Imagine you’ve just started ramping up your newly remodelled assembly plant when news comes that a current product has been cut 30% and another, completely different version needs to be accommodated.
June 30, 2011
by Eric Hutchenreuther
Imagine you’ve just started ramping up your newly remodelled assembly plant when news comes that a current product has been cut 30% and another, completely different version needs to be accommodated. Scenarios like this are common in the automotive industry, and Dürr’s FAStplant conveyor was originally conceived to help automotive OEM’s cope with changing production demands. However, this reconfigurable material handling system that’s independent of the building layout is also adaptable for use in the aerospace industry.
During WWII, Lockheed Martin Aeronautics Co. used ideas from the automotive industry to transform the nature of aircraft production. It continues to do so today with its F-35 Joint Strike Fighter aircraft program at its facility in Fort Worth, Tex. An increased emphasis on cost reduction has Lockheed Martin’s production engineers taking a renewed interest in using conveyors to optimize material flow through the assembly process. They decided on an overhead conveyor system for transporting partially completed wings between assembly stations in the plant that has been in a constant state of remodelling since it was built nearly 70 years ago.
FAStplant didn’t need a lot of infrastructure thanks to its scalability, adaptability to product and process changes, and its TTS belt drive system.
Designed for modularity, FASTplant’s layout can be changed easily because of a standardized interface between components. If Lockheed decides at some point to add more work cells or reconfigure the existing process flow, it’s a simple matter of moving the existing modules, or adding new ones. Field checks and as-built aren’t needed to understand the interface between old and new equipment.
The concept applies on a smaller scale as well. Rather than designing a different detail for every application, there are few unique parts. Although features not used in every application do add some costs, there are also economies of scale and the time spent tracking individual parts is reduced.
There’s only one PLC and main control cabinet, but each module is a discrete unit rather than a set of components and programming is broken into blocks (called tech schemes) for each one. When a new module is added, the tech scheme is pasted into the program and the system is ready to run. Rather than running a cable to each module from the main control panel, the units daisy chain into the adjacent modules. Each motor’s VFD and built-in IO for the switches that detect the carrier’s position are linked via a Profibus network.
Basic control functionality comes with a pendant for all modules, but more advanced functions are handled through the HMI, and Dürr’s EcoScreen software provides intuitive status display and control. As it would for a typical automotive application, the PLC coordinates the synchronous indexing of multiple cells. Carrier speed is fully programmable with ramp-up and ramp-down functions for smooth acceleration and deceleration in the cells where the carrier stops.
In aerospace, takt times (time allowed for each step in an assembly line) are comparatively slower than other types of manufacturing. The old line of reasoning was, give priority to accessibility rather than moving rapidly because of the enormous amount of work that must be accomplished before a work piece is ready to move to the next station.
Traditionally, transportation of parts was done with a crane or tow dolly along the main pedestrian aisle. Such moves are time consuming and cumbersome, often involving moving crew, spotters and specialized equipment, all of which must be scheduled in advance. Beforehand, any workbenches or toolboxes that need to be reconfigured would be moved to clear the path. But as production rates increase and the impact of pulsing the line becomes more significant, the conveyor needs to be more adaptable.
In other types of manufacturing, cycle times at each assembly station are so short it can be dangerous to walk or drive across the path of the moving product. Since dedicated moving space must be allotted, equipment that occupies part of the floor, or even partly blocks access, may still be acceptable if it simplifies the non-value added task of moving parts in and out. It’s possible to mount the conveyor in a pit so workers can walk around the part, but installation costs are quite high. In more cases the part must be mounted on a turntable so all sides can be accessed without stepping across the conveyor.
An overhead conveyor increases accessibility from the floor level, but when the roof has to support a material handling system, the structure of the building must be oversized to handle the weight of the conveyor, plus the hardware.
Roof trusses in auto plants are very heavy with short spans and if any type of modifications were made, it’s likely there’s an uncharted combination of old and new header steel and partially cut-out or still-live utilities. Though early plants were often fitted with skylights, the light is usually completely obliterated from the floor level.
Aerospace plants with their high clearances for tall assemblies and long spans between building columns would be particularly susceptible to the shortcomings of a roof-supported overhead conveyor. Moreover, it would be hard to give up the versatility of a house crane for moves that can’t be accomplished across the floor.
FAStplant provides a compromise between floor and overhead conveyor systems. Most of the equipment for transport and utility supply is supported by its own columns and out of the way of floor-based operations. Other than the area directly under the conveyor, the building crane has access to the entire floor. If that’s a problem, special “C” shaped load bars handle loads directly under the conveyor track.
Lockheed chose Dürr’s Twin Trolley System (TTS) conveyor for several reasons. Other conveyor types use hardened steel rollers in hot rolled steel beams or channels. TTS has polymer wheels on extruded aluminum track that smooth and quiet motion. Tapered steel wheels on typical conveyors constantly rub the track, causing paint, mill scale and rust to rub off and fall into the work area. Catch pans may be a necessary countermeasure to reduce foreign object damage (FOD). The TTS conveyor doesn’t require such protections.
No utilities needed
While it’s possible to provide electrical contacts to power the moving carrier, Lockheed did not require any utilities. The TTS drives the carriers via timing belts mounted above the track. Belts are spaced so there’s always one of the two spring loaded drive dogs in contact with a belt along any point on the line. This drive system is forgiving enough that slight differences in speeds and tooth synchronizations don’t cause any noticeable effect as trolleys get handed off from one belt to the next.
The TTS bearings are sealed and maintenance free, and the belt drive doesn’t wear out as quickly as a friction wheel or chain drive. Friction wheels are very sensitive to tension adjustment; too much tension and the wheel breaks down internally, too little and the contact surface rubs away. Chains have to be checked for correct tension and lubrication, and if something breaks it’s a big job to get them running again. On TTS conveyors, belt replacement is a simple but rarely needed.
In other applications Dürr has provided FAStplant systems with an electrified monorail system (EMS) conveyor. Electrified monorail also uses an extruded aluminum track, but positions a motor on each carrier. This set-up offers economic and operational benefits for systems with a relatively low count of carriers per length of track.
As the motor requires power, there’s some added complexity delivering power to the carrier. On newer systems, Dürr has steered toward inductive power transfer because of the reduced need for maintenance. Typically communication is always through inductive pickup even in cases where the power is delivered via bus bar.
In addition to providing access for the house crane, some parts of the F-35 wing assembly line were adapted to provide a smaller crane that covers each assembly station. The runways mount directly on the top of the structure and steel that supports the conveyor is low enough for the empty crane hook to pass over, allowing the crane to be used on either side of the workstation. In places where the line runs parallel to itself, a small bridge lets one crane move across to the other part of line as a backup.
Reducing or increasing production in an assembly plant on short notice presents a complex challenge. Lockheed’s engineers, not wanting a lot of infrastructure commitment, now use Durr’s FAStplant system to reconfigure the existing process flow by either moving existing modules or adding new ones when necessary. Scalability and adaptability to product and process changes makes this system a good fit for Lockheed.
Eric Hutchenreuther is an application engineer for the Aircraft and Technology Systems business unit at Dürr Systems Inc., a manufacturer of paint and assembly systems. The Dürr Group, based in Germany, manufactures equipment for all aspects of production processes. Visit www.durr.com.