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Introduction to reliability
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Reliability and Maintenance
(MANE 4015 )
Instructor: Dr. Sayyed Ali Hosseini
Winter 2015
Lecture #1
What is Reliability?
2MANE4015 Reliability and Maintenance Lecture #1
The RELIABILITY of an item/system is the probability that the
item/system performs a specified function under specified
operational and environmental conditions at and throughout a
specified time.
Reliability is the capability to operate as intended, whenever
used, for as long as needed.
Reliability is performance over time, probability that
something will work when you want it to.
3MANE4015 Reliability and Maintenance Lecture #1
Reliability
• Is a time dependent concept
• Can only be determined after an elapsed time
• But can be predicted at any time.
Reliability Characteristics
Reliability Measures
4MANE4015 Reliability and Maintenance Lecture #1
• Availability
• Mean Time to Failure (MTTF) or Mean Time Between Failures (MTBF)
• Mean Time to Repair (MTTR)
What is failure?
Failure is the termination of the ability of an item to perform its
required function.
What is Maintainability?
The ability of an item, under stated conditions of use, to be retained
in, or restored to, a state in which it can perform its required
function(s), when maintenance is performed under stated conditions
and using prescribed procedures and resources. The time takes to
repair or maintain an item is expressed as Mean Time To Repair
(MTTR).
Availability
5MANE4015 Reliability and Maintenance Lecture #1
The AVAILABILITY is the probability that a system is available for
use at a given time. In the other word, it is the probability that
the system is operational at any random time (�) . The
availability is a function of reliability and maintainability.
Availability
• Is used for repairable systems
When equipment is in a failed state it is no longer available for
work, and its reliability decreases. As the length of time in a
failed state (downtime ) increases, the maintainability of the
equipment decreases.
6MANE4015 Reliability and Maintenance Lecture #1
Mean Time To Failure (MTTF) or Mean Time Between Failures
(MTBF) is the average expected time to failure (or between
failures)
Remarks
• MTBF provides a reliability figure of merit for expected failure free operation
• MTBF provides the basis for estimating the number of failures in a given period
of time
• Even though an item may be discarded after failure and its mean life
characterized by MTTF, it may be meaningful to characterize the system
reliability in terms of MTBF if the system is restored after item failure.
MTTF or MTBF
7MANE4015 Reliability and Maintenance Lecture #1
Mean Time To Repair(MTTR) is the average expected time to
repair a damaged item after failure.
MTTR
Maintenance
8MANE4015 Reliability and Maintenance Lecture #1
Maintenance is a combination of all necessary actions for
retaining an item, or restoring it, to a serviceable condition.
• It may include servicing, repair, modification, overhaul, inspection and
condition verification.
• It increases the availability of a system.
• It keeps system’s equipment in working order.
Some Important Questions
9MANE4015 Reliability and Maintenance Lecture #1
Why do we need maintenance?
What are the costs of doing maintenance?
What are the costs of not doing maintenance?
What are the benefits of maintenance?
How can maintenance increase profitability of company?
Purpose of Maintenance
10MANE4015 Reliability and Maintenance Lecture #1
• Attempt to maximize performance of production equipment efficiently and
regularly
• Prevent breakdown or failures
• Minimize production loss from failures
• Increase reliability of the operating systems
Principle Objectives in Maintenance
11MANE4015 Reliability and Maintenance Lecture #1
• To achieve product quality and customer satisfaction through adjusted andserviced equipment
• Maximize useful life of equipment
• Keep equipment safe and prevent safety hazards
• Minimize frequency and severity of interruptions
• Maximize production capacity – through high utilization of facility
Problems in Maintenance
12MANE4015 Reliability and Maintenance Lecture #1
• Lack of management attention to maintenance
• Little participation by accounting in analyzing and reporting costs
• Difficulties in applying quantitative analysis
• Difficulties in obtaining time and cost estimates for maintenance works
• Difficulties in measuring performance
Why these problems exist?
• Failure to develop written objectives and policy
• Inadequate budgetary control
• Inadequate control procedures for work order, service requests etc.
• Infrequent use of standards
• To control maintenance work
• Absence of cost reports to aid maintenance planning and control system
Different Types of Maintenance
13MANE4015 Reliability and Maintenance Lecture #1
• Corrective or Breakdown maintenance
• Scheduled maintenance
• Preventive maintenance
• Predictive (Condition-based) maintenance
Corrective or Breakdown Maintenance
14MANE4015 Reliability and Maintenance Lecture #1
Corrective or Breakdown maintenance implies that repairs are
made after the equipment is failed and can not perform its
normal function anymore
Quite justified in small factories where:
• Down times are non-critical and repair costs are less than other type ofmaintenance
• Financial justification for scheduling are not felt
Disadvantages of Corrective Maintenance
15MANE4015 Reliability and Maintenance Lecture #1
• Breakdown generally occurs at inappropriate times leading to poor and hurriedmaintenance
• Excessive delay in production & reduces output
• Faster plant deterioration
• Increases chances of accidents and less safety for both workers and machines
• More spoilt materials
• Direct loss of profit
• Can not be employed for equipment regulated by statutory provisions e.g.cranes, lift and hoists etc.
Schedules Maintenance
16MANE4015 Reliability and Maintenance Lecture #1
Scheduled maintenance is a maintenance procedure that is
repeated on a regular basis. If scheduled maintenance is
neglected, breakdown and failure is likely to occur.
Scheduled Maintenance includes:
• inspection
• lubrication
• repair and overhaul of equipment
Examples:
• overhauling of machines
• changing of heavy equipment oils
• cleaning of water and other tanks etc.
Preventive Maintenance
17MANE4015 Reliability and Maintenance Lecture #1
“Prevention is better than Cure”
Preventive Maintenance
• locates weak spots of machinery and equipment
• Provides them periodic/scheduled inspections and minor repairs to reduce thedanger of unanticipated breakdowns
Advantages of Preventive Maintenance
18MANE4015 Reliability and Maintenance Lecture #1
• Reduces break down and thereby down time
• Less odd-time repair and reduces over time of crews
• Greater safety of workers
• Lower maintenance and repair costs
• Less stand-by equipment and spare parts
• Better product quality and fewer reworks and scraps
• Increases plant life
• Increases chances to get production incentive bonus
Predictive (Condition-Based) Maintenance
19MANE4015 Reliability and Maintenance Lecture #1
In predictive maintenance, machinery conditions areperiodically monitored and this enables the maintenance crewsto take timely actions, such as machine adjustment, repair oroverhaul. It makes use of human sense and other sensitiveinstruments, such as audio gauge, vibration analyzer, amplitudemeter, pressure, temperature and resistance strain gauges etc.
Examples:
• Unusual sounds coming out of a rotating equipment predicts a trouble.
• An excessively hot electric cable predicts a trouble.
• Simple hand touch can point out many unusual equipment conditions and thuspredicts a trouble.
Failure Cost VS. Maintenance Commitment
20MANE4015 Reliability and Maintenance Lecture #1
Maintenance Commitment
Co
st
Failure Cost
Maintenance Cost VS. Maintenance Commitment
21MANE4015 Reliability and Maintenance Lecture #1
Maintenance Commitment
Co
st
Maintenance Cost
Total Maintenance Cost
22MANE4015 Reliability and Maintenance Lecture #1
Maintenance Commitment
Co
st
Maintenance Cost
Failure Cost
Total Maintenance CostOptimum
Emerging Strategies for Maintenance Management
23MANE4015 Reliability and Maintenance Lecture #1
Conclusions
24MANE4015 Reliability and Maintenance Lecture #1
In general, reliability analysis and maintenance procedures are effective tools inthe hands of engineers to:
• Predict the expected life of systems and their major components.
• Predict the availability of systems
• Predict the expected maintenance activities
• Predict the support resources and spare parts required for effective operation
It must be noted that accurate prediction of the above mentioned items can onlybe achieved by careful consideration of reliability and maintainability factors, mosteffectively at the design stage.