In the traditional manufacturing system goods are built and passed on to the subsequent system which is known as Push production system. In a Pull production goods are built only when requested by a down stream process.
In Push system demand is anticipated and parts produced based on the anticipated demand, typically what happens in mass production environment where huge inventory is built up; whereas in a Pull system the parts are produced based on customer demand, which ensures low levels of inventory. Pull system reduces the schedule sheets, meetings to discuss shortages, expeditions and premium freight paid to ship materials; the material pull is created using Kanbans or sign boards. If we take an example of lap top manufacturing, if a customer places an order for a lap top, this would trigger off a series of ordering and manufacturing activities for producing parts and procuring raw materials just in time (JIT) and that's the reason companies like Dell are able to maintain very high inventory turns. In the simplest terms no one from upstream will produce goods or provide service until the down stream customer asks for it.
The concept is simple and understandable but what is difficult is to bring the change in a place where people are already working in a Push environment. It seems difficult to visualize a more efficient production using the Pull system and typically a shop supervisor believes that the machines and operator should work uninterrupted no matter if the downstream process needs goods or services. If operator speed is very high in China plants I think slowing it down is also very challenging. In one of the projects I was working in one of our Chinese plants wherein we wanted to reduce inventory and improve productivity, the biggest challenge was to ask operators to go slow and work on a Pull system, this was a conveyor driven line and operators were busy pulling parts off the conveyor, working on them and pushing it back on the conveyor. Finally we removed the conveyors and had asked operators to physically pick up parts from the upstream process; which brought in a rhythm and prevented push and build up of inventory. I think that was the turning point in that shop and subsequently in all lines the conveyor was pulled off and a pull system implemented.
Pull system brings a host of benefits including reduced inventory, which lead to reduced lead time to manufacture. In a pull system parts move one piece at a time or few pieces at a time and this prevents scrap in case of any quality issues. In case of mass production where in there is huge batch sizes and parts pushed to the subsequent process steps, any quality issues noticed downstream will lead to the whole batch being affected and causing higher percentage of defectives and scarp. The time for a part to move from upstream to downstream is higher in push system as the WIP (work in process) at each stage slows down the travel and hence increased lead time. Increased lead time directly affects flexibility of production planning. When WIP is low, lead times are low and the speed at which part numbers or models can be changed in a production line is faster. The rhythm created by the pull system also creates a very positive and motivating environment in the shop floor.
However to have a pull system in place it is essential that the equipments are capable and reliable, the operators multi-skilled (wherever needed), the quality high and no defectives in incoming parts . The line has to be balanced and in pull system parts are produced at a rate which customer demands and this rate is called the Takt time.
Let's look at some common examples of pull and push system in our day to day life to understand this concept better. A soda vending machine is a pull system because you get soda only when you drop the coins. The supermarkets typically work on a pull system, because when we pick up soaps or other consumable from the shelf, the gap in the shelf triggers supply of fresh material, which in turn triggers production of the material. Most of the doctors offices are push system, because we got to visit the doctor and wait for our turn, by introducing consultations based on appointment it makes things better for patients though that is not a pure pull system. The flight landing or take off must ensure that they will follow a pull system to prevent accident. In factory canteens when we serve rice on a buffet system it is pull, but when items are served by the caterer it is typically push. If we notice all that is pushed on to the plate would be get wasted, because the person serving is not aware of the consumers taste and also the consumer doesn't feel guilty of wasting food, because it was forced on to him or her. Though the purpose of push system followed by the caterer is to restrict consumption of high cost item, typically more of such food would get wasted.
A good pull system I noticed in a Sushi bar in Shanghai, wherein customers don't have to place an order but just pull-off plates of several standard items from the conveyor; a pull by the conveyor triggers manual introduction of more such plates on to the conveyor. The conveyor has a limited space and this system is called a CONWIP system or constant WIP system, wherein the work in process in the system is defined and fixed by the conveyor capacity. In CONWIP system the part numbers (or types) can he different and in the Sushi bar example too, each plate may have a different items but the cost per plate is standardized.
I am sure that you could see many such areas where in Pull system is used or pull system can be implemented to improve efficiency and make life better for all involved.
Pradeep Kumar E.T. A Master Black Belt in Six Sigma , is the Country Manager- Operational Excellence with Tyco Electronics Corporation India Pvt Ltd. Feedback can be
e- mailed to pradeep@ businessgyan.com
Issue BG87 Jun 08