Following a fixed path

Conveyors, overhead monorails, and wire- or rail-guided vehicles are among the many ways for moving materials.

By -- Modern Materials Handling, 9/1/2000

Say you need to move materials along a fixed path. Which handling methods and equipment types should you consider?

There are several handling technologies that can get this job done in manufacturing and in warehousing applications. Floor-mounted conveyors come first to mind, perhaps. Overhead conveyors and monorails are among the other major system categories for moving materials over a set course. Guided vehicles, running either with or without operators aboard them, also traverse a fixed, generally horizontal route. And they do so easily and reliably, with a load of materials.

Overhead systems move loads both horizontally and, to a degree, vertically, of course. Often, the choice of these systems responds to the need to free up shop or warehouse floor space below. In other words, your choice for horizontal movement of inventory may need to have some vertical movement in it too, if the cost/benefit equation works in your favor.

Following Henry Ford's fixed path

Henry Ford pioneered mass production of autos early in the last century. A key concept then involved moving autos progressively along an assembly line by chain conveyor.

In the intervening 80 or so years, conveyor makers have refined and improved upon this concept many times over. And so conveyors remain the materials handling backbone of auto assembly in the U.S. and elsewhere in the world.

DaimlerChrysler Corp., for example, now builds Jeep Grand Cherokees in Detroit using over 10 miles of power-and-free conveyor (Dearborn Mid-West Conveyor, www.dmwcc.com , 734-288-4400 ). Each Jeep vehicle undergoing assembly spends a day and a half riding on power-and-free and other conveyor systems before it's finally ready to roll away to the consumer under its own power.

Why power-and-free at Jeep? Plant engineering manager Mike Colburn says his Jeep plant needs power-and-free's buffering capability. The system's capability for accumulation and storage "protects us from excessive downtime. And it improves our throughput."

Similarly, PACCAR builds Kenworth and Peterbilt trucks near Montreal with overhead power-and-free conveyor as well as over-and-under conveyor (Acco Systems, www.accosystems.com , 810-755-7500 .) Operating on a mezzanine level much like "a very fancy clothesline," the power-and-free system, says plant manager Gilles Gervais, provides for an uncluttered and safer shop floor environment below it.

Much like the Jeep application, this system at PACCAR also delivers line buffering capabilities. And, under computer control, it correctly sequences specific truck units to the line just as they're needed for further assembly.

Behind another well-known brand name, Harley-Davidson, there are several fixed-path assembly systems at plants in Missouri and Wisconsin. Included is the 346 ft. tow line conveyor system at Harley's Kansas City, Mo. plant. This variable speed tow system (SI Handling, www.sihs.com , 610-252-7321 ) brings motorcycle units on ergonomic carriers to work cells for progressive assembly along the line. SI execs suggest there's a trend to using tow lines in these kinds of U.S. manufacturing operations.

Auto, truck, and motorcycle makers, of course, have no monopoly on manufacturing applications of fixed path conveyors.

"Keep it simple" is his guiding philosophy behind materials handling, says Dan Lewis, vice president, manufacturing, at Howard Computers, Laurel, Miss. A key system at Howard is a single powered roller conveyor line (Shuttleworth, www.shuttleworth.com , 219-356-8500 ). The system is laid out in a straight line. "Because the line's a straight shot, we have the visibility to see if we have any flow problems developing," Lewis says.

Fixed-path conveyors (both belt and roller models) combine at QSC Audio Products, Costa Mesa, Calif. so that the fast-paced tempo of making amplifiers keeps up with high demand levels. Conveyor spurs off the main roller conveyor line (Hytrol, www.hytrol.com , 870-935-3700 ) do provide some variability of routing to what otherwise is essentially a fixed path system.

Production of computers and amplifiers are but two more examples of fixed-path conveyorized assembly. And how many warehouses, in today's competitive environment, can thrive, indeed even exist, without using some kind of fixed-path conveyor technology? Conveyors figure in virtually every one of our Warehouse of the Month features.

Suspended from a rail overhead

Power-and-free conveyors can be inverted (floor mounted) as they are in some 8 plus miles of the systems at Jeep. Or they can be installed overhead, as they are at Jeep (2 plus miles) and at PACCAR.

Other overhead systems allowing materials to move on a fixed path include simple patented track and rail/trolley systems. These basic concepts of materials movement via a ceiling-mounted rail then extend into the technology behind the more sophisticated, automated electrified monorail (AEM) systems.

Harley, for one, has opted to install AEM systems (Assembly Test and Technology, www.dtindustries.com, 734-522-1900 ) at several of its Milwaukee-area manufacturing plants for motorcycle engines.

Starting in 1990 with its initial AEM, Harley subsequently modified and expanded these fixed-path systems five times, as of changes last made in 1998.

These five upgrades point up the fact that this supposedly "hard" automation by AEM does permit some revisions. In Harley's case, expansions of the AEM systems were necessary to keep up with rising demand for motorcycles. Thus, AEM installations can be flexible, to a moderate degree.

Along with floor space savings, the AEMs give Harley assembly line workers totally free, 360 deg access to work-in-process units transported on the systems carriers suspended from the overhead track or rail. Line rebalancing and pacing changes also can be quickly made with simple, software programming modifications-often with a phone call and modem link.

In another example of systems featuring a fixed path subsystem, a plastics manufacturer uses an AEM as a means to transport Super Sacks of raw material. The sacks are moved by trolleys operating on the monorail subsystem (Webb-Triax, www.asrs-webbtriax.com , 440-285-4630 ). At an automated storage and retrieval system (AS/RS) unload station, an AEM trolley picks up a palletized sack, lifts it off its pallet, and moves it to one of three mixer feed bins on a mezzanine some 20 ft. above floor level.

Guiding by wire, by rail

Automatic guided vehicles (AGVs) using navigational guidance from the fixed paths set by rails, wires, or tapes represent a third, major alternative to the conveying and overhead systems previously described.

Similarly, specialized industrial trucks such as orderpickers, sideloaders, and turret trucks often are rail- or wire-guided when operating in narrow- or very-narrow-aisle rack storage systems in warehousing. Due to the tight squeeze in these NA or VNA systems, there's little margin for error on the operator's part which is why guidance is important.

These AGV or guided lift truck systems can be configured with single or multiple vehicles. Generally, they're bidirectional in the paths they follow.

This issue's Manufacturing Spotlight feature illustrates how Unifi uses wire-guided AGVs to help boost its productivity. An earlier manufacturing story at a Caterpillar plant in Pontiac, Ill., showed how wire-guided AGVs (Schlafhorst Automation, www.saiautomation.com , 616-393-0101 ) transport trays of parts and interface with an AS/RS.

Meantime, at a hybrid, 284,000 sq. ft. distribution center in Ono City, Japan, the Kansai Co-op moves pallet loads of foods in cases by means of STVs (sorting transfer vehicles). This loop system (Eskay, www.eskay.com , 800-253-1003 ) has 22 STVs. The vehicles shuttle between receiving and a storage buffer over dual rails in this specific application. Single-rail guidance is suitable in other STV systems.

In the U.S., a floor-running STV loop system (Eskay) moves pallet loads among input conveyor, freezer storage, and output conveyor for Keystone Foods in Camilia, Ga.

True fixed-path AGV systems appear to be installed less and less frequently, however. Even wire-guided AGVs are acquiring more "smarts," more of a roaming capability.

Systems, for example, at the Seattle Times newspaper and at a Home Depot imports DC (AGV Products, www.agvp.com, 704-845-1110 ) run off wire guidance. But they add navigational flexibility through a traffic routing system. This routing system can "unlock" or "lock" a section of wire for movement by a specific AGV. This procedure avoids collisions between this AGV and others in the system.

From conveyors to AGVs, from overhead track and trolley equipment to more sophisticated AEMs, the systems highlighted here demonstrate the wide variety of options when it comes to finding the best system to traverse your particular fixed path from start to finish.