Just in time production systems are based upon the simple concept that excess work in progress is a waste of company resources. This is a part of a “Lean” manufacturing system. Toyota is one of the pioneers of this manufacturing style. It uses a “push” or a “pull” style assembly line and kanban cards to ensure that there are no excessive resources wasted on building inventory that is not needed. As with a classic assembly line, the product moves down the line and receives an additional part at each assembly node on the line. In either the push or pull style there are assembly stations that support each of these assembly line nodes. These support stations produce the parts to be added onto the finished product. In a “push” system the part station notifies the corresponding node on the assembly line when a part, or a certain amount of parts is ready. In a “pull” system the assembly node notifies the part station when a particular part or amount of parts is needed. Both systems are calibrated to ensure that everyone is working and there is no excessive build up of inventory at any one particular location.
The system uses what are called kanban cards. Kanban, when translate literally means “visual card.” These cards are used by the assembly line nodes to notify the part assembly support station when more parts are needed or by the support station to notify the assembly line node when more parts are ready. They are also used by the part assembly stations to make a timely request for materials. The aim is to keep everyone working in a steady flow with no down time.
The “pull” system works back from the end of the assembly line, through the assembly line nodes. The assembly line node receives their particular amount of parts from the part assembly station. As they attach their particular part to the finished product their parts inventory decreases. When it drops below a certain level, they get a Kanban reminder to request more parts from their part assembly station in order to have them in time to attach at their particular node on the product assembly line. The part assembly station immediately starts working on the next order of parts to send to their assembly line node. These parts are ready and moved so that the assembly line node receives them just in time to continue working with neither down time nor part build up. Materials are provided to the part assembly stations in a like manner. The part assembly station gets a set amount of materials that are just the right amount to make a set amount of part orders for their assembly line station. When their materials drop below a certain level, they get a Kanban reminder to order more materials. These materials arrive just in time to make the next set of parts, with no materials build up and no down time. .
Along the assembly line, the process works in much the same manner. At the end of the line there is a shipping container that holds a set amount of finished products. As the container is being filled there is a certain point when it is partly full that they get a Kanban card to let them know it is time to order a new container. As they are filling up the container, the new one arrives and there is just enough overlap for them to send a Kanban card to the end of the assembly line to let them know that there was an empty container and they should send more product to fill it. That call is sent up the line to the second to the last station that the final stage was clear and they should send down more product. At each node, there is a point when they receive a Kanban card to send more product down the line. Then there is another when they send a Kanban card up the line to let the station above them know that the way is clear below and they are ready for the next product to be sent down the line so that they may perform their particular function. In this manner, the finished products are pulled down the line from the empty container to the head of the line. Similarly, the materials are pulled into the part assembly stations as the parts are sent to the assembly line nodes.
The push system works in much the same way, except the process starts at the materials center at the head of the line. They let the first station know that there are all the materials needed for them to start building a unit of product and send it on down. When the product hits a certain point of completion that assembly node sends a Kanban card down the line to let them know the next unit is almost ready to be sent to them. This is just enough time for them to clear out their station from the last unit, when the next one arrives. In this manner the products are pushed down the line from supplies through the line nodes and into the waiting shipping containers.
The problem is of course when there are blockages due to machine failure or the cycle is halted by material shortages. American manufacturers decry that their unmotivated workers create too many of these problems for maximum success in the United States. The Japanese Toyota Company has enjoyed so much success from their highly motivated workers that they are reengineering the line and equipment so that the workers are less fatigued at the end of the day. .
References
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Cai-feng, L. (2009, 06 30). Research on a Fast Delivery Production System - Just-in-time production system. Canidan Social Science .
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Ohno, K. (2011). The optimal control of just-in-time-based production anddistributions systems and performance comparisons with optimized pull systems. European Journal of Operational Reserach , 124 - 133.
Sarker, B. R. (1989). Simulating A Just-In-Time Production System. Computers and Engng , 16, 127 - 137.
