Failure Mode and Effects Analysis
|Failure Mode and Effects Analysis|
|Methods and techniques|
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
- 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.
- 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.
- 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
- Stage 1. Preparation
- Definition of objectives
- Setting a team
- Setting scope of analysis
- Functional decomposition
- Data collection
- 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.
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 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 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
|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|
|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
|less than 1 per 8
less than 1 per 2
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.
FMEA for a PC
|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
|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.
- 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