Imagine you’ve just started production in your newly remodeled assembly plant, and news comes that production of the existing models will be cut 30 percent and another completely different variant will need to be accommodated. Scenarios like this are a nightmare for the facilitation of lean principals, but a common occurrence in the automotive industry. Unfortunately, it also seems that the higher the production rate, the more specialized the infrastructure needed to keep up.
Lockheed Martin Aeronautics Co. is no stranger to using innovative methods for increasing manufacturing output. During WWII Lockheed used ideas from the automotive industry to transform the nature of aircraft production. With the F-35 Joint Strike Fighter aircraft program’s increased emphasis on cost reduction, production engineers took a renewed interest in using conveyors to optimize material flow through the assembly process. For wing assembly, they decided on an overhead conveyor system for transporting partially completed wings between assembly stations. In a plant that has been in a constant state of remodeling since it was built nearly 70 years ago, Lockheed’s engineers wanted a conveyor solution that does not require a lot of infrastructure commitment. Dürr’s FAStplant® was a perfect fit because of its scalability, adaptability to product and process changes, and it’s highly reliable and easy to maintain TTS belt drive system.
The FAStplant concept achieves its flexibility because it was designed for modularity. Similar to building blocks, the layout can be changed without headaches about compatibility because of the standardized interface between components. If Lockheed decides to add more work cells or reconfigure the existing process flow, it is a simple matter of moving the existing modules, or adding new modules as required. There are no field checks or as-built drawings required 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, the design was carried out with the goal of creating the fewest number of unique parts. Using this philosophy, there are some added costs for features that may not get used in every application, but there are also great savings from economies of scale and reduced time spent tracking individual parts.
Modularity occurs in the controls design also. Although there is only one PLC and main control cabinet, the modules are each designed as a discrete unit rather than a set of components. Programming is broken into blocks called tech schemes for each module. When a new module is added to the system, 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 are designed to daisy chain into the adjacent modules. The VFD for each motor and its built-in IO for the switches that detect the position of the carrier are linked via Profibus network.
Although basic control functionality for driving the conveyor is provided with a pendant for all modules, more advanced functions can be handled through the HMI. Dürr EcoScreen software provides user-friendly status display and control. On a typical automotive application the PLC will coordinate the synchronous indexing of multiple cells. Carrier speed is fully programmable with ramp-up and ramp-down functions to provide smooth acceleration and deceleration in the cells where the carrier is brought to a stop.
Industrial Engineering Considerations
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 that priority must be given to accessibility over the ability to move rapidly because of the enormous amount of work that is accomplished before the work piece is ready to move into the next station. Transportation of parts was traditionally 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 work benches or toolboxes that need to be reconfigured are moved to clear the needed path. As production rates increase however, and the impact of pulsing the line becomes more significant, a conveyor solution like FAStplant® becomes more appealing.
In other types of manufacturing, the cycle times at each assembly station are so small that it can get 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 is possible to mount the conveyor in a pit so that workers can walk around the part, but the installation costs are quite high. More frequently the part must be mounted on a turntable so that all sides can be accessed without stepping across the conveyor.
An overhead conveyor increases accessibility from the floor level. Unfortunately, when a material handling system is supported by the roof, the structure of the building must be oversized to handle the weight of the conveyor plus the hardware to put a hanger wherever it may be desired. Roof trusses in auto plants can be characterized as being very heavy with short spans. Also if the plant has undergone any type of modifications, most likely there is 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, which frequently must have high clearances for tall assemblies and long spans between building columns would be particularly susceptible to the shortcomings of roof supported overhead conveyor. Moreover, it would be hard to give up the versatility of using the house crane for moves that cannot be accomplished across the floor.
The Twin Trolley System® (TTS®) conveyor was selected at Lockheed because it is clean and simple while being reliable and easy to maintain. Unlike other types of conveyor which use hardened steel rollers in hot rolled steel beams or channels, the TTS uses polymer wheels on extruded aluminum track. The improved surface finish of the aluminum, combined with the synthetic wheels results in very smooth and quiet motion. Also on typical overhead conveyors, the tapered steel wheels constantly rub the track, causing paint, mill scale and rust to rub off and fall into the work area. In an effort to reduce Foreign Object Damage (FOD), catch pans may be a necessary countermeasure. The extruded aluminum and plastic combination of the TTS conveyor is clean enough that it does not require such protections.
While it is possible to provide electrical contacts to power the moving carrier, Lockheed did not require any utilities supplied to the carrier. For this reason it was also favorable to choose the TTS, which drives the carriers via timing belts mounted above the track. The belts are spaced such that there is always one spring loaded drive dog, of the two on each carrier, 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 do not cause any noticeable effect as trolleys get handed off from one belt to the next.
The TTS requires almost no maintenance. All the bearings are sealed and maintenance free. The belt drive helps with reliability because it does not wear out as quickly as a friction wheel or chain drive. Friction wheels a very sensitive to tension adjustment; too much 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 is a very big job to get running again.
In addition to providing access for the house crane, some parts of the F-35 wing assembly line was adapted to provide a smaller crane that covers each assembly station. The runways mount directly on the top of the structure. The steel that supports the conveyor is low enough to allow 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 is included to allow one crane to move across to the other part of line as a backup.
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