TABLE OF CONTENTS Earlier, we defined availability of a system as the fraction of time it is able to function. System availability is clearly the consequence of subsystem availabilities which in turn are a function of assembly and part level availabilities.
Operating time can frequently be greater than available time. Available time therefore sets a limit on production.
The objectives of an availability analysis are:
To estimate system availability for comparison with a target value.
To identify low availability components, assemblies, or parts for improvement.
Availability analysis is an extension of FMEA. One specific effect, namely unavailability, is estimated. System availability analysis should therefore be performed after an FMEA has been completed.
System availability assessment is a tool that can be applied during all life-cycle phases of a machinery system. Its results can be used by management as availability control. It can identify deficiencies in certain areas, or compare alternative designs.
The work steps are as follows. The system outage time caused by each level of the hierarchy is estimated from failure and maintenance data and recorded on a work sheet. Three types of downtime are identified: forced maintenance, predictive-scheduled maintenance, and planned (turnaround) maintenance. The sum of all types of downtime is used to compute machinery system availability.
The availability ( A) of a component is expressed as:
| (10.1) |  |
The unavailability of a unit is a function of the time required for two modes of maintenance: breakdown-corrective and predictive-preventive. Unavailability resulting from breakdown maintenance or forced...
