Managing a production line has always been a demanding task. This article by Sabry Shaaban and Tom Mcnamara* offers a brief overview of the various production line management techniques available today.
PRODUCTION lines are by far the most commonly used method of mass production. Broadly speaking, they fall into two categories; paced and unpaced.
In paced lines work pieces move from one workstation to another on a mechanically paced line. The benefit here is that a production manager can determine the output rate by setting the pace at which the line moves.
In unpaced lines, workpieces move along the line either manually or with the assistance of some mechanical devices (e.g. conveyers, rollers, etc.).
While intuition would suggest that the paced line is better, research shows that when operators are allowed to work at their own rhythm, higher production levels can be achieved.
Production lines can also be operated as “push” or “pull” systems. In push lines an individual station processes another unit as long as there is a supply of work in front of it.
The drawback to this system is that it might require substantial buffer space to maintain production, resulting in increased inventory holding costs.
In pull lines production of another unit starts only when a downstream station asks an upstream station for another work piece. The merit of pull lines is that they require less work in process (WIP).
An efficient “lean” Japanese production method that is based on the pull line system is known as Just in Time (JIT), or Kanban.
Here floor space and WIP are kept as low as possible, hence the term “lean”. The main goal is to cut waste at every stage of the production process. JIT lines usually make use of small lot sizes, leading to a substantial reduction in WIP and buffer storage space.
JIT drawbacks
One major concern with JIT lines is the likely inadequacy of WIP, resulting in sizeable stockout costs. This could bring your operation to an immediate halt.
Another shortcoming is the relatively high percentage of worker idle time found in JIT lines (as high as 18%).
This is deemed excessive by most managers, though manufacturers adopting these systems would argue that higher quality and lower inventory keeping costs obtainable from JIT more than offset the losses incurred from having workers standing idly by.
Still another issue for line managers is the operation of a balanced line. Every year huge amounts of money, time, and effort are expended trying to bring lines into balance, with each station working as closely as possible to the same notional cycle time. But do unbalanced lines really provide inferior performance?
Some studies have shown that not only is imbalance not detrimental to the production rate, it can actually increase it. That is to say, the way in which you place workers having different average service times along a production line can augment output, decrease idle time, and reduce average WIP level.
One beneficial arrangement has been found to be a “bowl configuration”, in which the slowest workers are positioned at both the front and the end of the line, while the fastest workers are placed in the centre (creating a bowl-shaped pattern).
TOC/DBR concept
A school of thought that seizes upon the imbalance inherent in lines is referred to as the “Theory of Constraints” (TOC).
TOC involves identifying the slowest station (the bottleneck or constraint station) and assigning more resources to it, so that it will never be starved of work.
It was found that a line with a bottleneck could achieve higher volumes of output than a balanced line.TOC lines are based on the “drum-buffer-rope” (DBR) concept.
The drum represents the bottleneck station, which dictates the overall movement and throughput of the line. The main function of the buffers is to provide protection for the bottleneck against any fluctuations.
The rope is a signalling device from the bottleneck to drive all stations to work in harmony with the pace of the bottleneck.
Some researchers have found that DBR lines are more efficient than JIT lines. The primary shortcoming of this system is the need to insure that the bottleneck station will never suffer from starvation.
CONWIP concept
In conjunction with DBR is a production method known as “Constant Work In Process” (CONWIP).
In this approach higher efficiencies are achievable by maintaining inventory at a constant level. A new work piece is released into the system only when a completed work item exits the production line.
Just as with DBR lines, CONWIP lines have been shown to outperform JIT lines. But some researchers have found that in highly variable lines, with high machine failures, scrap rates, etc., JIT lines are superior to TOC lines.
A relatively new production method is known as “Bucket Brigade” lines, or lines that self- balance. Under such systems workers are ordered, from the slowest to the fastest, in terms of their mean operation times. Production is accomplished by each worker moving forward with their work piece along the stations until it is handed over to another worker. At this point the line is said to “reset” with the worker going back upstream to take an item from a predecessor.
This system is ideally suited for the apparel industry, order picking industries, or any other processes that involve simple repetitive tasks. Studies have shown this system to be extremely easy to implement (45 minutes in one case for workers with no previous training). Increases in output in the order of 30% were obtained in several cases.
This article has discussed various production methods that are available for you to choose from. All of these techniques can be adapted to various types of industries and operating environments. In the end it will be up to you to decide which one is best suited to your line — the one that generates the highest return on your investment.
* Sabry Shaaban and Tom Mcnamara are at the Department of Finance and Operations at the ESC Rennes School of Business in France: sabry.shaaban@esc-rennes.fr; tom.mcnamara@esc-rennes.fr.
