class 08- introduction to maintainability

7/30/2019 Class 08- Introduction to Maintainability

1/11

1

Maintainability

Various measures are used in maintainability analysis:

Mean Time To Repair (MTTR)

Mean Preventive Maintenance Time ( MPMT)

Mean Maintenance Downtime (MMD)

In addition to these measures, maintainability functions are usedto predict the probability that a repair, starting at time =0, willbe completed in a time .

Maintainability

Maintainability

Functions

Examples

Industrial Engineering


2/11

2

Maintainability

It measures the elapsed time required to perform a givenmaintenance activity.

MTTR is expressed by:

Where:

= number of units or parts

= failure rate of unit/part , for

= Corrective Maintenance or repair Timerequired to repair unit or part , for

Usually, times to repair follow exponential, lognormal, andnormal probability distributions

Maintainability

Maintainability

Functions

Examples



3/11

3

Maintainability

The mean preventive maintenance time is defined by:

Where:

= Mean Preventive Maintenance Time

= total number of preventive maintenance tasks

= Frequency of Preventive Maintenance task ,for

= Elapsed Time for Mreventive Maintenance

Task , for

Note that if the frequencies are given in maintenance taskper hour, then should also be given in hours.

Maintainability

Maintainability

Functions

Examples



4/11

4

Maintainability

Mean maintenance downtime (MMD) is described as the total

time required either to restore system to a given performancelevel or to keep it at that level of performance.

It is composed of corrective maintenance, preventivemaintenance, administrative delay, and logistic delay times.

The administrative delay time is the system or item downtimedue to administrative constraints.

Logistic delay time is the time spent waiting for a requiredresource such as spare part, a specific test, or a facility

Maintainability

Maintainability

Functions

Examples



5/11


6/11

6

Maintainability Functions

Maintainability functions predict the probability that a repair,starting at time , will be completed in a time .

The maintainability function for any distribution is defined by:

Where:

= time

= maintainability function (probability that arepair action will be finished at time t)

= probability density function of the repair time

Maintainability

MaintainabilityFunctions

Examples



7/11

7


Exponential p.d.f:

Exponential p.d.f is widely used in maintainability work torepresent repair times. It is expressed by:

The maintainability function in this case is:

Maintainability


Examples



8/11

8


Weibull p.d.f:

Weibull p.d.f can also be used to represent times to repair. It isdefined by:

where:

= shape parameter = scale parameter


When and , the reduces to the in thecase of exponential distribution

Maintainability


Examples



9/11

9


Normal p.d.f:

Normal p.d.f can also be used to represent times to repair. It isdefined by:

where:

= standard deviation of the variable maintenancetime t around the mean value

= mean of maintenance times


Maintainability


Examples



10/11

10


The mean of the maintenance times is:

where:

= total number of maintenance tasks performed

= ith maintenance time, for

The standard deviation is :

Maintainability


Examples



11/11

11

Series and Parallel System Reliability

Example

A mechanical machine has three important subsystems. The firstsubsystem has failure rate of 0.003 failures per hour and repair time of1 hour. The second and the third subsystems have failure rates of 0.005and 0.004 failures per hour and repair times of 4 hours and 3 hoursrespectively. The times to repair of this machine are exponentiallydistributed. Find:

1. The probability that a repair on this machine will be performed in 5

hours.

2. The MTTR of this machine

3. The MMD of this machine, if the authorization of the maintenance

work order takes 12 min and the time to get the required materials

and spare parts from the store is 20 min


Maintainability


Examples

class 08- introduction to maintainability

Documents