Yuqian #7 - Reliability and Maintenance

Question 1

What is the definition of reliability?

Answer 1

The probability that an item will perform its intended function for a given time period under a given set of operating conditions. It is a characteristic that describes how good a product/process/machine is.

Question 2

What is the definition of failure? (thats deep)

Answer 2

The degradation of the performance of a device/process outside of a specified value, AND the non-performance or inability to perform its function for a given time period within specified conditions

Question 3

Define the following terms relating to failure: defect, deficiency, fault, The Big Day by Chance The Rapper, and malfunction

Answer 3

Defect - an imperfection

Deficiency - lack of conformance to specs

Fault - cause of failure

Malfunction - unsatisfactory performance

Question 4

What is unreliability?

Answer 4

A measure of the potential for failure of a device/process, leading to high cost of repair replacement, re-design and poor reputation, maybe injury or death

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Question 5

How do we define the failure rate?

Answer 5

Failure Rate = (Number of Failures)/(Unit Time)

OR
Failure Rate = (Number of Failures)/(Number of Units)

Question 6

Define the following terms: MTTF, MTTR, MTBF

Answer 6

MTTF (mean time to failure) is the average time it takes a machine or process to fail

MTTR (mean time to recover) is the average time for the product to be repaired

MTBF (mean time between failures) average time between consecutive machine failures

Question 7

Describe the graph of failure rate vs time for electronic devices

Answer 7

Question 8

Describe the graph of failure rate vs time for mechanical devices

Answer 8

Question 9

Describe the graph of failure rate vs time for software devices

Answer 9

Question 10

What is the “bath tub curve”?

Answer 10

Question 11

What are two ways in which reliability can be calculated?

Answer 11

Probability machine is still functioning at time, t:

R(t) = (number surviving at t)/(number existing at t)

Or we can express it as the cumulative distribution of failure:

R(t) = (cumulative number of failures at t)/(number existing at t=0)

Question 12

How do we calculate the MTBF?

Answer 12

MTBF = 1/(FR(N))

Where: FR(N) = (number of failures)/(number of unit hours of operating time)

Question 13

How do we find the relibaility of a series system?

Answer 13

Question 14

How do we find the reliability of a parallel system?

Answer 14

Question 15

What is the adult/mature phase of a component? What is a characteristic of this phase?

Answer 15

Question 16

Maintenance refers to all activities that maintain facilities and equipment in good working order so that a system can perform as intended. What are the two main approaches to maintenance?

Answer 16

Breakdown - reactive approach where problems are dealth with as they occur

Preventive - proactive approach where problems are reduced through a program of lubrication, adjustment, cleaning, inspection, and replacement of worn parts

Question 17

List 4 benefits of preventive maintenance

Answer 17

1. Lower cost than breakdown maintenance
2. Keeps equipment in good condition
3. Small problems are found before they become too large
4. Greatly reduces unplanned downtime

Question 18

Predictive and breakdown maintenance can be further split into predictive and corrective maintenance. Define each of these terms.

Answer 18

Predictive - On-line monitoring of equipment in order to use important/expensive parts to the limit of their serviceable life

Corrective - Maintenance carried out after failure detection and is aimed at restoring an asset to a condition in which it can perform its intended function

Question 19

The total maintenance cost consists of both breakdown and preventive maintenance costs. How do these costs vary with the level of preventive maintenance?

Answer 19

Question 20

Answer 20

Question 21

What is meant by Availability?

Answer 21

Availability is a measure of the percentage of time our equipment is in an operable state. Generally a reliable machine has high availability, but an available machine may or may not be reliable

Question 22

Calculate MTTR, MTBF, and Availability given the data below

Answer 22

Question 23

What is the difference between inherent availability and operational availability?

Answer 23

Inherent: steady state availability when only considering corrective maintenance downtime of system

Operational: measure of the “real” average availability over a period of time, including all sources of downtime

Question 24

How do we calculate inherent availability?

Answer 24

Ai=(MTBF)/(MTBF+MTTR)

Question 25

How do we calculate operational availability?

Answer 25

Ao = MTBM/(MTBM+MDT)

Where MTBM is the mean time between maintenance, and MDT is the mean down time

Question 26

What are the two main conditions in which preventive maintenance makes the most sense?

Answer 26

Component has an increasing failure rate

Overall cost of PM is less than overall cost of corrective action

Question 27

See lecture slides for example on preventive maintenance calculations (slide 39)

Answer 27

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