Equipment: Overhead handling basics

Overhead handling systems can include elevated roller, belt, chain, power and free conveyors, and monorails. Here's a primer on how they work.

By Allison Manning, Associate Editor -- Modern Materials Handling, 12/1/2008

One of the biggest challenges for distribution center managers is efficiently fitting their people, equipment and product within four walls. Pickers and packers shouldn't be bumping into one another while maneuvering around equipment. It's inefficient and unsafe. So, many mangers look up for guidance—to an overhead conveyor system.

Overhead conveyor systems can include elevated roller, belt, chain, power and free conveyors, and monorails. These products are often used in assembly or manufacturing environments, according to Tom Carbott, Conveyor and Sortation Systems managing director at the Materials Handling Industry of America (704-676-1190, www.mhia.org).

OVERHEAD CONVEYORS

Adding an overhead conveyor system can be a good idea for a warehouse that's looking to expand, and already has a ground-level conveyor system in place. Locating product and equipment overhead provides minimum interference with those working below, as long as proper safety precautions are taken.

“Floor space is money,” says Stephen Folsom, manager of engineering at VARGO (877-876-6384, www.vmhi.com). “You want to use the upper cube of a building where there aren't people working to maximize what that facility can do. Transportation chews up floor space, and if you can do it in a place that's not utilized, there's savings.”

An overhead system also means fewer people are needed to handle products, whether it's by hand, lift truck or pushing along a regular conveyor. When equipment and product is elevated, it stays out of the way of workers and can travel at much greater speeds as a result.

Which overhead system you choose depends on the individual application, says Don Derewecki, president of Gross & Associates|TranSystems (732-636-2666, www.grossassociates.com).

“The size and weight of what you're transporting and the throughput requirements should dictate the actual materials handling technology,” he says.

Consider some facility specifications before installing an overhead system including:

• Product load, weight, size and nature—not every building is able to structurally support an overhead system

• Where the product needs to go, and how fast

• Plant layout and clear space

Belt and roller conveyor

The most simple and familiar type of overhead system is an elevated belt or roller conveyor. Almost identical to similar conveyor systems of the same name that would be based on the floor, the overhead version allows a higher throughput rate, higher speeds and greater queue densities, says HK Systems vice president of engineering Phil Kaffenberger (800-457-9783, www.hksystems.com). All of the cases on the conveyor can be close together or touching, allowing greater density, unlike with chain conveyor.

A roller conveyor is also a better choice for products that aren't easily suspended, like a corrugated case filled with eaches in preparation for shipping.

For picking processes, some facilities use elevated conveyors for ergonomic purposes. Multiple levels of conveyors can be placed even with a shelving unit, where a picker stands on a mezzanine. The order pickers fill the boxes as they pass, and the boxes are transported from one level to another, before being sorted, wrapped and shipped. Instead of climbing up and down ladders or having to lug totes, the packaging comes to the person.

Chain conveyor

Overhead chain systems are by far the most common type of overhead handing and have the ability to move a variety of product weights, from 5 to 1,500 pounds, says Brent Brosch, director of industrial conveyors at Jervis B. Webb (248-553-1000, www.jervisbwebb.com).

In this versatile system, the load is attached to a pendant spaced evenly along a single chain, which pulls the load forward through an enclosed track. The rate of flow is determined by the spacing of the pendants, with the loads constantly moving at the same speed and pace.

The sequence of the product is also static, operating on a first-in, first-out basis. This is why the system is usually best for several of the same item, such as car parts or empty boxes.

Consisting usually of a drive motor and wheel pulling the chain through a tube, dragging the load along with it, this type of conveyor is relatively simple to maintain. But the systems tend to be slow, especially if there are a number of curves, inclines or declines along the route. But because it doesn't require a lot of control configuration, it's rather economical to install, according to Brosch.

Chain conveyors have been used extensively in the paint and finishing market, transporting auto parts and other unwieldy loads for more than 50 years, says Brosch.

Power and free

Chain conveyors run at a constant speed all the time. However, there is a more upscale, intelligent version of a chain conveyor: power and free systems. These systems allow one part to idle while other parts continue on their route, Brosch says.

While the product hanging from a chain conveyor is static in its sequence, power and free systems allow the load carrier to separate from the drive chain and idle while the chain continues to move other loads. This allows accessory parts, such as in an automotive plant, to move to a production area and remain stationary while a worker uses the parts.

There are two tracks in a power and free system, a chain track on top that provides the power, and the carrier track sitting below, which moves the carrier to assembly or processing areas.

“Power and free allows you complete flexibility. It's an intelligent system,” Brosch says.

One of the oldest, most “tried and true” type of technologies, Carbott says, power and free is found frequently in the automotive industry, especially in paint lines, where it can carry loads from 25 to 25,000 pounds.

By being able to disengage, the loads can be diverted into a queuing lane, away from the main drive chain and held there indefinitely. The loads can also be diverted to other tracks and areas of a warehouse or DC. This feature provides a dimension of storage, and allows the product to be resequenced—an option that provides flexibility.

HK's Kaffenberger says this system is often used in handling garments for the retail industry, similar to the system a dry cleaner would use. Garments hang from hooks or hangers, idling in storage, until called out to fulfill an order. This set-up eliminates unwieldy rolling racks, which cause clutter on the floor. “That's all about saving space,” Kaffenberger says.

MONORAILS

Similar to the monorail that transports visitors across Disney World's theme parks, electrified monorail systems are best for moving high-value products across long distances. The system consists of a single rail or I-beam from which a hook or hoist hangs, providing both lateral and vertical movements of loads.

Eisenmann sales manager Rich Goelz (815-455-4100, www.eisenmann.de) says of all the overhead systems, electrified monorail is the most flexible and durable, with load weights of up to 30,000 pounds. Each carrier has its own drive unit and can run at variable speeds in different areas, topping out at 600 feet per minute. A chain conveyor rarely exceeds 60 feet per minute.

Rather than a chain moving the product attached along a set track, the product transported by a monorail is powered by a bus bar, or power rail, inside the monorail beam. The system is easy to integrate with lifting devices or other electrification.

Monorail can be more expensive than other systems, and should be seen primarily as a high speed, long distance transportation system, Goelz says.

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