Reliability Block Diagrams
Reliability Block Diagrams are a powerful reliability analysis tool. They are visual representation of reliability and risk that exists within machinery or work processes.They allow the user to notate the reliability of each part or task and calculate the reliability for the whole scenario. The use of Reliability Block Diagrams extends to all aspects of operational processes and procedures, both as series and parallel arrangements.
Once a Reliability Block Diagram is created, you can do “what-if” scenarios to identify the defects that can happen to each part. This information allows you to then make informed decisions about to how to proactively eliminate the causes of each risk or defect, and by extension, eliminate that cause of failure.
Below is a series of examples where Reliability Block Diagrams are applied to a range of situations, and aid in identifying risks, defects, and causes of failure.
Reliability Block Diagrams for Machinery
The engineering logic of how machine parts work together is show in their Reliability Block Diagram.
When you look at a machine you see a complete piece of equipment. How this machine is built, and where each part is interconnected is detailed in its engineering drawings. It is an assembly of many parts working together in a series arrangement.
We can draw this series arrangements of parts on paper as a row of boxes connected by arrows. An example of this, known as a Reliability Block Diagram, is shown below. The engineering logic of how the parts of this electric motor drive end bearing work together is show in their Reliability Block Diagram. Each component detailed in the engineering drawing is placed into the Reliability Block Diagram in the order it works when the machine operates.
Figure 1. Example Reliability Block Diagram for an electric motor drive end bearing, illustrating how machines are made of parts in a series arrangement.
This is a visual representation of the reliability and risk that exists in this machine sub-assembly. The reliability for each part is notated and used to calculate the reliability for the whole series. For example, if the boxes that represented the lubricant had a reliability of zero, then the reliability of the whole series is zero. This means the whole assembly reliability is zero, and the machine will breakdown. If individual parts contain defects or faults, they will fail at some time in future when the circumstances arise that trigger the defect to become a failure.
Once you have a Reliability Block Diagram, you can do “what-if” scenarios to identify the defects that can happen to each part. Then you can make informed decisions about to how to proactively eliminate the causes of each risk or defect, and by extension, eliminate that cause of failure.
Work Tasks
Below is an example of a technician repairing bearings. The job requires varies tasks and activities to be performed in sequence. Ideally, he has a comprehensive and clear procedure to follow in which each task, and its sub-activities, are listed and explained. From the procedure we can draw a Reliability Block Diagram of the whole job. Because every job is a series of tasks and activities performed sequentially, we can estimate the reliability of the completed job from the reliability of completing each task. The job reliability equation to use is shown under the work task flow diagram.
In this example there are only five tasks for the job. Each task is completed one after the other. The job is only done 100% correctly if each task is itself done 100% right. Because the job is a series of tasks and activities performed sequentially, the job is right if, and only if, each task is first done right.
Figure 2. Example Reliability Block Diagram for a work task, illustrating how work tasks are made of activities in series arrangements
If a Job is a Series of Tasks, What’s Your Chance of Getting Them All Right?
The reliability of a task is the chance that it will be completed correctly. When work tasks are not done right an error or defect is left behind. Each error or defect caused by a task mistake makes the task reliability fall from 100% to a lesser value. If a task reliability is 90%, then for every 10 times the task is done once it will be done wrong. We know from the risk equation shown that unless task reliability is 100%, the error in the task will cause risks in the future. Once a defect is left behind in a job it sits there waiting to become a future problem or failure.
Because work is a series of tasks and activities performed sequentially everyone who does work can produce errors, mistakes, defects, and failures in every task and activity they do. No job is certain of being successful until every task is known to be done right.
We must control the quality of individual work tasks, so they are always completed right, if you want to ensure the job is 100% successfully done.