Vacuum devices play an important role in a wide range of applications. Typically, they are used either in pick and place systems, for moving items ranging from small electronic components to plate glass, or for holding products in position, while other operations are carried out.
Pumps and Ejectors
A vacuum is generally defined as a space which is devoid of all matter. In practice, this is difficult to achieve, so that in terms of pneumatic technology a vacuum generally represents a contained area from that of the surrounding atmosphere. The difference in pressure, or the under pressure, as it is sometimes known, is measured either in Torr (named after the Italian physicist, Torricelli) or in standard SI units of bar or Pascal.
There are two methods of applying a vacuum: by means of a special pump, which evacuates a tank and requires dedicated circuits routed to each work station, or by using the normal compressed supply.
Although dedicated pump systems are relatively expensive, they can provide high levels of vacuum over large work surfaces. For most applications, however, a more cost effective method is to use vacuum ejectors, which rely on the kinetic energy of a jet of air to evacuate a vacuum cup or suction pad.
Vacuum ejectors have largely superseded the older venturi-type generators and are constructed as shown in the diagram below.
Compressed air is forced through a nozzle into the mouth of a larger tube or diffuser, causing turbulence within the chamber. This has the effect of drawing air from the vacuum port into the diffuser. As can be seen from the diagram, changes in the relationship between the size of the nozzle and diffuser will affect both the level of vacuum created and the rate of flow through the vacuum port, with larger diameter nozzles and diffusers giving higher flow rates but a reduced vacuum.
This phenomenon can, however, be utilised to produce highly efficient two-stage vacuum ejectors, such as SMC’s ZM devices.
The vacuum inlet is split between two chambers, the first ofwhich provides a high level of vacuum, while the second increases the volume of air removed. By installing a check valve between the two chambers it is therefore possible to control the relationship between vacuum and flow, with the
Valve closing as the vacuum increases or as the flow rate falls. In addition, air consumption is minimised, as the flow rate can be reduced once the vacuum has been applied to the work piece.
Vacuum ejectors such as the ZM incorporate silencers, suction filters, valves and switches within a single compact unit, making them simple to install, even on robotic arms or other moving assemblies, where weight must be minimised to reduce inertia. They also use carefully calculated flow paths and nozzle and diffuser orifices, to provide a level of vacuum which is up to 40% greater than that of conventional devices.
Having developed a vacuum or, to be more specific, a suction force, it is then necessary to create a seal between the ejector and the workpiece, so that it may be moved or secured. This is generally achieved using vacuum or suction pads, located where possible close to each ejector, to minimise the volume of air which must be evacuated and to improve response times.
Under normal operating conditions each ejector should be connected only to a single pad, to prevent the failure of one pad from affecting the vacuum applied to the remainder.
Vacuum pads are manufactured from a variety of rubber compounds. Typically, these include nitrile (NBR) silicon, urethane and Viton (FPM). For most applications, conventional NBR pads provide a suitable combination of surface compatibility and rigidity; softer silicon pads are useful with items that have deeply contoured surfaces, while viton or other anti-static materials are required in areas, such as the electronics industry, where components are easily damaged.
The size and type of vacuum pad required for a given task depends on a variety of factors, of which the most important are the weight, surface finish and shape of the work piece. In addition, however, the ability of the pad to retain an optimum profile as the vacuum is applied, will also have a direct impact on the performance of the system.
Unsupported vacuum pads can deform as vacuum is applied, with exterior air pressure forcing the outer sections of the pad into contact with the surface of the work piece. This dramatically reduces both the effective pad area and, therefore, the level of vacuum which can be created. Conversely, if the pad material is too hard, air will leak under the rim, so that the supply pressure has to be increased, simply to maintain a static vacuum.
The solution is both to use relatively soft, ribbed pads, capable of retaining the maximum volume of chamber between the pad and workpiece, and to ensure that the devices used to create the vacuum are also capable of generating a high rate of flow. This draws the pad down rapidly onto the work surface before a significant leakage flow is set up. Two stage ejectors are ideal for this purpose.
Ribbed pads should also be used with flexible items, such as plastic sheets, which are likely to deform as vacuum is applied; in this instance, both the size of the pads and the level of vacuum should be reduced, to prevent the work surface from creasing. Flat, rigid work surfaces generally require low profile pads, while curved surfaces can only be moved using pads with a deep, conical profile.
Rough surface finishes will cause air to bleed under the rim of vacuum pads. To overcome this it is necessary to increase the level of vacuum applied and to use pads with smaller diameters, to minimise the air loss. Larger numbers of pads will, therefore, be necessary to retain the same overall pad contact area. With deeply textured surfaces a softer pad material will also reduce air loss, as the pad will mould itself to the contours of the surface; a softer material can, however, affect the level of grip which is achieved, as the pad is likely to deform under pressure.
The time taken to evacuate each pad will affect the sequencing of events which are subsequently carried out. Calculations for response times must take into account a variety of factors. These include the flow rate through the ejectors, and hence the available vacuum pressure, the size of pad, leakage of air from around the pad, the porosity of the work piece and the size and length of air tubes, which will affect the operating pressure.
Tables showing the lift force available from different sized ejectors are available from vacuum equipment manufacturers. One point which should be remembered, however, is that the figures supplied do not always provide a reasonable safety margin; for example, an allowance is not always made for factors such as the sheer or lateral force acting on each pad as the work piece is moved. Depending on the application, it may therefore be necessary to reduce the lift force quoted by up to 50%, if suction is being applied from above the work piece, or by as much as 75%, if the vacuum pad is applied to the side of the work piece.
The following is a brief summary of some of the main points which should be considered when designing or specifying vacuum systems.
a) Use a series of small pads to spread the suction force over as wide an area as possible.
b) Do not allow pads to protrude beyond the edge of the work piece.
c) Use multiple pads to prevent the work surface sagging as it is lifted.
d) Reduce the time taken to reach operating vacuum by mini-mising the amount of tubing used between the ejector and the suction surface.
e) Use one ejector per pad. Several pads driven by a single ejector will all be affected if one pad fails to function correctly.
f) Protect multiple ejector and pad systems with a throttle valve, to ensure that a constant flow of air is maintained across the pads, even if one or more suffer from a loss of vacuum.
g) Always install extra pads, so that in the event of several pads malfunctioning, the work piece continues to be held firmly in position.
h) Minimise the diameter of air tubes, to reduce flow resistance, leakage and response times.
i) Use and correctly maintain in-line filters. Efficient filtration will prevent solid debris or air borne particles of oil and other contaminants fouling the vacuum pump or internal orifices in vacuum ejectors. Ejectors should be protected by filters both in the vacuum intake and in the main supply lines.
j) In certain applications, it is necessary to provide both a suction force and a positive air pressure through thevacuum pad. This can be achieved simply by incorporating a valve into the system, so that when deactivated the valve provides suction. Switching the valve causes air to be blown through the pad, either to dislodge the work piece or to prevent debris entering the vacuum port.
The various standard ISO symbols, used to define vacuum devices, are shown below. These can be used in a similar fashion to the symbols for other pneumatic equipment and follow similar conventions.