When it comes to materials handling, most think of the warehouse or plant floor in two dimensions: width and length. The third dimension - the space above the floor - is often ignored.

Sure, some facilities run shelf and storage systems to the rafters. But overhead is not always considered for moving materials. That's where overhead conveyor, power-and-free systems, electrified monorail, and cranes come into the picture.

'The number one reason for going overhead is to better utilize floor space,' explains Tom Wolsos, business development manager, Siemens Dematic (616-913-6200, www.siemens.com).

While space is important, there are also applications where it makes sense to deliver the product overhead. For instance, overhead conveyors are used to remove corrugated from an assembly line or deliver cartons to a packaging area. 'Another example might be using an overhead monorail system to replenish a flow rack with totes,' says Dave Koch, senior vice president, Daifuku America Corp. (800-253-1003, www.daifukuamerica.com).

Despite the advantages of going overhead, there are important considerations. For example, as the overhead system becomes more sophisticated, costs tend to rise too. Furthermore, not every building is able to support the added weight of overhead materials handling equipment without additional reinforcement.

Safety also needs to be addressed when materials arrive at a work area from above. Maintenance is typically more difficult to perform above the ground.

Finally, not every product can be conveyed overhead. 'You want to avoid products that might get stuck since someone has to go up there to fix the problem,' says John Kuhn, product manager, warehousing and distribution, FKI Logistex (877-935-4564, www.fkilogistex.com).

Nevertheless, overhead movement of materials relies on proven technologies that, when applied correctly, maximize efficient use of the plant or warehouse.

Overhead conveyor systems are often used to move product from one end of the plant or warehouse to the other without disrupting floor traffic. For instance, overhead roller and belt conveyors transport totes of work-in-process as well as cases and containers of finished goods.

Mounting sortation and accumulation lines on a mezzanine is also common, especially at the shipping dock. 'It's just too expensive to consume all your floor space for something that doesn't have enough value in and of itself,' says Don Kloosterhouse, manager of Ermanco, a Paragon Technologies brand (231-798-4547, www.conveyor.net).

Providing a safe operating environment may be the biggest challenge to taking conveyors overhead. 'You have to have high side guards or spill netting underneath to contain products,' says Kloosterhouse.

What's more, overhead conveyor is better suited to some products than others. 'If you have a sloppy product or an unstable product that doesn't transport well at high speeds, overhead is not the way to go,' says Mike Kotecki, senior vice president, HK Systems (800-457-9783, www.hksystems.com).

The options for overhead conveyor don't stop with roller and belt systems. In fact, overhead chain conveyor, which consists of a single continuous loop of chain and carriers, is one of the technologies Henry Ford used to automate assembly lines.

Decades later, automakers still use chain conveyor to deliver a steady stream of parts to an assembly line or paint finishing area.

The simplicity of chain conveyor is also a drawback. At 30 feet per minute, the speeds are relatively slow. And since all carriers are permanently fastened to the chain, if one carrier stops, they all stop.

Power-and-free takes chain conveyor to the next level. While a chain still powers these systems, the carriers run in a separate track. That allows an individual carrier to mechanically disengage from the chain and remain idle at a workstation or accumulate in a buffer staging area while the rest of the chain moves forward. Where overhead chain conveyor travels in a predetermined route, carriers in a power-and-free system can be diverted from one spur to another. That allows them to travel on multiple paths.

While chain conveyor travels at a fixed speed, power-and-free can speed up and slow down. It also operates at speeds of up to 60 feet per minute, and moves loads weighing up to 8,000 pounds.

Carriers, however, have no onboard intelligence. Instead, carriers are typically identified by a metal bar code or sometimes even by a radio frequency identification tag. At a decision point, an external reader identifies the carrier and the system makes routing decisions.

The sweet spot for monorail is what is known as automated electrified monorail (AEM), which is sometimes referred to as an overhead electric monorail system (EMS). Typically, an AEM consists of an I-beam track that hangs from the ceiling. Instead of chains, power is provided by a bus bar, or power rail, inside the I-beam. Communication lines linked to the host computer are also mounted on the I-beam.

Each carrier has its own motor and drive unit as well as onboard intelligence. That allows every carrier on the system to act independently of the others. Carriers don't have to travel a single fixed path but can go to any location along the I-beam to pick up or drop off a load. The message directing the carrier originates at a host computer and is sent along the communication line mounted on the I-beam to the carrier's onboard intelligence unit.

Another advantage of monorail is precise positioning of carriers. 'Monorail's accurate positioning makes it a good choice when you have to interface with another piece of equipment, like a robot,' says Rich Goelz, sales manager for general industrial systems, Eisenmann (815-455-4100, www.eisenmann.com).

While monorails can be used to carry loads weighing several tons, they are more often used to transport lighter loads over long distances at high speeds. A carrier, for instance, typically travels at speeds of 200 feet per minute and can reach 600 feet per minute.

There are disadvantages, however. The cost per carrier and per foot of track is high compared to other overhead systems. That makes monorail too expensive to be used for buffering parts, as is common with a power-and-free system. Instead, monorail should be looked at as a high-speed transportation system.

The biggest challenge to the adoption of monorail, say experts, might be resistance from maintenance personnel. 'In the United States, plant people have been working with chains forever, and that's the mindset,' says Siemen's Wolsos. 'However, in new plants, it's catching on.'

Overhead cranes are often called the workhorses of overhead materials handling. While jib and workstation cranes are used to improve ergonomics in small areas, bridge and gantry cranes do the heavy lifting over long distances.

'The competing technology for a crane is usually a forklift and not roller or chain conveyors,' explains Doug Maclam, vice president, sales and marketing for Konecranes America Inc. (800-231-0241, www.konecranesamerica.com). 'Not only can they move heavy loads, cranes allow you to locate more material in a confined area where you would otherwise have to create aisles and access for other means of handling.'

Bridge cranes are ceiling mounted and allow movement of loads in six directions: up and down, forward and backward, and sideways in both directions.

Although there are several types, bridge cranes consist of a trolley that runs across a bridge (which can have one or two girders) that moves across a bay along a runway. The largest capacity units, known as top-running, double-girder cranes, mount the bridge on top of the runway and have two girders for the bridge. Gantry cranes are a close cousin to bridge cranes, and run on one or two floor-mounted legs rather than suspended from the ceiling.

Distance is no impediment when working with cranes. 'We've done 1,000-foot long bays,' Maclam explains. 'The difficulty comes if you have to transfer product from one bay to another.'

In those instances, it's not uncommon for cranes to work in conjunction with other materials handling systems, especially in high capacity production facilities with heavy products and repetitive movements.

Since cranes have been used for years, some of the most significant advances being made involve ease of use and maintenance of the equipment, according to John Paxton, president, Demag Cranes and Components (440-248-2400, www.demag-us.com). 'On-board diagnostics is coming to cranes,' Paxton explains. 'The technology includes small onboard computers that record the duty cycle, maintenance data, and load spectrum reporting to enhance the maintenance and safety of the crane.'

The technology allows maintenance personnel to easily download the information they need to a handheld PDA.

Click on the icon for a list of pros and cons of specific types of overhead handling equipment.