injected molded plastic parts, and nonrigid packaged goods or shrink-wrapped products.

Both types come in a number of shapes, including round and oval. Various sizes make them suited for handling products weighing from a fraction of an ounce to several pounds. And they come in many different rubber and elastomer materials to suit specific application requirements, from FDA compliant cups for handling food, abrasiveresistant materials for moving bricks to oilresistant types in metalworking operations.