Q 467. What is a Reliability Block Diagram? In which phase of a DMADV project will it be used?   

 

Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

• All questions so far can be seen here - https://www.benchmarksixsigma.com/forum/lean-six-sigma-business-excellence-questions/
• Please visit the forum home page at https://www.benchmarksixsigma.com/forum/ to respond to the latest question open till the next Tuesday/ Friday evening 5 PM as per Indian Standard Time. Questions launched on Tuesdays are open till Friday and questions launched on Friday are open till Tuesday. 
• When you respond to this question, your answer will not be visible till it is reviewed. Only non-plagiarised (plagiarism below 5-10%) responses will be approved. If you have doubts about plagiarism, please check your answer with a plagiarism checker tool like https://smallseotools.com/plagiarism-checker/ before submitting. 
• The best answer is always shown at the top among responses and the author finds honorable mention in our Business Excellence dictionary at https://www.benchmarksixsigma.com/forum/business-excellence-dictionary-glossary/ along with the related term

                   The Reliability Block Diagram is used for industrial and commercial power systems. It is a graphical presentation of a system diagram based on reliability or functional logic; i.e., connecting subsystems or components according to their function or reliability relationship. The significance of RBD is that it is easy to read. It is easily understood by customers who purchase the critical power systems, by the people who sell the systems, by engineers who design and test the systems, and by managers who make decisions on the systems. With knowledge of the system design, engineers can easily construct, verify, and modify the RBD, and also communicate with those of different functions.  It is generally drawn from top-down, left-right approach, depicting the system element functions. The 3 types of RBD are Series, Parallel and Combination of Series & Parallel elements. 

 

                                              An example of Reliability block diagram

Reliability is typically calculated as below:

Reliability = e-λt

λ (lambda) = Failure rate = 1/MTBF

t =  time in cycles, hours, miles, etc.

e = natural logarithm = 2.71828

 

RBD can be used in the following phases of DMADV:

1. Analyze Phase - To identify the to identify potential areas of poor reliability and where improvements can be made to lower the failure rates for the equipment
2. Design Phase - To construct / design the system & subsystems thus improving the reliability from the functional perspective
3. Verify Phase - To check if the newly designed system / subsystem is able to eliminate the potential bottlenecks identified with respect to reliability are improved.  

This provides a useful benchmark to refer to when looking back at how the system used to perform, and whether the system’s current performance is as expected and is based on the model created.

 

 

 

Reliability block diagram, is a representation of component reliability, to contribute to success or failure of a system.

These Reliability Block Diagram or, “Dependence diagram”, are often referred to as a series of blocks connected in parallel or series configuration. Each block in Reliability Block Diagram represents a component of the system, with a failure rate.

A parallel block in a dependence diagram plot, represents “redundant subsystems or components”, that contribute to a lower failure rate.  

The Reliability Block Diagram method, may be used for modelling the cumulative effect of component and sub system failure, in developing into a system failure.

Alternatively, a representation of switches instead of blocks may be sketched on paper, with open switch representing a working component and a closed switch representing a failed component. A path drawn between beginning to end through network of such switches, represents a functional system.

 

The success in production activities may studied by calibrating operating assumptions, over a period of time, while accounting for the level of accuracy and complexity inherent in production system. The number of operating assumptions is increased, corresponding to higher level of complexity, in production system.

 

A measure of system or sub-system component characteristics, (e.g., equipment failure rates, operational and maintenance strategies), are assessed in quantitative assessments, during production run, to determine likely effect on production or performance of the system.

 

These scaled values of production or system performance, are analysed to estimate, each component reliability, to contribute to success or failure of the system.

 

The estimated values of component reliability are represented on paper as, “diagram of blocks” to highlight resultant effect on output performance-- system efficacy and efficiency.

 

 

In the above diagram, a portion of blocks, say two out of total displayed three block constituting subsystem B, need to be succeed for system to succeed.  Adding corollary to this assertion, it can be concluded that a failure along a series path, effectuates, the entire series path to fail.

 

Quantitatively, reliability for a component may be represented with use of either probability or failure rate

 

Series Path Reliability for components in series path is estimated by multiplying the reliability (a probability) of the series components:

 

R_SYS= R₁(t)*R₂(t)*R₃(t)…. R_n(t)

 

Parallel Path Reliability for components in parallel path is estimated by multiplying the unreliability (Q) of the series components, where Q = 1 – R, if only one unit needs to function for system success:

 

Q_SYS = Q₁(t)*Q₂(t)*Q₃(t)…. Q_n(t)

 

Failure rates in series are calculated assuming Poisson Distribution or random distribution of the series components:

 

λ_SYS = λ₁ + λ₂+ λ₃… λ_n

 

Failure rates in parallel are calculated with formula given hereunder:

 

λ = n! λ^q+1/(n-q-1)!µ^q

 

The process may be utilized to estimate fault tree analysis FTA on the critical components of industrial robots.

Determining reliability of production asset, machinery, tool, operational measure as contribution from one or more factor in production block or operative scenarios is critical to Reliability Block Diagram method. The production tool and assets contributing most to the perceived risk, are identified & targeted for improvement using Reliability Block Diagram.

 

The scenario-based analysis of individual component reliability in Reliability Block Diagram facilitates, an unbiased estimate of “Projected Performance Levels”, targeted with designate changes in the test environment.

Reliability Block Diagram may be put to practice to estimate bottlenecks impacting production activity, with the biggest risk potential/risk threat. A bottleneck may be defined as, “an element in production activity, including- physical asset as “single point of failures”, or, an operational measure, such as “lack of control values”, that slows down progress.”

 

The process may result in simplification in levels of production hierarchy, as complex system design is synergised to generate better component reliability; through check in place, for the present regulations, functional directives, and knowledge guidelines related to production process. This may lead to better estimation of reliability of a modular or functional block, in comparision to individual system component/machinery/tool.  

 

The data on failed initiatives, production failures, maintenance records for production activity and Root Cause Analyse for identified instance, may be interpolated, to underline instances of dips in production activities. The expertise of skilled and technical specialists, operating in different departments, may be thus cohesively utilized, to realise better production. A study based on Reliability Block Diagram method results in list of changes that may be listed or grouped together for implementation at a later stage.  

 

Reliability Block Diagram method may be used in Improve phase of DMAIC methodology to design a new system or to describe-analyse-improve an existing system design used in production. The high-level process overview of process components physical asset, process key participants-operational aspects and interrelationship between physical asset and operational principle is implemented to get better output from production design.

 

A Reliability Block Diagram (RBD) illustrates the state of a specific function in a system with several elements.

 

The diagram is made up of functional blocks represented as blocks and connected by lines. Reliability Block Diagram (RBD) has a single starting point (A) and single ending point (D), as shown in the following figure:

RBDs are also known as dependence diagrams (DDs).

RBDs are represented by series or parallel connections of blocks. Parallel blocks refer to redundant subsystems or components that contribute to a lower failure rate. Each block represents a component of the system with a high failure rate. The RBD will indicate the type of redundancy in the parallel path. For a group of parallel blocks to succeed, two out of three components would have to be successful. On the other hand, any failure along a series path causes the entire series path to fail.

A RBD may be drawn using switches in place of blocks, with a closed switch representing a working component and an open switch representing a failed component. As long as there is a path through the network of switches from beginning to end, the system still works.

 

DMADV stands for Define, Measure, Analyse, Design and Verify. All five phases can be addressed with RBD. At the define stage, to identify which system needs further analysis using RBD, at the measure stage, to study the opportunity for failure rate at each subsystem, at analyse stage, to quantify the probability of failure rate from study and historical data analysis, then at the design stage, to indicate which system requires modification or updating and at verify stage, to ensure that the right subsystem is affected by the action. In this way, RBD can be used as a tool to keep the team focused on the right problem throughout the entire process.

The Reliability Block Diagram is helpful to identify possible areas of poor reliability and where enhancements can be made to lower failure rates for the equipment. Reliability Block Diagram method can be used in both the design and operational phase to recognize poor reliability and provide targeted enhancements.

 

The Reliability Block Diagram shows the logical networks of components within a piece of equipment. It is not predictably the schematic diagram of the equipment, but the functional components of the system. The equipment is made up of multiple modules/systems in series, parallel and a combination of the two. Arrangement of modules/systems provides us with the inherent reliability of the equipment. 

 

Each block in the Reliability Block Diagram signifies a component or subsystem of the system. The organization and connecting lines represent the reliability structure of the system and may or may not be characteristic of the system’s functional block diagram.

 

The below image is basic Reliability Block Diagram with 4 subsystems the R values are the probability of success (reliability) values for that element over a specific time period.

 

 

In difficult systems, the Reliability Block Diagram model in the above image may be a summary of the major subsystems.

 

Each subsystem have an associate Reliability Block Diagram with its own diagram of its subsystems and components. Include sufficient detail with each system or subsystem level to support decision making.

 

For example, it is not necessary to show every component on a circuit board when the decisions are under consideration do not comprise any deviations to the circuit board.

Signals of a useful Reliability Block Diagram include:

 

·         Reliability statement is inclusion of all four elements: function, environment, probability, and duration

·         Existing reliability performance estimate with reference on source of estimate

·         Identification of Bottlenecks and Low Hanging Fruit, to know which areas are likely to be the most problematic and which assets are causing these issues based on frequency and period of issues created by each asset compares to others in the model

·         Evidence of prioritization and design decisions based on RBD information includes in design review meetings

 

·         The Reliability Block Diagram should receive regular updates as improved reliability estimates become available

 

Reliability Block Diagram is important for the design team to understand the options available to create a reliability system.

 

If there are multiple ‘what-if’ situations should have substitute Reliability Block Diagram’s created to evaluate system reliability performance.

 

Reliability Block Diagram will be used in Analyze and Design Phases of a DMADV project:

 

As 6O% of failures and safety problems can be prohibited by ensuring there is a strong equipment design and that Maintenance and Reliability is taken into account during the design phase. Equipment will be designed with the following parameters/thoughts:

 

·         For Fault Tolerance

·         To Fail Safely

·         To give early warning of the failure to the user

·         Built-in indicative system to identify fault location

·         To eliminate all critical failure modes cost effectually

 

To ensure that the equipment design is capable and ensure the system or process can deliver requirements of the design. There are various model available to perform the analysis, but a relatively simple and widely accepted approach is the Reliability Block Diagram.

Edited May 3, 20224 yr by Dharanesh Mysore
Spelling mistake

While all answers are well drafted, there are two that stand out - P Balakumaaran and Manish Manjhi. Both answers are a must read and both answers are selected as winners. 

 

P Balakumaaran - answer for its simple yet powerful way of explaining RBD plus its usage.

Manish Manjhi - for highlighting its valid usage across 4 phases of DMADV.

 

Congratulations to both!!