The next time you’re walking by a fire hydrant, look at the stem nut on the top of it: it is pentagon-shaped. This means that your standard crescent or pipe wrench can’t grip it. While this does act as an anti-tampering feature, it also conveys that this is a special-purpose application.
This mirrors poka-yoke—Toyota’s system for designing out the possibility of mistakes. Poka-yoke was used everywhere: from requiring you to press the brake pedal before starting a car to ensure a known state, to making assembly jigs for switches and welded assemblies that wouldn’t allow for missing parts or incorrect orientations.
I think we can all use the poka-yoke method more to increase efficiency and safety while intuitively instructing users without using a single sticker or manual.
Interrelated Design Themes
- Poka-yoke: Design prevents incorrect use, immediately highlighting errors.
- Affordance: Design to communicate intended usage through intuitive cues.
- Keying/Indexing: Geometric constraints enforce a single correct orientation.
- Interlocks: Mechanisms enforce sequential actions to ensure safe operation.
| Themes | Design Examples |
|---|---|
| Poka-yoke | Diesel fuel nozzles (too large for gasoline tanks), Medical gas connectors (distinct shapes per gas type), RJ45 Ethernet cables (click-fit verifies correct connection) |
| Affordance | Door handles (shaped for intuitive grasping/pulling), Push plates on doors (flat surfaces suggest pushing), Teapot handles (ergonomic grip clearly indicating hold location) |
| Keying/Indexing | USB connectors (shape prevents reversed insertion), Shaft keys (align mechanical parts accurately), HDMI plugs (unique shape ensures proper orientation), Printer ink cartridges (brand-specific fittings), Apple Lightning cables (proprietary shape limits compatibility), Power tool battery packs (unique interface restricts brand interchangeability) |
| Interlocks | Microwave doors (prevents microwaving unless door is closed), Circuit breaker boxes (prevent opening unless power off), Elevator doors (only open when elevator is safely aligned) |
Poka-yoke in Practice
It seems one of the easiest ways to integrate poka-yoke into the science setting is through better operating or assembly procedures. This might require adding features to a part (e.g., keys) or creating simple custom jigs or part holders.
There are a few dimensions to consider these themes against in an application.
- Proprietary designs often increase the manufacturing cost and the need for specific tools or hardware to use them. Do you want users to be stuck if they misplace a special tool?
- Safety should assess the cost of adding poka-yoke principles or keying features against how catastrophic the misuse would be (e.g., installing a microchip 180° off).
- Efficiency should evaluate if design cues, asymmetric features, or tactile interlocks would make assembly or deployment faster or simpler.
Poka-yoke is not limited to designing something for only one specific use. Many people have likely attempted to insert an older USB device incorrectly into the socket, wondering why it even has a polarity. The introduction of USB-C solves the orientation problem while also dynamically assigning pins based on a host-peripheral handshake, a concept that adds cost and complexity, but is now worth it in a world of so many types of digital devices.