Reliability - Professor: Eyran Gisches PDF

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Professor: Eyran Gisches...

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Chapter 4s

MIS 373

Lecture Notes

Reliability o Reliability  The ability of a product, part, or system to perform its intended function under a prescribed set of conditions  Reliability is expressed as a probability  *The probability that the product or system will function when activated  The probability that the product or system will function for a given length of time o Reliability – When Activated  Finding the probability under the assumption that the system consists of a number of independent components  Requires the use of probabilities for independent events  Independent Event: Events whose occurrence or non-occurrence do not influence one another  Rule 1  If two or more events are independent and success if defined as the probability that ALL of the events occur, then the probability of success is equal to the product of the probabilities of the events  P = P1 * P2  Though individual system components may have high reliabilities, the system’s reliability may be considerably lower because all components that are in series must function  One way to enhance reliability is to utilize redundancy  Redundancy: The use of backup components to increase reliability  Rule 2  If two events are independent and success is defined as the probability that AT LEAST one of the events will occur, the probability of success is equal to the probability that (either) one occurs plus 1.00 minus that probability multiplied by the probability the other occurs  P = P1 + (1 – P1)(P2)  Rule 3  If two or more events are involved and success is defined as the probability that AT LEAST ONE of them occurs, the probability of success is 1 – P(all fail)  1 – (#1 fails AND #2 fails AND #3 fails and so on…)  1 – [(1 – P1)*(1 – P2)*(1-P3)…]  Can also be calculated by using Rule 2 multiple times

Chapter 4s

MIS 373

Lecture Notes

 Rule 2 & Rule 3 are interchangeable o Example: System Reliability

o Reliability Over Time  Reliabilities are determined relative to a specified length of time  Commonly used when establishing warranty periods o Improving Reliability  Redundancy/backups  Component Design  System Design  Production/assembly techniques  Testing

Chapter 4s  

MIS 373

Lecture Notes

Preventive maintenance procedures User education

WILL NOT BE ON EXAM: o Maintenance  All activities that maintain facilities and equipment in good working order so that a system can perform as intended  Goal: keep the production system in good working order at minimal cost  Reactive/Breakdown maintenance  Dealing with breakdowns or problems when they occur  Proactive/Preventive maintenance  Reducing likelihood of breakdowns o Total Maintenance Cost  Goal: strike a balance between cost of preventive maintenance and cost of breakdown  The amount of preventive maintenance is a function of the expected frequency of breakdown, the cost of breakdown, and the cost of preventive maintenance

o Preventative vs. Breakdown Maintenance  Looking at historical data, monthly breakdown frequency is:

Repair cost is $1,000 Cost of preventive maintenance is $1,250 If preventive maintenance is performed, the probability of a machine breakdown is negligible  Should preventive maintenance be done, or would it be cheaper to repair the machine when it breaks down? o Preventative vs. Breakdown Maintenance  Expected number of breakdowns per month:   

Chapter 4s

MIS 373

Lecture Notes

 0.2*0+0.3*1+0.4*2+0.1*3 = 1.4 breakdowns/month  Expected repair cost per month:  1.4*1,000 = $1,400/month  Cost of preventive maintenance is $1,250 per month  Since $1,250 < $1,400, preventive maintenance is cheaper than breakdown maintenance o Total Maintenance Cost Including Downtime

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QUESTIONS

1. The reliability of a Rube Goldberg machine can be calculated using: a. Rule 1 2. Adding a component to a system will affect reliability in this way: a. All are possible (increase, decrease, not change) i. Increase by a backup ii. Decrease by adding more features that must function iii. Not change by adding a component with 100% reliability or by adding a backup with 0% reliability...