Taking the high road

Overhead conveying gets inventory off the floor and keeps it on the move.

By -- Modern Materials Handling, 11/1/2000

With space at a premium in manufacturing and warehousing facilities, overhead conveying offers many advantages over floor level systems.

"The main reason for overhead conveying is to keep transportation off of the floor," says Ken Ruehrdanz of Rapistan Systems.

Monorails, cranes, and overhead chain conveyors, such as power-and-free units, reduce traffic commonly caused by lift trucks, pallet jacks, tow tractors, and guided vehicles. Besides eliminating floor congestion, overhead systems better utilize space above crowded machinery that is typically not used for any other purpose. They also reduce manual handling, provide movement of product from assembly to storage, allow for conveying and buffering for work-in-process manufacturing, and improve ergonomics and ease of access to work.

Overhead equipment additionally rates among the most flexible of materials handling systems by moving products of most shapes and sizes. Depending on the units chosen, full movement can also occur in three dimensions-up and down, forward and back, and side to side.

As with any materials handling system, selection of which system a facility chooses depends on application, speed, product to be carried, and cost.

Facilities that have large distances to cover may choose overhead electrified monorail systems.

"Companies using these systems want to leverage the design aspects of monorails, which are speed, the ability to accumulate, and the intelligence of the individual carriers," explains Mike Kotecki of HK Systems. "And being overhead, you buy yourself a lot of free real estate down below."

Besides space savings, another factor in selecting a system is speed.

"Throughput plays a huge part in the decision of using monorail or a power-and-free system," says Fintan Mulligan of Murata.

Monorails are among the most versatile of overhead systems, and rank as the fastest overall, attaining speeds up to 600 ft./min.

Typically, systems consist of an I-beam track from which carriers are suspended. Each carrier contains its own power and communicates in real time with execution and management software systems. As a result, inventory location is known at all times. Monorails also tend to be quieter than chain-driven overhead systems, and can run forward or reverse. That helps in backing carriers out of workstations.

"At a whim you can change the direction of the carrier," says Paul Hopersberger of Jervis B. Webb Co.

Monorails are ideal for handling larger payloads, and they usually feature hoists or grab arms built into the carriers to receive and discharge product. They can change elevations easily, while freeing up precious floor space.

A major tire manufacturer uses one of Murata's systems in a tire plant in Asia. Mulligan says the entire facility is covered with machines, and the only way to reach them was with overhead conveying.

The monorail system employs about 60 carriers with hoists that descend to pick up pieces of wire used in the manufacturing of the tire's steel belts. Three monorail lines form ovals that loop through the facility, offering a variety of paths for the carriers. This is typical of monorails, where spurs or loops permit access to workstations while allowing other carriers to flow by unhindered. Such spurs can also make repairs easier, as a carrier can be diverted to a maintenance spur for routine evaluation. If a carrier breaks down elsewhere in the system, it is simply removed from the rail and does not stop the other carriers from moving.

"Monorail carriers are like putting a slot car on a race track," says Kotecki.

Overhead monorails also easily interface with storage systems, as well as transfer product quickly to automatic guided vehicles. Most monorail systems are more expensive than chain-driven overhead conveyors, but the advantages of speed, flexibility, and easier maintenance often outweigh the cost factor.

Manufacturing facilities with a large number of assembly processes often utilize power-and-free conveyors. There are two major types of power-and-free-overhead units and floor level models, which are known as inverted power-and-free conveyors.

The overhead units typically consist of a double-channel track with a moving chain in one track and carrier mounts in the other. Pusher dogs engage the chain to move a carrier forward. They disengage at points where work on the product is desired. This makes power-and-free one of the most popular choices for work-in-process assembly.

"Power-and free systems allow you to accumulate loads at workstations to balance workloads," adds Jervis B. Webb's Hopersberger. "You also have the ability to control and track every carrier in the system."

Typically these systems run about 150-200 ft./min. and often have multiple chains to increase flexibility. A major advantage is in the wide range of products units can carry, with loads ranging from 25 to 25,000 lbs. These consist of everything from a small part transported through a paint shop to an airplane fuselage.

As with monorails, power-and-free conveyors can easily adjust height, providing good ergonomics for the manufacturing environment.

"You can move the part relative to the man," says Jeff Homenik of Dearborn Midwest. "That allows a worker to get right under an assembly without a track getting in the way."

Ford Motor Company uses a number of Dearborn Midwest units at its Wixom, Mich. assembly facility. Overhead power-and-free systems there carry small parts, doors, side panels, and even complete auto assemblies.

The Continental, Town Car, and LS models produced in Wixom all have different engines that are installed at separate areas of the plant. Bodies ride overhead power-and-free units as they leave the body shop and are then diverted using track switches to the areas where the three different engines are inserted. The bodies will later meet up on the main assembly line, also utilizing a power-and-free.

Another type of overhead system that provides a less costly alternative to power-and-free units is the trolley conveyor. These conveyors typically have just one chain instead of the two chains that comprise power-and-free units. Carriers simply move forward when the trolley conveyor is powered. To stop them, the operator shuts down the conveyor. These units are best suited for simple conveying of materials over long distances where work does not need to stop.

Some trolley conveyors are also manufactured without power. An assembly station worker completes work on the product, then simply pushes the carriers ahead by hand to the next line worker.

Short-distance moves within facilities are often accomplished with overhead cranes. Jib and gantry cranes usually service stationary workstations while bridge cranes move materials between workstations.

Jib cranes are single-girder underhung cranes cantilevered from a wall or mounted on the floor. Gantry cranes are mounted on one or two legs. Both pivot, allowing a full range of motion within the workstation area.

Bridge cranes are similar to monorails, except that typically there is only one carrier on the I-beam track. These systems are designed for very heavy lifting applications, but unlike monorails that travel forward and reverse, cranes coupled with hoists allow full-dimensional motion.

"You can get from point to point in a number of ways," says Al Romig of Mannesmann Dematic. "You can lift first, or have one axis move before another, or both simultaneously."

Such variety of movement provides increased flexibility, making cranes extremely versatile.

Nova Steel gets a lift from its crane

Handling large coils of steel is a difficult lifting task for any facility. Nova Steel of Louisville, Ky. uses two large overhead cranes to accomplish the work. The company is a steel processor, taking in large coils of steel and slitting them to various widths and lengths for use by manufacturers, particularly those producing automotive parts.

Repair parts fly at American Airlines

American Airlines uses an overhead monorail system to deliver aircraft parts to four maintenance hangars at Dallas-Ft. Worth International Airport.

The system has eliminated a large amount of ground equipment and personnel that the airline had used to truck parts to the maintenance areas.

Most aircraft parts are stored in a mini-load automated storage and retrieval system at the main maintenance warehouse. When a particular part is required, the mini-load retrieves it and places the part into black totes, which are then deposited onto a takeaway conveyor.

Four arms extending from each monorail carrier then reach down to pick the totes from the conveyor. It then begins the ride to the four hangers at a speed of 600 ft./min.

The half-mile long monorail system is built within an overhead tunnel that crosses two streets on its way to the maintenance hangars. The track passes directly through Hangars 1 and 2. These are the small hangars, each holding two aircraft. If the part is needed here, the carrier drops the tote onto an elevator. The part descends down to a conveyor that then takes the tote to the repair area.