Heijunka: What it is and how to level Lean production
What is Heijunka and why is it key in Lean Manufacturing?
Heijunka is a Japanese term, which translates as “leveling” and is one of the fundamental principles of the Toyota Production System.
To better understand what Heijunka is, its main objective is to keep the production of a product or its variants at regular levels as sustained as possible, and produced in as small batches as possible with frequent deliveries to the following processes.
What are the benefits of implementing Heijunka?
- Waste Reduction: By avoiding overproduction, reducing excess inventory, minimizing waiting times, lowering the number of necessary changeovers, and decreasing the number of defects produced.
- Improved Production Flow, making it more predictable.
- Greater Flexibility to Demand.
- Optimization of Existing Resources.
- Increased Stability for Workers.
How to implement Heijunka step by step
To explain the implementation of Heijunka more clearly, let's use an example.
In a home appliance factory, three types of washing machines are manufactured. The monthly demand for these washing machines is:
- Washing Machine A: 800
- Washing Machine B: 900
- Washing Machine C: 100
Considering this, the first step is to determine the basic leveled sequence. To do this, we must divide the production volumes of each item by the greatest common factor, which is the Greatest Common Divisor (GCD).
We start by breaking down the different volumes into their prime factors:
- Washing Machine A: 2 × 5 × 2 × 5 × 2 × 2 × 2 = 800
- Washing Machine B: 2 × 5 × 2 × 5 × 3 × 3 = 900
- Washing Machine C: 2 × 5 × 2 × 5 = 100
The GCD, common to all three products, is highlighted:
GCD = 2 × 5 × 2 × 5 = 100
To obtain our leveled sequence, we divide the total volumes by 100, resulting in:
- Washing Machine A: 800 / 100 = 8
- Washing Machine B: 900 / 100 = 9
- Washing Machine C: 100 / 100 = 1
A leveled production sequence would be:
A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-C
This sequence should be repeated 100 times to reach the total demand.
Assuming that the available monthly working time for this production line is 300 hours, we proceed to calculate the cycle time of the basic sequence:
Cycle Time (Basic Series) = (300 × 60) / 100 = 180 minutes.
