Yuqian #7 - Reliability and Maintenance Flashcards
What is the definition of reliability?
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.
What is the definition of failure? (thats deep)
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
Define the following terms relating to failure: defect, deficiency, fault, The Big Day by Chance The Rapper, and malfunction
Defect - an imperfection
Deficiency - lack of conformance to specs
Fault - cause of failure
Malfunction - unsatisfactory performance
What is unreliability?
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
Kanye West album announcements
How do we define the failure rate?
Failure Rate = (Number of Failures)/(Unit Time)
OR
Failure Rate = (Number of Failures)/(Number of Units)
Define the following terms: MTTF, MTTR, MTBF
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
Describe the graph of failure rate vs time for electronic devices
Describe the graph of failure rate vs time for mechanical devices
Describe the graph of failure rate vs time for software devices
What is the “bath tub curve”?
What are two ways in which reliability can be calculated?
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)
How do we calculate the MTBF?
MTBF = 1/(FR(N))
Where: FR(N) = (number of failures)/(number of unit hours of operating time)
How do we find the relibaility of a series system?
How do we find the reliability of a parallel system?
What is the adult/mature phase of a component? What is a characteristic of this phase?
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?
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
List 4 benefits of preventive maintenance
- Lower cost than breakdown maintenance
- Keeps equipment in good condition
- Small problems are found before they become too large
- Greatly reduces unplanned downtime
Predictive and breakdown maintenance can be further split into predictive and corrective maintenance. Define each of these terms.
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
The total maintenance cost consists of both breakdown and preventive maintenance costs. How do these costs vary with the level of preventive maintenance?
What is meant by Availability?
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
Calculate MTTR, MTBF, and Availability given the data below
What is the difference between inherent availability and operational availability?
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
How do we calculate inherent availability?
Ai=(MTBF)/(MTBF+MTTR)
How do we calculate operational availability?
Ao = MTBM/(MTBM+MDT)
Where MTBM is the mean time between maintenance, and MDT is the mean down time
What are the two main conditions in which preventive maintenance makes the most sense?
Component has an increasing failure rate
Overall cost of PM is less than overall cost of corrective action
See lecture slides for example on preventive maintenance calculations (slide 39)
Its 9:34pm at the time of making this card and jumping out my window seems like a really nice alternative to this whole study thing
