No matter how perfectly you’ve planned and executed your manufacturing process, there will always be limitations and constraints that will negatively affect manufacturing productivity.
Excessive idle time, poor configurations or design, and overscheduling are just some out of so many challenges that haunt manufacturing efficiency and productivity, and line balancing is one of the most effective ways to address these challenges.
In this guide on how to perfectly balance a manufacturing line, we’ll learn all you need to know about line balancing, including:
What is line balancing in manufacturing?
The benefits of balancing your production lines
Step-by-step guide on how to implement line balancing
Example implementations of line balancing
Without further ado, let us begin this guide right away.
What is line balancing?
In the manufacturing context, line balancing, also often called load balancing and production leveling, refers to a production technique to optimize machine time and operator time to eliminate bottlenecks so it can be as efficient as possible.
Originally called heijunka (平準化) in Japanese (which directly means leveling), the initial concept of line balancing is to reduce mura (unevenness), which in turn can reduce muda (waste.) We can define line balancing as the technique to align customer demand with production output through leveling (heijunka) of cycle times.
In practice, line balancing simply means balancing the production lines.
For example, suppose there are three production lines: lines A, B, and C:
Line A can produce 10 units per minute
Line B can produce 20 units per minute
Line C can produce 30 units per minute
The pieces flow from A to B to C, and since line A has a lower capacity, line B will remain idle for 50% of its available production time and line C for 66.66% of its available production time.
Obviously, the three lines are unbalanced, and the objective of line balancing is to distribute tasks evenly over the machines to minimize the idle time of the machines, as well as the operators handling the machines.
The very basic line balancing approach is to simply have three machines of type A, with every two machines of type B and one machine of type C.
Alternatively, we can simply add other tasks to machines B and C throughout their idle times so that they do not remain idle.
In practice, however, a real-world production line may face complex line-balancing problems, and we’ll need more sophisticated ways to achieve balance. Further below, we will discuss different line balancing methods we can apply to solve various balancing problems.
However, let us first discuss why we need to do line balancing.
Benefits of line balancing: why level your production?
Line balancing has many benefits in regard to improving manufacturing productivity, but the main objective is to ensure manufacturing operations are properly aligned with users’ (or customers’) demands.
In turn, this alignment can provide the following benefits:
Line balancing allows operators to leverage unused capacity and minimize machine downtime, which in turn will translate into improved productivity and efficiency. Idle time occurs every time a manufacturing process exceeds Takt time (more on this later.)
In this modern business landscape, customer demand often changes rapidly due to changes in tastes, the introduction of new products, seasonability, or other reasons.
When these demand changes happen, the takt time of the production line will also change, which can skew the existing calculation.
Line balancing allows the company and its managers to adjust the production line and especially adjust its takt time.
Reduce waste production
When production lines are out of balance, they can result in increased waste, which can come in various different forms, including but not limited to:
Line balancing can help minimize waste in the various areas of the manufacturing process.
Improved employee morale
By streamlining and consolidating the otherwise lengthy and stressful manufacturing processes, the company can achieve higher employee morale, which will result in improved productivity.
Avoiding excessive investment
A balanced production line can help the company assess the right number of machines required and the number of operations in each workstation.
In short, balanced production lines can facilitate a streamlined flow of the manufacturing process, reducing the idle time of each machine, which in turn translates into an improved rate of production and the quality of the produced units.
Performing line balancing
The line-balancing problem
Theoretically, the line-balancing problem is about how to arrange the assembly tasks and individual processing so that the total time required at each workstation is more or less the same, to minimize idle time.
The perfect balance is achieved when all the times spent at the workstations are exactly equal. When the workstation times are unequal, the slowest station determines the overall production rate of the line.
Line balancing terminology
To properly explain the line-balancing problem, we’ll need to first discuss the terminology and relationships in line balancing, and we’ll do so with the following example:
A manufacturing process is designed to assemble car toys. The total job of assembling the product has been divided into five work elements:
- Minimum rational work element
The minimum rational work elements are the smallest possible, indivisible tasks into which the manufacturing job can be divided. These work elements cannot be divided further.
For example, drilling a hole in the material cannot be divided further, so this is a minimum rational work element.
The time required to carry out this minimum rational work element is symbolized as Tej.
On the other hand, Te symbolizes the total time required to finish all work elements, in which the time to perform two or more work elements is the sum of the times of the individual work elements.
- Total work content
Refers to the aggregate of all work elements that must be performed on the production line. Symbolized by Twc.
- Workstation process time
The term ‘workstation’ refers to a physical location along the production flow line where work is performed. Each workstation will involve at least one work element. Symbolized by Tsi for station i of an n-station line.
- Technological sequencing requirements
Refers to the possible order in which the work elements can be accomplished. Also known as ‘precedence constraints.’
For example, brackets must be attached to the frame first before motors can be attached to the brackets.
In nearly every assembly job, there will be technological sequencing requirements that restrict the sequence in which tasks can be accomplished.
- Precedence diagram
Related to technological sequencing requirements (or precedence constraints), a precedence diagram is a graphical representation in which the sequence of work must be accomplished as defined by the technological sequencing requirements.
Work elements that appear on the nodes on the left side of the diagram must be accomplished before those positioned on the right.
- Cycle time
The maximum (ideal) theoretical speed at which the production line produces its parts/products. Symbolized by Tc, the value of Tc must be greater or equal to E / Rp, in which E is the line efficiency and Rp is the required production rate. The line efficiency of a manual line is typically closer to 100% than an automated line, where mechanical malfunctions will be more common.
For example, if the required production rate (Rp) is 30 units per hour or 0.5 units per minute at a line efficiency (E) of 100%, the Tc would be 0.5 minutes or 30 seconds. If the line efficiency is lower than 100%, then the ideal production rate (Rc) must be increased to compensate for the idle time.
With this equation, the minimum possible Tc must be greater or equal to the time spent at the bottleneck workstation with the largest value of Tsi (max Tsi.) Otherwise, if Tc = max Tsi, the production line will experience idle times at all workstations with larger Ts values than Tc.
- Balancing Delay
Also referred to as ‘balancing loss,’ balancing delay is the measure of the production line’s inefficiency due to idle time as a result of unbalanced lines.
Balancing delay is symbolized as d, which can be calculated with the following formula:
d = (nTc - Twc) / nTc
Whereas n stands for the number of workstations, Tc is cycle time, and Twc is total work content (as discussed above.)
Balance delay is more often expressed as percentage rather than the decimal fraction used on the other calculations here. A balance delay of 0 means the line is perfectly balanced.
For example, if the total work content (Twc) is 10 minutes and the cycle time Tc is 1 minute, thenit’s possible to achieve balance with n= 10 workstations, achieving perfect balance:
d= (10 (1.0) - 10.0) / 10 (1.0) = 0
On the other hand, if balance only achieved with 11 workstations for the 1.0 minute cycle, the balance delay would be:
d= (11(1.0) - 10.0 )/11(1.0) = 0.0909 or 9.09%
In this second assumption with 11 workstations, the production line is less efficient not only due to the additional workstation—resulting in delay—but also because it will require an additional operator. For this second solution, a viable way to improve efficiency is to decrease the cycle time (Tc.)
In this example the production line could be balanced at a cycle time of 11.0 minutes, so:
d= (11(1.0) - 11.0 )/11(1.0) = 0
When attempting to find perfect balance, however, one should look at the constraints put in place by the technological sequencing requirements (precedence constraints) and the values permitted by Tej.
Due to this problem created by the constraints, in the next section, we will discuss methods that may provide solutions.
Popular Line Balancing Methods
While there are various methods available for solving the line-balancing problem, we can generally categorize those methods into two big groups: heuristic and computerized.
I. Heuristic Methods
The term ‘heuristic’ here means the methods are based on logic (common sense) rather than on mathematical proof. With that being said, heuristic methods don’t guarantee the mathematically/theoretically optimal solution, but they are likely to have good solutions that approach the optimal one.
There are three primary heuristic line-balancing methods you can implement:
Largest candidate rule
Kilbridge and Wester
Ranked positional weights
Below, we will discuss them one by one while using the following simple work elements example.
Work elements example
Work element description
Must be preceded by
Assemble brackets to frame
Assemble mechanism plate to bracket
Align blade and attach to motor
Attach cover and test
- Largest candidate rule
The most basic heuristic line-balancing method and typically the easiest to comprehend.
The basis of the largest candidate rule is to choose assignments to workstations based on the size of the Te values of the workstations. The largest Te is prioritized, hence the name.
The steps used in this method are:
List all work elements. The largest Te at the top of the list, in descending order of Te value.
Assign work elements to the first workstation, starting at the top of the list and working down while also paying attention to the technological sequencing requirements (precedence constraints.) The sum of the Te values at each workstation must not exceed the cycle time Tc.
Continue assigning work elements to the workstation as in step 2 until no further work elements cannot be added without exceeding cycle time Tc.
Repeat steps 2 and 3 for the other workstations in the production line until all elements have been aligned.
- Kilbridge and Wester
The Killbridge and Wester’s method was introduced in 1961 for balancing a process in a manufacturing system.
It is an iterative method that uses trial-and-error to balance the production line by adjusting the cycle times of the different operations. The goal is to achieve a balanced line, where all stations have equal cycle times and, therefore similar production rates, while also minimizing the total cycle time of the line. It is one of the most widely used methods for balancing a manufacturing process.
To use the Kilbridge and Wester's method, follow these steps:
Identify the production line: Determine the sequence of operations that make up the production line and the cycle time for each operation.
Determine the critical path: The critical path is the sequence of operations that takes the longest time to complete. This is the path that determines the overall cycle time of the line.
Determine the ideal cycle time: Divide the critical path cycle time by the number of stations on the line to determine the ideal cycle time.
Assign cycle times: Assign the ideal cycle time to the critical path operations, and then iteratively assign shorter cycle times to the non-critical operations.
Check for balance: Check if all stations now have equal cycle times and if the total cycle time is minimized.
Repeat steps 2 to 5, adjusting cycle times as necessary, until all the stations are balanced and the total cycle time is minimized.
Implement the new cycle times: Once the cycle times have been determined, implement them in the production line and monitor the performance to ensure that the line is truly balanced.
It's important to note that Kilbridge and Wester's method is an iterative process, and it may take several cycles to achieve balance. It's also important to keep in mind that there may be other factors that can affect the balance of the line, such as the availability of resources or changes in demand, so the process may need to be repeated periodically.
- Ranked positional weights
Ranked positional weights is a method used to assign a weight or value to different positions in a manufacturing line. The goal of this method is to optimize the performance of the line by assigning higher weights to the more critical positions and lower weights to the less critical positions.
The process of ranked positional weights generally involves the following steps:
Identify the positions: First, identify all the positions that make up the manufacturing line.
Assign weights: Next, assign a weight or value to each position based on its importance or criticality to the overall performance of the line. The weight can be assigned based on factors such as the level of skill required, the complexity of the task, or the impact on the overall performance of the line.
Rank the positions: Once the weights have been assigned, rank the positions based on their weight. The position with the highest weight is considered the most critical and is assigned the highest rank. The position with the lowest weight is considered the least critical and is assigned the lowest rank.
Optimize performance: Use the ranked positions to optimize the performance of the line. For example, assign the most skilled or experienced workers to the higher-ranked positions, and focus on improving the performance of the lower-ranked positions.
This method allows the company to optimize the performance of the line by focusing on the critical positions and ensuring that the right resources are being used in the right places. Additionally, it can also be used to identify potential bottlenecks in the production process, which can be addressed to improve the overall efficiency of the line.
II. Computerized Methods
Computerized line balancing method is a technique for balancing a manufacturing line that uses computer software to analyze and optimize the production process. This method typically involves the following steps:
- Data collection: The first step is to collect data on the production process, including the time required to complete each task, the number of workers required, and the number of products produced per hour.
- Modeling the line: The data is then used to create a computer model of the production line. This model includes information on the tasks, workers, and machines involved in the process.
- Optimization: The computer model is then used to analyze the production process and identify potential bottlenecks and inefficiencies. The software can then be used to optimize the line by adjusting the sequence of tasks, the number of workers, or the speed of the machines.
- Simulation: The computer model can also be used to simulate the production process under different scenarios, such as changes in demand or the introduction of new equipment. This allows manufacturers to test different scenarios and determine the best course of action.
- Implementation: Once the optimal configuration for the line has been identified, the changes can be implemented in the actual production process.
This method allows manufacturers to quickly and accurately balance the production line, and can be used to identify and solve problems more quickly and efficiently than traditional methods. Additionally, the computerized method allows for more flexibility in testing different scenarios and evaluating the results.
Other line balancing methods to consider
Other methods to balance a manufacturing line include:
Heijunka: This is a Japanese term that refers to leveling the production rate to match customer demand. The goal is to produce a consistent quantity of products at a steady rate, rather than producing in large batches.
Drum-Buffer-Rope (DBR): This method uses a "drum" (the rate at which the market is demanding the product), a "buffer" (a safety stock of work-in-progress inventory), and a "rope" (the pace of production) to balance the production line.
Theory of Constraints (TOC): This method focuses on identifying and managing the constraints that are limiting the performance of the production line. Once the constraints are identified, TOC aims to optimize the performance of the entire system, rather than individual stations.
Lean Manufacturing: This method is focused on eliminating waste, reducing variability, and increasing efficiency in the manufacturing process. It is an approach that emphasizes continuous improvement and focuses on creating a "pull" system where production is based on actual customer demand.
Six Sigma: This method is a data-driven approach to improving the quality of the production process. It uses statistical analysis to identify and eliminate defects, reduce variability, and improve overall performance.
This is a non-exhaustive list, and there are other available methods you can implement, including continuous flow manufacturing, cellular manufacturing, and so on.
Ultimately, you should experiment with different balancing methods and find the most ideal one according to the type of product you are manufacturing, the company culture, the production process, and your company’s overall objective.
Using LineView for line balancing
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