POKAYOKE

1 Definition

The Japanese expression "Poka Yoke" ( �jJ 3 J:r : "preventing unforeseen

incorrect handling") describes a principle which uses technical precautions
to prevent errors from occurring.

2 Basis

The starting basis for Poka Yoke is the recognition that no-one is able
always to prevent errors or failures from occurring.

Poka Yoke uses simple and effective systems to ensure that mishandling
(for example, in the production process) is prevented or detected
immediately, so that defects cannot be carried forward into the end­
product.

Employees can find terms such as "fool-proof' or "idiot-proof' degrading

and the expressions "error-proofing" or "proof against mishandling" are
preferred.

3 Reactive POKAYOKE

The first published examples (Shigeo Shingo) referred to the elimination of

errors or failures which occurred in processes as a result of unforeseen
mishandling. It was in this connection that the POKA YOKE failure list was
developed, together with the POKA YOKE decision-making matrix and the
POKA YOKE system matrix, also referred to as the "ideas module".

4 The failure list

The failure list describes ten typical categories of unforeseen failures. In

seeking the possible cause of mistakes this failure list is used as a check­
list.

Poka Yoke 3
1. Mishandling:
Parts installed the wrong way round, wrong parts fitted

2. Forgetfulness:
If people do not concentrate, things can often be forgotten.

3. Errors caused by misunderstandings:

Sometimes people think they understand the situation before they
understand the problem.

4. Errors caused by overlooking something:

Sometimes mishandling occurs because people jump too quickly to a
conclusion or are too far to see the problem.

5. Errors by beginners:
People can make mistakes if they lack experience.

6. Items are overlooked:

Mistakes occur when people are inattentive and then they themselves
do not know how the mistakes could have occurred.

7. Errors caused by working too slowly:

Sometimes mistakes occur when handling slows down or stops
unexpectedly.

8. Errors caused by a lack of standards:

Errors sometimes occur if process instructions or work procedures are
not provided or if they are not comprehensive or are unsuitable.

9. Surprise errors:
Mistakes sometimes occur if a process goes other than expected.

10. Deliberate mistakes:

Mistakes sometimes occur because people deliberately ignore certain
rules and regulations - for example, driving through a red light because
no other vehicle is coming.

This does not mean the person is carrying out sabotage with the aim of
creating damage. It is more a disregard for the reason for a regulation -
"Why should I stop at traffic lights at 2 o'clock in the morning if nothing
is coming ?").

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5 The decision-making matrix

The decision-making matrix is used to check whether the cause of a defect

is restricted to such an extent that POKA YOKE actions can be taken
successfully.

YES NO
Do we know where the defect has occurred?
Do we know which part has caused the defect?
Do we know the activity which has caused the defect?
Does the defect have a binary/volatile nature?

If the answer is "No" to more than one of these questions, action must be
taken to restrict the defect further. If this further action does not result in a
"Yes" to all the questions, other problem-solving methods must be used.

6 The system matrix; the ideas module

The system matrix serves as support for the development of creative

actions. Initially, in order to be absolutely sure that a failure is prevented, a
separate test method, initiating function and regulatory function should be
determined for each failure. This creates redundancy; however, if one
action fails, one of the others will always take effect.

Test method Initiating function Regulatory function

Check the source of Contact methods Action method
the failure
Test with direct Constant value methods
feedback Warning method
Test with Step-by-step methods
indirect feedback

Fig. 1 : System matrix

5
Explanation of the system matrix

6.1 Test method

Check the source of the failure

A check on the source of the failure makes it impossible for the cause
leading to a mishandling to occur.
Example:
When material is fed to a process a physical stop is fitted, to prevent the
item from being inserted the wrong way round into the process.

Without Poka Yoke

Lathe

Work.piece (inserted Work-piece (inserted

Chuck
wrong way) correctly)

Fig. 2: Example of a check to determine the source of a failure

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Check with feedback (direct)

A check with direct feedback prevents the failure from occurring because
the mishandling is detected immediately.

Example:

The design of the component in conjunction with the jig means that
incorrect installation is immediately prevented (direct feedback).

Locating peg Cut-out

�-D-D--�-o�'
DD
DD
0
• DD 0

inserted correctly inserted wrongly

Fig. 3: Example of a test with direct feedback

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Check with feedback (direct)

A check with indirect feedback prevents an error from proceeding to the

next process.

Example:

A symmetric item must be processed on two sides. A check-peg is fitted in

a cut-out so that, if the cut-out is not present, the item cannot be processed
in the next stage of operations.

Problem: The cut-out is occasionally omitted and

the defect is not detected before assembly.
Tapping drill

Cut-out

Solution: A test peg in the drilling jig prevents parts without

a cut-out being inserted. The parts are sent for rework at
an earlier stage.

Fig. 4 : Example of a check with indirect feedback

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6.2 Initiating function

Contact methods

Unacceptable deviations from the ideal condition are measured using

sensors. Depending on the type of sensor, the measurement may be
made by contact or a non-contact method.

Example:

A limit switch which detects that an item has been set in position and a
counter which counts the number of operations carried out.

Problem: the number of drilled holes is counted by t

operator; holes are sometimes forgotten.

Corr�ct
/0000?

I ,
Wrong

Solution : A limit switch detects the installation of

each item and a counter counts the holes. A buzzer
sounds if a hole is not present.

Fig. 5 : Example of the contact method

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Constant value methods

Where deviations or irregularities occur in the course of a production

process, these can be detected by checking that a certain number of
operations has been carried out on each item.

Example:

The technical equipment must be simple and effective - e.g., limit switches
and interlocks.

Interlock switch
Problem: Sometimes a part is placed in a jig but
is then removed without being processed and is
forwarded to the next work-station.

D
s,.,;on2

Work-piece

\jPifJ
1 I
Stat;on 3 Flow of'
parts
Stat;on 1 Limit switch

Solution : The following station is interlocked and

will not operate if the previous process has not
been completed.

Fig. 6 : Example of the constant value method

10
Step-by-step methods

The standard sequence of movements in a work-process is detected and

checked using the simplest possible equipment.

Example:

The parts required for an assembly can be called up only in a given

sequence. During assembly, the parts required are dispensed only in the
correct quantities and in the correct sequence.

Before After

J
Parts container
Window for selecting parts
/
r----.=;;:::;---,

Work�urface

Rails

Fig. 7 : Example of the step-by-step method

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6.3 Regulatory function

Action method

If deviations or mishandling is detected, the system is stopped immediately

or the process is rendered impossible.

Example:

The plate cannot be installed the wrong way round.

Problem:
Because the pin is central, it is possible to
install the plate the wrong way round.

a a

12 k
4-------.2.________,./
7
Poka-Yoke design:
The pin is located to one side to that the plate
cannot be installed the wrong way round.

Fig. 8 : Example of the action method

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Warning method

These may be any type of optical and acoustic signals which indicate that
mishandling is taking or has taken place.

Example:

A red light flashes if a part has not been processed or has been wrongly
positioned.

Problem : An unprocessed part may

cause a production line to come to a
standstill.

Solution : A limit switch detects the

difference in height of unprocessed parts
and initiates an alarm via the signal lamp

Fig. 9 : Example of the warning method

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7 Preventive POKA YOKE

The principle that human error (or at least the damage caused by human
error) can be prevented by early detection is, of course applied in the
course of process planning and product development in form of preventive
Poka Yoke.

In product development consideration is given not only to the

manufacturing or assembly process in question but also to the handling of
the product by the customer, in service and when it is to be disposed of.

In this the following POKA YOKE check-list is used:

developed
manufactured
assembled the wrong way round
operated with mistaken parts
stored incompletely
What can be: built up with forgotten parts/operations
used in the wrong way
repaired manipulated
fitted or removed wrongly
disposed of

Each user can change or expand the list to suit the project in question.

8 "Hard" and "Soft" POKA YOKE

Hard POKA YOKE

The term "hard" POKA YOKE describes actions which make it impossible
for a process to be carried out other than in the desired manner - for
example, a coded shape means that a part can be assembled only in one
specific position and orientation and it is not possible to fit any other parts.

Hard POKA YOKE has the advantage that incorrect handling as the result
of human error is prevented. On the other hand, hard Poka Yoke makes
the process inflexible and takes from the operator his or her responsibility
for the job.

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Soft POKA YOKE

"Soft" Poka Yoke describes actions which inform operators of errors or

failures but leave it to the operators to decide how to react to the
information. For example, a warning signal will operate if a component is
not fitted.

The disadvantage is that the signal may, of course, be overlooked or not

seen or heard, so that the defect continues to the next process. As the
signal is normally noticed by other operators, the person who does not
notice it feels embarrassed.

The advantage is that responsibility for detection and correction remains

with the operator. A further advantage is flexibility, if it is necessary to
continue work (for example, if assembly can continue even if some parts
are in short supply). In these circumstances the warning signal can be
ignored.

Literature

1 c.a.r.s (publisher): Poka-Yoke ...oder mit einfachen Mitteln zur

Null-Fehler-Produktion (CD-Version);
(Poke Yoke ... or simple means to zero-defect production (CD version)
Frankfurt am Main: c.a.r.s. GmbH, 1999
2 Hirano, H.: Poka-Yoke -Verbesserung der Qualitat durch
Vermeiden von Fehlern;
(Improving quality by preventing defects)
Landsberg/Lech: Verlag moderne lndustrie, 1992
3 Kamiske, G., Brauer J.-P.: Qualitatsmanagement von A-Z;
(Quality management from A to Z)
Munich: Hanser, 1993
4 Shingo, S.: Poka-Yoke -Prinzip und Technik fur eine Null-Fehler­
Produktion;
(Principles and techniques for zero-defect production)
St. Gallen: gfmt, 1991
5 TQU, Internet documents, 2007
6 Wilhelm Karmann GmbH, training documents, 2008

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