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Levelling is one of the foundations of the Lean house. In order to smooth the customer demand, it reduces the variability and the effect of the whip to better control the production.

Introduction

The levelling (Heijunka-平準化), or smoothing, is part of the tools forming the fundamentals of the house TPS. It is a prerequisite to the implementation of the Just In Time.

In general, when you are trying to apply the TPS, you have to start to distribute or smooth out the production. This responsibility lies primarily with the people responsible for controlling or managing the production. The smoothing of the production program may require anticipating or postponing shipments, sometimes asking some customers to wait a few days. Once the level of production is more or less constant for a month, it is possible to implement drawn systems and to balance the work on the chain. But if the levels of production vary from one day to the next, it is pointless to try to apply these other systems, because standardising the tasks is simply impossible in these conditions. » F. Cho, President of Toyota Motor Corporation

Levelling consists in planning production with the smallest possible batch size by distributing them over time, and thus being able to produce several types of product within a defined period of time and on the same chain of Production. With this operation, we are recovering the fluctuation of customer demand, downward and upward, and we protect against the Bullwip effect.

Without levelling, if Monday’s orders are twice as high as Tuesday’s, it would mean that you pay overtime on Mondays and send them home on tuesday1.

Historical

In the years 50, T. Ohno had developed a planning approach to diversify products and maintain a high level of machine utilization. At that time, the production batches were not sufficient to occupy a production line with a single product2. In this context, the Kanban Had to be leveled in order to avoid the presence of stock or the overcapacity of the equipment or the manpower to cope with peak periods. This practice allows, among other things, the low level of stocks to identify the causes of excess capacity within the assembly line.

The implementation of the production leveling has led with time to the assembly of different models on the same line instead of assigning a given model to an assembly line. This practice has been refined with the years allowing, by example, to the factory of Tsutsumi in Toyota City to produce in 2005, 500 000 vehicles divided between eight different models according to a rotation rate of one model at the minute3. Thus the principles of the mixed scheduling are still applied today.

The slowest but consistent tortoise causes less waste and is much more desirable than the speedy hare that races ahead and then stop occasionally to doze. The Toyota Production System can be realized only when all the workers become tortoises.“Taiichi Ohno

The interest of levelling

It reduces the level of stocks

The leveling is based on reducing the production batch to their smallest size, to the unit if necessary. Reducing this batch size reduces inventory levels:

  • Before: We order the raw materials at once to meet the order of a customer. This induces the fact that on delivery, we have all the quantity of material.
  • After: with the leveling, the same control is distributed on several small batchdelivered to defined sequences. Thus, instead of having a complete delivery, we have X smaller batchdeliveries and thus reduce the stock and associated costs (surface, cash…)

Improve service rate

The challenge of levelling is to be able to reduce the batch size to the maximum is to be able to produce it as many times as necessary to meet the customer order.

At the most the batch is small, at the most the rate of service is good. Indeed, instead of “contenting ” a customer with a full order, we can deliver 3 customers with almost complete orders and thus reduce the impact of the problem by distributing it to all customers rather than a part.

Improve quality and traceability

In the same way as before, if the problem is related to the quality of the products, the leveling allows to identify it earlier. Let’s take an example:

  • Without leveling: 1000 parts is produced with a machine adjustment and a batch of raw material. If at the end of 500 parts It is indicated that the material batch or the settings are not good, the 500 parts are to be restarted.
  • With leveling: the 1000 parts per batch of 10 Parts produced. In case of problems with machine or material adjustments, the search and the recovery work will only be done on the corresponding 10 part produced.

Better manager and plan production

Thanks to a more flexible production, we will be able to manage the customers ‘ risks better. In the case of an urgent change in the delivery schedule, flexibility makes it easy to reconfigure the schedule and priorities and reduce the impact of this change on other productions.

Steps to Leveling

Switching from batch corresponding to a command to small batchs made several times a day can only be done in one step. It is necessary to go gradually in several steps4 which represent the 6 levels of the leveling:

Point 0: A batch of product is manufactured per month according to the customer’s schedule.

Minimum Change of series

Maximum batch size

Step 1: The size of batchs is reduced by 2

Same sequence

2 cycles per month

 

Step 2: The size of batchs is further reduced by 2, so production is at the week and not in the month

Each product every week

Same sequence

Step 3: Switching to the production of a batch of each product per day

1 batch per day

Same sequence

Each product every day

Step 4: Switching to identical batchsizes

Several batch of the same product per day

Same batch size for all products and calculated on minimum

Step 5: Synchronized Production

Reducing the batch size to the part

Synchronizing with demand via the Takt Time

The information flow

The setting up of the leveling on a production line consists of putting in place different physical tools:

Step one. Translating a sales order into a Kanban

First phase of the levelling, design the Kanban system. There is a choice between the different types of Kanban Depending on the specifics of production. In general, it is a good thing to ” fumble ” a little before finding a successful Kanban that suits you.

Step 2. The Logistic box

The Kanban represents the ” firm ” customer production. But in the case where firm orders do not correspond to what is expected, the Logistic box to cushion this variability.

Step 3. The Leveling box

The Leveling box is used to translate the customer needs of the Logistic box In production planning. In other words, the customer needs generally set for the week or month are translated into demand for daily production.

Step four. The batchs building box

The batch building is designed to bring together several batch Kanban to reduce the number of changeover or collect combinations of parts to form sub-sets.

Step 5. The sequencer

The sequencer, located closest to the operator, defines the production order of the different Kanban. Final stage of the levelling, it is it that is representative of the actual work of the operator.

Source

1 – J. K. Liker (2004) – The Toyota Model: 14 principles that will make the success of yourRecovery

2 – E. Daito (2000) – Automation and the organization of production in the Japanese automobile industry: Nissan and Toyota in the 1950s

3 – C. Chandler (2005) – Full Speed Ahead

4-I. Glenday, R. Sather (2005) – Breaking through to flow

A. Smalley (2004) – Creating level Pull

B. Vallespir, T. Alix (2010) – Advances in production management systems: New challenges, new approaches

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