February 14, 2002
This article describes how power-and-free conveyors are used in manufacturing. It outlines new themes in conveyor design, systems design, control systems, and simulation software.
The metal fabricating industry uses a variety of conveyors. One choice for processing, pretreatment, and finishing operations is the versatile power-and-free conveyor, which can be supplied in an overhead or inverted (floor-mounted) configuration.
These systems get their name because they use multiple rails. A continuously driven top, or power-rail carries dogs that engage carriers in the bottom, or free, rail to move them through the plant.
Other types of conveyors—overhead chain, towline, belt, and powered roller—also are used.
Automated guided vehicle systems (AGVs) and self-guided vehicles (SGVs) serve many of the same applications as power-and-free conveyors but provide more sophisticated control. They offer fixed-path configuration, but provide flexibility if work flow and layouts are changed frequently.
Recent developments in power-and-free conveyors have reduced costs, improved system design, integrated control systems, and enhanced simulation.
"The bulk of our development efforts in conveyor products has involved increasing value to the customer and reducing cost of the system," said Mark Longacre, marketing manager at FMC Corp., a supplier of power-and-free and other overhead chain conveyor systems and AGVs.Paul Hopersberger, director of marketing at the Jervis B. Webb Co., an overhead conveyor and AGV supplier, echoes this approach.
"Our 3-in. power-and-free conveyor has been enjoying increased popularity in automotive stamping and finishing operations," he said.
"It can be traced back to the use of lighter-gauge, higher-load-capacity steels for conveyor track and chains that permit the use of less conveyor support steel. Not only does that reduce our price to the customer by about 25 percent, but it contributes to increased design versatility by permitting tighter radiuses for horizontal and vertical curves.
"Not only can you reduce the overall floor space required for the conveyor path, you can often shorten the linear area between process equipment at downstream workstations," he said.
Conveyor systems intended for just-in-time (JIT) manufacturing and other advanced manufacturing methods must be flexible enough to be reconfigured when production requirements change. Most adaptable are AGV systems that follow easily changed, laser-guided paths.
Not all conveyors have to be expensive or sophisticated to be adaptable to changeover. Basic enclosed-track chain conveyors manufactured by Pacline are designed to be bolted together in the field rather than welded. While some degree of rigidity is lost, it will offer the ability to make changes without taking the conveyor apart. It also allows users to operate it in any configuration.
Power-and-free conveyors can accomplish many of these functions, but not with the same changeover ease. Typically, route variety and control versatility must be designed into the conveyor during initial planning and installation.
Power-and-free conveyors can be used with robots, process equipment, and other systems, offering greater flexibility and control. Engineering and production management can address productivity and changing needs by gathering data, tracking the status of work-in-progress, and keeping records through computers and readers.
At a basic level, mechanical switches coupled with overhead power-and-free conveyors direct products through various routes of the manufacturing process. At a more sophisticated level, a programmable logic controller or PC will process programmed data in real time, communicating between the programmable logic controller and automated, electrified monorail units, for instance, to provide pause-proceed, route, and dwell time information at various processing points.
Programmable logic controllers, traditionally used to manage power-and-free systems, can accommodate the newer, flexible manufacturing concepts. Integrated with a firm's mainframe, programmable logic controllers provide data for work-in-process (WIP), verification of work standards, and inventory management.
"We've focused our efforts on making controls easier for customers to use, providing information when they want it, not when it is convenient for us to provide that information," Longacre said.
Also used in metal fabricating are radio frequency data communication (RFDC), which employs bar-coded labels, and radio frequency identification (RFID), which uses transponders embedded in tags or labels.
The two formats use different technologies—optical scanning in the first instance and radio frequency recognition in the second—to collect product data, WIP status, and inventory data. Both methods employ data tags either on the conveyor unit or on the product, depending on whether the purpose is data collection or process control.
The operational features and benefits of using RFDC and RFID with an advanced AGV system are shown in the following example from an automotive operation:
This system can provide throughput of 1,000 racks of metal stampings in an eight-hour shift, 25 percent higher productivity than a manually serviced processing line, and flexibility for the plant's production demands.
Soft controls, or PC systems that offer easier connectivity to mainframes, are challenging the use of programmable logic controllers in the market. The issues are between traditional uses of the programmable logic controller and the flexibility offered by new PC-based systems.
Simulation software mimics the operation of a conveyor system, enabling the user to debug process control software or ensure that the system will perform as intended.
"It provides a real-time display of the system, showing production cycles for products that integrate with other material handling or manufacturing equipment," said Sue Niemiec, marketing manager at Richards-Wilcox, a supplier of power-and-free conveyors.
Most suppliers have emulation software or simulation capability that matches the layout of the system, duplicating transfer speeds, acceleration times, dwell time at processing points, and other system-critical data. Programmable logic controller code can be emulated and parameters changed, and conditions altered and equipment added or removed to optimize the system so it meets production goals and schedules.
Companies caution users, however, that simulated results are only as reliable as the entered data. The customer must have an intimate knowledge of material flow requirements and the anticipated data into and out of control systems.
Initial data requirements for a good simulation result include:
"This software allows a customer to see alternative system configurations, visually displaying ... a customer's production cycle in motion, including storage lanes, traffic routing, throughput statistics, load-unload processes, and other critical operations," Niemiec said.