The Pros and Cons of Paced Conveyor Lines

Steve asked in an e-mail:

What are the productivity advantages, specifically in terms of operator efficiency, with respect to a moving, paced conveyor line and a non-moving, non-paced line?

We’re looking at three scenarios:

1) Non paced, non-moving line: workstations (stalls) balanced per takt time, however they are non-paced

2) Paced non-moving line: workstations (stalls) balanced per takt time, with “something” pacing their work (i.e. clocks, audible tones, etc.)

3) Moving assembly line: assembly stations on moving conveyor line balanced per takt time and conveyor moves at a pace equal to (or slightly faster than) takt time

As long as the work content on the lines are balanced equally well to takt time the productivity differences between paced, moving lines and stationary lines may not be so different. The logistics of materials traveling to and from the line, the ability of the organization to rapidly detect and address problems, and the cost of operating and maintaining a conveyor steadily are the more important longer-term success factors than operator efficiency.

The question is basically, “Should the line move or not?” and the answer is that it should. The so-called “takt image” or sense of the pace of work is much clearer for people when the line is moving. Physical flow is the most visible and undeniable evidence of progress. When the product has not made it to the appointed pitch mark at the appointed time, we know we have a problem. This is visual management at its simplest and most direct form, and one of the main pros of a moving line.

Some may argue, “I already know we have a problem” due to flags, lamps, or LCD screens even without a moving line. However the fact that a moving line has stopped CAUSES a problem since it affects all processes upstream and downstream. We could look at this as either a pro or a con, depending on whether we are prepared to respond and fix the problem quickly. Disconnected workstations may stop and not cause problems upstream or downstream. A linked flow line forces management to put in an escalation system and a response team to address problems quickly. Non-moving lines bury problems easily and don’t force the issue of an escalation system.

Although lines, whether stationary or conveyor-paced, should be designed around people and how the work flows, we need to start by asking how the work needs to flow and only then select the appropriate technology for conveyance. There are various factors to consider such as safety, flexibility, size, energy use, cost, maintenance, down time, connectivity with other equipment etc. One of the cons of a moving line is that they can be a hassle to set up and manage. These considerations require that we broaden the discussion beyond issue of moving versus stationary and review the entire management system around the value stream (entire process). In the words of the architect Eliel Saarinen:

Always design a thing by considering it in its next larger context – a chair in a room, a room in a house, a house in an environment, an environment in a city plan.

When this is not done there is a risk that the design of one section of the business is sub-optimized and hides waste, or worse, large investments turn wasteful processes into monuments that you are forced to live with for years. After all, the main purpose of a conveyor is… conveyance, which is one of the seven types of waste. The trap of automating waste is one of the major cons of conveyors especially when this adds cost rather than reduces cost. Intelligent, simple automation or methods of conveyance that improve the process are always welcome.

As with many things lean, there is the ideal and there are the practical steps in that direction. This discussion becomes more than theoretical when it is time to decide on capital investment. It’s important to understand the so-called “true north” so that all improvements are made in that direction and not against it. As long as tatk-flow-pull and jidoka (stop and fix, escalation system etc) are in place, it is not so important whether the line moves continuously, moves by pulse (remains stopped and advances every takt), or move the people / jigs / materials while keeping the work pieces in-line but stationary – sometimes used for massive products like mining trucks.

If we say that option 3 above is near ideal, but compromise and go with options 1 or 2, it should be based on understanding and agreement of why 3 is the ideal. As opportunities present themselves for future investment, the direction should be towards true north – moving flow, balanced to takt, immediate exposure of problems, stable and reliable 4M elements, and human systems to support this such as andon response escalation, team leaders / group leaders with smaller span of control, visual controls, cross training, and persistent continuous improvement.

1 Comment

  1. Joseph

    August 19, 2010 - 4:02 pm

    A very interesting subject. Being production based I have never designed systems. Is this on the correct lines.
    1) Non paced, non-moving line.
    In this system operators work is free effort. There is no measure of how fast or slow they are producing so it is easy for them to loose track of time resulting in lost volume.
    You should get good quality as the operators are under no pressure to complete their work.
    A drawback with this system is that they usually have a longer cycle time. This means that operators have more complex processes to learn. You need a very highly skilled operator on these systems.
    * In order to get the best volume with 1-2-3-4-5 people on the equipement you need very well planned ( M/A ) Multiple Activity Charts. Even the best charts will show some waiting time while the group perform their individual duties.
    2) Paced non-moving line.
    In this system operators work to a cycle time and it is obvious if they are loosing volume. Action can be taken quickly to prevent volume being lost.
    Quality can suffer as operators are under pressure to pull back lost volume.
    * As above ( M/A ) Charts are required with their waiting issues.
    3) Moving assembly lines.
    With this system the operator has a short cycly time that is easy to learn. Most good operators get into a rhythm and the day passes quickly but if there are quality problems in either their own work or the quality of parts coming at them it can cause quality issues or at worst volume losses because there is no slack in the allocations to fix these problems.
    If the line stops then every operator on the line looses the same amount of time.
    This system needs very good work allocations to balance every station as close as possible to the cycle time.
    The cycle time is usually shorter than the takt ( Overspeed ) to prevent volume losses. This results in more operators that is waste.
    Time losses can be reduced by decoupling production lines by using buffers between lines. This allowes most lines to keep running when one line is stopped. Depending on the size of the buffer 15 – 20 minute stops can be absorbed.
    Moving lines can normally produce the highest volumes as it just need more labour to be thrown onto the line to increase the output.