Failure Mode and Effects Analysis

Failure Mode and Effects Analysis
See also
FMEA analysis PFMEA Six sigma Quality control Statistical process control Risk management process Why why analysis Fault tree Fish diagram

Failure mode and effects (critical) analysis (FMEA, FMECA) method was developed in USA in late 50s. It's aim was to prevent product failures already at the stage of product design. Early prevention of failures was especially important in space, aviation and military industries. As analyses have shown, over 3/4 of all failures during production and using of product can be prevented during product design process.

The method was popularized in USA and later in Europe and other countries. It was used first in high technology industries, but it appeared to be successful also in other industries and even in services. However, for services SERVQUAL may give better results. Since 90s it is used also in process improvement and as a simple tool of risk management. Despite over half of century since development of the method, it still finds new applications. The method can be easily digitalized and used in semiautomatic mode in MRP/ERP systems.

Range of FMEA applications

1. Product optimization
   • at the design stage - best moment for analysis, best effects achieved due to easiness of implementing changes into the product,
   • before implementation into production - limited effects due to limited possibility of changes in product design, but still better to use it than not,
   • later stage - very limited effects, because almost all parameters are fixed. Use it before upgrading your product.
2. Process optimization
   • implementation of new process - very good moment, however it happens very rarely in enterprise,
   • improvement of existing process - very good moment, process improvement is not as limited as product improvement. Moreover, employees know the process and its limits, which can lead to very good effects.
3. Risk management
   • as simple method of risk identification and dealing with risk. It can be enough for most small and medium enterprises.

Process optimization can be achieved also using modified FMEA: PFMEA.

Steps of using FMEA

1. Stage 1. Preparation
   • Definition of objectives
   • Setting a team
   • Setting scope of analysis
   • Functional decomposition
   • Data collection
2. Stage 2. Analysis
   • Qualitative analysis
   • Quantitative analysis
   • Planning preventive/corrective actions
   • Implementation of actions

The most important steps will be described below:

Setting scope of analysis

The scope of analysis is essential for the results. If the analysis will be limited to only several elements, it can add no value. But if the scope is too wide, the time of analysis and cost of it will be very high. Probably there is no need to analyse parts that are well known from earlier products, unless they can be incompatible with new solutions. Also level of decomposition should be considered. Very deep decomposition not always is proper perspective, as it hides problems coming from relations between elements.

Each analysis is performed in defined environment. The team should decide what are the environment variables, e.g. availability of nominal power, number of failures at a time (usually only 1 allowed), availability of necessary inputs, possibility to utilise outputs.

Functional decomposition

The object under investigation should be decomposed into elements. If it is a product, it should be decomposed into parts or even raw materials. In case of process, it should be decomposed into tasks. FMEA is very low level analysis. For each element data should be collected, including:

• number of failures,
• types of failures,
• known problems (e.g. analysis of literature),
• limits of use (e.g. temperature, humidity).

Qualitative analysis

Qualitative analysis concerns possible failures, their causes and effects:

• failure - what wrong can happen to the element under investigation,
• cause - what has to happen to make the failure occur; there can be several causes and each should be analysed separately,
• effect - what will happen if failure occur (not cause! - common mistake); there may be several short-term as well as long-term effects. Each should be analysed separately.

Set of failure + cause + effect is a failure mode. All data should be entered into table (see examples below).

Quantitative analysis

Quantitative analysis refers to three variables:

• (P) Probability of occurrence (related to cause of failure),
• (D) Detection difficulty (related to the failure itself),
• (S) Severity (related to effect).

Product of those three variables gives overall rating of the analysed failure mode.

$R=P\cdot D\cdot S$

Each variable is rated in scale from 1 to 10, where 10 is maximum. The enterprise using FMEA should define own evaluation tables that help in assigning ranks. Example table of probability is shown below:

Table 1. Example of probability table for mass production

Description	P value	Frequency
Almost impossible	1	less than 1 per million
Very rare	2	less than 1 per 20 000
Rare	3	less than 1 per 4 000
Average	4 5 6	less than 1 per 1 000 less than 1 per 400 less than 1 per 80
Often	7-8	less than 1 per 40 less than 1 per 20
Very often	9 10	less than 1 per 8 less than 1 per 2

Source: [1]

Corrective and preventive actions in FMEA

Since the R combines three areas in corrective action there are three different ways of repair:

• Reducing the probability,
• Improve detection,
• Reduce nuisance of failures.

The choice depends on the type and complexity of the failure and the product. Success relies in this case only on the experience and knowledge of the team members using this method. For ease of analysis.

Examples

FMEA for a PC

Element	Defect	Effect	Cause	P	S	D	R	Corrective action
system unit	the system will not load	computer does not work	improper system program	3	10	9	270	exchange program
monitor	Wrong colors	green and red unavailable	inadequate graphics card	2	3	2	12	check cards and exchange
hard drive	disc unreadable	data loss	bad hard drive	4	8	10	320	installation of the drive
keyboard	locks	possible transmission of data	improper connection	4	2	5	40	Test the keyboard, check. Connection
printer	Errors Print	printing can not be read	damage to the controller	6	3	3	54	exchange control
Disk Drive	read errors	you can not store data	dirty disc or drive mechanism	3	5	2	30	cleaning or replacement of the drive mechanism
expansion card	expansion card operation error	missed opportunities Cards	card not connected	7	1	8	56	Testing connections, the right mix after the test

Source: A.P. Mühlemann, J. S. Oakland, K.G. Lockyer, 1995, p. 138

The example given is very simplified and in practice would have to be greatly expanded. With several dozens or even hundreds of potential defects is useful to apply Pareto chart to carve out those that should be addressed at the earliest.

FMEA for the bathtub faucet with spray

Element	Defect	Effect	Cause	P	S	D	R	Corrective action
body	leak	leakage	bad casting, micropores	5	10	4	200	battery change
	bad threads	assembly difficulties	wrong setting machine	3	1	1	3	exchange of body
head	spiking	flooding the bathroom	a little thread	2	10	8	160	head replacement
	leak	leakage	bad seal	3	8	6	144	replacement of seals
spout	mounted askew	unsightly appearance	incorrect assembly	7	3	1	21	reassembly
spray switch	difficult switching	overexertion	too hard seal	8	5	3	120	change seals
	incomplete switching	Water poured from the spout and shower at the same time	incorrect assembly	6	4	4	96	reassembly
snake	interleaving	unsightly appearance	?break? hose	3	6	3	54	Hose replacement
	leak	leakage	trimming curves hose	3	7	8	168	Hose replacement

Source: own study

The analysis found that the most serious of these flaws are micropores in the bodies battery leakage hoses, knocking heads and their leaks and difficult to turn on the shower. In further work should focus on minimizing the likelihood of these problems.

References

• Examples and templates for FMEA
• Stamatis DH (2003) FMEA from Theory to Execution, ASQ Quality Press
• Arabian-Hoseynabadi H, Oraee H, Tavner PJ (2010) Failure Modes and Effects Analysis (FMEA) for wind turbines, International Journal of Electrical Power & Energy Systems, Volume 32, Issue 7

Author: Slawomir Wawak