Reliability Block Diagrams
Reliability for a product is estimated by building a reliability modelA simplified description of a system or product that represents reliability performance. The main types of reliability model are reliability block diagrams and fault trees.. They are simpler than functional models because there is no ordering within the parts. There are two types of reliability models, reliability block diagramsA diagram that links the elements of a product that need to be working for the product to work. So long as there is a path from the start of the diagram to the end of the diagram the equipment can provide the service. The diagrams can be hierarchical so that blocks at one level can be decomposed into an RBD at a lower level. shown in this section and fault treesA graphical method of modelling the reliability of the system as AND and OR gates and failure events. shown in the next section. They both represent the 'and' and 'or' aspects of product reliability performance.
It is essential to build a large reliability model as a hierarchical structure. This has already been started
in this site in the reliability source data section. The natural hierarchy for
reliability modelling is system – module – component. The split into modules and components
often reflects the repair processes in an organisation well since first and second level
maintenanceMaintenance activities for a large organisation are usually organised into levels.
First level maintenance uses relatively unskilled labour and covers cleaning, inspections and other simple preventative maintenance activities.
Second level maintenance involves the diagnosis of faults and replacement of failed modules.
Third level maintenance is usually outsourced and repairs failed modules.
is often at the module level (failed modules are replaced). Particular industries will have their own names
for these hierarchy levels. For example in an electronic product system, module and component might be product,
rack, and items such as boards, disks, amplifiers etc. Large models require additional levels in the model.
Reliability block diagrams (RBD) connect together the parts of a product that need to be working for the product to be able to provide a service. Protection by redundancy is shown as parallel branches.
A simple RBD model for the air conditioning of a room is shown below.
Air conditioning will be available if there is a path from A to B through functioning modules. In the hierarchical reliability model the 'Air Conditioning unit' block may break down into the boards, motors, controllers etc. that make up the unit. Alternatively the specification for the unit may directly provide the failure data.
The fault treesA graphical method of modelling the reliability of the system as AND and OR gates and failure events. diagram for the same air conditioning service is shown in the next section.
Optimal Solutions have considerable experience in developing reliability models, and welcome enquiries on Reliability. We would be pleased to provide consultancy tailored to your requirements. You can get in touch by sending a message from our Contact Us page, or by calling us on the number below.