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Optimizing Assembly: A Case Study

  • Michael A. M. Bork
  • Peter J. Sherman
This article is a sidebar to "A Primer on Attaining High-Impact Flow."

Proterra Inc. is a designer and manufacturer of zero-emission heavy-duty vehicles, which are in high demand. With the goal of 100 percent customer satisfaction, delivery dates can be rigid and aggressive, leaving little room for inefficient flow of product through its assembly processes.

Originally, assembly comprised seven main stations, which were

  1. body prep
  2. underbody assembly
  3. interior electrical assembly
  4. rooftop assembly
  5. interior mechanical assembly
  6. interior fit and finish plus battery installation
  7. vehicle commissioning.

The demand is 50 electric buses per year, making takt time about 1 bus every week. Of the seven stations, body prep was Proterra's bottleneck because it required 99 labor hours by six workers per bus. This put capacity at only .40 units per week and, hence, capacity utilization at 100 percent. Although body prep was by far the biggest bottleneck, many other stations also were bottlenecks relative to takt time. In fact, the only station with available capacity was vehicle commissioning.

Due to the nature of Proterra's processes, the line could not be balanced by reallocating work. Instead, it was necessary to disaggregate or distribute tasks across a longer assembly process. Many of the operations that happen within each station require serial completion. This means workers must wait for certain tasks to be complete by other associates before they can proceed with their own work.

Proterra decision-makers chose to split assembly processes into 13 smaller, more manageable stations. With this more balanced approach, work was redistributed effectively and processes rearranged to allow for tasks to be done in parallel.

Moreover, after reviewing the new operations setup, it was discovered that excess time was being spent moving materials from the warehouse to the point of use. The warehouse was trying to maintain and distribute components all the way down to the fastener level; meanwhile, delivery to some stations forced workers to cross the entire operations center. Seeing the opportunity to eliminate several non-value-added activities, Proterra rearranged its warehouse and fulfillment centers. Mobile carts were developed to give workers easy access to tools. And large quantities of small, low-value parts, such as nuts and bolts, now are stored in the work stations on racks to eliminate wasted time.

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