Why use air to move things
Moving goods is mostly about three jobs: transporting material from place to place, picking up and placing items, and raising loads. Compressed air can do all three. It suits handling because the equipment is light, the motion is quick, and the system tolerates dust and starts and stops without harm. The recurring cost is the energy needed to compress air, so pneumatic handling is a good fit when loads are light to moderate and movements are frequent.
Pneumatic conveying
Pneumatic conveying moves bulk material, such as powders, granules, pellets or small parts, through a closed pipe using a stream of air. There are two broad families.
Dilute phase conveying suspends the material in a fast-moving air stream. It suits free-flowing, non-fragile materials and can be run by positive pressure (pushing), by vacuum (pulling) or by a combination.
Dense phase conveying moves material in slower slugs or plugs, at higher pressure and lower air speed. It is often chosen for abrasive or fragile materials, because slower travel causes less wear and breakage.
Pneumatic conveying has real advantages: pipes can be routed around obstacles, the material is enclosed so dust is contained, and one line can serve several destinations through diverter valves. The limits are the energy needed to produce the air flow, the wear on bends and pipe walls from abrasive material, and the need to keep moisture out of materials that clump. Dust-handling systems also need attention to combustible dust, a hazard that applies to many fine powders and is covered in more depth in material on combustible dust awareness.
Pneumatic conveying should not be confused with belt systems; a comparison of conveyor types explains where belts, rollers and screws are a better match than an air stream.
Pneumatic grippers
A gripper is the “hand” on a pick-and-place machine or robot. Air-powered grippers are very common because they are small, fast and simple.
- Parallel grippers close two flat jaws straight toward each other, good for gripping blocks and cylinders.
- Angular grippers swing their jaws like a pair of pincers and suit parts that are easier to approach from an angle.
- Three-finger grippers center round parts.
- Vacuum grippers use suction cups instead of jaws, covered below.
Air grippers close with a force set by pressure, so changing the regulator setting changes the grip. It is important to choose a gripper whose force is comfortably above what is needed to hold the part during acceleration, and to design the system so the part is not dropped if air supply is lost. Many grippers include a spring or mechanical lock for this reason, though that is a design matter for qualified engineers.
Vacuum handling
Vacuum cups hold flat or smooth items, such as sheets, boxes, panels, bags or trays. Vacuum is produced either by a pump or, commonly in small stations, by a compressed-air ejector, which creates suction as air flows through a nozzle. Ejectors are compact and have no moving parts, but they use compressed air continuously while they run, so efficient ones that shut off when the vacuum level is reached are common. Details of the equipment behind this are in the topic of vacuum pump types.
The holding force depends on cup size, the vacuum level, the surface of the load, and how fast the load accelerates. Porous or oily surfaces hold poorly, and cups wear and need replacing. Designers typically include a generous safety margin.
Air-powered lifting
Pneumatic lifting comes in a few forms.
- Lift tables and cylinders use an air cylinder or bellows to raise light loads over a short travel.
- Air bellows (air springs) lift or push loads with a flexible rubber and fabric bag that inflates. They are useful where limited stroke is acceptable, and they also isolate vibration.
- Balancers and vacuum tube lifters let an operator lift items by hand with the weight largely offset by air pressure. They are used in packing, assembly and warehouse work and help reduce physical strain.
- Air hoists use an air motor and are chosen in places where electric motors are undesirable, such as some hazardous areas.
These are best for light and medium loads. For very heavy loads, hydraulic lifting or conventional cranes and hoists are generally used, and the planning for overhead cranes and material handling equipment is a separate topic.
Choosing and keeping it reliable
Questions worth asking when evaluating a pneumatic handling task include:
- How heavy, how fragile and how clean is the load?
- How many cycles per hour, and over what distance?
- What happens to the load if air pressure drops?
- Are cleanliness, dust or washdown conditions a factor?
- Is the air supply large enough, including during peak demand?
For maintenance, keep the air clean and dry, replace worn vacuum cups and seals before they fail, check hoses and fittings for cracks or chafing, and listen for leaks, since leaking vacuum and cylinder lines waste energy and weaken the grip. Isolate and depressurize equipment and follow lockout procedures before any adjustment. Stand clear of overhead loads, and use only equipment rated for the load, following the manufacturer’s manuals. This article is general information, not engineering advice.