Guidelines for Continuous Moving Lines

This article is in response to a question posted by a reader about guidelines for continuous moving lines as part of a Lean manufacturing implementation. This is not a comprehensive list of all things to consider when designing and managing moving lines, but it is a good start.
1 – 100% efficiency is not the goal at Toyota. There’s a good explanation of this idea by Brad Schmidt in a previous article. The premise is that if you have a line that runs at 100% you are hiding your problems. It is better to expose and eliminate these problems and work 5% overtime to catch up.
2 – Product and production line stability. Part of the design of any line should be determining the frequency and severity of failures and line stops using either past data from comparable lines, or by conducting FMEAs and Process FMEAs, verifying with trials before run at rate. This provides two benefits. First, you can eliminate, prevent or mistake proof potential failures. Second, you can determine the appropriate level of support needed on the line to rapidly address these problems and keep the lost time under control.
3 – Appropriate span of control for supervisors, line leaders, and team leaders. Industrial Engineers can tell you that theoretically, the line will run at 80%, 90% or whatever the standard is based on allowances. The truly Lean approach is to design the line and the support needed to keep this line running at 95% or better. This includes the appropriate number of workers per team leader and supervisor. If the frequency and severity of problems cannot be determined as in 2 above, start with 5 workers per team leader and expand their span as the process stabilizes. If the span is smaller than 5, revisit the stability issue and reconsider your process design.
4 – Fixed position stop vs. continuous moving line. Question the type of moving line you need. Some pacing mechanism is needed, but whether the line should move continuously, work in a stopped position and move for conveyance only, or whether it should move and stop is worth testing, depending on line speed, size and complexity of product, and the material logistics to the line. A fixed-position stop system at Toyota helps resolve problems on continuously moving production lines by allowing a duration and length of line between stations to address problems. If problems are not resolved before a fixed point, the production line stops.
5 – Straight line vs. u-shaped line. The main benefit of a curved or u-shaped line is to allow workers on the line to flexibly work at both beginning and end of line stations. For lines with fewer than 8 people, this also allows the team leader or supervisor to have a view of the total line and step in easily in any position. Material logistics often straightens out the line.
6 – Material quality and availability. Don’t plan on a smoothly moving line if your materials don’t move smoothly. Make sure that your logistics systems supports delivery quality materials on-time to the point of use. Often this is a smaller investment with a larger impact, overlooked until the new production line is up and running. Even the perfect moving line will stop without parts.
7 – Building on-line vs. using feeder lines. While it may seem easiest to line up all of the parts and build from star to finish on a moving line, in reality there can be variation in cycle times due to product variation or simply because some operations are not as stable as others. It is a good idea in these cases to create sub-assembly feeder lines adjacent to the moving line, synchronized with the main line or using one-piece buffers. If the process is simple and has little variation, building it on the same line may work just as well.
8 – Appropriate use of automation. One of the biggest heartbreaks in the Lean business is to arrive at a new client site just weeks or months after they have proudly (and unnecessarily) expanded their plant or installed a disastrous (in Lean terms) bit of automation. We got there too late! Not only does this reduce the available cash for proper Lean training and investment in people and methods design, misguided accounting principles demand that these bits of inappropriate automation continue to be utilized. And of course there is the loss of face when clients do come around and begin to dismantle these systems, not to mention the removal and disposal cost. Start simple. Design and build the line as if you had no money. Use what you have. Build bare bones equipment. Take a zero off of the budget and see what happens.
All you smart people out there feel free to add on to this list.

2 Comments

  1. Kevin Young

    October 11, 2007 - 7:32 pm

    I think that we have finally decided on the line speed for our work stations. Our takt time is 120 seconds but our line will move through each station at 118 seconds. I still think this is a little high but I don’t have any say in the matter. The next problem we have is how many seconds worth of work each operator should have. We are trying to balance all the jobs to 116 seconds but that doesn’t really give anyone enough time to try and fix a problem before it gets out of their work zone. The andon system will make the line stop but shouldn’t there a little more time to try and fix a problem before this happens.

  2. Jon Miller

    October 12, 2007 - 3:19 am

    Try to target 95% of takt for cycle time at each station. That gets you to 114 seconds at 120 seconds takt, or 112 at 118 seconds takt.
    I think you will find that much higher than that and you will lose more than you will gain.
    Most likely this will require some kaizen to take out the waste and balance each operation closer to 95%.
    Try it out and let us know what you learn.