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.