What is a Water spider (or Mizusumashi)
The Water Spider (“Mizusumashi”) System is one of the improvement specialties in internal logistics flow. This Japanese word indicates “water beetle,” and this internal logistics worker is often called a “water spider” in English. This term probably was chosen for this concept because of the water beetle’s agility as it swims across the water. Here a mizusumashi is a logistics worker who does the internal transportation of goods using a standard fixed cycle route.
The Water Spider is a critical element of the creation of internal logistics flow. A worker moves all the information related to production orders (kanban) along with all containers. However, a water spider moves the flow containers between supermarkets and the border of lines by repeating the same movements in a fixed cycle (which usually runs for 20 or 60 minutes). During this cycle, the water spider will stop in a certain number of stations along the route and check whether they need materials. The water spider uses a small train with a suitable load capacity to serve all the stations on its fixed route, delivering information to several points along the way.
The mizusumashi fixed cycle time is also called the pitch time. This pitch time is a multiple of the takt time. If the mizusumashi is moving one piece at a time, the pitch time would equal the cycle time. Because the mizusumashi is moving small containers, the pitch time is designed to carry several small boxes to many points of use at the border of several lines.
The customers are the production operators on the lines. They have a reliable logistics provider who comes every 20 or 60 minutes, looks to see if more material is needed, and removes the empty containers and any garbage generated during the process (also known as reverse logistics). Production is assured of a reliable and frequent supply.
How Waterspider help in building successful lean factories?
The mizusumashi system is one of the most important means of creating internal logistics flow to build successful lean factories. The water spider operates like a shuttle service at an airport. The shuttle service has a fixed route (e.g., Arrivals 1, Arrivals 2, Hotel 1, Hotel 2, and Hotel 3) that it keeps on the following cycle after cycle. The cycle timing can be calculated—if we allow 4 minutes for each shuttle stop and 20 minutes for the driving time between visits, we have a cycle of 40 minutes. There will be a schedule at every shuttle stop that shows the estimated time of arrival. The users know that every 40 minutes, the shuttle will arrive. Once they are on board, they know what time they will arrive at their destination.
The mizusumashi system operates the same way. It has the following characteristics:
The mizusumashi “shuttle” stops are at supermarkets (i.e., picking supermarkets, border-of-line supermarkets, kitting supermarkets, or finished- goods delivery supermarkets).
The cycle is calculated in the same way, by measuring the work to be done at several stops and adding the travel time.
At this level of organization, the containers to be moved onboard the shuttle service are the equivalent of customers or passengers.
The water spider’s definitive work means a fixed route (i.e., a plan that shows the travel route and the stopping points) and a constant cycle time determined by the sum of the times involved. Because we are using supermarkets and flow containers, we can improve the productivity of the mizusumashi by enhancing the operator’s standard work, just as we improved routine work to achieve production flow.
As well as moving materials and empty containers and doing other driving tasks, the mizusumashi also moves the information associated with replenishment and different synchronization needs.
Figure: The advantages of using Mizusumashi over Forklifts