POKA-YOKE

Poka-yoke (ポカヨケ) is a Japanese term that means “mistake-proofing”. A poka-yoke is any mechanism in a Lean Manufacturing process that helps an equipment operator avoid (yokeru) mistakes (poka). Its purpose is to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur.

The concept was formalised, and the term adopted, byShigeo Shingo as part of the Toyota Production System. It was originally described as baka-yoke, but as this means “fool-proofing” (or “idiot-proofing”) the name was changed to the milder poka-yoke.

HISTORY 

The term poka-yoke was applied by Shigeo Shingo in the 1960s to industrial processes designed to prevent human errors. Shingo redesigned a process in which factory workers, while assembling a small switch, would often forget to insert the required spring under one of the switch buttons. In the redesigned process, the worker would perform the task in two steps, first preparing the two required springs and placing them in a placeholder, then inserting the springs from the placeholder into the switch. When a spring remained in the placeholder, the workers knew that they had forgotten to insert it and could correct the mistake effortlessly.

Shingo distinguished between the concepts of inevitable human mistakes and defects in the production. Defects occur when the mistakes are allowed to reach the customer. The aim of poka-yoke is to design the process so that mistakes can be detected and corrected immediately, eliminating defects at the source.

USAGE 

More broadly, the term can refer to any behavior-shaping constraint designed into a process to prevent incorrect operation by the user. Similarly, a constraint that is part of the product (or service) design is considered Design for Manufacturability or Design for X.

A simple Poka-Yoke example is demonstrated when a driver of the car equipped with a manual gearbox must press on the clutch pedal (a process step, therefore a poka-yoke) prior to starting an automobile. The interlock serves to prevent unintended movement of the car. Another example of poka-yoke would be the car equipped with an automatic transmission, which has a switch that requires the car to be in “Park” or “Neutral” before the car can be started (some automatic transmissions require the brake pedal to be depressed as well). These serve as behavior-shaping constraints as the action of “car in Park (or Neutral)” or “foot depressing the clutch/brake pedal” must be performed before the car is allowed to start. The requirement of a depressed brake pedal to shift most of the cars with an automatic transmission from “Park” to any other gear is yet another example of a poka-yoke application. Over time, the driver’s behavior is conformed with the requirements by repetition and habit.

PRACTICAL APPLICATION

Poka-yoke can be implemented at any step of a manufacturing process where something can go wrong or an error can be made. For example, a jig that holds pieces for processing might be modified to only allow pieces to be held in the correct orientation, or a digital counter might track the number of spot welds on each piece to ensure that the worker executes the correct number of welds.

Shigeo Shingo recognized three types of poka-yoke for detecting and preventing errors in a mass production system:

1. The Contact Method identifies product defects by testing the product’s shape, size, color, or other physical attributes.
2. The Fixed-Value (or constant number) method alerts the operator if a certain number of movements are not made.
3. The Motion-Step (or sequence) method determines whether the prescribed steps of the process have been followed.