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Balance your line with Optio

Balancing your line will decrease leadt-ime and WIP. Use Optio to get real-time data on throughput and cycle time, identify bottlenecks, and get your line balanced.

Nikolaus Correll
August 2, 2021

Balance your line

A manufacturing line is only as strong as its weakest link. If some stations are running at a higher rate than others, the line is unbalanced and flow is disrupted. As WIP will build up in front of slower stations, the overall WIP in the line will increase, thereby increasing lead-time. Also, the OEE of the stations after the ”bottlenecks” will decrease, leading to waiting or machine downtime. Likewise, stations working at a much faster pace will lead to WIP build-up after them with the same negative side-effects. Balancing your line therefore involves identifying stations that are working much faster or slower than the rest and then adjust your line to customer demand.

Measuring production rate and cycle time

The image below shows sample throughput for our cut-and-sew manufacturing line that we set up in the "map the value stream" article. as available from the statistics pane in Optio™. We can see that the printer produces around 60 items per hour, the press around 10 items per hour, the laser cutter 20 items per hour, and the sewing stations 5 per hour. It is clear that no matter how fast the printer, cutting and press operate, the hourly output is limited by the sewing station to 5 items per hour.

Sample throughput in a cut-and-sew manufacturing line with 25% variation on cycle-time for each station.


The output over time is known as the production rate:

It can be measured in products per minute, per hour, per day, or per week, whatever works best for your business. For example, if you measure lead-time in days, you might want to measure production rate in products per day. Optio™ allows you to choose different time-scales in the top left of the statistics view, for example the figure below shows the same data as above, but normalized per day.

Sample throughput per day in a cut-and-sew manufacturing line with 25% variation on cycle-time for each station.

Tip: Production rate of a line
The production rate of a line is defined by the production rate of its slowest station.

The production rate is directly related to the cycle time, that is the amount of time it takes to make one item. In the example above, 60 items per hour is equivalent to a cycle time of one minute, 10 items per hour to six minutes, and so on. The cycle time is therefore simply the inverse of the production rate, or:

Care needs to be taken as the cycle time might not accurately reflect an underlying batch process. It is for example likely that the production rate of 60 per hour arises from the printer producing 10 items every 10 minutes, leading to a cycle-time of one minute per item, which is only correct on average. This is why Optio™  shows the production rate by default.

Tip: Cycle time of a line
The cycle time of a line is defined by the cycle time of its slowest station.

We can now compare either production rates or cycle times to assess whether a line is balanced or not. In this example, the line is highly unbalanced with the printer producing items at a rate that is much too high, while sewing is performed much too slow. If operated this way, there will be large amounts of WIP building up in front of the press and the sewing station. There will be no build-up in front of the laser cutter as it produces at a higher rate than the press.

This can also be seen in the Kanban view of this process shown below after running it for a few hours. Widely differing amounts of WIP in each column is usually a clear indicator of an unbalanced line.

Kanban view of a cut-and-sew line with ”unbalanced” cycle times.

Line balancing tips: Takt time

We have now seen an example of a very unbalanced line and established that sewing is ”too slow” while printing is ”too fast”. What are the right numbers, however? Ultimately, your line needs to be balanced to meet customer demand. Customer demand determines your target production rate and thereby your target cycle time. The target cycle time is known as Takt time. In German, a ”Takt” is the beat of a musical piece. Mathematically speaking, the Takt time is given by:

Example: Takt time
A high-performance valve manufacturer expects demand for 200 valves per week or 40 valves per day. They therefore need to produce 40 valves during an 8 hour work day, resulting in a Takt time of 8/40 hours or one valve every 0.2 hours (12 minutes).

You can think about Takt as a big drum to which your manufacturing line is synchronized. Indeed, many factories use an audible signal to indicate that a Takt is over and the next step needs to begin. Although a seemingly slow Takt can lead to workers being idle, reducing the Takt will prevent overproduction and bring all the other benefits of a Pull system. (This is not very surprising as implementing Takt is a pull system.)

In this context, a balanced line is a line in which the average cycle time of each station is exactly the Takt time. The more a station deviates from the optimal cycle time, the more waste will you produce. This can be summarized by the following steps:

  1. .Determine the required production rate to meet customer demand and the associated Takt time.
  2. Identify the stations with the lowest and highest production rates.
  3. Use the ”five steps to lean” to increase production rate at the slowest station.
  4. Identify cost-effective ways to slow a fast station down (energy, tool tear-and-wear, decreased maintenance, etc.), and implement them, otherwise ignore them.
  5. 5.If differences vary widely, consider splitting or merging stations.6.Start over at step one.
Example: Balancing a line
A cut-and-sew manufacturer has average customer demand for 80 of its products a day. They establish their production rate to be limited to 40 a day due to the production rate of their sewing process. They decide to print only on Mondays and operate the laser cutter only on half-days to adjust their production rate to 80/day. Freed up labor is now allocated to the sewing process to increase its production rate. As customer demand further increases, they buy additional sewing machines and re-allocate workers to printing and pressing.

Summary

Balancing your line is quite easy once you can see cycle times or production rates in real-time. You will find that a balanced line will allow you to operate with less inventory and immediately increase your lead-time. Unless you can achieve balance by splitting and merging stations appropriately, balancing will require constant management oversight and reallocation of workers — something that Optio™  can help with.

ABOUT THE AUTHOR
Nikolaus Correll

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