KANBANS (Kanban Ststem)
KANBANS: (Kanban Ststem)
Learning Objectives of the Article:
- Define and explain KANBAN. Where and how KANBANS are used.
- What are advantages and disadvantages of using KANBAN system?
Definition and Explanation:
A Kanban system is a means to achieve just in time (JIT) production. It works on the basis that each process on a production line pulls just the number and type of components the process requires, at just the right time. The mechanism used is a Kanban card. This is usually a physical card but other devices can be used Two types of such cards are usually used.
A withdrawal Kanban: Specifies the kind and quantity of product which a manufacturing process should withdraw from a preceding process. This card (illustrated below) shows that the preceding process which makes this part is forging, and the person carrying this card from the subsequent process must go to position B-2 of the forging department to withdraw drive pinions. Each box of drive pinions contain 20 units and the shape of box is “B”. This Kanban is the 4th of 8 issued. The item back number is an abbreviation of the item.
Example of Withdrawal Kanban
A production ordering Kanban: Specifies the kind and quantity of the product which the preceding process must produce. The one illustrated below shows that the machining process SB-8 must produce the crankshaft for the car type X50BC-150. The crankshaft produced should be placed at store F26-18. The production ordering Kanban is often called an in-process or simply a production Kanban.
Example of Production Kanban
Using KANBANS on production lines:
Each process (area, cell) on the production line has two KANBANS “post-boxes”, one for withdrawal and one for production ordering KANBANS. At regular intervals a worker takes withdrawal KANBANS that have accumulated in his process post-box, and any empty pallets, to the location where finished parts (components, assemblies) from the preceding process are stored. Each full pallet has attached to it one or more production ordering KANBANS which he removes and puts in the appropriate post-box belonging to the process that produced the parts. The worker now attaches a withdrawal Kanban to the pallet and takes it back to his own process area. When this new pallet begins to be used, its withdrawal Kanban is put back into the withdrawal post-box. At each process on the line, production ordering KANBANS are periodically removed from their post-box and used to define what parts and quantities to produce, next.
When and how KANBANS are effective:
KANBANS help simply planning and to fine tune production to meet changing customer demand of up to + or – 10%. The system requires planned monthly and weekly production schedules. KANBANS simplify day to day flexibility, and changes to the production schedule need only to be given to the final assembly process and will then automatically work their way back up the line. Kanban systems can be tightened by removing cards or by reducing the number of parts on pallet. The effect will be to speed the flow through the process and hence reduce lead time. However it also makes the system more vulnerable to breakdowns and other causes of dislocation. By identifying the areas within the line that are causing disruption, efforts can be made to improve them. Thus the overall efficiency of the line is raised by tackling the key points.
Other types of Kanban also used are supplier kanbans -to withdraw goods from external suppliers, and two types of Signal Kanban, which are inserted near the bottom of a stack of item. These automatically initiate production of batch produced items when the stock reaches a preset order level.
Single card KANBAN system:
In a single card KANBAN system, parts are produced and bought according to a daily schedule, and deliveries to the user are controlled by a “conveyancing” (withdrawal) KANBAN. In effect, the single card system is a push system for production coupled with a pull system for delivery to the point of use. Single card KANBAN system controls deliveries very tightly, so that the using work center never has more than a container or two of parts and and the stock point serving the work center is eliminated. Single-card systems work well in companies in which it is relatively easy to associate the required quantity and timing of component parts with the schedule of end products. These are usually companies with a relatively small range of end products, or products which are not subject to rapid, unexpected changes in demand levels.
Advantages of KANBAN:
- Low costs associated with the transfer of information
- Provides quick response to changes
- Delegates responsibility to line workers
- It is a simple technique not involving computers so its cost is low.
- Lead times are reduced.
Disadvantages of KANBAN:
- It is less effective in shared-resource situations. Suppose the upstream station made several parts. Then a request to make more of the part needed by the downstream station will have to wait if other parts have to be made. A buffer is needed to ensure the downstream station doesn’t run out meanwhile. And, because each part needs a separate signaling card, the system becomes more complex than if the resources were dedicated.
- Surges in mix or demand cause problems because KANBAN assumes stable repetitive production plans. It is less suited to industries where mix and volumes fluctuate.
- KANBAN in itself doesn’t eliminate variability, so unpredictable and lengthy down times could disrupt the system; poor quality in terms of scrap and rework also affect its good functioning.
- KANBAN systems are not suited for manufacturing environments with short production runs, highly variable product demand, poor quality products, and a multitude of product types.
- A breakdown in the KANBAN system can result in the entire line shutting down.
- The throughput of a KANBAN system is not managed but is instead a result of controlled WIP and known cycle times.
