What is Water Spider in Lean?

A water spider is a Lean production personnel role centered around timely and accurate stock replenishment. The water spider team member is similar to a mobile Kanban system that refills the production line with the required materials to maintain a steady flow.

The person filling a water spider position needs to have a good understanding of the process, excellent communication skills, and has got to follow the standards of each process stage.

The water spider term was inspired by an insect known as a whirligig beetle, Mizusumashi in Japanese. Mizusumashi appears to hang between the water and the air while quickly going from place to place, keeping both the environments in his sight simultaneously. The swiftness of a water spider stock manager moving about the production floor, and their ability to perceive all production steps at once resembles the beetle behavior, giving the role its name.

What does a Lean water spider do?

The goal of the water spider is to shorten the process lead time and reduce waste.

On some production lines, engineers or assemblers need to step away from their station to gather a new batch of raw materials. This causes a delay in the process and is a waste in movement. The Mizusumashi can reduce this by carrying around a mini shopping cart, equipped in a Kanban system with the materials needed by all the assemblers. This prevents assembly line workers from performing the non-value-adding work since the required materials are delivered to them in a way that does not disrupt their workflows.

A water spider needs to follow a standardized, sequenced route: passing workers at routine times, thereby decreasing variability. Workers would typically use a bin system indicating which items have been depleted and need refilling. It’s common for manufacturing lines to have their Mizusumashi constantly connected to a work management and parts replenishment system, which instantly alerts them about where inventory is needed and at what speed.

Furthermore, the water spider role does not have to end at material replenishment - they can do several other things. E.g., transport the finished products, move Kanban cards, update workflow monitoring systems, or supervise job execution, especially for the untrained staff.

The skills of the water spider

It would be a mistake to compare the water spider role to an errand runner, a duty typically dropped on a junior apprentice. Mizusumashi is a crucial role, filling which demands extensive knowledge of the process and an awareness of what is happening across all stations, at all times. Besides maintaining flow on the production line, a water spider is also performing Gemba multiple times a day, which makes them invaluable when it comes to Kaizen initiatives.

In many ways, a water spider is a mobile Scrum Master on steroids, supplying the team with what they need when they need it, contributing to removing any impediments. The part of being “on steroids” may not be all that exaggerated, given how intense a Mizusumashi’s typical workday can be. They need to be on the move and alert at all times, and their ability to maintain the same pace throughout the day transfers directly onto how the process flows. It is not an easy “show your face here and there” job, despite potentially appearing as such to an uninformed observer!

A crucial aspect of a water spider role is that it is system-centric, not person-centric, in the sense that all of their work revolves around the efficiency of the system, not their own. This matters, as it will often be acceptable for the Mizusumashi to do a few half-empty rounds, not delivering material to all work stations, for the sake of servicing those few in need. It benefits the system, even if it doesn’t make the best out of the water spider’s time and capability 100% of the time.

The benefits of designating a water spider for your process

A steady flow of production and better standardization of stock delivery to the line will most likely come with an increase in production efficiency, hence a decrease in cost. One company has reported a 40% time saving thanks to introducing a water spider to its production floor.

Besides maintaining an even flow and reducing lead times and variability, a water spider duty can be an opportunity for leaders to become more familiar with the process and to get to know each person working on the line. That, in turn, works towards improved communication between the team and management.

A water spider is certainly one type of bug you would like to have on your production line!