What is line balancing?
Line balancing also referred to as load balancing, production leveling, or originally Heijunka (Japanese) describes a technique to align production output with customer demand through leveling of cycle times. Using the line balancing method several sources of waste (jap. Muda) are reduced.
How to balance an assembly flow line?
Manufacturing operations that use (assembly) flow lines forward products step after step from one station to another. Each station needs a certain amount of time to conduct the necessary actions before the product can be passed to the next station. If stations forward products directly after they are finished with a process, stocks and waiting time is generated between stations, if processing time differs (see figure 1).
Figure 2 displays the processing time for the stations within an assembly line. Due to the difference between processing times efficient use of the line’s capacity is hindered. To avoid a pileup of stock between stations a cycle or takt time is introduced. The cycle time reflects the longest time needed to finish processes at all stations. After one cycle all stations begin assembling the next product.
This procedure ist leading to idle time at station 1, but avoids stock generation in-between stations as figure 3 shows.
Through the division of the available time (7 hours = 420 minutes = 25.200 seconds) in a shift by cycle time (4 seconds) the output per shift of such an unbalanced line is calculated.
To align the as-is output with the target output the cycle time needs to be adjusted in most cases. By dividing target output by available shift time, the target cycle time is calculated. The use of line balancing shifts parts of work between stations to lower the longest processing time and therefore to lower cycle time. Figure 4 depicts the processing and cycle time after applying the line balancing method to an assembly line.
Figure 5 depicts the flow of goods in such an assembly line. Due to line balancing, now every 3 seconds a product leaves the line.
Before applying the line balancing method a product left the line every 4 seconds. Therefore an efficiency gain of 25% was realized using line balancing.
If the new as-is cycle time is still too high to reach the demanded output, additional stations need to be ramped up to shorten cycle times or building a whole new line. In a balanced assembly line, all stations work with the same processing and cycle time and therefore do not produce stocks or waiting time between stations.
This easy example levels one product on one line. In reality, most manufacturers assemble multiple products on the same assembly line with fluctuating cycle and processing times. Multi-product assembly lines are more difficult to balance and more error-prone to devitations from cycle time. Due to frequent changeovers, issues occur more often making it tough to follow the targeted cycle time.
How to improve line balancing with digital tools?
Digital tools as the WORKERBASE Andon system enables line operators to stay within cycle time and improves the availability of the whole line. Through real-time connections between the line operator and cross functions such as maintenance, logistics, and quality transparency is increased and downtime reduced. Once an issue occurs, that cannot be handled by the operator alone, the operator triggers a workflow that informs the next available and skilled cross-function. Due to faster reaction times, the issue can often be handled in-line without ejecting the product from the assembly line nor slowing or shutting down the line. Therefore allowing smaller buffers in cycle times, reduced costs of rework, and more stable operations.
Author: Marius Maier, Digital Transformation Consultant, WORKERBASE