Chapter 5:Corrective Maintenance

1
Chapter 5Corrective Maintenance

• Overview
• Corrective Maintenance
• Mathematical Models

2
Corrective Maintenance

• Corrective maintenance is the remedial action
  carried out due to failure or deficiencies
  discovered during PM, to repair an equipment/item
  to its operational state
• Corrective maintenance is an unscheduled
  maintenance action
• Classifications

Servicing
Rebuild
Corrective Maintenance
Salvage
Overhaul
Fail-repair
3
Corrective Maintenance Steps
Recognition of existence of a failure
Localizing the failure within the system to a
specific item/equipment
Diagnosing within the item/equipment to identify
specific failed part/component
Replacing or repairing failed item./part/componen
t
Checking out and returning the system to service
4
Model 1 Single Failure Mode CM for Single
Machine

• Problem The system can either be in up or down
  state. Corrective maintenance is performed on the
  failed system to put it back into its operating
  state. Determine the probability in failed state,
  availability, MTTF
• Assumptions
• Failure and CM rates are constant
• The system is as good as new after CM is
  performed
• System failures are statistically independent

5
CM Markov Model

• Model
• Solution Using Laplace transformation
• At the steady state (t ?8)

6
Model 2 Two Failure Modes CM for Single Machine

• Problem The system can either be operating
  normally or failed in two mutually exclusive
  failure modes. Corrective maintenance is
  performed on either failure mode of the system to
  put it back into its operating state. Determine
  the probability in failed state, availability,
  MTTF
• Assumptions
• Failure and CM rates are constant
• The system is as good as new after CM is
  performed
• All system failures are statistically independent

7
CM Markov Model

• Model
• Solution Using Laplace transformation
• At the steady state (t ?8)

8
Model 3 Two Operating Modes CM for Single
Machine

• Problem The system can either be operating
  normally, operating in degradation mode, or
  failed completely. Corrective maintenance is
  initiated from degradation and completely failed
  modes of the system to repair failed parts.
  Determine the probability in failed state,
  availability, MTTF
• Assumptions
• Partial and complete failures, CM rates are
  constant
• The system is as good as new after CM is
  performed
• All system failures are statistically independent

µc2
System working Normally 0
µc3
µc1
Complete
Partial
?3
?1
?2
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CM Markov Model

• Model
• Solution Using Laplace transformation
• At the steady state (t ?8)

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Model 4 CM for Two Identical Parallel Machines

• Problem The system includes two independent and
  identical parallel machines. At least one machine
  must operate normally for system success.
  Corrective maintenance to put it back into its
  operating state begins as soon as any one of the
  machines fails. Determine the probability in
  failed state, availability, MTTF
• Assumptions
• Partial and complete failures, CM rates are
  constant
• The system is as good as new after CM is
  performed
• No CM is performed on the failed system

System working Normally 0
System failed 2
1 unit failed 1
µcm
?
2?
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CM Markov Model

• Model
• Solution Using Laplace transformation
• The system MTTFS
• Where MTTF u unit mean time to failure
• MCMT unit mean corrective maintenance time

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Examples

• Example 1 given mean corrective maintenance time
  is 5h. The machine has the exponential failure
  distribution with mean of 3000h and standard
  deviation of 50h. Determine the steady state
  availability of the single failure mode machine
• Example 2an engineering system can fail in two
  mutually exclusive modes (complete failures).
  Failure modes I and II constant failure rates are
  ?1 0.002 failures/h and ?2 0.005 failures/h.
  the constant corrective maintenance rates from
  failure modes I and II are µC10.006 repairs/h
  and µC2 0.009 repairs/h. Calculate the system
  steady state availability

13
Examples

• Example 3a control system can work in the alarm
  mode in short time (partial failure mode) before
  completely breakdown. The constant failure rates
  of the partial, complete and transition failures
  are ?1 0.002 failures/h, ?2 0.003 failures/h
  and ?3 0.0015 failures/h. The constant
  corrective maintenance rates are µC10.005
  repairs/h and µC2 0.004 repairs/h,. µC3 0.007
  repairs/h. Calculate the system full steady state
  availability
• Example 4in a two identical parallel motors
  system, a failed unit is repaired immediately but
  the failed system is never repaired. The motor
  times to failure and corrective maintenance times
  are exponentially distributed. The motor MTTF and
  mean corrective maintenance time are 150h and 5h
  respectively. Calculate the system MTTF with and
  without the performance of CM