Increasing Assembly Line Flexibility with Automatic Guided Carts

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A Historical Look at the Conventional Conveyor Assembly Line.

In 1919, Jervis B. Webb, a young The assignment of tasks to work- Paced and Unpaced engineering graduate from Detroit, stations is done to ensure that the Lines adapted a forged rivetless chain con- assembly line can meet the demand veyor used in the mining industry to rate. Thus, each workstation is given Since the task times allotted to the rugged needs of the automotive a fi xed amount of time to complete workstations may be unequal, parts industry, allowing automobiles to its task. Depending on the de- are produced at different speeds on effi ciently move on an assembly line. mand rate and task times, it may be the line. Accordingly, stations may Henry Ford quickly recognized the necessary to duplicate one or more either be starved or a queue may possibilities that existed with this workstations. When the demand is build up in front of a station. To innovative technology and installed high enough, it is not uncommon regulate the fl ow of parts, assembly 30 miles of Webb conveyor systems to duplicate the entire assembly line. lines are often paced. In a paced line, in his plants. As a result, this single This shortens the assembly line, but each workstation is given a fi xed invention helped revolutionize mass may require more equipment and amount of time called “C.” Material production. tooling. Also, because each worksta- handling systems are designed so that tion has a larger amount of time to after every C time unit, the system Today, nearly 90 years later, assembly complete its tasks, more tasks can indexes, advancing the part to the lines are the backbone for manufac- be assigned to the station, thereby, next station. If a workstation fi nishes turing and production plants all over enriching the work content. Also, in less than C time units, it is idle for the world. They’re the conventional if equipment problems occur at a the remaining period. On the other choice for increasing productivity station, other lines can continue to hand, if the workstation is unable to and reducing costs. The balancing run. A single serial line would have complete the assigned tasks in C time act of assigning tasks to workstations, to be shut down whenever there was units, the part will still be indexed however, has been a problem for as- a failure at any workstation. forward. The incomplete part will sembly lines for many years. Because index through the remaining work- each task may require a different While the assembly line balancing stations with little or no work being amount of time, the time spent at problem is typically solved after the done at the stations. To ensure that each workstation is rarely equal. This assembly process has been designed, this does not happen, it is common leads to idle time at workstations. it is a good practice to revisit the to provide some “slack time” at each One of the objectives of assembly assembly process to redesign tasks workstation to reduce the chance of line balancing is to minimize this idle and remove any problem areas. A incomplete work. These slack times time. A secondary objective is bal- bottleneck station could be avoided are more important during the earlier ancing workloads across workstations by redesigning some of the tasks and stages. Typically, if a part does not so that no workstation has excessively their associated tooling. get completed on the assembly line, high or low workloads. it is taken offl ine to a repair and Smart Warehouses, Smarter Productivity

rework area for completion. It is not Indexing units over identical work from Jervis B. Webb Company, a unusual for a queue of repaired parts zone distances allows time for the subsidiary of Daifuku Co., Ltd., to be kept available to feed the line worker to pick parts and/or perform which is ideal for moving products whenever an empty slot develops on other duties during the index cycle. on an assembly line. the line. High speed index travel minimizes lost time and, in most cases, is equal What makes SmartCart AGCs so If there is too much variability in the to walk-back time on fi xed-speed fl exible and cost-effi cient? Smart- task times, it is preferable to have un- lines. Indexing allows permissive Cart AGCs are guided by magnetic paced or asynchronous lines. In such safety interlocks to assure workers tape, allowing for quick installation lines, each station works at its own and tools are returned to a safe posi- or modifi cation of the guidepath in pace and advances the part to the tion before index travel. as little as a few hours. In addition, next station whenever it completes SmartCart has numerous load han- the assigned tasks. Experience has proven real through- dling options to meet specifi c needs. put is the same or improved on the Over time, the load handling carrier Mixed Models indexing line due to stable work can be replaced to accommodate dif- position, support, environment, ferent products, protecting the initial The demand for a single product reduced worker stress and fatigue, as investment. is often not suffi cient to justify an well as system control over the events independent assembly line. In such at each individual workstation. The Modern Approach cases, it is common to design mixed To Increasing Produc- model assembly lines. Typically, the The Dilemma Facing tivity And Reducing products being made on the assembly Conventional Conveyor Costs line tend to have very similar tasks Assembly Lines and precedent diagrams. One may design a single line for the assembly A prime area for cost savings in man- While the assembly line provides of a product that has several differ- ufacturing plants is through in-pro- undeniable benefi ts, three challenges ent options. In designing such lines, cess inventory reduction. Simply by can’t be ignored. First, scalability is different models are appropriately adding or removing SmartCart AGCs an issue because assembly lines are sequenced on the line to ensure a from the line, assembly production typically designed for the maximum smooth work fl ow. rates can be increased or decreased on potential production rate up front. demand. This is a substantial benefi t Second, installation generally takes Indexing vs. Fixed- over conventional conveyor methods, many weeks or months to complete. which require the assembly line to be Speed Operations Third, modifi cations are typically designed and installed up front for very expensive and also take many the maximum potential production Theoretically, a fi xed-speed assembly weeks or months to complete – not rate. And, because of their fl exibility, line will produce a set number of including potential building modifi - SmartCart AGCs are an excellent units for a given period of time at a cations. choice for companies adhering to given speed. Therefore, the model continuous improvement initiatives. complexity required to work and Automatic Guided support the line allows for little Carts: The Flexible As- SmartCart AGC provides the as- human error and no interruptions. sembly Line Solution sembly line designer with a variety of In other words, everything must be problem-solving tools, such as: almost perfect to make throughput. The SmartCart® Automatic Guided Workers must constantly adapt dex- • conforming layout to existing Cart (AGC™) is a fl exible and cost- terous skills to the work unit as they physical geometry, which is not lim- effective material handling solution try to keep pace with the line. ited to straight line operation Smart Warehouses, Smarter Productivity

• providing workstation confi gu- optimum balance among all worksta- ration and spacing to fl ow around tions. SmartCart AGC technology columns and other fi xed installations provides this fl exibility while allowing organizational input to become • allowing parallel workstations in operational reality with the least number or physical relationship to amount of downtime and disrup- serial workstations tion – making SmartCart the newest innovation to help revolutionize the • dedicating mixed-model worksta- assembly line. tions to single or multiple product assembly tasks Recap: SmartCart AGC Strengths vs. Conventional Conveyor Assembly • accomplishing continuous Lines improvement adjustments during breaks, off shifts and weekends • Scalable Material Handling: SmartCart allows you to increase • indexing synchronous or non- throughput by simply adding AGCs. synchronous systems based on take time and/or permissive input signals • Easy Installation: SmartCarts can be installed in as little as a few hours; • optimizing throughput for pre-set conveyor system installations can speeds between main line operating take weeks or months to complete. speeds • Easy Modifi cation: Changes to • reducing operator fatigue and the SmartCart guidepath can be stress with fi xed workstation lo- made in as little as a few hours, while cations that allows operators to conveyor system modifi cations can perform intermittent duties during be very costly and take weeks or AGC index months to complete.

• providing individual workstation • No building modiﬁ cations are safety and ergonomics required with SmartCarts.

• increasing reliability because AGCs are not subject to main line prime-mover failure.

SmartCart AGCs: The Ideal Assembly Line

The ideal assembly line is meant to be very ﬂ exible in nature – contract- ing or expanding by deleting or adding individual workstations to suit the ever-changing work and product requirements, while also maintaining